Why is heat machining a core technology for high-end manufacturing?

In high-end industries such as aerospace and automotive manufacturing, we often hear the term “hot working.” What kind of technology is it? Why can we solve the problem of material forming that is difficult to overcome by cold processing? This article will dismantle thermal processing technology in an all-round way, from basic concepts to industry frontiers, and help you clarify its core logic, practical scenarios, and future directions – whether you are an engineer, technical decision-maker, or industry researcher, you can find truly valuable references.

1. Basics of hot processing: from definition to core principles, read in one article

1.1 What is Hot Machining? Core definitions and boundaries

The definition of thermal processing essentially refers to the general term for plastic forming or cutting of metals and other materials under conditions higher than the recrystallization temperature of the material. Unlike cold working, the material is recrystallized during hot working, avoiding work hardening and resulting in better formability.

The thermal working temperature range needs to be clarified: the recrystallization temperature varies significantly between materials, such as mild steel with a thermal working temperature of around 800-1250°C, aluminum alloys with around 300-500°C, and superalloys with a temperature of over 1000°C. To determine whether it is hot working, the key is whether the temperature exceeds 50% of the recrystallization temperature of the material (calculated according to the absolute temperature).

1.2 Hot vs. Cold Working: Core Differences and Applicable Scenarios Comparison

Many practitioners confuse the application boundaries of the two, and the following is a clear comparison through the table:

Real case: An auto parts manufacturer once tried to produce high-strength steel crankshaft with cold processing, but due to the high hardness of the material, the mold loss was fast, and the pass rate was only 60%; After switching to the hot forging process, the plasticity of the material is improved, the mold life is extended by 3 times, and the pass rate reaches 98%.

1.3 Core principle of thermal processing: material thermal deformation mechanism

The essence of thermal processing is the combination of the thermal deformation mechanism of the material and the recrystallization process. When the material is heated above the recrystallization temperature, the kinetic energy of the atoms increases, the lattice slip resistance decreases, and plastic deformation occurs under the action of external forces. At the same time, the dislocation caused by the deformation is eliminated by the recrystallization process, forming new equiaxial grains and avoiding material hardening.

In simple terms, hot processing is like “kneading hot dough”: the dough is hard and difficult to knead at room temperature (cold working), the dough becomes soft after heating (hot working), and does not harden during the kneading process (recrystallization eliminates hardening), and can finally be shaped into various shapes.

1.4 Process classification and historical development of hot processing

The classification of hot working processes mainly includes forging, extrusion, rolling, stamping, stretching, welding, etc., which will be analyzed in detail in the following chapters. From the perspective of the historical development of thermal processing, the earliest thermal processing of mankind can be traced back to the Bronze Age (about 3000 BC), which smelted metals by heating ores with charcoal; After the Industrial Revolution, the heating furnaces and presses driven by the steam engine promoted the birth of modern thermal processing technology. Since the 21st century, automation and intelligent technology have made a qualitative leap in the accuracy and efficiency of thermal processing.

2. Core process methods of thermal processing: technical types and operation points

2.1 Hot Forging Process: “Strong Shaping” for Metal Forming

The hot forging process is a process in which a forging press applies impact or pressure to the heated metal billet to cause plastic deformation. According to the forming method, it can be divided into free forging and die forging:

• Free forging: no fixed mold, suitable for large, simple parts (such as machine tool beds, large gear blanks);
• Die forging: Using special molds, it can produce parts with complex shapes and high precision (such as aero engine blades, automobile connecting rods).

Operation key: Before forging, the billet needs to be heated to the specified temperature (such as 45 steel die forging temperature 1100-1150°C), and the insulation time is calculated according to the thickness of the material (generally every 100mm for 1-2 hours) to avoid cracking caused by uneven temperature.

2.2 Hot extrusion technology: efficient forming of tubes and profiles

Hot extrusion technology is a process in which the heated metal billet is placed into the extrusion cylinder and pressure is applied through the extrusion rod to extrude the material from the die hole to form a tubular, rod or special-shaped profile. Its core advantage is the ability to produce profiles with complex cross-sections, continuous material fibers, and excellent mechanical properties.

