On the Origin of the Local Hardening Zone on Welded Stainless Clad Steel Plates

The joining of stainless clad steel plates (SCSPs) by welding processes is relatively difficult due to differences in the chemical compositions and the physical and mechanical properties between both the carbon and the stainless steels comprising the clad material. These welded structures often suffer from several structural integrity problems such as bulging phenomena that can appear after bending tests, in the welded zone, due to the presence of a local hardening zone (LHZ). The main purpose of this paper is to investigate the origin of the LHZ typically produced in the welded joint of SCSPs after the bending operation. Optical micrographs revealed the presence of a typical pearlitic-ferritic structure in the welded zone filled with E7018 metal and a dendritic δ-ferrite structure solidified under a skeletal form in the welded zone filled with ER316L metal. The microstructure of the weld metal transition zone (WMTZ) filled with ER309L metal shows the presence of martensitic laths as well as cellular and columnar structures. In addition, the WMTZ revealed the presence of three types of grain boundaries, which are formed during the gas tungsten arc welding process: solidification sub-grain boundary, solidification grain boundary, and migrated grain boundary. Vickers microhardness measurements performed along the thickness of the welded joint showed that the highest microhardness value (406 HV) was observed at the WMTZ. The significant increase of the microhardness value in this transition zone was attributed to the presence of martensitic laths as well as cellular and columnar structures.