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Two distinct heat treatments, that is, intercritical quenching combined with low‐temperature tempering (QT) to control the initial quenching temperature, and salt‐bath quenching combined with partitioning (Q&P) to regulate the final quenching temperature, were employed on high‐strength low‐alloy (HSLA) steels to achieve multiphase microstructures characterized by high strength, low yield ratio, and good impact toughness. Comprehensive experiments involving tension test, low‐temperature impact, microstructural observation, and in situ tension have been conducted to compare the microstructures and mechanical properties. It is found that both kinds of specimens can achieve a good match between high strength‐toughness and low yield ratio. The microstructures are composed of lath martensite with ferrite for QT specimen and tempered martensite with bainite for the Q&P specimen, a combination of “soft” and “hard” phases thus resulting in a yield ratio lower than 0.85 for 900 MPa low‐alloy steels. Notably, the Q&P specimen exhibited a markedly superior uniform elongation of 6.1% compared to the QT counterpart of 3.2%, a phenomenon attributed to the work hardening rate during deformation. A combination of strong and weak lath structures in tempered martensite and bainite can induce dislocation propagation and the carbides can act as obstacles to dislocation motion, jointly enhancing work hardening. [ABSTRACT FROM AUTHOR]

This investigation presents the development of third-generation advanced high-strength steels (AHSS) for automotive industries, focusing on materials with higher strength, plasticity, and crashworthiness. Three alloy steels Fe-6Mn-(1, 1.6, 2.0) Si were developed via a melting route using an arc melting furnace followed by homogenization at 1200 °C, hot rolling at 1100 °C, and then air cooling. The developed steels are characterized using FE-SEM, XRD, microhardness tester, and universal testing machine. The FE-SEM micrographs exhibit a complex phase microstructure containing pearlite, retained austenite, martensite, ferrite, and bainite. The microhardness values of novel alloys are measured as 423, 441, and 475 VHN. The tensile strengths of alloys were achieved at 1409 MPa, 1497 MPa, and 1438 MPa with elongations of 18, 15, and 14% respectively. Atom probe tomography results confirmed the presence of retained austenite of film thickness 25–27 nm. The fracture surface of alloy steel containing 1 wt% Si and 6 wt% Mn exhibits dimples, causing ductile fracture, while alloy steel containing 1.6–2.0 wt% Si and 6 wt% Mn combines dimples and facets, confirming intergranular fracture with a 14 % ductility limit. The average dimple size decreases from 0.82 µm to 0.64 µm with an increase in silicon from 1 wt% to 2 wt%. [ABSTRACT FROM AUTHOR]

This article investigates the precipitation behavior of Mo element in the continuous cooling phase transformation process of weathering steel of bridge construction and its influence on microstructure and hardness. The results demonstrate that the addition of Mo leads to the narrowing of the phase transformation interval of proeutectoid ferrite and pearlite, the expansion of the bainite transformation zone, a delayed bainite phase transformation process, and the refinement of bainite microstructure. Moreover, theoretical calculations of bainite phase transformation kinetics reveal that the average activation energy for bainite transformation in Mo‐free and Mo‐containing steels is 94.8 and 134.0 kJ mol−1, respectively. The new grains of Mo‐free steel mainly grow in a 2D mode, while Mo containing steel mainly grows in a 1D mode, which confirms the retarding effect of Mo on bainite grain growth. Additionally, Mo significantly increases the number density and refines the size of precipitated phases in weathering steel of bridge construction. Within the cooling rate range of 0.1–30 °C s−1, the hardness of weathering steel of bridge construction is enhanced by Mo, with the magnitude of hardness increment decreasing as the cooling rate increases. This behavior is primarily governed by the combined influence of phase transformation strengthening and precipitation strengthening. [ABSTRACT FROM AUTHOR]

The overall structural integrity and longevity of modern high-performance ship structures are significantly affected by the mechanical properties of plate-beam connectors, which are critical components in these structures. As global maritime strategic needs continue to evolve, understanding the behavior of high-strength steel structures under extreme dynamic loading has become increasingly crucial. This paper investigates the effects of heat input on microstructural evolution and dynamic fracture behavior in 8NiCrMoV high-strength steel T-joints using laser-MAG hybrid welding. A comprehensive multi-scale analysis is achieved through the development of a temperature field model for the laser-MAG hybrid heat source, coupled with high-speed camera and acceleration sensing technology. The results show that T-joints under low heat input (13.81 kJ/cm) exhibit superior dynamic mechanical properties, exhibiting 81.4% higher impact fracture energy and 47.2% greater maximum displacement compared to T-joints welded at 18.62 kJ/cm. The microstructure of weld metal primarily comprises acicular ferrite, granular bainite (GB), and plate-like bainite. Temperature distribution during the laser-MAG hybrid welding indicates that the microstructure gradient and microhardness within the heat-affected zone (HAZ) emerge as key determinants of impact toughness. The higher heat input leads to grain coarsening and precipitation of GB, with a 384.1% increase in average grain size when the heat input rises from 13.81 to 18.62 kJ/cm. The existence of hardenable lath martensite and GB in the HAZ results in reduced impact toughness. Especially under high heat input, the precipitation of these microstructures and high-hardness second-phase particles can cause the initiation and propagation of cracks in the HAZ. [ABSTRACT FROM AUTHOR]

