Pearlite growth kinetics in Fe-C-Mn eutectoid steels

Fe-C-Mn共析鋼のパーライト成長速度

Zhang, Y.*; 梅田 岳昌*; 諸岡 聡  ; 宮本 吾郎*; 古原 忠*

Zhang, Y.*; Umeda, Takemasa*; Morooka, Satoshi; Miyamoto, Goro*; Furuhara, Tadashi*

Essential understanding of the pearlite growth kinetics is of great significance to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, a series of eutectoid steels with Mn addition up to 2mass% were isothermally transformed at a temperature range from 873K to 973K to investigate the growth kinetics and the underlying thermodynamics at the migrating interface during pearlite transformation. The microscopic observation revealed that the pearlite growth rate in each alloy becomes increased while the lamellar spacing becomes decreased by lowering the transformation temperature. Mn addition decelerates the growth rate, accompanied by a relatively wider lamellar spacing at each temperature. After analyzing the element distribution in the vicinity of migrating austenite/pearlite interface via three-dimensional atom probe, Mn was found to be enriched at the austenite/pearlitic ferrite interface, whereas the Mn partitioning among the three phases is negligibly small in the 2mass% Mn added alloy isothermally transformed at 873K. Based on the estimation of energy dissipated by various factors, the driving force for pearlite transformation in the Mn-free alloy was found to be consumed by interface friction, carbon partitioning and ferrite/cementite interfacial energy, whereas neutron diffraction analysis indicated that the influence of transformation strain is relatively small. The retardation effects of pearlite growth kinetics in the Mn-added alloy, which is partly due to the reduced driving force for pearlite transformation, can be well explained by further considering the energy dissipation caused by solute drag effects of Mn.