The mechanism of brittle fracture in a microalloyed steel Part I Inclusion-induced cleavage

The mechanism of brittle fracture in a microalloyed steel Part I Inclusion-induced cleavage

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Unformatted text preview: The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage D.P. FAIRCHILD, D.G. HOWDEN, and W.A.T. CLARK The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to- brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch- tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents. I. INTRODUCTION bcc iron is a borderline material. [18,19] Dislocation flow is essential for cleavage prevention, and, depending primarily S EVERAL authors [1–4] in the 1950s demonstrated the on the microstructure, service temperature, and loading rate, detrimental effect of Fe 3 C (cementite) on the cleavage tough- dislocations in steel can move with relative ease or with ness of steel. A few authors [5,6] during this period speculated much difficulty. Third, structural steels will always be fusion that cementite was directly responsible for fracture, but they welded, and this guarantees the presence of microstructures reported such events as special cases and did not identify (the weld metal and heat-affected zone (HAZ)) which are the exact mechanism of initiation. In 1965, McMahon and typically inferior to the highly processed parent material. Cohen [7] convincingly demonstrated that the cracking of By 1980, the study of cleavage in steels was perceived cementite particles located at ferrite grain boundaries repre- by many as a mature science, and, in general, the capabilities sents a primary cleavage initiation mechanism in steel. This of modern structural steel were in high regard. Then, in established that brittle fracture in steel often begins in or 1982, local brittle zones (LBZs) were discovered.* [20] The near some minute particle that is harder or more brittle than the surrounding ferrite matrix. Subsequently, this concept *The term “discovered” is used here to denote detection, definition, and contributed to the formulation of the “shear-cracking” the- labeling. Low-toughness HAZ regions in steel welds were not new as oflabeling....
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The mechanism of brittle fracture in a microalloyed steel Part I Inclusion-induced cleavage

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