Bainitic Microstructure Research Articles

Microstructure refinement and enhanced mechanical properties of wire arc additively manufactured RAFM steel via post heat treatments

Reduced activation ferritic/martensitic (RAFM) steel with high strength and neutron radiation damage resistance is deemed one of the promising structural materials for fusion blankets. In this study, RAFM steel tube was prepared through wire arc additive manufacturing, and the evolution of phase composition, microstructural features and mechanical properties during the as-built and heat treatment processes were investigated. The results show that the as-built state is predominantly composed of α-Fe, and the microstructure's morphology from bottom to top consists of alternating columnar grain regions, equiaxed coarse grain regions, and fine grain regions. Normalizing and tempering treatment eliminates the heterogeneous microstructure, resulting in the presence of M23C6 carbides enriched with Cr and W, as well as MX phases enriched with Ta and V. Partitioning heat treatment results in the formation of ultrafine martensitic, quenched martensite and bainitic microstructures. The mechanical properties of wire arc additively manufactured RAFM steel are influenced by variations in microstructure. The coarse-grained region formed in the as-built state represent weak links. Normalizing and tempering treatment reduces the strength while improving ductility. Partitioning heat-treated RAFM steel exhibited a high strength of 1.1 GPa and an elongation of approximately 10.9%. This study provides a method of combining additive manufacturing with heat treatment to enhance the strength and ductility of RAFM steel simultaneously.

Effects of Heat Treatment on Microstructures and Mechanical Properties of a Low-Alloy Cylinder Liner.

Cylinder liners, considered a crucial component of internal combustion (IC) engines, often require excellent mechanical properties to ensure optimal engine performance under elevated temperatures, pressures, and varying loads. In this work, a new low-alloy cylinder liner, incorporating a low content of molybdenum, copper, and chromium into gray cast iron, was fabricated using a centrifugal casting process. Subsequently, the heat treatment processes were designed to achieve bainite microstructures in the cylinder liner through rapid air cooling, isothermal transformation, and tempering. The effects of different air-cooling rates and tempering temperatures on the microstructure evolution and mechanical properties of cylinder liner were investigated. The results revealed that during the supercooled austenite transformation process, rapid air cooling at a rate of 14.5-23.3 °C/s can effectively bypass the formation of pearlitic structures and directly induce the formation of bainite structures. Once the temperature exceeded 480-520 °C, hardness and tensile strength increased with the temperature increase owing to the enhancement of the lower bainite content, the reduction of residual austenite, and the precipitation of the fine hard carbides in the matrix. With temperatures above 520-550 °C, the carbide and lower bainite organization coarsened, thereby reducing the hardness and tensile strength of the material. Therefore, the optimal heat treatment parameters were rapid cooling at 14.5-23.3 °C/s rate to obtain bainite, and tempering of 480-520 °C for finer and more uniform bainite. In addition, the results of the characterization of the mechanical properties of the cylinder liner after heat treatment showed that the hardness, tensile strength, and wear resistance were improved with the refinement of the bainite.

Non-transferring arc and wire additive manufacturing: microstructure, mechanical properties and bulk texture evolution of deposits

PurposeThis study aims to demonstrate a modified wire arc additive manufacturing (AM) named non-transferring arc and wire AM (NTA-WAM). Here, the build plate has no electrical arc attachment, and the system’s arc is ignited between tungsten electrode and filler wire.Design/methodology/approachThe effect of various deposition conditions (welding voltage, travel speed and wire feed speed [WFS]) on bead characteristics is studied through response surface methodology (RSM). Under optimum deposition condition, a single-bead and thin-layered part is fabricated and subjected to microstructural, tensile testing and X-ray diffraction study. Moreover, bulk texture analysis has been carried out to illustrate the effect of thermal cycles and tensile-induced deformations on fibre texture evolutions.FindingsRSM illustrates WFS as a crucial deposition parameter that suitably monitors bead width, height, penetration depth, dilution, contact angle and microhardness. The ferritic (acicular and polygonal) and lath bainitic microstructure is transformed into ferrite and pearlitic micrographs with increasing deposition layers. It is attributed to a reduced cooling rate with increased depositions. Mechanical testing exhibits high tensile strength and ductility, which is primarily due to compressive residual stress and lattice strain development. In deposits, ϒ-fibre evolution is more resilient due to the continuous recrystallisation process after each successive deposition. Tensile-induced deformation mostly favours ζ and ε-fibre development due to high strain accumulations.Originality/valueThis modified electrode arrangement in NTA-WAM suitably reduces spatter and bead height deviation. Low penetration depth and dilution denote a reduction in heat input that enhances the cooling rate.

