Effect of post-weld heat treatment on the tensile strength of laser beam welded Hastelloy C-276 sheets at different heat inputs

Abstract

Laser beam welding of 2.6-mm-thick Hastelloy C-276 sheets was carried out at two different values of linear heat input, i.e., 60 J/mm and 120 J/mm. Uni-axial tensile test of as-welded (AW) Hastelloy C-276 sheets showed failure (fracture) occurring mainly at the weld zone, irrespective of the linear heat input. Following that, AW Hastelloy C-276 sheets were subjected to post-weld solution heat treatment (PWSHT). After PWSHT, it was observed that the samples welded at 60 J/mm failed mainly at the weld zone, while all the samples welded at 120 J/mm failed at the base metal. X-ray diffraction (XRD) analysis carried out in the weld zone of PWSHT sample of 120 J/mm, fractured at the base metal, showed the presence of relatively higher density of chromium-rich carbide precipitate phases, especially Cr3C2 and Cr7C3. Scanning electron microscopic (SEM) analysis of the weld zone for samples welded at 120 J/mm, both AW condition and PWSHT condition, shows the presence of chromium-rich carbide precipitates. However, size and distribution density of the precipitates were found to be relatively more in PWSHT condition. Also, the amount of chromium-rich carbide precipitates in the base metal of PWSHT sample was found to be less than that of the weld zone. The hardness of the weld zone after PWSHT was increased significantly as compared that of the AW counterparts. Hence, it might be possible that cumulative effect of (a) higher number of grain boundaries, (b) increase in hardness, and (c) high density of chromium-rich carbide precipitates, the PWSHT sample of 120 kJ/mm could have resisted the failure at weld zone. Analysis of XRD patterns of the weld zone for different welded samples showed the formation of various stoichiometries of chromium-rich carbide precipitate (i.e., Cr23C6, Cr3C2 and Cr7C3) due to the variation in the heat input and treatment condition. Transmission electron microscopic (TEM) analysis showed the formation of Mo-rich p-type (phase) precipitates in the weld zone of PWSHT sample.