What specific effects does the tempering process have on the properties of carbon structural steel?
The tempering process directly alters the internal structure of carbon structural steel (such as martensite decomposition and carbide precipitation morphology) through temperature control (low temperature 150-250°C, medium temperature 350-500°C, high temperature 500-650°C) and holding time. This in turn has a differential impact on core properties such as hardness, strength, toughness, plasticity, elasticity, and dimensional stability. The specific impacts require explanation in conjunction with tempering temperature classification. Typical carbon structural steels (such as 45 steel and 65 steel) will be used as examples to provide a more intuitive understanding of the performance changes:
1. Changes in Core Mechanical Properties: Hardness and Strength - A "Gradual Decline" with Increasing Temperature
After quenching, carbon structural steels exhibit extremely high hardness and strength due to their martensite structure (a supersaturated solid solution). However, during tempering, the martensite gradually decomposes into stable carbides (cementite) and ferrite, alleviating lattice distortion. Consequently, hardness and strength decrease regularly with increasing tempering temperature, and the magnitude of the decrease is positively correlated with temperature.