Investigation of Peritectic Behavior of Steel Using a Thermal Analysis Technique
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DUCTION
DURING solidification, the peritectic reaction can occur in a wide variety of materials, including ferrous and non-ferrous alloys.[1–10] The term peritectic reaction is derived from the fact that during solidification a secondary phase grows peripherally on the primary phase. This distinct solidification phenomenon occurs at certain composition ranges where both the primary and secondary phases coexist for some time. The difference between the thermal coefficient of contraction of the primary and secondary phases makes peritectic steels challenging to cast. In Fe-C alloys, the peritectic reaction occurs over a carbon range of 0.09 to 0.53 pct.[11,12] These two extreme ends of the peritectic range are referred to as the hypoperitectic and hyperperitectic points, respectively. However, peritectic
SUNDAY ABRAHAM and RICK BODNAR are with the SSAB Americas, 1755 Bill Sharp Boulevard, Muscatine, IA 52761. Contact e-mail: [email protected] JOHAN LONNQVIST and FATEMEH SHAHBAZIAN are with the Swerim, Isafjordsgatan 28A, 164 40 Kista, Sweden. ANDERS LAGERSTEDT and MAGNUS ANDERSSON are with the SSAB Oxelo¨sund AB, 80 Oxelo¨sund SE613, Oxelo¨sund, Sweden. Manuscript submitted September 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
reactions are described by some researchers as occurring mostly between a tighter range of carbon contents, for example, 0.080 to 0.180 pct. The hypoperitectic and hyperperitectic points for given steel will change depending on the concentrations of other dissolved solute elements. In the ‘‘peritectic’’ range, once a certain temperature is reached, part of the primary phase (delta ferrite) that precipitated from the liquid reacts with the residual melt to form a secondary phase (austenite). The austenite precipitates around the remaining delta ferrite, preventing its access to the remaining liquid. The austenite then begins to grow inward into the delta ferrite and outward into the liquid. The transformation to austenite takes some time to finish and hence, the peritectic reaction itself refers to the initial precipitation of austenite on the periphery of the delta ferrite. The gradual transformation of delta ferrite to austenite at lower temperatures is referred to as a peritectic transformation.[13] The peritectic reaction and transformation during solidification are illustrated in Figure 1.[14–16] It should be noted that in certain cases, the secondary phase, austenite, may nucleate in the liquid in lieu of on existing primary phase, delta ferrite. Due to the tighter atomic packing of the austenite (FCC structure) compared to the delta ferrite (BCC structure) during the peritectic reaction, the volume of austenite contracts more than the delta ferrite.[17] This difference in the thermal contraction of the two phases
makes peritectic steel prone to cracking, particularly longitudinal cracking during solidification.[18] As a result, some steelmakers attempt to avoid casting peritectic steels, and others take steps to avoid peritectic-related defects in the as-cast product. These measur
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