Visualization of Microstructural Factor Resisting the Cleavage-Crack Propagation in the Simulated Heat-Affected Zone of
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e grain size can be defined as the dimensions to which a crack can propagate without a large deviation,[1] and it is an important length-scale for the weld toughness. Although high heat input welding (such as submerged arc welding with high heat input) has a high efficiency, it results in a less-tough, large effective grain.[2] To ensure weld toughness, it is important to
HIDENORI TERASAKI, Professor, and YU MIYAHARA, Undergraduate, are with the Graduate School of Science and Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan. Contact e-mail: [email protected] MITSURU OHATA, Associate Professor, is with the Graduate School of Engineering, Osaka University, Osaka, Japan. KOJI MORIGUCHI, Chief Researcher, and YUSAKU TOMIO and KOTARO HAYASHI, Researchers, are with the Technical Research & Development Bureau, Nippon Steel & Sumitomo Metal Corporation, Tokyo, Japan. Manuscript submitted on June 24, 2015. Article published online September 28, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
identify the effective grain necessary for resistance against cleavage-crack-propagation. Gogeruous et al.[3,4] applied electron backscatter diffraction (EBSD) to assess cleavage-crack-propagation in low alloy steel having bainite microstructure and the Bain zone was defined. The Bain zone consists of ferrite variants in Kurdjumov–Sachs orientation (K–S OR) that belong to the same lattice correspondence.[2,5–7] They verified cleavage-crack-resistant nature of Bain zone. The relation between crystallographic properties of bainite and cleavage-crack propagation has also been discussed by other researchers.[8–14] It was clarified the microstructural factor to arrest the microcrack such as high-angle boundaries[10,14] and the second phase.[12] Our research group visualized the Bain zone using Bain-zone map and also verified that the Bain-zone boundaries provide crack propagation resistance within the simulated heat-affected zone of bainitic steel.[15] Furthermore, our group visualized using the closepacked plane (CP) group map that boundaries of the CP group also provide a resistance against cleavage-crack-propagation. The CP group consists of ferrite variants in K–S OR belonging to the same CP of austenite ((111), (1-11), ( 111), and (11-1)).[6,16] However, our visualization study of crack resistance only investigated when the coarse-grained heat-affected zone (CGHAZ) of a weld with a high heat input was simulated. From viewpoint of crystallographic analysis, it was the condition in which the CP group was smaller than the Bain zone (hereafter referred as ‘‘CP < Bain’’). The present study continues in an effort to verify the resistance nature of Bain and CP group boundaries against cleavage-crack-propagation by examining the path of secondary cleavage cracks (observed on a fractured V-notch Charpy specimen) when the coarsegrained HAZ (CGHAZ) of a weld with a low heat input was simulated. From viewpoint of crystallographic analysis, it was the condition in which the Bain zone was smaller than CP group (CP > Bain).
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