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Grain Size Dependence of Austenite Decomposition in Air-Cooled 16MnCr5 Steel
SEOK-JAE LEE and CHESTER J. VAN TYNE
A hot-rolled and controlled rolled 16MnCr5 steel was analyzed after similar industrial cooling conditions. The hot rolled steel had a ferritebainite microstructure whereas the controlled rolled steel had a ferritepearlite micro-structure. The prior austenite grain size was found to be the controlling factor based on a cooling analysis. The eect of prior austenite grain size on the bainite start temperature had to be considered in the transformation model.
DOI: 10.1007/s11661-013-1706-y The Minerals, Metals & Materials Society and ASM
International 2013
The various phases or microstructures produced from austenite decomposition in carbon and low-alloy steels during cooling depend on various factors.[1,2] The
primary factor that controls the nal microstructure is generally considered to be the cooling rate from a high temperature, where austenite exists as a stable phase, to room temperature. The diusion-controlled transformation, e.g., polygonal ferrite, occurs at a relatively high temperature near the Ae3 temperature during cooling, while the diusionless transformation, e.g., martensite, occurs at a low temperature, where the diusion of substitutional alloying elements is dicult. A slow cooling rate increases the volume fraction of high-temperature products, such as ferrite and pearlite. A cooling rate that is rapid enough to avoid the high-temperature products created by diusional transformation causes austenite to decompose into bainite and/ or martensite at lower temperatures.
The second factor that aects the phase transformation is the chemical composition of the steel itself. The phase transformations from the austenite decomposition during cooling can be delayed to lower temperatures by lowering the equilibrium phase transformation temperatures through the addition of the austenite-stabilizing alloying elements (e.g., Mn, Ni, etc.). Such elements expand the austenite stable region in the Fe-C phase diagram. In an extreme case, austenite can exist at room
temperature as a stable phase (as in TWIP steels) due to a high Mn content. In contrast, the addition of the ferrite-stabilizing alloying elements (e.g., Si, Cr, etc.) reduces the austenite stable region, resulting in an increase in the temperatures for the austenite decomposition reactions.
The grain size of austenite also plays an important role in the phase transformation from austenite decomposition during cooling. Many investigations have reported the eect of austenite grain size...