THE SMALL PUNCH TOUGHNESS TEST - SOME DETAILED FRACTOGRAPHIC INFORMATION

The present paper describes a detailed fractographic survey on a serie s of small punch tests conducted on a Cr-Mo-V bolting steel over the t emperature range -200 degrees C-ambient. Two specific material conditi ons were investigated, viz, fully and partially embrittled bolts which experience toughness degradation by reverse temper embrittlement (RTE ) during service at elevated temperatures approaching 490 degrees C. T he 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 th e transition region an initial, limited amount of slow ductile MVC gro wth occurred before final failure ensued; and (3) at the lower shelf b rittle region, wholly brittle fracture, which was predominantly transg ranular cleavage containing some isolated facets of intergranular fail ure, 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(p) e xhibited a smooth continuous transition curve over the temperature ran ge ambient--200 degrees C; also the level of fracture strain explained the change in the amount of initial ductile MVC growth through the tr ansition region.