Loading...
Reheat cracking susceptibility in welds of creep resistant alloys for power generation applications
Pickle, Timothy J.
Pickle, Timothy J.
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2025
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2026-11-11
Abstract
Power generation industries utilize creep-resistant materials for elevated temperature components (>500°C) and various corrosive environments, e.g. molten salts. Transfer pipelines, pressure vessels, and thermal energy storage (TES) tanks may utilize austenitic stainless steel (SS) alloys, e.g. type 347H, 347AP (347LN), and Therma 4910 (316LNB). Higher temperature components(>720°C), such as supercritical CO2 primary heat exchangers (PHX), may require nickel base superalloys with superior creep strength, such as Haynes®282 (H282) and Inconel®740H (IN740H). Arc welds of these components may be susceptible to stress relaxation cracking (SRC) or strain age cracking (SAC) during post weld heat treatment (PWHT) or service. These reheat cracking (RC) phenomena are attributed to a combination of susceptible microstructures, sufficiently high tensile residual stress and reheating.
The first objective of this work is to analyze the influence of weld-induced residual stress/strain and temperature on RC susceptibility of the three abovementioned creep-resistant stainless steels, including their heat-affected zones (HAZs), and fusion zone (FZ) made with matching fillers and alternative filler E16.8.2 FZ. The influence of PWHT, service environment and repair welds on RC susceptibility is evaluated as well. Stress relaxation tests (SRT) are primarily conducted with a Gleeble® 3500 to simulate RC in these various microstructures and provide a susceptibility ranking based on stress thresholds and time to failure as a function of temperature. Residual stress measurements, using the high intensity diffractometer for residual stress analysis (HIDRA) beamline at Oak Ridge National Laboratory (ORNL), were performed to understand the influence of PWHT, repair welding, and weld filler on residual stress in 347H SS weldments. A combination of metallurgical characterization techniques is used to assist in failure mechanism analysis. In 347H SS weldments, it is observed that the matching filler E347 FZ in as-welded (AW) condition is more susceptible to RC than single pass 347H HAZ and AW E16.8.2 FZ based on stress- and time-to-fracture at 750-1050°C test temperatures. Lower tortuous grain boundaries in E347 FZ could explain lower critical stress threshold compared to 347H HAZ, but E16.8.2 filler with leaner composition (no Nb (C, N)) is observed to be more RC resistant. PWHT with properly designed parameters not only reduces residual stress and strain reductions, but it also reduces RC susceptibility in E347 FZ microstructure compared to AW E347 FZ. SRT of ex-serviced 347H SS welds generally indicate similar failure times and cracking susceptibility to fresh welds. For repair welds, the FZ longitudinal residual stress using E16.8.2 filler is observed to be lower than that with E347 filler, although the transverse residual stress increased in the previous HAZ that is further away from the repair. Repair welds with alternative E16.8.2 filler exhibit better RC resistance in the FZ compared to repair welds with matching E347 filler at 800-850°C test temperatures, due to a more creep ductile E16.8.2 FZ in contrast to adjacent 347H HAZ. Microcracks in adjacent E347 FZ (HAZ of repair) show δ-ferrite dissolution laced with interdendritic Nb (C, N) precipitates. The impurity segregation of sulfur to γ/γ boundaries because of δ-ferrite dissolution can lower the grain boundary surface energy and facilitate easier cracking. These SRT results of failure in adjacent HAZs correlate with increased transverse tensile residual stresses after repair welding, regardless of the weld filler used. 347AP and Therma 4910 SS HAZs using similar SRT techniques demonstrate higher RC resistance (longer times to failure) compared to 347H HAZ. Using matching fillers, FZ of 347AP outperforms E347. All samples that failed by RC at temperature reveal intergranular/interdendritic fracture characteristics. Despite residual stress, the lack or reduction of Nb carbonitrides (contributing to grain deformation resistance during creep or stress relaxation in 347H) in 347 AP HAZ and FZ, Therma 4910 HAZ, and E16.8.2 FZ could explain the enhanced RC resistance in alternative materials. More creep resistance in AW 347H HAZ and E347 FZ attributed to the Nb carbonitrides strengthening may explain the high RC compared to alternative HAZ and FZ microstructures with higher creep strains but no RC failure.
The second objective of this work is to use the SRT methodology developed above to investigate SAC susceptibility of the welded H282 and IN740H laser-powder bed fusion (L-PBF) components. The influence of weld HAZ of L-PBF (and additive manufacturing) components has not been studied in literature. The impacts of heating rate during post weld aging treatment and build orientation on HAZ cracking susceptibility are evaluated. Overall, the vertically built components are more resistant to cracking than horizontal build components for both alloys due to grain morphology difference. Intergranular SAC failures are observed along migrating grain boundaries (MGBs) distinguished by stress-assisted elongation of grain boundary secondary phases (γ’ or carbides). IN740H HAZ shows extensively wider precipitate denuded zones within MGB region compared to H282 HAZ, likely explaining higher SAC susceptibility in IN740H HAZs. Fast heating rates (3480°C/h and above) for both H282 and IN740H L-PBF HAZ microstructures exhibit low SAC susceptibility, manifested by no failure in H282 microstructure and extended failure times in IN740H compared to slower heating rates. The shortest time-to-failure, or worst SAC resistance, is observed during the intermediate heating rates (100-333°C/h) for H282 and slow heating rates (40-100°C/h) in IN740H. The competing mechanism between strengthening by phase transformation and stress relaxation as a function of time at temperature determines the SAC susceptibility. Non-isothermal CALPHAD simulations show higher γ’ volume fraction development during heating with slower heating rates, which could inhibit further bulk stress relaxation prior to reaching aging temperature at 800°C and cause faster strain accumulation along MGBs and γ’ denuded zones.
Associated Publications
Rights
Copyright of the original work is retained by the author.