Factors affecting reheat cracking in the HAZ of austenitic steel weldments

Reheat cracking in the heat affected zones of austenitic stainless steels can occur during high temperature service and is thought to arise from the relaxation of welding residual stresses, resulting in creep damage. Key features of reheat cracking are the magnitude of the residual stresses, degree of triaxiality, extent of stress relaxation behaviour and the influence of triaxial stress on creep ductility. A thermo-mechanical pre-conditioning technique has been developed for testing specimens to represent the welding cycles experienced between 200 and 1100 °C by the ‘strain affected zone’ of a weldment in a steam header during manufacture. Subsequent creep tests have been carried out at 550 °C on pre-conditioned plain and notched specimens to identify the behaviour as regards creep rate and ductility in both the uniaxial and triaxial stress states. The results have been compared with data in the literature and with models of the influence of multiaxial stress on creep ductility. It has been found that certain types of pre-conditioning give a marked reduction in creep ductility as strain rate is reduced, which helps to explain the reheat cracking process.