This study employed rapid, short-time duration (1 s) tempering to improve the dynamic performance of highstrength steel (HSS) and ultra-high-strength steel (UHSS), compared to quenched and conventionally (1800 s) tempered microstructures. Rapid tempering significantly improved the Charpy toughness of both steels compared to the conventionally tempered condition at an equivalent tempering parameter (TP) or hardness level. For conventionally tempered conditions, tempered martensite embrittlement was observed within the tempering temperature range of 200-400 degrees C, whereas rapid tempering exhibited increased Charpy toughness with increasing tempering temperature across all tempering conditions. Dynamic compression experiments indicated that rapid tempering led to improved ductility compared to the conventionally tempered condition at an equivalent TP. This work shows that the same rapid tempering strategy used for HSS is directly transferable to a higher-carbon UHSS and provides the first systematic evidence of improved strength-ductility combinations and cracking resistance under dynamic compression, supported by Kolsky bar experiments.

Quenching and partitioning can produce excellent combinations of strength and ductility. Comprehensive knowledge regarding the effect of isothermal holds below M_s is relevant to plate processing but is currently lacking. Microstructure and mechanical properties of a 0.3C–1.5Si–1.5Mn–0.3Cr steel held from 10 seconds to 3 hours at 280 °C were investigated. The results show increased austenite stability at shorter hold times. Longer hold times resulted in bainitic constituents and coarser retained austenite, with reduced tensile ductility.

The cost and reliability of hydrogen (H) storage and delivery infrastructure present significant challenges to the widespread adoption of H energy technologies. High-nickel (Ni) austenitic stainless steels, known for their excellent resistance to hydrogen embrittlement (HE), are relatively expensive. This work explores a low-cost alternative in the form of high manganese (Mn) duplex (austenite-ferrite) steel. Alloy design was guided by thermodynamic calculations to achieve austenite stacking fault energy (SFE) of 47.7 mJ·m-2. Cold rolling and post-heat treatment were performed to enhance both strength and ductility. HE sensitivity was assessed using circumferentially notched tensile specimens under rising displacement tests with in-situ electrochemical H charging. The high Mn duplex steel in the as-hot-rolled condition exhibited lower strength compared to a reference commercial duplex stainless steel but also showed lower strength loss when fractured in H. Cold rolling increased the strength of the high Mn duplex steel to a level comparable to the commercial duplex stainless steel, without degradation of HE resistance compared to the lower strength as-hot-rolled condition. Deformation mechanisms and fracture behavior in both air and H environments were characterized using electron microscopy, fractography, and diffraction techniques. Results suggest that cold rolling improves the combination of HE resistance and strength in the high Mn duplex steel, likely by reducing H transport kinetics and minimizing stress incompatibility between austenitic and ferritic regions.

This work explores key aspects of designing and optimizing the quenching and partitioning process integrated with galvanizing (GI Q&P) for developing next-generation automotive steels. A conventional CMnSi steel was quenched below the martensite-start temperature, followed by partitioning at 460 degrees C for time durations (t(P)) between 3 and 60 s, to simulate the GI Q&P process using a dilatometer. Carbon redistribution between martensite and remaining austenite was observed during initial quenching, which continued during isothermal holding at the quenching temperature (T-Q) and subsequent partitioning at 460 degrees C, leading to the stabilization of austenite. Transition carbides and/or cementite and bainite formation were observed during different stages of Q&P. Transition carbides formed during initial quenching and holding at T-Q, and then dissolved during partitioning at 460 degrees C after 30 s, followed by precipitation of cementite. The amount of retained austenite increased with increasing T-Q, while its carbon content was not affected much. With increasing t(P), both the amount and carbon content of retained austenite first increased, attained a maximum value (similar to 15 vol pct after t(P) of 10 s and similar to 0.82 wt pct carbon after t(P) of 30 s), and then decreased, due to austenite decomposition into bainite, and ferrite and cementite.