Tuning Bainitic Microstructures by Complex Thermo-mechanical Treatments under Constant Stress

Tuning Bainitic Microstructures by Complex Thermo-mechanical Treatments under Constant Stress

Texture Evolution of Bainitic Steels Processed from Nickel Lateritic Ores: A Neutron-Diffraction Investigation

This study investigated the texture evolution of bainitic steel samples processed from lateritic nickel ores with varying heat treatment times. The samples were austenitized, air-cooled, and subjected to bainitic heat treatment at 532°C for 15 and 30 minutes. Scanning electron microscopy (SEM) analysis revealed that the sample treated at 532°C for 15 minutes, developed bainitic plates within the austenitic matrix due to the relatively rapid air cooling. Conversely, the sample, heat-treated at 532°C for 30 minutes, exhibited a combination of bainitic and pearlitic microstructures. Neutron diffraction analysis using software showed moderate-to-weak textures in both samples, indicating dislocation polygonization resulting from prolonged heat treatment. The results suggest that further research is needed to investigate the texture evolution of bainitic steels from nickel lateritic ores after plastic deformation. This work contributes to understanding the microstructural characteristics of bainitic steel derived from nickel lateritic ores, which may have significant implications for the production and application of these steels in the future.

Effect of austenitizing temperature on isothermal quenching microstructure and mechanical properties of 52100 bearing steel

In general, isothermal quenching heat treatment in salt bath can improve the toughness but sacrifice the strength and hardness of 52100 bearing steel. Many previous studies focusing on the isothermal quenching process have been carried out to balance the strength and toughness. And the optimal mechanical properties are limited to approximately 60–61 HRC hardness and 100 J impact toughness. In this study, in order to further improve the mechanical properties, the influences of austenitizing temperature on microstructural evolution and strengthening-toughening mechanism in isothermal quenched bainite in 52100 bearing steel have been systematically investigated. The increase of austenitizing temperature decreases the volume fraction of undissolved carbides, and increases the carbon concentration in austenite matrix. These changes not only strongly influence the kinetics of bainite phase transformation and bainite microstructure, but also cause the conflict between the coarsening of prior austenite grains, as well as bainite sheaves, and the refining of substructure of lower bainite. Austenitizing at 870 °C can balance this conflict well and obtain a good combination of hardness and toughness. It is found that the carbon content and dislocation density in bainite ferrite reach the peak value at 870 °C austenitized specimens, which effectively strengthens the bearing steels. At the same time, the 870 °C austenitized specimens retain appropriate content of undissolved carbides, which avoids stress concentration and inhibits crack propagation, and finally improve the impact toughness. Therefore, the optimal austenitizing temperature (870 °C) achieves the improvement of mechanical properties with the hardness and toughness of 59.7 HRC and 273 J, respectively.

Wear Behavior of Microstructurally Modified Elastic Rail Clip Steels with Bainitic Microstructure

Wear Behavior of Microstructurally Modified Elastic Rail Clip Steels with Bainitic Microstructure

Constitutive Modeling for Flow Stress and Processing Maps of a 0.2C–2.0Mn–0.5Cr Microalloyed Steel

Low‐carbon microalloyed steel with the microstructure of bainite is frequently utilized to produce automotive parts. The systematic investigation of the hot deformation behavior can serve as a reference for the optimization of deformation process parameters. The hot deformation of 0.2C–2.0Mn–0.5Cr microalloyed steel is studied. Based on the stress–strain curve, an Arrhenius model with strain compensation is established. The correlation coefficient R2 and the average absolute relative error calculated according to this model are 0.962 and 4.871%. The microstructure evolution at the strain of 0.90 is studied. In the stable region with substantial power dissipation, the microstructure is uniformly fine. The strain rate is high in the stable region with low‐power dissipation, and the room‐temperature microstructure after deformation retains a large amount of deformation storage energy and fine substructures. During deformation, the banded structures are formed in the instable region. The difference in dislocation density between the band structure and the nearby regions promotes the formation of necklace microstructures. Therefore, the 0.2C–2.0Mn–0.5Cr microalloyed steel has an ideal deformation process window in the range of a hot deformation temperature of 880–920 °C and a strain rate of 0.1–0.3 s−1.

In-situ heating-stage EBSD validation of algorithms for prior-austenite grain reconstruction in steel

Prior-austenite grain (PAG) structures play a critical role in determining steel microstructures and properties. Over recent years, the use of algorithms that reconstruct PAGs from electron backscatter diffraction (EBSD) measurements of martensitic or bainitic microstructures have become commonplace. These appear to work well, but there has yet to be a comparison made of the reconstructed PAG microstructure to the true PAG microstructure of a steel measured above Ae3 before transformation. Here, in-situ EBSD was used to record the austenite grain structure (at 1000 °C) before transformation, as well as the progress of subsequent martensitic and bainitic transformations. The resulting dataset, which we have made available to the community here: https://doi.org/10.5281/zenodo.8348372, was used to assess of two popular algorithms for PAG reconstruction. They were found to be accurate enough for PAG size measurement and general texture analysis, although the exact morphologies of some PAG and twin boundaries are sometimes lost.

