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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]

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]

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]

In this study, an analysis of current welding technologies for new highstrength low-alloyed AHSS sheet steels is presented. Over the course of it, a series of welded joints of sheet metal with a thickness of 1.2 mm made of CR1000Y1370T-CH steel (composition: 0.23% C, 2% Si, 3% Mn, 0.015-2% Al, 1% Cr + Mo, 0.15% Ti + Nb, 0.2% Cu) by Voelstalpine (Austria) is investigated. The joints are produced using laser, spot, and argon-arc spot welding under specified conditions. High-quality weld joints are obtained through laser welding with the formation of a dispersed ferrite-bainite structure with an acceptable hardness level (up to 4000 MPa) without martensitic-type structures and zinc evaporation from the sheet coating along the entire length of the welded joint. As discovered, due to delamination and remnants of zinc coating, it is not possible to achieve a high quality of weld joints while using spot welding, as well as argon-arc spot welding. The selected welding conditions for these technologies are found to be ineffective and require further refinement. [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]

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]

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]

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]

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.

An air‐cooled low‐carbon Mn–Si–Cr alloyed steel with multiphase microstructure of carbide‐free bainite/martensite (CFB/M) is developed. The kinetics of bainitic transformation during air‐cooling process is discussed via considering the transformation latent heat with help of the measured temperature‐cooling curves. Results show that the CFB formed during air cooling can effectively refine the multiphase microstructure and stabilize the film‐like retained austenite that contributes to transformation‐induced plasticity effect upon deformation. An optimum combination of strength, ductility, and toughness of the air‐cooled bainitic steel is achieved (tensile strength: 1418 MPa, total elongation: 15.8%, and Charpy V‐notch impact energy: ≈71 J). The newly developed Mn–Si–Cr alloyed air‐cooled bainitic steel exhibits superior mechanical properties to the commercial 23CrNi3Mo steel but has a much lower alloy cost. [ABSTRACT FROM AUTHOR]

In this study, the microstructure evolution of laser solid forming (LSF) 34CrNiMo6 steel with different deposition thickness was investigated. The microstructure of 10 layers (1.35 mm) sample from bottom to top presented the martensite with a large amount of upper bainite, the mixture of martensite and lower bainite, and the mixture of martensite and retained austenite. When deposition thickness reached 20 layers (3.12 mm), the microstructure of the deposition zone consisted of a large amount of bainite (needle-like lower bainite and feather-like upper bainite) and a small amount of martensite. Besides, the number and size of the feather-like upper bainite gradually increased from the top to the bottom of the deposition, and the morphology of martensite changed from lath-shaped to island-shaped. Two types of carbides morphology existed in lower bainite at the top of 20 layers (3.12 mm) sample: a short rod-shaped with an angle of 55-60° from the main axis and a strip-shaped approximately parallel to the main axis. The carbides in lower bainite can be identified as Fe3C. [ABSTRACT FROM AUTHOR]

Temper embrittlement is a major challenge encountered during the heat treatment of high-performance steels for large forgings. This study investigates the microstructural evolution and mechanical properties of Cr-Ni-Mo-V thick-walled steel, designed for large forgings with a tensile strength of 1500 MPa, under different tempering cooling rates. Optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to analyze the microstructural features. The results demonstrate that the embrittlement occurring during air cooling after tempering is attributed to the concentration of impurities near Fe3C at the grain boundaries. The low-temperature impact toughness at −40 °C after water quenching reaches 29 J due to the accelerated cooling rate during tempering, which slows down the diffusion of impurity elements towards the grain boundaries, resulting in a reduced concentration and dislocation density and an increased stability of the grain boundaries, thereby enhancing toughness. The bainite content decreases and the interface between martensite and bainite undergoes changes after water quenching during tempering. These alterations influence the crack propagation direction within the two-phase microstructure, further modifying the toughness. These findings contribute to the understanding of temper embrittlement and provide valuable guidance for optimizing heat treatment processes to enhance the performance of high-performance steels in large forgings. [ABSTRACT FROM AUTHOR]