High Mn austenitic steels are considered an economical alloy system for hydrogen storage and transport applications. This study used stacking fault energy (SFE) as a design parameter to achieve hydrogen embrittlement (HE)-resistant high Mn austenitic alloys. The role of hydrogen on the deformation mechanisms of low (29 mJ/ m2) and high SFE (49 mJ/m2) alloys was evaluated through in-situ neutron diffraction during tensile loading. Hydrogen-precharging increased yield strength, partly due to hydrogen-induced lattice distortion (i.e., solute strengthening). Hydrogen accelerated the increase in defect density, including dislocations and stacking faults. The formation of planar deformation structures (twins and stacking faults), relative to dislocations, plays a critical role in promoting hydrogen-assisted fracture. The stacking fault frequency parameter obtained from neutron diffraction quantifies planar deformation tendencies, correlated with HE sensitivity. The higher SFE alloy exhibited greater resistance to HE, associated with the reduced propensity to form stacking faults and twins upon deformation in the hydrogen-precharged condition.

Mn alloying is widespread in structural steels as it increases strength and toughness. However, the use of high amounts of Mn is challenged by the marked segregation during continuous casting, which increases the degree of banding after rolling, and the reduction of weldability of steels. In order to decrease the Mn contents in structural steels, steelmakers are seeking to partially replace it by other alloying elements, for example, Nb. This work evaluates the effect of Mn and Nb additions on the hardenability of a structural microalloyed steel by austenite decomposition dilatometry testing. Continuous cooling transformation diagrams of three different alloys and two prior austenite grain sizes of 10 and 50 µm were developed. It is shown that Nb microalloying has a strong effect on hardenability, lowering the austenite decomposition temperatures and obtaining finer microstructures with increased hardness. Both Mn and Nb additions have a greater effect on transformation temperatures for larger prior austenite grain size. The Nb micro addition evaluated has a similar effect on hardenability than the Mn addition.

In this investigation, the low cycle fatigue properties of carbide free bainitic microstructures are being studied, with particular interest in comparing the performance of controlled cooled microstructures to that of microstructures created via austempering. Carbide free bainitic microstructures developed via different thermal processing routes exhibit microconstituents of different scale and morphology. Bainitic microstructures were formed by either a controlled cooling process or an isothermal austempering process to evaluate the relationship between microstructure and mechanical properties in a 0.2C - 2Mn - 1.5Si - 0.8Cr steel containing small amounts of Nb, Ti, B, and N. When compared to microstructures produced via austempering, which are composed of lath-like bainitic ferrite, microstructures produced with a controlled cool exhibit more variation in the size and distribution of martensiteaustenite constituent in the lath-like or granular bainitic ferrite matrix. These carbide free bainitic conditions were tested under strain-controlled uniaxial low cycle fatigue with a total strain amplitude of 0.4 pct and R = -1. The low cycle fatigue results are compared to those of a quenched and tempered 4140 steel. Results indicate that carbide free bainitic microstructures outperform quenched and tempered microstructures of the same hardness in low cycle fatigue, and increased variability in the scale and morphology of CFB microstructure results in less variation in fatigue lives.

Alloying considerations for 3rd generation advanced high strength steels (3rd Gen AHSS) are being partially driven by mitigation of liquid metal embrittlement (LME) in these Zn-coated steels. This work, therefore, assesses the effect of substrate Al concentration on the LME susceptibility of electrogalvanized (EG) 3rd Gen AHSS with quench and partitioned (Q&P) microstructures. Two alloys were investigated: 0.5Si-0.05Al and 0.5Si-1.38Al. The 1.38Al alloy exhibited lower LME susceptibility in hot tension testing at 800 degrees C and less Zn enrichment ahead of the LME cracks. To elucidate the mechanisms creating disparate behavior between the two alloys, diffusion and thermodynamic simulations were employed. Due to limited solubility of Al in the liquid Zn coating, diffusion simulations predict an enrichment of Al on the substrate side of the coating-substrate interface. The enrichment of Al would stabilize the Fe-Zn intermetallic Gamma (Fe3Zn7) phase at the expense of the liquid phase, such that a physical barrier to Zn diffusion into the substrate and the consumption of the liquid phase may mitigate the LME response. The solubility and diffusivity of Zn in the substrate also decreases with increasing Al concentration, resulting in a decrease in the flux of Zn into the substrate. Thus, multiple factors contribute to a decrease in the stress-assisted, solid-state diffusion of Zn ahead of LME cracks, such that the embrittlement of the 1.38Al alloy was lower than that of the 0.05Al alloy. A mechanistic sequence of events occurring during hot tension testing of both alloys is also proposed.