Herein, the effects of peak temperature on the microstructure and low‐temperature impact toughness of coarse grain heat‐affected zone (CGHAZ) of EH460 steel are investigated by Gleeble simulation welding. The low‐temperature impact toughness of CGHAZ decreases with the increase of peak temperature. With the increase of peak temperature, the microstructure of CGHAZ gradually changes from bainite to a mixed structure composed of bainite, intragranular ferrite, side lath ferrite, and a small amount of grain boundary ferrite. The inclusions size decreases as the peak temperature increases. The existence of large‐sized inclusions is conducive to the initiation and propagation of cracks, reducing the low‐temperature impact toughness of the material. [ABSTRACT FROM AUTHOR]

Simultaneously enhancing the strength, plasticity, and toughness of metal materials has been a prominent research objective. In this study, the precipitation of nano-scale vanadium carbide particles at a lower tempering temperature is controlled to achieve three key benefits: reducing lattice distortion in bainitic ferrite, strengthening through precipitation, and enhancing the toughness of bainitic ferrite, while also maintaining the content and stability of retained austenite. The adopted procedure increases the comprehensive mechanical properties of steel by 17%. In particular, the room-temperature impact toughness is increased by 41%. This comprehensive microstructure optimization endows bainitic steel with an excellent combination of mechanical properties. [ABSTRACT FROM AUTHOR]

This study investigates the role of coalesced bainite in enhancing the hydrogen embrittlement resistance of tempered martensitic steels. By analyzing the microstructural characteristics and mechanical properties under varying cooling rates, it was found that the presence of coalesced bainite significantly impedes hydrogen accumulation at prior austenite grain boundaries. This leads to a transition in the fracture mode from intergranular to transgranular, thereby improving the overall resistance to hydrogen embrittlement in steels. Slow strain rate tests (SSRTs) on both smooth and notched specimens further confirmed that steels cooled at lower rates, which form a higher fraction of coalesced bainite, exhibiting superior hydrogen embrittlement resistance. These findings suggest that optimizing the cooling process to promote coalesced bainite formation could be a valuable strategy for enhancing the performance of tempered martensitic steels in hydrogen-rich environments. [ABSTRACT FROM AUTHOR]

Bainitic microstructures in high‐strength steels are obtained either by continuous cooling or isothermal holding. Both scenarios necessitate faster cooling to keep the parent austenite phase untransformed till the bainite‐start temperature. The present study reports the development of bainitic microstructure in a low‐carbon steel with minimal alloying additions, under continuous cooling at very slow rates, similar to furnace cooling. For understanding the related transformation pathways, samples from the forged‐steel ingot are austenitized and cooled at different rates, viz. water quenching, air cooling, and furnace cooling. Microstructural characterization reveals development of acicular microstructures in all samples including the forged one, with gross absence of carbides. X‐ray diffraction confirms the ferritic nature of acicular plates and also indicated retained austenite present in some samples, the content of which could be correlated to the extent of bainitic transformation. Thermodynamic calculations together with microstructural observations (e.g., ferrite plate size) and hardness data established the development of fully martensitic microstructure on water quenching, while that of a mixed microstructure comprising predominantly of bainite in the forged, air cooled, and furnace‐cooled condition. The aforementioned findings could have wider implications in developing fully bainitic microstructures in large components, where uniform rapid cooling is not practically feasible. [ABSTRACT FROM AUTHOR]

The microstructure and hydrogen embrittlement sensitivity of two X65 pipeline steels at different t 8/5 (the time required to cool from 800 °C to 500 °C) were investigated employing thermal simulation. With the increase of t 8/5 for both steels, the martensite transferred to acicular ferrite (AF) and bainite, and further converts to bainite and martensite/austenite (M/A) constituents. Elongated polygonal Mn–Nb–S inclusions appear in steel A, which are easy to fall off and the hydrogen embrittlement sensitivity is thus increased. However, spherical Al–Ca–Nb–N–O–S inclusions appear in steel B, which can promote nucleation of AF, thus obtaining lower hydrogen embrittlement sensitivity under lower t 8/5. • The HE sensitivity of two X65 pipeline steels at different t 8/5 were investigated. • The elimination of M and the nucleation of AF can reduce HE sensitivity. • Elongated polygonal Mn–Nb–S inclusions easily shed and increase HE sensitivity. • Spherical Al–Ca–Nb–O–S inclusions promote AF nucleation and reduce HE sensitivity. [ABSTRACT FROM AUTHOR]

The influence of austempering temperature on the microstructure evolution and tensile deformation behaviour of 24CrNiMo samples prepared by selective laser melting (SLM) were studied in this study. The results showed that the content of lower bainite increases by 25.2% as the austempering temperature increases from 200 to 250 °C. The samples at 250 °C (below MS) have obtained the best mechanical properties, with the microhardness (536.89 HV0.2), ultimate tensile strength (1714.29 MPa), and elongation (14.3%). The microscopic movement mechanism of the crystal showed that during the plastic deformation process of the sample, a large amount of dislocation slip occurred in the {110}<111> slip system inside the grains, resulting in higher strain energy in the grains. [ABSTRACT FROM AUTHOR]

