EFFECT OF MICROSTRUCTURE ON THE SUSCEPTIBILITY OF A 533-STEEL TO TEMPER EMBRITTLEMENT

In ferritic steels, brittle fracture usually occurs at low temperature by cleavage. However the segregation of impurities (P, As, Sn etc...) along prior gamma grain boundaries can change the brittle fracture mo de from transgranular to intergranular. In quenched and tempered steel s, this segregation is associated with what is called the temper-embri ttlement phenomenon. The main objective of the present study is to inv estigate the influence of the as-quenched microstructure (lower bainit e or martensite) on the susceptibility of a low alloy steel (A533 cl.1 ) to temper-embrittlement. Dilatometric tests were performed to determ ine the continuous-cooling-transformation (CCT) diagram of the materia l and to measure the critical cooling rate (V-c) for a martensitic que nch. Then subsized Charpy V-notched specimens were given various cooli ng rates from the austenitization temperature to obtain a wide range o f as-quenched microstructures, including martensite and bainite. These specimens were subsequently given a heat treatment to develop temper embrittlement and tested to measure the V-notch fracture toughness at -50 degrees C. The fracture surfaces were examined by SEM. It is shown that martensitic microstructures are more susceptible to intergranula r embrittlement than bainitic microstructures. These observed microstr uctural influences are briefly discussed. (C) 1998 Published by Elsevi er Science B.V. All rights reserved.