Carbon steel forging is a time-tested metalworking process in which carbon steel is shaped into desired forms through a combination of heat and applied pressure. Essentially, the steel is heated to a specific temperature to achieve plasticity, then repeatedly hammered or pressed to form parts with the intended geometry. Unlike machining, which removes material, forging relies on plastic deformation in the solid state, producing components with superior structural integrity.

Carbon steel is an iron-carbon alloy with a carbon content typically ranging from 0.05% to 2.0%, though most industrially used carbon steels contain no more than 1.2% carbon. Apart from carbon and iron, other elements are present only in trace amounts, primarily residual impurities from production. Its low cost, wide availability, and versatility make carbon steel forging one of the most commonly used metals in industrial manufacturing.

Carbon steel can be divided into four major categories based on carbon content, each offering unique mechanical properties and applications.

Low carbon steel contains approximately 0.05%–0.25% carbon and up to 0.4% manganese. Commonly referred to as “mild steel,” it has relatively low strength but excellent formability and is cost-effective. Surface hardness can be improved through carburization, while the core maintains toughness and ductility. Common grades include AISI 1018 and 1020, widely used for components where ease of machining and forming is prioritized over high strength. Applications include automotive panels, structural sections, and simple mechanical components.

Medium carbon steel contains 0.29%–0.54% carbon and 0.60%–1.65% manganese. This type strikes a balance between strength and ductility while offering superior wear resistance. AISI 1045 is a representative medium carbon steel, frequently used in large parts, industrial machinery components, and automotive applications where load-bearing capacity and durability are critical.

High carbon steel contains 0.55%–0.95% carbon and 0.30%–0.90% manganese. Its high carbon content confers excellent strength and shape retention, making it ideal for springs, high-strength wires, and cutting tools. Although the hardness of high carbon steel is advantageous, machining and forming it require greater skill and attention.

Ultra-high carbon steel contains 0.96%–2.1% carbon. These steels are extremely hard but inherently brittle, requiring careful handling during forging and machining. They are primarily used in the production of specialty tools, dies, and applications demanding extreme surface hardness.

The forging process transforms carbon steel into mechanically superior components through controlled heating and deformation. Hot forging is the most common method for carbon steel and involves several key steps:

Steel ingots or billets are first cut to suitable sizes, then heated in a furnace to a temperature range of 1050°C–1250°C. This temperature ensures the steel achieves good plasticity while avoiding excessive grain growth, which could negatively impact the final mechanical properties. Proper temperature control is crucial to ensure uniformity and prevent defects in the forging process.

Heated billets are transferred to forging equipment such as hammers or presses, where compressive forces shape the steel into the desired form. Depending on the complexity of the component, two primary forging methods are used:

Open-Die Forging: Suitable for simple shapes, open-die forging allows flexibility in forming while maintaining high material integrity.

Impression-Die (Closed-Die) Forging: Ideal for complex, precise components, this method uses dies to produce intricate shapes with minimal post-forging machining.

Forged parts typically exhibit flash (excess material) and oxide scale on the surface. These are removed through trimming and cleaning processes, which prepare the parts for machining and further processing. Proper surface treatment enhances dimensional accuracy and ensures higher quality in the finished product.

Forged components may undergo heat treatments such as annealing, normalizing, quenching, and tempering. These processes refine the steel’s microstructure, enhancing mechanical properties, improving hardness, and optimizing machinability. Heat treatment ensures the forged part meets specific performance requirements, whether it is strength, toughness, or wear resistance.

Precision machining via CNC equipment brings the part to final dimensions, while strict quality inspections ensure compliance with design standards. Inspection methods include:

Dimensional checks

Hardness testing

Non-destructive testing (ultrasonic, magnetic particle)

These measures guarantee reliability and longevity, particularly for critical load-bearing applications.

Carbon steel forging is favored in industrial manufacturing for its multiple, irreplaceable advantages:

During forging, repeated plastic deformation compresses internal pores and eliminates shrinkage defects, while refining the grain structure. This results in forgings with higher structural integrity than castings, enabling them to withstand higher pressures and temperatures, even under harsh operating conditions.

Forging aligns metal grains along the component’s contours, creating continuous, fiber-like structures. This directional grain flow enhances fatigue resistance and impact toughness, providing superior performance compared to cast or machined components. Properly designed forgings maximize the material’s natural strength along the flow lines, delivering exceptional durability in high-stress applications.

Carbon steel is widely available and significantly cheaper than materials like stainless steel. While forging equipment requires investment, high-volume production achieves competitive unit costs. Combined with the long service life of forged components, carbon steel forging often results in lower total lifecycle costs.

Many low- and medium-carbon steels offer excellent machinability after forging. Certain grades can also be welded, although high-carbon steels may require preheating or post-weld heat treatment. Forgings provide flexibility for subsequent machining, assembly, and fabrication.

Forging and casting are two primary methods of producing metal components, each suited for different applications:

Forged parts typically exhibit 15–20% higher strength and toughness than castings. Forging eliminates internal porosity and produces a denser, more uniform grain structure, enhancing fatigue and impact resistance. Castings can achieve more complex geometries, but they are more prone to defects, limiting their load-bearing capacity.

Casting generally requires lower initial investment and unit costs, making it suitable for large-scale production of complex shapes. Forging involves higher tooling and equipment costs, but superior reliability, longer service life, and enhanced mechanical performance justify the investment for critical components.

Forging is preferred for shafts, gears, connecting rods, flanges, and other high-stress or fatigue-prone parts. Casting is suitable for complex housings, enclosures, and components with low mechanical load requirements.

Carbon steel forgings are widely used across industries due to their exceptional mechanical properties, reliability, and cost-effectiveness.

Forgings are heavily used in vehicles, often exceeding 250 components per car. Critical parts such as crankshafts, connecting rods, camshafts, drive shafts, gears, differential gears, and universal joints rely on forging to ensure durability, smooth operation, and performance under dynamic loads.

In the oil and gas sector, carbon steel forgings are used for drill bits, drill collars, valve bodies, flanges, and pipe fittings. These components must withstand high pressure, high temperature, and corrosive environments. Forged components provide the structural integrity and pressure resistance required for safe and efficient operation.

Excavators, bulldozers, cranes, and other heavy equipment rely on carbon steel forgings for structural components, hydraulic cylinders, and frames. These parts endure tremendous stress and impact, and forging provides the strength and toughness necessary for reliability and long service life.

Plowshares, cultivators, tractor drive shafts, connecting rods, and gears benefit from forging, which delivers durability, wear resistance, and impact tolerance essential for working in harsh environments like soil, sand, and rocks.

Forged carbon steel is widely used in wrenches, hammers, pliers, knives, axes, and saw blades. High-carbon steel achieves superior hardness and wear resistance after forging and heat treatment, maintaining sharpness and performance over prolonged use.

• AISI 1018: Low-carbon steel with moderate strength, excellent ductility, and low cost.
• AISI 1045: Medium-carbon steel with high strength and wear resistance, ideal for load-bearing parts.
• AISI 4140: Alloy structural steel with high strength for heavy-duty applications.
• A36/A105: Standard structural forging steels with well-rounded properties.
• 8620 Alloy Steel: High surface hardness with tough core, suitable for wear-resistant parts.

Material selection depends on load requirements, operating conditions, and whether heat treatment is needed.

Quality control in carbon steel forging spans the entire production process:

1. Material Certification: Confirm chemical composition and traceability.
2. Process Monitoring: Control heating temperature, forging ratio, and deformation rate.
3. Dimensional Inspection: Verify critical dimensions against design specifications.
4. Non-Destructive Testing: Detect internal or surface defects using ultrasonic or magnetic particle inspection.
5. Hardness Verification: Confirm post-heat-treatment hardness meets design requirements.
6. Custom forgings often optimize material usage, geometry, machining allowances, and dimensional precision, reducing overall production costs while ensuring performance.

Carbon steel forging is a mature, reliable, and cost-effective metal forming process. By combining the advantages of carbon steel with the benefits of forging, manufacturers can produce components that are strong, tough, and durable. These qualities meet the rigorous demands of automotive, oil and gas, heavy machinery, and agricultural industries.

While forging may be less suitable for extremely complex shapes, it remains the preferred method for critical load-bearing parts. By carefully considering part complexity, mechanical requirements, production volume, and budget, carbon steel forging delivers structural integrity, reliability, and long service life, making it a trusted and indispensable choice in industrial manufacturing.

Forging is a metal forming process in which a metal workpiece is placed between two dies and pressure is applied to change its shape. This process is usually performed on a forging press, which slowly applies pressure until the workpiece is shaped into the desired new form. Compared to other forging processes, forging press has obvious cost advantages and is widely adopted in industry. More importantly, the forging process allows the final product's grain flow to align with the workpiece contour, a feature that can significantly enhance product strength.