By means of microstructure observation, phase analysis, and mechanical‐property tests, the effect of martensite–bainite (M–B) duplex microstructure on carbide precipitation and mechanical properties of M50 steel is studied. In that results, it is shown that the distribution of secondary carbides in specimens with M–B duplex microstructure is more uniform and finer, and the stability of retained austenite (RA) in the steel is also improved, so that the content of RA in specimens with M–B duplex microstructure is 2.34%, which is higher than the 0.94% of the specimens with full martensite microstructure. The M–B duplex microstructure leads to the reduction of tempering hardness of M50 steel to 60.9 unit of Rockwell hardness (HRC), compared to the 61.6 HRC of the specimens with full martensite microstructure, but the wear resistance is slightly enhanced. Moreover, the M–B duplex microstructure effectively improves the impact toughness and fatigue properties by refining the microstructure and carbides in the steel, and the increase amplitude is 47.4% and 41.0%, respectively. [ABSTRACT FROM AUTHOR]

By means of salt‐bath austempering treatment, microstructure observation, X‐Ray diffraction analysis, microhardness measurement, and friction and wear performance tests for 8Cr4Mo4V steels with standard composition and with trace addition of 0.005% boron, the effect of boron on the phase composition and mechanical properties of the steel is studied. The results show that trace element boron greatly increases the content of needle‐like lower bainite in the quenched microstructure, promotes the dissolution of undissolved carbides, reduces the content of M/A island, but coarsens grains. After tempering, trace element B increases the amount of bainite generation, refines the lath martensite, and significantly promotes the precipitation of carbides, whose size is finer, and the distribution is more dispersed. In terms of mechanical properties, trace element B improves the hardness by promoting the precipitation of the second phase and refining the lath martensite, and the increase in the bainite content changes the wear mechanism and enhances the wear resistance. [ABSTRACT FROM AUTHOR]

Pipe steel 06Mn2MoNb is studied after hot rolling and subsequent heat treatment, i.e., austenitizing at 980°C, 30 min and cooling at different rates. By means of orientation microscopy and crystallographic analysis using various orientation relationships the possibility is demonstrated of creating structural and textural characteristics based upon transformation products (martensite, bainite) of deformed austenite within the steel. It is shown that the crystallographic texture resulting from a multivariant γ → α-transformation may be explained by α-phase generation at crystallographically ordered boundaries between deformed γ-phase grains. [ABSTRACT FROM AUTHOR]

Hot stamping is a well‐established process in the automotive industry for structural parts to decrease the weight of a vehicle without reducing the safety and stiffness of components. In this work, a bainitic approach during the hot stamping process is studied by anisothermal annealing. The effect of different strategies on the microstructure and mechanical properties of the hot‐stamped parts is investigated. The microstructure is analyzed using light microscopy (LM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) techniques. Tensile and hardness tests are applied to analyze mechanical properties. A microstructure consisting of a carbide‐free bainite matrix containing martensite, fine‐grained ferrite, and retained austenite is obtained. SEM analysis of fracture surfaces from tensile samples reveals that the bainitic microstructure showed mainly a ductile fracture by dimples and voids along with some quasi‐cleavage fracture. [ABSTRACT FROM AUTHOR]

Overcoming the trade-off relationship between strength and ductility has always been a challenge. In this article, a neatly arranged ultrafine bainite that is composed of multi-scale retained austenite was obtained by introducing weak chemical heterogeneity in low-alloy steel. The optimized microstructure makes transformation-induced plasticity (TRIP) behavior and coordinating deformation behavior between each phase to be smoother. This results in better comprehensive properties, with 28.5% and 9% increases in uniform elongation and toughness, respectively, and a similar strength. This study provides a new way to improve the properties of low-alloy steels. A strategy of weak chemical heterogeneity has been proposed in low-alloy steel, which effectively optimizes the microstructure and enhances the plasticity and toughness of steel. [ABSTRACT FROM AUTHOR]

SA516 Gr.70 steel exhibits an excellent combination of toughness and plasticity, which is widely used in the cryogenic field. However, the presence of coarse grain in fusion-welded joints degrades the toughness and ductility of the joint. In this work, SA516 Gr.70 steel was welded using friction stir welding at rotation rates of 400–800 rpm, and the microstructural evolution and mechanical properties of joints were investigated. At 800 rpm, coarse martensite was found in the nugget zone (NZ), and with the decrease of rotation rate, some bainite was also observed besides martensite. The yield strength of joints at different rotation rates was significantly higher than that of base metal (BM). With decreasing rotation rate, the elongation of the joint increases significantly, and the highest elongation of 13.6% is obtained at 400 rpm since the joint with low hardness difference has more areas participating in coordinated deformation, alleviating the concentration of strain at BM. In addition, with the decrease of rotation rate, the toughness of NZ is improved. Finally, the maximum impact energy of 109 J/cm2 was obtained in the NZ at 400 rpm, depending on the finer grains and the presence of bainite. [ABSTRACT FROM AUTHOR]

