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.