Superbainitic Steel Research Articles

Roles of cooling rate of undercooled austenite on isothermal transformation kinetics, microstructure, and impact toughness of bainitic steel

At present, the application of extra thick super bainite steel has attracted considerable attention. In this paper, medium carbon bainite steel is taken as the research object, and the effects of the cooling rate of undercooled austenite on the transformation kinetics, microstructure and mechanical properties of bainite steel are studied systematically. The results show that the incubation period of bainite shortens and the transformation kinetics accelerates with the decrease of the cooling rate of undercooled austenite. The segregation of solute atoms at a slow cooling rate shortens the incubation period. When bainite nucleates and grows by an autocatalytic method, the size of the untransformed austenite is large and the strain compatibility is strong, leading to low activation energy of its autocatalytic nucleation and accelerating the kinetics of bainite transformation. The size of block structure and the length of bainite sheaves decrease with the increase of cooling rate. The impact toughness of the sample with a cooling rate of 30 °C/s is twice as high as that of the sample with a cooling rate of 0.3 °C/s. As the smallest unit to control the toughness of bainite steel, block structure affects the impact toughness. The impact toughness of bainite increases with the decrease of its block size. The content of film-like retained austenite increases with the increase of cooling rate, which is also one of the factors of excellent impact toughness of bainite at high cooling rate.

Accelerated alloy discovery using synthetic data generation and data mining

The search for new alloys with improved properties is never ending with infinite combinations and amounts of alloying elements in the alloy. Advancements in machine learning have made navigating this enormous search space feasible. However, training the machine learning models and tuning their hyper-parameters to make accurate predictions can be time-consuming and often require high-performance computing resources. Furthermore, the quality of the predictions depend on the availability of sufficient training data. Here, we present a generic approach to accelerate alloy discovery by coupling high throughput CALPHAD calculations, synthetic data generation, and data mining. As a demonstration of the approach, we design super bainitic steels that form bainite at 200∘C in lower transformation times.

Micromechanical simulation of plastic deformation behavior and failure commencement in high silicon bainitic steel after austempering

The plastic deformation and failure initiation of super bainitic steels containing different amount of bainite were analyzed using a real microstructure-based micromechanical finite element method (FEM). Real micrograph of such phase steels obtained by microscope were utilized as the representative volume elements (RVEs) in the micromechanical approach. Ductile mode of failure throughout selected RVE was simulated in term of strain localization for the duration of deformation. Calculations were carried out on the representative volume to numerically analyze the effects of characteristics of every individual constituents phase on the macroscopic mechanical properties of steels regarding their quantitative volumetric amount. The results obtained from simulation were assessed against experimental records and it was determined that the real microstructure-based FEM can predict strength and also ductility of the studied bainitic steels. Furthermore, shear failure mode occurs in all heat treated state of the studied steel which correlate quite well with experimental findings.

Leaf spring model for heavy load vehicle using solid works and ANSYS analysis

In present generation, suspension plays most important aspects in heavy automobile vehicles the optimization of suspension, the leaf spring is introduced with new material selections in this paper. Leaf spring is a simple form of spring, it is widely used for the suspension in wheeled vehicles. Leaf spring is long narrow plates attached to the body of the trailer axle. Leaf spring plays major role in automobile industries. Multi leaf spring in Indian Automobile Industries are very large automobile industries in Indian manufacturing companies majorly producing multi leaf spring for heavy load vehicles because this leaf spring it acts as an energy absorbing material on the virtue of its deflection. In this paper the study includes, the version of leaf spring is created in stable works of different materials FRP, Titanium Alloy, Super Bainite and Structural Steel. The 3D modeling of leaf spring is carried out using SOLIDWORKS 2013 and the analysis is done using ANSYS 17.2 software.

