Impurities, segregation and creep embrittlement

Abstract

Reported studies of creep embrittlement in low alloy steels at 550 °C and of stress relief cracking at 700 °C show conflicting evidence for the importance of residual impurities. Calculations of the relative effects of impurities, through segregation to grain boundaries and also to the internal surfaces of cavities, show that rupture life and ductility can be affected. Auger electron spectroscopic measurements of these segregations in commercial ½CrMoV steels give the relative importance for all active elements. For stress relief cracking, the equilibrium surface segregation results agree with cracking measurements and allow extension to all important elements. Analysis of published data shows that, in practice, impurities are as important as microstructure in causing embrittlement, the most important impurities being, equally, tin, copper and arsenic. A similar analysis for creep shows that impurities are more important than microstructure with copper dominant and phosphorus and tin also detrimental. Here, the creep ductility falls but rupture life increases with impurity content, consistent with a model involving grain boundary segregation. The stress relief cracking and creep embrittlement are both sensitive to impurities but, involving different segregations, are dominated by different elements.