The selection of the tempering temperature is the key to formulating the tempering process. The overall impact of tempering temperature on the mechanical properties of wheel forgings is that as the tempering temperature increases, its strength and internal stress decrease, and its plasticity and toughness increase. Due to the differences in steel types and sizes of wheel forgings, their performance changes are different, as shown in the following: the specific values of various performance indicators are different; the degrees of performance increase or decrease are different; same.
In the heat treatment of large wheel forgings, two temperings are often required. The second tempering is often to accurately adjust the mechanical properties after the first tempering to meet the design requirements. Obviously, the second tempering temperature at this time should be determined based on the measurement results and design requirements of the mechanical properties after the first tempering, and by referring to production experience.
At this time, the second tempering temperature is often higher than the first tempering temperature. In addition, for the second tempering of some high alloy steels, the problem of tempering of residual austenite decomposition products is considered. Residual austenite becomes more unstable after the first tempering, and then decomposes into martensite or other transformation products during subsequent cooling. The presence of such untempered residual austenite transformation products will reduce the yield ratio and impact toughness of the wheel forging.
At this time, the purpose of the second tempering is to temper these transformation products to improve the yield ratio and impact toughness of the wheel forging. In wheel forgings, retained austenite is often more than in small pieces due to segregation and other reasons. Of course, in this case, the second tempering temperature must be lower than the first tempering temperature.