HYDROGEN-INDUCED CRACKING IN HIGH-STRENGTH STEEL

Initiation of quasicleavage (QC) cracks in delayed fracture of hydroge n charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed f racture of an unnotched specimen is found not to be caused by intergra nular cracking as is commonly found in notched specimens, but by QC cr acking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specime n. Furthermore, the tensile component of stress causes a QC crack to g row in the plane of maximum tensile stress. The conditions under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress. (C) 1996 The Institute of Mater ials.