Simulated thermochemical controlled processing (TMCP) was performed on four microalloyed plate steels with Nb and Mo contents varying from 0.03 to 0.045 and 0.03 to 0.15 wt pct, respectively, to investigate influences of both processing and alloying on transformation behavior and microstructural evolution. Dilatometry was performed in situ in a Gleeble((R)) 3500 during thermomechanical simulation to construct continuous cooling transformation (CCT) diagrams for all alloys. A range of cooling rates between 2 degrees C/s and a target 100 degrees C/s along with two deformation levels, - 0.4 total true strain and - 0.6 total true strain in the austenite regime, were employed to create a range of microstructures. Increased deformation in the austenite non-recrystallization region promoted both polygonal ferrite and acicular ferrite transformation through an increase in nucleation sites. The increase in nucleation sites also resulted in a finer resultant microstructure with increased deformation. Increased cooling rates reduced transformation start temperatures and favored non-polygonal transformation products. Intermediate cooling rates led to the more desirable microstructures consisting of acicular ferrite and bainite. Both Nb and Mo increased the hardenability of the steel through interactions with the polygonal ferrite transformation. Nb and Mo retarded the polygonal ferrite transformation and favored an acicular morphology. Molybdenum alloying also favored bainite transformation. Desirable microstructures of acicular ferrite and bainite were able to be produced with the combination of higher deformation, intermediate cooling rates, and increased Nb/Mo alloying.

In this work, the influence of secondary soaking parameters on microstructural evolution in medium manganese steels is evaluated with a specific focus on the mechanisms driving austenite formation and solute partitioning at an intermediate secondary soaking temperature of 750 degrees C. The secondary soaking heat treatment represents the latter step of the double soaking heat treatment where it is used, after an initial intercritical annealing heat treatment, to form additional, solute-lean austenite. Here, data are obtained on a 0.14C-7.17 Mn steel for secondary soaking temperatures between 700 degrees C and 800 degrees C, and for times up to 1200 s. Austenite formation during secondary soaking is characterized using dilatometry and XRD, while Mn redistribution is experimentally observed using STEM-EDS. Manganese partitioning during secondary soaking, as well as calculated changes in carbon partitioning, are further explored through a DICTRATM TM model. Experimental and simulation results suggest that the ferrite-to-austenite transformation during the secondary soaking step may proceed under a partitioning, diffusional transformation at the 750 degrees C secondary soaking temperature. Manganese partitioning was demonstrated to primarily occur from ferrite to newly formed secondary austenite, while carbon diffusion principally occurred from primary to secondary austenite. Transformation of secondary austenite to martensite upon quenching along with some fraction of primary austenite, whose transformation was attributed to a reduction in C concentration, was shown demonstrated. The results of this study are expected to be of interest to those seeking to design multi-step heat treatments which produce austenite of variable solute concentrations and stabilities.

In this article, the multifaceted challenges of abrasive wear are explored, delving into its impact on components across diverse industries such as automotive, aerospace, mining, manufacturing, and energy production. The introduction of hard eutectic niobium carbide (NbC) networks into a high C martensitic steel and its effect on abrasive wear properties specifically is investigated. Four ingots are casted with Nb contents of 0.01, 0.25, 0.5, and 1.0 wt% and a base compositions of Fe–1.0C–0.96Mn–0.22Si–0.26Cu–0.11Ni–0.50Cr–0.005V–0.012Nb to determine the influence of eutectic NbC on hardness and wear resistance. Microstructural evaluation performed using scanning electron microscopy, electron dispersive spectroscopy, and X‐ray diffraction reveals that the eutectic NbC networks are broken up and distributed in bands within the microstructure upon hot‐rolling. A general increase in material hardness and wear resistance with an increase in Nb content is observed as measured by dry sand/rubber wheel (DSRW) testing. Nb alloying leads to a 65% decrease in DSRW mass loss between the 0.01 and 1.0 wt% Nb alloys at an approximate rate of 6% per every 0.1 wt% Nb added.

