Bainite Formation Research Articles

Kinetics and microstructural change of low-carbon bainite due to vanadium microalloying

Noticeable strength increases, up to 120 MPa, have been obtained in hot strip and linepipe bainitic steels by microalloying with vanadium. However, there is no consensus on the mechanisms proposed in the literature to explain this effect. As such, this study has investigated the root cause and various strengthening contributions of vanadium additions to bainitic linepipe steels. Two variants of a X100 linepipe steel with identical compositions but different vanadium contents, i.e. a reference steel with residual amount of vanadium and a microalloyed variant with 0.06 wt% vanadium, have been subjected to dilatometric study to determine the kinetics of bainite formation during continuous cooling and cooling-coiling scenarios. Further, a novel in-situ hot compression experiment was developed to determine the strength of fresh bainite (prior to coiling) and its softening during simulated coiling at 450 °C. Various microstructural features of the bainite, including the morphology, dislocation density and lath size, as well as the presence of microalloyed carbonitrides have been quantified by TEM characterization. It was found that the kinetics of the bainite transformation during cooling and the tempering of fresh bainite upon coiling simulations were substantially influenced by vanadium microalloying. Vanadium in solution shifted the bainite transformation to lower temperatures (by 30–40 °C) during cooling at 1–50 °C/s, refining the lath structure and increasing the dislocation density of bainitic ferrite. The V-added steel demonstrated a higher strength of fresh bainite (about 50 MPa) compared to the reference alloy. This superior strength has been explained explicitly by accounting for the contributions from the increased dislocation density and the refined lath structure of V-added variant. No evidence of fine VC or VN precipitation occurring during coiling at 450 °C was found. Further, the complex TiNbV(CN) particles that were observed were too coarse and too scarce to make any strengthening contribution. The fresh bainite of both X100 variants tends to soften rapidly upon coiling at 450 °C; about 110 MPa strength drop was noticed for the reference alloy. The implications of the findings for alloy design and processing of advanced linepipe steels are discussed.

High hardness and retained austenite stability in Si-bearing hypereutectoid steel through new heat treatment design principles

Austempering, and quenching and tempering heat treatments of high Si hyper-eutectoid steels are investigated using both dilatometry and salt-bath heat treatments. Changes in retained austenite content are measured using X-ray diffraction. Results are compared to the prevailing theory for bainite formation and published data. From the understanding of retained austenite evolution during tempering, a novel heat treatment is designed which is technologically identical to quenching and tempering with the exception that the principles guiding the selection of tempering temperature and duration are different. This heat treatment is shown to lead to a combination of hardness, retained austenite content and thermal stability entirely unprecedented for this category of alloys. In addition, this result is achieved for heat treatment durations that can be considered standard. In comparison, low temperature austempering is shown to require considerably longer holding durations to achieve somewhat lower combinations of hardness and retained austenite content.

ДОСЛІДЖЕННЯ КІНЕТИКИ РОЗПАДУ АУСТЕНІТУ ПРИ БЕЗПЕРЕРВНОМУ ОХОЛОДЖЕННІ СТАЛІ К76Ф ДЛЯ РЕЙОК ЗВИЧАЙНИХ ДЛЯ ЗАЛІЗНИЦЬ ШИРОКОЇ КОЛІЇ

The aim of the work is to study the kinetics of decomposition of supercooled austenite with continuous cooling of steel for railway rails with a high content of manganese and micro-alloyed vanadium. The relevance of the work is the need to determine the possibility of producing rails from K76 F steel according to foreign standards, in particular EN 13674-1-2011. During the research, a thermo-kinetic diagram was constructed with different cooling rates for samples of K76F steel heated by the heating mode for quenching the rails in an induction unit at the heat treatment site at Azovstal Iron and Steel Works. Dilatometric studies have shown that when samples are heated at a rate of 300С/min in steel for railway rails, polymorphic  -transformation begins at temperature 7350С (Ас1), ends at 7600С (Ас3). It is established that the temperatures of the beginning and completion of the formation of ferrite, perlite and bainite decrease with increasing cooling rate. With an increase in the rate of cooling, the morphology changes and the dispersion of perlite increases - from the middle plate of frequent perlite to highly dispersed sorbitol structures and quenching troostite. It was concluded that in the studied steel grade K76F with 0.80%С, 0.25%Si, 0.97%Mn, 0.055%V, it is impossible to achieve hardness above 400НВ without the formation of bainite. For the manufacture of high-strength rails of category R400HT EN 13674-1-2011, it is necessary to create steel of a new chemical composition. It is shown that it is possible to develop grounded reinforcing heat treatment mode and to increase the operational reliability of rails by defining critical points and studying the kinetics of decomposition of supercooled austenite with continuous cooling of steel.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

