FORMATION OF HYDROGEN-ASSISTED INTERGRANULAR CRACKS IN HIGH-STRENGTH STEELS

The fracture behaviour of high strength steels under the influence of hydrogen was studied, with special emphasis on the critical condition for the formation of intergranular (IG) cracks. Mechanical tests were carried out on cathodically charged specimens subjected to both a cons tant load and a constant displacement under a variety of hydrogen-char ging conditions. Experimental analyses show that a high local hydrogen concentration plus a high stress intensity at a quasi-cleavage (QC) c rack tip are required to initiate intergranular cracking. The conditio n for continued intergranular crack propagation, leading to micro-void coalescence (MVC), is determined by the combined effect of the speed of crack propagation together with the rate of hydrogen diffusion. A q uasi-cleavage crack triggers the onset of intergranular crack growth i rrespective of the test control parameters, i.e. constant loading or c onstant displacement conditions. A fracture map is proposed relating t o the boundary conditions between QC, IG and MVC cracking, which will assist further research.