Bainitic Microstructure Research Articles

Weldability of austempered rail steel using the flash-butt process

Abstract In this study, bainitic microstructure was formed via heat treatmenton R260 rail steel, which is generally used in railways. Bainitic steel, which is considered more advantageous than current rail steel, waswelded by flash butt welding, which is often used for joining rails andthe mechanical and microstructure of the samples were thenexamined and compared. Bainitic structural steel obtained by austempering heattreatment with normal rail steel was welded by flash butt welding. Flash-butt welding parameters were kept constant during the experiment. The welding capabilities of the joints were compared and the results wereevaluated. It was determined that the bainite structure obtained as a result of austempering heat treatment changes the microstructuralproperties of the samples and affects the mechanical values ​of the joints.

Scale-bridging Microstructural Analysis – A Correlative Approach to Microstructure Quantification Combining Microscopic Images and EBSD Data

AbstractA comprehensive description of complex material structures may require characterization using different methods and observations across several scales. This work will present a correlative approach including light optical microscopy, scanning electron microscopy and electron backscatter diffraction, enabling microstructure quantification which combines microscopic images and electron backscatter diffraction data. The parameters obtained from electron backscatter diffraction such as misorientation parameters or grain and phase boundary data are an ideal source of information, complementing microscopic images. Two case studies performed on bainitic microstructures will be presented to demonstrate practical applications of this approach.

Effect of Ausforming on the Macro- and Micro-texture of Bainitic Microstructures

The reason why variant selection phenomena occur in ausforming treatments is still not known. For that reason, in this work, the effect of compressive deformation on the macro and micro-texture of a bainitic microstructure was analyzed in a medium-carbon high-silicon steel subjected to ausforming treatments, where deformation was applied at 520 °C, 400 °C and 300 °C. The as-received material presented a very weak leftlangle {3, 3, 1} rightrangle fiber texture along the rod axis, due to prior thermomechanical processing. For the samples isothermally heat-treated, it was detected that the bainitic ferrite inherited a leftlangle {1, 0, 0} rightrangle fiber texture from the leftlangle {1, 1, 0} rightrangle fiber texture present in the prior austenite. The intensity of this transformation texture was more pronounced as the deformation temperature decreased. Also, variant selection was examined at different scales by combining Electron-Backscattered Diffraction and X-ray Diffraction. The quantification of the fraction of crystallographic variants under certain conventions for every condition revealed variant selection in samples subjected to ausforming treatments, where these phenomena were stronger as the deformation temperature was lower. Finally, some of the theories proposed so far to explain these variant selection phenomena were tested, showing that variants were not selected based on their Bain group and that their selection can be better described in terms of their belonging to packets, if these are defined according to a global reference frame. This suggests that the phenomena might have to do with the effect of deformation mechanisms on the prior austenite.

Hydrogen-induced cracking and corrosion behavior of friction stir welded plates of API 5L X70 pipeline steel

The use of friction stir welding (FSW) has proven to be an excellent alternative to join engineering components. Although FSW has had a significant development in recent years, challenges for new applications have been raised, such as offshore steel parts suffering hydrogen embrittlement in the gas and oil industry. Therefore, in this work, the microstructure, corrosion, and hydrogen-induced cracking were investigated in a two-pass FSW welded joint of API 5L X70 pipeline steel. The electrochemical results indicate an inhibitory effect on corrosion reaction because of a carbonate product generation in the steel surface. The polygonal ferritic and degenerated pearlite bands microstructure in the base metal fixed carbonate deposits in the steel surface. In the welded regions, the bainitic microstructure and the carbide particle distribution are less efficient in setting the weld surface carbonate deposit. HIC tests showed cracks initiation and propagation to be more prone in hard phases.

Computational analysis of austenite film thickness and C-redistribution in carbide-free bainite

In this work, a methodology for the computational analysis of some essential microstructural features of a bainitic microstructure is developed. The focus lies in the accurate prediction of the ferritic subunit size, the thickness of the residual austenite films, their corresponding C-enrichment and the accompanying stabilization of the residual austenite. Basis of the approach is the T 0-temperature concept in combination with the numerical simulation of C-diffusion profiles utilizing the cell diffusion module of the thermokinetic software package MatCalc. This methodology gives the opportunity to predict the C-distribution under consideration of consecutively forming subunits, which is necessary to estimate the C-content of austenite films. The simulations also take into account the effect of C trapping at the dislocations formed inside the ferritic platelets due to plastic deformation and its influence on the chemical potentials. Good agreement is achieved between measured and predicted retained austenite layer thickness and the C-enrichment of the layers accompanying the C redistribution process.

