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Austenite Stability and Its Influence on Mechanical Properties of 18-8 Stainless Steel at Cryogenic Temperatures (SS 304LN)

机译：低温下18-8不锈钢的奥氏体稳定性及其对力学性能的影响（ss 304LN）

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Stress/strain induced martensitic transformation has been found to play an important role on the mechanical properties of metastable austenitic stainless steels. Typical two-stage stress-strain curves were observed for AISI 304L and 304LN stainless steels containing 18% Cr and 8%Ni. The martensite phase induced in the high-work-hardening stage was found, by transmission electron microscopy, to be accommodated by mechanical twinning. The drop of 0.2% offset yield strength caused by the dilation and induced slip associated with the martensitic transformation of the low-carbon-content 304L was observed at liquid nitrogen temperature. Low temperature favorable fatigue properties were observed for 304L but not for 304LN. Below the M/sub d/ temperature, the fatigue crack propagation in metastable austenitic steels can be assumed to be the same as crack growth in an austenite and martensite composite. Although fatigue cracks grew faster in the martensite phase than in the austenite phase, the compressive stress near the crack tip due to the martensitic transformation appeared to reduce the effective stress intensity range which resulted in a decreased crack growth rate. The controlling factor became K/sub max/ instead of delta K if the load ratio R was not large. The addition of interstitial nitrogen, which stabilizes the austenite phase, reduced the compressive stress at crack tips and resulted in the temperature-insensitive fatigue properties. Both the austenite phase and the martensite phase were found to be ductile at liquid nitrogen temperature. (ERA citation 08:049495)

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