Effect of retained austenite amount on copper properties and control of retained austenite amount and Stability of retained austenite.

There is always a certain amount of retained austenite in steel after quenching. The influence of retained austenite on the properties of steel is a complex problem affected by many factors. At present, there are few reports on the role of retained austenite in steel from the aspects of quantity, morphology, stability and distribution of retained austenite, which affect the deepening of understanding. In this paper, the influence of the quantity of retained austenite and the stability of retained austenite on the properties of steel is discussed.

1. Effect of retained austenite amount on copper properties and control of retained austenite amount

Document (1) uses 30CrMnSiNi₂A Steel as a sample. The final heat treatment is 900 ℃ * 40min heating and heat preservation, 230 ℃ * 60min isothermal quenching +280 ℃ * 2H tempering, and +50 ℃ * 15min~-60 ℃ * 15min cyclic cooling with hot water and dry ice. Three and nine times respectively. The amount of retained austenite was 5.8% and 4.5% respectively, and then the multi impact fatigue test was carried out. The test results show that the thin layer of retained austenite exists between the martensitic laths and becomes the crack initiation site under multi impact loading. The more the amount of retained austenite, the lower the multi impact fatigue initiation life. It is also pointed out that the fatigue crack propagation is the result of the connection between the main crack and the sliding crack of martensite lath. The thin layer of retained austenite not only does not prevent crack propagation, but also becomes the path of crack propagation. The higher the residual austenite content is, the faster the crack growth rate is. However, literature (2) holds the opposite conclusion. After studying the morphology and distribution of residual austenite, G. Thomas believes that thin-layer residual austenite reduces the fatigue initiation life, but there is a large amount of residual austenite in the structure that can absorb crack energy and prevent crack propagation. After studying the effect of retained austenite on the fatigue crack growth behavior of 40Cr steel, reference (3) points out that the existence of an appropriate amount of retained austenite between martensitic laths is not only conducive to increasing the plastic tearing work when the crack passes through the lath boundary, but also to alleviating the stress concentration at the tip and passivating the crack tip. Therefore, the existence of retained austenite film is beneficial to reducing da/dn and increasing the number of fatigue cracks and propagation cycles. The amount of retained austenite in the sample used by the author is 1.8%~2.3%. The different conclusions in the above literature are closely related to the different content of retained austenite. When studying the fatigue life of bearing parts, reference (4) points out that the use of cryogenic treatment to reduce the amount of retained austenite will not improve the fatigue life of bearing parts. When bearing steel is quenched by hot liquid at 200 ℃ ~250 ℃, the content of retained austenite increases to about 15%, but the fatigue life increases slightly. It is also pointed out that 9% retained austenite has the highest fatigue life. Some literatures think that 5%~10% retained austenite has a beneficial effect on the fatigue life of bearings. According to foreign statistics, about 31% of the gears were damaged by pitting corrosion. The pitting damage is related to the contact state of the tooth surface. If the contact area is increased, that is, the surface compressive stress is reduced, which can significantly improve the service life. It is generally considered that the most appropriate content is 10%~15%. These retained austenite will produce plastic deformation when loaded, which will mesh the tooth profile surface, resulting in an ideal contact state, and can maintain the accuracy of the tooth profile. At the same time, the surface of retained austenite is strengthened due to plastic deformation under stress. When discussing the fatigue damage of residual austenite to gears, some people think that when the residual austenite is less than 10%, the fatigue strength decreases. When the residual austenite is more than 10%, the fatigue strength is almost a fixed value. In low cycle fatigue under high stress, the fatigue life can be increased with the appropriate increase of retained austenite, while in high cycle fatigue under low stress, the fatigue life decreases with the increase of retained austenite. From the above analysis, it can be seen that only an appropriate amount of retained austenite can have a beneficial impact on the properties of steel. It is very necessary to control the amount of retained austenite when using retained austenite.

To control the amount of retained austenite, we can find out the main factors affecting the amount of retained austenite and put forward control methods. The factors affecting the amount of retained austenite are (1) the composition of steel (2) the heat treatment conditions of steel. The practice shows that the lower MS, the more retained austenite in the quenched steel. With the increase of austenite carbon content and the decrease of temperature MS, the amount of retained austenite after quenching also increases. The amount of retained austenite in high carbon steel after quenching to room temperature can reach 10%~15%. After the alloy elements except drill and aluminum are dissolved into austenite, the MS and MF points of martensite are reduced, and the amount of residual austenite in steel is increased, among which manganese, chromium and nickel have the strongest effect on increasing the amount of residual austenite. After the steel grade is determined, the residual austenite content can be controlled by changing the austenitizing and cooling conditions, selecting a lower quenching temperature, and using continuous cooling to a lower temperature can effectively reduce the residual austenite.

