Features of heat treatment and surface modification technology

The performance of the material does not depend solely on the type and composition of the material. Changing the internal structure of the material through heat treatment and surface modification will greatly change the material performance. For example, the hardness of high-speed steel in the annealed state is not higher than 280HB and has quite good plasticity and toughness. After quenching and tempering, it has high hardness, red hardness and wear resistance. Since the content of alloying elements dissolved in the matrix and the grain size of austenite are related to the quenching temperature, the trend is that the hardness and red hardness increase with the increase of the quenching temperature, the toughness decreases, and the strength increases first. Descending (Figure 1). Using this rule, the best quenching temperature can be selected according to the use characteristics of different tools and molds (Table 1). The blades and handles of the turning tools are relatively thick, and the strength requirements are not high, and the impact load is lighter. The quenching temperature close to the melting point can be used to dissolve as much alloying elements and carbon as possible into the austenite, thereby improving the red hardness and wear resistance. The cutting edge of the drill bit is not easy to cool when drilling. It is hoped to increase its red hardness as much as possible. However, in order to prevent twisting, the drill bit needs to have higher strength, so its quenching temperature is slightly lower than that of the turning tool. The cutting edges of milling cutters and reamers are relatively thin. In order to avoid chipping, sufficient toughness is required. The quenching temperature should be appropriately lowered. The main damage method of small drills is twisting or breaking. In order to ensure higher strength, the quenching should be further reduced. Heating temperature. The cold extrusion die is subject to high stress, but does not require high red hardness, so select the quenching heating temperature that appears the peak strength, and for some slender or complex shape, subject to greater impact load, you should choose Lower quenching temperature. Table 1 shows that for tools or molds made of the same kind of steel, different quenching temperatures should be selected according to use conditions and failure modes, and the range of changes reaches 150°C. But for a specific workpiece, only ±5°C deviation is allowed.

Structural steel and low-alloy tool steel have a similar situation. The pre-heat treatment structure, quenching heating temperature, cooling method, and tempering temperature all have a significant impact on the performance of the steel. The different combinations between them can make the material obtain different comprehensive properties. . The strength, hardness, toughness, plasticity and elastic limit of structural steel all change with the tempering temperature after quenching. For workpieces that require high plasticity, high toughness, especially low notch sensitivity, high temperature tempering (tempering and tempering) Treatment), and the workpieces that require high strength and higher hardness should be tempered at about 200 ℃, such as 30CrMnSi, 40CrNiMo after quenching at 200 ℃, the tensile strength can be as high as 1600″1800MPa, which is about 1 times higher than quenching and tempering. Springs and other elastic components are usually tempered at medium temperature showing the peak of elastic limit. In addition, processes such as austempering, two-phase zone heating quenching and deformation heat treatment can make structural steel good strength and toughness. As for various chemical heat treatment and surface coating technologies The concentration and depth of the carburized layer surface and the control of the concentration gradient and performance gradient can be adjusted by adjusting the process parameters to adapt to the requirements of different service conditions on the overall performance of the workpiece. For example, the carburizing treatment of different parts should be There are different technical requirements to obtain good performance (Table 2). For oil drilling roller cones, the surface concentration of the carburized layer is reduced from 0.9″1.0%C to 0.7″0.8%C, and the concentration distribution curve is flat. , The service life is increased from 27 hours to 52 hours, and the effect that one drilling team can reach two drilling pairs is received. For example, the surface modification treatment by ion implantation can greatly improve the overall strength and toughness. Improve wear resistance, reduce friction coefficient, improve corrosion resistance, apply to bearings and various friction parts in various transmission mechanisms on spacecraft, wear parts in hydraulic motors on aircraft, and sleeves for mud pumps in the petroleum industry Good results have been achieved in cylinders, etc. In some cases, some seemingly “informal” heat treatment processes are used for the characteristics of the workpiece, and surprising results can be obtained: the quenching temperature range of 3Cr2W8 hot mold steel is generally 1050″1120℃, but The boiler steel pipe hot extrusion die, which is equivalent to the two sides of the steel pipe radiating ribs in the die cavity, bears great stress and is easy to yield in the hot state and cause the die to fail. After the experiment, the quenching temperature is increased to 1170″1180℃, and the water is cooled to “650℃ during quenching cooling, and then transferred to a low-temperature salt bath for cooling. The mold life is increased several times; the blade of the rice harvester is treated with high-concentration carbonitriding; A large amount of carbides and retained austenite appear on the surface layer, which is regarded as unqualified according to conventional inspection standards. However, due to its high wear resistance and good corrosion resistance, the service life of rice harvester blades is longer than that of conventional carburizing Deal with several times higher.

A cursory review of the above-mentioned facts that have long been well-known is just to illustrate an easily overlooked view: the optimal heat treatment process cannot be the same, and the properties of the same material will change due to the heat treatment method and process parameters. , And various performance indicators often fluctuate one after another. Only by selecting appropriate heat treatment process parameters and obtaining the best comprehensive performance compatible with the use status and failure modes of the workpiece can high-quality products be manufactured. This is the characteristic, difficulty and charm of heat treatment and surface modification technology. , Full of space and leeway for people to exert their subjective initiative.