AbstractMachining 42CrMo4 martensite—ferrite dual phase steel is challenging due to its high hardness and it is essential to determine the favorable requirements for the optimum machining condition. The ability to alter the martensite quantity in dual phase structure of steel leading to variation in bulk hardness is the motivation for machinability investigation. In heavy duty machining, the determination of tool life and surface roughness at various conditions of machining plays an important role in the manufacturing industry. In the present work, machinability tests are carried out on 42CrMo4 martensite—ferrite dual phase steel to assess the tool life and surface roughness. Speed, feed, and depth of cut are varied in different levels. The tests are carried out as per the full factorial method. A microstructure study is performed to correlate the various mechanical properties with phase morphology. The main objective of this study is to obtain the optimized machining process parameters for the turning operation of 42CrMo4 martensite—ferrite dual phase steel. All cutting tests are carried out under dry conditions using a carbide insert. Microstructure and mechanical property analysis shows an increase in the martensite quantity with the increase in the dual phase processing temperature. From the ANOVA results, it is found that the depth of cut is the major contributing factor to the variation of tool life and surface roughness within the range of values considered for the study. Microstructure analysis revealed the distribution of ferrite and martensite phases evenly. The optimum combination of machining parameters is calculated for obtaining a superior combination of tool life and surface roughness. The 42CrMo4 DPS steel may be used for structural applications with wide variation in the property range.

Intermediate transformation structures in the fusion zone of a weld joint of the X80 steel plates 8 mm thick are studied. It is shown that laser welding leads to the formation of predominantly bainite structures of various types and a small fraction of martensite in the fusion zone. The microhardness measurements in different structural phases have been carried out. The fine bainite and martensite structures have been analyzed using scanning tunnel microscopy. [ABSTRACT FROM AUTHOR]

A dilatometric study of the bainite transformation in 20Kh2G2SNMA steel during isothermal holding in the temperature range of 280–380°C was carried out. The kinetics of the bainite transformation is determined, and the values of the coefficients k and n of the Austin–Rickett equation are established. The fraction of retained austenite in the steel structure following isothermal hardening was estimated using X-ray diffraction analysis. It is shown that, at 320°C and higher, the carbon content in retained austenite is in satisfactory agreement with the theoretically predicted values, and below 320°C, there is a discrepancy associated with slower bainite transformation and carbon partitioning. A correlation has been established between the impact strength, the fraction of bainite and retained austenite, and the carbon content in the latter. It is shown that with an increase in the fraction of bainite during isothermal holding, the fraction of retained austenite in steel increases, and the carbon content in it decreases, which adversely affects the impact strength of steel. [ABSTRACT FROM AUTHOR]

The steel 30CrMnSiNiWMo used in the aircraft industry is studied. The temperature and time ranges of austenite transformation during continuous cooling are established using the dilatometric method. The dilatometric modeling of various modes of quenching–partitioning and austempering heat treatment is carried out. The features of transformations occurring in the studied steel at different stages of the heat treatment are revealed. The analysis of mechanical properties after austempering, quenching–partitioning and oil quenching and tempering is carried out. It is shown that the implementation of isothermal hardening does not lead to the expected increase in the level of mechanical properties of the studied steel. [ABSTRACT FROM AUTHOR]

Bainitic transformation goes through three stages: incubation period, nucleation, and growth. The long transformation time is not conducive to industrial production, which restricts its development. Therefore, research on accelerating bainitic transformation is based on the situation of energy saving and efficiency saving. The methods to accelerate bainitic transformation include the control of alloy elements, heat treatment control, etc. This paper focuses on the influence of the pre-formed phase on the kinetics of accelerated bainite transformation. Combined with existing research and experiments, it clarifies the influence of pre-formed ferrite, pre-formed martensite, and precipitated second-phase particles on the kinetics of bainitic transformation. Moreover, it clarifies the characteristics of bainitic transformation in terms of microstructure characterization. [ABSTRACT FROM AUTHOR]

The current state of iron polymorphism is considered as the most significant phase transformation in the industrial technology of iron-carbon alloys, which manifests itself in four forms in the processes of heating and cooling iron. The analysis of literature data revealed the lack of information about the essence of the mechanism of polymorphic transformation of pure iron in domestic basic textbooks and in popular foreign publications on materials science. The main historical stages of the formation and development of the doctrine of iron polymorphism from the moment of its discovery by D.K. Chernov in 1868 to the introduction of the generally accepted diagram by M. Hansen and K. Anderko in 1949 are considered. It is shown that two questions were at the center of scientific discussions: the question is whether iron has polymorphism, and the question of the nature and degree of carbon solubility in polymorphic modifications of iron. The problem of carbon solubility in austenite and ferrite has been a key issue for steelmaking technology. The question of the mechanism of the occurrence of polymorphic transformation, due to its low significance for theory and practice of iron-carbon alloys, has not been widely raised or considered. Consequently, study of the mechanism of polymorphic transformation of pure iron under equilibrium conditions was not relevant for both science and technology of iron-carbon alloys. At present, many researchers believe that, firstly, polymorphic transformation α-Fe ↔ γ-Fe is less studied than similar transformations in other systems, i.e., it is indirectly stated that direct studies of the polymorphism of pure iron have not been carried out at all. Secondly, mechanism of martensitic transformation in steel, which is currently quite well studied, has been arbitrarily transferred to the α-Fe ↔ γ-Fe polymorphic transformation of pure iron. In fact, it is recognized that the mechanism of transformation of supercooled austenite into martensite and polymorphic transformation of pure iron are identical, which is unacceptable. It is well known that the martensitic transformation proceeds by a diffusionless shear mechanism. There are two generally accepted models for the polymorphic transformation for α-Fe ↔ γ-Fe: the shear martensitic mechanism according to the Bain model (tetragonal lattice distortion) and the Kurdyumov-Sachs model (double lattice shift). However, the paradox lies not in these two models, but in the fact that shear martensitic mechanisms were identified with the mechanism of polymorphic transformation of pure iron under equilibrium conditions. On the basis of the performed studies, a logical conclusion is obvious: the mechanisms of both the metastable martensitic transformation in steels and the polymorphic transformation of pure iron in its equilibrium state are based on a single, common diffusion-free deformation transition, but this transition is "triggered" by completely different physical processes, which are absolutely not identical to each other. [ABSTRACT FROM AUTHOR]

