The deformation heat treatment process is commonly referred to as “thermomechanical processing”.
In the field of machine manufacturing, the combination of pressure processing techniques (such as rolling, forging and rolling) and heat treatment can result in linear strengthening and heat treatment, leading to comprehensive mechanical properties that cannot be achieved through a single reinforcement method. .
This combined strengthening process is known as thermomechanical treatment.
In addition to providing exceptional mechanical properties, thermomechanical treatment also eliminates the need for high-temperature heating during heat treatment, resulting in significant energy savings, reduced use of heating equipment and workshop space, and reduced occurrences of treatment defects. thermal, such as oxidation of materials, decarburization and distortion.
Therefore, the thermomechanical treatment process not only provides excellent strengthening effects, but also offers significant economic benefits.
For reference, the following 36 examples of thermomechanical treatment methods are listed.
1. 16 cases of hot forging quenching process
(1) Quenching of oring cutter after forging
A boring machine with a cutter body diameter of 4 mm, a head diameter of 6 mm and a total length of 40 mm is immediately quenched after forging and immediately tempered. This results in an increase in cutting life of more than 30% compared to conventional treatments.
(2) Quenching of lathe tool after forging
A self-service turning tool made of M2 steel from a domestic electrical machine tool factory is quenched in oil immediately after forging and tempered at 550°C. This results in a service life that is more than once longer than that of lathe tools on the market.
Jialong Company's proprietary 9341 steel 12-square turning tool is oil-cooled after free forging, resulting in a relatively long service life.
(3) Hot forging quenching of crusher hammer
The hot forging hardening process for a 355mm x 98mm x 33mm crusher hammer made of 65Mn steel is as follows:
The initial forging temperature is 1050°C and the final forging temperature ranges from 840°C to 860°C. After final forging, the hammer must be cooled in air for 2 to 3 seconds and then quenched under running tap water. It must then be tempered at a temperature range of 180°C to 200°C, resulting in a surface hardness of 50 to 55 HRC within 10mm of the surface.
This hot forging hardening process increases hammer life by more than 50% compared to conventional heat treatment.
(4) Tempering socket wrenches by forging
A 40Cr steel socket wrench from a domestic hardware tool factory uses forging quenching instead of traditional salt bath quenching. This method is not only energy-efficient and environmentally friendly, but also produces high-quality results.
(5) Chisel forging temper
The 55MnSi steel chisel is forged using a 2500N air hammer and specialized die. The ideal temperature for deformation is between 920-950°C, with a deformation rate of approximately 75%. The final forging temperature is around 900°C.
To maintain optimal hardness and toughness, the workpiece must be quickly quenched in water and cooled in oil within 30 seconds of deformation (based on the surface color of the workpiece). The chisel must then be tempered to a temperature range of 220-270°C.
After undergoing thermomechanical treatment, the chisel has improved hardness and toughness, resulting in longer service life.
(6) Forging quenching of thread ring gauge
The 230mm x 120mm CrMn steel blank, weighing approximately 40kg, is forged into 90mm x 90mm x 600mm square bars. Suppression is then performed according to the ring gauge size.
The plate will be heated to a temperature of 1050 to 1150 ℃ with suitable insulation. It will then undergo rapid extrusion formation in the high temperature deformation area.
The shape variation will be between 35% and 40%, with a final forging temperature of 920 to 900 ℃.
Immediately after forging, the square bar will be cooled in oil at a temperature of 40 to 70 ℃ for 40 to 60 seconds.
After air cooling to approximately 100℃, the square bar will be tempered.
The surface hardness of the measuring ring should be ≥62HRC.
(7) Forging quenching of 45 steel sprockets
The initial forging temperature ranges from 1070 to 1150°C, while the final forging temperature is set at 850°C. The deformation variable varies from 35% to 75%. The tempering temperature can vary from 200 to 350°C.
Compared to heating and quenching in a salt bath firebox furnace, strength increased by approximately 30% and wear resistance increased by 26% to 30%.
(8) Forging tempering of GCr15 steel bearing
The deformation temperature varies from 930 to 970°C, with a variable deformation of 30%. Cooling is done with oil and the tempering temperature varies from 150 to 180°C.
Compared to conventional heat treatment methods, this process results in an increase of almost 20% in resistance and a 23% increase in contact fatigue life.
(9) Forging tempering of 40Cr steel diesel engine connection
The initial forging temperature is between 1150 and 1180°C with tread forging, and the deformation time is between 13 and 17 seconds, with a deformation rate of approximately 40%.