Austenitic steels have relatively high resistance to hydrogen embrittlement and play a critical role in hydrogen service applications. In particular, high Mn austenitic steels are considered economically viable alloy alternatives for these applications. The current study employed in-situ and ex-situ neutron diffraction techniques combined with diffraction line profile analysis (DLPA) to investigate the influence of hydrogen on deformation and embrittlement mechanisms in a high Mn (approximately 30 wt pct) austenitic steel. Investigation using both neutron diffraction and electron backscatter diffraction revealed the presence of extensive deformation twins and stacking faults within the steel microstructure after tensile deformation in the non-charged condition. These microstructural features suggest planar deformation behavior, which is expected from the relatively low stacking fault energy (SFE) of the alloy (approximately 29 mJ/m2). Hydrogen pre-charging resulted in apparent increases in both dislocations and stacking faults, contributing to macroscopic hardening and embrittlement mechanisms. Overall, numerical parameters obtained through neutron DLPA were used to elucidate the underlying mechanisms associated with hydrogen effects on the mechanical behavior, i.e. macroscopic strengthening, strain hardening rate, and embrittlement.

Historically, steels with carbon contents above about 0.45

Abstract Carbide free bainitic microstructures can be developed via different thermal processing routes, and the details affect the scale and morphology of the microstructural constituents. In this study, bainitic microstructures are formed by either a controlled cooling process or an austempering process to evaluate the relationship between microstructure and mechanical properties in a 0.2C - 2Mn - 1.5Si - 0.8Cr steel containing small amounts of Nb, Ti, B, and N, and the results are compared to a 4140 steel processed via quenching and tempering. The resulting microstructures are characterized with scanning electron microscopy. When compared to microstructures produced via austempering, microstructures produced with a controlled cool exhibit an increased variety of transformation products, specifically regarding size and distribution of martensite-austenite constituents within a lath-like bainitic ferrite matrix. Nanoindentation testing shows that different transformation products exhibit significantly different local hardness. In all (primarily) bainitic conditions tested for these materials, the martensite/austenite constituent exhibits the highest hardness, followed by the lath bainitic ferrite/retained austenite constituent. Granular bainite and coarse bainitic constituents exhibit the lowest relative hardness in the conditions where they are observed.

Abstract Quenching and tempering (Q&T) allows a wide range of strength and toughness combinations to be produced in martensitic steels. Tempering is generally done to increase toughness, although embrittling mechanisms result in temperature ranges where strength and toughness may decrease simultaneously. Tempered martensite embrittlement (TME) represents one such mechanism, associated with the decomposition of retained austenite and precipitation of cementite during tempering, usually between 250 and 450 °C. The use of induction heating allows for time-temperature combinations, previously unobtainable by conventional methods, that have been shown to improve properties. The present work shows a beneficial effect of rapid tempering in alloy 1045, with an increase in energy absorption of about 50% when measured at room temperature via a three-point bending fracture test in the TME regime. Phase fraction measurements by Mössbauer spectroscopy showed that increased energy absorption was obtained despite essentially complete decomposition of retained austenite during tempering. Scanning electron microscopy (SEM) investigation of the carbide distribution showed refinement of the average carbide size of approximately 15% in the rapid tempered conditions. SEM characterization of the fracture surfaces of the rapid tempered three-point bend samples showed that, despite an increase in energy absorption in the TME regime, increased microscopic ductile fracture appearance was observed only at the highest test temperature.

