Research on Mechanism of Internal Cracking of Welds Caused by Carbon Migration of Welding Joints

Abstract

The dissimilar steel joints of austenitic stainless steel and low alloy steel are commonly used in superheaters and super heaters of the boiler. Carbon migration is an important factor affecting the life of welding joints during service. In this paper, the carbon migration of T23 and TP304h dissimilar steel of welding joints was studied. The results show that carbon-rich zone and carbon-depleted zone are formed near the interface between the T23 bases and weld metal. The carbon-rich zone is located near the weld line in weld metal. On the other hand, the carbon-depleted zone is located at the T23 steel base metal. Significant carbon migration phenomenon can be observed by the optical micrographs of the welding joins. The morphology of the carbon-rich zone along the fusion line can be used as a “feather” for identification and extends toward the center of the welding joints. In the carbon-depleted zone, it is found that the microstructure of the carbon-depleted zone near the fusion line is inhomogeneous and relatively coarser, and the micro hardness of the carbon-rich zone is significantly different from that of the surrounding zone. The carbon-poor zone is becoming a weak link to promote the process of crack propagation.