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2009 MAGNETO-SCIENCEChanges in the microstructure resulting from a high cooling rate in Fe-xC-Mnalloys in strong magnetic fieldThe impact of magnetic field on the microstructures obtainedafter rapid cooling is observed in Fe-xC-1.5wt%Mnsteels with x = 0.2 and 0.3 wt%C. The experiments aredone in a furnace in which alloys are water cooled in magneticfield from the austenite state [T.Garcin et al., Patent955380, 2009]. After holding at 1173K for 5min, the poweris switched off and the sample is quenched. More than200 K/s is obtained by water quenching between 1173 Kand 373 K. The Figure show the SEM micrographs afterquenching under 0T and 16T.These modifications in the resulting microstructure by theapplication of a static magnetic field may be interestingwhen a mixture of ferrite, bainite and martensite is desired.Figure 1(a) shows the 0.2wt%C steel grades cooled under0T and 16T. In the 0T sample, the microstructure is composedof a mixture of bainite (α B ) and martensite(α’). Bainiteis a two-phase structure that contains ferrite-cementiteregions formed from the eutectoid decomposition of austenite(γ). Martensite is a product of diffusionless transformationand can occur in the form of thin, lenticular plateswhich often extend right across the parent gamma grains, oras packets of approximately parallel, fine laths whose sizeis generally smaller than that of the γ grains. Only severalislands of allotriomorphic ferrite are observed in the structure.The allotriomorphic ferrite (α) grows rapidly alongthe austenite grain boundary (which is an easy diffusionpath) but thickens more slowly. Application of 16T leads toa considerable increase in the allotriomorphic ferrite fractionalong the prior austenite grains boundaries. In addition,a large amount of Widmansttten ferrite (α W ) is detected bythe presence of thin-wedge plates which have grown fromthe prior austenite grain boundaries.Figure 1(b) shows the 0.3wt%C steel grades cooled under0T and 16T. In the 0T sample the microstructure iscomposed of a mixture of bainite and martensite. No allotriomorphicferrite is observed probably due to the relativelyhigh carbon content in alloy. When the transformationoccurs under 16T, the microstructure is predominantlymartensite but also has allotriomorphic ferrite, Widmanstttenferrite, bainite and even pearlite. The martensite structurecan be here referred to as lath martensite which consistsof martensite needles with different but well-definedorientations. Notice that the spherical shape of pearlitecolonies is obvious in this sample because of the lack ofimpingement with other pearlite colonies. Vickers hardnessmeasurements have been performed for the different alloyscomposition after thermomagnetic treatment. The applicationof a strong magnetic field results in a general decreasein the hardness values. This reduction of hardness is relatedto the enhancement of the ferrite precipitation in the alloystreated under static magnetic field.Figure 151: SEM micrographs showing the structure after waterquenching under 0 and 16 T for the Fe-0.2C-1.5Mn (a) and forFe-0.3C-1.5Mn (b)(wt%). Original magnification 4000x, 4% nitaletching sample.F. DebrayT. Garcin, S. Rivoirard, R. Morgunov, E. Beaugnon, (CRETA, CNRS, Grenoble, France)107