Carbide-free Bainitic Steels Research Articles

Data-driven predictions of retained austenite content and yield strength and elucidation of the sliding wear performance of carbide-free bainitic steels

Carbide-free bainitic (CFB) steels have drawn much attention due to their high strength and toughness. The present work aims to build machine learning (ML) based surrogate models to predict retained austenite (RA) content and yield strength (YS) of isothermal transformed CFB steels and study the sliding wear performance. The input data for ML prediction were related to compositions, heat treatment process, and phases. The Random Forest regression and Gaussian process regression were respectively selected to build the predictive models for RA content and YS. Result shows that the predicted RA content agrees well with the experimentally determined ones in steel with high stability of untransformed austenite. For YS prediction, the present surrogate model can predict YS of CFB steels even without additional microstructural information such as dislocation and effective substructure size. Steels with different YS and RA content were designed to study the sliding wear performance. Result shows that the dominant wear mechanism of the tested steels is abrasive wear and the weight loss is inversely proportional to hardness when the load and wear distance are constant. Increasing YS and RA content can increase the initial hardness and work hardening ability at the worn surface, thereby enhancing wear resistance.

Microstructural evolution and mechanical performance of friction stir spot welded ultrafine carbide-free bainitic steel: Role of dwell time

This study focuses on unraveling the role of dwell time on microstructural evolution, mechanical performance, and failure characteristics of friction stir spot welded ultrafine carbide-free bainitic steel. Results clarify that the microstructures of the different weld zones exhibit a receptive behavior to extended dwell time, by which not only coarser martensite is acquired within the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and fine-grain heat affected zone (FGHAZ) but also sub-critical heat affected zone (SCHAZ) experiences a more pronounced tempering. Applying a dwell time of 3 s is sufficient to make an ample bonding width containing refined martensite, being effective in retarding the crack propagation, and in turn, delivering a joint with the highest load-bearing capacity. However, enlarging the bonding width at dwell times beyond 3 s fails to provide higher peak loads owing to the formation of coarse martensite within the SZ. The failure modes of the welds are classified into two distinct modes: interfacial and partial pullout. The reduced upper sheet thickness (∼685 μm) paves the way for crack propagation along the thickness direction, allowing for partial pullout failure to occur.

High-silicon spring steel transformation to carbide-free bainitic (CFB) steel: a mechanical and tribological approach under rolling/sliding conditions

The present study concentrates on the development of carbide-free bainite steel and explores its tribological and mechanical characteristics using a newly devised disk-on-disk tribometer. The microstructure of the treated samples was examined through heat treatment at various holding temperatures (300, 350, and 400 °C) and durations (10, 20, and 30 min), along with isothermal transformation. The wear rate demonstrated significant sensitivity to holding duration, transformation temperature, phase fraction, and the stability of retained austenite. The microstructural analysis encompassed atomic force microscopy (AFM), FE-SEM, x-ray diffraction (XRD), and energy-dispersive x-ray spectroscopy (EDS). The study extensively investigated the robust correlation among mechanical properties, microstructure, wear behavior, and the presence of retained austenite (RA) and bainite volume fraction. The findings indicate that carbide-free bainite steel holds promise for replacing conventional railway wheel track steel in the steel industry. The structural property correlations have developed of high silicon steel, hardness reduced with increasing temperature and isothermal preserving time; resulting in a gradual increase in the wear rate of CFB steel. The salt bath soaking temperature of 350 °C with a period of 30 min was found to be the preeminent wear properties condition for CBF steel.

