Enhancing the transformation-induced plasticity effect in steel-based binders through deformation-induced martensitic transformation requires careful control of the austenite stability. This study investigated the effect of binder fraction on the thermal stability of austenite in the WC-(Fe,Ni) system with different binder contents within the WC + binder + graphite equilibrium region. Binder martensite fractions and carbon content across different binder fractions were compared using electron microscopy. Thermodynamic-based calculations, accounting for variation in binder carbon content, provided a baseline for distinguishing the roles of the binder fraction and chemical composition on the thermal stability of the austenitic binder. Samples with lower binder fractions and comparable binder carbon content revealed relatively similar martensite fractions, whereas the one with the highest binder fraction and the lowest carbon content exhibited the highest value. The predicted martensite fractions were comparable and in good agreement with the measured values for lower binder fractions.

This work explores the effect of varying punching parameters on the high-cycle fatigue (HCF) response of thick-plate high-strength low-alloy (HSLA) steel intended for heavy-duty truck chassis. A combination of microstructural characterization, tensile testing, HCF experiments on both punched and unpunched specimens, and neutron diffraction-based residual stress measurements was conducted. The punching operation induces pronounced microstructural modifications, including grain refinement, defect generation, tensile residual stresses, development of a hardened shear-affected zone, and a rough fracture surface inside the punched hole. At higher stress amplitudes and shorter fatigue lives (approximately 10⁵ cycles), the HCF behavior after punching remains comparable to that of unpunched specimens and exhibits lower sensitivity to punching-induced changes. In contrast, under lower stress amplitudes and longer lifetimes (around 10⁶ cycles), fatigue strength decreases considerably due to the combined influence of surface roughness and, more critically, tensile residual stresses, with fatigue cracks initiating near mid-thickness—the region of highest measured tensile residual stresses. Furthermore, localized deformation during punching promotes microstructural refinement and defect formation, further influencing fatigue resistance. The optimization of punching parameters to balance the hardening benefits with minimal defect and sub-grain formation can improve fatigue performance. These insights offer strategies to enhance fatigue performance of HSLA steel in heavy-duty truck chassis components.

To exploit the full potential of advanced high-strength steels (AHSS), a more in-depth understanding of the complex micromechanical interaction of thin-film retained austenite (RA) and lath martensite is indispensable. Inspired by the medium-Mn steel microstructure, a three-dimensional micromechanical modeling approach is therefore proposed in the present work, embedding the thin RA films explicitly into the hierarchical lath martensite structure. This enables systematic studies of the effect of RA film thickness and volume fraction on the local stresses and strains as well as their partitioning within the microstructure. The investigations reveal that with shrinking RA volume fraction, both stress and especially strain heterogeneity in the thin-film RA intensifies. In the martensite blocks, stress and strain heterogeneity also intensifies, although stresses are generally more heterogeneously, and strains much more homogeneously, distributed than in RA. The results underline the key role of RA with thin-film morphology for further optimizing AHSS microstructures.

This study compares the role of microstructure on post-punching fatigue properties in three advanced high- strength steels (AHSSs) with a high-strength low-alloy (HSLA) steel commonly used in heavy-duty truck chassis. Microstructure characterization, tensile testing, high cycle fatigue (HCF) testing, fatigue crack growth rate (FCGR) testing, and neutron residual stress measurements are conducted. Punching significantly alters the microstructure, causing microstructure refinement, sub-grain formation, defect creation, tensile residual stresses, and a work-hardened shear-affected zone (SAZ) around, and a rough fracture zone, inside the punched hole. At 105 cycles, the HCF performance is primarily governed by the fatigue crack growth resistance of the as-rolled microstructure, with minimal sensitivity to punching. However, near the fatigue limit, post-punching fatigue failure is strongly related to strain localization when significant strength difference exists between micro- constituents (e.g., martensite and ferrite). Strain localization also promotes sub-grain formation, reducing the local threshold stress intensity factor range (Delta Kth), thus facilitating fatigue crack initiation. In microstructures with smaller strength differences (e.g., ferrite and bainite), sub-grains, together with surface roughness and residual stress, contribute significantly to the post-punching fatigue limit reduction. These findings provide insights into mechanisms of punching-induced fatigue performance degradation, offering potential strategies to optimize fatigue performance of AHSS for new applications.