Current conventional methods of evaluating microstructures are characterized by a high degree of subjectivity and a lack of reproducibility. Modern machine learning (ML) approaches have already shown great potential in overcoming these challenges. Once trained with representative data in combination with objective ground truth, the ML model is able to perform a task properly in a reproducible and automated manner. However, in highly complex use cases, it is often not possible to create a definite ground truth. This study addresses this problem using the underlying showcase of microstructures of highly complex quenched and quenched and tempered (Q/QT) steels. A patch-wise classification approach combined with a sliding window technique provides a solution for segmenting entire microphotographs where pixel-wise segmentation is not applicable since it is hardly feasible to create reproducible training masks. Using correlative microscopy, consisting of light optical microscope (LOM) and scanning electron microscope (SEM) micrographs, as well as corresponding data from electron backscatter diffraction (EBSD), a training dataset of reference states that covers a wide range of microstructures was acquired in order to train accurate and robust ML models in order to classify LOM or SEM images. Despite the enormous complexity associated with the steels treated here, classification accuracies of 88.8% in the case of LOM images and 93.7% for high-resolution SEM images were achieved. These high accuracies are close to super-human performance, especially in consideration of the reproducibility of the automated ML approaches compared to conventional methods based on subjective evaluations through experts. [ABSTRACT FROM AUTHOR]

Herein, the microstructure evolution and mechanical properties of the 40CrNiMo steel treated by different austempering temperatures are investigated. With the decrease in austempering temperatures from 270 to 180 °C, the volume fraction of bainite decreases, and the martensite fraction increases. The austenite content is about 8% at 270 °C, and then increases over 12% at 240≈180 °C. The mean grain size decreases from 270 to 180 °C, which is related to the reason that the development of bainite/austenite interface is restricted for the martensite formation. The yield strength increases from 1153 to 1323 MPa by refinement hardening and dislocation hardening, and ultimate tensile strength increases from 1193 to 1731 MPa at from 270 to 180 °C. The elongation is about 13% at 210≈270 °C, and then decreases at 180 °C. [ABSTRACT FROM AUTHOR]

Steel 30Kh2GSN2VM (VL1) used in the aircraft industry for heavy-duty parts is studied. The temperature and time ranges of the transformation of supercooled austenite under continuous cooling from the temperature of complete austenitization are determined by the method of dilatometry. Different variants of interrupted and isothermal quenching are simulated to establish the special features of the transformations occurring in the steel in different stages of heat treatment. The mechanical properties are analyzed after isothermal and interrupted quenching and after oil quenching and tempering. It is shown that the isothermal quenching and the stage quenching (the quenching-partitioning technology) do not provide the expected stabilization of a considerable content of retained austenite and elevation of the mechanical properties of the steel. [ABSTRACT FROM AUTHOR]

