Interstitial Carbon and Nitrogen Effects on the Cryogenic Fatigue Crack Growth of AISI 304 Type Stainless Steels

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Abstract
Constant-load-amplitude fatigue crack propagation (FCP) rate measurements are reported for AISI 304 (Unified Numbering System [UNS] S30400) type stainless steels having variable carbon-plus-nitrogen (C+N) contents. Rates at stress-intensity factors from 20 to 70 MPa ·m½ were measured using 25-mm-thick compact specimens. The Fe-18Cr-10Ni steels tested exhibited partial martensitic phase transformations during tests at 76 and 4K, but not during tests at 295 K. The behavior varied considerably with composition: at high C+N contents (0.187% by weight or greater) the FCP resistance was lower at 4 than at 295 K, whereas at low C+N contents (0.067% by weight) the FCP resistance was significantly higher at 4 than at 295 K. The improved cryogenic behavior at low C+N contents was associated with a transition in failure micromechanisms.