Effects of test temperature on internal fatigue crack generation associated with nonmetallic particles in austenitic steels

Internal crack generation associated with nonmetallic inclusions or precipitates has been investigated on high-cycle fatigue at 4 K, 77 K, and 293 k of 25 Mn-5Cr high-manganese austenitic steel and nitrogen-strengthened 25Cr-13Ni austenitic stainless steel. In both steels, the internal crack initiation typically occurred at 4 K or in long-life range over 10(6) cycles at 77 K. Particles such as inclusions and precipitates were responsible for the internal crack-generation behavior, and the origins were identified as mainly Al2O3 inclusions in 25Mn-5Cr steel and AlN precipitates in 25Cr-13Ni steel, respectively. We discuss the crack-generation stage I mechanism and the relationship between stress range and size of crack-initiation site. The generation of fatigue cracks associated with the nonmetallic particles in the specimen interior involved a stage I crack. A threshold condition assumption was proposed, that the crack propagation occurred at any stress level when the local stress intensity factor range reached over a constant at or around the initiation crack associated with defects.