Effect of residual stress on thermal fatigue in a type 420 martensitic stainless steel weldment

This paper aimed at the investigation of the effect of residual stress on thermal fatigue in martensitic stainless steel. In this study, a submerged arc welding was utilized. In order to obtain the different magnitude of residual stresses, the parallel heat welding, vibration stress relief, shot peening, and controlled low temperature stress relief processes were used for reduction of residual stresses due to welding. During welding, the thermal cycle of different locations in weldment were recorded. The residual stresses were determined by using the hole-drilling strain-gage method of ASTM standard E837. The retained ferrite content was determined with a digital optical microscope and image process software. The experimental results showed that the residual stresses decrease with increase of thermal fatigue cycle number. This is because that the formation of crack relieves residual stresses of specimen. Because of solidification structure difference, at low thermal fatigue cycle, the density of cracks on near fusion line is the higher than centerline. At high thermal fatigue cycle, the density of crack from near fusion line is lower than centerline, but very longer crack is found easily. The retained ferrite content and thermal fatigue crack tendency exist a critical value. The retained ferrite less than about 31.5 vol.% will increase crack propagation tendency. The retained ferrite become a harmful factor for crack propagation tendency when the retained ferrite more than this critical content. It is because that too much ferrite content in martensitic matrix may result in a lower strength structure and crack can propagate easily. In addition, the dual phase structure may introduce more harmful interior stress due to different volume shrinkage between retained phase and matrix.