Prevention of carbon migration in 9% Cr/CrMoV dissimilar welded joint by adding tungsten inert gas overlaying layer

High chromium (9–12% Cr) steels with excellent heat resistance and CrMoV steels with good toughness were potential candidates for combined rotor for steam turbine operated over 620 °C. Two welding techniques were used to fabricate 9% Cr and CrMoV dissimilar welded joint. The results show that the carbon migration only appears in the specimen using narrow gap submerged arc welding (NG-SAW) technique, yet it can be effectively prevented by adding tungsten inert gas (TIG) overlaying process before the NG-SAW. The carbon migration occurred in NG-SAW resulting from the sharp transition of the strong carbide-forming element Cr between the weld (~ 2.7 wt%) and the base metal (~ 9 wt%). On the contrary, the application of TIG overlaying layers can promote the diffusion of Cr element, and therefore result in its much smaller concentration gradient. That is to say, a gentle transition zone of Cr element can be created among the SAW weld, TIG overlaying layers and the base metal, which effectively prevents the carbon migration and therefore produces a decreased carbon concentration adjacent to the fusion line.