Can Pearlite form Outside of the Hultgren Extrapolation of the Ae3 and Acm Phase Boundaries?
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E best-known eutectoid reaction is the decomposition of austenite (c) into ferrite (a) and cementite (h) in steels. Pearlite is the dominant decomposition product in eutectoid and near-eutectoid carbon and low-alloy steels that are cooled at slow-to-moderate rates or are isothermally transformed above about 823 K (550 C). It is readily recognized due to the regularity of the alternating lamellae of a and h, which can be coarse enough to be resolved by light optical microscopy.[1–4] The various aspects of ferrous pearlite have been reviewed.[5–11] The occurrence of pearlite in off-eutectoid compositions is an interesting problem from both practical and fundamental perspectives. The competition between pearlite and proeutectoid constituents for a share of the final microstructure is a major consideration in
M.M. ARANDA and R. REMENTERIA Ph.D. Students, and C. CAPDEVILA, Senior Scientist, are with the Materalia Research Group, Centro Nacional de Investigaciones Metalu´rgicas (CENIM), Consejo Superior Investigaciones Cientı´ ficas (CSIC), Madrid, Spain. Contact e-mail: [email protected] R.E. HACKENBERG, Permanent-Staff R&D Scientist, is with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM. Manuscript submitted August 26, 2014. Article published online November 23, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
materials and process design. For example, there is interest in obtaining fully pearlitic structures in hypereutectoid steels of ever-higher carbon contents,[12–14] with the intention of maximizing the strengthening increment from pearlite while avoiding brittle, continuous-network morphologies of proeutectoid cementite. In hypoeutectoid steels, the formation of acicular, bainitic, and possibly martensitic constituents must be considered as competitors to allotriomorphic (polygonal) ferrite and pearlite when designing thermomechanical processing and welding schedules. Control of austenite grain size, hardenability, and carbon equivalent (for welding) becomes quite important.[15–17] At a minimum, pearlite can form only within the 2-phase a + h field below the Ae1 temperature, 1000 K (727 C) in the Fe-C binary. However, a more restrictive condition has long assumed[18–20] that pearlite can form only when average composition of austenite C0 lies within a window of composition-temperature space enclosed by the extrapolated Ae3 (c/a) and Acm (c/h) phase boundaries, often called the ‘‘Hultgren extrapolation.’’[21–23] The austenite compositions within this region are supersaturated with respect to both a and h. These supersaturations are with respect to the formation of a and h separately from one another, i.e., within the framework of c + a and c + h phase fields extending independently of each other, in these respective metastable domains below the eutectoid temperature. This assumed condition for pearlite formation will be termed the ‘‘mutual supersaturation’’ criterion. This is assumed to apply irrespective of whether pearlite is the first product to form or whether it forms
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