A forging press forms metal by applying gradual and controllable pressure to a workpiece in a die through a vertical ram. This process is similar to drop forging, but the core difference is that forging press uses constant pressure rather than repeated impacts.

The stable motion provided by the ram allows deeper penetration into the workpiece, producing uniform plastic deformation. This working method ensures stable and consistent product quality.

Forging presses can be classified in multiple ways. By pressure generation method, they can be divided into mechanical, hydraulic, screw, servo, pneumatic, and extrusion types; by frame design, they can be divided into upright and C-frame basic forms. Different types of forging presses differ in working principle, performance characteristics, and applicable scenarios, and users can choose according to specific production requirements.

• Mechanical Forging Press: Mechanical forging presses are the most basic type of forging equipment, driven by an electric flywheel that transmits energy to a slowly pressing ram. It should be noted that mechanical forging presses are often confused with hammer presses, but their working methods are different. Mechanical forging presses complete forging in a single long stroke through multiple rapid impacts, controlled by an air clutch. During the stroke, the crankshaft applies stable, continuous pressure to the ram. The ram reaches its highest speed in the middle of the stroke, but maximum pressure is fully applied only at the end of the stroke, completing the forming of the workpiece. Because the movement distance of the mechanical ram is fixed, a proper gap must be left at the end of the stroke to prevent the ram from sticking to the die, which could cause stoppage or equipment damage. Mechanical forging presses can reach pressures of up to 12,000 tons, suitable for high-volume production scenarios. With the development of robotic forging automation, many forging processes can be operated automatically through computer programming, ensuring consistency and reducing the risk of human error. Despite the high cost of mechanical forging presses, they are widely used in the automotive industry, mainly for forming transmission system components. Additionally, government departments use this equipment for coin minting. Modern mechanical forging presses are highly automated, capable of up to 70 strokes per minute, widely used for mass production of automotive parts, hand tools, and hardware components.
• Hydraulic Forging Press: Hydraulic forging presses generate downward pressure through a hydraulic system and are usually used for open-die forging. Open-die forging features a flat ram that presses the metal flat, in contrast to closed-die forging, which shapes the metal into a specific form using a mold. The greatest advantage of hydraulic forging presses is precise control of stroke speed and pressure, enabling the forming of complex workpieces without opening or closing molds. Large hydraulic presses can exert pressures up to 50,000 tons or even 75,000 tons, mainly used for metals other than steel, producing large aircraft components such as turbine shafts through open-die techniques. Hydraulic forging presses are also suitable for isothermal forging and large, complex-shaped production. It should be noted that hydraulic dies are prone to wear due to longer contact with heated workpieces, so die life must be fully considered in maintenance and production planning.
• Screw Forging Press: Screw forging presses combine mechanical and hydraulic drive features, with twin flywheels and a screw mechanism converting rotational energy into downward ram pressure. During the ram's downward movement, the screw continues to rotate, generating more pressure. After one stroke, the screw and flywheel reverse to return to the starting position. Modern technology allows precise control of ram downward distance and applied pressure, ensuring uniform thickness of metal sheets during flattening. Screw forging presses can reach pressures of 31,000 tons, suitable for mass production, and can process steel, copper, aluminum, titanium alloys, as well as special aerospace and medical materials. With increasing aerospace development needs, the importance of screw forging presses may further rise. Screw forging presses are suitable for producing non-ferrous alloys, tool steel, and heavy industrial components, widely used in manufacturing large valves, flanges, and heavy machinery parts.
• Servo Forging Press: Servo forging presses use a servo motor to drive an eccentric gear, allowing precise control of slide position, speed, stroke, and pressure. This precise control makes them especially suitable for precision forging and high-mix, low-volume production scenarios. Servo presses have multiple advantages: high production efficiency, high part accuracy (up to ±0.01 mm), low noise, precise control, and energy efficiency. They are also easy to maintain and support the requirements of Industry 4.0 smart factories.
• Pneumatic Forging Press: Pneumatic forging presses operate by compressed air or gas to drive the ram, suitable for light to medium precision forging, deep drawing, stamping, assembly, and shearing operations. This type of equipment is mainly used for non-ferrous metals, aluminum, and light steel processing.
• Extrusion Forging Press: Extrusion presses are used for horizontal extrusion forging, capable of shaping rods and tubes into complex bends. During hot extrusion forging, the rod is placed between two dies, and lateral pressure is applied to bend it. Modern technology allows multiple precise bends on a single tube. Because the dies are not vertically compressed, extrusion presses can produce multiple shapes, but sometimes multiple strokes are required to achieve the desired form, up to five strokes. Extrusion presses are suitable for producing various shaft components, such as flanged shafts, gear clusters, socket wrenches, and sometimes for preparing materials for hammer, mechanical, hydraulic, or screw forging presses. Extrusion forging uses horizontal ram presses to apply axial pressure to heated workpieces, changing the end shape of the workpiece, commonly producing bolts, nuts, flanged shafts, shaft components, and special parts for automotive, aerospace, and construction industries.

• Upright Press: Upright forging presses provide maximum rigidity, suitable for high-tonnage, high-volume forging operations. This design can withstand extremely high pressure and ensures stability during production.
• C-Frame Press: C-frame presses have an open side, facilitating small-batch production, maintenance, and die replacement. This design provides better operational convenience, suitable for scenarios requiring higher production flexibility.

Forging offers significant advantages in metal processing, mainly in material performance, production efficiency, and safety. Compared to casting and traditional machining, forging can utilize materials more efficiently and produce metal parts with higher strength and more stable performance.

Forging is more efficient and economical than casting and traditional machining, and the finished product has higher strength. Through plastic processing, forging rearranges the metal grain flow, improving mechanical properties and fatigue life. Continuous grain structures eliminate weak points, making forgings widely used in aerospace, automotive, oil & gas, agriculture, and heavy machinery industries.

Forging provides high strength-to-weight ratio, reduces defects, and improves subsequent machining accuracy. Preheating workpieces optimizes grain flow, reduces forging force, and maintains metal integrity, improving tensile strength, fatigue resistance, and durability.

Modern forging presses can achieve high output, producing 40 to 50 forgings per minute. Products include bolts, nuts, valves, shafts, and structural brackets. Die design enables complex shapes and precision control, with deep drawing up to six times the material thickness while maintaining tight tolerances.

Forging extends die life and saves costs. With CNC automation, forging presses can achieve precise control, traceability, and high repeatability, ensuring uniform plastic deformation and optimized mechanical properties such as yield strength, ductility, and impact toughness.

High operator safety is an important feature of modern forging presses. Modern presses are mostly automatic or semi-automatic, reducing manual operation and risk. Forgings have excellent mechanical properties, including strength, impact toughness, ductility, and fatigue life, and can be optimized for specific stress directions.

The forging process ensures part consistency and uniformity, and strict quality control meets high standard processing requirements. Forging presses can process carbon steel, alloy steel, stainless steel, aluminum, copper, brass, titanium, and other non-ferrous metals, with a very wide application range.

Forging technology, with its efficiency, economy, and high quality, has been widely used in multiple key industrial fields. From automotive manufacturing to aerospace, from heavy machinery to medical devices, different industries choose suitable forging equipment and processes based on production needs.

Mechanical forging presses are most widely used in the automotive industry, mainly for forming transmission system components. High automation of modern mechanical presses allows meeting high-volume production requirements, with speeds up to 70 strokes per minute ensuring efficient output.

Hydraulic forging presses play a key role in aerospace, especially large hydraulic presses with pressures up to 50,000 tons, used for large aircraft components such as turbine shafts. Screw presses are also widely used for processing special aerospace materials. With increasing aerospace demand, the importance of relevant forging technology continues to rise.

Screw presses are widely used for large valves, flanges, and heavy machinery parts. Friction-driven, direct electric, and gear-driven presses achieve high-tonnage forging through different transmission mechanisms, meeting heavy industrial manufacturing needs.

Screw presses can process specialty medical materials, providing high-precision, high-quality metal parts for medical device manufacturing.

Forging technology is used to produce bolts, nuts, flanged shafts, shafts, and special parts for construction. Mass production of hand tools and hardware also relies on the high efficiency of mechanical forging presses.

Government departments use mechanical forging presses to mint coins, reflecting the value of forging technology in precision manufacturing.

Choosing the appropriate forging process requires consideration of multiple factors:

First is production volume. Mechanical forging presses are suitable for large-scale production, while servo presses are suitable for high-mix, small-batch production. C-frame presses are convenient for small-batch production and die changes.

Second is workpiece material and size. Hydraulic presses are suitable for metals other than steel and large, complex shapes; screw presses are suitable for non-ferrous alloys and special materials; pneumatic presses are suitable for non-ferrous metals, aluminum, and light steel.

