Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe -C and Fe-C-Cu experimental steels have bern examined after isothermal tran sformation in a range just beneath the eutectoid temperature. Particular at tention was paid to the formation of so-called "abnormal ferrite", which re fers to coarse ferrite grains which can form, in hyper-eutectoid compositio ns, on the pro-eutectoid cementite before the pearlite reaction occurs. Thu s it is confirmed that the abnormal ferrite is not a result of pearlite coa rsening, hut of austenite decomposition before the conditions for coupled g rowth of pearlite are established. The abnormal ferrite formed on both allo triomorphic and Widmanstatten forms of pro-eutectoid cementite, and, signif icantly, it was observed that the pro-eutectoid cementite continued to grow , despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eu tectoid cementite to thicken concurrently with the abnormal ferrite is pres ented. The orientation relationships between the abnormal ferrite and pro-e utectoid cementite were also determined and found to be close to those whic h have been reported between pearlitic ferrite and pearlitic cementite. (C) 2000 Ada Metallurgica Inc. Published by Elsevier Science Ltd. All rights r eserved.