Industry case: When an aerospace company produces titanium alloy pipes, it uses a hot extrusion process to extrude titanium alloy ingots with a diameter of 50mm into pipes with a diameter of 50mm, with a yield rate of 85%, which is much higher than 30% of cutting processing, and the tensile strength of the pipe is increased by 20%.

2.3 Hot rolling method: large-scale production of plates and strips

The hot rolling method is a process in which the heated billet is rolled through one or more pairs of rotating rolls, so that its thickness is reduced and the length is increased. It is widely used in mass production scenarios such as steel plates, steel strips, and rails.

Technical points: The rolling temperature needs to be strictly controlled, for example, the hot rolling temperature of mild steel is usually 1100-1200°C, too high a temperature will lead to coarse grains, and too low a temperature will increase the rolling force. Modern hot rolling lines have automated temperature control with an accuracy of up to ±5°C.

2.4 Other key processes: hot stamping, hot drawing and high-temperature welding

• Hot stamping forming: heating the sheet material to an austenitic temperature (about 900-950°C), quickly transferring it to the mold for stamping and pressure quenching, which can produce high-strength and high-precision body structural parts (such as automobile B-pillars, bumper beams);
• Hot Stretching Machining: Mainly used for the length extension of pipes and bars, often used in the production of wires and cables, mechanical shaft parts;
• High-temperature welding technology: realizes the metallurgical bonding of materials in high-temperature environments, suitable for the connection of high-temperature resistant materials (such as nickel-based alloys, ceramics), and is widely used in boiler and pressure vessel manufacturing.

2.5 Heat treatment combined with processing: a “combination punch” for performance optimization

Some heat processing processes are combined with heat treatment to further improve the material’s properties. For example, hot-forged parts are quenched and tempered (quenched + tempered at high temperatures) to improve hardness and toughness; Annealing of hot-rolled steel plates can reduce internal stress and improve processing performance. This combination of “heat processing + heat treatment” is particularly common in high-end equipment manufacturing.

3. Material science of thermal processing: material selection, properties and microscopic changes

3.1 High-temperature material characteristics: the “material selection basis” of thermal processing

Thermal processing has clear requirements for the high temperature material properties of the material:

• Good thermoplasticity: ensure that large plastic deformation can occur at high temperature without rupture;
• Low deformation resistance: reduce the external force required for processing and reduce equipment energy consumption;
• Good oxidation resistance: avoid oxidation on the surface of the material during the heating process, affecting product quality;
• Stable microstructure: Prevents problems such as coarse grains and uneven organization during processing.

Common hot working materials include: mild steel, alloy steel, aluminum alloy, copper alloy, superalloy (such as Inconel series), titanium alloy, etc.

3.2 Selection of hot working materials: the core principle of matching according to needs

The selection of hot working materials should follow the principle of “process adaptation + performance requirements”, which can refer to the following logic:

1. Clarify product usage scenarios: for example, aerospace components need to withstand high temperature and high strength, and auto parts need to take into account strength and cost;
2. Matching processing technology: such as hot forging is suitable for high-strength materials, hot extrusion is suitable for materials with good plasticity;
3. Consider economy: Prioritize lower-cost materials (such as mild steel for ordinary structural parts, superalloys for high-end parts) while meeting performance.

Case reference: A new energy vehicle company produced a battery pack frame, which needed to take into account both strength and lightweight, and finally chose 6061 aluminum alloy for hot extrusion processing, which not only met the requirements of tensile strength ≥ 300MPa, but also reduced the weight by 40% compared with steel.

3.3 Thermoplastic behavior of metals and flow stress of materials

Metal thermoplastic behavior refers to the plastic deformation ability of metals at high temperatures, which is affected by factors such as temperature, strain rate, and material composition. Generally speaking, the temperature rises, the thermoplasticity increases; The strain rate increases and the thermoplasticity decreases.

Material flow stress refers to the stress required for the material to generate unit plastic deformation during the thermal deformation process, and is the key basis for designing thermal processing process parameters (such as pressure and speed). For example, at 1000°C and strain rate 0.1s⁻¹, 45 steel has a flow stress of about 150MPa; The superalloy Inconel 718 has a flow stress of more than 400MPa under the same conditions, requiring the use of larger tonnage equipment.

