Development of hardness-based model for remaining life assessment of thermally loaded components

In recent times hardness measurement for assessing remaining life of high temperature components undergoing microstructural degradation has become a popular method. However, most of the reported techniques work on the basis of a linear relationship presumed to exist between hardness degradation as a function of Larson–Miller parameter (LMP) in which time and temperature of service are combined. This is based on empirical correlationship obtained through data fittings of the results of controlled laboratory experiments and/or field data collections. However, if the degradation of hardness is based on simple but well-established physical models relating hardness-via-flow stress to the various microstructural parameters, and extending these models to include the kinetics of microstructural changes under high temperature and applied stress, a different form of functional relationship is obtained. This functional form resembles the nature of hardness change as a function of LMP, as reported in the literature, much better than a linear representation. It is also able to explain the effect of applied stress on this correlation. The model also highlights the need to develop a better functional correlationship between hardness and LMP, which is more representative of the actual observations and not based on the presumption of a linear relationship.