Weakening modes for the heat affected zone in Inconel 625 superalloy welded joints under different high temperature rupture conditions

Abstract

Inconel 625 superalloy (IN625) was generally welded as the base metal to fabricate the rotors in steam turbine with temperature over 700 °C. In this case the weakening modes for the heat affected zone (HAZ) in IN625 welded joints during the stress rupture test are systematically investigated in the present study. The stress rupture test results show a dependence of rupture time on stress lg (t) = −0.017 σ + 5.85, which can be achieved to predict the rupture life with different stress levels. Besides, all the samples are deemed as intergranular fracture with columnar grain morphology appeared on the fracture surface. The microhardness distribution reflects that all the specimens ruptured in the HAZ. It is deemed that the chain-like particles can weaken the binding force of the continuous grains and therefore cause the resultant fracture in the region with comparative low microhardness of HAZ. With the decrease of the applied stress, the formation of acicular δ phase (Ni3Nb) can reduce the deformation capacity of the HAZ by consuming the precipitate strengthened element Nb. Hence, the micro-voids tend to initiate along the grain boundaries. The coarse Laves phase particles with hard and brittle features become the dominated factor with the further increased rupture time. The micro-voids initiate and converge around the Laves phase under the stress to form micro-cracks, eventually cause the fracture of the ruptured specimens.