Bainite Transformation Research Articles

Microstructural Characteristics and Mechanical Properties of Nanostructured Bainite Processed through High and Low Temperature Ausforming and Isothermal Holding near Ms in a Medium Carbon Steel

Laboratory rolling simulations, comprising of thermomechanically controlled rolling followed by ausforming and isothermal holding close to Ms temperature, were conducted on a medium-carbon (C) steel in order to understand the combined effect of prior straining and low temperature holding on the bainite transformation characteristics and resultant properties. Field emission scanning electron microscopy (FE-SEM) was employed to examine the morphology, size and volume fraction of different phase constituents and the observed microstructural features were correlated with the mechanical properties of the steels. The morphology of bainite and/or retained austenite (RA) after low temperature ausforming was found to be extremely fine compared to the sample ausformed at high temperature. An excellent combination of high yield (~1200 MPa) and tensile (~1800 MPa) strengths, good ductility (~18 %) and reasonable ambient temperature impact toughness (~15 J/cm2) were achieved in the low temperature ausformed steel that was attributed to significant refinement of bainite sheaves and presence of high fractions of film-like RA. The dislocations introduced by ausforming hindered the growth of bainite and promoted enhanced carbon diffusion, resulting in high fractions of finely divided film-like RA with high stability. Recent results obtained on a nanostructured /ultrafine medium-carbon bainitic steel elucidating the ausforming conditions and corresponding microstructures and properties are illustrated here.

Effect of Cr–Mo Addition on the Microstructure and Mechanical Properties of Medium‐Carbon Steel

Herein, the effects of Chromium–Molybdenum (Cr–Mo) addition on the microstructural evolution and mechanical properties of medium‐carbon steel after spheroidization annealing are systematically studied through scanning electron microscopy, electron backscatter diffraction, and tensile testing. Cr–Mo addition hinders the proeutectoid ferrite + pearlite transformation, thereby promoting the bainite transformation. Moreover, it refines the pearlite lamellar spacing as well as decreases the average carbide diameter, increases the number of carbides per unit area, and hinders ferrite recrystallization. Compared with those in the B1 steel annealed for 8 h, the size of carbides and their number per unit area in the CM1 steel are 30% lower and 2.2‐fold higher, respectively. Due to finer ferrite grains, smaller carbides, and a higher amount of carbides, the strength of steel improves, and the plasticity slightly reduces after Cr–Mo addition. After 2 h of annealing, the yield strengths of Cr–Mo steels are 77.5–109.5 MPa higher than those of base steels; the elongations are above 20%. The contributions of the strengthening mechanism of steel to the yield strength are as follows (from high to low): grain boundary, precipitation, solid solution, and dislocation strengthening.

Enhancing the yield strength of intercritically annealed Q&P steel via bainite-based quenching and partitioning treatment

Intercritically annealed Q&P steels exhibit good strength-ductility synergies. Unlike fully austenized Q&P steels, intercritically annealed Q&P steels exhibit good ductility but contain a significant amount of ferrite, resulting in a lower yield strength of the material. To address this challenge, we propose a novel short-time isothermal method for bainite-based quenching and partitioning (B-Q&P) treatments to enhance the yield strength. The experimental results indicate that a short-term isothermal bainitic transformation generates a small quantity of preformed bainitic ferrite (PFB), accelerating primary martensite (PM) transformation and enhancing the PM content during the initial quenching stage. Moreover, the B-Q&P treatment significantly refines the retained austenite (RA) and improves its mechanical stability, thus resulting in a more sustained transformation induced plasticity (TRIP) effect. Furthermore, the results reveal that the yield strength increases from 632 MPa in the conventional Q&P sample to 792 MPa in the B270-Q&P sample, representing a ∼25.3 % improvement, the total elongation increased from 22.4 % in the conventional Q&P sample to 24.1 % in the B270-Q&P sample. Additionally, the impact of the bainite-based quenching temperature on the content and transformation kinetics of the primary martensite is examined.

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

Microstructure and mechanical properties of 0.2C-3.9Al-1.12Mn-0.3Mo δ-TRIP steel as a function of isothermal bainitic transformation temperature

The present investigation employed a two-stage heat treatment on a δ-transformation-induced plasticity (TRIP) steel comprising 0.2C–4Al–1.2Mn–0.3Mo (wt%). The mechanical and microstructural characteristics that result from varied isothermal bainitic transformation (IBT) temperatures following inter-critical annealing at 820℃ for 10 min are thoroughly analyzed. The microstructure of the steels consisted of δ-ferrite, α-ferrite, bainitic ferrite, retained austenite (RA), and martensite, resulting in an optimum combination of the ultimate tensile strength (UTS) and total elongation (TE). The results of the investigation showed that IBT temperature had an effect on the stability of RA and the martensitic transition. Due to the increased mechanical stability of RA, the study revealed that the TRIP phenomenon was more prominent at lower IBT temperatures. Both tensile and yield strengths, as well as elongation, decreased as a consequence of the increase in IBT temperature. Maximum values of UTS, TE, and the product of these two properties (PSE) are attained (860 MPa, 41%, and 35260 MPa∙%, respectively) under optimal processing conditions (at 350℃ IBT temperature in 10 min).