3.4 Microstructure changes and recrystallization phenomenon in hot working

During the process of thermal processing, the microstructure changes of materials mainly include:

1. Heating stage: the grains gradually grow, and the original tissues (such as pearlite and ferrite) are transformed into austenitic (steel-like materials);
2. Deformation stage: the grains are elongated, resulting in dislocation accumulation;
3. Recrystallization stage: dislocation density decreases, new equiaxial grains are formed, and material properties return to uniformity.

Recrystallization is the core difference between hot and cold working, and is also the key to avoiding work hardening of materials. Recrystallization temperature is an important parameter, for example: about 450°C for mild steel, 200°C for aluminum alloys, and 800°C for superalloys.

3.5 High-temperature oxidation protection and material performance optimization

During high-temperature processing, the surface of the material is easy to react with oxygen to form oxide scale, which affects the surface quality and performance of the product, so high-temperature oxidation protection measures need to be taken:

• Environmental protection: processing in inert gases (such as argon, nitrogen) or reducing atmospheres;
• Coating protection: apply antioxidant coating (such as ceramic coating, metal coating) on the surface of the material;
• Process optimization: shorter heating time and lower heating temperature (within permissible limits).

Optimization of material properties can also be achieved by adjusting thermal processing parameters: for example, by controlling the rolling temperature and speed to refine the grain and improve the strength of the material; Optimizing the forging process can make the direction of the material fibers consistent with the direction of stress, and improve the fatigue resistance.

4. Thermal processing equipment and tool system: selection, maintenance and safety

4.1 Hot processing machine tool design: key requirements for core equipment

The design of hot processing machine tools needs to meet the requirements of high-temperature, high-pressure, and high-precision working environments, and the core components include:

• Frame: It must have sufficient rigidity and strength to withstand the impact force during processing;
• Transmission system: ensure smooth movement and adjustable speed (such as hydraulic transmission, servo motor transmission);
• Temperature control system: precise control of processing temperature, error ≤± 10°C;
• Cooling system: prevents damage to machine parts due to high temperatures.

Common hot processing machine tools are: hot forging press, hot rolling mill, hot extrusion machine, hot stamping die, etc.

4.2 Heating Furnace Systems: The “Energy Core” of Thermal Processing

The heating furnace system is a key auxiliary equipment for thermal processing, which can be divided into according to the heating method:

• Gas heating furnace: low cost, suitable for mass production (such as hot rolling of steel plates);
• Electric heating furnace: high temperature control accuracy, suitable for high-end material processing (such as high-temperature alloy forging);
• Induction heating furnace: fast heating speed, low energy consumption, suitable for local heating (such as quenching of shaft parts).

Technical parameters: The temperature uniformity, heating speed, and energy consumption of the heating furnace are the core indicators. For example, high-end induction furnaces can heat up to 100°C/min, with a temperature uniformity of ±5°C and 30% lower energy consumption than traditional gas furnaces.

4.3 High-temperature mold materials and tool coatings

High-temperature mold materials need to have the characteristics of high temperature resistance, high hardness and high wear resistance, and commonly used materials include:

• Hot work mold steel: such as H13, 3Cr2W8V, suitable for general hot processing molds;
• Superalloys: such as Inconel 625, suitable for ultra-high temperature processing (such as above 1000°C);
• Ceramic materials: such as silicon nitride ceramics, suitable for thermal processing in corrosive environments.

In order to extend the life of the mold, thermal processing tool coating technology can be used, and the common coatings are:

• TiN coating: improves hardness and wear resistance, suitable for medium and low temperature molds;
• AlTiN coating: good high temperature resistance (up to 800°C), suitable for high-temperature molds;
• DLC coating: low coefficient of friction for molds where material adhesion needs to be reduced (e.g., aluminum alloy hot extrusion).

Case: A hot forging mold factory used AlTiN coating to treat H13 molds, and the mold life was increased from the original 5000 pieces/set to 15000 pieces/set, reducing production costs.

4.4 Temperature control system and automatic thermal processing equipment

The temperature control system is the key to ensuring the quality of thermal processing, and modern thermal processing equipment generally adopts a closed-loop control system: the temperature is monitored in real time through thermocouples, infrared thermometers and other sensors, and feedback to the controller to automatically adjust the heating power to ensure stable temperature.

