Rishi Kant - Research
This project addresses the issue of stress relief cracking in high temperature alloys used for energy applications. The project is sponsored by the National Science Foundation Manufacturing and Materials Joining Innovation Center and involves collaboration with Lincoln Electric, Special Metals, the Babcock & Wilcox Company, Electric Power Research Institute, Shell, Petrobras, and Ohio State University.
The project draws motivation from the problem that many alloys which are used at high temperature exhibit stress relief cracking (SRC) during welding and/or service. This form of cracking typically occurs along the grain boundaries and results in a low ductility fracture. The problem is often associated with precipitation strengthened alloys in which a locally soft precipitate free zone forms along the grain boundaries. In this case, the residual stress from welding is relieved by localized plastic deformation along the grain boundaries. In few other alloys, SRC is related to grain boundary precipitation of carbides and segregation of impurities. When failures occur in service, the applied stresses combine with the welding residual stress to promote failure.
The major objectives of the project are to develop a standardized test(s) that provides reproducible results between different laboratories. Also, to correlate laboratory test results to actual cracking behavior under field conditions. Additionally, identifying the cracking mechanism and susceptibility of SRC for each alloy through detailed microstructural characterization.
The research so far has been to devise a test procedure that can simulate SRC on Gleeble 3500 (thermo-mechanical simulator) with good control over the test parameters. Initial tests on ferritic alloys successfully produced intergranular failure mode that matched failures observed in the field, and work is in progress to evaluate the test procedure for austenitic alloys. This has been established by microstructural characterization of samples using Hitachi-4300 SE/N, light optical microscopy, and microhardness tester. The applicability of this procedure to all other alloys and testing its reproducibility at other labs is also underway.
Figure a: Stress-Temperature vs. Time showing the thermal and stress cycle for SRC test.
Figure b: Photomicrograph showing intergranular cracks.
Figure c: SEM fractograph showing intergranular failure.