A Study on the Role of Mn in the Isothermal Pearlite Transformation

The rate of growth and the interlamellar spacing of the isothermally transformed pearlite in a high purity eutectoid carbon steel and in two commercial manganese steels were measured, and the process of the pearlite growth was studied from the points of view of the carbon diffusion controlled mechanism and the interface migration mechanism. Referring to the reliable data of several experiments carried out by other writers, the following results were obtained:(1) It is reasonable to think that the rate controlling process of pearlite growth in the high purity eutectoid carbon steel is the diffusion of carbon in austenite along the interface between austenite and pearlite.(2) In the high purity eutectoid carbon steel, the interlamellar spacing measured shows a value ten times greater than the minimum interlamellar spacing calculated from the thermodynamical limit in the pearlite transformation. The interlamellar spacing which corresponds to the maximum growth rate of pearlite is two times greater than the minimum interlamellar spacing, and it does not agree with the measured spacing. A relation between the measured growth rate (r) and the calculated maximum growth rate of pearlite (rp) is given by r=0.36rp.(3) The growth rate of pearlite is decreased considerably by the addition of manganese. This phenomenon may in part be attributed to the decrease of ΔF0 with the increase in manganese content.(4) The interlamellar spacing of pearlite is scarcely influenced by the addition of less than about 1 weight percent manganese.(5) In the temperature range of the non-partitioned pearlite, it is considered that the rate controlling process of pearlite growth in the steel containing less than about 0.5 weight percent manganese is similar to that of high purity eutectoid carbon steel, but with more than about 0.5 weight percent manganese, the steel shows the austenite-pearlite interface migration mechanism; namely, its rate is controlled by the short range diffusion of iron atoms at the austenite-pearlite interface.