The small punch toughness test: some detailed fractographic information

Abstract

The present paper describes a detailed fractographic survey on a series of small punch tests conducted on a CrMoV bolting steel over the temperature range −200°C-ambient. Two specific material conditions were investigated, viz. fully and partially embrittled bolts which experience toughness degradation by reverse temper embrittlement (RTE) during service at elevated temperatures approaching 490°C. The following pertinent fractographic observations were established: (1) in the upper shelf regime, ductile failure by microvoid coalescence (MVC) occurred over the full through-thickness test section; (2) in the transition region an initial, limited amount of slow ductile MVC growth occurred before final failure ensued; and (3) at the lower shelf brittle region, wholly brittle fracture, which was predominantly transgranular cleavage containing some isolated facets of intergranular failure, occurred. The initial slow ductile MVC growth was semi-elliptical in nature and the extent decreased with decreasing test temperature. Finally it was shown that the level of biaxial fracture strain, q f , exhibited a smooth continuous transition curve over the temperature range ambient-−200°C; also the level of fracture strain explained the change in the amount of initial ductile MVC growth through the transition region.