Creep behaviour of IN 740 alloy after HAZ thermal cycle simulations

Abstract

In this study, the creep specimens of an age-hardened IN 740 alloy were subjected to heat affected zone (HAZ) thermal cycle simulations using a Gleeble 380 physical simulator. The short-term uniaxial creep tests in tension were carried out at temperatures of 973 and 1023 K and at applied stresses ranging from 220 to 600 MPa to evaluate the effect of welding simulations on the microstructural evolution, creep properties and behaviour of the IN 740 alloy. It was found that the stress exponent, n, of the minimum creep rate is not constant and the evaluated values of n are of the order of ~3 for lower stresses, with the gradual variation of n with an increase of the applied stress up to ~15 at the highest stresses, indicating that the creep testing was carried out in the power-law (dislocation) creep region. Analysis of the creep data leads to the conclusion that the creep behaviour of the simulated HAZ of the IN 740 alloy arises from a synergism between the deformation process that is based on intragranular dislocation processes, involving the glide and climb of mobile dislocations, and grain boundary mediated processes. Finally, an assumption was proposed that both creep deformation and fracture are controlled by the same mechanism(s). Examination of the creep fracture surfaces revealed that the main fracture mode is mixed-mode fracture with dominating intergranular character.