Extremely high temperature carbide precipitation induced intragranular acicular ferrite transformation of M2 steel during semi-solid cooling

摘要

Intragranular acicular ferrite (IAF) transformation of M2 steel occurred at extremely high temperature (1180 similar to 960 degrees C) during semi-solid cooling. The uncommon IAF transformation was observed in situ via confocal laser scanning microscope. Small VC carbide particles precipitated explosively during semi-solid cooling and they acted as the preferential site for the nucleation of IAF due to the low lattice misfit with ferrite. The length of ferrite lath showed a bimodal behavior that long primary ferrite laths and short secondary ferrite laths both existed. The acicular laths intersected with each other and terminated at the solid boundary or interface. The prior solid austenite grain was filled with acicular ferrite. The IAF might help refine the effective grain size and improve the strength of semi-solid M2 steel.