High-temperature transmission electron microscopy in situ study of lower bainite carbide precipitation
- PDF / 2,410,632 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 22 Downloads / 239 Views
means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree. I.
INTRODUCTION
W H E T H E R newly formed bainitic ferrite is supersaturated with carbon or not is an important criterion for determining when the mechanism of bainite transformation is that of the shear or diffusional type. r~] Unfortunately, direct measurement of the initial composition of bainitic ferrite in ordinary steel is almost impossible with current experimental techniques, since carbon can diffuse out of a substructure unit in a few milliseconds,t2] In steels, however, it is well known that the typical lower bainite carbides are distributed mainly inside ferrite, but the source of their precipitation is not clear. It is probable that austenite-originated or interphase-precipitated t~,3] carbides can be subsequently engulfed by ferrite, or carbides may be precipitated from bainitic ferrite, t4j An in situ examination of the carbide source is thus necessary, and the present work is one such attempt to this approach. In the course of high-temperature transmission electron microscopy (TEM) in situ observation of lower bainite carbide precipitation, two difficulties arise. If austenitizing and subsequent austempering of the thin foil specimens are directly carried out in the heating chamber of the transmission electron microscope, (1)serious bending of the specimens due to temperature changes will make recording of clear images almost impossible; tSl and (2) Widmanst~itten ferrite morphology in thin foils by what amounts to an ultrafine austenite grain size diminishes the generality of the evidence thus obtained. [5.6] In the present work, 40CrMnSiMoV steel was selected. The element Si in this steel has a strong effect of retardation of carbide precipitation, t71 The other elements, like Cr, Mo, and V, etc., reduce the diffusion coefficient of carbon in ferrite. [sj Therefore, the lower bainite transformation of this steel was found to take place in two distinct stages, i.e., the formation of carbide-free bainM.K. KANG, Professor, is with the Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, People's Republic of China. J.L. SUN, formerly with Northwestern Polytechnical University, is Lecturer, Kuming Institute of Technology, Kurning, Yunnan, People's Republic of China. Q.M. YANG, formerly with Northwestern Polytechnical University, is Postdoctoral Student, Zhejiang University, Hangzhou, Zhejiang, People's Republic of China. This paper is based on a presentation made in the symposium "International Conference on Bain
Data Loading...