Fatigue crack growth rate under mixed-mode loading conditions (I+III) of a carbide-free bainitic steel designed for rail applications

Carbide-free bainitic steel was designed for railway infrastructure application, focusing on high-speed and heavy-loaded freight tracks. Considering the complex state of stresses occurring on the rails running surface, mode III plays a significant role in the initiation and propagation of fatigue cracks of rails during service. Thus, the Fatigue Crack Growth Rate (FCGR) under mixed-mode loading conditions (I+III) was evaluated. It was revealed, that fatigue lifetime increases with loading angle modes. In the area of fatigue fracture, transgranular cracking mechanisms dominated. For the stable fatigue crack growth, a trend was observed related to the decrease in the fraction of intergranular fracture with the increasing loading angle modes (α). Secondary cracks indicated privileged cracking directions related to the crystallographic structure of bainite. The influence of the mechanical stability of retained austenite during mixed-mode FCGR requires further in-depth research. These studies contribute to understanding the factors influencing the reliability of railway tracks in terms of designing new materials and modeling the rate of crack growth to precise assessment of the life cycle of rails.

Microstructure–property correlation and strain partitioning behavior in medium-carbon carbide-free bainitic steel

Microstructure–property correlation and strain partitioning behavior in medium-carbon carbide-free bainitic steel

Study on the relationship between the microstructure characteristics, tensile properties and impact toughness of carbide-free bainitic steel

To understand the relationship between microstructure characteristics, tensile properties, and impact toughness of bainite structure, microstructure characterization, μ-DIC and mechanical tests were carried out on bainitic steel subjected to pre-quenching treatment, followed by austenitizing at 800 °C or 880 °C and then by austempering. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite. Compared with 880 °C-austenitized specimen, the final microstructure in the 800 °C-austenitized specimen is significantly refined, and the amount of large martensite/austenite (M/A) islands is also decreased. The finer microstructure of the 800 °C-austenitized specimen results in a relatively uniform strain distribution during the tensile process, delaying the occurrence of tensile necking. In contrast, the strain of the 880 °C-austenitized specimen tends to concentrate at the interface between the bainite region and M/A islands, leading to premature fracture and decreased plasticity. In addition, the smaller retained austenite size in the 800 °C-austenitized specimen enhances its stability, leading to persistent work hardening behavior and improved plasticity. Furthermore, the decrease in the amount of M/A islands in the 800 °C-austenitized specimen reduces the initiation of cracks, and finer packets and blocks increase the proportion of high angle grain boundaries, enhancing the resistance to crack propagation. Therefore, the total absorbed energy and toughness of the 800 °C-austenitized specimen are about twice higher than those of the 880 °C-austenitized specimen.

Microstructure and property evolution during tempering of a medium carbon bainitic steel

Tempering a medium carbon carbide-free bainite steel led to two distinct microstructures and properties below and above the bainitic start (Bs) temperature. Austenite decomposition during tempering, depended on carbon content, tempering temperatures and time, determining retained austenite (RA) stability. Up to 400 °C, most RA remained intact, but a significant reduction occurred above the Bs temperature. Film austenite primarily turned into carbides within the bainitic–ferrite matrix and form lines of carbide particles, while blocky austenite decomposed into troostite at 500 °C and sorbite at 600 °C. During tempering, on the other hand, bainitic ferrite lost tetragonality losing its carbon and resulting in the growth of bainitic–ferrite plates through grain boundary migration and misorientation annihilation. Above Bs, drastic microstructural changes caused significant alterations in tensile properties.

Initiation of internal fatigue crack in a carbide-free bainitic steel during high cycle fatigue

The crack initiation stage is crucial to the overall fatigue behavior of the carbide-free bainitic steels. Through S–N curve testing and fracture observation, it is demonstrated that internal crack initiation is the dominant mode during high cycle fatigue. The crack initiation region exhibits a tilted micro-facet feature, which is along the direction of the maximum shear stress. Through fine characterization of crack initiation region, we proposed the crack initiation mechanism of dislocation dipoles pileup at prior-austenite grain boundaries in slip bands. Finally, the lifetime of the crack initiation stage was estimated from the deformation energy stored by dislocations pileup. It is beneficial for life prediction of the whole fatigue process for CFB steels in the future.

Design of carbide free bainitic steels for hot rolling practices

ABSTRACT In 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.

