| A stdy has been made on tempered embrittlement of AISI 4340 steel having martensite-bainite duplex structure, as compared with that in tempered martensite, metallographically. For making martensite-bainite duplex structure, lower bainite formation was achieved in a salt bath held at 315℃, upper bainite at 400℃, and subsequently quenched into iced brine. After the isothermal transformation, they were tempered for lhr in the range between 200℃ and 500℃, and then the mechanical properties were examined. Impact toughness was measured by Charpy Impact Tester, microstructure was examined by means of optical and transmission electron microscope, and fracture surface scanning electron microscope. The lower bainite was precipitated acicularly so as to partition prior austenite grain and the upper bainite was precipitated massively so as to fill prior austenite grain. These shapes of bainite had an effect on the impact toughness, temper embrittlement, and brittle fracture resistance of martensite-bainite duplex structure. The 300℃-tempered martensite embrittlement was greatly improved by martensite-bainte duplex structure and the ductile-brittle transition temperature of martensite-bainite duplex structure was lower than that of martensite. The former, the improvement of tempered martensite embrittlement by martensite-bainite duplex structure, may have been attributed to the shape and distribution of carbide which precipitated differently compared with those of martensite structure during tempering. They also affected on the fracture behavior. And, it seemed that the latter, the increment of brittle fracture resistance, have been due to the partitioning of prior austenite grain, with the result of fining cleavage crack path by lower bainite. |
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