Subsequently, the part is immediately trimmed in a 2150N crank press, followed by immediate quenching (at which time the part temperature is between 900 and 950°C) and then tempered at 650°C.
(10) Pre-cooling quenching after 45Mn2 steel ball forging
When working with 45Mn2 steel balls with a diameter of 70 to 100mm, the initial forging temperature should be around 1200°C. The final forging temperature must be maintained between 1000°C and 1050°C.
The appropriate pre-cooling time after water quenching can be selected based on the specifications of the steel ball. Tempering steel balls at a temperature between 150°C and 180°C will result in a surface hardness of at least 57 HRC, with a hardened layer depth greater than 20 mm and a hardness greater than 50 HRC. This meets the requirements for large specification steel balls.
(11) 65 Mn steel coulter roller forging immediate tempering
The intermediate frequency induction heating temperature is between 1100°C and 1200°C. During the roll forging process, from the beginning of deformation to 20 seconds before quenching, the deformation of various parts of the plowshare varies from 56% to 83%. After deformation, the quenching thermal density is between 1.30g/cm 3 to 1.35g/cm 3 in an aqueous CaCl2 solution.
After quenching, the plowshare is tempered at a temperature of 460°C to 480°C for 3 hours, resulting in a hardness range of 40 to 45 HRC.
Compared with the traditional heat treatment process for plowshares, the number of heating cycles has been reduced from 4 to 5 times to just two times, leading to an increase in production efficiency of about 4 times. Product quality meets first-class requirements, resulting in significant economic benefits.
(12) Forging tempering of steering knuckle
For the 40Cr steel steering knuckle with a diameter of 60mm, it is forged by heating it to a temperature range of 1150 to 1200℃. The final forging temperature is then reduced to 900 to 850°C and oil cooling is conducted. The joint is tempered at a temperature of 600°C for 2 hours.
Using waste heat from the forging process for tempering not only saves energy and reduces costs, but also significantly improves the organizational structure and performance of materials, particularly in terms of impact resistance, which is crucial for automobile safety.
(13) Cr12MoV Steel Precision Plastic Hot Press Die Forging and Hot Quenching
The overall dimensions of the matrix are 70 mm x 20 mm x 10 mm.
There are 20 small holes in the plane of width 20 mm, with diameters of 1.5 mm, 2.5 mm and 3 mm. These holes require heat treatment with a hole spacing tolerance of ±0.006 mm, flatness of less than 0.01 mm and hardness between 56 and 60HRC.
Due to the severe segregation of eutectic carbides in Cr12MoV steel, there is a significant risk of cracking after billet rolling. The material is still distributed in strips along the rolling direction, with the core being distributed in mesh, blocks and piles, which become stress centers and sources of cracks. This leads to anisotropy of the material and increased distortion from heat treatment.
Thermal deformation of forging is the best solution to solve these problems.
The specific process is as follows:
The initial forging temperature range is 1050°C to 1160°C, with a final forging temperature range of 850°C to 950°C.
The material undergoes oil cooling when hot, followed by two tempering processes at 780°C for 3 hours each.
The final metallographic structure consists of martensite, lower bainite, dispersed carbide powder, and small amounts of residual austenite.
The specific volume is similar to that of thermally quenched sorbite.
Microdeformation does not require straightening after heat treatment, and all distortions meet technical requirements with a hardness range of 58 to 60 HRC and a qualified index of 99.99%.
This heat treatment process results in high heat resistance, thermal hardness, wear resistance and a long service life for the matrix.
Examples of residual heat quenching from forging and high-temperature tempering include hexagonal drawing dies, deep drawing dies, and cold punching dies, among others, but these are not mentioned here.
(14) Forging tempering of various metal utensils and tools
Various metal utensils and tools, such as wrenches, screwdrivers, pliers and scissors, were the first to be hardened by the residual heat generated during forging. This could be considered the first thermomechanical treatment prototype.
The tool parts were heated and then tempered in a coke oven, where the color of the fire was observed, a process known as in-line forging. Some required multiple heating cycles to reach the desired size, while the final forging step after forming did not require air cooling.
The appropriate coolant must be selected based on the material and then applied to the side of the kiln or tempered using its residual heat. A special tempering furnace is rarely used.
(15) Woodworking tool forging temper
After undergoing free forging, woodworking tools such as planes, axes, and chisels are typically quenched with residual heat. This method is economical as it saves electricity and time and is also highly efficient in terms of production.
(16) Immediate hardening of agricultural equipment
In some rural towns, coke ovens are still used.