Quantification of microstructure obtained during isothermal bainite transformation: A novel dilatometry-based model

Microstructural quantification of austenite and ferrite phases during isothermal bainite transformation (IBT) is paramount to tailor properties in low-carbon carbide-free bainitic steel. Such quantification carried out at the end of IBT heat treatment through microstructural analyses is fraught with limitations, such as non-consideration of transformation of the untransformed austenite on cooling post IBT. The current work has, therefore, devised a simple model to quantify volume fractions and the average carbon contents of bainitic ferrite and different austenite regions, namely film and blocky present between ferrite platelets and between bainitic sheaves, respectively, formed during IBT treatments at 375 to 450 °C from dilatometric strain itself. Such quantification, through other means would be very demanding in terms of experimentation and characterization. Microstructural quantification from the devised model is validated with experimental findings and highlights the difference in carbon contents of film and blocky austenite during IBT treatments.

Laser Powder Bed Fusion Fabrication of a Novel Carbide-Free Bainitic Steel: The Possibilities and a Comparative Study with the Conventional Alloy

A newly developed medium-carbon carbide-free bainitic steel was fabricated for the first time utilizing the laser powder bed fusion (L-PBF) technique. Process parameters were optimized, and a high density of 99.8% was achieved. The impact of austempering heat treatment on the bainite morphology and transformation kinetics was investigated by high-resolution microstructural analysis (SEM, TEM, and EDS) and dilatometric analysis, and results were compared with conventionally produced counterparts. Faster kinetics and finer microstructures in the L-PBF specimens were found as a consequence of the as-built microstructure, characterized by fine grains and high dislocation density. However, a bimodal distribution of bainitic ferrite plate thickness (average value 60 nm and 200 nm, respectively) was found at prior melt pool boundaries resulting from carbon depletion at such sites.

Effect of the cooling rate in the medium temperature zone on the phase transformation and microstructure of carbide-free bainitic steel

Controlling continuous cooling is considered an effective means of producing high-strength and high-toughness bainitic steel. A medium carbon bainite frog steel is designed. By combining a thermal dilatometer with in-situ high-temperature metallography, the influence of continuous cooling rate on the phase transformation of bainitic steel in the medium temperature range (500-365 °C) is studied. The microstructure and crystallographic orientation are characterized. The results indicate that with the increase of cooling rate in the temperature range for bainitic transformation, the initial temperature of bainitic transformation significantly decreases, and the transformation kinetics enhance, which causes the cooling rate for obtaining a maximum fraction of bainite significantly shifts to the left. As the cooling rate increases, the morphology of the bainite changes from a mixture of granular and coarse lath-like forms to fine lath-like forms. The volume fraction and average size of retained austenite as well as the size of bainite laths decrease with the increase of cooling rate. In addition, there are significant differences in the high-angle interface under different cooling rates, which is related to the variant selection mechanism of bainitic transformation at different cooling rates. The research results reveal the transformation process and microstructure evolution of bainite under different cooling rates in the medium temperature range, providing support for the microstructure control and performance optimization of bainite frog steel under continuous cooling control.

Design and Properties of Hot Rolled Tough Carbide Free Bainitic Steels

Carbide free bainitic microstructures of steels in hot rolled condition have high potential for automotive and structural applications, where both high elongation and toughness at a high strength level are needed. However, achieving a combination of these properties remains a challenge due to difficulties in ensuring a high stability of retained austenite while maintaining industrial processability. Therefore, an attempt has been made in this work to achieve combined high toughness and high elongation in hot rolled carbide free bainitic steels.

