Evolution of Pearlite Microstructure in Low-Carbon Cast Microalloyed Steel Due to the Addition of La and Ce
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f the common air-cooled cast structural steels have a mixed ferrite-pearlite microstructure.[1] The formation of pearlite is a process during which alternate layers of ferrite and cementite develop. The neighboring ferrite and cementite lamellae grow parallel to each other sharing a transformation front with austenite. As the ferrite grows, the excess carbon partitions into the austenite which will be absorbed by the adjacent cementite and vise versa. This cooperative growth creates a colony. As the colony grows, new lamella may nucleate from its sides and take a different growth direction. In this way, new colonies with different lamellae orientation from the adjacent colonies are generated, and eventually a larger reacted volume
H. TORKAMANI, SH. RAYGAN, and J. RASSIZADEHGHANI are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, 111554563, Iran. Contact e-mails: [email protected], [email protected] C. GARCIAMATEO and D. SAN-MARTIN are with the Materalia Research Group, National Center for Metallurgical Research (CENIM-CSIC), Avda Gregorio del Amo, 8, 28040, Madrid, Spain. Contact e-mail: [email protected] Y. PALIZDAR is with the Materials and Energy Research Center, Karaj, 3177983634, Iran. Manuscript submitted November 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
known as a pearlite nodule is formed. These constituents are illustrated in Figure 1.[2–5] It is generally agreed that, in hypoeutectoid steels, the presence of pearlite rather than proeutectoid ferrite is beneficial for increasing hardness/tensile strength but harmful to the toughness.[1,7–9] In eutectoid/fully pearlitic steels, the yield strength has been reported to follow a Hall–Petch type of relationship with respect to the interlamellar spacing,[1,10] which has been explained on the basis of a dislocation pile-up model.[9,11] In low-carbon steels, where pearlite coexists with ferrite in the microstructure, other pearlite features rather than interlamellar spacing would also affect the properties. For instance, the contribution of pearlite volume fraction to the strength is considered to obey the law of mixtures.[7,10,12] The nodule and colony size would also contribute to the strength of this type of steels according to the Hall–Petch relation.[1,13] It is also reported that the impact transition temperature decreases with a finer colony size.[12] Ray and Mondal[7] have also discussed that the strength of hypoeutectoid steels would be affected by the characteristics of proeutectoid ferrite. It is well known that the grain refinement of ferrite improves the strength and toughness at the same time.[1,10] In polycrystalline steels, pearlite formation is more likely to take place at the grain boundaries or at the junctions between several grains.[14] In hypoeutectoid
Fig. 1—Schematic illustration of the various constituents of pearlite.[6] Reprinted from Materials Science and Engineering A, Vol. 404, A.M. Elwazri, P. Wanjara, and S. Yue. The effect of microstructural characteristics of
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