Hydrogen-induced cracking in 4340-type steel: Effects of composition, yield strength, and H 2 pressure
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I.
INTRODUCTION
H I G H strength alloy steels generally exhibit brittle fracture along prior austenitic grain boundaries when stressed in hydrogen or a hydrogen-producing environment. This has been noted by practically all investigators of the subject. 1-10 However, most studies have been concerned with such aspects as crack growth kinetics and the dependence of the threshold stress intensity for crack extension, Kth, on variables like temperature, gas pressure, electrochemical potential, and steel composition or strength, rather than the significance of the fracture mode, per se. On the other hand, work in our laboratory5'H-15 has indicated that the fracture mode can be of central importance. The results have shown that the tendency for intergranular fracture is directly related to the concentration of impurities like phosphorus, sulfur, and tin, which are known to reduce intergranular cohesion in iron-based alloys. 16 Moreover, it has been shown that, when actions are taken to reduce the amount of impurity segregation, the tendency for intergranular fracture is reduced, the K~ is raised, and the crack growth rate at K > Kth is lowered. 5'11-15Since the latter two effects are presumably the practical goals of research on the effects of hydrogen on high strength steels, it is of critical importance that we understand them. With regard to 4340-type steels with yield strengths -> 1400 MPa (200 ksi), studies have been made on a highpurity laboratory steel containing nickel, chromium, molybdenum, and carbon (in the correct amounts for 4340 steel), but without manganese or silicon. It was found 11that in this steel, at a yield strength of 1380 MPa, intergranular fracture did not occur in 0.11 Pa H2 gas, and the apparent K~ level was > 120 MPa m 1/2. However, in a similar laboratory steel N. BANDYOPADHYAY and C.J. McMAHON, Jr. are both with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. JUN KAMEDA, formerly with the Department of Materials Science and Engineering, University of Pennsylvania, is now with Metals Development, Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted January 8, 1982. METALLURGICALTRANSACTIONSA
to which the commercial amounts of manganese and silicon were added, cracking in H2 under the same conditions was mainly intergranular, and the K0~ was
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