Laser surface treatment of Inconel 617 for next-generation nuclear reactors: A strengthening mechanisms study

Abstract

With the recent acceptance of Inconel Alloy 617 (IN617) for use under the ASME boiler and pressure vessel nuclear code, there is significant interest in developing manufacturing methods that can further magnify its effectiveness in intense thermo-mechanical loading scenarios. In this work, one such method, known as laser peening, was utilized in tandem with thermal aging to determine the relationship between the dislocation-rich microstructure following laser peening and the precipitation behavior of the strengthening phases. Results show that surface hardness after laser peening alone increased to 226 HV over the as-cast value of 195 HV. Following an aging heat treatment, the microhardness of the entire specimen and the laser peened zone increased by an additional 20%. Microstructural investigations revealed no change in grain size or orientation, though the precipitation of carbides throughout the microstructure resulted in a more homogenous dispersion after aging. Transmission electron microscopy revealed a rich dispersion of nanoscale γ’ phases with an average area fraction of 12.5%, in addition to a high density of dislocations (3 × 1014 lines/m2) near the laser peened surface. The Jackson-Reed model of precipitate strengthening was used in tandem with the Taylor hardening relationship to determine the effects of γ’ strengthening and work hardening respectively. Both models were able to closely approximate the experimentally observed hardness data, leading to the conclusion that major surface strengthening effects result from (i) the precipitation of γ’, and (ii) work hardening effects, with minor strengthening effects coming from (iii) residual stresses, and (iv) carbide strengthening. The results outlined herein present a promising, proof-of-concept for the use of laser peening to introduce surface mechanical property enhancement for IN617, a prime candidate material for next-generation nuclear reactor applications.