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.

Continuous Heating/Cooling Transformation Kinetics of a Novel CrNiMoV Steel for Large Structural Parts

Continuous heating transformation (CHT) and continuous cooling transformation (CCT) characteristics as well as phase transformation kinetics of a novel Cr–Ni–Mo–V steel for large structural parts are studied using a Gleeble‐3500 thermal–mechanical simulator. CCT curves are obtained. The results show that carbides in initial structure can be completely dissolved during CHT as the heating rates are not greater than 0.2 °C s−1. The final microstructure exhibits obvious sensitivity to cooling rate. It contains martensite with or without bainite. Besides, kinetic models of austenitization are established, and it is confirmed that diffusion is the major mechanism of austenitic transformation in steel during CHT. Meanwhile, bainitic phase transformations kinetic model is developed based on the J–M–A model. The fitted Avrami indexes n combined with transmission electron microscope reveal that the bainite nucleation sites of the steel are mainly located at the prior austenite grain boundaries, and the growth of bainite grains mainly follows a 2D mode. The kinetics of martensitic transformation is better modeled based on the K–M equation. A deep understanding of kinetics and microstructure characteristics during CHT and CCT is of great significance to regulate the final performance of the novel steel under development.

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.

EFFECT OF A MAGNETIC FIELD ON THE MICROSTRUCTURE AND PROPERTIES OF A 22MnB5-CP780 WELDED JOINT

A steady-state magnetic field-assisted laser welding test of 1-mm-thick automotive 22MnB5 and CP780 high-strength steel plates was carried out using a low-power fiber laser. The effects of different magnetic induction intensities on the microstructure and mechanical properties of welded joints were investigated under a heat input of 150 J/mm. When no magnetic field was applied, there was splashing on the welded joint surface, the width was wide and the main solid-state phase transitions in the weld center included ferrite transformation and bainite transformation. The application of a magnetic field resulted in an acceleration of the cooling rate of the molten pool due to the thermoelectric magnetic force. This, in turn, led to the predominance of the bainite transformation and martensite transformation as the primary solid-state phase transitions in the weld center. In a range of 5–25 mT, the welded joints were narrow, symmetrical, well-formed and free of defects such as splashing. The experiment proved that as the magnetic induction intensity increased, the laser energy became more concentrated. This led to an increase in the average hardness at the center of the weld, a gradual reduction of the softening in the heat-affected zone and an increase in both the yield strength and elongation of the welded joint. The optimal comprehensive mechanical properties of the welded joints were observed when B = 25 mT.

Study on the Influence of Pre-Formed Phase on Accelerating Bainitic Transformation

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

Acceleration of bainitic transformation in 0.28C-3.8Mn-1.5Si steel utilizing chemical heterogeneity

This study proposes a novel method for accelerating bainitic transformation by utilizing Mn heterogeneity. The process involves intercritical annealing to create a heterogeneous distribution of Mn prior to full austenitizing. Heterogeneous Mn distribution leads to the rapid nucleation of bainitic ferrite in the Mn-depleted austenite region, and the rapidly formed bainitic ferrite acts as a nucleation site for Mn-enriched austenite region. Consequently, the required austempering time to achieve a similar fraction of bainitic ferrite is nearly halved compared to conventional process. The effectiveness of Mn heterogeneity in accelerating the transformation is comparable to other acceleration methods, such as prior austenite grain refinement and introducing prior martensite. Moreover, the proposed method has an additional advantage in increasing the fraction of retained austenite. The retained austenite is stabilized not only by C partitioning during austempering but also by pre-partitioned Mn during intercritical annealing.

Thermal stability of microstructure and corresponding hardness variations in two high-carbon nanostructured bainitic steels

The autotempering phenomenon and carbon partitioning from bainitic subunits during the bainite transformation suggest that the tempering behavior of the nanostructured bainite may be different compared to that of martensite. This article aims to investigate the thermal stability of the microstructure in two low alloy nanostructured bainitic steels containing 0.89 wt% carbon content. Nanostructured bainite was obtained after austempering at 250 °C for optimum heat treatment time. Nanostructured bainitic steels were tempered at the temperature range of 300–700 °C for 30 min. Additionally, the effect of prolonged austempering beyond the optimum heat treatment condition at austempering temperature was studied. Microstructural characteristics were evaluated using optical and field emission gun scanning electron microscopes and X-ray diffraction methods. Hardness variations during tempering of nanostructured bainite compared with that of martensite microstructure obtained in the same primary steels after austenitizing and quenching. Results indicated that microstructural characteristics of nanostructure bainite didn’t change significantly at heat treatment times longer than that needed for bainite transformation completion at isothermal bainite heat treatment temperature. Moreover, results approved the higher tempering resistance of nanostructured bainite compared to that of martensite in the same steel. It has been shown that tempering the nanostructured bainite up to 400 °C didn’t change the microstructure and hardness, while the changes were pronounced at higher tempering temperatures. It has been suggested that cementite precipitation was more significant when a higher number of dislocations were present in nanostructured bainite before starting the tempering process.

