Influence of carbon content on superplastic behavior in Ti- and B-added Cr-Mo steels

Superplasticity has been investigated in various ferrous alloys and steels. However, in these materials, especially in hypoeutectoid steels below the A₁ temperature, the relationship between the content of carbon and elongation to failure is not obvious. In the present investigation, the influence of carbon content on superplastic behavior is studied using carbon steels based on Cr-Mo steel. In order to obtain the fine grain structure, a small amount of Ti and B were added and the content of carbon was controlled to be in the range from 0.24 to 0.83 pct. The largest value of elongation to failure was 644 pct, which was obtained for a specimen containing 0.58 pct carbon. The temperature and strain rate at which maximum value was obtained were 710°C and 5 × 10^-4 s^-1, respectively. Of all the specimens, this specimen had the minimum grain size. Moreover, the area fraction of carbide took the maximum value at the temperature where the largest elongation value was obtained. These results show that the addition of carbon has an effect on grain refinement by the formation of carbide, but excess amounts of carbon(>0.6 pct) bring about premature failure because of the resulting coarse microstructure and larger carbides.