A Eutectoid Reaction for the Decomposition of Austenite into Pearlitic Lamellae of Ferrite and M 23 C 6 Carbide in a Mn-
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INTRODUCTION
PHASE transformation products in steels during cooling from high temperatures include Widmansta¨tten side plate, massive, and martensitic phases, which form in the austenitic matrix.[1] The empirical equation for the martensite start temperature, Ms, is dependent on the alloy contents (wt pct) of the steels as follows:[2] Ms ðKÞ ¼ 812 423ðCÞ 30:4ðMnÞ 17:7ðNiÞ 12:1ðCrÞ 7:5ðMoÞ
½1
In phase transformations of the steels after being quenched from high temperatures and held isothermally at low temperatures, various carbides, such as MC, M3C, M5C2, M7C3, and M23C6, precipitate in alloy steels.[3–12] The M3C carbide, named cementite, with an orthorhombic crystal structure is the most well-known carbide in steels. When steels containing about 0.77 wt pct C are cooled below the eutectoid temperature, a eutectoid reaction takes place, i.e., c fi a + Fe3C. The product phases are pearlite comprising lamellae, or sheets, of cementite embedded in the ferritic grains. The colonies nucleate most easily at the grain boundaries of the original austenitic grains.[1] The orientation of the lamellae is identical within each colony. Orientation relationships between the ferrite and M3C carbide have been well documented. For example, Bagaryatsky’s orientation relationship between the ferrite and M3C (C) is [110]a // [100]C, [111]a // [010]C, and (112)a // (001)C.[13] For the eutectoid reaction in high manganese WEI-CHUN CHENG, Professor, and SHIN-MING HWANG, Graduate Student, are with the Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted September 27, 2010. Article published online January 11, 2011 1760—VOLUME 42A, JULY 2011
steels, the partitioning of Mn and C solutes was discovered in the pearlite. The M3C carbide contains a high concentration of Mn and C, and the ferrite has a low concentration of Mn and C.[13–15] The other pearlite with the lamellae of ferrite and M23C6 carbide was found in Cr steels. The M23C6 plates replaced the lamellar M3C grains and embedded in the ferritic grains of the pearlite.[7] A well-known Kurdjumov–Sachs (K-S) orientation relationship was proposed to exist between the layer ferrite and M23C6 grains, i.e., (110)a // (111)C6 and [111]a // [011]C6. When steels containing more or less carbon than the eutectoid composition are cooled slowly or isothermally held below the eutectoid temperature, the formation of pearlite is usually preceded by the precipitation of proeutectoid cementite or ferrite, respectively, at the austenitic grain boundaries prior to the eutectoid reaction.[1] The progress of the isothermal phase transformation can be conveniently represented by plotting the fraction of transformation as a function of time and temperature in a diagram, i.e., plotting the transformation curves in the time-temperature-transformation (TTT) diagram.[1] The constituent phases in the ternary Fe-Mn-Al alloys have been intensely studied for decades.[16,17] F
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