Influence of Carbon Content and Isothermal Heat Treatment Temperature on the Microstructure and Mechanical Properties of Ultra-High Strength Bainitic Steels

The effect of carbon content and isothermal heat treatment conditions on the microstructure evolution and mechanical properties of ultra-high strength bainitic steels was investigated. A reduction in carbon content from 0.8 wt% to 0.6 wt% in super-bainite steel with typical chemistry effectively improved not only the Charpy impact toughness but also the strength level. This suggests that reducing the carbon content is a very promising way to obtain better mechanical balance between strength and impact toughness. The higher Charpy impact toughness at a lower carbon content of 0.6 wt% is thought to result from a reduction in austenite fraction, and refinement of the austenite grain. The coarse austenite grains have a detrimental effect on impact toughness, by prematurely transforming to deformation-induced martensite during crack propagation. Mechanical properties were also affected by the isothermal treatment temperature. The lower isothermal temperature enhanced the formation of bainitic ferrite with a refined microstructure, which has a beneficial influence on strength, but reduces impact toughness. The lower impact toughness at lower isothermal temperature is attributed to the sluggish redistribution of carbon from the bainitic ferrite into the surrounding austenite. Higher solute carbon in the bainitic ferrite contributes to an increase of strength, but at the same time, encourages a propensity to cleavage fracture. Key words: high strength, bainitic ferrite, heat treatment, austenite, impact toughness

Effects of Undercooling and Transformation Time on Microstructure and Strength of Fe–C–Mn–Si Superbainitic Steel

A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation, microstructure, and strength of Fe–C–Mn–Si superbainitic steel. The results indicate that the strength of the samples decreases and the elongation increases with the isothermal transformation time, resulting in an increase in the product of the tensile strength and the total elongation. Therefore, the prolongation of the transformation time can improve the comprehensive property of the sample. In addition, the morphology of bainite changes from granular bainite to lath-like one with a decline in the isothermal transformation temperature, leading to an increase in strength. The elongation first increases and then decreases with a decrease in the isothermal transformation temperature. Finally, the sample shows the best comprehensive property when it is austempered at an intermediate temperature (350°C). The data presented here are instrumental in optimizing the technology of austempering treatment in industrial production.

Advanced high strength steel (AHSS) development through chemical patterning of austenite

A new approach for the development of medium Mn AHSS is proposed in this work. It exploits a fine scale (~100nm) chemical patterning of Mn in austenite that may be obtained by the austenitisation of a pearlite structure formed under conditions with strong Mn partitioning between ferrite and cementite. The resulting ‘ghost pearlite’ ferrite/austenite can be subsequently treated to exhibit an interesting combination of ultimate tensile strength (UTS) (1600–2100MPa) and ductility (7–10% elongation). The simple steel composition and very short processing provides a UTS-ductility combination similar to maraging steels and super bainitic steels.

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

Two-body abrasion wear mechanism of super bainitic steel

The abrasion wear resistance and wear mechanism of super bainitic steel austempered at different temperatures and time have been investigated by two-body abrasion testings, scanning electron microscopy, X-ray diffraction, transformation electron microscopy and electron backscattering diffraction. The results show that the two-body abrasion wear mechanism is predominantly micro-ploughing abrasion, and the wear resistance is decreased with increasing isothermal temperature which is attributed to the decreased hardness caused by a higher retained austenite content and the coarsening of the microstructure. The sample austempered at 230°C for 2 h with the higher amount of retained austenite and lower carbon concentration exhibits excellent wear resistance, and the transformation induced plasticity effect is observed during wear process which is beneficial for the improving of hardness and wear resistance.

Cyclic stress induced phase transformation in super-bainitic microstructure**Project supported by the National Natural Science Foundation of China (Grant Nos. 51171030 and 51604034), the Scientific and Technological Planning Project of Jilin Province, China (Grant No. 20150520030JH), and the Scientific and Technological Research Fund of Jilin Provincial Education Department during the Twelfth Five-year Plan Period, China (Grant No. 2015-95).

