Stress relief cracking phenomena in high strength structural steel

Abstract

Stress relief cracking (SRC) is a potential risk during the post-weld heat treatment of fabrication welds. The present study investigates the susceptibility of Q1N (HY 80), a high strength weldable steel, to cracking in a recently developed stress relaxation test designed to simulate stress relief conditions. It has been found in previous investigations of the behaviour of 2.25Cr-1Mo and Mn-Mo-Ni steels in this test that the coarse grained heat affected zone (HAZ) is the most susceptible microstructure. In the present work, a comparison has been made of cracking behaviour in the HAZ of a metal inert gas butt weld and also in plate material in which the coarse grained HAZ microstructure has been simulated by application of an appropriate thermal cycle. The micromechanisms of cracking have been examined in both conditions and the prediction of SRC behaviour in service weldments from the observed behaviour of simulated HAZs is evaluated. It was found that cracking occurred under applied stresses of a magnitude which might be typical of the residual stress levels in service. Two fracture modes (observed by previous workers in other susceptible steels) were present in both the ‘welded’ and simulated conditions. The severity of cracking was also found to be similar in both conditions when plotted as a function of available susceptible material. It is concluded that observations of simulated HAZ behaviour can provide a realistic estimation of service behaviour.