History has proved that improving heat treatment technology and making full use of the potential of materials are often catalysts for product upgrading. After quenching and tempering, the yield strength after high temperature tempering after quenching is about 600” 900 MPa. Both the strength and toughness are significantly better than normalizing treatment, so it became a common heat treatment process for structural steel. Before the Second World War Researchers in the Soviet Union found that after 30CrMnSi quenching and low-temperature tempering, or austempering, the yield strength reaches 1500 MPa and maintains sufficient toughness. It is used to manufacture aircraft landing gear. Medium and low-carbon structural steel quenching and low-temperature tempering treatment is also used Military products such as bullet-proof shields of artillery, and subsequent development of a series of “ultra-high-strength steels” characterized by quenching and low-temperature tempering treatments promoted the upgrading of many important products, such as the rotor of the hydraulic coupler of high-power gas turbines , It transmits tens of thousands to hundreds of thousands of kilowatts of power, and the speed is more than 20,000 revolutions per minute. The original design is SEA4340 steel quenching and tempering treatment, and the yield strength is 800MPa. Later, quenching and low temperature tempering are used to achieve a yield strength of 1800MPa. The weight of the entire coupling is reduced to 1/4 of the original. This is very beneficial for improving the performance of the ship.

Surface modification technology also plays an important role in the development of high-end products. As we all know, the thermal efficiency of gas increases with the increase of gas temperature, but the heat-resistant temperature of superalloys limits the increase in combustion chamber temperature. In foreign countries, due to the successful deposition of a composite coating containing honeycomb ZrO2 on the surface of the heat-resistant alloy, it plays a role of heat insulation, making the temperature of the heat-resistant alloy blades more than 150 ℃ lower than the gas temperature, and developed a higher combustion chamber temperature Gas turbines promote the upgrading of aero engines.

Even in the general machinery manufacturing industry, the technological progress of heat treatment and surface modification is also of great significance to product innovation. For example, the productivity of the cold heading machine for producing standard parts has now reached 600 pieces/min, compared to more than two decades The first 60 pieces/min is increased by 10 times. The outlook of the standard parts industry has been greatly changed. In fact, the cold heading machine is not complicated. It is not difficult to design and manufacture the 600 pieces/min cold heading machine. The problem is that the small hexagonal punch has a lifespan of less than 20,000 pieces. In this case, increase The speed of the cold heading machine is meaningless. Because standard parts are products with extremely large batches, it is usually required that the life of each punch must exceed one shift, otherwise it is difficult to carry out production management. In the early 1980s, through the improvement of the heat treatment process, the life of the punch was increased to more than 50,000 pieces, and the cold heading machine with 100 pieces/min was available. And in the 1990s, the hexagonal punch was made by vapor deposition of titanium nitride. Surface modification treatment has increased its life to more than 350,000 pieces. Become a catalyst for high-speed cold heading machines.

The thin-walled ring gear of a special gearbox is characterized by its ability to significantly reduce the volume and weight of the gearbox, but it is difficult to manufacture with conventional gear heat treatment methods. It is difficult to control carburizing quenching or induction heating quenching. Heat treatment distortion, while the conventional nitriding treatment can not meet the requirements of the contact fatigue strength of the gear, only the application of dynamic controllable nitriding process increases the contact fatigue strength from 1400 MPa to 1700 MPa, and the research has successfully controlled the nitriding distortion of the thin-walled ring gear The method that made the trial production of special gearboxes succeeded.

Only from these examples can be reflected: the technological progress of heat treatment has an important role in promoting product innovation.

In view of the above characteristics, in order to improve the technical level of heat treatment, a systematic study of the influence of heat treatment process parameters on the structure and properties of materials should be carried out first, and secondly, the research work should not only stop at the level of sample research. The heat treatment process research needs to be combined with the product bench test, installation test and failure analysis. After continuous exploration and improvement, the effect of greatly improving the life can be achieved. For example, the cold extrusion punch of truck piston pin shown in Figure 3. To withstand a unit pressure of about 2000MPa, it needs to have a high compressive yield strength, and its shape is slender and easy to break, and it requires sufficient toughness. The extruded metal strongly rubs against the ligament during the extrusion process, so it needs high resistance Abrasion and certain thermal stability. It is made of W6Mo5Cr4V2 high-speed steel. At first, the standard heat treatment specifications given in the manual were used for treatment, and the service life was less than 400 pieces. The failure mode is that the punch breaks during pressing. In order to improve the toughness of the material, the quenching temperature was reduced from 1225°C to 1190°C, which received significant results and the service life was increased to about 2500 pieces. Although the toughness can be further improved by further reducing the quenching temperature, the service life will fall. After careful analysis of the working conditions and failure modes of the punch, it is found that the cutting edge of the punch that is heated and quenched in the low temperature range is gradually drawn, and the resistance during demolding becomes larger and larger. During the demolding process, due to impact tensile stress The effect of causing fracture. For this specific situation, quenching at 1190°C and tempering at 560°C for 4 times, and then gas nitrocarburizing treatment. The hardness of the surface layer (about 0.02mm) is increased to more than 1000HV, while the overall strength and toughness are maintained, and the service life is increased to more than 10,000 pieces.

Furthermore, considering that the heat treatment process parameters are very sensitive to the effect of material properties, in order to ensure the reproducibility and consistency of quality, it is necessary to research and develop advanced heat treatment process equipment, precise and reliable heat treatment process control technology, reasonable design fixtures, and regulations. And strictly implement reasonable furnace installation and operation methods. So improving the quality of heat treatment and its reproducibility is a systematic project. It is not surprising that there are large gaps between different countries and different companies in this field.