Consequences of thermite welding on the microstructure of the heat affected zone of a carbide-free bainitic steel rail

Due to their excellent Rolling Contact Fatigue (RCF) resistance, Carbide-Free Bainitic (CFB) steels rails have recently received increasing attention to replace pearlitic steels in the railways sector. This is mainly attributed to the TRansformation Induced Plasticity (TRIP) effect, which mechanically transforms austenite into martensite during wheel/rail contact. However, the welding of these carbide-free bainitic steels could lead to a local decrease in mechanical properties, as well as RCF resistance, inside the Heat Affected Zone, which limits the use of these steels. The present work aims to explain the reason of the loss of RCF resistance of these steels after welding. To do so, the microstructure evolution along the Heat Affected Zone is investigated thoroughly, and is linked with its thermal history.In the most critical areas in terms of RCF resistance, the consumption of all retained austenite has been evidenced, thus suppressing TRIP effect. This loss of austenite is mainly observed in a critical range of temperatures and has been linked to the low thermal stability of austenite. The microstructural investigations performed lead to a proposal of improvements in terms of rail chemical composition and/or heat treatments, in such a way that carbide-free bainitic steels rails could be less sensitive to RCF resistance loss after welding.

Design of cooling route for carbide-free bainitic rail steels and resultant microstructures and properties

In the present work, two new low-alloy, carbide-free bainitic rail steels (steel A: 0.29C + 2.49MnCr & steel B: 0.19C + 3.18MnCr wt.%) were designed to match the controlled cooling production process that combines forced air-cooling and nature air-cooling. The effect of alloy design and continuous cooling rate on the bainite production processing window and resultant microstructure was investigated, followed by the establishment of controlled cooling routes. The microstructure, transformation kinetics, and crystallographic characteristics of the specimens under the process were studied, and cuboid specimens were employed to validate the processing feasibility and investigate the correlation between related microstructures and mechanical properties. The results showed that both steels exhibited ultrahigh strength levels, with a tensile strength approaching 1400 MPa and elongation exceeding 16 %. However, the impact toughness of Steel A deteriorated due to the formation of granular bainite. Due to the homogeneous lath bainite morphology and refined bainitic ferrite substructure, Steel B exhibits excellent strength and toughness, essentially meeting the Chinese technical specifications for the procurement of 43 kg/m ∼75 kg/m rail.

Influence of Austenitization Temperature on Tensile Properties and Impact Toughness of Niobium Micro-alloyed Carbide-Free Bainitic Steel

Influence of Austenitization Temperature on Tensile Properties and Impact Toughness of Niobium Micro-alloyed Carbide-Free Bainitic Steel

Effects of above- or below-AC3 austenitization on bainite transformation behavior, microstructure and mechanical properties of carbide-free bainitic steel

The isothermal bainite transformation kinetics, microstructure, crystallography and mechanical properties of a carbide-free bainitic steel, subjected to pre-quenching treatment, followed by austenitization at various temperatures above or below AC3 and then by above-MS austempering, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that, as the austenitization temperature decreases from 980 °C to 860 °C (above AC3), bainite transformation first decelerates and then accelerates; this is primarily a result of the competition between the availability of parent austenite grain (PAG) boundaries as nucleation sites for bainite transformation and the shear resistance of bainite growth, both of which increase with decreasing the PAG size but play a reverse role in bainite transformation rate. Further decreasing the austenitization temperature down to 820 °C and 800 °C (below AC3) even more notably accelerates bainite transformation and shortens the transformation completion time, since the intercritical ferrite (or tempered martensite), which is formed in the intercritical region, favor the nucleation of initial bainitic laths. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite, the feature of which depends on the austenitizing temperature below or above AC3. Below-AC3 austenitization largely refines the microstructure and decreases the amount of large, brittle fresh martensite/austenite blocks, mainly attributed to the synergetic role of the refinement of austenite grains and pre-existence of intercritical ferrite prior to austempering. In addition, the intercritical ferrite and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. As compared with above-AC3 austenitization, below-AC3 austenitization shows large increases in elongation, especially for specimens subjected to shorter time austempering, despite some decreases in yield and tensile strengths.

