抄録
The effect of titanium on the toughness of 30% chromium ferritic steels was investigated by a Charpy impact test at low temperatures. Carbon content is (0.023±0.002) mass%, and titanium contents vary from 0.051 to 0.26 mass%. The ratio of titanium content to carbon content, Ti/C, ranges from 2.1 to 12.4. Two kinds of heat treatments with or without solution treatment were adopted to obtain different types of morphology of carbide precipitates.
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 dependent of Ti/C ratio, while annealing without solution-treatment produces coarse globular carbide particles within the grains.
(2) Ductile-brittle transition temperatures (DBTTs) of the specimens containing grain boundary carbides are higher than those of the specimens containing carbide particles within the grains.
(3) In case of the morphology of grain boundary carbides, DBTT first decreased remarkably with increasing Ti/C ratio up to 5 and then increased with the ratio. While in case of the morphology of carbide particles within the grains, DBTT remained constant in a range of 2 to 8 of Ti/C ratio and then remarkably increased with the ratio.
(4) Initiation of microcracks for brittle fracture is considered to be caused by the intergranular fracture cracks which are brought about by the decohesion at the interfaces between the matrix and grain boundary carbide precipitates. Addition of excess titanium such as 12 of Ti/C ratio causes the increase of DBTT due to promoting brittle intergranular fracture and weakens the effect of morphology of titanium carbides on the DBTT.
