VACUUM THERMAL-MECHANICAL FATIGUE TESTING OF TWO IRON BASE HIGH TEMPERATURE ALLOYS

摘要

Elevated temperature ultrahigh vacuum low cycle fatigue and thermal fatigue tests on 304 stainless steel and A-236 alloy have shown significant effects of frequency and of combined temperature-strain cycling on cyclic life. At temperatures in the creep range, fatigue lives of both materials were lower at .0065 Hz than at .65 Hz. Metallographic examination of fractured specimens indicated mixed mode (intergranular and transgranular) fracture at the higher frequency and exclusively intergranular fracture at the lower frequency. In-phase thermal cycling (tension at high temperature and compression at low temperature) caused large life reductions in both materials. These life reductions were attributed to grain boundary cavita¬tion caused by unreversed grain boundary sliding. Out-of-phase thermal cycling (tension at low temperature and compression at high temperature) also caused large cyclic life reductions in both materials.