Abstract
The mechanisms behind the carbon enrichment of austenite during quenching and partitioning are still a matter of debate. This work investigates the microstructural evolution during the quenching and partitioning of a model Fe–C–Mn–Si alloy by means of in situ high energy X-ray diffraction (HEXRD) atom probe tomography, and image analysis. The ultra-fast time-resolved quantitative information about phase transformations coupled with image analysis highlights the formation of carbide-free BCT bainite, which is formed within a very short range during the reheating and partitioning step. Its transformation rate, which is a better indicator than the intrinsic volume fraction, depends on the quenching temperature (QT). It is shown to decrease with decreasing QT, from 45% at QT = 260 °C to 20% at QT = 200 °C. As a consequence, a significant part of the carbon enrichment observed in austenite can be attributed to bainite transformation. Furthermore, a large part of carbon was shown to be trapped into martensite. Both the formation of Fe2.6C iron carbides and the segregation of carbon on lath boundaries in martensite were highlighted by atom probe tomography. The energy for carbon segregation was determined to be 0.20 eV, and the carbon concentration on the lath boundaries was obtained to be around 25 at %. Therefore, the carbon enrichment of austenite is the result of competitive reactions such as carbon partitioning from martensite, bainite transformation, and carbon trapping in martensite.
Highlights
The need to improve fuel efficiency and safety has led to a high and growing demand for high-strength steels in the automotive industry [1]
From a kinetics point of view, it has been suggested that the temperature is too low for carbon diffusion and that carbon supersaturation in martensite can be eliminated by carbides precipitation during the partitioning step [3,11]
A large amount of carbon can be trapped into dislocations and/or lath boundaries and, as a consequence, it may limit the total amount of carbon partitioning from martensite to austenite
Summary
The need to improve fuel efficiency and safety has led to a high and growing demand for high-strength steels in the automotive industry [1]. The benefits of such treatment in terms of improved mechanical properties have been clearly demonstrated [3,4]. It depends strongly on the austenite stability and on the level of carbon enrichment in austenite during the partitioning step. The formation of bainite during partitioning cannot be completely ruled out and could explain the carbon enrichment measured in retained austenite, as the temperatures are consistent with those for bainite formation [7,10]. A large amount of carbon can be trapped into dislocations and/or lath boundaries and, as a consequence, it may limit the total amount of carbon partitioning from martensite to austenite
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