日本金属学会誌

J. Japan Inst. Metals, Vol. 16, No. 9 (1952),
pp. 517-521

On the Brittleness of Normalized Cast Steel Caused by the Grain Boundaries of Primary Austenite (1st Report)

Susumu Oki and Ren Obata


Abstract:

It is already well known that the grain boundaries of the primary austenite of low carbon steel usually do not coincide with those of the primary dendrites, and also that they have little effect on the mechanical properties after the heat treatment for grain-refining. But the authors have often experienced to observe a remarkable embrittlement occurring at some parts of castings such as spokes and test coupons attached to the bosses of driving wheels of locomotives, in spite of normal heat-treat- ment for grain-refining. The distinctive features of the appearances of this brittleness are as follows: (1) Coarse granular fractures are distinctly observed along the interfaces of grains of primary austenite. (2) In the microscopic specimens, etched with aqueous solutions of 10% HNO3-10% H2SO4 and repolished, network of pit-chains are clearly obeserved. (3) Loss in elongation is not so remarkable. The coarse fractures and pit-chains are gradually eliminated on eating the specimens, partly at about 1300° and entirely at about 1450°.
The authors tried to reproduce the embrittlement artificially by means of monotonous cooling of the specimens from very high temperatures at various cooling rates, but in vain. Later they, however almost succeeded in to do so by keeping the specimens at 1000° for one or two hours after fairly rapid cooling from solidifying temperature. From the results of these experiments, an idea was introduced for the reason why the embrittlement occurred only at the peculiar parts of castings. These parts of castings are rather thin in themselves but they are attached to some large masses, and so they cool rapidly just after the solidification and then very slowly between about 1000° and 800°. So the authors reached the conclusions that the embrittlement would be caused by precipitation of certain elements or compounds along the grain boundaries of primary austenite. This phenomenon may be substantially the same with so called ``Overheating''. The physical and chemical properties of the precipitates are not yet investigated.


(Received 1952/5/24)

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