There are always some retained austenite after quenching. Too much retained austenite is harmful to the service life and hardness of parts, which will cause the instability of soft spot and size. However, a proper amount of retained austenite can improve the fatigue strength of parts. We can control the quality and service life of the product by controlling the retained austenite to achieve the expected effect.
Influence of retained austenite on various parts
(1) The rolling bearing requires good wear resistance, high rolling fatigue strength and good dimensional accuracy stability. The residual austenite has little effect on the fatigue life under the common stress level. In the actual production, the bearing steel is usually not cooled after quenching.
(2) Gear generally does not need cold treatment. Retained austenite is beneficial to the increase of fatigue life.
(3) For tool steel, retained austenite can increase impact resistance. For cutting tools, retained austenite reduces the hardness and deteriorates the cutting performance. Proper amount of retained austenite is beneficial to the tools such as bit and tap which mainly bear torsional stress. It is beneficial to die steel processed by pressure, especially to the retained austenite of punch. Compared with martensite, retained austenite is sponge like, which can buffer impact, improve toughness, improve surface contact fatigue strength and prolong the service life of punch.
(4) For measuring tools, retained austenite is not conducive to ensuring dimensional accuracy, so it must be eliminated as much as possible by cold treatment.
Analysis of influencing factors of retained austenite
With the increase of alloying elements and carbon content, the slow cooling rate and the increase of quenching temperature, the retained austenite will increase. If the cooling is interrupted and the temperature stays at the same time during quenching, the final transformation of martensite will be reduced and the retained austenite will be increased, which is the thermal stabilization of austenite. The content of carbon is about 0.8 at eutectoid point, the residual austenite is below 25%, and the residual stress is compressive stress. After carburizing, the surface carbon content of parts is high, and the retained austenite is increased after quenching.
The main factors determining the content of retained austenite are as follows:
(1) Effect of alloy elements of raw materials: Mn, Ni, Cr alloy elements increase retained austenite after quenching.
(2) With the increase of carbon content in raw materials, retained austenite increases.
Measures to reduce retained austenite
Generally, martensitic transformation occurs after heat treatment and quenching, and retained austenite will inevitably appear at the same time. To eliminate or control the retained austenite, the industrial freezer is used for cold treatment. The essence of cold treatment is to reduce the end temperature of cooling and further transform retained austenite into martensite. It is widely used in the plunger coupling of GCr15, and it is the most effective method to promote the transformation of retained austenite. Generally, the retained austenite is controlled within 10%.