Yasunori Harada1, Masanobu Ohmori1 and Shin-ichi Ohnishi2
1Department of Mechanical Engineering, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima
The workability of cast pure chromium produced by an induction-slag-melting process was investigated through the study of rolling and isothermal upsetting. The cast material had large crystal grains elongated along the cylindrical ingot axis or the solidification direction. The effect of rolling, which was performed to change the coarse cast structure to a finer one, on the ductile-to-brittle transition temperature (DBTT) was also examined. The present results are summarized as follows.
DBTT of the as-cast specimen tested in tension at a strain rate of 2.4 × 10-2 s-1 was about 500 K. A minimum ductility and a maximum flow stress appeared at about 973 K owing to dynamic strain aging.
The forming limit in upsetting, percent reduction of height at beginning of cracking on the specimen wall, increased to 80% at 773 K, and no crack appeared at temperatures above 873 K.
At all temperatures, there occurred cracks in the as-cast specimen deformed by a conventional rolling. Cracks occurred slightly in the specimen even in a sandwich-rolling by which a sintered chromium was successfully deformed. Cracking in the as-cast specimem was mainly due the coarse structure with large crystal grains. Once the structure was changed to a finer one by the upsetting, the specimen was easily rolled to a thin sheet without crack occurrence.
DBTT of the specimen rolled to 80% and followed by annealing at 1273 K was higher by about 200 K than that of the as-cast specimen, while annealing at 1473 K yielded a slightly lower value than that of the ascast material.
cast chromium, workability, upsetting, conventional rolling, sandwich-rolling, ductile-to-brittle transition temperature (DBTT), dynamic strain aging
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