Reliable microstructure characterization is essential for establishing process–microstructure–property links and effective quality control. Traditional manual microstructure analysis often struggles with objectivity, reproducibility, and scalability, particularly in complex materials. Machine learning methods offer a promising alternative but are hindered by the challenge of assigning an accurate and consistent ground truth, especially for complex microstructures. This paper introduces a methodology that uses correlative microscopy—combining light optical microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD)—to create objective, reproducible pixel-by-pixel annotations for ML training. In a semi-automated manner, EBSD-based annotations are employed to generate an objective ground truth mask for training a semantic segmentation model for quantifying simple light optical micrographs. The training masks are directly derived from raw EBSD data using modern deep learning methods. By using EBSD-based annotations, which incorporate crystallographic and misorientation data, the correctness and objectivity of the training mask creation can be assured. The final approach is capable of reproducibly and objectively differentiating bainite and martensite in optical micrographs of complex quenched steels. Through the reduction in the microstructural evaluation to light optical micrographs as the simplest and most widely used method, this way of quantifying microstructures is characterized by high efficiency as well as good scalability. [ABSTRACT FROM AUTHOR]

Increasing the proportion of bainite and martensite phases is one of the effective ways to enhance the toughness of medium-carbon low-alloy steel(42CrMo). In this paper, the surface heat treatment of 42CrMo steel is carried out by using a laser heat source to achieve complete austenitization, then with electromagnetic induction heating for several times to implement insulation, and eventually to promote bainite phase transformation. In conjunction with the temperature history curves, OM, SEM, and EBSD are used to characterize the bainite morphology and percentage of bainite via different parameters of the hybrid process. The results demonstrate that the hybrid process combined with twice electromagnetic induction heating can yield a considerable proportion of bainite phase in 42CrMo steel. The resulting duplex structure, consisting of bainite and a minor fraction of martensite, exhibits a significant increase (81.1%) in tensile strength, while maintaining similar impact toughness. The elongation after fracture only has a slight reduction of 5.5% compared to that of the substrate. This innovative process provides a practical alternative to traditional bainite isothermal treatment method which must perform in a heat treatment furnace. Therefore, it is presented as a promising solution for enhancing the mechanical properties of medium-carbon low-alloy high strength steels.

The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200–650 °C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases. Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role. The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600–650 °C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate. This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis. This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels. It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.

Martensite and bainite are formed from austenite through the rapid application of Bain's strain. In several studies, martensite is considered as a body-centered tetragonal phase, but it can also be viewed as bcc ferrite supersaturated with carbon, subject to internal residual stresses from incomplete relaxation of Bain's strain. Recent electron backscatter diffraction measurements have revealed a broad spectrum of tetragonality in quenched martensite, which can be attributed to the diversity of internal stress rather than variations in carbon distribution. Therefore, a thermodynamic unit cell model is developed to calculate the kinetics of carbon atom occupancy in particular kinds of octahedral interstitial lattice sites, contributing to tetragonality in loaded ferrite. The model includes a Zener-ordering term that influences carbon atom distribution and consequently affects tetragonality. Simulations suggest that carbon redistribution among octahedral interstitial lattice sites reaches equilibrium with internal stress within an hour at room temperature. The presented model provides a framework for understanding tetragonality in martensite and bainite, incorporating the effects of internal stress and carbon atom distribution in particular kinds of octahedral interstitial lattice sites. [ABSTRACT FROM AUTHOR]

Recently carbide free bainitic steels, with microstructure consisting in nanoscaled bainitic ferrite plates and high carbon-enriched austenite, have been developed. These steels have found commercial use in manufacturing engineering components, nevertheless, their high carbon concentration poses a limitation as they cannot be welded, restricting their potential applications, therefore the actual research has shifted to the investigation to medium carbon steels. In this work, Butt weld – V groove weldment have been realized with a novel medium carbon high silicon carbide-free bainitic steel by means GMAW and GTAW. Optical microscopy, scanning and transmission electron microscopy and X-ray diffraction were used to characterize the microstructures of both the fusion and heat affected zone. Dilatometry was utilized to study the phase transformations during post-welding heat treatment. Tensile and micro-hardness tests were carried out to evaluate the mechanical properties in the as-weld state after post-welding heat treatment. The analysis shows the possibility to obtain high strength weldments with high integrity and high strength (>1 GPa) coupled with reasonable ductility and the absence of cold or hot cracking phenomena.