The microstructure and hardness along the thickness direction of a water-quenched, high-strength thick plate with a thickness of 40 mm were investigated with three specimens from the thick plate: surface, 1/4t, and 1/2t (center) thickness, and the phase transformation behavior of the thick plate according to the cooling rate was analyzed through dilatometric experiments. Finally, the cooling rate for each thickness of the thick plate was estimated by comparing the microstructure and hardness of the thick plate along with the thickness with those of the dilatometric specimens. Martensite microstructure was observed on the surface of the water-quenched thick plate due to the fast cooling rate. On the other hand, an inhomogeneous microstructure was transformed inside the thick plate due to the relatively slow cooling rate and central segregation of Mn. A small fraction of bainite was shown at 1/4t thickness. A banded microstructure with martensite and bainite resulting from Mn segregation was developed at 1/2t; that is, the full martensite microstructure was transformed in the Mn-enriched area even at a slow cooling rate due to high hardenability, but a bainite microstructure was formed in the Mn-depleted area owing to relatively low hardenability. A portion of martensite with fine cementite at the surface and 1/4t was identified as auto-tempered martensite with a Bagaryatskii orientation relationship between the ferrite matrix and cementite. The microstructure and hardness as well as dilatation were investigated at various cooling rates through a dilatometric experiment, and a continuous cooling transformation (CCT) diagram was finally presented for the thick plate. Comparing the microstructure and hardness at the surface, 1/4t, and 1/2t of the thick plate with those of dilatometric specimens cooled at various cooling rates, it was estimated that the surface of the thick plate was cooled at more than 20 °C/s, whereas the 1/4t region was cooled at approximately 5~10 °C/s during water quenching. Despite the difficulty in estimation of the cooling rate of 1/2t due to the banded structure, the cooling rate of 1/2t was estimated between 3 and 5 °C/s based on the results of an Mn-depleted zone. [ABSTRACT FROM AUTHOR]

In this work, we have investigated the effect of martensite/bainite dual phase content on the mechanical properties of EA4T high-speed axle steel. For evaluation and control of the strength, ductility, and toughness of steel, the microstructure of lath martensite (LM) and granular bainite (GB) was clarified through an optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Besides, the tensile fracture morphology was studied by scanning electron microscopy (SEM). For this purpose, this study conducted a quantitative analysis of the LM and GB fractions using the Pro Imaging software-2018 of OM. The remarkable effect of the LM/GB structure on mechanical properties is discussed. The results have shown that by increasing the volume fraction of the GB structure, the LM structure is refined and its microhardness and strength are improved. Meanwhile, the micro strength of LM follows the Hall–Petch relationship with the lath martensite packet size. Subsequently, the mechanical property prediction model of EA4T steel based on the LM/GB content was established by regression analysis of all experiment dates. When the LM fraction in the steel is about 40–70%, a superior combination of strength, ductility, and toughness can be obtained in EA4T steel. [ABSTRACT FROM AUTHOR]

This study investigates mechanical properties of accelerated cooled and self-tempered (AC-ST) H-type S275JR quality steel sections in HEA120 and HEB120 sizes. The cooling process is conducted with a specially manufactured system that sprays a coolant consisting of a water + compressed air mixture on the section surfaces. Cooling times were applied as 10 and 30 seconds using 4 and 12 bar compressed air + water at an average constant pressure of 5 bar and a constant flow rate of 0.08 kg/s. In the HEA120 sections, the highest cooling rate was obtained with 83°C/s in the web region under the cooling time of 30 s and the air pressure cooling condition of 12 bar. At the cooling rate up to 6°C/s, the microstructure is transformed to acicular ferrite and polygonal ferrite phase from Ferrite+Pearlite. But upper bainite phase was formed at a cooling rate of 30°C/s, and a small amount of martensite and lower bainite microstructures were observed at a cooling rate of 60°C/s and above. The hardness in the untreated sections, in the range of 106-120 HB, was increased to 195 HB at a cooling rate of 83 C/s in the web region of the HEA120 section. For a cooling rate of 23°C/s, the maximum compressive residual stresses of -352 MPa are measured in the crotch region of the HEB120. And for a cooling rate of 6°C/s, the maximum tensile residual stresses of 442 MPa were determined in the flange region of the HEA120 section. [ABSTRACT FROM AUTHOR]

Studied here in the essay are the physiochemical characteristics of the anti-corrosion pipe with Cr content ranging from 3% to 9%, and the effects by the contents of the alloy elements on the austenitizing temperature, phase transformation temperatures of cooling process and the micro-structures at room temperature. Results show that mixed structures of bainite/martensite are obtained, when the Cr content is no more than 5%, and the proportion of the two components depends on the cooling speed; the subtotal structures of the pipe with the Cr content more than 7% could transform to martensite at normal cooling speed. Furthermore the relations between the post-tempering micro hardness and the original structures of the pipe are also studied. The results indicate that the steel pipe with more volume fraction of martensite shows larger variation of the post-tempering micro hardness, which could be used to determine the heat treatment process, and control the mechanical properties of the anti-corrosion linepipe as well as the casings. [ABSTRACT FROM AUTHOR]

