Nucleation during phase transformations plays an important role in the crystal structure, the grain size and the texture of the forming product phase, and thus determines the properties of the obtained material. In this study, molecular dynamics simulation is employed to study the heterogeneous nucleation of bcc-phase in fcc iron. It is found that the bcc-phase nucleates at the dislocations in the fcc/fcc grain boundaries in a pseudo-cylindrical morphology. The energy change as a function of the bcc nucleus size conforms to the Cahn's classical model with no energy barrier, and provides interface energies and elastic constants comparable to theoretical calculations and experimental data. Nevertheless, there are aspects that cannot be explained by the classical Cahn nucleation theory, namely the stepwise “fcc→intermediate→bcc” nucleation process, and the aggregation of discrete subnuclei. This noclassical nucleation processes contribute to the decrease of energy barrier and the stabilization of the bcc nucleus.

Successful implementation of third generation advanced high strength steels (3rd gen AHSS) can be accelerated by developing steels that can be heat treated in existing industrial lines. Here, we develop new carbide free bainitic (CFB) steels in which bainite formation is accelerated by a 0.2 volume fraction of prior martensite and thus can be realized in 5 min, making them suitable for manufacturing in modern continuous annealing lines for bare steel strips. The resulting microstructure consists of bainitic ferrite, tempered martensite, and retained austenite. Carbon and silicon had the most pronounced effect on the mechanical properties among the studied alloying elements (manganese, niobium, chromium, and molybdenum) because of their influence on the fraction and stability of retained austenite. Our proposed treatment, which we call bainite accelerated by martensite (BAM), showed higher strength and lower global formability than traditional CFB without prior martensite (also called TRIP-assisted bainitic ferrite, TBF) and quenched and partitioned (Q&P) steels. Five of the designed steels showed tensile strength higher than 1370 MPa, a total elongation higher than 8%, and hole expansion capacity higher than 30%, and thus meet the requirements for the strongest commercial grades of complex phase steels with improved formability. This work broadens the possibilities of using existing industrial lines for manufacturing novel 3rd gen AHSS.

This study investigates the localised corrosion mechanisms in laboratory-processed Q&P-treated martensitic stainless steels. Two steel variants, one NbTi-free (alloy B) and the other micro-alloyed with Nb and Ti (alloy M) were investigated to elucidate the influence of microalloying on corrosion behaviour. Both NbTi-free and NbTi-micro alloyed martensitic stainless steels were examined using a combination of electrochemical methods (potentiodynamic polarisation and double-loop electrochemical potentiokinetic reactivation) and microstructural analysis (Transmission Electron Microscopy and scanning Kelvin probe force microscopy). Potentiodynamic polarisation results showed no significant differences between the alloys and no clear evidence of pitting corrosion. Optical analysis of the specimens showed preferential attack at grain boundaries. Double-loop electrochemical potentiokinetic reactivation measurements revealed a higher degree of sensitisation to intergranular corrosion in the microalloyed steel compared to the NbTi-free variant. Transmission Electron Microscopy showed that intergranular corrosion in both steels originated from chromium depletion zones adjacent to chromium carbides along grain boundaries. The increased susceptibility in the microalloyed steel was linked to the presence of TiN(Nb) particles. Scanning Kelvin probe force microscopy further revealed variations in surface potential at grain boundary precipitates and depleted zones, emphasising their role in intergranular corrosion initiation. These findings emphasise the critical influence of processing routes on the corrosion mechanisms of Q&P-treated martensitic stainless steel.