Third is precision requirements. Servo presses provide the highest precision, suitable for precision forging. For complex-shaped workpieces, extrusion presses may be considered, though multiple strokes may sometimes be required to achieve the desired shape.

Fourth is cost consideration. Mechanical presses are costly but suitable for large-scale production to spread costs, while hydraulic presses require consideration of die wear and maintenance costs.

Forging, as an important metal forming process, changes the shape of metal workpieces by applying pressure between dies, offering relatively low cost and high product strength. Forging presses are classified by pressure generation method into mechanical, hydraulic, screw, servo, pneumatic, and extrusion types, and by frame design into upright and C-frame types.

Different types of forging presses have distinct characteristics in industrial applications: mechanical presses are suitable for mass production, hydraulic presses for large complex parts, screw presses for special material processing, servo presses for precision control, pneumatic presses for light to medium operations, and extrusion presses for horizontal extrusion and bending.

The forging process rearranges grain flow through plastic deformation, improving mechanical performance and fatigue life, with wide applications in automotive, aerospace, heavy machinery, medical devices, construction hardware, and other fields. With the development of automation, intelligence, and precision control, forging technology will continue to provide efficient, economical, and high-quality metal forming solutions for modern manufacturing.

Metal forging is one of humanity's earliest mastered manufacturing processes and remains a critical, irreplaceable technology in modern industrial production. Simply put, forging is the process of shaping metal by applying external forces, typically through hammering or pressing. The production of automotive components, railway wheels, aircraft engine parts, and structural steel members all relies heavily on forging. At the heart of this process lies temperature. Metals become soft at high temperatures, allowing them to be shaped like dough. However, improper temperature control can either leave the metal too hard, causing cracks, or overheat it, degrading its material properties. This is why temperature monitoring equipment is as essential as the forging machinery itself in a workshop.

Forging can be broadly divided into two main types: hot forging and cold forging. Hot forging, the more common approach, requires heating the metal to elevated temperatures. Most steels are hot forged between 900°C and 1250°C (approximately 1650°F to 2280°F).

Different steels require distinct temperature ranges. Ordinary carbon steel typically forges best between 1200°C and 1450°C. Austenitic stainless steels demand higher temperatures, usually above 1038°C (1900°F). Alloy steels, with their complex chemical compositions, often require even higher temperatures and stricter heating control. High-alloy tool steels may sometimes need forging temperatures exceeding 1450°C.

Cold forging, by contrast, is carried out at or near room temperature. While it requires no heating, it demands greater force. Cold forging is suitable for certain smaller components, but most industrial forging is performed as hot forging.

Temperature control is critical throughout the forging process, directly affecting both the deformability of metal and the quality of the finished product. Low temperatures make metals hard and prone to cracking, while excessively high temperatures can coarsen grain structure and degrade mechanical properties. Precise temperature management is key to preventing defects and ensuring consistency.

Metal plasticity refers to a material's ability to deform under stress without fracturing. Higher temperatures increase atomic activity within the metal, allowing the crystal lattice to slide more easily and making the material more malleable. This is why hot forging can more easily produce complex shapes than cold forging.

If the metal is too cold, its strength remains high, making deformation difficult. Forced forging can introduce internal cracks, and in severe cases, the metal may fracture outright during hammering. Such defects from low-temperature forging are often irreparable, leading to scrap.

Conversely, excessive temperatures are equally problematic. Prolonged exposure to high heat can cause grain growth, reducing strength and hardness. Surface oxidation may occur, forming scale, and defects such as internal porosity, surface folding, or delamination can arise.

Forging temperature directly impacts the mechanical properties of the finished product in three main ways:

• Grain size control: Temperature affects the size and morphology of grains in the metal. Excessive heat can lead to grain coarsening, reducing hardness and strength, while slightly lower temperatures can refine grains and improve overall properties. This explains why high-strength parts require strict final forging temperature control.
• Grain boundary distribution: At different temperatures, the position and number of grain boundaries change. Grain boundaries influence plasticity and toughness. Proper temperature control optimizes grain boundary distribution, enhancing performance.
• Phase transformation regulation: Many metals undergo phase changes during heating and cooling, such as solid solution transformations or precipitate formation. These transformations directly affect hardness and strength. Precise temperature control enables regulation of these processes to achieve the desired microstructure.

Modern forging is not simply heating metal and striking it; it is a highly controlled process requiring continuous temperature monitoring.

Before placing billets into the furnace, the furnace itself must be preheated to ensure uniform temperature distribution and prevent localized overheating or underheating. Infrared thermometers provide precise furnace temperature monitoring, creating a stable environment for subsequent heating.

Preheating is often underestimated. Uneven furnace temperature leads to inconsistent billet heating, resulting in irregular deformation and internal stress during forging.

This stage is the most critical for temperature control. Billets must be continuously heated in the furnace until they reach the target forging temperature, using non-contact infrared sensors for ongoing monitoring.

Non-contact measurement is essential because contact thermometers are prone to damage in high-temperature environments and are slower. Infrared thermometers measure surface temperature from a safe distance in real time. Advanced models use dual-wavelength sensors to compensate automatically for surface emissivity variations, maintaining accuracy even on reflective surfaces.

This stage must avoid underheating, which makes forging difficult and crack-prone, and overheating, which degrades material, coarsens grains, and can even burn the metal.

Once removed from the furnace, the metal temperature drops rapidly. Forging must be completed before it falls below the critical range. Real-time temperature monitoring ensures the metal stays within the optimal forging window.

Heat loss occurs from contact with dies and exposure to air, particularly for large components with high surface areas. Monitoring helps operators adjust forging speed or determine whether reheating is necessary.

After forging, many components undergo heat treatment to enhance properties. Common processes include normalizing, quenching, and tempering, which often demand even stricter temperature accuracy than forging itself. For example, quenching requires heating to precise temperatures followed by rapid cooling, with even minor deviations impacting hardness. Tempering occurs at lower temperatures to relieve stress and adjust toughness.

Accurate temperature control requires reliable measurement, yet forging environments are harsh: high heat, dust, and vibration are standard. This has driven the development of industrial-grade temperature monitoring systems.

Infrared thermometers detect emitted infrared radiation, converting it into precise temperature readings. Advantages include non-contact measurement, fast response, durability in high-temperature and dusty environments, and the ability to measure very high temperatures (hundreds of °C up to over 2000°C). High-end models use dual-wavelength or colorimetric compensation to correct emissivity changes, which is critical when measuring reflective metals.

Thermal cameras generate detailed temperature distribution maps across large surfaces, showing temperature variations in color. They allow operators to identify overheated or underheated areas, adjust billet positioning, and fine-tune furnace parameters for uniform heating.

Modern forging lines often use automated temperature control systems comprising sensors, controllers, and actuators. Sensors collect data, controllers compute adjustments, and actuators (e.g., burners, valves, conveyors) maintain stable operation. This closed-loop control enables unmanned precise temperature management, prevents overheating, and triggers automatic extraction when the target temperature is reached.

• Enhanced product quality: Stable temperatures reduce defects like cracks, warping, uneven surfaces, and porosity. Consistency improves, increasing yield and reliability—critical for safety-sensitive sectors like automotive and aerospace. For example, railway wheels endure high loads; precise forging ensures consistent strength and durability.
• Reduced energy consumption: Precise monitoring avoids over-heating and excessive dwell times, lowering gas and electricity use. Infrared sensors detect anomalies promptly, preventing waste and improving furnace efficiency.
• Minimized downtime: Real-time monitoring provides early warnings for potential equipment failures, enabling preventive maintenance and avoiding costly production interruptions.
• Facilitated automation: Reliable temperature data is the foundation for automated forging lines, allowing fully automated heating, transfer, forging, and heat treatment with minimal human intervention. Automation increases efficiency and reduces human error, while digital data storage supports traceability and process optimization.

• Automotive: Engine crankshafts, connecting rods, gears, and half-shafts require high-strength, fatigue-resistant forging with strict temperature control.
• Aerospace: Turbine disks, blades, and landing gear are forged from high-temperature alloys with narrow temperature windows, often ±10°C, monitored using advanced thermal imaging and simulations.
• Railway: Wheels, axles, and couplers must endure impact loads; forging temperatures ensure optimal plasticity to avoid internal defects.
• Energy and heavy industry: Rotors, rolls, and large valves weighing tens of tons require multi-point monitoring to prevent internal cracks from temperature gradients.

• Inaccurate temperature measurement: Causes include oxide layer thickness variations, emissivity changes, angle misalignment, and environmental interference. Use dual-wavelength infrared sensors, maintain perpendicular measurement angles, calibrate with thermocouples, and clean surfaces as needed.
• Uneven workpiece temperature: Causes include non-uniform furnace temperature, overcrowding, and rapid surface heating. Solutions include optimizing furnace design, controlling load and spacing, staged heating, and thermal imaging inspections.
• Rapid temperature drop: Causes include large surface area, low ambient temperature, or slow transfer. Solutions include optimized production pacing, insulation or heated conveyors, larger equipment for faster forging, or reheating intermediate steps.