The impact of silicon and aluminum on phase transformation behavior, particularly bainite, and microstructure evolution in Fe–0.2C–2.5Mn steel are presented. Continuous–cooling–transformation (CCT) and time–temperature–transformation (TTT) diagrams are determined experimentally. An aluminum extended empirical formula is introduced to estimate the martensite start temperature (Ms) with a thorough assessment of existing formulae. Results show that aluminum significantly increases Ms and has a stronger influence on promoting ferritic microstructures than silicon. During continuous cooling, alongside bainite, formation of Widmanstätten structures is induced in aluminum‐alloyed steel at higher cooling rates due to increased prior austenite grain size. Silicon decelerates bainite transformation kinetics by enhancing austenite's chemical stability through carbon enrichment via preventing carbide precipitation and by strengthening austenite against displacive phase transformation via solid solution hardening. Although aluminum has similar effects, incubation time is shortened during isothermal treatment because of the increased driving force, which overcompensates for the retardation effects. A finer carbide‐free bainitic microstructure is achieved in aluminum‐alloyed steel with more pronounced film‐like retained austenite (RA) formation and superior carbon enrichment, improving RA stability and suppressing martensite–austenite island formation. Finally, with the proposed formula, an accurate approximation to experimental Ms is accomplished. [ABSTRACT FROM AUTHOR]

The welding thermal simulation experiment of Q1100 low alloy high-strength steel was conducted using high-temperature laser scanning confocal microscope(HT-LSCM) and Gleeble-3500 thermal simulation tester with different welding heat inputs. In situ observation showed that prior austenite grains merged and grew instantly when the temperature reached the peak temperature, and then continued to migrate and grow during the subsequent cooling process. The prior austenite grain sizes and the CGHAZ microstructure were similar for the different welding heat inputs between high-temperature laser scanning confocal microscope and Gleeble-3500 thermal simulation. The microstructure consisted of granular bainite and proeutectoid ferrite, and a small amount of martensite/austenite (M–A) constituents when at t8/5 of 300 s. With increasing cooling rate, the microstructure transformed into lath bainite and lath martensite. When t8/5 was reduced to 15 s, the microstructure became lath martensite. Microhardness increased gradually as the welding heat inputs decreased, and impact toughness first increased and then decreased. By in situ observation of the microstructure and impact toughness testing, it is feasible to rapidly determine appropriate welding parameters and evaluate the weldability of materials. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy, it is shown that a lath structure is formed in the heat-affected zone of the low-carbon, low-alloy 09G2S steel after electric arc surfacing. Analysis of the results of scanning tunneling microscopy revealed a number of subtle morphological features of the lath structure, which makes it possible to explain the mechanism of its formation and identify it as lower bainite. The lower bainite structure in the heat-affected zone of the base metal of the 09G2S steel is favourable for increasing the impact strength of the coating – base metal composition, which was established earlier in our work. [ABSTRACT FROM AUTHOR]

Effects of transformation temperature and carbon content on the effective grain bounded by high‐angle boundary, i.e., "Bain size", in bainite structure are studied in Fe–2Mn–(0.05–0.75)C (mass%) alloys transformed isothermally between 773 and 673 K. Bainitic ferrite (BF) nucleates at austenite (γ) grain boundary with strong variant selection for carbon content less than 0.2 mass%, resulting in formation of coarse BF regions consisting of the Kurdjumov–Sachs (K–S) variants belonging to the same Bain variant. As temperature is lowered, K–S variant pairs with large misorientation are frequently formed. Variant selection at γ grain boundary becomes weaker for higher carbon content due to frequent intragranular nucleation particularly at 673 K. The Bain size decreases generally as transformation temperature is lowered, which is ≈15 μm at 773 K but down to ≈5 μm at 673 K, and also slightly decreases at 773 K with increasing carbon content. At lower temperatures, it decreases with the increase of carbon content in low carbon range less than 0.2 mass%C but inversely increases in higher carbon range. The observed changes in the variant paring and the Bain size are well understood in terms of the driving force of BF formation and self‐accommodation of transformation strain. [ABSTRACT FROM AUTHOR]

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Based on recent research, it is known that the strength of pearlitic rail steels has reached its limit. On the basis of the literature analysis, the steels used in the world practice for production of bainite railway rails have been identified and their comparison with the chemical composition of steels used in various areas in the world has been carried out. Regularities of austenite decomposition kinetics of 30HGS steel grade with 0.28% C, 1.49% Si, 0.92% Mn, 0.99% Cr have been investigated by the method of mathematical modeling. Analytically, using the developed model, the intervals of cooling rates, within which changes in the mechanism of structure formation during the decay of austenite are observed, are determined. Chemical composition of experimental steels for railway rails was developed and ingots were melted in laboratory conditions with fixing of cooling rate with subsequent study of microstructures. [ABSTRACT FROM AUTHOR]

This study proposed the dynamic modeling and control strategy for GMAW and FCAW processes using two parallel-connected inverters in the power supply to weld an HSLA X70 steel pipeline using short-circuiting and spray transfer modes. The obtained results showed a great similarity between the welding current and voltage of the simulation model and the experimental process. A novel technique, the single-sided enveloped spectrum (SSES), proved highly efficient in estimating droplet detachment frequency (DDF). Mesh area calculations confirmed the high arc stability due to optimized welding parameters. The microstructure of a GMAW pass consisted primarily of coarse acicular ferrite, Widmanstätten ferrite, and polygonal ferrite, with minor traces of bainite. In contrast, the FCAW passes exhibited a wider variety of ferrite morphologies. Grain boundary ferrite (GBF) morphology was prevalent, while the capping passes showed a distinct feature: very fine grains of bainite and acicular ferrite. Vickers micro-hardness measurements revealed a significant reduction in hardness within the Heat Affected Zone and Weld Zone of the GMAW process, down to approximately 180 ± 5 HV. Conversely, the capping pass exhibited the highest micro-hardness, reaching around 230 HV. This increase is attributed to the presence of a fine, columnar microstructure composed of Widmanstätten and polygonal ferrite. [ABSTRACT FROM AUTHOR]

