Abstract
Tension-compression fatigue tests were conducted at room temperature, −196°C and −269°C for 18-8 stainless steel which is widely used as structural materials for cryogenic appliances, using a fatigue testing apparatus specially designed for cryogenic use. As a result, electron microscopy of the fracture surfaces and martensitic transformation near the fracture surfaces were discussed. The results may be summarized as follows. (1) In the range up to 106 times of stress repetition, the endurance limit is not observed on the S-N curve at any temperatures tested. The curve at −269°C almost coincides with the curve at −196°C. (2) On the fracture surfaces at −269°C, parts of smooth surfaces are found near to starting points of fatigue cracks in many cases, which are thought to be formed with rubbing of the crack surfaces; and some scratches like a nail-mark are observed here and there on these parts. (3) Almost no martensite is formed in the specimens tested at room temperature, but small amount of ε′ martensite and considerably large amount of α′ martensite are formed in the parallel portions and the parts near to fracture surfaces of the specimens tested at −196°C and −269°C. (4) The amount of α′ martensite formed increases with the increase of stress amplitude at the temperatures tested. The amount of α′ martensite formed in the parallel portions of the specimens which did not fracture even after 106 times of stress repetition is a little less than the amount of α′ martensite formed in the parts near to fracture surfaces of the spcimens fractured.