Simulation of bainite and martensite formation using a novel cellular automata method

A novel two-dimensional cellular automata model for simulating the formation of lath martensite and bainite in steels during cooling is presented. The model is parameterized using fitted Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Koistinen-Marburger equations and by comparing predictions to scanning electron microscopy images of the actual microstructures after cooling. The results of this simulation can be used to estimate the fractions, shapes and sizes of bainite and martensite for different cooling rates, which should offer new possibilities for the qualitative estimation of the mechanical properties of high-strength steels with bainitic – martensitic microstructures formed from recrystallized or thermomechanically deformed austenite.

Influence of die-forging deformation on microstructure of 1538MV non-quenched and tempered steel for crankshaft

Non-quenched and tempered steels offer many advantages such as energy saving, emission reduction, simple processing, short production time, and low cost. Demand for energy saving and emission reduction is increasing with the rapid increase of car production and ownership. As a result, the usage of non-quenched and tempered steel in automotive parts has attracted increasing attention. The main problem with non-quenched and tempered steel in actual production is the lack of a hot deformation process and a cooling process that can be used to precisely control the microstructure and properties of the material. The material 1538MV is a type of pearlite+ferrite non-quenched and tempered steel, and its use for building crankshafts has not been studied sufficiently thus far. The main factors affecting the structure of a crankshaft are deformation, final forging temperature, and metal flow and cooling after forging. In this paper, the forging process of 1538MV non-quenched and tempered steel for building a crankshaft was studied by means of numerical simulation, microstructures of the rolled material and finished crankshaft were analyzed, and influence of deformation on the microstructure of the crankshaft was discussed. The results show that the microstructure of a crankshaft forged at a higher temperature and with smaller deformation is coarser than that of a crankshaft made of rolled material. The inhomogeneity of the microstructure is caused by unevenness of temperature and strain distribution during crankshaft deformation. The ferrite content of the crankshaft and the pitch of the pearlite were lower than those of the rolled material, and the bainite structure appeared in a few parts, which indicated that the cooling rate was too fast during phase transformation. Therefore, the cooling process should be optimized further. In addition, the metal flow in the segregation zone during the crankshaft forging process significantly influences the forged microstructure of the crankshaft, which is another cause of the formation of bainite. Therefore, the quality of the rolled material should be strictly controlled. The above results provide directions for improving the quality of the rolled material, optimization of the crankshaft forging process, and optimization of the cooling process after forging.

In-situ Investigation of Bainite Formation with fast X-Ray Diffraction (iXRD)*

Abstract Using specific equipment for high temperature fast in-situ X-ray diffraction (iXRD) enables the collection of diffraction patterns over a range of 48 degrees in 2θ within one second. In combination with a high-temperature furnace, modifications in the crystallographic structure under a thermal treatment like phase transformations in steel can be observed nearly in real-time. We demonstrate the usefulness of fast high temperature iXRD using the ball bearing steel (100Cr6), which is of high industrial importance for many applications. The specimens were austenitized at 860 °C. Various isothermal annealings were carried out to develop a bainitic steel structure. The transformation was interrupted by further cooling at different stages. The specimens were further investigated with SEM and light microscopy using different etching techniques to correlate the microstructure with the diffraction patterns. We will show that iXRD can help to optimize heat treatments, validate time transformation diagrams and increase the understanding of phase transformations in steels.