In addition to conventional isothermal heat treatment, ultrafine carbide-free bainite can be obtained by continuous annealing of steels, which is of great interest to scientists and research communities. The resulting microstructure consists of bainitic sheaves composed of bainitic ferrites interwoven with austenite films in nanoscale, being separated by austenite microblocks resulting in GPa-strength level. This article aims to investigate the microstructural characteristics and impact toughness of high-carbon steel after continuous cooling heat treatment at cooling rates of 0.1, 0.15, 0.2 and 0.3 °C/min. Results indicated that bainitic ferrites and austenite films of less than 270 nm and different volume fractions of bainitic sheaves and high carbon retained austenite could be attained. Wider size distribution of bainitic ferrites was achieved by a slower cooling rate due to the progressive transformation of the primary austenite to bainite. Finally, it has been shown that impact toughness values were increased by decreasing the cooling rate of the steel due to a more effective TRIP effect and obtaining more bainitic sheaves with different orientations, while premature bainite transformation and the presence of martensite deteriorated the impact toughness when the highest cooling rate was applied. [ABSTRACT FROM AUTHOR]

In quality control, microstructures are investigated rigorously to ensure structural integrity, exclude the presence of critical volume defects, and validate the formation of the target microstructure. For quenched, hierarchically-structured steels, the morphology of the bainitic and martensitic microstructures are of major concern to guarantee the reliability of the material under service conditions. Therefore, industries conduct small sample-size inspections of materials cross-sections through metallographers to validate the needle morphology of such microstructures. We demonstrate round-robin test results revealing that this visual grading is afflicted by pronounced subjectivity despite the thorough training of personnel. Instead, we propose a deep learning image classification approach that distinguishes steels based on their microstructure type and classifies their needle length alluding to the ISO 643 grain size assessment standard. This classification approach facilitates the reliable, objective, and automated classification of hierarchically structured steels. Specifically, an accuracy of 96% and roughly 91% is attained for the distinction of martensite/bainite subtypes and needle length, respectively. This is achieved on an image dataset that contains significant variance and labeling noise as it is acquired over more than 10 years from multiple plants, alloys, etchant applications, and light optical microscopes by many metallographers (raters). Interpretability analysis gives insights into the decision-making of these models and allows for estimating their generalization capability.

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10). [ABSTRACT FROM AUTHOR]

To match the strength of ultra‐high‐strength steel (850 MPa) during laser beam welding (LBW), various filler wires having under‐matching (520–640 MPa) and comparable‐matching (890 MPa) strength grades are used. All‐weld metals with no filler or fillers comprise low‐temperature transformation microstructures such as martensite and bainite, while the base metal (BM) comprises bainite, ferrite, and pearlite. All tensile fractures occur in the BM having the lowest hardness as compared to weld metal and heat‐affected zone. Therefore, the hardness and tensile properties of LBW exhibit insignificant variation regardless of the use of filler and their strength grades. This is because the cooling rate of LBW is fast, where a large dilution of the BM with high hardenability and a small deposition of the under‐matching filler occur at zero gap butt joints. To examine these LBW characteristics, the microstructural and hardness behaviors are successfully discussed through simulation of a continuous‐cooling transformation diagram using JMatPro software and cooling rate T9/3 using Rosenthal's two‐dimensional solution. To obtain the effect of lowering weld metal strength in LBW, it is necessary to increase more deposition of under‐matching filler by creating a gap between BMs, and/or to develop a filler of lower hardenability suitable for LBW. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of steel 20Kh2G2SNMA after thermal treatment were investigated. The treatment included cooling at different rates starting from the austenitization temperature. Decrease in the cooling rate 100 times was shown to decrease the strength of steel only slightly; impact elasticity in this case decreases 2.5 times due to formation of bainite. Kinetics of the isothermal bainite transformation and of the martensite formation during continuous cooling of the investigated steel was studied by dilatometry. Parameters of the Austin–Ricket equation, which describes the experimental dependence of the bainite fraction on the exposition time at different temperatures, were found. Coefficients of the Koistinen–Marburger equation, which describes formation of martensite upon cooling, were also defined. A mathematical model for evaluating the microstructure formation upon continuous cooling of steel 20Kh2G2SNMA was developed. [ABSTRACT FROM AUTHOR]

