Effects of microstructural parameters on work hardening of pearlite at small strains
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C3 carbides for polygonal shape formation during the steel austenitizing in temperatures lesser than 1150 8C and shorter times (until 90 minutes), this phenomenon is the further development of the coagulation process, but the coagulation must proceed with high intensity during which the carbides of bigger size are also dissolved. This is accompanied with the high austenitizing temperatures, though slightly lesser than the solidus temperature. Further, individual carbides to form joints and conglomerates develop a polygonal form; this process is called traversing. Some of the primary carbides do not take a truly polygonal shape even after 720 minutes of austenitizing.[6] Difficulties connected with revealing the intergranular boundaries in joints and conglomerates, using typical reagent etching methods, are the result of better matching in crystallographic lattice and less amount of defects in the boundary than in austenite grains. It results from the studies carried out by the authors that formation of coagulation carbide joints is limited by diffusion of carbon and alloying elements in the grain boundaries. Generation of the joints is schematically presented in Figure 7. Figure 7(a) shows that the dissolution of fine carbides results in the growth of bigger ones and finally the direct contact of two neighboring carbides. Joints generated in are unstable due to the small area of curvature at the contact site. They will tend to increase the area, thereby minimizing the overall free energy. Thus, increased curvature is associated with surface diffusion. Moving the mass consisting of iron and alloying elements to the neck area leads to increasing the contact areas between carbides. In the final stage, each individual carbide takes up the polygonal form. The change of carbide shape from spheroidal to polygonal results in the increase of area and with the same surface energy. There are possible changes accompanied by kinetics. The decrease of the total free energy in this process is not governed by surface energy but rather by the growth rate of crystal face. In the compact hexagonal lattice in which the M7C3 carbide crystallizes, the carbide grain shape is dominated by the increase of (0001) planes, which grow most slowly. Polygonal carbide grain shape is very close to equilibrium. The process of generation of the polygonal form of carbides is a further evolution of the coagulation process. REFERENCES 1. J.P. Gill: Trans. Am. Soc. Met., 1936, vol. 24, p. 736. 2. C.J. McHargue, J.P. Hammond, and C.S. Grouse: Trans. Am. Soc. Met., 1954, vol. 46, p. 716. 3. D.J. Blickwede, M. Cohen, and C. Roberts: Trans. Am. Soc. Met., 1950, vol. 42, p. 1161. 4. Yu.A. Geller: Instrumentalnye Stali, Metallurgizdat, Moskva, 1955. 5. Z. Bojarski: Prace Instytutu Metalurgii Zelaza, Gliwice, 1969, No. 12, p. 263. METALLURGICAL AND MATERIALS TRANSACTIONS A
6. T. Nykiel and T. Hryniewicz: Proc. 6th Int. Seminar IFHT, Kyongju TEMF Hotel, Kyongju, Korea, Oct. 15–18, 1997, pp. 222-29. 7. Metals Handbook, 10th ed., ASM INTERNATIONAL, Materials Park,
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