Transactions Volume 62 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1920 Excerpt: ...expect ductility to be restored more quickly in the 1%-in. square billets than in the larger blooms. This may in part account for the fact that this phenomenon of brittleness was not noted in the case of the square billets but only in the larger blooms. Further, there is the matter of finishing temperature and rate of cooling after work was finished. The smaller sections cool more rapidly in the mill and therefore usually finish at a much lower temperature than the larger sections, and, cooling from a lower temperature, pass through their critical range at a slower rate than if the finishing temperature were high. To balance this there is the tendency to fast cooling due to the smaller section. It is possible, Metallurgist, U. S. Bureau of Mines. t Received Aug. 29, 1919. however, that the rate of cooling through the transformation, by preventing the complete separation of proeutectoid iron to the grain boundaries, may play a part in this brittleness. If the finishing temperature is low and the section small, a small grain size results and we may expect a small grain to give up its proeutectoid iron in a shorter space of time than a large grain. In this respect the small section is favored. Strains probably also play a part in the production of this brittleness. Bars which show this brittleness (16 to 20 per cent, extension) after 4 days at 120 C, give from 22 to 25 per cent, after cooling in air from 850 C, showing that an air cool from 850 C. does not produce this brittleness in a test bar. Why then should it occur in the blooms? Perhaps because of finishing the blooms at a higher temperature than 850 C, or perhaps the condition may be due to strains set up in the cooling of the large section. There are many factors at work in the cooling of a large secti...