Abstract Scope |
347H SS has a combination of good creep strength and sensitization resistance compared to other 3XX SS grades for mildly corrosive, elevated temperature applications. Stress relaxation cracking (SRC) may take place in 347H SS welds using E/ER347 filler during elevated temperature service with the presence of high tensile residual stresses and susceptible microstructures in the as-welded condition. Post weld heat treatment (PWHT) has been prescribed to mitigate SRC. However, reheat cracking could occur during PWHT in highly restrained welds with residual stresses exceeding a critical value. Thermally stable weld consumables (e.g., E/ER 16.8.2) could be another viable solution for mitigating crack susceptibility in 347H SS welds. In this study, cracking susceptibility tests were conducted for four sets of materials: 1) 347H HAZ, 2) 347H SS weld with E347, 3) 347H SS weld with E16.8.2, and 4) NUCL 167 SPH (i.e. 316L w/B) HAZ. The two main variables are pre-stress (ranging from 200-600 MPa) and temperature (750-1050°C) for all samples, while for the 347H HAZ samples, an additional variable of a peak HAZ temperature (1150, 1275, and 1335°C) was considered. The pre-stress level represents the weld-induced residual stress. Critical stresses as a function of temperature were identified for the first two sets of materials, while a few comparative tests using similar applied stresses were carried out for the last two sets of materials. Such information is critical for determination of allowable PWHT temperature range as a function of maximum residual stress. Furthermore, it was found that E347 WM contains a lower critical stress to failure than the 347H HAZ. In other words, the E347 WM is more susceptible to SRC than the 347H HAZ. 347H SS welds using E16.8. filler were demonstrated to be less susceptible to cracking than welds made with E347. Failure occurred preferentially in the 347H HAZ instead of the E16.8.2 WM. The NUCL 167 SPH HAZ samples did not show any indications of cracks up to 16% strain. |