This study investigates the microstructure after quenching and the precipitation behavior during subsequent tempering in Fe-2.5Ni-0.5Al, Fe-2.5Cu, and Fe-2.5Cu-2.5Ni-0.5Al (wt%) steels, with and without Mo additions. All alloys were solution-treated at 900 °C for 60 min, followed by quenching and tempering at 550 °C for up to 40 min. Microstructure and precipitation characteristics were analyzed using microscopy, atom probe tomography, and in situ small-angle X-ray scattering, supported by thermodynamic calculations and continuous cooling transformation diagram simulations. The Fe-Ni-Al steels (with or without Mo) exhibited a ferritic–bainitic microstructure. The Fe-Cu steel was primarily ferritic, while Mo addition promoted a ferritic-bainitic structure. The Fe-Cu-Ni-Al steel displayed a ferritic–martensitic microstructure, which transformed into a fully martensitic structure with Mo addition. During tempering, no precipitates were detected in the Fe-2.5Ni-0.5Al steel, whereas Cu-rich precipitates formed in both Fe-2.5Cu and Fe-2.5Cu-2.5Ni-0.5Al steels. The enhanced bainitic/martensitic transformation induced by Mo addition resulted in a higher dislocation density after quenching, which facilitated Cu precipitate nucleation during tempering. Hybrid Monte Carlo/Molecular Dynamics simulation confirm that Mo alters the matrix distortion in Fe-2.5Cu-2.5Ni-0.5Al steel, a key factor influencing nucleation and precipitation kinetics. Moreover, the addition of Mo reduced precipitate growth and coarsening, contributing to the retention of high hardness after tempering.

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.

An innovative heat treatment process at temperatures between 750 and 900 °C, i.e. above the upper limit of the miscibility gap (MG), is found to slow down the subsequent phase separation occurring in the bcc phase when a super duplex stainless steel (DSS) 2507 is further treated within the MG. This delayed phase separation postpones low temperature embrittlement of the DSS and provides unprecedented life time. The innovative heat treatment can be applied to other DSSs, and thus it could provide means to prolong the service life of DSS in general. The underlying mechanism of the delayed phase separation can be sought in the initial nanostructure, studied here through atom proble tomography and scanning transmission electron microscopy. The results lead us to suggest that the change in short-range order and solute interstitial elements are responsible for the reduced phase separation kinetic imposed by the innovative heat treatment.

In this study, we investigate the precipitation kinetics of Cu in 15–5 PH maraging stainless steel during high-temperature thermal treatments in the fully austenitic state. This provides direct evidence that Cu precipitation can occur in the austenite phase of martensitic or ferritic steels. The kinetics of Cu precipitation in austenite are examined at 700 and 800 °C using in situ synchrotron small-angle and wide-angle X-ray scattering, complemented by atom probe tomography investigations to analyze the precipitates, particularly their chemistry, following heat treatment. The resulting experimental data, which include the evolution of size, volume fraction, number density and chemical composition, are used to inform precipitation kinetics modelling using the Langer-Schwartz-Kampmann-Wagner (LSKW) approach coupled with CALPHAD thermodynamic and kinetic databases. The simulations accurately capture the experimental data by adjusting the interfacial energy in an inverse modelling approach. The insight that Cu precipitation occurs in austenite and subsequently in martensite paves the way for design of hierarchical structures with a bi-modal particle size distribution of Cu precipitates with varying crystal structures and compositions. Additionally, the validated LSKW modelling approach establishes a foundation for designing Cu-alloyed high-performance steels, taking into account various manufacturing routes.

Modeling the relationship of properties with composition, process, and microstructure is important to designing and developing new steel products. As traditional Machine Learning (ML) relies only on digital data, it is incapable of treating multimodal information. In this paper, a Deep Learning (DL) method is proposed to predict mechanical properties of High-Strength Low-Alloy (HSLA) steels, in which both microstructural evolution during hot rolling and transformations during cooling are taken into account. Continuous Cooling Transformation (CCT) diagrams are generated based on hot rolling parameters and compositions and superimposed with Cooling Path (CP) curves to represent the dynamic changes of transformed products, which is perceived and processed by the Convolutional Neural Network (CNN) as inputs. By doing so, the multimodal model for predicting mechanical properties of high-grade pipeline steels was developed, which demonstrates superior prediction accuracy and stability over traditional data-driven ML models. Also, reverse visualization is performed to work out hotspots in cooling processes, which clearly demonstrates the interpretability of the DL model. This framework provides useful guidance for designing production routes of HSLA steels and can also be implemented for other high-strength steels.

