A substructure characterizing parameter in creep

Abstract

A substructure characterizing parameter which is the ratio of applied stress, σ, in creep test to yield stress, σ_YS, of the material at the test temperature is introduced. A correlation is found to exist between this parameter and the creep rate for the data obtained in the temperature range 820–975 K when the initial yield strength is modified by (i) introducing different amounts of prior cold work by two modes of deformation at room temperature in a type 316 LN stainless steel and (ii) grain size, chemistry and grain size variation in a type 316 stainless steel. The correlation was found to exist also for a Cr-Mo-V steel at 823 K, in which different yield strengths were due to different heat treatments. Minimum creep rate when plotted against the substructure characterizing parameter yields an exponent similar to Norton's creep exponent and it is postulated that the value of the exponent reflects on the type of substructure developed in creep. Another parameter σ/F _ys where F _ys is a function of the ratio of the yield strength of a given microstructure to that of a reference microstructure (zero cold work for cold worked material, largest grain size when the microstructure variation is through grain size and solution annealed microstructure among heat treatments) also gives a unique correlation with the minimum creep rate at a test temperature with the exponent identical to Norton's creep exponent.