The selection of tempering temperature is the key to the formulation of tempering process. The effect of tempering temperature on the mechanical properties of forgings is: with the increase of tempering temperature, the strength and the internal force decrease, and the plasticity and toughness increase.
Because of the difference in the steel and size of forgings, the performance changes are different, which shows that the specific values of various performance indices are different, and the different performance increases or decreases; The heat treatment of large forgings often requires two times tempering. The second tempering is often used to accurately adjust the mechanical properties after the first tempering to meet the design requirements. Obviously, the second tempering temperature should be based on the first tempering mechanical properties after the determination of the results and design requirements, reference to the production experience to determine, 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 tempering problem of residual austenite decomposition products is considered. The residual austenite becomes unstable after the first tempering, and decomposes into martensite or other transition products during subsequent cooling. The existence of this kind of Cuchamby residual austenite transformation product will reduce the forging's strength and impact toughness. At this time, the purpose of the second tempering is to make these transformation products to temper, to improve the forging Cuchamby and impact toughness. In the large forgings, the residual austenite is often more than that of 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. For the rotor forgings with a central hole, the first tempering achieves mechanical properties, then the long-term hydrogen tempering at a slightly lower temperature can improve the plasticity and toughness index.