2. Stability of retained austenite

The stability of retained austenite includes thermal stability and mechanical stability. Thermal stability refers to the ability of retained austenite to resist tempering decomposition during tempering, while mechanical stability refers to the tendency of retained austenite to resist strain induced martensitic transformation under stress at room temperature. When considering the influence of retained austenite on the properties of steel, it must be clear whether it is the stable retained austenite or the part of retained austenite that has been transformed due to induced transformation. When studying the relationship between contact fatigue life of bearing steel and retained austenite, Yojiro Yashima of Japan pointed out that the retained austenite in the center of the bearing raceway was subjected to large contact stress during the contact fatigue test, resulting in deformation induced transformation and transformation into highly distorted martensite, which caused compressive stress during martensite transformation. The effect of these two factors improves the contact fatigue life of some shaft steels. The reasons for the increase of fatigue life of retained austenite are explained as follows in literature (6): (1) under the applied alternating contact stress, the retained austenite induces martensitic transformation, thereby improving the basic hardness of the transformation area, and the transformation occurs at the maximum shear stress on the sub surface, thus improving the fatigue life, (2) Residual austenite can prevent and alleviate the crack. When exploring the way to improve the service life of Cr12 Steel blanking die, Chen Wei believed that under the action of working stress, the blade surface of Cr12 Steel blanking die induced transformation and formed deformation induced martensite hardening layer. This kind of hardened layer has high hardness of martensite and is closely combined with the matrix, thus improving the service life of Cr12 Steel blanking die. And Japan's Tamura and others hold very different views. He believes that the lamellar martensite produced by the transformation of high carbon retained austenite at low temperature may be the cause of early fracture. Japanese Suzuki et al. Conducted impact test on steel with composition of 18%ni, 1~3%mn, 0.3~0.9% and found that the carbon content of the reverse transformation austenite in 9%ni steel containing 0.1%c can reach more than 0.3%, and the martensite formed by this austenite transformation is very brittle. Scholars pointed out that the stable residual austenite can significantly improve the toughness of low-temperature steel, but the transformation induced plasticity is applicable to steel with carbon content less than 0.05%. When studying the effect of heat treatment on the microstructure and mechanical properties of 4Cr5MoSiV steel, reference (9) pointed out that the residual austenite was distributed in the martensite boundary in the form of film, with a thickness of 100~200 ℃. After tempering at 350 ℃, the residual austenite still existed, so that the cracks propagating in martensite would be prevented by these residual austenite films, and the energy required for fracture would be increased. When there is residual austenite between the strips, the polymerization force between the residual austenite and the matrix is large, and the interface energy is low. When there is no residual austenite, the two adjacent strips are in close contact laterally, forming a rotating boundary. Such an interface has high energy, which is conducive to crack propagation, impurity segregation and carbide precipitation. At this point, it is not pointed out that the role of retained austenite in steel is indeed a very complex problem. When considering the stability of retained austenite, we should also consider the composition of retained austenite, the brittleness of martensite formed by deformation induced transformation, the orientation relationship with the original matrix, the bonding force and many other factors.

The stability of retained austenite is related to its composition and heat treatment system. The residual austenite decomposes and precipitates carbides and phase during tempering. Therefore, any factor that can prevent the precipitation of carbides and the formation of phase a can improve the thermal stability of residual austenite. The mechanical stability of retained austenite depends on the level of MD point. All factors that reduce MD point can improve the mechanical stability of retained austenite. Angel proposed that MD30 should be taken as a rough criterion for austenite stability. MD30 is the temperature at which 50% martensite is formed at 30% tensile deformation

Md30（℃）=413-462（C+N）-9.2Si-8.1Mn-13.7Cr-9.5Ni-18.5Mo

The above formula reflects the relationship between austenite stability and austenite composition.

3. Conclusion

The role of retained austenite in steel is a complex problem affected by many factors, especially the quantity and stability of retained austenite. A certain amount of stable retained austenite is favorable for toughness. It should be very careful to use retained austenite to induce transformation to martensite to improve the service performance of another part. The morphology, distribution and toughness of martensite obtained by induced transformation, the binding force with matrix, the influence on crack propagation and other factors, as well as the loading mode and stress state under service state should be paid great attention to.

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