Effect of microstructure on the impact toughness transition temperature of direct-quenched steels

A sufficient level of toughness at low temperatures is paramount for the use of structural steels intended for arctic applications. Therefore, it is important for the steel industry to identify the factors that control brittle fracture toughness. In this study, the quantitative effect of microstructure on the impact toughness transition temperature has been investigated with 18 different thermomechanically rolled and direct-quenched low-carbon ultra-high-strength steels with varying martensite and bainite contents. The steels were produced by altering their chemical composition, the finish rolling temperature and the total reduction of the prior austenite grains in the non-recrystallisation temperature regime, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions and texture as well as by using Charpy-V impact and tensile testing. It is shown for the first time that the impact toughness transition temperatures T28J and T50 closely follow a dynamic reference toughness, defined by yield strength and the size of the coarsest grains in the effective grain size distribution at 80th percentile. Decreasing the area fraction of {100} cleavage planes oriented within 15° of the macroscopic fracture plane by increasing austenite pancaking is also shown to improve T28J. The best toughness is achieved with the lowest finish rolling temperatures that are nevertheless high enough to avoid the subsequent formation of granular bainite, which weakens both the toughness and strength. The results show that it is perfectly possible to produce untempered ultra-high-strength martensitic and martensitic-bainitic structural steels with adequate low-temperature toughness when the grain size is properly controlled.

Chemistry and three-dimensional morphology of martensite-austenite constituent in the bainite structure of low-carbon low-alloy steels

The martensite-austenite constituent (MA) originates from carbon-enriched and untransformed austenite surrounded by bainite and is known to deteriorate the toughness of bainitic steel. The relationship between MA and bainite formation in low-carbon low-alloy steel was investigated with respect to the effects of cooling rate and Nb addition, as well as the relationship between the MA distribution and surrounding bainite. Nb addition enhanced the formation of MA over a wider range of cooling rates, and the fraction of MA became maximum at a cooling rate of 5 K/s. A microstructure consisting of carbide-free coarse bainite with few high angle boundaries is formed near the prior austenite grain boundary, and fine bainite with a high density of carbide and high angle boundaries is formed within the austenite grains. MA tends to be located in the coarse bainite region. Three-dimensional observations of the bainite and MA indicated MA is elongated along bainitic ferrites with the same growth direction or the same habit plane, while MA is blocky when the surrounding bainitic ferrites have different growth directions. Thus, the shape of MA is strongly affected by the crystallography of the surrounding bainite structure. The carbon content in austenite revealed that the carbon content of MA is close to the T0 composition at the transformation temperature, which suggests that MA formation is caused by a reduction of the driving force for transformation by carbon enrichment, which results in incomplete bainite transformation. Nb addition further inhibits the decomposition of austenite and promotes MA formation.

The Formation of Carbide-Free Bainite in High-Carbon High-Silicon Steel under Isothermal Conditions

It is shown that a carbide-free bainite structure can be formed in high-carbon steel of the Fe–Si–Mn–Cr–V system using a traditional furnace facility. The structural aspects of bainitic transformation developing under isothermal conditions at 300°C have been studied by the methods of X-ray diffraction and transmission electron microscopy. Orientation relationships between crystalline lattices of γ and α phases have been established. A superequilibrium carbon concentration in the bainite α phase has been determined.

Phase field modeling of the simultaneous formation of bainite and ferrite in TRIP steel

In transformation-induced plasticity (TRIP) steels, Si and/or Al delay cementite precipitation during bainite transformation leading to a transformation product that is usually referred to as carbide-free bainite. The present work proposes an approach to calibrate the parameters of a multi-phase field model to simulate the austenite decomposition into ferrite and bainite in a TRIP steel. Ferrite nuclei are introduced at austenite grain boundaries and suitable interfacial mobilities are selected to reproduce experimental ferrite formation kinetics. Bainite nucleation occurs for a sufficiently high undercooling at available interface sites (i.e. austenite grain boundaries, austenite-ferrite interfaces) and/or within austenite grains. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The proposed model is benchmarked and validated with experimental continuous cooling transformation data.

Transformation Stasis Phenomenon of Bainite Formation in Low-Carbon, Multicomponent Alloyed Steel

The transformation stasis phenomenon of bainite formation in low-carbon steel was detected using a high-resolution dilatometer. The phenomenon occurred at different stages for different isothermal temperatures. In combination with microstructural observation, the calculated overall activation energy of transformation and interface migration velocity shed new light on the cause of formation of the stasis phenomenon. The temporary stasis formed at the initial stage of phase transformation for high isothermal temperature was attributed to the drag effect of substitutional atoms, which leads to low-interface migration velocity and large overall activation energy.

