Effect of Retained Austenite on Hydrogen Induced Cracking of 60Si2Mn Steel

Abstract

The effects of volume and mechanical stability of retained austenite in 60Si2Mn steel austempered on the resistance to hydrogen-induced cracking (HIC) is studied in the paper. The hydrogen embrittlement properties are shown by the threshold stress intensity factor Kth of sustained load hydrogen embrittlement and brittleness ratio IH of dynamic load hydrogen embrittlement. The results show that Kth is not affected by volume of retained austenite (8.4 to 17.9%) in the steel austempered remarkably, the mechanical stability of retained austenite is decreased and the resistance of the steel to hydrogen embrittlement is affected by tempering and charging after austempering. The hydrogen–induced cracking happens easily and the voids form in the martensite transformed from the blocky retained austenite under the action of stress. The stress at the tip of carck is relaxed and the resistance to HIC is improved since the voids form in the martensite. On the other hand, the fracture happens easily since the voids in martensite serve as the path of crack propergation. The difference between Kth and IH is also discussed in the paper. There have been some reports on the effects of austempering and retained austenite on the resistance hydrogen embrittlement. There exists plenty of retained austemite in the microstructure of 60Si2Mn steel austempered since the traneformation from austenite to bainite can not be finished completely during austempering. Ritchie (1978) considered that retained austenite had no remarkable effect on thresold stress intensity factor K1scc but decreased crack growth rate in region II ▪ II by one to two orders of magnitude. During deformation, retained austenite transformed to martensite which affected greatly the hydrogen embrittlement cracking. The paper aims to study the factors which affect the mechanical stability of retained austenite and the effect of the mechanical stability of retained austenite on the resistance to hydrogen embrittlement.