The intergranular fragility index — an engineering materials parameter

Abstract

Residual elements affect the performance of engineering alloys through their ability to concentrate at grain boundaries, and this in turn may lead to certain forms of intergranular failure in various metallurgical situations. In order to identify the most potent residuals occurring in alloys of commercial purity, an evaluation exercise was conducted on a range of commercial engineering alloys which had exhibited various forms of intergranular failure in service. The results were assessed according to several behaviour categories: high temperature ( e.g. creep, stress relief cracking), low temperature ( e.g. temper brittleness) and reaction with the environment ( e.g. intergranular stress corrosion cracking). For each such distinguishable category there exist a different hierarchy of effective elements. Here we introduce a generalized engineering parameter, the fragility index, which allows a rapid assessment of the quality of a steel cast in relation to the above specific behaviour categories and in terms of an assessed dominant impurity equivalent. Thus for the above behaviour categories this index is expressed as a tin or phosphorus or copper equivalent depending on the phenomenon. Another engineering parameter, the relative fragility, forms the basis of a screening test for identifying the most probable embrittling elements. These parameters may be derived either from metallurgical test data or from the appropriate model presented here which requires only a knowledge of the bulk composition and segregation data.