In sparsely populated areas single-track railway lines are still common. Despite the low-density traffic and low axle loads, rail damage is observed to initiate in the rails of these lines. Not only large cracks requiring repair are found, but also newly initiated, post-grinding damage is observed. Rail specimens from the track containing representative damages are extracted to identify the reasons for the damage initiation in the R260Mn steel rail. At the rail surface, three years after the preventive grinding maintenance, the characteristic grinding facets and roughness patterns are still present. White etching layers are observed to surround the residual grinding grooves, maintaining the roughness as a result of the high wear resistance of these layers. The strain orientation at the gauge side of the rail is uni-directional due to lateral creep in the wheel–rail contact while in the center of the contact surface the strain patterns evidence shear stress reversal associated with the bi-directional traffic. The damage initiation mechanism after preventive grinding is associated with low-wear conditions and stresses concentrated around long-lasting grinding grooves. The findings show that preventive grinding maintenance specifications for regional single-stock railway lines must be improved. Specific points of interest are stringent requirements on the number of facets and surface roughness, as well as directions for the removal of corrugation.

Increasing train speeds and the reduction of maintenance slots places high demands on the railway rails. To meet the challenging demands, producers regularly introduce new steel types. In this experimental investigation is the mechanical behavior of an air-cooled vanadium-alloyed hypereutectoid rail steel presented. The rail is produced applying conventional hot rolling of a reheated bloom and is then cooled on a cooling bed. The mechanical behavior is determined by performing standardized linear elastic fracture mechanics tests. The necessary specimens are extracted from new rails that are made in series production. Monotonic tensile test results have shown that the strain-hardenability of the steel is comparable to standard grade eutectoid rail steel and is higher than that of an accelerated-cooled eutectoid rail grade. The fracture toughness test results showed, statistically, no difference when compared with the fracture toughness values of the accelerated-cooled eutectoid rail grade. The tests were performed at room temperature. The fatigue crack growth rates are, in the linear Paris-regime, slightly higher than in the previously mentioned steels. The results are explained considering the distinct microstructural characteristics of the air-cooled vanadium-alloyed hypereutectoid steel and the fractured surface of the specimens. This experimental investigation contributes to selecting railway steels and predicting the actual in-service behavior.

Preventive grinding of rails is a recurring maintenance routine to remove damage initiated in the wheel–rail contact. The grinding routine increases the service life of rails and reduces operational costs. Despite these benefits, grinding-related defects are observed. In this work a field test is performed to investigate the contact-surface formation and to better understand its durability. Surfaces after grinding are studied at different stages of the test to characterize wear mechanisms and deformation. The freshly ground surface exhibits a higher roughness and is composed of facets. It is determined that roughness asperities are extruded and fill grinding grooves in the process. High contact stresses at the facet transitions accelerate the extrusion of roughness asperities and the fast formation of the contact surface. The analysis further shows that deeper grinding grooves prevent homogeneous deformation. Strain concentrations arise due to the inhomogeneous deformation leading to damage initiation sites. These grooves are still present in the rail surface after the test. The evolution of the ground surface is captured in a schematic wear model.

This work discusses the microstructure evolution observed in a quenching and partitioning (Q&P)-processed martensite/austenite stainless steel during the partitioning step at 400 °C for 300 s, where distinct microstructural bands rich in austenite due to elemental segregation, evolve into a uniform distribution of austenite grains. This phenomenon is characterised and investigated using a model for the carbon partitioning from martensite to austenite coupled with the movement of the martensite-austenite interface. The observed elimination of microstructural bands is found to be related to the topological distribution of austenite grains and the heterogeneity of the thermodynamic equilibrium regime at the various interfaces governing the partitioning process. Furthermore, the concurrence of banding elimination (local equilibrium) and phase growth towards the global equilibrium phase fractions is investigated in the simulations in terms of the role of Mn. It is found that the local equilibrium-negligible partitioning (LENP) conditions lead to the most realistic outcome.