Machining 42CrMo4 martensite--ferrite dual phase steel is challenging due to its high hardness and it is essential to determine the favorable requirements for the optimum machining condition. The ability to alter the martensite quantity in dual phase structure of steel leading to variation in bulk hardness is the motivation for machinability investigation. In heavy duty machining, the determination of tool life and surface roughness at various conditions of machining plays an important role in the manufacturing industry. In the present work, machinability tests are carried out on 42CrMo4 martensite--ferrite dual phase steel to assess the tool life and surface roughness. Speed, feed, and depth of cut are varied in different levels. The tests are carried out as per the full factorial method. A microstructure study is performed to correlate the various mechanical properties with phase morphology. The main objective of this study is to obtain the optimized machining process parameters for the turning operation of 42CrMo4 martensite--ferrite dual phase steel. All cutting tests are carried out under dry conditions using a carbide insert. Microstructure and mechanical property analysis shows an increase in the martensite quantity with the increase in the dual phase processing temperature. From the ANOVA results, it is found that the depth of cut is the major contributing factor to the variation of tool life and surface roughness within the range of values considered for the study. Microstructure analysis revealed the distribution of ferrite and martensite phases evenly. The optimum combination of machining parameters is calculated for obtaining a superior combination of tool life and surface roughness. The 42CrMo4 DPS steel may be used for structural applications with wide variation in the property range. [ABSTRACT FROM AUTHOR]

Effect of partitioning on structure and mechanical properties of a Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel processed by quenching-partitioning (Q&P) was considered. This Q&P steel exhibited a superior combination of the yield stress (YS) of ∼1150 MPa, ultimate tensile strength (UTS) of ∼1650 MPa and elongation-to-failure (δ) of ∼21 %. The product of strength and elongation (PSE – UTS × δ) was >30 GPa × % after partitioning, with a duration ranging from 60 to 500 s. During partitioning, three main processes occurred: the carbon partitioning between retained austenite (RA) and martensite/bainitic ferrite (BF), the transformation of blocky RA into bainite, and the precipitation of transition η–Fe 2 C carbides in the martensitic matrix. The enrichment of RA by carbon changed the mechanism of bainitic transformation and then suppressed the formation of BF and induced the reverse growth of RA upon 500 s partitioning. The rate of carbon partitioning depends on the diffusion path that produces a wide distribution of carbon concentration in areas of RA. The formation of secondary martensite and bainite takes place in areas of untransformed austenite with relatively low carbon content. The effect of RA volume fraction on ductility and the PSE is insignificant. Carbon enrichment of RA ceased its transformation into strain-induced martensite and induced twinning during tension. [ABSTRACT FROM AUTHOR]

A novel approach to enhance the ductility of the transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel was proposed in this work. Lamellar lath Mn-heterogeneity was introduced into the single austenite grain through austenite-reversion from martensite by slow-heating or intercritical annealing. Acicular austenite is formed accompanied with apparent Mn enrichment occurring during reversion, and such a lamellar lath Mn-inhomogeneity is remained even after full-austenitization. Highly stable retained austenite (RA) is formed at the original Mn-enriched region after austempering with higher Mn and carbon contents than those of traditional austempering from Mn-homogenized austenite. In addition, higher total fraction and larger ratio of film-like RA is obtained in the heterogeneous-austenite than homogeneous one. These contribute to the ductility improvement of the TBF steel. [Display omitted] [ABSTRACT FROM AUTHOR]

• Spall strengths of steels involving phase-transition can be obtained using traditional acoustic methods by employing flyers without phase transitions with asymmetric impact configurations. • Higher spall strength results in an increased peak stress to activate additional spall planes, which can be attributed to the α-ɛ phase transition in steels. • Quasi-brittle fracture mode with mixed brittle (transgranular cleavage crack) and ductile (void nucleation and growth) fracture characteristics is observed for the dynamic spall of both bainitic and martensitic steels. • The dynamic spall damage behaviors of steels depend on the coupled effects of microstructure and phase transition. Microstructures determines the spall strength and fracture modes, while phase transition control the occurrence and morphology of multiple spall planes. This work presents a comprehensive and in-depth investigation of spall damage behaviors of bainitic and martensitic steels involving phase transitions. Plate-impact experiments, postmortem characterizations, molecular dynamics simulations considering realistic microstructures of bainitic and martensitic steels, and one-dimensional hydrodynamics simulations incorporating phase transition and spall damage are conducted and investigated. Origins of the multiple spall planes and their characteristics within steels are elucidated. Both steels exhibit a quasi-cleavage fracture mode involving phase transitions, with damage morphologies dependent on the microstructures and validated by molecular dynamics simulations. One-dimensional hydrodynamics simulations successfully replicate the dynamic spallation evolutions in steels considering phase transitions. [ABSTRACT FROM AUTHOR]