In this work we investigate the development of functionally graded cemented carbides, featuring macro gradients on the millimeter scale, where Fe, or Ni is the binder phase. Two composites, WC-Ni and WC-Fe with 20 % binder by volume were produced by addition of TiC on the surface of the samples before sintering at 1475 °C for 1 h. The sintered samples were analyzed using electron microscopy and microanalysis. For the Ni-binder sample, the results show a clear WC grain size gradient, with the smallest average grain size close to the TiC layer. This sample also exhibits compositional gradients, where Ni increases while Ti and C decrease from the added TiC layer and outward. The same effect of TiC addition on WC grain growth is observed in the Fe-binder sample, however, the effect is much smaller. The addition of Ti is known to influence the morphology of WC grains in Co-binder systems, and this effect is observed here in both Ni- and Fe-binder samples. WC growth ledges areobserved on the WC facets near the applied TiC layer where Ti levels in the binder are high. This suggests that the WC grain growth inhibition mechanism imposed by Ti is similar in these alternative binders as what has previously been reported for conventional Co-binders.

Functional gradient sintering of WC-TiC-Co cemented carbides was studied to reveal the effect of powder metallurgical processing conditions on compositional and microstructural, as well as hardness and toughness, gradients. The samples were created by local addition of TiC prior to sintering. Two different starting WC powders, one sub-micron and one medium-coarse, were used. Sintering at two different temperatures of 1410°C and 1475°C was compared. The local addition of TiC created a Ti/Co gradient that affected the structural evolution during sintering. The measured Ti/Co gradient after sintering at 1410°C and 1475°C differed, and the difference was especially apparent for the sample prepared using a sub-micron powder. After sintering at 1475°C, the sample prepared from the sub-micron powder exhibited a large WC grain size gradient and elongated or plate-like WC grains where the Ti concentration was high. In contrast, the sample prepared from the medium-coarse powder only showed a WC grain size gradient and no plate-like WC grain formation. It was also observed that the WC grain surfaces had growth ledges in both samples when the Ti content was enhanced, indicating the influence of Ti on the WC grain growth mechanism.

In this study, laser-material interactions during laser-powder bed fusion of WE43 magnesium alloy were characterized through numerical and experimental analyses. Various melting modes (i.e., conduction, transition, and keyhole) were induced through deposition of laser tracks at powers ranging from 80 to 130 W, and used as input parameters for a thermo-fluid model. Results of microscopy demonstrated good agreement between numerical and experimental measurements of melt pool depth, as well as a strong correlation between melt pool microstructure and the thermo-fluid conditions predicted by the model. Specifically, for conduction mode at 80 W, a predominance of cellular subgrains within the melt pool was consistent with the predicted steep thermal gradients, while for keyhole mode at 130 W, low thermal gradients correlated with high presence of equiaxed dendrites. Moreover, convection currents attributed to high recoil pressure in keyhole melt pools, were in agreement with locations of numerous subgrain boundaries having non-uniform morphologies, while under conduction, outward Marangoni flow led to a unique alignment of cellular subgrains and fewer subgrain boundaries. This study demonstrates the interplay among processing, thermal history, fluid flow and microstructure in WE43, and provides a basis for future design of microstructures for improved material properties.

In-situ small-angle neutron scattering (SANS) experiments, with and without an applied magnetic field of 1.5 T, were performed for two duplex stainless steels: 22Cr-5Ni and 25Cr-7Ni (wt.%) during isothermal heat treatment at 450 ∘C. The kinetics of phase separation was suppressed by the external magnetic field in both steels; however, the suppression was much more pronounced in 25Cr-7Ni, where phase separation was nearly eliminated. The difference in magnetic energy contributions from the external field in each steel explain their different degrees of phase separation. The findings are believed to have large technical implications for mitigating low-temperature embrittlement in Fe-Cr-Ni based alloys.

In duplex stainless steels (DSSs), phase separation of iron and chromium is a well-known phenomenon causing low-temperature embrittlement, which greatly limits the lifetime of components in service conditions at temperatures above 250 °C–300 °C. Hence, means of mitigating the underlying phase separation causing this embrittlement is highly interesting to extend the service life of DSSs in certain applications. In this work, we investigate the effect of intermediate heat treatments (5 minutes annealing at temperatures between 700 °C and 900 °C), performed after the conventional solution treatment, on the kinetics of phase separation super duplex stainless steel 2507. Using in situ small-angle neutron scattering at accelerated aging conditions (i.e., aging at 475 °C), we show that the application of intermediate heat treatments, which change the “initial state” of the material, can slow down development of the concentration fluctuation amplitude by up to 65 pct during aging inside the miscibility gap. This indicates great potential to delay the embrittlement process of duplex stainless steel. All intermediate heat treatments, conducted prior to aging, change the phase separation kinetics but to different extent. The 800 °C intermediate heat treatment shows the largest reduction in phase separation kinetics as compared to the reference sample. This sample also correspondingly shows the lowest hardness increase after aging. These findings show that intermediate heat treatments can be effective to reduce phase separation kinetics in duplex stainless steel and thus mitigate low-temperature embrittlement during service. The origin of the intermediate temperature treatment effect on phase separation kinetics is discussed in relation to the short-range atomic order introduced during intermediate heat treatment, prior to accelerated low-temperature aging.