日本金属学会誌

J. Japan Inst. Metals, Vol. 17, No. 3 (1953),
pp. 105-108

Studies on the Microstructure of Steels at High Temperatures by Means of the Emission-type Electron-microscope (II). On the Grain Growth of Austenite

Ryuzo Ueda


Abstract:

The grain growth of austenite at high temperatures as observed by means of the emission-type electron microscope proceeds as follows: (a) On heating a specimen of plain carbon steel or Ni-Cr-Mo steel from room temperature to a certain temperature above A3 point, fine austenite grains filling the whole surface at first diminish rapidly in number, but the rate of diminution falls suddenly at a certain stage. The grain growth accompanied by the smoothing of zigzag boundaries becomes sporadic (Photo. 1, 2). (b) If then the specimen is cooled for a few seconds just below A3 point and at once restored to the previous temperature, the original pattern is preserved except at boundary regions between grains where vigorous recrystallization occurs (Photo. 4a-e). This fact means that the decomposition of austenite grains on cooling below A3 point begins at their peripheries. (c) Austenite grain boundaries as seen electronmicrographically (1) below about 1000° are only geometrical lines between light and shade, but (2) when heated at about 1150° for 10 ∼ 30 minutes, they begin to have actual breadth (Photo. 4g, h) and (3) at the same time some uniformly emitting grains begin to take on network pattern on the whole surface (Photo. 4g, h). Correspondingly on the cooled specimen surface we observe respectively (1) very fine grooves, (2) broader ones and (3) irregular furrows about 30 μ apart. (d) Twin bands grow differently from the ordinary grains in that (1) they change only in length but not in width (Photo. 4c-h) and their breadth suffers no alternation even when cooled for a few seconds below A3 point (Photo. 4a, b) and (2) the straight parallel lines marking twin boundaries are either entirely not or only slightly perceptible as grooves on the cooled specimen surface, and do not broaden when heated at a very high temperature (1200°) (Photo. 4g, h).


(Received 1952/10/24)

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