Agricultural machinery cooled by the residual heat of forging includes sickles, shovels, rakes, crushing hammers, as well as kitchen utensils such as spoons, spatulas and knives.
2. 2 cases of forging normalization
(1) 3Cr3Mo3W2V Steel Hot Forging Die Normalization Treatment to Eliminate Chain Carbide
Steel tends to develop chain-like carbides upon slow cooling during forging, resulting in brittle matrix fracture, cracking, or thermal cracking failure.
Heating to normal temperatures may dissolve M6C.
When air cooled at a speed greater than 15℃/min, which exceeds the critical cooling speed for the formation of chain carbides, eliminates chain carbides and anneals the steel through spheroid annealing to obtain a fine and uniform distribution of carbides.
The recommended normalization temperature is 1130 ℃. This modification results in a reduction in forging normalization impact strength from 26J/ cm2 to 23J/ cm2 and an increase in service life from 1,500 parts to 2,000 parts.
(2) 20CrMn steel forged hot normalization
The process of normalizing high-temperature deformation involves heating the part to its final forging temperature of approximately 850°C and then allowing it to cool in air. This not only increases the strength of the steel, but also significantly improves its impact toughness, wear resistance, fatigue resistance and reduces its brittle transition temperature.
The blank for forging in 20CrMnTi steel has dimensions of 80mm x 80mm x 40mm.
After forging, the part is cooled by air and the cooling speed is carefully controlled to improve its mechanical properties and facilitate cutting.
Some domestic machinery companies that produce 20CrMnTi steel automotive gears use the waste heat generated during forging to perform normalization. This process can save more than 300 kWh of electricity per ton of gears produced.
3. 2 cases of forging thermal annealing
(1) Fast spheroid annealing of HSS forgings
Some domestic units immediately place the high-speed steel at Ac1 (20-30°C) for 2-3 hours after forging, allowing the furnace to cool to 550°C and then air-cool. This simplifies the process, shortens the production cycle and saves 70-90% on electricity costs, reducing production costs and improving working conditions. Furthermore, this process improves the quality of forgings and facilitates mechanized operations.
For high-speed steel parts processed by rolling, forging and isothermal processing, there is no need to follow the traditional annealing process. This example can be used as a reference.
(2) 8Cr2WMnMoVS (referred to as 8Cr2S) system of precision cold forging, residual heat annealing
The matrix dimensions are 250 mm x 200 mm x 42 mm. The initial forging temperature is between 1150-1100°C and the final forging temperature is between 900-850°C.
The annealing process involves heating the die to 800-820°C for 4-6 hours and then letting the furnace cool to 500°C using air cooling.
4. 5 cases of residual thermal quenching of rollers, laminating and extrusion
(1) Set mechanical blade cooling
In the wood processing industry, some rotary and flat knives are manufactured by the flanging method. The blade of these knives is made of tool steel alloy such as 5Cr8W2MoVSi while the body or back is made of 45 Q235A steel. The body is heated to the forging temperature of the steel blade and then the two are welded together using a rolling mill.
This process is known as solid phase welding, and the sheet is rolled to the desired size before being controlled to the final rolling temperature and then promptly quenched and cooled.
The blades produced using this method are of high quality, with high hardness and long useful life, and offer the additional benefits of saving time and electricity during the manufacturing process.
(2) Rolling hot quenching of M2 steel lathe tool
Hot rolling tempering is a heat treatment process that uses the residual heat generated by the rolling of various profiles to temper them. This process produces the same strengthening effect as hot quenching in forging.
For example, M2 steel can be rolled at a temperature of 1220°C (mill 250, 50 r/min) to the desired size and then directly quenched, resulting in a hardness of 65HRC or higher. This results in longer cutting life for lathe tools compared to salt bath quenching.
(3) Rolling hot quenching twist drill for 45 steel machine woodworking
The author successfully carried out a thermomechanical treatment process using helical drills from a national machine tool company.
The high-frequency heating device was used to conduct four-roll hot rolling.
The austenitizing temperature was regulated between 950°C and 1000°C, and the deformation temperature was between 880°C and 950°C, with a strain rate of approximately 30%. The quenching cycle was carried out using an aqueous solution of two nitrates, with the water temperature maintained below 70°C.
The resulting hardness after quenching was ≥54 HRC, and after tempering at a temperature of 240°C to 260°C for 1 hour, the hardness was ≥50 HRC, which meets the technical requirements and exceeds the deformation requirements in more than 95%.
