EFFECT OF THERMO-MECHANICAL HISTORY ON REHEAT CRACKING IN 316H AUSTENITIC STAINLESS STEEL WELDMENTS

Reheat cracking has been observed in the heat affected zone of the 316H austenitic stainless steel thick section weldments during service at a temperature of similar to 500 degrees C. This has been attributed to the creep dominated relaxation of the highly triaxial residual stresses. Here the role of thermomechanical variables that contribute to the susceptibility of thick section 316H austenitic stainless steel weldments is briefly reviewed. The influence of the plastic strain, carbide precipitation and impurity element segregation on the subsequent creep deformation behaviour and the susceptibility to creep cavitation damage is discussed. A systematically designed experiment which includes these parameters has been undertaken for a 316H austenitic stainless steel. In addition, residual stress profiles have been introduced into cylindrical pre-treated specimens and the relaxation of these profiles with heat treatment has been measured by neutron diffraction. The experimental results are considered with respect to the effect of the microstructure on subsequent creep deformation and stress relaxation. The susceptibility to intergranular brittle fracture is discussed and an attempt is made to correlate the microstructure and stress state factors encountered in the HAZ with the accumulation of the creep cavitation associated with reheat cracking.