Evaluation of Simulated Heat Affected Zone of Molybdenum- and Niobium Alloyed Ultra-High-Strength Steels

Gleeble 3800-thermomechanical simulator was used to simulate the heat affected zone of quenched and tempered 0.16 wt.% C steels with variation of molybdenum-and niobium contents. The purpose of the study was to evaluate the effect of alloying content on the properties of the coarse-grained zone of HAZ region (CGHAZ) and partially re-austenitised inter-critical zone (ICHAZ) with two different t8/5 times (5 s and 15 s). Results showed that Mo and Nb decreased the amount of softening in the HAZ-region, especially with longer t8/5 -time (15 s). 0Mo steel had mixed microstructure of bainite and martensite in the CGHAZ region with t8/5 time of 15 s, which led to higher degree of softening compared to other steels. Shorter t8/5 time of 5 s produced martensitic microstructure in CGHAZ region in all cases leading to higher hardness values. Impact energy values at-40 °C were at least 34 J/cm2 in all cases. Highest hardness values in the ICHAZ-region were achieved in the case of 0.5Mo steel. Also, at-40 °C impact energy values of ICHAZ were at least 34 J/cm2 in all cases, however Mo-free steel achieved clearly higher impact energies in ICHAZ region, which is result from softer microstructure with relatively low hardness compared to other steels. Overall, it can be concluded that longer t8/5 time can be used, which corresponds to higher heat input in welding, if Mo and/or Nb alloying is used.

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.

Effect of N Content on the Microstructure and Impact Properties of Normalized Vanadium Micro-Alloyed P460NL1 Steel

In this work, the effect of nitrogen doping on vanadium micro-alloyed P460NL1 steel is studied in terms of microstructures and impact toughness. As the nitrogen content increased from 0.0036% to 0.0165%, the number of V (C,N) particles increased. The fine precipitates of V (C,N) effectively pin the prior austenite grain boundary, resulting in the refinement of the austenite grain. The intragranular and intergranular V-containing coarse particles enhanced the nucleation of intragranular ferrite and the grain boundaries of polygonal ferrite during air cooling. Accordingly, the proportion of heterogeneously nucleated ferrite increased, and the grain size of ferrite decreased. Notably, the size of the pearlite microstructure decreased, and the bainite microstructure appeared with a high doping of N. With the increase in N content, the impact toughness of vanadium micro-alloyed P460NL1 steel was enhanced. This can be attributed to the refinement of ferrite and the reduction in pearlite, which, in turn, was ascribed to the increase in nitrogen.

Influence of Austempering Conditions on Hardness and Microstructure of Bainite in Low-Alloyed Steel

The influence of austempering temperature and time on the microstructure and hardness of a low-alloyed bainitic steel is investigated in the temperature range 275 °C to 375 °C for up to 24 hours. It is shown that the dislocation density and coarseness of the bainitic microstructure are affected by the austempering temperature, while only the dislocation density is significantly affected by the austempering time. The hardness of the steel is estimated based on microstructure–property relations and is in good agreement with the measured hardness. In conclusion, the decrease in dislocation density is the main reason for loss in hardness upon increasing austempering temperature and/or time for the studied temperature range.

Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.

Evolution of ultrafine bainite microstructure during rapid heating process and the role of retained austenite

Rapid heat treatment is an important method to improve the properties of steel. Moreover, the study of the phase transformation process plays an important role in the formulation of rapid heat treatment. This paper investigates phase transformation behavior and microstructure evolution of ultrafine bainite at varying heating rates, and reveals the influence mechanism of retained austenite on austenitizing. Results showed that the decomposition of retained austenite was influenced by the diffusion of Si element. Therefore, the decomposition temperature of retained austenite is severely affected by the heating rate. Retained austenite completely transformed into cementite and ferrite when heating rate was below 5 °C/s. A small amount of retained austenite was kept when the heating rate was higher than 5 °C/s. The kept austenite effectively suppressed the hysteresis of Ac1 and resulted in a unique mechanism of austenite grain formation. When the heating rate range was 5–10 °C/s, some new austenite skipped the nucleation process and grew directly on the kept austenite, forming the large austenite grains. And other new austenite was obtained via nucleation and growth process. Thus, the grain size of bimodal distribution is formed. This leads to the phenomenon of austenite memory effect. When the heating rate was increased to 50 °C/s and higher, the austenite grain size was less than 14 μm. Therefore, rapid heating makes the phase transition lag. When the heating rate is high enough, part of the austenite is kept, which has an important influence on austenitizing process of the steel.