The effect of austempering temperatures varied from 300 to 400°C to form bainite in the structure on the corrosion resistance and mechanical properties of two newly developed FeNi steels containing 1 and 2 wt.% Al is investigated. Corrosion tests of the steels are carried out in a 3.5 wt.% NaCl solution. It is found that an increase in the Al content and in the austempering temperature promotes formation of a bainite lath structure in the steels; however, this has an insignificant effect on the hardness. The results of an electrochemical test show that the corrosion resistance of the steel in marine-like atmosphere increases upon increasing the Al content from 1 to 2 wt.%. The increase of the austempering temperature tends to improve the corrosion resistance of both steels, except for the steel with 1 wt.% Al processed at the highest temperature. It is shown that the corrosion resistance of the steels is more affected by the hardness than by the volume fraction of bainite laths in the structure. [ABSTRACT FROM AUTHOR]

The effect of continuous cooling on the development of the bainite transformation process and the mechanical properties of low-carbon (0.17% C) steel alloyed with 1.73% Mn, 1.35% Si and microalloyed with Nb (0.02%) and Ti (0.04%) is studied. The microstructure and phase analysis is carried out at various continuous cooling conditions. The work hardening behavior under various conditions is examined using the Hollomon equation and modified Crussard–Jaoul work hardening model. The steel is subjected to austenitization in the intercritical range and above the A3 temperature between 790 and 900°C for 5 and 30 min, followed by continuous cooling to form a dominantly bainitic microstructure, which makes it possible to achieve an ultrahigh strength (1100 MPa) at a high ductility (20%). A longer austenitization time contributes to an increase in the strength and ductility of the steel. [ABSTRACT FROM AUTHOR]

Low-carbon Cr-Ni-Mo steels widely used in mechanical engineering are studied: 18Kh2N4MA, 25Kh2N4MA, 25KhN3MA. Thermokinetic diagrams are plotted by a dilatometric method. Temperature-time ranges for microstructure constituent formation are established. It is shown that bainite within the steels studied may be formed both above and below the Ms temperature. Features of isothermal bainite transformation are investigated, kinetics of bainite transformation are determined, as well as the quantitative ratio of microstructure constituents formed as a result of austempering. It is established that the largest amount of bainite within the steel structures studied (80–95%) is achieved at a temperature near Ms. Mechanical properties (strength, ductility, impact strength) of the steels being studied are analyzed after various heat treatment methods: various cooling intensities, upper and lower bainite austempering. It is shown that formation of upper bainite has an ambiguous effect on steel impact strength. [ABSTRACT FROM AUTHOR]

To measure textures of hot rolled martensitic and bainitic steels, electron backscatter diffraction (EBSD) is applied to large areas, each covering about a thousand of prior grains. A scanning step is selected to keep reasonable time of the data acquisition still providing in any grain a significant number of analyzed points. Contributions to the transformation texture appearing from deformation and recrystallization components of the parent texture are estimated with the Bain relationship. To assess the parent austenite state, their proportion is then expressed by a scalar parameter. The results are verified by independent metallographic data. Avoiding the reconstruction of prior grains, this statistical approach will facilitate evaluation of complex austenite structures with comparable fractions of dissimilar material states. [ABSTRACT FROM AUTHOR]

The mechanical properties of ausferritic ductile iron are strongly influenced by the ability of the austenite to undergo martensite formation. Strain-induced martensite formation occurring under the right circumstances results in transformation induced plasticity (TRIP) that improves ductility and strength. TRIP has been shown to depend on the austenite's stacking fault energy (SFE), which describes the crystals micromechanical behaviour. In austenitic FeMnAlSi-TRIP steels, TRIP as a reaction to mechanical load only occurs for SFE <20 mJ/m2. For carbon-stabilised austenite as in ADI, the relationship between stacking faults, mechanical properties and martensitic transformation has not yet been established. To investigate the TRIP-effect in ADI, unalloyed ADI with 3.43 wt% C, 2.52 wt% Si and 0.21 wt% Mn was ausferritised and subjected to tensile tests at temperatures between −180 °C and 200 °C. The amount of martensite produced by thermal and mechanical activation, crystalline microstrain and stacking fault density were measured on deformed and undeformed regions of the specimen by XRD and the resulting SFE calculated. Between −70 and 20 °C, the elongation at fracture exceeded 10 % compared to below 2 % in the temperature range of −130 to −180 °C. At temperatures above 80 °C, elongation at fracture gradually decreased to 4.5 % at 200 °C. High sample ductility was associated with lower SFEs as low as 35 mJ/m2, indicating a correlation of stacking fault formation and strain-induced martensite formation in ADI. [ABSTRACT FROM AUTHOR]

