Study On High Temperature Behaviour Of Nickel-based And Austenitic Cr5Mo Dissimilar Welded Joints

Dissimilar weld joints are widely used at elevated temperature in the process industry such as petro-chemical, chemical and power plants. Service experiences show that carbon migration from welding heat affect zone(HAZ) to weld metal(WM) may take place during high temperature service due to significant content difference in carbide forming e!ements(Cr, Mo etc.) between weld metal and base metal(BM) in A302/Cr5Mo joint. A decarburised zone in HAZ and a carburised zone in WM are thus formed. The high temperature properties of A302/Cr5Mo dissimilar weld joint decrease due to decarburization, which leads to premature failures of the weldments and endangers the long term safety of high temperature plants.In order to improve service performance of Cr5Mo dissimilar weld joints at elevated temperature, high temperature behaviour of Ni-based Cr5Mo dissimilar joints welded by Inconel82 has been studied based on microstructure analysis and mechanical testing. A comparison with A302/Cr5Mo dissimilar weld joints is made, which provides basis to ensure a long term and safe operation of Ni-based Cr5Mo dissimilar weld joints at elevated temperature. The research is supported by the Natural Science Foundation of P. R. China under contract No. 10172046.Carbon migrations of Inconel82/Cr5Mo and A302/Cr5Mo dissimilar weld joints are studied in terms of aging of the weld specimen, microstructure analysis and hardness measurement. The results show that carbon migration happens in A302/Cr5Mo dissimilar weld joint and carburised zone and decarburised zone are formed near the fusion line(FL). However, almost no carbon migration is observed in Inconel82/Cr5Mo dissimilar weld joint, which indicates that Ni element improves the activity of carbon, reduces the stability of carbides and restrain carbon migration effectively.The long term local deformation measuring technique is first used to measure local creep deformation near FL in dissimilar weld joints. Because of carbon migration in A302/Cr5Mo dissimilar weld joints, higher creep strain rate is found in the decarburized zone, which makes the decarburised zone a weak location in weld joints. However, higher creep strain rate is not observed near fusion line of Inconel82/Cr5Mo dissimilar jointswhich further verifies that Ni element restrains carbon migration and final fracture would not happen near the fusion line.It is found from instantaneous tensile test at high temperature that Inconel82/Cr5Mo joints and A302/Cr5Mo joints have the same tensile strength. Inconel82/Cr5Mo weld joints meet the strength requirement. Under lower stress, the creep time to rupture and failure mode of A302/Cr5Mo dissimilar weld joint is very different from that of Inconel82/Cr5Mo dissimilar weld joints, with rupture occurred at decarburised zone in the former case and at base metal in the later case. The life of A302/Cr5Mo joints is only about 40-50% of that of Inconel82/Cr5Mo joints. In summary, carbon migration may result in the reduction of creep strength of dissimilar weld joints at elevated temperature, while Ni-based Chrome steel weld joints have better creep resistance at elevated temperature in comparison with the dissimilar weld joints welded by austenitic electrode.