The effect of boron on stress-relief cracking of alloy steels

High-purity vacuum-melted laboratory heats of the MnMoNiCr forging steel designated by the ASTM as A508-class 2 were studied to determine the effects of additions of boron and of aluminum on the susceptibility to stress-relief cracking (SRC). The highly deleterious effect of boron was reproduced, and the aluminum addition was found to be at least as deleterious as the boron. High-resolution examination of the fracture surfaces showed that post-fracture faceting occurred on the intergranular fracture surface of the pure steel, but not on the boron-doped steel, indicating a difference in surface composition. From previous AES studies on other steels, it was deduced that the effect of both boron and aluminum is to scavenge nitrogen and to prevent surface segregation of nitrogen with chromium at the test temperature of 550°C. This would allow more sulfur segregation to the surface and thus provide a higher surface concentration of this element, which is known to be the cause of the diffusion-controlled intergranular decohesion that occurs during SRC at high stresses, a process known as dynamic embrittlement. It is concluded that the cosegregation of nitrogen and chromium retards SRC by suppressing sulfur segregation, but this protective effect can be eliminated by scavenging of nitrogen by either boron or aluminum. It is suggested that the susceptibility of a heat of steel to SRC could be assessed by examining the surface-segregation behavior by means of AES.