Temperature control in metal forging is a sophisticated integration of physics, equipment technology, and practical experience. Within the 900°C–1450°C range, even small variations can impact the final product. Understanding temperature effects on plasticity and microstructure, leveraging infrared monitoring technologies, and establishing robust control systems are essential for forging companies seeking competitive advantage.

Whether for small specialized shops or large integrated manufacturers, investment in temperature control yields benefits in quality, efficiency, and cost. For those wishing to deepen their expertise, mastering heat treatment principles and material-specific forging characteristics lays the foundation for tailored temperature control solutions. As forging technology evolves, temperature control will remain the driving force for higher quality and greater efficiency in the industry.

High-temperature alloy forgings are essential advanced materials in modern industry and are widely used in extreme operating environments such as aero engines, spacecraft, and petrochemical equipment. These materials must maintain excellent strength, creep resistance, and corrosion resistance at temperatures ranging from 600°C to 1200°C. As a result, their manufacturing process is far more complex than that of ordinary steels. This article focuses on the core technical challenges in high-temperature alloy forging and systematically discusses key processes including heating control, deformation resistance breakthrough, microstructure regulation, and advanced manufacturing methods.

The primary difficulty in high-temperature alloy forging lies in the precise control of the heating stage. Compared with carbon steel, high-temperature alloys exhibit poor thermal conductivity. If conventional heating methods are used, excessive temperature differences between the surface and the interior of the billet may easily occur, generating severe thermal stress and ultimately leading to cracking and scrapping.

To address this challenge, the industry generally adopts a stepwise heating process. Specifically, the billet is gradually heated from room temperature at a rate of approximately 5–10°C per minute until the temperature exceeds 1200°C. This slow heating process, combined with real-time monitoring using infrared temperature measurement technology, can strictly control the temperature difference within ±20°C across the billet.

Taking GH4145 alloy production as an example, the application of this process reduced the grain coarsening rate from 30% under traditional methods to about 5%, significantly improving material strength.

The control of holding time is equally critical and must be managed at a minute-level precision. Excessively long holding times may cause abnormal grain growth and degrade material performance, while insufficient holding time prevents uniform diffusion of alloying elements, directly affecting subsequent deformation quality. Such stringent time requirements reflect the high level of precision required in high-temperature alloy manufacturing.

High-temperature alloys exhibit extremely high deformation resistance. While ordinary steels can be formed under 1000-ton press pressure, high-temperature alloys often require multi-thousand-ton hydraulic presses to achieve effective plastic deformation. This characteristic poses significant challenges to equipment capability and process design.

Isothermal forging technology provides an effective solution to this problem. The “two-pass ring rolling” process adopted in patent technologies represents a typical example. In the first pass, upsetting and punching operations are used to form a pre-shaped billet. In the second pass, final forming is completed under conditions where the die temperature is close to the billet temperature, with the temperature difference controlled within ±15°C.

This method increases the reduction of cross-sectional area of GH4145 alloy to 44%, achieves a tensile strength of 1268 MPa, and reduces energy consumption by 15%.

The application of die preheating technology effectively suppresses surface chilling effects, increasing the qualification rate of thin-walled components from 65% to 88%. This refined control of temperature and stress fields reflects the transformation of modern forging technology from experience-based operation to scientific precision manufacturing.

Microstructure regulation represents the ultimate objective of forging technology and directly determines the final service performance of high-temperature alloys. Controlling dynamic recrystallization to refine grain size is a core approach to improving high-temperature performance.

Taking aero-engine turbine discs as an example, multi-pass forging of 3–5 heats is usually adopted during manufacturing. Deformation energy is gradually accumulated to fully break down the coarse dendritic structure in the original casting, ultimately forming an equiaxed grain structure with an average grain size not exceeding grade 5.

In the forging of DD6 single-crystal turbine blades, a “high-temperature deformation followed by rapid cooling” process is used to achieve more uniform carbide distribution along grain boundaries, extending blade service life by approximately 20%.

For powder metallurgy high-temperature alloys, the combination of hot isostatic pressing and multi-directional forging is required to eliminate particle boundaries and ensure density levels above 99.5%. This pursuit of microscopic structural perfection reflects the core philosophy that “details determine success” in high-end manufacturing.

High-temperature alloys possess high alloying degrees and multiphase complex structures, resulting in relatively low process plasticity. In particular, impurities such as sulfur, lead, and tin may weaken intergranular bonding at high temperatures and further reduce plasticity.

The total content of strengthening elements such as aluminum and titanium is commonly used as an indicator of alloy plasticity. When the combined aluminum and titanium content reaches or exceeds 6%, the material plasticity becomes very low. The process plasticity of nickel-based high-temperature alloys is generally lower than that of iron-based high-temperature alloys.

Such plasticity is highly sensitive to deformation speed and stress state. Some alloy ingots and intermediate billets must be formed using special processes such as low-speed deformation, canned upsetting, canned rolling, or even canned extrusion.

Due to complex compositions, high-temperature alloys exhibit high recrystallization temperatures and slow recrystallization rates. During deformation, they show high deformation resistance and work-hardening tendencies. Their deformation resistance is typically 4–7 times that of ordinary structural steel.

Compared with carbon steel, high-temperature alloys also have lower melting points. Excessively high heating temperatures may cause overheating or burning, while stopping forging at too low a temperature reduces plasticity and increases deformation resistance, often leading to mixed hot-and-cold deformation and coarse grain formation. Therefore, the forging temperature range of high-temperature alloys is very narrow, usually about 200°C, and for nickel-based heat-resistant alloys it is even narrower, mostly within 100–150°C, and sometimes less than 100°C.

In addition, the thermal conductivity of high-temperature alloy forgings at low temperatures is much lower than that of carbon steel. In the temperature range of 700–800°C, slow preheating is required; otherwise, large thermal stresses may occur, placing the metal in a brittle state and increasing the risk of cracking.

In response to the special characteristics of high-temperature alloys, multiple forging methods have been developed in industry, each with its own applicable scope and characteristics.

Traditional forging methods include free forging and die forging. Free forging shapes metal materials through manual or mechanical hammering, while die forging forms materials within a forging die to maintain specific geometries during compression. These methods are characterized by simple equipment and low cost, but have lower production efficiency and poorer quality stability.

Isothermal forging is carried out under constant temperature conditions to maintain stable microstructure and uniform material properties. This method effectively reduces internal stress and improves fatigue life, making it particularly suitable for manufacturing critical components such as turbine discs and blades.

Rapid forging completes the forging process within a very short time, preventing excessive internal stress development during deformation and thereby improving forging quality. This method offers high productivity and excellent mechanical properties, making it suitable for large high-temperature alloy forgings.

Superplastic forging utilizes the superplasticity of metal materials under specific temperature and pressure conditions to achieve large deformation and enhance material performance. It is mainly used in manufacturing high-performance high-temperature alloy components.

Powder metallurgy forging first compresses and sinters metal powders to form billets, and then performs forging operations. This process effectively increases density and strength while reducing internal porosity and defects, making it suitable for producing high-performance high-temperature alloy forgings.

Composite material forging combines two or more materials into composite billets through specialized processes before forging, allowing the final forgings to possess multiple material advantages and making it suitable for manufacturing high-temperature alloy components with complex structures and performance requirements.

Manufacturing high-quality high-temperature alloy forgings requires the establishment of a comprehensive quality assurance system. The first step is material selection, where suitable high-temperature alloys must be chosen based on specific application scenarios and operating temperatures to ensure excellent high-temperature oxidation resistance, creep resistance, and corrosion resistance, while maintaining stable and reliable supply channels.

The second step involves process control, including precise management of preheating, forging pressure, forging temperature, and forging speed. Proper preheating eliminates internal stresses and reduces deformation resistance. Appropriate forging pressure and temperature ensure dimensional accuracy and consistency. Suitable forging speed helps achieve grain refinement and uniformity.

Heat treatment is a critical step in improving forging quality. Processes such as annealing, normalizing, quenching, and tempering are used to eliminate internal stresses, enhance strength and hardness, and improve heat resistance. Specific methods must be determined based on alloy type and forging size.

Finally, inspection and quality control are essential, including metallographic analysis, mechanical property testing, nondestructive testing, and ultrasonic inspection, ensuring that every production batch and every component meets design specifications and customer requirements.

High-temperature alloy forging manufacturing is a systematic engineering process involving multiple disciplines including materials science, thermodynamics, mechanical engineering, and automatic control. From precise heating control to effective breakthroughs in deformation resistance, and from microstructure refinement to the comprehensive application of digital technologies, every process link requires strict quality management. As aerospace and other high-end equipment industries continue to demand higher material performance, high-temperature alloy forging technology will continue to evolve toward precision and intelligent manufacturing, providing strong support for the transformation and upgrading of the manufacturing industry.

