Post weld heat treatment of bisalloy 80 steel: Mechanics and industry safety code compatibility

Quenched and tempered (Q&T) steels are the preferred alloy in applications where both toughness and strength are two concerned properties and welding is the main method of fabrication. This is more prevalent in the defence industry and shipyards. Development of high tensile residual stresses during welding of Q&T steels requires Post-Weld Heat Treatment (PWHT) to alleviate the generated stresses. The present study utilized both in-situ and ex-situ neutron diffraction residual stress measurements to evaluate the evolution of strains/stresses prior-to, during and after PWHT for Q&T Bisalloy 80 steel welded by pulsed Gas Metal Arc Welding (GMAW-P). The ex-situ measurements looked at strains and stresses before and after PWHT. The in-situ measurements looked at these strains and stresses during PWHT at the most highly stressed area. It was found that strain/stress relaxation mainly occurred during the reheating step with a high relaxation rate and steep slope (~70% of strain relaxation) while linear strain relief was observed during holding (soaking) time. Most of the strain relief occurred within the temperature range of 450°C-600°C which is believed to be due to creep strain development occurs far earlier than the component reaching to the isothermal holding temperature. The ex-situ neutron diffraction measurements were similar to in-situ results confirming the applied PWHT is effective in mitigating the residual stresses.The measurements were compared with existing literature data and the current fitness of safety assessment codes (BS7910 and R6). It was found that both assessment codes were conservative for both the transverse and longitudinal residual stresses in the region close to the weld toe. Furthermore, both standards may underestimate through-thickness residual stresses in the transverse direction.