Abstract
Bainitic microstructures obtained in high-carbon (HC) and high-silicon (HSi) steels are currently of great interest. Microstructural evolution and the bainitic transformation kinetics of a high-carbon and high-silicon cast steel held at 280, 330, and 380 °C was analyzed using dilatometry, X-ray diffraction, optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). It is shown that the heterogeneous distribution of silicon (Si), manganese (Mn), and chromium (Cr) associated to microsegregation during casting has a great impact on the final microstructure. The transformation starts in the dendritic zones where there is a lower Mn concentration and then expands to the interdendritic ones. As Mn reduces the carbon activity, the interdendritic areas with a higher Mn concentration are enriched with carbon (C), and thus, these zones contain a greater amount of retained austenite plus martensite, resulting in a heterogeneous microstructure. Higher transformation temperatures promote higher amounts of residual austenite with poor thermal/mechanical stability and the presence of martensite in the final microstructure, which has a detrimental effect on the mechanical properties. Tensile tests revealed that the ultra-fine microstructure developed by the transformation at 280 °C promotes very high values of both tensile and yield stress (≈1.8 GPa and 1.6 GPa, respectively), but limited ductility (≈2%).
Highlights
Nowadays, metal casting is widely used to produce high-performance parts or components [1,2]
Microsegregation occurs during solidification due to different partition coefficients of alloying elements in solid and liquid phases, and it produces a heterogeneous distribution of solute at the dendritic scale
In the case of the solidification of steels in sand molds, microsegregation can be a problem, especially when the cast parts need to be subjected to heat treatments to achieve a desired final microstructure
Summary
Metal casting is widely used to produce high-performance parts or components [1,2]. It is widely recognized that the microstructures and mechanical properties of castings differ from those obtained on rolled or forged parts of the same chemical composition, which is mainly due to the typical casting defects (non-metallic inclusions, porosity, voids, micro-shrinkage, etc.) and to the macro and microsegregation of alloying elements present in the structure. This point has not been reported yet, and it is considered of utmost importance when considering the production of castings with carbide-free bainitic microstructures This work addresses this subject and continues this line of research, evaluating the effect of the microsegregation on the bainite transformation kinetics, microstructures, and mechanical properties for this type of steel.
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