The mechanism of brittle fracture in a microalloyed steel: Part II. Mechanistic modeling
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I. INTRODUCTION
IN Part I of this study, the base-metal and weld heataffected zone (HAZ) cleavage-fracture resistance of two microalloyed steels was investigated, and it was determined that one of them (steel B) had significantly lower toughness. Scanning electron fractography revealed that the toughness degradation was caused by cuboidal or angular-shaped, nonmetallic inclusions that acted as cleavage initiators. These inclusions were tentatively identified as TiN. It was surmised that these inclusions were particularly potent initiators because they were well bonded to the ferrite matrix. The current article describes additional work on steel B, including chemical and structural characterization of the offending inclusions, fracture-specimen analysis, and scanning electron fractography. These results are combined with existing micromechanical cleavage models to produce a specific model of cleavage fracture in steel B. In developing a cleavage model for steel B, particular emphasis is given to the physical phenomena influencing the instant of initiation. With respect to cleavage fracture in steel, at one moment the component is a relatively flawless continuum, then, an instant later, it is severed into two or more pieces. Given these bounding observations, it can be reasoned that, at a single instant in time, something within the microstructure, some tiny constituent, snaps first. The metallurgical feature that is first to break, the forces which cause it to break, and how its failure causes the entire component to fail catastrophically are among the subjects addressed. [1]
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 November 25, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. BACKGROUND: CLEAVAGE FRACTURE IN STEELS A. Early Cleavage Modeling The topic of cleavage fracture is often introduced by discussing the theoretical strength of a solid. This strength can be estimated using the force-displacement laws governing the attractions between the atoms.[2,3] Conceptually, adjacent atomic planes are forced apart until new, free surfaces are created. Calculated strengths are up to 1000 times greater than those actually observed. However, engineering materials are not an ideal continuum of atoms. They contain minute discontinuities such as dislocations, grain boundaries, and second-phase particles. Under applied loads, these elements create local stress elevations that eventually cause some microstructural feature to break. This is the moment of cleavage initiation, and this minute crack may propagate to cause complete failure of the component. Griffith[4] was perhaps first to quantify that tiny stress concentrations can cause cle
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