Effect of bainitic and retained austenite's length scale on the aqueous corrosion behaviour of nanostructured bainite in 3.5 wt% NaCl

Microstructural refinement offers immense advantages in terms of enhancement of strength, hardness and wear resistance in carbide-free bainitic steels. Lath thickness in bainite can be changed while maintaining the similar volume fractions of bainitic ferrite (BF) and retained austenite (RA) by modifying the grain size of austenite by changing the austenitizing temperature. However, the effect of this microstructural refinement on the corrosion response is difficult to predict and has not been studied. Hence, the current study aims to study the corrosion response of carbide-free bainitic steels formed by austenitization at 900°C (NSB_900) and 1000°C (NSB_1000) respectively and austempered at 250°C. Finer BF lath thickness and RA was obtained in NSB_1000 while the volume fractions of both BF and RA were the same in both the blocks of steels. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) technique have been used to evaluate the corrosion behaviour of the two steel blocks. Lower corrosion current density was found for NSB_1000 which had a finer microstructure although corrosion potential was similar for both the specimens due to similar bainitic volume fraction. The side-section SEM of NSB_1000 showed a thinner corroded layer and the top surface showed a better connected and adhered corrosion film as compared to NSB_900 which showed the corroded layer as disconnected patches leading to a significant part of the sample still being exposed to subsequent corrosion. This manifested in NSB_1000 showing higher polarisation resistance, solution resistance, charge transfer resistance and higher tendency towards an ideal capacitive behaviour.

Promoting ductility and formability in a carbide free bainitic steel via pre-annealing treatment

This study aimed to overcome the existing trade-off in strength–ductility in carbide free bainitic steel and improve ductility without compromising strength. Herein, we proposed a novel and simple method to obtain Mn-rich cementite via a pre-annealing (PA) treatment. PA promoted the partitioning of Mn into cementite particles. The Mn-rich cementite increased the Mn content in the retained austenite (RA) and enhanced the RA stability. Moreover, the initial number of Mn-rich cementite microstructures is positively correlated with RA. The effect of PA on the bainitic ferrite (BF) content was evaluated via comparison with the non-PA sample, revealing that the PA treatment promoted BF nucleation and increased the BF content. The PA-treated samples exhibited the optimal transformation induced plasticity (TRIP) effect and mechanical properties; in particular, the PA-3 sample (with 3 h treatment period) improved ductility and formability, with the total elongation increased from 11.9% in the non-PA sample to 18.0% in the PA-3 sample, corresponding to an increase of ∼51.2%. Additionally, the Erichsen cupping value increased from 7.52 mm in the non-PA sample to 9.78 mm in the PA-3 sample, corresponding to an increase of 30.1%. The overall applicability of PA for the improvement of formability of the carbide free bainitic steel is evident in all PA-treated samples.

Unforeseen influence of the prior austenite grain size on the mechanical properties of a carbide-free bainitic steel

The reasons behind the limited impact toughness of medium manganese, medium carbon carbide-free bainitic steels has been discussed in this manuscript. Various hot-rolling schedules have been employed to obtain microstructures with three different prior austenite grain sizes (PAGSs): coarse (CG = 45 ± 5 μm), medium (MG = 18 ± 4 μm) and fine (FG = 8 ± 2 μm) grain sizes. Although the coarse and middle grain size microstructures present similar levels of strength (tensile strength ∼1000 MPa) and ductility (36% and 41%, respectively), the impact toughness measured at room temperature improves significantly from 17 J to 83 J in the middle grain size one because of the lower ductile-to-brittle transition temperature (DBTT) temperature. Compared to the other microstructures, the fine grain size one contains higher contents of blocky unstable retained austenite, which are responsible for the enhanced strain hardening behavior under tensile testing but can be harmful for the toughness. Besides, during the first stages of straining these blocky austenite islands transform effortlessly, deteriorating the ductility (30%) and toughness (58 J) in spite of keeping a similar DBTT as the microstructure with the medium grain size.