The annealing response of 316L stainless steel manufactured with both laser powder bed fusion (PBF-LB) and wire-arc directed energy deposition (DED-Arc) was investigated in the context of microstructural evolution and mechanical properties. Because additive manufacturing (AM) comprises several technologies that differ in heat source, feedstock, and scan strategy, among other build variables, the final products can experience a range of thermal histories and defect (dislocation) populations. These unique thermal histories can subsequently influence as-built properties and annealing response of AM materials, most notably those manufactured with different AM deposition processes. The PBF-LB process (with higher cooling rates) yielded finer austenite microstructures having more lattice strain, higher dislocation densities, and higher yield strength values than the DED-Arc 316L process (with lower cooling rates). Electron backscattered diffraction (EBSD) data of the kernel average misorientation (KAM), boundary density, and geometrically necessary dislocation (GND) density support the differences in lattice strain. As a result, the PBF-LB 316L exhibited more recovery and recrystallization after annealing at elevated temperatures above 873 K based on changes to yield strength, work hardening behavior, and microstructure evolution. The DED-Arc 316L exhibited a delta- ferrite/austenite microstructure with microsegregation that influenced deformation mechanisms active after annealing. Tensile data, deformed microstructures and misorientation histograms from EBSD showed a decreasing amount of deformation twinning when comparing the as-built condition to annealed conditions (up to 1473 K/1h) of PBF-LB 316L. The opposite trend was noted in DED-Arc 316L. The behavior was interpreted to be due to the differences in chemical segregation during solidification and the effects of heat treatment on chemical homogenization and local stacking fault energy.

It is well known that the element boron (B) improves the hardenability of low- to medium-carbon steels, reducing the need for expensive alloying elements. This study focuses on investigating the effect of quenching rate on B segregation at prior austenite grain boundaries (PAGBs) and Charpy impact energy in medium-carbon AISI 4130 steel with a tempered-martensitic microstructure. NanoSIMS data after quenching from austenite at different rates, including water quenching (WQ), He-gas quenching (HeQ), and Ar-gas program quenching (ArQ), were used to analyze B-doped 4130 steel specimens. The results revealed that the concentration of B along the PAGBs, and the extent of B segregation and boro-carbide precipitation increased with decreasing quenching rate. This phenomenon could be explained by the critical time required for non-equilibrium segregation of B during quenching. Charpy impact test data showed that the B-doped 41B30 specimens and slowly quenched specimens exhibited higher absorbed energy than the non-B-doped 4130 specimens and rapidly quenched specimens, respectively, particularly in the impact transition temperature region. This improvement in impact energy can be attributed to several factors. First, B doping enhances the resistance to brittle quasi-cleavage fracture by strengthening the grain boundaries through cohesive bonding. Second, B facilitates slip transfer across the grain boundaries, promoting plastic deformation and enhancing toughness. Finally, B doping may suppress the formation of transition carbides during tempering, further contributing to improved impact properties.

Silicon (Si) aggravates liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) by inhibiting intermetallic formation between liquid Zn and the AHSS substrate at elevated temperatures. This study performed detailed microstructural and elemental characterization of an LME-susceptible Si-bearing AHSS to reveal the fundamental mechanism by which Si inhibits Fe-Zn intermetallic reactions. Notably, the interaction between Si-alloyed AHSS and liquid Zn at elevated temperatures results in pronounced Si enrichment in a thin region of the substrate in contact with the liquid. This Si enrichment behavior is at the core of the retarded Fe-Zn reactions, and is explained from a fundamental standpoint by analyses of the phase equilibria in the Fe-Zn-Si ternary system and diffusion kinetics using DICTRA®. Specifically, the liquid dissolves the surface layers of the AHSS substrate upon initial contact at elevated temperatures. The liquid phase, however, has no appreciable solubility for Si; hence, almost all of the Si atoms from the dissolved substrate layers are “back diffused” into the substrate leading to Si enrichment. The Si-enrichment of the substrate reduces the thermodynamic driving force for nucleating intermetallic phases at the steel/coating interface, making it difficult to form a “protective” interfacial layer barrier between the liquid and the substrate. Further, Si enrichment results in an increase in the chemical potential of Zn and reduction in the chemical potential of Fe in the substrate; both these factors slow the kinetics of substrate dissolution and Fe-Zn alloying, leading to an increased liquid fraction in the coating available to activate LME.