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| SUPERLEDGES AND CARBIDES IN BAINITE |
| XU Zuyao(T. Y. HSU) (Shanghai Jiaotong University); GU Wengui; YU Xuejie (Shanghai Institute of Iron and Steel Research) |
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Cite this article:
XU Zuyao(T. Y. HSU) (Shanghai Jiaotong University); GU Wengui; YU Xuejie (Shanghai Institute of Iron and Steel Research). SUPERLEDGES AND CARBIDES IN BAINITE. Acta Metall Sin, 1983, 19(1): 12-132.
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Abstract The existence of superledges on the broad faces of bainite formed isothermally at 320 and 420℃ has been observed with TEM on the specimens of a Si-modifled AISI 4340 steel. The height, α, and the width, b, of the superledges are about 320—850 and 1500—1800 A respectively. From the thickening rate of bainire, 0.006μm/s, the width of the superledge, b, is calculated to be approximately 0.5μm, this being of the same order of magnitude as width of the superledges observed. These ledges did exist at the habit plane and act as the growing superledges, ε-carbide and cementite were also found in bainite formed during 320 and 420℃ reaction. The orientation relationship between ε and α is the Jack relationship. Therefore, the existence of a eutectoid decomposition γ→α+ε at about 350℃, as suggested by Hehemann, is questionable. As the holding duration was prolonged there appeared cementite existing between γ and α phases. The orientation relationship between Fe_3C and γ is the Pitsch, Fe_3C and α-the Isaichev and γ/α is K-S relationship, from which the orientation relationship among γ-α- γ(or γ-α-Fe_3C) can also be established. However, it is uncertain whether or not the carbides are interphase boundary precipitates. The results have shown that part of the γ/α interface boundaries extended in a curve shape. The morphology of such interfaces and the existence of the growing superledges indicate that the growing interface is not a coherent one, so it is difficult to ascribe the growing mechanism of bainite to coherent shearing mechanism and it is more likely that bainite grows through the lengthening of the ledges.
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Received: 18 January 1983
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