Elevated temperature fatigue crack growth in incoloy alloy 800 in sulfidizing environments

Rates of fatigue crack growth in INCOLOY alloy 800 were measured in He-SO₂ and He-H₂S environments at a frequency of 0.1 Hertz over a temperature range of 316 to 650 °C. Increasing concentrations of sulfur and increasing temperatures increased the growth rates at higher △K values. H₂S environments were more aggressive than SO₂-containing gases, probably because of the inhibiting effects of oxygen when SO₂ reacted with the alloy. At lower crack growth rates, sulfide particle formation at the crack tips caused cracks to decelerate and stop due to crack tip blunting and inhibition of crack closure. Sulfide particle formation also promoted the formation of periodic crack opening jumps. Under static loading, cracking could be produced only by a combination of prior cyclic deformation and the presence of a sulfidizing environment. The results suggest that sulfidizing environments enhanced fatigue crack growth through enrichment of sulfur ahead of the crack tip, producing a time-dependent crack growth process.