Tempering Behavior of a Si-Rich Low-Alloy Medium-Carbon Steel

Owing to the addition of Si, 0.33C-1.8Si-1.44Mn-0.58Cr steel exhibits a unique tempering behavior. The tempering takes place in two distinct sequential stages that are significantly different from those in steels containing 0.2-0.5 wt.% of Si. Stage I is associated with the precipitation of transition carbides in a paraequilibrium manner, can take place in temperatures ranging from similar to 200 to similar to 474 degrees C, and concurrently increases strength, ductility, and toughness. Stage II is associated with the decomposition of retained austenite to bainitic ferrite and transition carbides. As a result, no significant effect of overlapping of Stage I with Stage II takes place. Stage III does not occur at temperatures below similar to 474 degrees C, since the precipitation of cementite in a orthoequilibrium manner is suppressed by the addition of 1.8 wt.% of Si. It was shown that a major portion of carbon atoms redistributes to Cottrell atmospheres under quenching. During low-temperature tempering at 200-400 degrees C, the precipitation of transition carbides consumes a large portion of carbon atoms, thereby increasing the number of ductile fractures and improving the impact toughness without strength degradation. The formation of chains of cementite particles on boundaries takes place in Stage IV at a tempering temperature of 500 degrees C. This process results in the full depletion of excess carbon from a ferritic matrix that provides increased ductility and toughness but decreased strength.

Automated summary

The addition of Si in 0.33C-1.8Si-1.44Mn-0.58Cr steel leads to a unique tempering behavior, characterized by two distinct sequential stages. Stage I involves precipitation of transition carbides, increasing strength, ductility, and toughness. Stage II involves decomposition of retained austenite to bainitic ferrite and transition carbides. Stage III does not occur due to Si addition suppressing the precipitation of cementite. Low-temperature tempering leads to the formation of transition carbides, increasing ductile fractures and improving impact toughness without strength degradation.