The role of plastic strains on variant selection in ausformed bainitic microstructures studied by finite elements and crystal plasticity simulations

Abstract The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.

Hydrogen Diffusion and Trapping in Laser Additively Manufactured Ultra-High Strength AerMet100 Steel

Hydrogen trapping and the permeation behavior of laser additively manufactured (LAM) AerMet100 (UNS K92580) steel with an as-deposited specimen (AD) and after three types of heat-treated specimens (bainite microstructure [BM], tempered bainite and martensite microstructure [TBMM], and tempered martensite [TM]) was investigated. At least three types of different hydrogen traps were identified in each microstructure of the LAM steel, including both reversible and irreversible H traps. For as-deposited microstructure, the main reversible H trap states are related to the precipitation of M3C carbides associated with a detrapping activation energy (Ed) of 17.3±0.2 kJ/mol. After heat treatment, the dominant reversible hydrogen trap states in the tempered martensite microstructure have a different Ed value of 19.3±0.5 kJ/mol, which is attributed to the precipitation of highly coherent M2C carbides. In comparison with the reported Ed value of approximately 21.4 kJ/mol for main reversible hydrogen traps in wrought UNS K92580 steel, the lower Ed value in the LAM steel is closely related to the composition change of M2C carbides. In all of the H precharged samples, the diffusible and total H concentration of the TM specimen and the TBMM specimen are about three to four times higher than that of the AD specimen and the BM specimen. The TM specimen with tempered martensite microstructure has the highest diffusible and total H concentration due to its high density of dominantly reversible H traps. The effective hydrogen diffusion coefficient (Deff) of the LAM steel is on the order of 10−9 cm2/s, and decreases with increasing density of the dominant reversible H traps brought about by heat treatment. The LAM steel has a comparable Deff of about 2.8 × 10−9 cm2/s compared to the wrought steel of a similar yield strength (∼1,750 MPa),

Role of the Ni-based filler composition on microstructure and mechanical behavior of the dissimilar welded joint of P22 and P91 steel

The manuscript described the effect of Ni-based filler on microstructure and mechanical properties of the dissimilar welded ASTM A335 grade P22 and P91 alloy steel plate. The dissimilar welds joint (DWJ) was produced using the Gas Tungsten Arc Welding (GTAW) process. The DWJ was characterized for metallographic and mechanical testing for as-welded and post-weld heat treatment (PWHT) conditions. For mechanical properties, microhardness, tensile test and Charpy impact test were performed. The DWJ showed the heterogeneity in microstructure and mechanical properties (microhardness, impact toughness) along the welded joint. The weld metal showed the formation of the austenitic microstructure consisted of the equiaxed dendrite along with segregation of the alloying elements like Cr, Mo, and Ti at the inter-dendritic boundaries, while P91 and P22 HAZ showed the formation of coarse-grained, fine-grained, and inter-critical heat-affected region depends on the temperature experienced during the welding cycle. The region of P91 HAZ was characterized by untempered and tempered martensite, while P22 HAZ was characterized by coarse and fine bainitic microstructure. The PWHT showed a negligible effect on the microstructure of the weld metal, while a significant change in microstructure of P91 HAZ and P22 HAZ was observed. PWHT also led to the formation of the soft and hard zone along the interface of the P22 steel due to the carbon diffusion. At the interface of the weld metal and base material, a sharp gradient was observed for the elements Cr, Fe, Ni and Mo. The weld metal near the interface showed the columnar dendritic structure having segregation of the alloying elements Mo, Ti and Cr along the inter-dendrite boundaries. A variation in hardness was measured along the weldments for as-welded and PWHT conditions. The peak hardness was measured in P91 coarse-grained heat-affected zone (CGHAZ) for both as-welded and PWHT, while the poor hardness of 202 HV and 195 HV was measured for weld metal in as-welded and PWHT conditions. The strength of the welded joint was measured higher than the strength of P22 steel for both conditions; however, the fracture was observed in P22 base metal for the as-welded joint, while in PWHT, it was in P22 HAZ near the interface. A variation in Charpy impact toughness was observed along the weldment for both as-welded and PWHT conditions. In as-welded, the minimum Charpy impact toughness (CIT) of 95.8 J was measured in weld metal, while the maximum was 152 J in P91 HAZ. After the PWHT HAZ region of P22 and P91 steel showed a drastic increase in CIT, however reduction in CIT of the weld metal was measured.