Forging is one of the fundamental processes in modern manufacturing, and the cooling stage is often overlooked despite being a critical factor in determining product quality. After forging, the hot steel billet must be cooled from high temperature to room temperature. Although this appears to be a simple physical process, it actually involves complex mechanical transformations. Improper cooling methods may generate excessive internal stress, leading to cracks or white spot defects. In severe cases, this may result in product scrapping or even production safety accidents. More importantly, improper cooling can prolong production cycles, reduce manufacturing efficiency, and increase production costs. Therefore, a deep understanding of the mechanism of internal stress formation during forging cooling is of great practical significance for improving product quality and optimizing processing technology.

Compared with the heating stage, forged workpieces are more vulnerable to internal stress during the late cooling stage when the material is in a low-temperature elastic state with poor plasticity. This makes proper control of cooling stress even more critical, requiring engineers to master the evolution law of cooling stress and adopt scientifically optimized cooling strategies.

Temperature stress is the first type of internal stress that appears during forging cooling, mainly caused by temperature gradients across the cross-section of the workpiece. At the initial cooling stage, the outer layer of the forging is in direct contact with air or cooling media, resulting in rapid heat dissipation and a sharp temperature drop, accompanied by significant volumetric contraction. In contrast, heat transfer from the core to the surface requires time, leading to slower cooling and delayed contraction in the core.

This asynchronous contraction between the surface and the core is similar to two people pulling an elastic band—while the surface layer tends to shrink, it is restrained by the core, and the core tends to maintain its original state but is pulled by the surface. As a result, tensile stress is generated in the outer layer, while compressive stress forms in the core. This stress distribution persists during the early cooling stage and represents the first phase characteristics of temperature stress.

Different materials exhibit significantly different temperature stress evolution paths, primarily determined by their deformation resistance.

Soft steel forgings, such as low-carbon steel, have low deformation resistance and good plasticity. During the initial cooling stage, tensile stress generated in the surface layer can be released through localized plastic deformation, resulting in relatively low stress magnitude. As cooling continues into the later stage, surface temperature decreases substantially and volumetric contraction nearly stops. However, the core temperature remains relatively high and continues to contract but is constrained by the already hardened surface layer.

At this point, the stress direction reverses: the surface layer changes from tensile stress to compressive stress, while the core changes from compressive stress to tensile stress.

This stress reversal is particularly important for soft steel forgings. Since the final stress state is characterized by tensile stress in the core and compressive stress on the surface, cracks, if they occur, usually initiate in the core and propagate outward, forming what is known as internal cracking.

Hard steel forgings, such as high-carbon steel and alloy steel, exhibit completely different behavior. These materials have high deformation resistance and are difficult to plastically deform. During early cooling, surface tensile stress cannot be relaxed through deformation and thus remains relatively high. In the later cooling stage, although core contraction generates additional compressive stress on the surface, it only reduces surface tensile stress without reversing the stress direction. The final stress state still presents tensile stress on the surface and compressive stress in the core.

Therefore, crack risk in hard steel forgings is mainly concentrated on the surface, where external cracking may occur. Such cracks usually initiate at the surface and propagate inward, making them visually detectable and relatively easier to inspect.

It should be noted that temperature stress during cooling exists in a three-dimensional stress state, consisting of three mutually perpendicular stress components. Among them, axial stress (along the longitudinal direction of the forging) is the largest and is the primary cause of longitudinal cracking. Therefore, when designing cooling processes, special attention must be paid to the longitudinal direction of the forging, and appropriate supporting and protective measures should be implemented.

When solid-state phase transformation occurs during forging cooling, another type of internal stress is generated in addition to temperature stress—microstructural stress. This stress arises from mechanisms similar to temperature stress in that it is caused by asynchronous internal and external transformations. However, the fundamental cause is different. Microstructural stress originates from changes in specific volume before and after phase transformation, as well as differences in transformation timing between the surface layer and the core.

Common phase transformations in steel during cooling include austenite to pearlite, austenite to bainite, and austenite to martensite transformations. Among these, martensitic transformation causes the most significant volumetric expansion and generates the highest structural stress, making it a primary concern in industrial production.

Taking martensitic transformation as an example, the formation process of structural stress can be analyzed as follows:

As temperature decreases, the surface layer of the forging first reaches the martensite start temperature (Ms point) and begins transformation. Since the specific volume of martensite (0.127–0.131 cm³/g) is significantly larger than that of austenite, volume expansion occurs in the surface layer. At this time, the core temperature remains high and the material is still in the austenitic state without transformation. The core constrains the expansion of the surface layer, resulting in compressive stress on the surface (restricted expansion) and tensile stress in the core (stretched deformation).

Fortunately, the core remains at a relatively high temperature at this stage, and austenite possesses good plasticity, allowing stress relaxation through localized plastic deformation. Therefore, structural stress in the first stage generally does not cause cracking.

As cooling continues, the core temperature drops below the Ms point and martensitic transformation begins, accompanied by volumetric expansion. However, by this time, the surface transformation has already been completed, and the surface has formed a hard and brittle martensitic structure with little further volume change. The surface layer strongly restricts core expansion, creating a new stress state: compressive stress in the core and tensile stress on the surface.

This stage is extremely dangerous. Since the forging is already in a low-temperature elastic state with poor plasticity, structural stress cannot be released through deformation and continues to accumulate until martensitic transformation is completed. If the stress exceeds the material strength limit, cracking will occur.

It is important to note that the specific volume of all steel phases is greater than that of austenite. Whether it is pearlite, bainite, or martensite, their specific volumes exceed that of austenite. Therefore, regardless of the phase transformation type, similar structural stress patterns are generated: the region that transforms first is under compression, while the region that transforms later is under tension. The final stress state shows compressive stress in the later-transformed region and tensile stress in the earlier-transformed region.

Structural stress is also a three-dimensional stress state, but unlike temperature stress, the circumferential (hoop) stress component is the largest. This is the main reason for longitudinal cracks forming on the forging surface along the length direction. In practical production, cracks parallel to the forging axis are often observed on the surface, which are typically caused by structural stress.

Residual stress is the internal stress "left behind" during the forming process. During forging, metal undergoes plastic deformation, but due to non-uniform deformation—such as uneven deformation distribution, differences in deformation speed, and frictional conditions—as well as work hardening effects, internal stresses are generated.

If these stresses are not eliminated through timely recrystallization softening (microstructure recovery at high temperature) during forging, they will remain as residual stress after forging. The distribution of residual stress depends on the specific deformation conditions. It may appear as tensile stress on the surface and compressive stress in the core, or vice versa, forming a complex stress distribution pattern.

Unlike temperature stress and structural stress, residual stress already exists before the cooling process begins. It does not follow a clear evolution pattern like the other two types of stress but acts as a background stress superimposed on them. Although residual stress alone may not immediately cause cracking, it reduces the load-bearing capacity of the forging, affects dimensional stability, and may combine with other stresses during subsequent processing or service, becoming a potential crack initiation source.

The three types of internal stresses do not exist independently but interact and superimpose during the cooling process, jointly determining the quality of the forging. When their resultant force exceeds the material strength limit, cracking becomes inevitable. Therefore, understanding the superposition mechanism of these stresses is the key to developing effective prevention measures.

In actual cooling processes, temperature stress, structural stress, and residual stress are superimposed to form the total internal stress inside the forging. These stresses may reinforce each other (additive effect) or partially cancel each other (subtractive effect), depending on the cooling stage and material properties.

When the combined stress exceeds the material strength limit at a given temperature, cracks will form in the corresponding region. Since the material is in a low-temperature brittle state during the late cooling stage and has insufficient plastic deformation reserve, even a temporary exceedance of the strength threshold may cause irreversible cracking damage.

Based on the above analysis, the following general rules can be summarized:

For soft steel forgings, temperature stress is relatively small during the initial cooling stage and may reverse direction later. Structural stress tends to dominate. Since the final stress state usually features tensile stress in the core, internal cracking is more likely to occur. Internal cracks are difficult to detect and pose greater safety risks.

For hard steel forgings, temperature stress remains tensile on the surface throughout the cooling process. When combined with structural stress, which also results in surface tensile stress, stress levels on the surface become significantly higher, increasing the risk of surface cracking. Although surface cracks are visible and easier to detect, they may still lead to serious surface quality degradation and stress concentration problems.

Different cooling strategies should be adopted for different types of forgings.

For soft steel forgings, prevention of internal cracking is the primary objective. Relatively slow cooling methods should be adopted to reduce temperature gradients and temperature stress. Meanwhile, cooling rates should be controlled to ensure more uniform phase transformation and reduce peak structural stress.

