A Study of the Divorced Eutectoid Transformation, DET, in Eutectoid Composition Plain Carbon Steel

Experiments on the divorced eutectoid transformation, DET, in eutectoid steel have produced the following conclusions. The maximum rate of continuous cooled steel that produces the DET rather than the pearlite eutectoid transformation, PET, is 0.7 ± 0.4 °C/s. The product microstructure of the DET is not just spheroidite but rather regions of three different microconstituents which will be labeled F, L and S. The F regions consist of pure ferrite, L regions, a coarse lamellar pearlite and S regions spheroidite. The product of the DET is strongly influenced by the initial microstructure of the steel; starting with spheroidite generally produces a spheroidite product while starting with pearlite produces the F, L, S product. In the L regions, the pearlite-like structures are growing into a ferrite matrix; experiments show that the pearlite growing into austenite at 700 °C is finer and harder than that growing into the ferrite during a DET at 700 °C Experiments were done on commercial steels containing levels of carbide forming elements of 900 ppmw. Similar experiments on a high-purity steel found a significant increase in the arrest temperature of the DET and decrease in hardness of the DET product; this result is postulated to be due to a combination of reduced amounts of carbide forming elements as well as S and P in the high-purity steel. Finally, hypotheses are presented for the cause of these results along with possible models.