Effect of Silicon Addition on Carbide Transition in Tempered Martensite of Middle Carbon Steels

Mechanical properties of tempered martensitic steel are controlled by precipitation conditions of carbides. In high strength medium carbon martensitic steel, carbide precipitation at low tempering temperature is strongly affected by silicon addition. Silicon addition prevent carbide transition from ε carbide to cementite, also prevent tempering softening and low temperature temper embrittlement. In this paper, we investigated the carbide transition in medium carbon steel with various Si content, using chemical analysis and XRD analysis of residue carbide after electrolytic extraction, DSC thermal analysis and FE-SEM observation. We found three step carbide transition from ε carbide to para-cementite and ortho-cementite. Especially, para-ortho transition of cementite is not only change in chemical composition of carbide, but also change in the cementite morphology and precipitation site from platelet shape inside of martensite block to granular shape at martensite block boundary. Silicon addition inhibited the formation of para-cementite, and shifted higher the transition temperature of ortho-cementite.