The Bainite reaction in Fe-Si-C Alloys: The primary stage
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INTRODUCTION
THEformation of bainite in steels takes place essentially at temperatures ranging from the pearlite and proeutectoid ferrite temperature ranges to the martensite temperature range. According to the classical work of Mehl 1there are two forms of bainite: upper bainite and lower bainite. Upper bainite consists of lath or plate shaped ferrite units arranged in packets, and of interlath carbide precipitates. 2'3 Lower bainite differs from upper bainite in that the ferrite is usually coarser and contains internal carbides. The carbides are reported to form an angle of approximately 60 deg with the long direction of the plates. 3 Moreover, the habit plane of upper bainite is close to {111}f, while that of lower bainite is irrational. 4 There have also been some other forms exhibiting nonacicular morphologies reported, as granular bainite, 5 columnar bainite, 6 and bainitic grain boundary allotriomorphs. 7 Because these forms are formed only under special conditions, the major forms of bainite are upper bainite and lower bainite. According to Kinsman and Aaronson, s three different definitions of bainite can be used: the microstructural definition, the kinetic definition, and the surface relief definition. Since all of the above mentioned forms of bainite consist of both ferrite and carbide, the microstructural definition describes bainite as a nonlamellar aggregate of ferrite and carbide. The kinetic definition is based on the fact that bainite has a separate C-curve. 9 There are also separate C-curves for upper and lower bainite.10 The surface relief definition is based on the fact that bainite exhibits a martensitic surface relief. Single planar tilt 11 as well as more complex surface relief morphologies 12 have been observed. Although these features are all well established, there remains much controversy as to whether bainite is formed by a diffusion controlled or by a martensitic transformation. 8 By the diffusion controlled reaction mechanism s the growth of bainitic ferrite is considered to take place by the diffusion controlled movement of ledges, i.e., by the same mechanism as has been proposed for the formation of proeutectoid ferrite with Widmanst~itten morphology. This B.P.J. SANDVIK, formerly with Laboratory of Physical Metallurgy at Helsinki University of Technology, Finland and Ovako OyAb, Imatra, Finland, is now with the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted January 12, 1981. METALLURGICAL TRANSACTIONS A
mechanism requires the initial carbon content of the ferrite to be low, and the formation of carbides should take place on the austenite side of the ferrite/austenite interface. Indirect analysis of electron micrographs 13 indicates that upper bainitic carbide is formed from austenite. Also, the orientation relationship between lower bainitic ferrite and epsilon carbide may be interpreted as precipitation on the austenite side of the interface. 14According to a limited analysis of the orientati
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