For hard steel forgings, prevention of surface cracking is more important. Surface cooling rate should be carefully controlled by employing preheating, insulation, or staged cooling processes to prevent premature entry into brittle temperature ranges. For high-alloy steels, isothermal transformation or controlled cooling paths may be adopted to avoid the severe temperature zone associated with martensitic transformation.

General measures include optimizing forging structural design to avoid sudden cross-sectional changes, improving deformation uniformity to reduce residual stress, selecting appropriate cooling media (such as air cooling, pit cooling, furnace cooling, or controlled cooling), and applying intermediate holding or stepwise cooling processes for large forgings.

Internal stress control during forging cooling is a comprehensive technology involving heat transfer, materials science, and mechanics. The interaction of temperature stress, structural stress, and residual stress jointly determines the final quality state of the forging. Understanding their formation mechanisms, evolution laws, and superposition effects is the foundation for developing scientifically optimized cooling processes.

In practical production, engineers should select appropriate cooling methods and process parameters based on material characteristics, cross-sectional dimensions, and geometric complexity of the forging, keeping internal stress within a safe range to ensure product quality, improve production efficiency, and reduce manufacturing costs. Only by paying sufficient attention to the cooling stage can high-quality and efficient forging production truly be achieved.

In the metal processing and heat treatment industries, cracks are a common quality defect. However, not all cracks are the same; there are significant differences between forging cracks and heat treatment cracks (including quenching cracks, tempering cracks, etc.) in terms of formation mechanisms, appearance, and root causes. Accurately distinguishing between these two types of cracks not only helps quickly identify the problematic process stage but also enables companies to improve manufacturing techniques, reduce scrap rates, and save production costs.

This article systematically explains the differences between forging cracks and heat treatment cracks from multiple perspectives, including formation temperature, microstructural characteristics, and causes. It also provides practical metallographic analysis methods to help technicians accurately determine crack types in real-world applications.

Forging cracks refer to crack defects that occur during the metal forging process. Forging involves heating metal to high temperatures and applying pressure to deform it. If the process is improperly executed or the material contains defects, cracking can easily occur.

Forging cracks generally form at high temperatures. When metal undergoes plastic deformation within the forging temperature range (typically far above room temperature), cracks begin to develop if the stress exceeds the material’s capacity. Because these cracks form and propagate at elevated temperatures, they come into direct contact with air, resulting in oxidation inside the cracks.

When observed under a microscope (at 100× or 500× magnification), forging cracks display several typical features:

Oxide scale filling: The interior of the crack is filled with oxide scale, serving as direct evidence of high-temperature oxidation.

Decarburization on both sides: The microstructure adjacent to the crack consists of ferrite, showing clear decarburization. This occurs because carbon reacts with oxygen at high temperatures, reducing the carbon content.

Thick morphology: These cracks are relatively wide and often appear in multiple forms without obvious tips or clear directionality.

Partial decarburization: Some forging cracks are thinner, and instead of full decarburization, the surrounding area shows a semi-decarburized state.

The primary causes of forging cracks include:

Raw material defects: Surface or internal defects such as hairline fractures, sand holes, cracks, inclusions, subsurface bubbles, shrinkage cavities, white spots, and laminations can act as crack initiation points during forging.

Improper process operations:

• Overheating: Excessively high heating temperatures lead to coarse grains.
• Overburning: Heating temperatures approaching or exceeding the solidus cause grain boundary oxidation or melting.
• Low final forging temperature: Reduced metal plasticity increases deformation resistance.
• Excessive cooling rate after forging: Generates excessive thermal and structural stresses.

Heat treatment cracks occur during processes such as quenching, tempering, and cold treatment, with quenching cracks being the most common. Compared with forging cracks, heat treatment cracks form at much lower temperatures.

For structural steels, heat treatment temperatures are generally far lower than forging temperatures. Even for high-speed steels and highly alloyed steels that require higher quenching temperatures, the heating and holding times are significantly shorter than those in forging. This temperature difference fundamentally determines the distinct formation mechanisms of the two crack types.

If process parameters are not properly controlled during the quenching heating stage, early cracking may occur:

Excessive heating temperature or prolonged holding time: Early cracks may form during heating, distributed along coarse grain boundaries, with slight decarburization on both sides.

Excessive heating rate: Rapid heating can also cause early cracking. These cracks typically show no obvious decarburization on either side, but oxide scale is present inside the crack and at its tail.

Instrument malfunction causing overheating: Faulty high-temperature instruments may result in abnormal temperature rises, producing extremely coarse microstructures with cracks distributed along grain boundaries.

Quenching cracks are macroscopic cracks primarily caused by macroscopic stresses. In actual production, steel workpieces often develop quenching cracks due to the following factors:

Design factors: Unreasonable structural design leading to stress concentration.

Material factors: Improper steel selection or actual carbon content exceeding the specified grade.

Process factors: Incorrect quenching temperature control or inappropriate cooling rates.

These factors increase internal quenching stress, allowing existing microcracks to expand into macrocracks or increasing the number of microcracks, thereby reducing resistance to brittle fracture and raising the likelihood of quenching cracks.

Raw material defects: Undetected surface or internal cracks and inclusions may develop into quenching cracks. Severe internal inclusions or hidden cracks will expand further during quenching.

Poor original microstructure: Non-uniform structures or severe segregation increase cracking risk.

Improper quenching temperature:

Instrument readings lower than actual temperatures may cause overheating quenching, resulting in coarse grains and coarse martensite.

Actual carbon content higher than the specified grade effectively raises the quenching temperature under standard processes, promoting overheating and grain growth.

Improper cooling: Excessive or uneven cooling generates high internal stresses.

Machining defects: Rough, deep tool marks can lead to cracking or early failure during quenching, even outside stress concentration areas.

Part geometry: Unreasonable shapes or large cross-sectional differences cause stress concentration during quenching.

Delayed tempering: Failure to temper promptly after quenching leaves excessive residual stress that can cause cracking.

After understanding the formation mechanisms, how can these cracks be quickly distinguished during inspection? This is a central issue in failure analysis. Misjudgment can lead to incorrect process improvements; for example, treating forging cracks as quenching cracks may result in unnecessary heat treatment adjustments, while the true defect source remains unresolved.

Forging cracks form at high temperatures, contain oxide scale, and exhibit obvious decarburization layers on both sides. Quenching cracks form at lower temperatures (below the Ms point), so fracture surfaces are not oxidized and show no surrounding decarburization. This is the most fundamental difference.

Observing the fracture surface allows accurate differentiation:

Quenching cracks: White, off-white, or light reddish (water rust from water quenching).

Forging cracks: Dark brown, often with visible oxide scale.

Forging cracks: Thick, often multiple, without obvious tips or directionality.

Quenching cracks: Usually deep and large, often straight and knife-cut in appearance, unrelated to grinding direction.

When observing the fracture path under a microscope:

Quenching cracks: Intergranular fracture along grain boundaries.

Fatigue cracks: Transgranular fracture (not along grain boundaries).

Pre-forging cracks: Surrounded by a decarburized layer.

This is one of the most important identification criteria:

Decarburization around the crack → Not a quenching crack; it existed before quenching (formed during forging or rolling).

No decarburization → Possibly a quenching crack, as cracks formed during quenching cooling will never decarburize.

The ultimate purpose of crack identification is prevention. Targeted control measures should be implemented according to the characteristics of forging and heat treatment processes.

Strictly inspect raw materials and eliminate defective billets.

Control heating temperatures to avoid overheating and overburning.

Ensure adequate final forging temperatures.

Manage post-forging cooling rates; use isothermal annealing when necessary.

Design part structures rationally to avoid sharp corners and abrupt section changes.

Select appropriate steel and control actual carbon content.

Strictly control quenching temperatures and regularly calibrate instruments.

Choose suitable cooling media and methods.

Temper promptly to eliminate residual stress.

Improve machining quality and avoid deep tool marks.

Control grinding parameters to prevent excessive temperatures.

Use sufficient cutting fluid to reduce surface temperature.

Select grinding wheels with appropriate hardness and grit size.

Apply specialized grinding processes for high-hardness components.

Accurately distinguishing between forging cracks and heat treatment cracks is an essential skill in metal processing quality control. By examining formation temperature characteristics, oxidation levels, decarburization, fracture surface color, and microstructural morphology, technicians can effectively determine the crack type and the process stage in which it originated. Mastering these analytical methods not only helps resolve current quality issues but also enables fundamental process improvements, defect prevention, and enhanced product quality and production efficiency. In practical work, it is recommended to establish a comprehensive quality traceability system, strictly control each stage, from raw materials to forging, heat treatment, and machining, and maintain complete process records. When crack defects occur, combining metallographic analysis with process investigation allows the root cause to be identified quickly and accurately, enabling effective corrective actions.

