| This study was attempted to improve the mechanical properties such as strength and elongation in martensitic high carbon steel by increasing Ms temperature. In order to raise the Ms temperature cobalt was used, and high carbon steel was selected to be one containing 1% C (wt.). For the purpose of investigating the effect of cobalt addition, three alloys were prepared, such as Fe-1% C alloy, Fe-1% C-14% Co alloy, and Fe-1% C-23% Co alloy, by means of a high frequency induction furnace in air. The optimum austenitizing temperature for obtaining martensitic structure was 900℃ in Fe-1% C alloy and 1250℃ in the other alloys having cobalt. As-quenched strength of the alloy having no cobalt was lower than the strength in the two alloys having cobalt. The ultimate tensile strength of Fe-1%C steel was increased with increasing tempering temperature showing a maximum strength around 300℃, and then decreased with further tempering temperature. In the case of Fe-1%C-14%Co alloy, the strength was gradually increased until tempering of 400℃, and then decreased with tempering temperature above the 400℃. However, in the case of Fe-1%C-23%Co alloy, as-quenched strength was maintained nearly constant up to 450℃ and then decreased gradually with further tempering. The different change in strength for these three alloys during tempering can well be explained by the variation of Ms temperature due to cobalt addition. It was noted that the cobalt contained alloys showed little improvement in elongation during tempering. Thus, although the combination of strength and elongation at tempering temperatures below 200℃ or 300℃ was better in Fe-1%C steel than in the cobalt contained alloys, it was reverse on tempering at temperatures above the 300℃. |
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