Effect of Electrode Coatings on Diffusible Hydrogen Content, Hardness and Microstructures of the Ferritic Heat Affected Zones in Bimetallic Welds

Abstract

Bimetallic welds (BMWs) have been a necessity within the steam generators of nuclear power plants, where the heavy section low alloy steel components are usually connected to stainless steel primary piping systems. These welds represent zones with metallurgical discontinuities, which tend to localize the strains and cause failures. The other critical issues which must be taken care of while welding of bimetallic joints include solidification cracking, thermal fatigue and residual stresses in welds. The occurrence and severity of all these problems associated with bimetallic welds depend upon the type of welding process used along with the other variables like welding consumables, heat input etc. In shielded metal arc welding (SMAW), welding electrode consumables/coatings play an important role in deciding the weld metal quality. The development of welding electrode coatings is quite difficult due to very rapid complex reactions taking place during welding and the involvement of many controlled process variables. The present paper is an effort to highlight the general issues and challenges for the systematic and scientific development of welding electrode coatings for bimetallic welds. The experimental procedure contains three sets of coated electrodes having varying ferro-alloys, silica and carbon content. In the first set of electrodes, non-ferrous elements are also introduced into the weld metal through flux addition. Further, the welded joints are subjected to various tests and then evaluated to ascertain their mechanical as well as metallurgical behaviour.