On the Toughness and Micro-Structure in Low Carbon Steels Subjected to Weld Thermal Cycles

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With a view to making clear some of the relations between toughness and metallurgical structure in a weld joint subjected to a large heat input, toughness was studied through careful examination of micro-structure, of two steels A and B (A: 0.10C, 0.25Si, 1.15Mn; B: 0.10C, 0.25Si, 1.15Mn and 0.30Mo) subjected to various weld thermal cycles. Test results are summed up as follows: (1) There appear high carbon martensite islands, M^*, between the needles of an acicular ferrite in the steel containing 0.3% Mo, if cooled at medium cooling rales from 1,350℃. (2) If the structure of a specimen contains M^*, toughness deteriorates seriously. (3) If a specimen is free from M^*, variation in toughness is rather gradual with respect to the cooling rale. This is due to the fact that an austenitic grain size or cleavagefacet size gradually changes with the cooling rate.
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