The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon super-bainitic steel were investigated. Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite, which will obviously enhance the tensile strength and the product of tensile strength and ductility. The higher the bainitic transformation temperature, the lower the transformation rate of the retained austenite. The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation.

Isothermal Transformation of a Commercial Super-Bainitic Steel: Part I Microstructural Characterization and Hardness

The effects of isothermal treatment on the microstructure and hardness of commercial super-bainitic steel were investigated. A series of isothermal treatments were carried out at temperatures of 210-250 °C for different time periods. The results indicate that the bainitic reaction and hardness were very sensitive to the isothermal transformation temperature. The fine super-bainitic microstructure, containing the carbide-free bainitic ferrite lath and the carbon-enriched retained austenite film, can be produced by heating to 210 °C for 30 h, resulting in a hardness of 662 HV. By increasing the isothermal transformation temperature, the bainitic transformation kinetic is accelerated; however, this is at the expense of coarsening bainitic ferrite laths and decreasing the bainitic ferrite quantity. The relationship between hardness and microstructures obtained under different isothermal treatments, which is correlated with the carbon concentration, dislocation density, bainitic amount and super-bainite size, is discussed in detail.

Synchrotron analysis of toughness anomalies in nanostructured bainite

High-resolution synchrotron X-ray diffraction has been used to characterise the notch root regions of Charpy impact test specimens of a superbainitic steel, both before and after loading. The changes in the volume fraction of austenite induced by the application of a three-point-bending load were quantified. Analysis of diffraction peak shifts revealed the extent of residual tensile and compressive strains present due to both machining and an applied load. The results lend support to the hypothesis that the comparatively low energies absorbed during Charpy impact testing of superbainitic steels, <7 J, are due to the formation of stress-induced martensite at the notch root, prior to crack initiation.

Feasibility study for thixoforming nanostructured bainitic steels

Super Bainite Steel (0.74% C, 1.64% Si, 1.82% Mn, 1% Ni, 0.36% Mo, 0. 21% Cr, 0.047% Al bal. Fe) after cold plastic deformation was used as the starting material for bainitic treatment preceded by controlled cooling from the thixoforming temperature range. Differential Scanning Calorimetry was used to determine the amount of liquid phase fraction vs temperature in the solidus–liquidus range. The steel was heated up to 1430°C, which corresponded to about 30% of liquid fraction. It was then rapidly cooled in oil at three different temperatures: 240°C, 270°C and 300°C. The samples were then held at those temperatures for 5h. The microstructure of the samples after treatment at 240°C showed globular grains (average size of 115µm), containing carbide-free bainite as a mixture of ferrite plates with average thickness of 63nm, and retained austenite plates of 40nm in thickness. Plates of martensite and blocks of austenite were visible in some grains. X-ray studies confirmed the presence of 20.1% of austenite and 79.9% of ferrite with martensite. The sample revealed the highest compression strength of 6651MPa, at yield strength of 1780MPa and compression strain ε=38%. With the increase of isothermal treatment temperature to 270°C and 300°C, yield strength decreased to 1370MPa, 1375MPa and compression strength was 5243MPa, 4138MPa, respectively, while plastic strain reached 38.5% and 25%, respectively. Higher temperature of bainitic treatment led to coarsening of super bainite plates and as a consequence, a decrease in mechanical properties. Initiation of cracks was observed inside grains. They propagate through the globules and eutectic, unlike in thixoformed tool steels, in which the crack path runs along the eutectic mixture between the globular grains. Preliminary studies of the corrosion resistance of Super Bainite steels containing a carbide-free bainite structure were carried out. Results indicate slightly a higher resistance in chlorides environment of the sample isothermally treated at 240°C/5h preceded by controlled cooling from thixoforming temperature range than the sample treated in the conventional way (1000°C/15min/240°C/20h).

