Effect of environment on crack growth behavior in austenitic stainless steels under creep and fatigue conditions

Crack growth behavior at high temperatures under cyclic, static, and combined loads was studied in annealed and 20 pct cold-worked Type 316 and 20 pct cold-worked Type 304 austenitic stainless steels in air and vacuum. Under cyclic load, crack growth rates in annealed Type 316 steel are slightly lower in vacuum than in air, but this difference decreases with increase in crack growth rate. Most importantly, the effect of temperature on crack growth is present even in vacuum and arises mostly from the variation of elastic modulus with temperature. In the cold-worked Type 316 steel, the pronounced hold-time effects on fatigue crack growth in air reported in the literature persists even in vacuum. This implies that at high crack growth rates these hold-time effects arise mostly from creep-fatigue interaction rather than environment fatigue interaction. Environment has a negligible effect also on crack growth under static load. Thus, time dependent crack growth in these steels is due to creep processes. Crack growth behavior in annealed and cold-worked materials are compared and reasons for the enhanced time dependent crack growth in cold-worked material are discussed in detail.