The present study aimed to investigate the impact of the as-quenched microstructure and tempering temperature on the final microstructure and hardness of a medium-carbon, low-alloy steel using dilatometry, Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The results revealed substantial differences in the continuous heating stage of tempering in bainitic and martensitic samples, primarily attributed to the auto-tempering process during cooling. Tempering was carried out at 550 and 620 °C, and dilatometry results, along with microstructure analysis, indicated incomplete decomposition of retained austenite (RA) at both temperatures during a 30-min hold in the bainitic sample. The results show that non-decomposed RA, following the tempering of bainite, transformed into blocky fresh martensite, while no evidence of fresh martensite was observed in the martensitic sample. A new approach using EBSD and SEM images revealed that the decomposition of M/A (martensite/austenite constituent) zones in the bainitic sample resulted in the formation of a chain of aggregated chromium carbide zones at the grain boundaries. In contrast, the martensitic zone exhibited a uniform distribution of carbides in the microstructure. The stability of the phases was examined using the TCFE10 (thermodynamics) and MOBFE5 (mobility) modules of the DICTRA Themo-Calc software. Hardness measurements on all samples indicated decreases by about 18–24 % in the martensitic sample after tempering, while the bainitic sample exhibited a 5 % increase in hardness after tempering, attributed to secondary hardening and fresh martensite formation. [Display omitted] • Martensite reacts differently to non-isothermal tempering than bainite. • Formation of aggregated carbide zones correlate to incomplete bainitic phase transformation and retained austenite. • Fresh martensite formation during the final cooling of bainite tempering increases hardness compared to as-quenched bainite. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of engineering steels with high supercooled austenite stability after cooling at different rates from the austenitizing temperature are studied. Thermokinetic diagrams are plotted for supercooled austenite transformation and temperature-time intervals are determined for structural component formation during continuous cooling. In order to evaluate mechanical properties trajectories and cooling rates are established by experiment for specimens in various media, i.e., oil, still air, and a special container filled with cast iron shavings. It is shown that when the experimental cooling path is superimposed on a thermokinetic curve it is necessary to consider instantaneous cooling rates in different temperature ranges. The effect of cooling intensity on mechanical properties of steels of different composition is established quantitatively. It is found that the impact strength of steels of different alloy systems depends in different ways on cooling rate. [ABSTRACT FROM AUTHOR]

The combined effect of the C and Si contents on the microstructure and low-temperature impact toughness of heat-affected zone (HAZ) of the offshore engineering steel. As the C and Si content is decreased from 0.09 to 0.07 wt.% and 0.12 to 0.03 wt.%, the HAZ toughness at −40 °C is increased from 36 to 180 J, and the hardness in the coarse-grained heat-affected zone (CGHAZ) is decreased from 325 to 297 HV0.05. The higher C and Si content promote the precipitation of M23C6 and cementite, but restrain the formation of MC and M2C carbide in the present developed steels. The microstructures in the CGHAZ are comprised of a mixture of martensite and bainite after the welding with the heat input of 50 kJ/cm and are similar with the different C and Si contents. As the C and Si contents are decreased, the density of the geometrically necessary dislocation (GND) is increased. The fracture is changed from brittle to ductile manner, and the cleavage facet size is decreased from 23 to 11 µm. Reducing the C and Si contents significantly decreases the packet size from 25 to 17 µm, thereby leading to the improvement of HAZ toughness. [ABSTRACT FROM AUTHOR]

The mixed structure of lath bainite and martensite was obtained by isothermal transformation tests at 698 K, 673 K and 623 K in ultra-high-strength steel. The interweaving mode of lath bainite and martensite was revealed by colored metallography and band contrast maps based on Gaussian fit. The crystallographic characteristics were analyzed in terms of variant pairing. The twin-related V1/V2 variant pairs were found in the region with high band contrast (BC) values, inferred as lath bainite. While in the low-BC value region, which was inferred as lath martensite, besides the dominated twin-related V1/V2 variant pair, V1/V4 variant pairs with low-angle grain boundaries were occasionally revealed. According to the classification by Takayama et al., both variant pairings of lath bainite and martensite in this work correspond to a type II mode. The present work has confirmed, even with complex microstructure, i.e., mixed lath bainite and martensite were formed depending on the processing route, bainite and martensite still follow their own rules of variant pairing. [ABSTRACT FROM AUTHOR]

In this study, a novel precipitation-quenching & partitioning (P-Q&P) process was proposed by combining a proper intermediate holding treatment with the Q&P process, which successfully increased the strength of a V–Ti–N microalloyed steel without sacrificing the plasticity. However, the impact toughness of the P-Q&P samples is lower than that of the Q&P sample. Compared to the Q&P sample, the P-Q&P samples have more retained austenite. In addition, coarser substructures of martensite and bainite were formed in the P-Q&P samples. All the P-Q&P and Q&P samples contain two types of carbonitrides, which are the large-size particles (enriched in Ti) formed or undissolved in austenite and the small-size particles (enriched in V) formed in martensite and bainite. The P-Q&P samples have a smaller size and larger volume fraction of the large-size particles than the Q&P sample. The increase in the strength of the P-Q&P samples is attributed to the precipitation strengthening of the carbonitrides formed in austenite during the intermediate holding treatment. And the maintained elongation is mainly caused by the higher austenite content in the P-Q&P samples. The poor toughness of the P-Q&P samples is mainly resulted from the coarser substructures. [ABSTRACT FROM AUTHOR]

