Microstructure and Performance of Ultrahigh Carbon Steel Processed by Two-stage Austempering

Microstructure and impact toughness of the ultrahigh carbon steel processed by two-stage austempering were investigated.Compared only one stage austempering with the two-stage austempering in which a low temperature was adopted during the first-stage,the bainite grown slowly.As the holding temperature rised of the first-stage holding,bainite transformation greatly accelerated in the second-stage holding.When bainite transformation occured in very small extent or not during first-stage,the final morphology of bainite depended on mainly the holding temperature of the second-stage.Tearted by 250 ℃×10 min +300 ℃×10 min of two stage austempering,ultrahigh carbon steel exhibited complex microstructucture containing bainite and a small amount of residual austenite or martensite with high hardness(56 HRC) and high impact toughness(120 J).