25Cr-13Ni 오스테나이트 스테인리스강의 고온 크리프-피로거동에 미치는 유지시간과 변형속도의 영향

Abstract

Exhaust manifold is subjected to an environment in which heating and cooling cycle occur due to the running pattern of automotive engine. Therefore, main failure mechanisms are thermal fatigue, thermal mechanical fatigue, creep-fatigue which cause to fatigue fracture. Effects of hold time ( hold time tension fatigue test : HTTF ) and strain rate (Frequency control fatigue test : FCF ) have been investigated on the creep-fatigue behavior at 800℃ with a 25Cr-13Ni cast austenitic stainless steel for advanced exhaust manifold material. Before creep-fatigue test, microstructure was observed, and thermal expansion coefficient was analysed. Room and high temperature tensile test, high temperature low cycle fatigue test have been performed on the test material. High temperature creep-fatigue test and high temperature low cycle fatigue test were performed under the condition of a strain controlled and triangular waveform. The creep-fatigue test was performed under the total strain ranges of 0.6% and 0.7%, hold times of 0min, 1min, 10min, 30min, and strain rate of 0.1Hz, 0.017Hz, 0.0017Hz. Generally as known by traditional theory, longer hold time and slower strain rate result in a shorter fatigue life. HTTF test with hold time cause to a shorter fatigue life. The creep-fatigue test under the hold time of 30 min at total strain range of 0.6% were remarkably shorter fatigue life compared with hold times of 1 min or 10 min. Deficient creep effect was shown under the hold time of 1 min and 10 min. FCF test with faster strain rate showed a shorter fatigue life. Consequently longer hold time and faster strain rate caused to a shorter fatigue life. Lower peak stress was observed under the slow strain rate, and cyclic softening was observed rather than cyclic hardening in the creep-fatigue test, which was not expected as a creep effect. 25Cr-13Ni cast austenitic stainless steel did not show a creep effect under the tested strain rate.