Solute segregation and hydrogen-induced intergranular fracture in an alloy steel

Abstract

Hydrogen-induced intergranular fracture of laboratory heats of a 3.5 Ni-1.7 Cr steel doped with P, Sn, or Sb and having a yield strength of 840 MPa and a prior austenite grain size of 120 μm has been compared with that of an undoped steel at a hydrogen pressure of 0.17 MPa (1.68 atm). The intergranular concentrations of the impurities were controlled by varying the time of aging at 480 °C. Cracking of the undoped steel tested in hydrogen occurred along martensitic lath boundaries at high stresses. However, the susceptibility of the doped steels to hydrogen-induced intergranular cracking increased precipitously with impurity concentration. The susceptibility was measured in terms of the threshold stress intensity Kthfor the first detectable crack extension in precracked specimens and in terms of the threshold stress σth for microcrack formation in notched specimens. A comparison between the intergranular strength in hydrogen and in air revealed that absorption of hydrogen produced a profound intergranular weakening when the grain boundaries contained even a small amount of a segregated embrittling element. The relative embrittling potencies of P, Sn, and Sb in hydrogen gas were the same as in air. The combined effects of hydrogen and the impurities in reducing intergranular cohesion are discussed in terms of a newly proposed dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.