Microalloyed low-carbon steels strengthened by vanadium carbide (VC) nanoprecipitates are receiving increasing attention, particularly in the automotive industry. A clear understanding of the nanoprecipitate chemistry is essential for optimizing the alloy composition and processing routes, thereby enhancing the mechanical properties of such advanced steels. The chemical evolution of VC precipitates, especially regarding the incorporation of iron into the nanoprecipitates, remains uncertain. Here, a model vanadium-microalloyed low-carbon steel is studied by atomic-resolution scanning transmission electron microscopy (STEM) techniques. The steel contains nanoscale VC precipitates formed either as interphase precipitates (IP) at the austenite/ferrite interface during the austenite-to-ferrite phase transformation, or as randomly distributed precipitates (RP) in the ferrite matrix during bainite tempering. The first-time observation of carbon sublattice atoms in VC is achieved using integrated differential phase-contrast STEM (iDPC-STEM). Non-equilibrium compositions are identified under both precipitation mechanisms, with no correlation between precipitate size and associated elemental contents. Most interphase VC nanoprecipitates contain higher amounts of not only iron but also manganese compared to random VC nanoprecipitates. Complementary ex-situ small-angle neutron scattering (SANS) analysis and solute-drag effect (SDE) modeling support the co-segregation of iron and manganese into the precipitates. Manganese typically appears to form a core–shell-like structure within VC. Experimental evidence is presented for the SDE-assisted formation of manganese-rich–core (fibrous) interphase VC precipitates, and a mechanism is proposed for iron–manganese co-enrichment in random VC precipitates. This study offers new insights into future strategies to tune nanoprecipitate chemistry in microalloyed steels.

The influence of carbon concentration variations on pearlite formation (20 h at 600 °C) in a case-carburized steel is investigated. The resultant microstructure shows three distinct regions: carburized case, a transition region, and the original core. The microstructural transition from the case to the core regions is observed to be relatively sharp. The investigated region of the carburized case (0.9 wt.% C) contains two types of pearlite: ferrite + cementite and ferrite + M₂₃C₆, where the pearlitic aggregate with M₂₃C₆ shows faster formation kinetics. The kinetics of pearlite formation in the transition region (0.3 wt.% C) is very slow and is observed with only M₂₃C₆ carbide. Only around 40% austenite decomposes into pearlite in the transition region, which, in comparison to the carburized case region of 0.9 wt.% C is a fraction that is lower by a factor of two. Pearlite is absent in the investigated core region (0.16 wt.% C). The microstructure in this region is predominantly martensite and pro-eutectoid ferrite, with a fraction of ferrite well below the equilibrium fraction. Ferrite formation in this region is limited by the redistribution of mainly Ni, Mn, and Cr, and their resulting solute drag effect on the austenite/ferrite interface. A thermodynamic and kinetic argumentation of these observations is provided with the help of thermodynamic data, precipitation simulations, and a general mixed-mode Gibbs energy balance model.

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

The influence of soaking and cooling rates on the final microstructure of a R260Mn pearlitic railway steel subjected to fast heating is investigated. Fast-heating experiments followed or not by soaking and cooling at different rates were performed using quenching dilatometry on railway steel specimens obtained from the rail head. Additional cyclic heating and quenching experiments were done to investigate the evolution of the microstructure during thermal cycling, which is relevant for railway applications. The final microstructure is characterized via microhardness measurements, optical microscopy and scanning electron microscopy. The microstructural features are distinguished and the influence of each condition is detailed. The study allows the construction of transformation diagrams during cooling after fast heating. Furthermore, comparison between the final microstructures obtained in controlled laboratory conditions and field White Etching Layers is presented. The obtained results can serve as a guideline for future reproduction of White Etching Layers in laboratory conditions and interpretation of field conditions.

In this research, in situ high-temperature electron backscattered diffraction (EBSD) mapping is applied to record and analyze the migration of the α/3 interfaces during cyclic austenite-ferrite phase transformations in a medium manganese steel. The experimental study is supplemented with related 3D phase field (PF) simulations to better understand the 2D EBSD observations in the context of the 3D transformation events taking place below the surface. The in situ EBSD observations and PF simulations show an overall transformation behavior qualitatively similar to that measured in dilatometry. The behavior and kinetics of individual austenite-ferrite interfaces during the transformation is found to have a wide scatter around the average interface behavior deduced on the basis of the dilatometric measurements. The trajectories of selected characteristic interfaces are analyzed in detail and yield insight into the effect of local conditions in the vicinity of interfaces on their motion, as well as the misguiding effects of 2D observations of processes taking place in 3D.

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.