Mechanisms of stress relief cracking in a ferritic steel

Abstract

The mechanism of stress relief cracking was studied in three heats of commercially produced MnMoNiCr pressure vessel steel (A508-2) using notched specimens austenitized briefly at high temperatures to simulate a weld heat-affected zone (HAZ) and then loaded in pure bending after a fixed stress-relief temperature had been reached. Thus, the effects of applied stress and temperature could be studied separately. Stress relief cracking always began by microcrack formation ahead of the notch. At lower loads at all temperatures (823–923 K), microcracking occurred mainly by cavitation nucleated on re-precipitated sulfides. As the initial load was raised cavity growth became more crack-like and a transition occurred to a brittle cracking mode, which was the mode of macrocrack growth in all specimens heated to a sufficiently high HAZ-simulation temperature. Surface analyses by Auger electron spectroscopy indicated that sulfur was the element primarily responsible for the brittle cracking, which occurred quasi-statically at a rate controlled by temperature and stress intensity. A qualitative model is proposed which treats this sulfur-induced diffusion-controlled brittle fracture in the framework of the theory of plasticity-aided crack-like growth of creep cavities. The effect of the HAZ-stimulation temperature is rationalized in terms of the extent of dissolution of sulfides and of alloy carbides, the latter being important because they control the rate of stress relaxation.