Observations of Microstructure and Fracture Surface of Stainless Steels Fatigued at Cryogenic Temperatures

Abstract

The measurements of fatigue crack growth characteristics at room and liquid nitrogen temperatures as well as the observations of microstructure and fracture surface topography of metastable 304L were carried out. The results obtained were compared with those of stable austenitic stainless steels (310 and high Mn stainless steel). The effective stress range ratio of 304L steel at liquid nitrogen temperature was larger than that at room temperature or that of 310 steel. The blocks of α' martensite transformed from austenite via ε martensite were observed in the close neighborhood of cracks in 304L steel, whereas no α' was found in the high Mn stainless steel. The fracture surface of 304L steel at liquid nitrogen temperature was smooth and flat. Based on these observations, the fatigue crack growth mechanisms in stainless steels at cryogenic temperatures were briefly discussed with emphasis on the crack closure.