Effect of Volumetric Energy Density and Part Height on the Material Properties of Low‐Alloyed Steels Manufactured by Laser‐Based Powder Bed Fusion of Metals

The layer‐by‐layer manufacturing approach in laser powder bed fusion of metals (PBF‐LB/M) leads to heat accumulation in the workpiece with increasing part heights. The effect of this heat accumulation on the resulting material properties has, however, only barely been studied for low‐alloyed steels. The goal of this work is to analyze the influence of different PBF‐LB/M‐specific boundary conditions like varying part heights and volumetric energy densities (VED) on the resulting material properties. It isfound that lower part regions possess similar hardness (380–410 HV1) and retained austenite values (7%–8%), independent of the applied VED. Higher energy inputs lead to higher retained austenite contents of up to 20% due to an incomplete transformation upon cooling. This rise in retained austenite content is also linked to a decreased material hardness down to 320 HV1. In higher part regions, this effect is reversed as the retained austenite content starts to decrease for the highest investigated VED. This is caused by the in situ preheating temperatures caused by heat accumulation, which favor a bainitic transformation. The part height‐specific properties indicate that the microstructure formation forms through a continuous transformation in lower part regions and through an isothermal transformation in higher regions.

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel: Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels. Here, we investigate the isothermal bainite transformation in a 10Mn steel at 450 °C experimentally and theoretically, focusing on the effect of dislocations introduced by warm deformation. We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics (on a time scale of days), concurrent with the pearlite formation. The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction. This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels. With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work, we propose a new ‘carbon depletion mechanism’, in which dislocations-stimulated pearlite formation makes a twofold contribution: facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite. On this basis, a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation, revealing good agreements between model descriptions and experiment results. Our findings, herein, offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation, which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

Effect of recrystallization on bainite transformation and mechanical properties of complex phase steel with high formability (CH steel)

Herein, the effect of recrystallization on the bainite transformation and mechanical properties of CH steel were investigated using SEM, TEM, EBSD, dilatometry, and tensile testing machine. First, precipitation was introduced to hinder recrystallization by batch annealing. Subsequently, the competitive relationship between austenite transformation and recrystallization was revealed using different annealing processes. Finally, the effect of recrystallization on bainite transformation was analyzed, mainly including bainite morphology, fraction, transformation rate, bainite nucleation rate of untransformed austenite and austenite-bainite orientation relationship, as well as the effect on the mechanical properties. After batch annealing, a large amount of cementite and nanoscale (Nb, Ti)C precipitated in the hot-rolled sheet, the austenite transformation start temperature (Ac1) and the pearlite to austenite transformation end temperature (Ac1f)of the cold-rolled sheet increased, and the austenite transformation end temperature (Ac3) of the cold-rolled sheet decreased, and the austenite transformation process accelerated. The microstructure of annealed sheets is consisted of ferrite, granular bainite, martensite, and a small fraction of retained austenite. Carbide hindered the recrystallization process during the annealing process, resulting in significant refining of the grains. The lower degree of recrystallization promoted bainite transformation, providing more nucleation sites, higher grain boundary and autocatalytic nucleation activation energy, improving bainite's fraction and transformation rate. The bainitic ferrite was mainly lathy in the recrystallized zone and polygonal in the non-recrystallized zone. Both a K-S and an N-W relationship between retained austenite and bainite, while the N-W relationship in the non-recrystallized zone was relatively higher. Without severe loss of plasticity, hindering recrystallization increased the dislocation, grain refinement, and precipitation strengthening values of the samples, and the strength significantly improved. However, with the increase of annealing temperature, recrystallization fraction increased gradually, and the increase in strength became less noticeable.

