Analysis of Retained Austenite and Residual Stress Distribution in Ni-Cr Type High Strength Steel Weld by Neutron Diffraction

Abstract

Prevention of weld cracking is necessary for ensuring the reliability of high strength steel structures. Tensile residual stress in the weld metal is one of the major factors causing the weld cracking, therefore, it is important to clarify the residual stress distribution in the weld metal. Conventional stress measurement, the stress relief method using strain gauges and the X-ray diffraction technique, can only provide the stress information in the surface region of the steel weld. The neutron diffraction is the only non-destructive method that can measure the residual stress distribution inside the steel weld [1-3]. The neutron stress measurement was applied for the 980MPa class high strength steel weld and it was revealed that high level of tensile residual stress can affect the weld cracking to a significant degree [4-5]. Recently, it was reported that Ni-Cr type steel weld exhibit higher resistance to the weld cracking compared with conventional low alloy type weld. Increase of tensile residual stress is prevented by lower transformation temperature of the Ni-Cr type weld metal and retained austenite phase is dispersed in the martensite microstructure. It is considered that lower level of tensile residual stress and the existence of retained austenite may prevent hydrogen accumulation in the weld metal [6]. However, retained austenite and the residual stress conditions in the Ni-Cr type high strength steel weld is not well understood. In this study, neutron diffraction analysis was conducted on the Ni-Cr type steel weld joint with the tensile strength level of 980MPa in order to investigate the effect of the retained austenite and the residual stress distribution on the weld cracking.