Automated thermal processing equipment has become an industry trend, such as:

• Automated hot forging production line: integrating heating, forging, cooling, testing and other processes to achieve unmanned production, increasing efficiency by more than 50%;
• Robot-assisted thermal processing: Robots can complete billet handling, mold replacement, and other operations to reduce manual labor intensity and improve safety.

4.5 Energy consumption management and equipment maintenance safety of thermal processing

Thermal processing energy consumption management is an important direction for enterprises to reduce costs and increase efficiency, and the following measures can be taken:

1. Optimized heating process: shortened heat holding time, waste heat recovery system;
2. Select energy-saving equipment: such as frequency conversion heating furnace, high-efficiency insulation materials;
3. Reasonable production arrangement: avoid equipment idling and improve equipment utilization.

Equipment maintenance and safety should be paid attention to:

• Regularly check the wear of the mold and repair or replace it in time;
• Maintain the heating elements and insulation layer of the heating furnace to ensure heating efficiency;
• Operators need to wear high-temperature protective equipment (such as protective shoes and gloves) to avoid high-temperature burns;
• The equipment should be equipped with an emergency shutdown device to deal with sudden failures.

5. Application fields and industry cases of hot processing: from aviation to construction

5.1 Aerospace Component Machining: Technological breakthroughs in extreme environments

The aerospace field has extremely high requirements for high temperature resistance and high strength of parts, and hot processing is the core manufacturing technology. For example:

• Aero engine blades: using the hot forging process of superalloys (such as GH4169), forged at temperatures above 1100°C to ensure the strength and toughness of the blades in high-temperature gas environments;
• Rocket body structural parts: Titanium alloy hot extrusion process is used to produce high-strength and lightweight tubes and profiles to reduce rocket launch weight.

Data support: The fatigue life of the turbine blades produced by an aero engine company has been increased from the original 2,000 hours to 5,000 hours by optimizing the thermal processing process, reaching the international advanced level.

5.2 Thermoforming of Auto Parts: Balancing Safety and Lightweight

In the automotive industry, hot stamping is widely used in the production of body structural parts. For example:

• Automotive B-pillar: Made of 22MnB5 boron steel hot stamping, the tensile strength can reach more than 1500MPa after processing, which can effectively improve the safety of side impact on the body;
• Engine crankshaft: Uses a hot forging process to ensure the strength and wear resistance of the crankshaft when running at high speeds.

Industry trends: With the development of new energy vehicles, hot processing is increasingly used in the production of lightweight parts, such as aluminum alloy battery pack frames, magnesium alloy body components.

5.3 Energy equipment manufacturing: the test of high temperature and high pressure resistance

Energy equipment (such as power plant boilers, nuclear power equipment, oil and gas pipelines) need to withstand high temperature and high pressure environments, and thermal processing is a key manufacturing process:

• Boiler superheater tube: Hot extrusion process of heat-resistant steel (such as P91, P92) is used to ensure corrosion resistance and creep resistance at high temperatures above 600°C;
• Nuclear power pressure vessel: Thick steel plate hot forging and welding process is used to ensure the tightness and structural strength of the equipment.

5.4 Other application areas: heavy machinery, military industry and construction

• Heavy mechanical hot processing: such as excavator sticks and crane booms, using high-strength steel hot forging process to ensure load-bearing capacity;
• Military product applications: such as tank tracks and shell shells, using thermal processing technology to improve impact resistance;
• Hot processing is used in the construction industry, such as hot rolling of steel bars and hot welding of steel structural parts, ensuring the stability and safety of building structures.

6. Advantages and challenges of thermal processing: efficiency, cost and technological innovation

6.1 Core advantages of thermal processing: efficiency and forming ability

Thermal processing efficiency advantages and material forming capabilities are its core competencies:

• Strong forming capacity: it can process high-strength, high-hardness materials that are difficult to form by cold processing, and produce complex shape parts;
• High production efficiency: good plasticity of the material, small processing force, can achieve large-scale and continuous production;
• Excellent product performance: the grain is refined after recrystallization, the mechanical properties of the material are uniform, and the fatigue resistance is good;
• High material utilization: Compared with cutting processing, there is less waste of hot machining materials, and the yield rate can reach 70%-90%.

