Tensile cracking of a chromia layer on a stainless steel during thermal cycling with hold periods

This study has used thermogravimetric, acoustic emission and finite-element modelling techniques to examine the tensile cracking of a chromia layer on a 20Cr25Ni steel during thermal cycling from an oxidation temperature of 900 °C. It is found that the process is highly sensitive to the temperature amplitude of the thermal cycle and that hold periods at the bottom temperature result in enhanced oxide cracking whereas hold periods at the peak temperature can inhibit the onset of cracking. For standard saw-tooth cycles, the finite-element analysis shows that these effects can be explained by a creep hysteresis effect. This arises from stress relaxation during the cooling half-cycle, which results in the development of in-plane tensile stresses within the oxide layer on the return to temperature. Low-temperature hold periods in the creep regime increase the in-plane tensile stress in the oxide on the return to temperature. High-temperature hold periods tend to reduce the peak tensile stress produced in subsequent cycles but, for the example examined here, the effect is small.