Effect of helical rolling on the bainitic microstructure and impact toughness of the low-carbon microalloyed steel

Ferrite-bainite microstructures and impact toughness of the X65 low-carbon microalloyed steel were investigated after helical rolling at 1000, 920, 850, and 810 °C followed by continuous cooling in air. After helical rolling at 1000 °C, granular bainite with large areas of the massive-shape martensite-austenite constituent (d = 1.5 μm) and a high fraction of twinned martensite (d > 2.0 μm) were observed in the steel. This caused a decrease in impact energy at low test temperatures (for example, 70 J at –70°С). Lowering the helical rolling temperature contributed to a reduction of dimensions of both ferrite-bainite and martensite-austenite constituent areas, as well as the replacement of the latter by a slender type one and an improvement in fracture toughness at the low temperatures. The highest impact energy level (210 J at –70 °C) was achieved after helical rolling at 850 °C due to the formation of a homogeneous microstructure, which included dispersed ferrite grains, granular bainite and small fractions of the slender type martensite-austenite constituent (d = 0.1–0.7 μm). In this case, areas of twinned martensite were absent.

Influence of carbonitriding conditions on phase composition and residual stresses for 20MnCr5 low alloy steel

This paper reports an investigation of the influence of carbonitriding conditions for 20MnCr5 low alloy steel. Three gaseous carbonitriding conditions were investigated based on different carbon and nitrogen potentials to attain varying levels of carbon between 0.62 and 0.93% mass, whereas for nitrogen between 0.19 and 0.26% mass at the surface. Analysis of retained austenite and residual stress distributions was conducted using X-ray diffraction technique. The effective case depth varied between 900 and 1200 µm. The case microstructures were characterized by varying proportions of retained austenite and martensite, while the core contained essentially bainitic microstructures. The maximum amount of retained austenite which occurred at a depth of 50 µm from the subsurface ranged between 30 and 70% mass and significantly influenced the level of surface micro-hardness whereas the core hardness remaining relatively constant at 450 HV1. High values of residual stresses in martensite phase were observed. The signs, magnitudes, distributions and location of maximum compressive residual stresses were highly influenced by the maximum fraction of retained austenite. Retained austenite of 30%, 50% and 70% mass at the surface lead to peak compressive residue stresses of -280, -227, and -202 MPa at depths of 555, 704, and 890 μm, respectively.
 Keywords: Carbonitriding, retained austenite, martensite, residual stress, XRD.

Effect of TMCP rolling schedules on the microstructure and performance of X70 steel

Four laboratory rolling schedules were devised and produced from a thick-wall API X70 steel plate. The microstructure and crystallographic texture were characterized followed by an evaluation of the performance of the rolled steels by investigating the mechanical properties and SCC susceptibility. High pH Stress Corrosion Cracking (SCC) is a damaging form of environmental corrosion that can cause rapid failure of buried gas transmission pipelines. It is a key question to ascertain if the manufacturing procedure has any bearing on the resistance of line pipe steels to SCC. Finish rolling above the recrystallization lower temperature, RLT (Tnr), resulted in the formation of large, equiaxed grains of upper bainite and granular bainite. Despite resulting in the highest Yield Strength (YS) and Ultimate Tensile Strength (UTS), the steel manufactured through this process had the highest SCC susceptibility. Finish rolling in the two-phase region increased the polygonal ferrite formation and deteriorated mechanical properties of the rolled steels. Finish rolling above the Ar3, combined with severe rough rolling, produced the lowest SCC susceptibility, with a primarily granular bainite microstructure and a high intensity of {332}<113> texture. Rolling strategies that balanced strength and ductility were shown to positively affect SCC performance.

Influence of Thermomechanical Treatments on Corrosion of Carbon Steel in Synthetic Geopolymer Fly Ash Pore Solution

In this study the effect of thermomechanical treatments in chloride induced pitting corrosion is presented for carbon steel rebars exposed to synthetic fly ash (FA) pore solution. Due to the likely phase transformations that steel reinforcements in concrete experience during the event of a fire, the understanding of the corrosion behavior of such phases is key in predicting the stability of the structure. The motivation for this study arrives from the scarce literature regarding the corrosion behavior of thermomechanically treated steel reinforcements in FA environments and the need for further investigation to understand its mechanism. In order to better understand the effects on the corrosion behavior electrochemical measurements including cyclic potentiodynamic polarization curves (CPP) and electrochemical impedance spectroscopy (EIS) were used. It was found that quenched specimens showed enhanced corrosion kinetics as their icorr values were higher, being of 3.18 × 10−5 and 2.20 × 10−5 A/cm2 for water and oil quenched compared to 2.13 × 10−6 A/cm2 for the as-received. Furthermore, the effective capacitance of the double layer (Ceff,dl) showed the lower stability of the passive film for the quenched specimens, with values of 1.11 × 10−3 µF/cm2 for the as-receive sample that decreased to 8.12 × 10−4 µF/cm2 for the water quenched sample. The anodic charge transfer coefficient in the synthetic FA alkaline pore solution changes from 0.282 to 0.088, for the as-received and water quenched rebars specimens, respectively. These results indicate a lower energy barrier for the anodic dissolution reaction of quenched specimens, indicating that martensite and bainite microstructures promote corrosion process. Enhanced corrosion was found on quenched samples presenting martensite and bainite microstructure as showed by the increased pith depth, with values of 5 μm compared to 1 μm observed in the as-received samples.

