The Influence of Gas Environments on Fatigue Crack Growth Rates in Types 316 and 321 Stainless Steel

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Abstract
Fatigue crack growth rates of Types 316 and 321 stainless steel were studied as a function of stress intensity, temperature, relative humidity, and gas environment. At 25°C it was shown that humidity in the presence of oxygen will accelerate crack growth rates by a factor of ∼1.7, as compared to an inert environment such as dry nitrogen or argon. In addition, a threshold level is implied for the effect of humidity above which further increases in moisture content have no additional influence on crack growth rates. At 649°C an air environment was found to increase crack growth rates by factors of ∼22 and ∼5 for Types 316 and 321 stainless steel, respectively, over rates determined in either argon or nitrogen. Crack growth rates in nitrogen and argon at 649°C were comparable to crack growth rates at 25°C, leading to the conclusion that increased growth rates observed at 649°C in air are principally a result of environmental interactions with the crack front rather than a result of reduced mechanical properties at 649°C. These results also show that Type 321 stainless steel is less susceptible than Type 316 to aggressive environments at 649°C such as room air, suggesting that Type 321 might be a better choice of material for some elevated temperature applications. Fractography of specimens tested at 649°C revealed the absence of striation formation for specimens tested in inert environments, while pronounced striations were found in environments such as air.

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