Microstructure details and impact toughness for dissimilar 9Cr/CrMoV welded joints fabricated by NG-SAW and NG-TIG techniques

Narrow gap submerged arc welding (NG-SAW) and narrow gap tungsten inert gas welding (NG-TIG) with multilayer and multi-pass techniques were used to join the advanced 9Cr and CrMoV steels. The relationship between the impact toughness and microstructure of the dissimilar welded joint fabricated by these two different welding techniques was systematically investigated. By the impact tests, the results indicated that the average impact energy of the specimen welded by NG-TIG (154.0 J) was five times as high as the one (29.2 J) welded by NGSAW. Interestingly, an amount of fine second phase particles were detected on the basis of the results. In particular, the precipitates in the welded joint by NG-TIG were much smaller than those by NG-SAW attributing to the faster heat input. Generally, NG-TIG was operated at a faster manufacturing speed, which could lead to a faster cooling of the weld pool and therefore refine the microstructure of the weld. However, no significant differences in the metallographic structure of the whole welded joint could be traced even they suffered different welding techniques. The main microstructure of the WM was tempered martensite in both kinds of specimens. Therefore, the fine dispersed phases could account for the much higher impact toughness of the dissimilar joint prepared by NG-TIG than that via NG-SAW.