The Influences Of Cooling Speed On Growth Behaviors Of Proeutectoid Ferrite In Low Carbon Steels

The mechanical properties are directly determined by the shapes and distributions ofproeutectoid ferrites, which are decided by the growth process of proeutectoid ferrites. Thestudies about the growth process of proeutectoid ferrites are less because of the limitations ofthe research tools. In this paper, the growth process of proeutectoid ferrites are in-situobserved and dynamically tracked by using the high-temperature confocal laser scanningmicroscope (HTCSLM) during continuous cooling process of Fe-0.08%C steel, and thechanges of the growth ways, growth directions and growth speeds are researched. The phaserelationships between grain boundary ferrite and widmanstatten ferrite, betweenwidmanstatten ferrites are studied applying the electron backscatter diffraction (EBSD)technique. The influences of cooling speeds on the pro-eutectoid transition temperature andchanges quantity are analyzed adopting the thermal expansion analyser. The hardness of thesamples is measured under different cooling conditions. The results show that:(1) The proeutectoid ferrites mainly grow in some ways including "polygonal","lath","needle","cellular" and "dendritic" during the continuous cooling of low carbon steel, andthere exists "interlocking" phenomenon during the growth process of acicular ferrites. Theferrites grow in the fashion of "polygonal" and "lath" under slower cooling, whereas the"needle","cellular" and "dendritic" ferrites increases with the enhancement of the coolingspeeds.(2) The growth directions of acicular ferrites will be deflected during the growth process.The deflection manners include one deflection, continuous twice deflections, batchdeflections and branching. Through calculating the growth speeds before and after thedeflection, it is found that the growth speed after the deflection is obviously increased thanthat of before the deflection, then the growth speed is rapidly decreased.(3) One side of the grain boundary ferrite of lath-shaped is relatively flat, clear andcontinuous, the other side of the interface is roughly sketched by the acicular ferrites. Thegrowth speed of clear interface side extremely slow, but the moving speed of the unsharpinterface side is faster during the phase transition. The phase transition is mainly completedby the unsharp interface. The faster the cooling rate is, the more narrow the width of grainboundary ferrite is.(4) It is found that the growth speeds of proeutectoid ferrites increase with theenhancement of cooling speeds through counting the average growth speeds under differentcooling speeds. The relationship between cooling speed and growth speed is determined bythe method of fitting.(5) The onset temperature of proeutectoid ferrite transition is decreased with increasingthe cooling speed. The reduce amplitude is gradually decreased. EBSD analysis shows thatsmall-angle grain boundaries is increased and large-angle grain boundary is reduced with increasing the cooling speeds. There exists5°～10°phase difference between between grainboundary ferrite and widmanstatten ferrite. Ferrites is not completely parallel, and there alsoexists the phase difference among ferrites and the phase difference is less than12°.