Formation of grain boundary ferrite in eutectoid and hypereutectoid pearlitic steels

Abstract

Formation of grain boundary ferrite (GB-α) in high carbon pearlitic steels deteriorates ductility of steel wires. Since it is generally believed that fully pearlitic structure forms in eutectoid steels, GB-α formation in eutectoid or even hypereutectoid steels is curious. Therefore, effects of temperature and carbon content on the formation of GB-α in Fe-1mass%Mn-(0.75 and 1.05) mass%C alloys transformed isothermally at temperatures ranging from 873 K (600 °C) to 973 K (700 °C) were investigated to clarify the formation mechanism of GB-α. It was found that volume fraction of GB-α increases with decreasing transformation temperature, carbon content and prior austenite grain size. Pearlite nucleated at prior austenite grain boundary usually grows into only one of austenite grains separated by the grain boundary, and GB-α forms on the other austenite grain. Orientation analyses revealed that GB-α and pearlitic-α hold the same orientation, indicating that one α grain grows as pearlite into one of austenite grains, and as GB-α into the other austenite grain. It was shown that such morphology difference is caused by the difference in orientation relationship (OR) between ferrite and austenite such that near K–S OR and non K–S OR correspond to GB-α and pearlite, respectively. Consequently, it was proposed that suppression of cementite nucleation at ferrite/austenite boundary holding near K–S OR is a reason for the formation of GB-α in the transformation at low temperature. Furthermore, degenerated pearlite (DP) and Widmanstatten ferrite (WF) is formed as well as GB-α at lower temperature, and OR dependency of GB-α, WF, DP and pearlite were clarified.