Among several separate challenges, the major one for replacing cobalt in cemented carbides is the difficulty to obtain alternative binder materials with a C-window broad enough to be robustly processed under conventional industrial control on the C content. The C-window is defined as the C content range for which phases that are detrimental to the mechanical properties are avoided. The present paper has two main objectives: first, to show that the processing C-window of Fe-Ni based systems is in fact wider than what thermodynamic equilibrium calculations predict, and that its width can be controlled moderately by tweaking the initial WC grain size and the cooling rate used in the material's processing. Secondly, in case those detrimental phases are not avoided, this work gives insight on how to make their appearance less detrimental for the mechanical properties. The morphology, volume fraction and particle size distribution of the detrimental phases, specifically η6-carbides at low C contents, are investigated to explore desirable combination of hardness and toughness of alternative binder cemented carbides. During this study it was also discovered that in samples with carbon contents below the low-C limit of the C window a carbide with hexagonal lattice known as κ, not commonly seen in cemented carbides, appeared and formed the core of a core-rim structure together with the more common η6-phase. It is believed that the κ-carbide form due to local high concentrations of tungsten during solid state sintering and that it has an impact on the precipitation characteristics of the η6-phase.

The stability of retained austenite is a key factor in the design of advanced high-strength steels that exhibit excellent mechanical performance, including high strength and high ductility/toughness. However, the contribution of certain microstructural factors, such as the morphology and size of the austenite, and the surrounding matrix, to this stability is still not fully understood, partly due to the inherent difficulties in separating these factors in multiphase microstructures. Therefore, this study uniquely compared the stabilities of retained austenite in two-phase microstructures with bulk austenitic microstructures of the same composition, across four medium-Mn steels upon quenching. By fixing the austenite chemical composition, we could exclude the influence of composition and examine the influence of other factors, such as morphology, size, and the surrounding matrix, on the stability of austenite. Our experimental results showed that retained austenite in the two-phase microstructures was more stable than the bulk austenitic microstructures of the same composition, regardless of morphology and size. Analysis using thermodynamic calculations revealed that neither the steel composition nor the size alone could explain the high stability of the retained austenite in the two-phase microstructures. Instead, we propose that microstructural factors, including size, morphology, and matrix, have a significant influence on the metastable austenite in two-phase microstructures. While these factors have been studied previously, our study introduces a novel perspective by excluding the influence of the austenite composition, thus contributing to a more comprehensive understanding of retained austenite stability. These findings may guide the design of advanced steels and highlight the importance of considering the contribution of these microstructural factors in tailoring the stability of metastable austenite.

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.

Isothermal information is rarely available for the formation of martensite in Fe or Fe alloys due to a very high rate of transformation compared to the rate of heat conduction. Such information has now been extracted for lath martensite in some sets of Fe alloys from available information on ultra-rapid quenching but only at a single temperature for each alloy, related to its two MS temperatures. The temperature dependence could, thus, be studied only on binary sets of alloys. Those results have been applied to mathematical models based on the Arrhenius equation and illustrated with Arrhenius plots. For three sets of binary Fe alloys, a large group of rates came close to the rate of an almost pure and carbon-free Fe-C alloy. It illustrated that Cr, Ni, and Ru in low contents have relatively small effects on the rate of formation of lath martensite in Fe. It also demonstrated that the present measurements have considerable reproducibility. In contrast, a set of Fe-C alloys did not give a straight line in the Arrhenius plot. Using a new mathematical model based on the concept of the Arrhenius equation to express the effect of carbon, it was possible to predict the rate of formation of lath martensite for Fe-C alloys with fixed C content and their temperature dependencies which are not available experimentally due to the very high rate of formation.

The influence of austempering temperature and time on the microstructure and hardness of a low-alloyed bainitic steel is investigated in the temperature range 275 °C to 375 °C for up to 24 hours. It is shown that the dislocation density and coarseness of the bainitic microstructure are affected by the austempering temperature, while only the dislocation density is significantly affected by the austempering time. The hardness of the steel is estimated based on microstructure–property relations and is in good agreement with the measured hardness. In conclusion, the decrease in dislocation density is the main reason for loss in hardness upon increasing austempering temperature and/or time for the studied temperature range.

Cemented carbides, which are one of the most important composites produced by powder metallurgy, exhibit an excellent performance within metal cutting and rock drilling tools when their hard phase, tungsten carbide, is bound by cobalt. However in recent years, due to health and ethical concerns related to cobalt, there has been a significant focus on designing alternative binders. The martensitic transformation in high-strength steel and its subsequent transformation-induced plasticity effect present a solution to substitute cobalt and improve the overall properties of cemented carbides. However, factors including residual stresses induced by tungsten carbide grains and confined dislocation mean free path significantly affect the martensitic transformation in these composites. In this study, a thermodynamic-based model for the martensitic transformation in steels has been utilized to predict the martensitic start temperature in cemented carbides. The model coupled with a CALPHAD-based approach presents a systematic solution for designing new steel-based binders.