Designing a Rig for Investigating Thermal Fatigue of Bimetallic Welds

Abstract

The bimetallic welds between ferritic plain carbon steels or low alloy steels and austenitic stainless steels being used in nuclear power plants impose a challenge towards the structural integrity assessment for researchers not only due to the different metallurgical zones, having a gradient in chemistry and mechanical properties but also due to the high temperature operating conditions and the temperature variations over the length of operation. Thermal fatigue is a phenomenon where a structural damage occurs when a material is subjected to cyclic thermal changes that may lead to initiation and growth of cracks and eventually lead to fracture after an exposure to sufficient amount of cycling. The thermal strains and associated thermal stresses are generated in the material when exposed to a temperature gradient, over a number of repeating cycles. This process is referred to as thermal fatigue. The magnitude of the thermal stresses generated in the case of thermal shock, is greater due to steeper temperature gradients in comparison to that observed for gradual change in temperature. To investigate the effect of the resulting stresses on mechanical behaviour of the reactor components, it is necessary to perform fatigue test under thermal test loading. The present work addressed the problem of evaluation of mechanical behaviour of bimetallic welds under thermal fatigue conditions by developing a test rig for simulating thermal fatigue conditions at laboratory scale and using the experimental test rig to study the mechanical behaviour of bimetallic welds exposed to thermal fatigue conditions. The effect of testing parameters such as the number of cycles, notch radius in specimen (defining degree of constraint) and heating time (corresponds to temperature gradient during heating) was observed in bimetallic weld zone, heat affected zone on the ferritic side of the plain carbon steel and the base materials namely stainless steel 304 L and plain carbon steel SA516 grade 70.