The mechanism of brittle fracture in a microalloyed steel: Part I. Inclusion-induced cleavage
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I. INTRODUCTION
SEVERAL authors[1–4] in the 1950s demonstrated the detrimental effect of Fe3C (cementite) on the cleavage toughness of steel. A few authors[5,6] during this period speculated that cementite was directly responsible for fracture, but they reported such events as special cases and did not identify the exact mechanism of initiation. In 1965, McMahon and Cohen[7] convincingly demonstrated that the cracking of cementite particles located at ferrite grain boundaries represents a primary cleavage initiation mechanism in steel. This established that brittle fracture in steel often begins in or near some minute particle that is harder or more brittle than the surrounding ferrite matrix. Subsequently, this concept contributed to the formulation of the “shear-cracking” theory, whereby cleavage in steels was observed to initiate at cracking cementite lamellae in pearlite,[2,8–12] and of the theory that nonmetallic inclusions often act as cleavage initiators in steel weld metals[13–16] and in pearlitic steels.[17] Over the past few decades, there has been a steady decrease in the carbon content and the impurity (P and S) level of structural steels, and as a result, typical cleavage initiators like cementite particles and nonmetallic inclusions have been reduced in number and size. This has contributed to improved brittle-fracture resistance. However, despite these advances, three facts virtually guarantee that brittle fracture in steel will always remain a concern. First, because of continuing improvements in structural steels, users have responded by selecting these materials for more-severe service conditions: applications that are less tolerant of oversights or unfortunate mishaps. Second, structural steels have a bcc iron matrix, and, with respect to cleavage resistance, D.P. FAIRCHILD, formerly Graduate Fellow, The Ohio State University, is Research Specialist with Exxon Production Research Co., Houston, TX 77252. D.G. HOWDEN, Associate Professor, Department of Industrial, Welding, and Systems Engineering, and W.A.T. CLARK, Professor, Department of Materials Science and Engineering, and Associate Dean, Graduate School, are with The Ohio State University, Columbus, OH 43210. Manuscript submitted August 29, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
bcc iron is a borderline material.[18,19] Dislocation flow is essential for cleavage prevention, and, depending primarily on the microstructure, service temperature, and loading rate, dislocations in steel can move with relative ease or with much difficulty. Third, structural steels will always be fusion welded, and this guarantees the presence of microstructures (the weld metal and heat-affected zone (HAZ)) which are typically inferior to the highly processed parent material. By 1980, the study of cleavage in steels was perceived by many as a mature science, and, in general, the capabilities of modern structural steel were in high regard. Then, in 1982, local brittle zones (LBZs) were discovered.*[20] The *The term “discovered” is used here to denote dete
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