(4) Hot quenching by reinforced bar rolling
Bars reinforced with 20MnSi steel require supply hot rolled and must meet the performance requirements of having a tensile strength of ≥ 510MPa, a flexural strength of ≥ 335MPa and an elongation of ≥ 16%.
A 60mm x 60mm billet is wrapped around a 16mm diameter reinforced bar. The initial rolling temperature varies from 1100 to 1200°C and results in a reduction in the rolling shape of around 93%. The final rolling temperature is between 950 and 900°C, which is the temperature for tempering steel with low carbon martensite.
After rolling, the bar is cooled with water within 1 to 1.26 seconds. It then undergoes self-tempering at temperatures between 550 and 600°C.
The reinforced bar that has undergone the above rolling, quenching and tempering process exhibits mechanical properties that exceed those specified in GB1499 and also exceed the mechanical properties specified in the British standard BS4449.
(5) Immediate xtrusion hardening of 35CrMo steel oil jumper joints
The extrusion deformation temperature ranges from 1100 to 1200°C, and the tempering temperature is between 570 to 580°C.
The hardness of the material is between 300 to 335HBW, with tensile strength ≥ 1068MPa, flexural strength ≥ 960MPa and elongation ≥ 14.5%, which meets the standards set by the Ministry of Standardization.
Experience shows that for large thermal quenching extrusion parts such as gaskets, it is crucial to carefully select the deformation temperature, the time elapsed before quenching after deformation, the quenching medium, the cooling time of the part in the quenching medium and tempering temperature, among other process parameters.
5. 4 cases of transformation superplasticity heat treatment
(1) Superplastic thermomechanical treatment of 9SiCr steel
The purpose of the 840°C x 2h oil cooling and 200°C x 2h quenching process is to achieve double fabric refinement.
Then, during the superplastic deformation process at 800°C, the strain rate is 2.5 x 10s and the tensile strain variable is 250%. After deformation, oil cooling is carried out.
The results of the steel superplastic deformation test, including flexural strength, multi-stroke service life and hardness indicators, showed that the flexural strength was 28% higher than with conventional treatment. Multi-stroke life increased by 38.6% and hardness was ≥ 60 HRC, equivalent to that achieved through conventional hardening.
(2) Low temperature thermomechanical treatment
The flexural strength of H11 steel is 1852 MPa and, after undergoing two tempering cycles at 482°C in conventional quenching, its elongation rate is 12.5%.
By performing a low temperature creep quench and two tempers at 482°C, followed by 2% strain aging at around 316°C and a final temper at 482°C, the flexural strength of the steel increases to 2548 MPa , an increase of 37.5%, while its elongation rate remains unchanged.
(3) Combined high and low deformation treatment
This composite thermomechanical treatment is a process in which high-temperature deformation quenching is followed by a small amount of deformation and tempering at a specific temperature.
Conducting strain aging of martensite after high temperature strain quenching can make the steel obtain much higher strength properties than any other heat treatment.
For example, the mechanical properties of 50CrVA after conventional quenching and tempering at 200°C are a tensile strength of 2,119 MPa, a flexural strength of 1,497 MPa, and a 41.7% reduction in section.
After undergoing high temperature strain quenching, 200°C tempering, 3% strain and 200°C tempering, the mechanical properties of 50CrVA are a tensile strength of 2597 MPa and a flexural strength of 2254 MPa.
This composite thermomechanical heat treatment, combining high-temperature strain quenching and martensitic strain aging, increased the tensile strength and flexural strength of 50CrVA steel by 22.6% and 50.7%, respectively.
(4) Mechanical blade roller straightening
Jialong Company heats and tempers mechanical blades, such as flat knives and rotary knives longer than 2 meters, in a furnace with a protective atmosphere at a temperature of approximately 500°C.
After the workpiece cools to about 200℃, it is rolled back and forth several times on a roller press using the principle of phase change superplasticity. This process allows immediate adjustment of straightness to ≤0.30mm after bending 10-15mm.
This deformation reinforcement not only straightens a previously bent insert, but also creates a residual compressive stress approximately 5 mm deep in the rolled surface. This helps to increase tool life.
6. 7 cases of chemical thermomechanical treatment
(1) Cold deformation carburizing
The process involves carburizing after cold deformation of the part, as cold deformation creates several structural defects that can accelerate the carburizing process.
For example, after cold deformation, the deformation of 20CrNiMo is 25%. If the workpiece is gas carburized at a temperature of 930-950°C for 2 hours, the depth of the carburizing layer will reach 0.84 mm. If the deformation increases to 50%, the coating depth will reach 0.88 mm. The greater the deformation, the deeper the penetration layer.