Wear Resistance of Medium Carbon Steel with Different Microstructures.

In this research, the tribological properties of different microstructures of medium carbon steel produced by either an austempered process or quenched-tempered process are investigated. The as-received samples with annealed microstructure (spherodized) are austempered to obtain a bainite microstructure or quenched-tempered to obtain a tempered martensite microstructure. The tribological performance of these microstructures was studied using a ball-on-disk UMT3 tribometer. The results indicated that both bainite microstructures and tempered-martensite microstructures produced better wear resistance than pearlite microstructures. At the same hardness level, the austempered disk specimens have less cracking due to higher fracture toughness compared to quenched and tempered steel. For the disks, tempered martensite microstructures produced more plastic deformation compared with bainite microstructures. Mild abrasive wear was observed on the harder disks, however, smearing wear was observed on the softer disks. Adhered debris particles were observed on the balls.

Microstructure and Phase Transformations in High-Strength Bainitic Forging Steel

In this work, the microstructural evolution and the phase transformations on a 0.22 wt.% carbon bainitic forging steel are investigated using a deformation dilatometer. The microstructure evolution was characterized using light microscope (LM), scanning electron microscope (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), and electron backscatter diffraction (EBSD). It was tried to predict the phase transformations via a continuous cooling transformation (CCT) diagram calculated by JMatPro using the results of prior austenite grain size measurement. In addition, critical temperatures were calculated using Thermo-Calc, JMatPro, and empirical formulations. Based on the dilatometric tests, hardness, and microstructural investigation, a deformation-CCT diagram was plotted. It was found that a bainitic microstructure can be obtained at cooling rates from 0.15 to 8 K/s. The microstructures varied from a mixture of bainite and blocky martensite/austenite islands at lower cooling rates to a bainitic ferrite with martensite at higher cooling rates. Finally, an industrially hot-forged sample cooled at 1 K/s was investigated. The microstructural evolution showed a microstructure of carbide-free bainitic ferrite along with film and island of retained austenite.

Microstructural Characterizations and Kinetics Model of Isothermal Bainite Formation with the Presence of Prior Martensite in High‐Carbon Nanostructured Bainitic Steel

Quenching and subsequent isothermal bainite transformation (QBT) and direct isothermal transformation (DIT) procedures are performed to comparatively evaluate the impact of the introduced prior martensite on bainite transformation behaviors and microstructural characterizations in a high‐carbon nanostructured bainitic steel. A thermodynamic model under a QBT process based on displacive theory is also developed to expound the bainite formation kinetics. The results indicate that the modified kinetics model, by considering austenite/martensite interface nucleation, austenite grain boundary nucleation, and autocatalytic nucleation, evolved here can be used to accurately describe the bainite formation behavior in the presence of prior martensite. A certain fraction of prior austenite/martensite interface can serve as the preferential nucleation site for the bainite transformation and assist in an acceleration of transformation rate. The bainite nucleation event initially takes place at prior martensite plate boundary in the QBT process instead of austenite grain boundary in the DIT process and subsequently continues through autocatalytic nucleation at austenite/bainite interface generated at either austenite/martensite interface or austenite/austenite interface. The introduced prior martensite slightly refines bainite sheaves and lessens the blocky retained austenite size, presenting a feasible approach to shorten the required time for producing a nanosized bainitic microstructure.

Effect of prior martensite on bainite transformation and microstructure of high-carbon nano-bainitic steel

Bainite transformation kinetics, microstructure, and mechanical properties were comparably investigated by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy observation, and tensile and impact tests for two treatments: with and without quenching before isothermal bainite transformation at 300 °C. Both the incubation time and growth stage of bainite transformation are accelerated by the introduction of prior martensite, especially the former. In addition, the bainite microstructure around the prior martensite is refined and shows a similar orientation with the martensite due to the strain field caused by the existence of prior martensite. The finer bainite microstructure and prior martensite improve the strength and hardness of the high-carbon nano-bainitic steel. Although the presence of prior martensite and low misorientation relationship between the martensite and the adjacent bainite microstructure are prejudicial to impact toughness, the nanoscale bainite microstructure can inhibit the crack propagation and then improve the impact toughness of the high-carbon nano-bainitic steel.

