Effect of Carbon on the Low Temperature Brittleness of Iron

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  • 鉄の低温脆性に対する炭化物の影響
  • テツ ノ テイオン ゼイセイ ニ タイスル タンカブツ ノ エイキョウ

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The brittleness of iron containing 0.001∼0.44% carbon has been investigated by the charpy U-notch impact test at various temperature and tensile test at 77 K. Impurities (manganese, oxygen etc.) in the specimen, which influence the ductile-brittle transition temperature, were minimized.<BR>In the charpy test, the shape of absorption energy vs temperature curves depended on the carbon content and the test temperature. The value of elongation in the tensile tests depended on the quenching temperature (Tq) after slow cooling from the austenizing temperature. The maximum elongation was obtained in the specimen quenched from the solvus temperature (Ts) of carbon in ferrite. When Tq>Ts, the intergranular fracture occurred, while in the case of Tq<Ts the cleavage fracture took place.<BR>The transition temperature, in the charpy test, first rose remarkably with increasing carbon content, and then appeared to rise slightly for carbon contents above 0.013%, whereas the cleavage fracture in ferrite was more strongly dependent on the presence of cementite in the tensile test. The higher sensitivity in the tensile test may be seen due to a higher probability of the existense of the favorable cementite for cleavage fracture in the gauge length of the specimen.

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