Partition and non-partition transition of austenite growth from a ferrite and cementite mixture in hypo- and hypereutectoid Fe-C-Mn alloys

Abstract The growth of austenite from a ferrite and cementite mixture in low Mn steel of hypo- and hypereutectoid composition is investigated with focus upon the Mn partitioning between dissolving cementite (or ferrite) and austenite. Under the assumption that austenite is nucleated on cementite, two critical temperatures which characterize the transition between Mn-partitioned and non-partitioned growth of austenite are noticed; below the 1st and lower critical temperature the austenite grows with redistribution of Mn from the beginning, and above the 2nd and higher critical temperature, without Mn redistribution until completion. Between them the growth mode switches from carbon-diffusion to Mn-diffusion control during growth. The influence of carbon and/or Mn diffusion through the matrix becomes progressively more significant with time, but may not affect the growth mode transition temperatures. Above the 2nd critical temperature, which is at most ca. 50 °C higher than Acm or Ae3 in alloys studied, the distribution of Mn in as-transformed or spheroidized pearlite is preserved at the completion of austenitization irrespective of the last dissolving phase, leading to the formation of an ultrafine mixture of martensite and austenite upon quenching.