Improving the fatigue performance of vanadium and silicon alloyed medium carbon steels after nitriding through increased core fatigue strength and compressive residual stress

The fatigue performance of nitrided medium carbon steels, alloyed with V and Si, and heat treated to form martensite and bainite microstructures, was evaluated using cantilever bending fatigue testing. Hardness testing and X-ray diffraction residual stress measurements were used in conjunction with an analysis of the applied stress distributions to determine vulnerable regions of crack initiation. The analysis was correlated with experimental scanning electron microscopy observations of crack initiation location. Increases in V and Si content lead to higher core hardness and higher core fatigue strength which increase the applied stress needed to initiate fatigue failure originating subsurface below the case. Increases in V and Si content also increase the magnitude of compressive residual stress, which increases the applied stress needed for surface initiated failure. The combination of increased core fatigue strength and magnitude of compressive residual stress improve the fatigue performance after nitriding compared to other nitrided medium carbon steels reported in literature.

Effect of introduced vanadium carbide at the bay region on bainite transformation, microstructure and mechanical properties of high-carbon and high-silicon steel

Obtaining a fine bainitic ferrite for high carbon steel at low temperature requires a long period. Thus, creating strategies to accelerate bainite transformation is important. This investigation aims to explore the introduction of vanadium carbide at the bay region on bainitic transformation kinetics and mechanical properties by comparing isothermal bainitic transformation treatment directly and in combination with bay treatment prior to bainite transformation. Results show that introduced fine VC precipitation during bay treatment notably accelerates the initial bainite transformation with the incubation period being reduced by 91%. Decrease in activation energy for bainite nucleation at vanadium carbide/austenite interfaces and generation of preferred nucleation sites lead to a favorable acceleration of bainitic formation kinetics. However, indirect refinement of grain size for intragranular bainitic ferrite nucleation and consumption of available vanadium carbide/austenite interfaces result in overall rate of the subsequent bainite transformation. For the sample with prior bay treatment, the lath directions of bainitic ferrite are more complex and the size of block RA is smaller than that of the conventional sample. Moreover, the introduction of vanadium carbide at the bay region can simultaneously enhance the tensile strength and impact toughness of the steel.

Failure Analysis of Drive Axle Shaft failed under Torsional Stress

Drive axles are an essential component for power transmission in any automobile. It transfers the power from the differential to the hub of the wheels which drives the vehicle. The axles are subjected to torsional stresses due to the torque from the differential and the resisting torque present due to the frictional resistance from the ground on the wheels. Hence, they are prone to torsional failures under fatigue with and without shock loading. In this paper, a comprehensive engineering failure analysis of the failed drive axle shaft of material EN 24-T, custom designed for a 200kg BAJA SAE all-terrain vehicle was performed. Destructive, non-destructive testing and material analysis along with finite element simulations using Ansys were carried out to better understand the nature and cause of the failure. Failure analysis data hints at the flaws in design considerations and scanning electron microscopy (SEM) images reveal the formation of bainitic microstructures within the component due to improper heat treatment which lowers the ultimate tensile strength of the material.

Fracture Behavior and Mechanical Properties of a Thermomechanical TRIP Steel Under Simulated Heat Treatment Conditions

In this research, hypereutectoid Si–Mn TRIP steel was prepared via casting method followed by thermomechanical processing and different heat treatments. Subsequently, fracture behavior and mechanical properties of the prepared steel were investigated. The time–temperature–transformation curves of the specimens were predicted by a simulation software. Their mechanical properties were studied through tensile, hardness, and impact tests. The microstructure and fracture behavior of the specimens were evaluated using an optical microscope, scanning electron microscope and fractographic analysis. Results indicated that the heat-treated sample containing a multi-phase microstructure of bainite, martensite, pearlite and retained austenite, had the highest ultimate tensile strength around 1,300 MPa. The software predictions of the heat treatments were in good accordance with the microstructural results. The samples have undergone thermomechanical process and heat treatment including isothermal transformation showed brittle behavior with intergranular and cleavage fracture surfaces. In addition, the two-stage quenching cycles resulted in dual-phase microstructure composed of martensite embedded in retained austenite with a combination of brittle and ductile fracture behavior.