Silicon carbide was employed as a source of silicon and carbon to produce sintered silicon steels, offering the advantages of silicon addition. This work explored the effects of low SiC contents of 1.0 and 2.0 wt.% on as-sintered and as-tempered microstructures and mechanical properties of sintered silicon steels. The experimental sintered steels were produced from mixtures of pre-alloyed Fe-1.50Mo and SiC powder. The mixtures were compacted to tensile bars, sintered at 1553 K for 2.7 ks, and cooled at 9.0 K/s with N2. Sintered specimens were tempered at 873 K and held for different times. The microstructures and mechanical properties of the sintered and tempered specimens were characterized. It was revealed that the sintered silicon-bearing steels showed low temperature phase transformations under cooling at 9.0 K/s. The sintered steel produced by 1.0 wt.% SiC addition had degenerate upper bainite microstructure and its tempered microstructure was upper bainite. The sintered steel produced by 2.0 wt.% SiC addition had a microstructure consisting of bainitic-ferrite plates, martensite plates, and austenite blocks. Its tempered microstructure was upper bainite. The tempered microstructures resembled those of sintered Si-free Fe-Mo-C steels. Tempered specimens exhibited tensile strength higher but elongation lower than sintered ones attributed to carbide precipitation strengthening. [ABSTRACT FROM AUTHOR]

The paper studies the influence of accelerated cooling after helical rolling on the structure and mechanical properties of X70 steel in different sections of the rod. Accelerated cooling is carried out in two modes: cooling and holding at a temperature of 530°C (mode I) and cooling to a temperature of 200°C without holding (mode II). It is found that conditions of cooling affect the fraction and morphology of the bainitic phase. This in turn determines different microhardness and strength in different sections of the rods. [ABSTRACT FROM AUTHOR]

Steel with a combination of strength and plasticity is prevalently demanded for lightweight design and emission reductions in manufacturing. In this study, a high-strength Cr-Ni-Mo martensitic steel treated by quenching and partitioning (Q&P) and ultrasonic surface rolling (USR) processes was studied for both strength and plasticity enhancement. Specimens were austenitized at 850 °C and then quenched to 240 °C via cooling by water, oil, and normalization in quenching. This was followed by partitioning, in which two groups of specimens were heated to 370 °C and 350 °C for 45 min, respectively. At last, all the specimens were quenched to room temperature with the same methods of quenching. The highest tensile strength increased from 681.73 MPa to 1389.76 MPa when compared to as-received (AR) steel after the Q&P process. The USR process with a static force of 800 N further improved the tensile strength of specimens with high tensile strength after the Q&P process, which improved from 1389.76 MPa to 1586.62 MPa and the product's strength and elongation (PSE) increased from 15.76 GPa% to 15.9 GPa%, while the total elongation showed a mitigatory decrease from 11.34% to 10.02%. Tensile fractures were also studied and verified using a combination of strength and plasticity after a combined process of Q&P and USR. [ABSTRACT FROM AUTHOR]

It is frequently found in the literature that an acicular ferrite (AF) dominant microstructure has unsteady or even low‐impact toughness. It is necessary to analyze whether they are qualified for real AF. Herein, a Ti‐killed high strength low alloy steel is prepared and subjected to heat‐affected zones simulation tests to induce various AF‐like microstructures. The morphological differences of these microstructures are compared, and their crystallographic characteristics are analyzed. It is found that even within a single prior austenite grain, the transformed microstructure can be heterogeneous. Local areas having clear lath boundaries and a chaotic or interlocking morphology should be classified as AF. Adjacent laths in these areas are from different close‐packed plane and Bain groups. In contrast, local areas having vague lath boundaries and a granular‐bainite‐like morphology should be classified as granular bainite. Neighboring laths in these areas are from the same Bain group, and a relatively low density of high‐angle grain boundaries can be found. The heterogeneous microstructure consisting of granular bainite and AF has a wider grain size distribution and a higher area fraction occupied by large grains, which can lead to the scatter of Charpy impact energy. Criteria for the identification of AF are also proposed in this research. [ABSTRACT FROM AUTHOR]

Nanostructured bainitic steel obtained through austempering or bainitic transformation of high Si steels consists of bainitic ferrite and carbon‐enriched retained austenite. Secondary tempering of those microstructures tailors their mechanical properties through its unique nanostructure and austenite stability. Herein, fatigue and tensile properties of a nanobainitic steel in different secondary tempering conditions are presented and the results are discussed based on the deformation mechanisms taking place due to the microstructure evolution during secondary tempering. Influence of secondary tempering and elevated testing temperature on tensile and fatigue properties at room temperature and 250 °C is presented. Based on the present deformation, mechanism results are discussed supported by microstructure evolution results obtained through dilatometry, X‐ray diffraction, and microscopy. Depending on secondary tempering temperature, main influencing mechanisms are martensite formation after secondary tempering during cooling, transformation‐induced plasticity effect, and precipitation hardening obtained by secondary tempering. [ABSTRACT FROM AUTHOR]

