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Effect of PWHT and filler metal on stress relaxation cracking susceptibility in 347H stainless steel welds for elevated temperature service
Pickle, Timothy J.
Pickle, Timothy J.
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2021
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Pickle_mines_0052N_12319.pdf
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Pickle_mines_0052N_316/Figures 2.12 and 2.13 permission_Materials Science Forum.pdf
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Pickle_mines_0052N_316/Figure 2.7 permission_Springer_Metallurgical and Materials Transactions A.pdf
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2023-04-14
Abstract
347H stainless steel (SS) welds, used in commercial concentrating solar power (CSP) thermal energy storage at temperatures of 565 °C, are reported to be susceptible to stress-relaxation cracking (SRC) within months or years of service. Without prior stress relief via post weld heat treatment (PWHT), stress relaxation cracking may occur. The welded heat-affected zone (HAZ) and fusion zone (FZ) of 347H SS, particularly in thick sections greater than a half-inch, may be susceptible to reheat cracking during PWHT. The goal of this project is to determine the effect of PWHT and alternative alloys/filler consumables on reheat cracking in 347H SS welds to avoid SRC during service.
Welding experiments on 347H SS substrates, using GTAW and SMAW, were completed to duplicate the susceptible microstructure for crack susceptibility testing using E347 and E16.8.2 weld consumables. Finite element modeling (FEM) was performed to reveal residual stress profiles. Reheat crack tests, using a Gleeble© 3500, were performed as a function of temperature, stress/strain, and microstructure. Four different sets of microstructures and materials were investigated: 1) simulated HAZ on 347H SS substrate material, 2) simulated HAZ on a modified 316L substrate material, 3) transverse cross-welded samples of E347-347H SS, and 4) transverse cross-welded samples of E16.8.2-347H SS. Finally, elevated temperature tensile tests at 600°C using the Gleeble© 3500 were used to determine the effect of PWHT on mechanical properties for all four sets of materials.
The reheat crack tests indicate more rapid failure in higher pre-strains/stresses and higher test temperatures. The E347 FZ was demonstrated to be the most susceptible to cracking. The 347H HAZ samples exhibited a higher critical stress to failure threshold than E347 FZ and is deemed to be less susceptible to reheat cracking. Plastic strain (1%) in the E16.8.2 FZ was needed to cause cracking. Furthermore, failure occurred preferentially in the 347H HAZ instead of the E16.8.2 FZ. No cracking was detected in the modified 316L SS samples up to 16% strain. Careful PWHT design is needed to prevent cracking in 347H welds, and E16.8.2 consumable demonstrates to be the best SRC mitigation technique in FZ among the filler tested.
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