This study investigates the microstructural development of commercial low-alloyed AISI 4340 steel through the synergistic application of Binder Jetting and Quenching and Partitioning (QP) processes. The material in the as-sintered condition exhibited significant variations in microstructure and mechanical properties, primarily influenced by the processing route. Carbon content was influenced by the building technique as decarburization was observed at different intensities mainly during the heating stage of sintering, driven by carbothermic reduction. Vacuum-debinding was found to be optimal, leading to the most homogeneous microstructure, predominantly granular bainite with superior hardness and tensile strength. Different QP treatments were optimized considering the decarburization effect on the optimal as-sintered condition, stabilizing 4–8 % retained austenite in a martensitic matrix, with optimal results observed after isothermal holding at either 220 °C or 240 °C for 30 min. These conditions resulted in high UTS values of 1231 MPa and 1151 MPa, respectively, compared to 750 MPa in the as-sintered state. Despite high tensile properties, A% was limited by the presence of residual porosity. This study highlights the critical importance of controlled debinding and sintering atmospheres as well as decarburization-informed QP treatments in achieving desirable microstructural and mechanical properties in additively manufactured AISI 4340 steel components.

While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.

Carbon segregation to defects in martensite is a phenomenon known for its occurrence and interference with mechanisms such as carbon partitioning in multiphase steels. Especially in martensite–austenite partitioning processes, carbon trapping at/de-trapping from martensite defects plays an important role since it interacts with the austenite enrichment. In this work, we develop a physics-based model in which we incorporate the concurrent evolution of carbon partitioning and trapping at/de-trapping from martensite defects. The model describes the global and local, time-dependent distribution of carbon between three lattice types, namely martensite defects, martensite solid solution, and austenite. We implement the model in mean-field and full-field descriptions, and discuss the interaction between carbon enrichment in austenite and segregation to martensite defects, on the basis of global equilibrium as well as on the carbon kinetics. We apply the model in several martensite — austenite microstructures and discuss the dependence of the interaction between carbon partitioning and trapping at/de-trapping from defects on specific microstructural features, i. e. phase fractions and microstructural banding.

This work presents an investigation of the microstructure development during the application of the quenching and partitioning (Q&P) process to two stainless steels with different Mn content. The results are compared with calculations based on the constrained carbon equilibrium theory, paying special attention to the presence of reactions competing for the carbon available for partitioning and to the effect of alloying element segregation. Results show that chromium carbides must be considered when accounting for the carbon available for austenite stabilisation. Moreover, manganese/chromium segregation bands play an important role in the microstructure development, particularly in martensite formation, with important consequences in the microstructure development during the following processing steps.

The present article investigates the influence of chemical composition and phase fractions on the corrosion behaviour of industrially produced quenching and partitioning (Q&P) martensitic stainless steels. Localised corrosion was analysed by scanning Kelvin probe force microscopy (SKPFM) and scanning electrochemical microscopy (SECM) in 3.5 wt.% NaCl solution. SKPFM revealed a Volta-potential difference of around 40 mV between inclusions and the matrix, which is larger than the Volta potential variations within the matrix. This difference in surface potential is a driving force for selective dissolution (corrosion initiation) at inclusions and inclusion/matrix interfaces. SECM detected early pitting initiation, particularly in alloys containing MnS and TiN inclusions. Results suggest that pitting initiation and propagation occur at those specific regions. This study emphasised that irrespective of chemical composition and phase fraction, localised corrosion initiation in Q&P-processed martensitic stainless steels is predominantly governed by the presence of inclusions.

Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. High cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high cycle fatigue performance in Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.

This work presents constitutive equations and a dataset of a thermal model for the prediction of temperature fields and heating rates during the application of localized laser treatments to a Fe-C-Ni alloy. The model considers transient material properties and the coupling between temperature and microstructure, with emphasis on the phase dependence of the thermal parameters and the hysteresis in the phase change. The model can predict temperature fields that are in agreement with the experimental microstructures at the laser-affected zones. This model can be applied to other materials exhibiting solid-state transformations upon the application of laser treatments.

