NMC2006 Accepted 28/11/2006
MICROSTRUCTURAL MAPPING OF HADFIELD MANGANESE STEEL-3401 IN AGING TREATMENT
Introduction
During the late of 1920s, in the course of pioneering studies on the isothermal transformation of austenite at temperatures above that at which martensite first forms, but below that at which fine pearlite is found, Davenport and Bain (1939) discovered a new microstructure consisting of an ‘acicular, dark etching aggregate’ which was ‘Bainite’ in honour of their colleague E. C. Bain. The high-range and low-range variants of bainite were later called ‘upper bainite’ and ‘lower bainite’ respectively (Mehl, 1939) and this terminology remains useful. Both upper and lower bainite were found to consist of aggregates of parallel plates, aggregates which were later designated sheaves of bainite (Aaronson and Wells, 1956). isothermal transformation experiments led to the clarification of microstructures, describes as massive ferrite, grain boundary ferrite, acicular ferrite, Widmannstätten ferrite, etc. High–carbon steels can sometimes transform to plates of lower bainite which do not have a homogeneous microstructure (Okamoto and Oka, 1986). Bainite grows in the form of clusters of thin lenticular plates or laths, known as sheaves. The observed characteristics of bainitic ferrite proved that it grows by a displacive transformation mechanism.
Results and Discussion
The ensuing microstructures obtained are shown in figs 1a-g. Hadfield’s manganese steel with a composition of Fe-1.2%C and 13%Mn has a structure of metastable austenite phase after water-quenching after an annealing temperature of 1050ºC. It results in the solid solution of carbides the production of almost pure austenite. The original manufactures of the steel called this treatment “water toughening’. It results in the solid solution of carbides causing brittleness and the production of almost pure austenite. The austenite grain boundaries are well defined and of approximately uniform thickness. has also reported that the Hadfield’s manganese steel with a composition of Fe-1.2%C and 13%Mn, normally has a structure of metastable austenite phase which is obtained by water-quenching the steel from annealing temperature of 1050ºC. It results in the solid solution of carbides causing brittleness and the production of almost pure austenite.When aged, partial decomposition of the austenite occurs. The extent of this decomposition depends on the time and temperature of the tempering treatment. The austenite grain boundaries are well defined and of approximately uniform thickness. As the temperature and holding time is increased, several microstructures developed; an appearance of bainite in austenite structure, then massive ferrite and grain boundary ferrite. The nomenclatures become confused since the ferrite which formed first was variously describes as massive ferrite, grain boundary ferrite, acicular ferrite, Widmannstätten ferrite, etc. Some of these micro constituents are formed by a ‘displacive’ or ‘military’ transfer of the iron and substitutional solute atoms from austenite to ferrite, and are thus similar to carbon-free bainitic ferrite, whereas others form by a ‘reconstructive’ or ‘civilian’ transformation
