Strain-life curves of pressure vessel steel in high-temperature water

Abstract

Strain-life(S-N) curve tests of SA508Cl.3 pressure vessel steel were performed in room temperature, 288℃ air and 288℃ oxygenated water. S-N curve test system was constructed to be used in various environments. This system was composed of hollow cylindrical specimen, new grips and water loop system. The hollow cylindrical specimen itself performed functions of the autoclave and the new grips could apply a tensile and compressive load to the hollow cylindrical specimen. In room temperature air, the cyclic stress responses showed little initial hardening and moderate cyclic softening during testing. On the other hand, at 288℃, the cyclic stress responses showed an enhanced hardening, a secondary hardening due to dynamic strain aging(DSA). It could be observed that S-N curves were not changed with decreasing strain rate in room temperature air. S-N curve in 288℃ air had smaller slope than that in room temperature air. The longer fatigue life at 288℃ than that at room temperature was considered to be due to DSA. Fatigue life in 288℃ oxygenated water was shorter than that in air. This result may imply that fatigue resistance of test material was dependent on temperature and environments. Experimental results were similar to fatigue life estimated by the statistical model except for higher cycles. In comparison with the ASME design fatigue curves, SA508Cl.3 pressure vessel steel possesses safety margins in high-temperature water.