Transformation Characteristics of Ferrite/Carbide Aggregate in Continuously Cooled, Low Carbon-Manganese Steels
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PROCESS heat treatments for carbon and low alloy steels generally involve continuous cooling. Therefore, phases do not always form sequentially but rather can evolve simultaneously and under the influence of local temperature fluctuations. The nature of the phases formed can be difficult to identify. Microstructure classification schemes involving optical microscopy have been developed historically to facilitate this activity. Dube´ et al.[1] and Aaronson[2] first characterized ferrite morphologies in isothermally transformed steels, but recognized that continuous cooling could render distinguishing morphological features indistinct. Allotriomorphic ferrite and various sideplate morphologies (often classed as bainite) were readily identified, but Widmansta¨tten ferrite was difficult to place and was regarded as a generically similar structure to bainite. A comprehensive classification scheme for weld metals (International Institute of Welding (IIW)) was devised in the 1980s.[3] This scheme was applicable to weld metals where intragranular transformations dominate, but it could also be used for steels where austenite grain boundary transformations dominate. Ferrite sideplate and acicular ferrite were readily identified using the scheme, but associated principal structures e.g., Widmansta¨tten ferrite or bainite were difficult to define in S.F. DI MARTINO, Research Associate, is with the School of Materials, Loughborough University, Loughborough LE113TU, U.K. Contact e-mail: [email protected] G. THEWLIS, formerly Principal Metallurgist, with the Steel Metallurgy Department, CORUS, Swinden Technology Centre, Moorgate, Rotherham S60 3AR, U.K., is now retired. Manuscript submitted May 7, 2013. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
weld metals. The same was true of ferrite/carbide aggregate, where pearlite, bainite, or tempered martensite were not easy to recognize. Anelli and Di Nunzio[4] provided guidance on identifying transformation products associated with sideplate structures in steels with some success, but stereological effects i.e., the way constituents are orientated in space were not treated in depth. Thewlis[5] devised a comprehensive scheme (MiClass) for classifying and quantifying microstructure constituents and the associated principal structures in carbon and low alloy steels with the aim of aiding investigation of microstructure evolution during continuous cooling of industrial products and providing calibration data for theoretical models. Extensive evaluation exercises were carried out, which showed that a reasonable degree of consistency could be obtained between operators in identifying the principal structures primary ferrite, pearlite, and martensite, and also Widmansta¨tten ferrite and bainite constituting ferrite sideplate. The greatest discrepancy between operators occurred when attempting to determine the nature of ferrite/carbide aggregate. A major source of confusion was the presence in some steels of large grains of a dark etching phase containin
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