Multi-directional die forging is an advanced metal forming process primarily used to manufacture precision forgings with complex geometries. The process is carried out on a multi-directional die forging hydraulic press, utilizing a combined die with multiple parting surfaces. In a single pressing stroke, the heated metal billet is shaped into the desired form. One of the most notable features of multi-directional die forging is its ability to produce complex components with no flash or minimal flash, as well as little to no draft angle.

Unlike traditional single-direction die forging, multi-directional die forging overcomes the limitation of forming force from only one direction. Pressure can be applied simultaneously from multiple directions, allowing the metal to flow fully within the die cavity and fill intricate features. This method is particularly suitable for manufacturing parts with multi-directional holes, bosses, or branch-like structures, such as high-pressure valve bodies, pipe fittings, and aerospace structural components.

The multi-directional die forging hydraulic press is the core equipment that enables this process. Compared with conventional die forging presses, it features a more complex structure and broader functionality.

A standard multi-directional die forging hydraulic press is equipped not only with a vertical working cylinder but also with two or four horizontal working cylinders. Additionally, a piercing cylinder is installed at the center of the crossbeam or moving crossbeam. This configuration provides the press with four or six independent slides, each capable of operating individually or in coordination.

The advantage of this multi-cylinder design lies in its ability to apply pressure simultaneously or sequentially from mutually perpendicular directions. It supports various forming modes, including vertical parting, horizontal parting, and combined vertical–horizontal parting. This flexibility allows the equipment to adapt to forging requirements for components of different shapes.

Typically, the pressure of the vertical working cylinder is greater than that of the horizontal cylinders. This characteristic is crucial when selecting a parting method. For large forgings that require substantial clamping force, horizontal parting is often preferred because it enables the vertical cylinder to provide stronger mold-closing force, ensuring tight die closure and preventing flash formation.

Dies are central to the multi-directional forging process, and their structural form directly determines forging quality and process feasibility. Based on the position and combination of parting surfaces, multi-directional forging dies can be classified into four primary types.

In this structure, the parting surface is parallel to the horizontal plane. The forging is primarily formed through bidirectional extrusion by horizontal working plungers.

Two die cavities are secured to the upper and lower die holders using wedge plates and screws. Two horizontal punches are mounted on the clamping seats of the press’s horizontal plungers. Alignment accuracy is ensured by the guiding sections of the punches as well as guide pillars and bushings when the dies close.

Horizontal parting is the most widely used structure in multi-directional forging, particularly suitable for forgings with deep holes at both ends. It is also applicable when only one end contains a deep hole. For large forgings requiring significant clamping force, horizontal parting is typically the preferred solution.

The parting surface in a vertical parting die is perpendicular to the horizontal plane, and forging is mainly achieved through downward extrusion by the vertical plunger.

In this configuration, one end of each horizontal push rod connects to the plunger clamping seat of the horizontal working cylinder, while the other end is pinned to the die cavities. The rods drive the two die halves along the base plate to open and close the die.

A locating block at the center of the base plate ensures precise positioning, keeping the punch aligned with the cavity center. Guide pins between the die halves prevent misalignment during closure. Vertical parting is typically used when three or four die blocks must open and close within the same plane.

Combined parting dies incorporate both vertical and horizontal parting surfaces. Two lower die blocks form the vertical parting, while together with the upper die they create a horizontal parting, resulting in a complex combined structure. A guide hole is located at the center of the upper die to direct the punch during extrusion.

This structure is ideal for extremely complex forgings that require simultaneous forming from multiple directions, fully leveraging the advantages of multi-directional die forging.

When the three basic structures cannot meet forging requirements—or when equipment limitations must be compensated—special die designs are necessary. For example, fork-shaped or other customized structures may be adopted when unique vertical opening motions are required or when the forging geometry is highly irregular.

Die design directly affects forging quality and die service life. Several critical factors must be considered.

When arranging cavity positions, it is important to recognize that the center of die-opening force often does not coincide with the geometric center of the cavity. Designers should align the opening-force center as closely as possible with the clamping-force center to avoid localized separation and flash formation. Proper cavity placement is particularly important for asymmetric forgings with complex bosses or branch-like features.

The guiding and centering functions of die guidance systems are essential for ensuring dimensional accuracy. Mating surfaces should provide positioning capability, and dedicated guiding components must be incorporated to prevent misalignment.

Cylindrical guide pins are commonly used. One pin is press-fitted into one die half, while a guide hole—or a bushing pressed into the hole—is machined into the mating half. The typical clearance is about 0.5 mm, and most dies require two guide pins. When three die blocks are combined, cylindrical pins with guide grooves may be used.

The die's guide holes must be sufficiently long to maintain concentricity of the horizontal punches during forging. Insufficient guidance can cause punch deflection, negatively affecting the dimensional accuracy and surface quality of internal holes.

A certain clearance must be maintained between the punch and the die. Factors influencing this clearance include the concentricity of the horizontal cylinders, the precision of die and worktable heights, deformation of the moving crossbeam, elastic die deformation, and installation misalignment of the punch.

Excessive clearance may produce longitudinal burrs, while insufficient clearance can accelerate wear or cause seizure. Proper clearance ensures forging quality and extends die life.

The working dimensions of the punch are determined based on the forging’s internal diameter. Key factors include the nominal internal diameter, the material’s linear shrinkage coefficient, and tolerance requirements. Calculations must ensure that the cooled forging meets drawing specifications.

Shear-bearing surface design: When extracting a punch from a deep hole, a certain pulling force is required, which may deform the forging. Therefore, sufficient shear-bearing surfaces must be designed between the forging and the cavity to prevent tearing or distortion during punch removal.

Dimensional relationships between processes: For complex forgings requiring multiple operations, critical inter-process dimensions must be carefully defined. The opening width formed in the previous step must be greater than that of the semi-finished part in the next step. This ensures that forming cores can be inserted smoothly and removed easily—an essential principle in multi-stage multi-directional forging.

In vertical parting structures, horizontal push rods drive die opening and closing. Insufficient rigidity can lead to deformation under high pressure, resulting in poor die closure and flash defects. Therefore, push rods must be designed with adequate stiffness to maintain precise positioning under maximum load.

Rigidity design should consider working load, stroke length, and material strength. High-quality alloy steels are typically used, combined with appropriate heat treatment to enhance strength and wear resistance.

Because multi-directional forging presses operate relatively slowly, dies remain in contact with hot forgings for extended periods, experiencing high temperatures and repeated thermal cycling. These conditions can cause surface fatigue cracks, and extrusion-dominated deformation leads to significant wear.

As a result, die materials must offer high hardness and heat resistance, strong hot-hardness retention, excellent resistance to thermal fatigue, and good thermal conductivity for rapid heat dissipation.

Common materials include hot-work tool steels such as H13 and 3Cr2W8V. With proper heat treatment, these materials can meet the demanding requirements of multi-directional forging.

For complex forgings with large deformation, billet pre-treatment is essential for ensuring forming quality. Parts with significant bending deformation are especially prone to cracking during forming, making pre-treatment of cut round steel billets necessary.

The recommended method is localized low-temperature annealing at the bending areas. This process relieves internal stresses generated during cold drawing and enhances plastic deformation capacity, preventing bursting during subsequent forming.

Full high-temperature annealing should be avoided, as it easily produces oxide scale on the billet surface. Such scale is difficult to remove and can negatively affect surface quality—particularly for products requiring electroplating.

In practice, simplified annealing equipment can be used. A coke furnace with a small blower at the air inlet increases oxygen supply and fire intensity. Placing an iron plate with multiple 25–30 mm holes over the furnace opening allows the billet’s forming area to be heated for several minutes. This method reduces investment in high-frequency electric furnaces, lowers energy consumption, simplifies production setup, shortens preparation cycles, and helps reduce product costs.

The key reason multi-directional die forging can produce shapes that are difficult—or impossible—for other forging methods lies in its dies with multiple parting surfaces. This design allows metal to flow simultaneously or sequentially from several directions, filling complex cavities and forming features such as multi-directional holes, bosses, and branches.

Its main advantages include:

Capability to produce extremely complex forgings, especially parts with holes in multiple directions

High dimensional accuracy with little or no flash and minimal draft, reducing subsequent machining

High material utilization due to the absence of large flash allowances

Dense internal structure with well-distributed metal flow lines and excellent mechanical properties

Multi-directional die forging is a technologically advanced forming process with broad application prospects. Mastering this technology requires a thorough understanding of equipment characteristics, die structures, and critical design principles. Through optimized die design, strict process control, and appropriate billet pre-treatment, manufacturers can produce high-quality complex forgings that meet the demands of high-end industries such as aerospace, energy, and automotive manufacturing. As manufacturing technologies continue to evolve, multi-directional die forging is expected to expand into even more fields and applications.

