Characterization And Analysis Of Three Dimensional Morphology Of Intergranular And Intragranular Ferrite In Low Carbon Micro Alloyed Steel

In this paper, three-dimensional morphology of grain boundary and intragranular ferrite and the relations between intragranular ferrite and inclusions in an Fe-0.09%C-1.48%Mn alloy was revealed by means of serial sectioning in conjunction with computer-aided three-dimensional reconstruction techniques. Orientation relationships and nano hardness of allotriomorphs, idiomorphs and laths or plates nucleated at inclusions were examined by Electron Back-scattering Diffraction (EBSD) technology and nano indentation test, respectively.The characterization of three-dimensional morphology of pro-eutectoid ferrite was obtained in low carbon low alloy steels on the basis of observations of specimens transformed at various transformation temperatures for different holding times. The morphology of grain boundary ferrite allotrimorphs and primary sawteeth was quite different from that in literatures. Grain boundary ferrite allotriomorphs had a variety of morphology based on their different nucleation sites and growth mechanisms. One of which have the shape of lathy ellipsoid, and the other has the shape of prolate ellipsoid. Primary ferrite sawteeth had the shape of a long triangular prism. Primary ferrite sideplates were much smaller than that of the secondary sideplates. The morphology of ferrite allotriomorphs nucleated at grain boundary edges was a prolate ellipsoid. Results of serial sectioning and three-dimensional reconstruction showed that three-dimension morphology of allotriomorphic and Widmanst?tten ferrite could not be deduced from two-dimensional observations alone. Based on the observation and the quantitative ananlysis of the three dimensional(3D) morphologies of ferrite, the classification of 3D morphologies of grain boundary fettite was revised.The intragranular ferrite formed at low under-coolings had the shape of idiomorphs, which always consist of sub-grains with low-angle boundaries between them. At high under-coolings several laths or plates emanated from inclusions. At later stages of austenite-to-ferrite transformation austenite was sectioned by the transformed ferrite laths or plates, which resulted in grain refinement.Results of Nano-hardness indentation showed that the hardness and plastic modulus of intragranular ferrite idiomorphs formed at higher temperatures was less than that of intragranular ferrite laths or plates formed at lower temperature. Intragranular precipitates show high Nano-Hardness and Elasticity Modulus values than intergranular precipitates, regardless of transformed under high or low temperature. And the differences in dislocation densities and carbon or solute content in ferrite laths or plates may be the most possible reasons.