Tailoring microstructure evolution and austenite stability of TRIP steels by Rare-Earth micro-alloying

The microstructure evolution and austenite stability of Rare Earth (RE)-alloyed Transformation Induced Plasticity (TRIP) steels subjected to intercritical annealing and isothermal bainitic transformation have been investigated for superior strength-ductility performance. Structure morphology, austenitizing temperature and phase proportion during thermomechanical process were carefully compared, confirming that minor RE additions indeed refined the microstructure, and affected the thermodynamics of austenite and bainite transformation. Austenitizing ranges of TRIP steels were lifted with increasing RE content then stabilized when exceeding 360 ppm; the intercritical austenite reduced but dissolved more CMn atoms, hence the bainite transformation in RE-alloyed TRIP steels was significantly inhibited, which elevated the retained austenite quantity and thermal stability simultaneously. Therein, more austenite nucleated from the structure fragments with RE-refined lamellar morphologies and strictly grew into acicular counterparts or aggregated into clusters. The aggregated austenite, composed of several subgrains with [011¯] coaxial relations or Σ3 60°〈111〉 misorientation but coherent interfaces, were engulfed or being swallowed up by coalescing ferrites in sluggish growth, guaranteeing the higher mechanical stability than intergranular austenite. In this study, the product of strength and elongation of TRIP steels after optimized RE-alloying was increased by 38% as compared to that without RE additions. The micro-alloying effect of RE elements in TRIP steels highlights the innovations of new-generation automotive steels.

Enhanced stretch flangeability and crack propagation behavior of an 1100 MPa grade TRIP-aided bainitic ferrite steel

In order to face the challenges of lightweight, safety, and emission reduction in the automotive industry, transformation-induced plasticity aided bainitic ferrite (TBF) steel was fabricated by controlling the size and shape of bainitic ferrite and compared with quenching and partitioning (Q&P) steels. The effects of bainite transformation on the mechanical properties, microvoid formation, and crack propagation were analyzed by dilatometry measurement and microstructure characterization. At lower isothermal bainite transformation (IBT) temperature, the lathy bainitic matrix can prevent crack formation and limit the crack propagation inside its laths, in favor of improving the strength and stretch flangeability simultaneously. In addition, granular bainite, coarse martensite, and austenite (M-A) islands would increase the risk of void formation at higher IBT temperatures. These voids provided the initial sites for crack formation, which was also responsible for the reduction of the hole expansion ratio (HER). In conclusion, the outstanding properties were achieved in the T-375 samples with the HER of 33.5%, the total elongation of 16.9%, the tensile strength of 1191 MPa, and the yield strength of 934 MPa.

Effect of Chromium on Microstructure and Mechanical Properties of Hot-Dip Galvanized Dual-Phase (DP980) Steel

Generally, the addition of Chromium (Cr) into the dual-phase (DP) steels can suppress bainitic transformation. In this work, the continuous hot-dip galvanization of DP980 steel is thermally simulated with various Cr contents of 0/0.3/0.6 wt.%, and the effect of Cr on bainitic transformation and properties of steels is studied. The results indicate that the bainitic transformation is obviously inhibited. The fraction of bainite decreases with the increasing Cr content. The incubation time is prolonged in the 0.6Cr steel with a slower bainitic transformation rate. Compared to that of 0 and 0.3Cr steels, the 0.6Cr steel exhibits a high tensile strength of 1033 MPa and uniform elongation of 9.1% due to the rapid strain-hardening rate. As a result, the mechanical properties of 0.6Cr steel satisfy the requirements of hot-dip galvanized DP980 steel.

Bainite Transformation Behavior and Mechanical Characteristics of Medium-Carbon High-Silicon Steel via Austempering

Bainite Transformation Behavior and Mechanical Characteristics of Medium-Carbon High-Silicon Steel via Austempering

Effect of austempering time on multiphase microstructure evolution and properties of carburized Cr-Mn-Si alloyed steel subjected to bainitization quenching & partitioning heat treatment

An innovative multistage heat treatment, that combines Bainitization and Quenching & Partitioning processes (BQ&P) can be applied to obtain multiphase microstructure in the carburized surface layer comprising nanobainite, ultra-fine martensite and retained austenite. In this study, a carburized medium-carbon Cr-Si-Mn alloyed steel was treated by BQ&P treatments with 20%, 40% and 100% of advancement of bainitic transformation in the top surface to investigate the effect of isothermal holding time on microstructure evolution and properties.Extending the bainitization time leads to an increase in the bainitic ferrite content in the microstructure and more significant fragmentation of the residual austenite blocks. It was shown that multiphase microstructure consisting of nanobainite, ultra-fine martensite and retained austenite results in the higher hardness and wear resistance of the case as compared to the fully bainitic microstructure. The enhanced properties results from the strengthening effect of additional martensite fraction and higher amount of stabilized austenite prone to transformation under friction (transformation induced plasticity, TRIP effect). Furthermore, adjusted parameters of the process lead to favorable mechanical properties in a medium-carbon core (YS ∼ 1430 MPa, UTS ∼ 1820 MPa, KV ∼ 20 Jcm−2). The ability to tailor mechanical and service properties of steel by controlling the fraction of particular phases along with a significant reduction of heat treatment time might designate the BQ&P process to industrial application, e.g. carburized gear wheels.