CCS and Power Systems
Crosscutting Research - Plant Optimization Technologies
Joining of Advanced High-Temperature Materials
Performer: PNNL - Pacific Northwest National Laboratory
Project No: FWP-12461
Researchers continued creep testing of FSW creep enhanced ferritic steel A387-Gr91-M (V modified) plate. A sample taken along the weld line (longitudinal, weld-only material) ruptured after 9247 hours of testing at constant-load creep conditions of 130 MPa and 625 °C. Based on creep strength vs. time to rupture for T91(mod) from Kimura et al. ("Long-Term Creep Strength Property of Advanced Ferritic Creep Resistant Steels," Advances in Materials Tech. for Fossil Power Plants: Proc. Sixth Intl. Conf., 732-751, 2010) and assuming that A387-Gr91 behaves similarly to T91(mod), the parent plate material should have a rupture life of approximately 800 hours at 625 °C. This weld metal specimen has surpassed the expected parent material life. The microstructure and hardness were evaluated to determine why the weld-only material has superior creep performance compared to the base material. Initial results indicate that the martensite-lath length of the FSW nugget material is smaller compared to the base material.
A cross-weld FSW sample ruptured after 198 hours during the first cross-weld FSW Gr91-M creep test at 130 MPa and 625 °C. This is a shorter duration than the team’s base metal results (670 hours) but almost twice that of published data of fusion-welded 9Cr1MoNbV pipe with a post-weld heat treatment (PWHT) of 760 °C for 2 hours that ruptured between approximately 55 and 100 hours (V. Gaffard et al., Nuc. Eng. Design, v235, 92547-2562, 2005). (The project’s FSW cross-weld samples did not receive a PWHT.) Because the cross-weld specimens fail sooner than the base metal, the team did not test cross-weld specimens at 175 MPa and 625 °C, but instead conducted a cross-weld FSW Gr91-M creep test at 100 MPa and 625 °C. This sample ruptured after 789 hours, approximately three times longer than the approximately 250 hours for published values for fusion welds with PWHT (V. Gaffard et al.).
Creep tests at the more accelerated condition of 175 MPa and 625 °C are also underway for Gr91 base and FSW weld-only specimens to determine if the trends are the same as those observed as in the 130 MPa tests. The base material ruptured after 34 hours, which fits the extrapolated trend from the work of Kimura et al. The weld-only specimen is currently at over 2600 hours without rupture, indicating that the trend of higher creep resistance for FSW Gr91 weld-only material holds at both 130 MPa and 175 MPa at 625 °C.
Kanthal APMT, an ODS alloy, was successfully friction-stir welded up to ¼ inch in thickness with current commercial FSW tools. FSW welds of Kanthal APMT had creep performance at 750 °C, which was virtually identical in the weld nugget as in the parent rolled plate (in the longitudinal or rolling direction).