In-situ SEM/EBSD investigation of tensile deformation-driven microstructural changes in low-carbon HSLA steels with ferrite-bainite microstructures

In this study, we aimed to investigate how microstructural constituents affect the plastic deformation behavior of low-carbon HSLA steels during a uniaxial tensile test using the in-situ SEM/EBSD technique. We fabricated three types of low-carbon HSLA steels, each with distinct ferrite-bainite microstructures, by altering the thermomechanical control processing conditions. All the steels were composed primarily of polygonal ferrite (PF) and bainitic microstructures. As the finish rolling temperature and the cooling rate decreased, the volume fraction of PF increased. The in-situ SEM and EBSD analysis during deformation revealed a greater concentration of plastic strain and an increased mobile dislocation density in the PF than in the bainitic microstructures. Most notably, as the volume fraction of the bainitic microstructure increased, so did the degree of strain gradient between the soft PF and the hard bainitic microstructure. This suggests that at the initial yield stage the soft PF plays a more significant role in mitigating the yield point phenomenon than the hard bainitic microstructure. Our findings offer a deeper understanding of the microstructural factors that influence plastic deformation, and may serve as a guide for designing optimal microstructures to ensure the mechanical properties of low-carbon HSLA steels for a range of applications.

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 phase transformation sequence on the low cycle fatigue properties of bainitic/martensitic multiphase steels

The three microstructures of bainite-fresh martensite, pre-martensite-bainite and carbide-free bainite were obtained by modulating the phase transformation sequence by the heat treatment process in the medium-carbon multiphase steel. The effects of martensitic and bainitic phase transformation sequences on the low cycle deformation behavior of multiphase steels were investigated. The results showed that the sample of the fresh martensite transformation process (295 °C–15 min) exhibited the best fatigue life among all processes under strain control. The samples of the preformed martensite transformation process (250 °C–295 °C) and the carbide-free bainite transformation process (295 °C–120 min) exhibited similar fatigue life. The sample of 295 °C–15 min possesses good resistance to plastic destabilization, resulting in higher fatigue life. The results of the EBSD crack propagation path analysis demonstrate that high carbon M/A islands block crack propagation in the sample of 295 °C–15 min, and high angular grain boundaries skew the crack propagation path. Fracture scans observed that secondary cracks in the sample of 250 °C–295 °C extend inward with aggregation of voids, which are more likely to form crack sources.

The effect of vanadium micro-alloying on the microstructure of welded joints in high-strength structural steels

The balance between high strength and toughness in high-strength-low-alloy (HSLA) steels can be defined by the thermal cycles in the heat-affected zone (HAZ) of a welded joint, during a double-pass welding process with secondary heating in the inter-critical zone (IC CG HAZ). After multiple heating cycles in the temperature range between Ac1 and Ac3, the steel undergoes a strong loss of toughness and resistance to fatigue, mainly caused by the formation of residual austenite (RA). This study aims to investigate the influence of vanadium addition on the behavior of IC GC HAZ in S355-grade HSLA steel. The welding thermal cycles were simulated, considering five different inter-critical temperatures, between 720 and 790 °C. The addition of vanadium as a micro-alloy to an S355 structural steel was found to increase the mechanical strength of the IC GC HAZ zone of a welded joint without compromising toughness and fatigue resistance. This result is obtained through the generation of a bainitic microstructure with dispersion of fine regions of residual austenite and a fine and uniformly distributed precipitation.Graphical abstract

Characterization of the mechanical properties and microstructural evolution of martensitic steel in repeated tempering cycles

The purpose of this study was to understand the behavior of martensitic H13 steel in accordance with the microstructural evolution, mechanical properties and wear in repeated tempering cycles. The microstructures were characterized by axio image observer microscope, scanning electron microscope (SEM), x-ray diffraction (XRD). Uniaxial tensile test, charpy v-notch impact test, rockwell hardness test and wear test were conducted to analyze the changes in mechanical properties, impact properties, hardness and wear in repeated tempering cycles. The specimen prepared were subjected to the hardening at 1030 °C for 20 minutes, oil quenched and subjected to repeated tempering cycles at 570 °C for 2hrs holding time each. The mechanical properties recorded indicates that the maximum ultimate tensile strength obtained was at double tempering due to secondary hardening effect i.e., alloy carbides precipitation offering strength to the matrix and corresponding wear was found to be minimum. The annealed specimen revealed bainitic microstructure and with hardening and repeated tempering cycles, fine needle like structure and carbides was observed in microstructure and retained austenite was converted into martensite and martensite was converted into tempered martensite. Carbide size and martensite lath distribution controls the strength and fracture rate.