Environmentally controlled microstructures and correlation with creep rupture

The interaction of environment with the mechanical properties can generally be traced to the modification of microstructure induced by changes in alloy chemistry. Evidently, such changes will depend critically on the activity of the gaseous species at the metal surface as well as the diffusivity of the active components. The authors examine the extent to which microstructural changes induced by carburization are critical in terms of the mechanisms of creep and stress rupture. Two temperatures for carburization pretreatment were selected, 950 and 1150°C, which resulted in two distinctly different microstructural forms, even at the same levels of carburization. Creep experiments carried out at 1000 and 1050°C at stresses of 17, 21, and 28MN m^-2 clearly show that prior carburization modifies the creep and stress rupture properties of the alloy. Carburization at 950°C increases the creep strength and enhances the time to rupture but significantly reduces the rupture ductility, while carburization at 1150°C drastically reduces both the creep strength and the time to rupture but provides a marked increase in ductility. The results are discussed in terms of the influence of the alternative carburized microstructure on the nucleation and growth of critical cracks and the accommodation of stress concentrations due to boundary sliding.