(2) Cold deformation nitriding
The process is a composite heat treatment in which the part undergoes nitriding after being cold deformed at room temperature.
Cold strain nitriding is distinct from cold strain carburizing.
Cold deformation decreases the rate of nitrogen penetration and decreases the thickness of the diffusion layer, and this trend becomes more pronounced as the level of deformation increases.
This phenomenon can be caused by nitrogen atoms preventing the diffusion of other nitrogen atoms by fixing dislocation sites or trapping displaced nitrogen atoms.
However, cold deformation nitriding can increase the toughness of pure iron.
The temperature and duration of nitriding depend on the type of steel, for example, 38CrMoAl steel and 20 steel require temperatures of 650°C and 550°C respectively.
(3) Boron infiltration by cold deformation
This is a combined heat treatment in which the part undergoes deformation at room temperature followed by boron infiltration.
For example, 20 pieces of steel are rolled and deformed in an oven, then subjected to a holding period of 900°C and solid boron infiltration at varying heating rates.
Tests have shown that cold deformation significantly increases the depth of the boron infiltration layer.
The optimal strain level for maximum penetration depth varies based on the heating rate and retention time during the boron infiltration process.
This phenomenon is caused by the cold deformation of the steel structure, which accelerates the atomic adsorption process of boron on the steel surface.
(4) Cold deformation of carbon and nitrogen co-infiltration
Cold deformation carbonitriding is a compound heat treatment process where medium temperature carbonitriding is carried out after a room temperature deformation process.
The cold deformation pretreatment step has a significant impact on the steel carbonitriding process, as it increases the C and N content on the surface and increases the thickness of the penetration layer.
For example, when the cold-rolled deformation of 20CrMnTi steel is 15%, the thickness of carbon and nitrogen co-infiltration after the processes of 860°C×2h and 860°C×4h is 0.65mm and 0.80mm, respectively.
(5) Titanium infiltration by deformation
Deformation at room temperature affects not only the diffusion process of interstitial atoms in the steel, but also the penetration process of substitutional atoms.
By way of illustration, the cold deformation of 16Mn steel was studied to examine the effect on the solid infiltration process of titanium. The results showed that the best temperature for titanium infiltration was 900 to 950°C, with a deformation of 30%.
Furthermore, as the carburizing temperature of titanium increases, the retention time also increases, leading to a thicker penetration layer.
(6) Thermal carburizing quenching forging
The thermomechanical heat treatment process involves heating the blank to the initial forging temperature for forging, followed by carburizing in a carburizing furnace and finally direct quenching.
The forging-carburizing-quenching method conserves electrical energy that would otherwise be required to heat the part during carburizing and increases the speed of carburizing. This results in better surface hardness, wear resistance and makes it suitable for medium modulus gears and other carbureted parts.
Another form of combined carburizing and thermomechanical treatment is called carburizing-forging-quenching, which involves carburizing followed by hot die forging and quenching.
This process can significantly increase the thickness of the effective hardened layer on the workpiece, increase the surface compressive stress, improve the breaking resistance and extend the service life of the product.
(7) Composite tempering itriding heat treatment of 9SiCr steel circular bolting die
The hardness of a 9SiCr steel circular thread die after heat treatment is normally between 62 and 65 HRC. The conventional heat treatment process involves heating in a salt bath at a temperature of 860 to 880°C, followed by quenching and tempering at 150 to 180°C.
To improve tool hardness and wear resistance, surface chemical heat treatment can be used. However, this process requires a temperature of at least 400°C, which is not suitable for 9SiCr steel tools. Nitriding, on the other hand, can provide a solution to this problem.
The nitriding process involves heating the tool in a 60kW LD ionic nitriding furnace, followed by a 100kW medium temperature salt bath furnace, cooling the oil, cold treating and finally tempering at 150 to 180° W.
Tests have shown that the hardness at a depth of 0.10 to 0.80 mm is greater than 927HV5, with a peak hardness of 974 to 986HV5. The hardness at the depth of 0.20 to 0.60 mm is ≥857HV5, which improves the anti-tempering properties of the hardened area and prolongs the service life of the material.
Conclusion
The thermomechanical treatment process is widely used.
From a material perspective, it is suitable for a wide range of metal materials, including various carbon steels, alloy steels, alloy structural steels and nickel-based alloys.
In terms of processing methods, it can combine the benefits of both to meet specific strength and toughness requirements, significantly improving the quality and longevity of deformed components.
The future prospects for thermomechanical treatment are positive.
