Observations of material behavior under isothermal and thermo- mechanioal loading

Deformation and damage behavior of a 1070 steel (class U wheel steel) under thermo-mechanical and isothermal loading have been examined. Fatigue lives under thermo-mechanical and isothermal loading were compared for similar conditions of strain and temperature. For most cases, maximum tensile stresses developed in thermo-mechanical tests exceeded those obtained under isothermal conditions (at T_min) for the same strain amplitude. Cyclic hardening was observed at 200 to 300° C in isothermal tests. Under thermo-mechanical loading, static strain aging resulted in added hardening due to alternate exposure of the material to high and low temperatures. When thermal recovery effects become dominant at the high temperature end, strengthening upon cooling was suppressed. Oxide scales readily formed at high temperatures (≥ 400°C) and resulted in an increase in damage accumulation rates. Oxygen penetration into crack flanks and the ensuing loss of carbon was identified using Auger spectroscopy. The severity of oxide penetration into the base metal increased with increase in surface crack density, longer oxidation times and higher temperatures.