In the field of metal processing, steel forging is a critically important manufacturing process. By applying pressure, the metal undergoes plastic deformation, thereby producing forgings with specific shapes and properties. During the forging process, the forging ratio is an extremely important parameter, as it is directly related to the quality, performance, and production cost of forgings. This article provides an in-depth discussion of the definition, function, influencing factors, and the importance of reasonably selecting the forging ratio.

The forging ratio refers to the ratio of the cross-sectional area of the metal before deformation to that after deformation during the forging process. It is an important indicator used to measure the degree of metal deformation. The calculation method of the forging ratio varies depending on different forging operations, but in general, it is based on the change in cross-sectional area before and after deformation.

The drawing forging ratio refers to the ratio of the cross-sectional area before drawing to that after drawing. The calculation formulas are as follows:

The forging ratio y is equal to F1 divided by F0, or y is equal to L0 divided by L1.

Here, F0 and L0 are the cross-sectional area and length of the ingot or billet before drawing, while F1 and L1 are the cross-sectional area and length after drawing. Through this ratio, the degree of metal deformation during the drawing process can be clearly understood.

The upsetting forging ratio is also referred to as the upsetting ratio or compression ratio. It is expressed as follows:

The forging ratio y is equal to F0 divided by F1, or y is equal to H1 divided by H0.

Here, F0 and H0 are the cross-sectional area and height of the ingot or billet before upsetting, while F1 and H1 are the cross-sectional area and height after upsetting. The upsetting forging ratio reflects the degree of compression during the upsetting process and is one of the important indicators for evaluating forging quality.

The forging ratio has a significant influence on the mechanical properties and structural characteristics of forgings. As the forging ratio increases, internal pores in the metal are compacted, cast dendritic structures are broken up, and both longitudinal and transverse mechanical properties of the forging are significantly improved.

However, when the drawing forging cross-sectional ratio exceeds 3–4, with further increases in the forging ratio, an obvious fibrous structure forms. This causes a sharp decline in transverse plasticity indices, leading to anisotropy of the forging. Therefore, when selecting the forging ratio, it is necessary to fully consider the performance requirements and process characteristics of the forging in order to achieve optimal forging quality.

An increase in the forging ratio helps improve the metal’s microstructure. During forging, as the forging ratio increases, internal pores are gradually compacted and cast dendrites are broken up, resulting in a denser microstructure. This improvement in microstructure directly leads to enhanced mechanical properties of the forging, including strength, hardness, toughness, and plasticity.

For example, when forging high-quality carbon structural steel and alloy structural steel, a reasonable forging ratio can increase the strength of the forging by 20%–30%, while toughness is also significantly improved.

The forging ratio also has an important influence on anisotropy. When the forging cross-sectional ratio exceeds 3–4, an obvious fibrous structure forms inside the forging. This fibrous structure causes differences in mechanical properties between the longitudinal and transverse directions, resulting in anisotropy.

In practical applications, such anisotropy may have adverse effects on service performance. For example, in components subjected to multi-directional stresses, anisotropy may lead to stress concentration and thus reduce service life. Therefore, when selecting the forging ratio, it is necessary to reasonably control the ratio according to the specific service requirements of the forging in order to reduce the impact of anisotropy.

The selection of the forging ratio is influenced by multiple factors, including metal material properties, forging process, forging temperature, and the shape and size of the forging.

Different metal materials have different flowability and deformation capabilities during forging, and therefore require different forging ratios. Materials with good flowability, such as low-carbon steel, can use smaller forging ratios, while materials with poor flowability, such as high-alloy steel, require larger forging ratios. This is because materials with poor flowability require greater deformation during forging to achieve the desired structure and performance.

Different forging processes also affect the forging ratio. For example, in cold forging processes, due to poor material flowability, larger forging ratios are required to ensure sufficient deformation. In hot forging processes, better flowability allows for smaller forging ratios.

In addition, different forging equipment and die designs also influence forging ratio selection. For example, in free forging on hammers, greater flexibility allows the use of larger forging ratios, whereas in die forging, forging ratio selection must be more cautious due to die constraints.

Forging temperature is one of the important factors influencing the forging ratio. At higher forging temperatures, metals exhibit better flowability and deformation capacity, allowing for smaller forging ratios. At lower forging temperatures, metals have poorer flowability and deformation capacity, requiring larger forging ratios to achieve the required deformation.

Therefore, in actual production, appropriate forging ratios must be selected based on the specific forging temperature.

The shape and size of the forging also influence forging ratio selection. Forgings with complex shapes or large sizes require larger forging ratios to ensure sufficient deformation and the required structure and properties. Forgings with simple shapes and small sizes can use smaller forging ratios.

In addition, service requirements also affect forging ratio selection. Components subjected to high stress require larger forging ratios to improve strength and toughness, while general-purpose components may use smaller forging ratios to reduce cost.

When selecting the forging ratio, the following principle should be followed: on the premise of meeting all forging requirements, the smallest possible forging ratio should be selected. This reduces forging difficulty and cost while helping to improve forging quality and performance.

For free forging on hammers, forging ratios should be selected according to forging type and service requirements.

• Shaft forgings produced directly from ingots:
• Forging ratio based on the main section ≥ 3
• Forging ratio based on flanges or other protrusions ≥ 1.75
• Shaft forgings produced from billets or rolled materials:
• Forging ratio based on the main section ≥ 1.5
• Forging ratio based on flanges or other protrusions ≥ 1.3
• Ring forgings: forging ratio generally ≥ 3
• Disc forgings:
• Produced directly from ingots: upsetting forging ratio ≥ 3
• Other cases: upsetting forging ratio generally > 3, but the final operation should be > 2

For high-alloy steel billets, forging must not only eliminate structural defects but also ensure relatively uniform carbide distribution. Therefore, larger forging ratios must be used.

For example:

• Stainless steel forging ratio: 4–6
• High-speed steel forging ratio: 5–12

This is because greater deformation is required during forging to break up carbides and distribute them uniformly, thereby improving forging performance.

For large forgings, forging ratio selection is particularly important. The raw materials for large forgings are mainly steel ingots, which inevitably contain defects such as looseness, inclusions, and gas pores.

The purpose of forging is to break up the cast structure, refine grains, homogenize chemical composition, and improve mechanical properties. To ensure internal quality, the forging ratio must be controlled between 3 and 7. Ratios below 3 result in insufficient deformation, while ratios above 7 lead to shape distortion.

In actual production, concave upsetting plates are recommended for ingot upsetting, as they provide better load distribution and help improve forging quality.

At present, three categories of process schemes are used to improve forging ratio uniformity: changing billet–die boundary conditions, changing billet shape, and changing upsetting deformation modes. These methods effectively improve forging ratio and geometric uniformity, thereby enhancing forging quality.

By changing billet–die boundary conditions, metal flow behavior during forging can be improved, thereby enhancing forging ratio uniformity. For example, in die forging, optimizing die design to create smoother die surfaces reduces friction and promotes more uniform metal flow.

Additionally, adjusting die temperature and lubrication conditions further improves metal flow and forging ratio uniformity.

Changing billet shape is another effective method to improve forging ratio uniformity. For example, in large forgings, billets can be designed with pre-deformed shapes to promote more uniform deformation.

Optimizing billet dimensions and shapes allows better adaptation to die geometry, thereby improving forging ratio uniformity.

Changing the upsetting deformation mode is also an important method to improve forging ratio uniformity. For example, modifying flat-anvil upsetting deformation can effectively improve forging ratio and geometric uniformity.

In actual production, advanced upsetting processes such as multi-directional upsetting and stepped upsetting can be adopted to achieve more uniform deformation and improved forging ratio consistency.

The forging ratio has important practical significance in forging plant production, especially for large forgings such as wheel forgings, ring forgings, shafts, and cylinders.

Wheel forgings are typical large forgings, and forging ratio selection must comprehensively consider material properties, shape, size, and service requirements. Generally, the forging ratio for wheel forgings should be controlled between 3 and 5.

A reasonable forging ratio ensures internal quality and improves strength and toughness, meeting the service requirements of transportation equipment.

Ring forgings are widely used in aerospace and machinery manufacturing. Their forging ratio should be selected according to specific service requirements and is generally ≥ 3.

A reasonable forging ratio ensures internal quality and improves strength and toughness under complex service conditions.

Shaft forgings are common products in forging plants. Their forging ratio selection depends on material properties, shape, size, and service requirements and is generally ≥ 3.

A reasonable forging ratio ensures internal quality and improves strength and toughness to meet mechanical equipment requirements.

The forging ratio is an important parameter in metal forging processes and is directly related to forging quality, performance, and production cost. Reasonable selection of the forging ratio effectively improves internal quality and mechanical properties while reducing cost and improving productivity.

In actual production, forging ratio selection must comprehensively consider metal material properties, forging process, forging temperature, and forging shape and size. Optimizing forging ratio selection provides important technical support for forging production and promotes technological progress and sustainable development in the forging industry.