Low cycle fatigue of four stainless steels in 20% CO–80% H2

Four stainless steels, 253 MA, 304L and Kanthal APM and APMT have been subjected to low cycle fatigue (LCF) at 750 °C. The testing was carried out in a newly developed test rig that allows for testing in a controlled environment from room temperature up to 850 °C. The atmospheres used in the testing were laboratory air, a carburising atmosphere (20% CO–80% H2) and an inert gas (Ar).The results from the testing showed that the fatigue life for 253 MA decreased in the carburising atmosphere and in argon compared to normal air. This feature was insignificant at low strains but increased with higher strains. For APM, APMT and 304L the carburising atmosphere did not significantly alter the fatigue life. 304L showed a significant pick-up of carbon at the surface of the specimen but this effect did not influence the material properties. No significant pick-up of carbon could be detected in APM or APMT. It is demonstrated that the crack size increases exponentially with increasing number of cycles and the rate at which this takes place increases exponentially with the plastic strain range. The strain enhancement factor for the crack propagation is about 50% higher in 20% CO + 80% H2 than in air.