Microstructural Evolution of Inverse Bainite in a Hypereutectoid Low-Alloy Steel
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Furthermore, the crystallographic orientation deviations between the individual phases of inverse bainite microstructure suggest that the secondary carbide nucleation occurs from the inverse bainitic ferrite. Thermodynamic driving force calculations provide an explanation for the observed nucleation sequence in inverse bainite. The degeneracy of inverse bainite microstructure to upper bainite at prolonged transformation times is likely due to the effects of cementite midrib dissolution at the early stage and secondary carbide coarsening at the later stage. DOI: 10.1007/s11661-017-4373-6 The Minerals, Metals & Materials Society and ASM International 2017
I.
INTRODUCTION
THE formation of bainite is the most studied phase transformation in steels. There is evidence for the diffusionless shear transformation of bainite,[1,2] as proposed by Zener[3] and Ko and Cotrell.[4] Likewise, there is experimental evidence for the diffusion-controlled[5,6] transformation of bainite. The idea of the diffusional transformation of bainite dates back to the work by Hultgren[7] in 1947, where he believed that bainite forms through the initial nucleation of Widmansta¨tten ferrite followed by the nucleation of cementite on the sides of the Widmansta¨tten ferrite plates, which leads to sidewise growth of the ferrite plates. There are two conflicting theories on the transformation of bainite. With two schools of conflicting transformation mechanisms, the transformation mechanism of bainite is still a disputed subject. Upper bainite and RANGASAYEE KANNAN, YIYU WANG, and LEIJUN LI are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada. Contact e-mail: [email protected] Manuscript submitted February 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
lower bainite are the two common bainite morphologies in engineering steels, but the microstructure of bainite is complex with several other possible morphologies such as carbide-free bainite, nodular bainite, granular bainite, pearlitic bainite, and inverse bainite.[8,9] Though the transformation of other forms of bainitic microstructures can be explained in terms of either of the theories of bainite transformation, the transformation of inverse bainite is still unclear.[9] Inverse bainite was first suggested by Hillert[10] in 1957 when he proposed that there was a symmetry among the eutectoid transformation products in the iron-carbon system: pearlite forming as a result of cooperative growth of ferrite and cementite and bainite forming as a consequence of ferrite being the leading nucleating phase along the growth direction of Widmansta¨tten ferrite at lower carbon contents. Hillert proposed that there must be a third eutectoid transformation product with cementite as the first phase to form along the growth direction of Widmansta¨tten cementite at higher carbon contents. In ‘‘conventional’’ bainite in hypoeutectoid steels, ferrite nucleation is
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