For thermal energy storage concentrating solar power (CSP) applications, 347H SS maintains the necessary creep strength and sensitization resistance compared to other 3XX SS grades. However, reheat cracking in 347H SS weldments, sometimes called stress relaxation cracking (SRC), creates a problem for high temperature applications involving molten nitrate salts. Weldments that experience high temperature service without any post weld heat treatment (PWHT) have been reported to undergo reheat cracking, due to welding induced tensile residual stresses and susceptible microstructures. Different cracking mechanisms were found to occur in coarse grained heat-affected zone (CGHAZ), partially melted zone (PMZ), and fusion zone (FZ), respectively. For instance, failure in CGHAZ can be attributed to cracking along precipitate free zones as coarse Nb (C,N) reprecipitates along grain boundaries and fine-precipitate strengthens the grain interior. In the PMZ, liquated grain boundaries (Nb- eutectic) may be the root cause of failure. Therefore, multi-step post weld heat treatment (PWHT) procedure is needed for thick 347H SS weldments to mitigate cracking by reducing residual stress at a lower temperature range and eliminating susceptible microstructures at a higher temperature range. The proper temperature ranges were determined by Gleeble simulation of welding induced thermomechanical history and service conditions.
For this study, two 2” thick 347H SS plates were welded with E347 weld consumables using GTAW and SMAW with 40 passes using a 50o single v groove with approximately 12” weld length for development of a thermomechanical finite element model (FEM). A temperature dependent mechanical property database was implemented on the FEM to determine the as-welded residual stress and plastic strain development. Neutron diffraction experiments on the two welds provided validation for residual stress calculations before and after PWHT.
The plastic strain development in the weld provided by the FEM was used for Gleeble 3500 thermomechanical stress relaxation tests in the HAZ/PMZ of 347H SS. Two different weld thermal cycles were initially applied to a sub-size round tensile specimen: 1) 1325°C peak temperature to simulate grain boundary liquation in PMZ, or 2) 1275°C peak temperature with no liquation to duplicate microstructures in CGHAZ. After the PMZ thermal cycle, the specimen was pulled to two different true strain conditions, i.e., 0.174 and 0.1, at room temperature based on finite element (FE) analysis. The specimen is then heated up to a peak temperature ranging from 600-1050°C for failure time measurement. Weld metal tensile specimens were extracted from three 40-pass welds using E347, E309L, and E16.8.2 consumables for stress relaxation Gleeble testing with a similar method. 100% yield strength was used as a substitute to the 0.174 and 0.1 flow strain step. The Gleeble testing results showed that a single step PWHT above 800°C may lead to fast failure within the PMZ. Therefore, a two-step PWHT simulation was designed based on the Gleeble tests including a low temperature initial stress relief (500-700°C) followed by a higher temperature of 1000-1150°C to provide complete stress relief while solutionizing sigma Cr23C6, and δ-ferrite and stabilize γ-austenite and Nb (C,N).