Emerging grades of press-hardening steels such as Ductibor® 1000-AS are now commercially available for use within tailor-welded blanks (TWBs) to enhance ductility and energy absorption in hot-stamped automotive structural components. This study examines the constitutive (hardening) response and fracture limits of Ductibor® 1000-AS as functions of the as-quenched microstructure after hot stamping. Three different microstructures consisting of bainite and martensite were obtained by hot stamping with die temperatures of 25 °C, 350 °C, and 450 °C. Mechanical characterization was performed to determine the hardening curves and plane-stress fracture loci for the different quench conditions (cooling rates). Uniaxial-tension and shear tests were conducted to experimentally capture the hardening response to large strain levels. Shear, conical hole-expansion, plane-strain notch tension, and Nakazima tests were carried out to evaluate the stress-state dependence of fracture. A mean-field homogenization (MFH) scheme was applied to model the constitutive and fracture behavior of the mixed-phase microstructures. A dislocation-based hardening model was adopted for the individual phases, which accounts for material chemistry, inter-phase carbon partitioning, and dislocation evolution. The per-phase fracture modelling was executed using a phenomenological damage index based upon the stress state within each phase. The results revealed that the 25 °C hot-stamped material condition with a fully martensite microstructure exhibited the highest level of strength and the lowest degree of ductility. As bainite was formed in the final microstructure by quenching at higher die temperatures, the strength decreased, while the ductility increased. The predicted constitutive and fracture responses in the hot-stamped microstructures were in line with the measured data. Accordingly, the established numerical strategy was extended to predict the mechanical behavior of Ductibor® 1000-AS for a broad range of intermediate as-quenched microstructures. [ABSTRACT FROM AUTHOR]

Small components can obtain microstructure of martensite and stable retained austenite (RA) by quenching and traditional partitioning (Q&P) process, thereby demonstrating excellent performances. However, the Q&P is difficult to be realized in large-size parts with complicated multiple microstructures, especially the big property differences from surface to working depth. A novel dynamic quenching–partitioning process for large products was developed by water spraying and critical bainite transformation kinetics was taken into consideration in this process. Typical as-treated microstructure is composed of triplex phases mainly including martensite, bainite and RA. Good combination of strength and toughness have been gained because of multiphase structure and film RA between bainite plate by it. Corresponding ultimate tensile strength and impact energy are 1363 MPa and 81 J at 10 mm depth from the surface as well as 1182 MPa and 65 J at 50 mm depth, respectively, keeping high strength level of over 1100 MPa and demonstrating relatively low gradients. Simulation results also showed that spraying parameters can adjust temperature variation and phase transformation near the surface, realizing the novel process in large-size parts. And it is suitable for multiphase steels, providing a new idea for large-size parts with energy and cost savings. [ABSTRACT FROM AUTHOR]

Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 °C, the microstructure changes from tempered martensite + bainite + blocky retained austenite (RA) to martensite + coarse bainite plate + blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 °C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 °C increased by 702 MPa, but the toughness and ductility levels are almost the same. [ABSTRACT FROM AUTHOR]

The effects of Cr addition and cold deformation in increasing strength and altering the austempered structure of a FeNi steel is discussed. Cr content in the steel was varied from 2 to 3 wt% whilst cold deformation was undertaken on the material at either, 10% or 50%. The cold deformed steel was then subjected to austempering treatment at 400 °C for 30 min to obtain bainite structure in the samples. Post-austempering scanning electron microscopy (SEM) images of the FeNi-Cr samples showed the formation of lower bainite (LB) and martensite/retained austenite (M/RA) in the microstructure, whilst, in the FeNi sample showed the presence of ferrite in addition to LB and M/RA. The hardness and strength of the austempered FeNi-Cr samples increased significantly with increasing Cr content. Hardness and strength values of up to 46 HRC (from ⋘<20 HRC in FeNi) and above 1400 MPa (from ~800 MPa in FeNi), respectively, were obtained in 50% cold-deformed austempered FeNi-3 wt% Cr sample. However, the elongation decreased with increasing Cr content from 22% in the FeNi to 17% in the FeNi-3 wt% Cr sample. These findings suggest that while strength and hardness increase in cold-deformed austempered FeNi with increasing Cr content, the ductility decreases. [ABSTRACT FROM AUTHOR]