6.2 The main challenges of thermal machining: energy consumption, accuracy and tool life

Thermal machining also faces many challenges:

• Energy consumption issues: The heating process consumes a lot of energy, such as hot forging consumes about 500kWh per ton of steel, which is 5-10 times that of cold working;
• Tool life challenges: In high-temperature environments, molds are prone to wear and oxidation, and replacement costs are high.
• Difficulty in precision control: after hot processing, the cooling and shrinkage of the material is easy to deform, and the accuracy control is difficult (the general tolerance level is IT10-IT12);
• Cost-benefit analysis: large equipment investment, high energy consumption, and high cost when producing small batches.

6.3 Technological innovation and industry development trend of hot processing

To meet the challenges, thermal processing technology is innovating in the following directions:

1. Intelligent processing: AI algorithms are used to optimize processing parameters, monitor temperature, pressure, and other data in real time to improve processing accuracy and efficiency.
2. Energy-saving technology: develop new heating technologies (such as microwave heating, plasma heating) to reduce energy consumption;
3. Mold technology upgrade: research and development of new mold materials and coatings that are resistant to high temperature and wear to extend the life of the mold;
4. Composite processing process: such as “thermal processing + 3D printing” composite process to produce complex structural parts;
5. Green environmental protection: reduce the emission of exhaust gas and waste residue in the heating process to achieve cleaner production.

Authoritative forecast: According to the “Industry Report of the Thermal Machining Branch of the Chinese Society of Mechanical Engineering”, in the next five years, the market size of intelligent thermal processing equipment will grow by 15% annually, and the penetration rate of energy-saving thermal processing technology will increase to more than 60%.

7. Yigu Technology’s view

As the core technology of high-end manufacturing, the value of thermal processing lies not only in solving the problem of material forming, but also in promoting the development of industrial products in the direction of high performance, lightweight, and low cost. At present, the challenges faced by the industry such as energy consumption and precision are essentially mismatched between technological upgrading and industrial demand. Yigu Technology believes that the breakthrough point of future thermal processing lies in “intelligence + material innovation”: precise control of the processing process through AI and Internet of Things technology, combined with new high-temperature resistant materials and mold technology, can not only improve product quality, but also reduce energy consumption and costs. For enterprises, they should rationally choose thermal processing processes and equipment according to their own product needs, avoid blindly pursuing high-end technology, and balance costs and benefits in order to gain an advantage in market competition.

8. FAQ: FAQ

1. What is the difference between heat working and heat treatment?

Answer: Hot working is a plastic forming process (such as forging, rolling) at a temperature higher than the recrystallization temperature, and the core is “forming”; Heat treatment is to change the structure and properties of the material (such as quenching, annealing) through heating, insulation, and cooling in the solid state, and the core is “modification” without changing the shape of the part.

2. Does the hot-processed product need to be processed later?

A: Usually required. Hot processed products have low surface accuracy (such as hot forgings surface roughness Ra=6.3-25μm), and need to be improved by cutting (such as turning, milling, and grinding), and some products also need to be heat treated to optimize performance.

3. What materials are not suitable for thermal working?

Answer: Brittle materials (such as ceramics, cast iron) are not suitable for hot processing because of their poor plasticity at high temperatures and easy to crack; Some materials with low melting point (such as lead and tin) have a narrow temperature range and rarely use thermal processing technology.

4. How accurate can thermal processing be achieved?

Answer: The accuracy of ordinary hot processing is IT10-IT12, and the surface roughness Ra=6.3-25μm; Adopting precision hot processing technology (such as precision hot forging, hot extrusion), the accuracy can be improved to IT8-IT9 grade, and the surface roughness Ra=3.2-6.3μm.

5. How to reduce the energy consumption of thermal processing?

Answer: Energy consumption can be reduced by 10%-30% by optimizing the heating process (shortening the holding time, using waste heat recovery), selecting energy-saving equipment (such as inverter heating furnaces), and improving equipment utilization (avoiding idling).