Micro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel are investigated through experimental simulation and microstructural characterization. The results show that the deformation strengthening effect is fully exerted by thermo‐mechanical control process in steel containing 0.015 % lanthanum. Finally a kind of multi‐layer bainite ferrite microstructure featured by 3.5 μm blocks, 0.41 μm plates and 108 nm sub‐plates with ultrafine sub‐subunits and 55 nm θ‐M3C inside is achieved, which enhances the strength and toughness synergistically. And the nanoscale refinement mechanism of bainite ferrite plates lies in the formation of massive ultrafine sub‐subunits with the average size of 20 nm ×32 nm. Besides, a large number of twinning martensite with the size of 2 nm to 20 nm and high‐density entangled dislocations can be found on the boundaries of ultrafine sub‐subunits. Further, the density of dislocation is increased by 2.92×1014 cm−2 and its contribution to the strength is calculated to be 18 MPa. Moreover, micro‐alloying effects of lanthanum in thermo‐mechanical control process are explored to be that lanthanum enhances the interaction between bainite ferrite plates and dislocations, strengthens the entanglement of carbon and θ‐M3C with dislocations, and promotes the pinning effect of θ‐M3C on ultrafine subunits. [ABSTRACT FROM AUTHOR]

A hot rolled low carbon (0.17%) steel strip with 1.72% Mn, 1.35% Si and micro-alloyed with 0.04% Ti, 0.02% Nb was subjected to single stage quench partitioning followed by air cooling resulted in ultra high strength levels between 1039 and 1178 MPa with a total elongation between 21.6 and 29.9% with the product of strength and elongation exceeding 30 GPa at selected conditions. The yield ratio varied between 0.39 and 0.51. To achieve this range of properties, the steel was subjected to austenitization at 925 °C for 2 min followed by quenching in salt bath. The partitioning was carried out between 291 and 391 °C for 5 min in the salt bath followed by air cooling. The steel microstructure showed ferrite-bainite as the major phase with small amount of martensite and retained austenite. [ABSTRACT FROM AUTHOR]

The structure and mechanical properties of rolled sheets with a thickness of 40 mm made of low-carbon low-alloyed steel after thermomechanical treatment of different modes are investigated. The structural factors that affect obtaining high mechanical properties are established. It is shown that a complex treatment including quenching from the rolling temperature and high-temperature tempering with a subsequent additional cycle of quenching and tempering leads to formation of rolled sheet uniform over the cross section with a fragmented bainite structure with disperse niobium carbides and carbides of cementite type. Such structure provides a yield strength of no less than 630 MPa in combination with high values of the impact energy at negative temperatures and a level of plasticity of δ ≈ 20%. [ABSTRACT FROM AUTHOR]

This study evaluates the feasibility of producing high-strength low-alloy (HSLA) steel using advanced wire-fed laser additive manufacturing (LAM-W) and wire arc additive manufacturing (WAAM) technologies. Optimized parameters were independently developed for each heat source, utilizing a self-designed HSLA steel wire as the feedstock. Microstructural features and mechanical properties of the fabricated steels were characterized and compared, taking into account differences in heat input and cooling rates. LAM-W samples exhibited a finer columnar grain microstructure, while both LAM-W- and WAAM-produced steels predominantly showed lower bainite and granular bainite microstructures. LAM-W demonstrated higher strength and hardness, lower ductility, and comparable low-temperature toughness compared to WAAM. Both processes demonstrated an excellent balance between strength and ductility, with absorbed energy exceeding 100 J at −40 °C. The study confirms the feasibility of producing high-strength and tough HSLA steel parts using LAM-W and WAAM technologies, and compares the advantages and disadvantages of each method. These findings assist in selecting the most suitable wire-fed AM process for HSLA steel fabrication at high deposition rates. [ABSTRACT FROM AUTHOR]

In this study, we investigate the impact of prior ferrite on the bainite transformation kinetics and microstructure of medium-carbon steel interrupted by an intercritical annealing (IAA) process. It was found that the incubation time and completion time decreased from 687 s and 6018 s to 20 s and 4680 s, with the volume fraction of ferrite increasing from 9.5% to 28.6%, while the maximum transformation rate increased from 00271 μm/s to 0.0436 μm/s. The ferrite/austenite interface is introduced, and the nucleation sites are increased to accelerate the subsequent bainite transformation due to the formation of prior ferrite. However, there is a competitive relationship between the number and activation energy of bainite nucleation. According to the experimental results and theoretical calculations, the activation energy of the bainite transformation in the medium-carbon bainite steel decreases gradually with an increase in the volume fraction of prior ferrite. [ABSTRACT FROM AUTHOR]

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

ABSTRACTIn this work, we provide with an overview of the main theories, metallurgical and microstructural concepts used for the design of carbide free bainitic steels adapted to industrial hot rolling and coiling practices. The selected alloys were cast and tested at pilot plant scale and the obtained good combinations of properties tend to validate the thorough and careful approach undertaken to predict the microstructures during the alloy design process.

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C. [ABSTRACT FROM AUTHOR]