Effects of Impurity Segregation and Hydrogen on Intergranular Brittle Fracture in an Alloy Steel

Abstract

Recent progress in the study of the effects of impurity segregation and hydrogen on intergranular embrittlement is reviewed. A method of measuring the intergranular fracture strength, σ*, as a function of the grain boundary concentration of metalloid elements (antimony, tin, and phosphorus) in a nickel-chromium steel is developed, using a combination of micromechanisms of intergranular failure and statistical analyses of impurity segregation. The observed embrittling potencies of elements of Groups IVA to VIA are rationalized in terms of a simple hypothesis deduced from the atomistic binding components in transition metals. The relationship of the microscopic toughness, kIc, (derived from the foregoing measurements of σ*) to the fracture toughness, KIc, is discussed in terms of a proposed model for stress-gradient control of brittle fracture. The author has found that the measurement of KIc gives a distorted view of the progress of intergranular embrittlement. In addition, the mechanism of intergranular cracking is studied in terms of the combined effects of the previously segregated impurity and the hydrogen absorbed during tests. This study shows that the presence of hydrogen produces a profound intergranular weakening when the grain boundaries contain even a small amount of a segregated embrittling element. A dynamic decohesion model, which takes into account an atomistic mechanism of local hydrogen accumulation adjacent to a moving microcrack tip, is proposed to rationalize the pronounced effect of impurity segregation on hydrogen-induced intergranular cracking.