Effect of titanium on the tensile ductility of 30 mass% chromium ferritic steels

The effect of titanium on the ductility of 30% chromium ferritic steels was investigated by a tensile test at low temperatures. Carbon content is 0.023 +/- 0.002 mass%, and titanium content varies from 0.051 to 0.26 mass%. The ratio of titanium content to carbon content, Ti/C, ranges from 2.1 to 12.4. Ductility was evaluated by ductile-brittle transition temperature (DBTT) in reduction of area and fracture modes. The results obtained are summarized as follows: (1) Aging after solution-treatment gives rise to grain boundary precipitation of coarse Cr carbides and numerous fine Ti carbides depending on Ti/C ratio, while annealing without solution-treatment produces coarse globular carbide particles within grains. (2) The DBTTs for the specimens containing grain boundary carbides are higher than those for the specimens containing carbide particles within the grains independently of Ti/C ratio. The Effect of Ti/C ratio on the DBTT determined by the tensile test is not as remarkable as that on the DBTT determined by a Charpy impact test. (3) The effect of Ti/C ratio on fracture modes of the specimens containing grain boundary carbides is more remarkable than that on fracture modes of the specimens containing carbide particles within the grains. In case of the specimens containing grain boundary carbides, the amount of intergranular fracture first decreases with increasing Ti/C ratio up to 8 and then increases with the ratio at testing temperatures above 200 K, while at 153 K it first increases with increasing Ti/C ratio up to 8 and then decreases with the ratio.