Characterization of tempered martensite microstructure and embrittlement by acoustic and magnetic Barkhausen signal measurement

Abstract

It has been well recognized that ultrahigh strength alloy steels exhibit embrittlement when they are tempered in the temperature range from 300 to 400/sup 0/C. This phenomenon is referred to as tempered martensite embrittlement. The mechanism for tempered martensite embrittlement has been extensively studied by many investigators. Tempered martensite embrittlement is attributed to the occurrence of intergranular and/or cleavage fracture. The fracture mode depends on the alloy composition and the impurity content of steels. The intergranular embrittlement is caused by two events: segregation of impurities such as sulfur and phosphorus, and carbide precipitation at grain boundaries. The intergranular segregation of impurities during austenitization weakens grain boundaries and the precipitation of carbides during tempering promotes the nucleation and propagation of cracks. A major cause for tempered martensite embrittlement associated with cleavage fracture is the tramsformation of retained austenite at martensite lath boundaries to form coarse carbides. For both cases, the concurrent occurrence of softening and carbide precipitation, which mitigates and enhances embrittlement effects, respectively, gives rise to a toughness trough during tempering treatments. Therefore, it is important to investigate nondestructively the evolution of tempered martensite microstructure which causes embrittlement. The objective of this study is to evaluate nondestructively the characteristics ofmore » tempered martensite microstructure and embrittlement in an ultrahigh strength steel by measuring the two types of Barkhausen signals.« less