Identification of Inverse Bainite in Fe-0.84C-1Cr-1Mn Hypereutectoid Low Alloy Steel
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se bainite was first reported by Hillert[1] in 1957 when he proposed that there is a symmetry among the eutectoid transformation products in the iron–carbon system. According to Hillert, pearlite is a eutectoid transformation product resulting from cooperative growth of ferrite and cementite near the eutectoid compositions, and bainite is formed as a consequence of ferrite being the leading nucleating phase along the growth direction of Widmanstatten ferrite at low carbon contents. He proposed that there must be a third eutectoid point with cementite as the leading nucleating phase along the growth direction of Widmanstatten cementite at higher carbon contents. The bainite formed with cementite as the leading nucleating phase was named inverse bainite. Inverse bainite has been discussed only a few times since Hillert’s proposal. Kinsman and Aaronson[2] provided TEM micrographs indicating the characteristics of inverse bainite; they have shown that in inverse bainite, the primary nucleating phase is cementite and the ferrite nucleates at the
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 October 3, 2016. Article published online January 5, 2017 948—VOLUME 48A, MARCH 2017
pro-eutectoid cementite–austenite interface surrounding the primary cementite unit. Lee et al.[3] included inverse bainite in their microstructure morphology map. In 2011, Kolmskog and Borgenstam[4] studied the eutectoid transformations in a hypereutectoid alloy steel and used inverse bainite as an evidence for the bainitic transformation being a diffusional–displacive-controlled transformation. In a recent study of fracture toughness of a hypereutectoid steel grinding media, we have found inverse bainite to have a significant effect on fracture toughness.[5] Dilatometry is a well-established technique that quantifies the phase transformations in the sample into corresponding dilatation changes.[6,7] Several works have been carried out incorporating dilatometric technique to quantify bainitic transformation, and it is widely accepted that isothermal bainite transformation (IBT) is accompanied by volumetric expansion,[8–10] indicating the formation of bainitic ferrite. Recently, Goulas et al.[11] observed inverse bainite in their dilatometry experiments and reported a net increase in dilatation during the inverse bainite transformation at 783 K (510 °C). In this work, we characterized the interrupted isothermal and quenching microstructure obtained for Fe-0.84 pct C-1 pct Cr-1 pct Mn steel in the bainite transformation temperature, and proposed an explanation for the initial drop and subsequent increase in the dilatation recorded for the isothermal bainite transformation. Isothermal holding experiments were conducted for the alloy using a RITA L78ä high-speed quench dilatometer. Samples were heated up to an austenitization temperature of 1323.15 K (1050 °C) (at a heating rate of 10 K/s) an
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