The effects of external compressive stress on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel

Abstract The effects of external compressive stress below the yield strength of austenite on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel were studied through dilatometry, scanning electron microscopy and X-ray diffraction analysis simultaneously. The results show that even under a relatively low isothermal temperature of 300°C, compressive stress less than yield stress can clearly accelerate bainitic transformation because of the increase in total driving force for bainitic transformation and anisotropic transformation strain. Bainite ferrite becomes longer, the orientations of bainite sheaves become more uniform and the transformation strain becomes anisotropic under external compressive stress because of variants selection. The amount of bainite increases while the amount of martensite decreases with the application of external stress.

Dynamic observation of bainite transformation in a Fe-C-Mn-Si superbainite steel

The dynamic observations of bainitic transformation in a Fe-C-Mn-Si superbainite steel were conducted on a high temperature laser scanning confocal microscope. It is indicated that the mutual intersection of bainite sheaves often occurs during growth of bainite ferrite, resulting in an interlocked bainite microstructure. Moreover, bainite transformation is promoted by higher austenization temperature and the longer and finer bainite platelets are obtained. Further, The average growth rate of bainite after austenization at 1 100 ° is calculated as 5.8 µm·s−1. In situ observation investigation makes it possible to identify bainite transformation in real time during isothermal holding.

Effect of Undercooling and Austenitic Grain Size on Bainitic Transformation in an Fe–C–Mn–Si Superbainite Steel

Superbainite steel is a new high strength steel, widely used in mechanical machinery, structure, bridge engineering etc. The effects of transformation and austenization temperatures on bainitic transformation in an Fe–C–Mn–Si superbainite steel were investigated by the combination of high temperature metallography and dilatometry. The experimental results indicate that the overall kinetics of bainite transformation is promoted by higher austenization temperature and lower isothermal transformation temperature. In conclusion, the nucleation of bainite nuclei of the steel under current study is mainly controlled by carbon diffusion, whereas the growth of bainite nuclei is dominated by displacive mechanism. Moreover, isothermal transformation temperature has a far more significant effect on bainitic transformation than austenization temperature.

Estimation of the Volume Percent of Normal and Nanostructured Bainite in Bainitic Steels

Estimation of the volume percent of bainite has been conducted using a powerful computational technique known as neural network. The model is proposed for ordinary bainitic steel in addition to the more recent class of super bainite steel which are attractive for many applications. The model has been checked for a wide range of steels, including super-bainite steels. The proposed model is based on chemical composition, heat treatment and hardness. The predicted results have shown good agreement with the experimental results.

Microstructures and Mechanical Characteristics of a Medium Carbon Super-Bainitic Steel After Isothermal Transformation

The influence of isothermal treatments on microstructure, hardness, and tensile properties of a new designed medium carbon super-bainitic steel was studied by using the optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and mechanical property tests. The results show that the isothermal transformation temperature and holding time combine to determine the variations of microstructure and mechanical properties in the low temperature range of 140-260 °C. The size of bainitic laths is refined with the decrease in transformation temperature, but the bainitic transformation kinetic becomes lower. Longer treatment time can promote the transformation of complete bainite and also make the retained austenite more stable. The desired microstructure consisting of nanoscale carbide-free bainitic ferrite and thin film-like retained austenite located between the ferrite laths can be achieved at the temperature of 260 °C for 10 h, where the excellent combination of strength, ductility and hardness can be obtained.

In situ measured growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel

The growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demonstrated diversity. The bainite plates initiating at grain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation decreased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewpoint of growth rate.

Acceleration of the super bainite transformation through a coarse austenite grain size

The effect of austenite grain size on the kinetics of the isothermal bainitic transformation in a high-carbon super-bainitic steel was investigated. Experimental results showed that the transformation of super bainite was accelerated by a coarse austenite grain size. This is because while coarse austenite grains provide less nucleation sites, it is beneficial for bainite sheaf growth. Meanwhile, there is a critical austenite grain size below which there is a distinct grain size effect and above which it is not evident.