Grain Coarsening and Boundary Migration during Welding of Invar Fe-36Ni Alloy

To assess the metallurgical reasons for the susceptibility of Invar Fe-36Ni alloy to reheat cracking, a number of specimens have been prepared and welded in a weld simulator, with a gas tungsten arc welding procedure and heat inputs ranging from 1.3 to 7.5MJ/m. Grain coarsening in the heat affected zone was predicted with fair accuracy by using a diffusive model. Samples with two beads-on-plate were used to study the effect of reheating of the first bead by the weld thermal cycle of the second pass. Grain-boundary migration was found to be sensitive to boundary orientation and morphology. Filler metal containing Ti-Mn was responsible for the further impeding of boundary migration owing to the presence of precipitates. A clean surface and the use of oxygen-free shielding gas were compulsory requirements to obtain sound weldments; otherwise preferential diffusion of oxygen along grain boundaries and subsurface inclusions were observed in the joint.