In many commercial steel processing routes, steel microstructures are reverted to an austenitic condition prior to the final processing steps. Understanding the microstructure development during austenitization is crucial for improving the performance and reliability of the microstructure that forms from austenite. In this work, austenite formation in a high-C steel (0.85 wt%) from a microstructure containing martensite/austenite and bainite bands is investigated. It is shown that austenite formation from bainite results in a refined austenite grain structure, and the martensite matrix thus obtained on quenching has a homogeneous distribution of carbides with a relatively low fraction of retained austenite (24%). On the other hand, a coarser austenite microstructure is obtained when austenite forms from a mixture of martensite and retained austenite. The reason for the coarse austenite grains is argued to be a memory effect, which is substantiated by in situ X-ray diffraction analysis. After quenching, an inhomogeneous carbide distribution and a higher retained austenite fraction (30%) are observed in the regions that were initially martensite/austenite. The global microstructure, hence, has a bimodal size distribution of prior austenite grains and carbide-dense bands. The causes for these heterogeneities are discussed with the help of interrupted quench experiments, equilibrium phase calculations, and DICTRA simulations.

Localised laser treatments enable the creation of sophisticated austenite/martensite mesostructures in Fe–Ni–C steel with the potential of achieving enhanced mechanical performance. The control of phase topology is essential to modify the properties of these structures on demand and requires a profound understanding of the effect of the processing parameters on the development of the different phases upon the application of laser treatment. In this work, the microstructure evolution under exceptional gradients in temperature and heating rates is thoroughly investigated. The extent of the laser-affected zone and the heat input were tailored by varying laser parameters and specimen thickness, based on a model that considers transient material properties and the coupling between temperature and microstructure. The predicted temperature fields resulted in a complex interplay between martensite to austenite phase transformation and martensite tempering. Considering the high heating rates of up to 25000 K/s and the observed microstructures, it is suggested that austenite was formed by a pseudo-displacive mechanism and subsequently fully recrystallised in the zones most directly affected by the laser heat source. A smooth strength transition from austenite to martensite, affected by the laser parameters, could be exploited for more effective deformation mechanisms and improved material mechanical properties.

Quenching and partitioning (Q&P) treatment has been proven effective in manufacturing advanced high strength steels with high content of retained austenite, showing the improved balance of high strength and sufficient ductility. This method has been very well elaborated for carbon steel processing over the last two decades. Though it can also be potentially applied for processing other steel families, this has been scarcely studied. This article focuses on the effect of chemistry and heat treatment parameters on the microstructure and properties of Q&P treated martensitic stainless steels. Three different martensitic stainless steels with different contents of alloying elements are subjected to Q&P processing with varying quenching temperature or partitioning temperature and partitioning time. The tensile behavior of the Q&P treated steels is studied. The effect of chemistry and Q&P treatment parameters on the microstructure and tensile properties is analyzed. The effect of plastic deformation on the microstructure of the Q&P treated steels is also investigated. It is demonstrated that the Q&P treated martensitic stainless steels can show a good combination of enhanced strength and sufficient tensile ductility. Their uniform elongation increases with the increasing volume fraction of retained austenite due to the transformation induced plasticity (TRIP) effect. The ability of the martensitic matrix to accumulate plastic deformation also plays an important role. The Q&P process - microstructure - property relationship is discussed.

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.

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (z_case) and 0.16 wt% C (z_core). The microstructure of z_case is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For z_core, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of z_core are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.

The martensitic transformation in pure Fe and its alloys has been studied over many decades. Several theoretical models have been proposed to describe the atomic motion that leads to the fcc-to-bcc martensitic transformation. However, such models do not account for the effect of pre-existing planar defects such as twin boundaries and stacking faults, present in the high-temperature austenite phase prior to the transformation process. This work systematically studies the role of nano-spaced planar faults with different inter-spacing on the martensitic transformation using molecular dynamics simulations. Research shows that the investigated planar defects affect the nucleation and growth mechanisms during martensite formation, the morphology of the resulting microstructure, the specific atomic path leading to the phase transformation, and the martensite start temperatures. Martensite variants were identified by the analysis of the atomic shears and slip systems during the transformation process. A crystallographic analysis is done to explain the existence of different shear mechanisms of martensite transformation at different locations in the fcc austenite. The present investigation provides fundamental insights into the martensitic transformation process in presence of pre-existing planar defects and can be applied to other material systems, e.g., Fe alloys.