The Morphology of Intermediate Structures Formed During Bainite Transformation in HSLA Steels

The paper deals with the structure of bainite formed under the influence of thermal deformation cycles of welding in low-carbon bainitic class steels. Morphology features associated with the formation of mesoferrite and granular bainite determines the high cold resistance of welded joints.

Microstructural evidence of nano-carbides in medium carbon high silicon multiphase steels

High silicon steels containing 1.5–2wt% silicon are considered to form carbide free bainite. However, the present work shows experimental evidence for the formation of nano-carbides in a series of medium carbon (0.61wt%) high silicon (1.71wt%) low alloy mixed phase steels with the help of transmission electron microscopy. The nano-carbide formation in bainite is attributed to the formation of inter-critical ferrite during continuous cooling, resulting in partitioning of carbon and silicon at the austenite-ferrite interface, leading to an increase in carbon and decrease in silicon at the austenite side of the interface. Due to diffusionless growth of supersaturated bainitic ferrite lath under para-equilibrium conditions, formation of bainite proceeds with higher carbon and lower silicon, allowing carbide formation in the bainite during subsequent holding at bainitic transformation temperature. Nano-carbides along with mixed phase microstructures are attributed to the excellent combination of high strength and high ductility in the present steels.

Modeling Bainite Transformation and Retained Austenite in Hot Rolled TRIP Steel by Instantaneous Carbon Enrichment

The rate model inherited from Leblond and Devaux's concept has been applied to multiphase transformations in hot‐rolled TRIP steel. The phase fraction is collected from different laboratory scale process simulations, both multi‐pass rolling for repeated recrystallization and single‐pass cycles. The latter is also investigated by in situ synchrotron X‐ray diffraction (XRD) to acquire instantaneous data for the phase fraction and carbon content. The different cycles allow different austenite conditioning inputs for the model to enlarge the range of thermomechanical parameters. Bainite transformation model is a function of the instantaneous average carbon content in the remaining austenite, which is measured from the shift in the 1‐dimensional diffraction peaks. The modeling can map different process windows for a specific required microstructure of TRIP steel. The bainite formation is remarkably favored by low carbon content. But, the promotion of ferrite formation in the former step increases the carbon content dramatically and yields smaller bainite fraction that consequently larger fraction of retained austenite is allowed. A modified Koistinen–Marburger equation has been established from the in situ XRD data to model the martensite formation during the final cooling to reveal the stability of austenite. Above an average carbon content of 1.475 mass%, the retained austenite becomes stable.

Continuous cooling transformation behavior and the kinetics of bainite formation in a bainitic–martensitic steel

Abstract The continuous cooling transformation diagram of a low carbon bainitic–martensitic steel was constructed using dilatometry and metallographic methods. It was found that as cooling rate increases, the structure changes from granular bainite to lath martensite. Three regions of different kinetic behavior were discerned for the bainitic–martensitic steel. One of the regions conformed to martensite formation and the other two comprised transformation to bainite. A non-isothermal type of Johnson–Mehl–Avrami–Kolmogorov kinetic equation of reaction rate was used to analyze the transformation behavior during continuous cooling of bainite formation. The Avrami exponent and activation energy values for different regions at cooling rates of 0.1 and 0.4 K s−1 varied from 1.5 to 4.7 and 71 to 84 kJ mol–1 respectively. Models obtained from such kinetic coefficients closely corresponded to experimental results.

Effect of microalloying elements on microstructure and properties of quenched and tempered constructional steel

The effects of microalloying elements Nb, V and Ti on microstructure and properties of quenched and tempered steel were studied. Results showed that the addition of microalloying elements led to the formation of bainite and increased strength, while the austenization and ferrite transformation temperature was barely affected, i.e. 10°C. Microalloying elements shortened the incubation time for bainite transformation by refinement of austenite grain, and decreased the hardenability by forming carbides and therefore reducing the carbon content of super-cooled austenite. Either of them promoted the bainite transformation. The better tempering stability was ascribed to the as hot-rolled bainite microstructure and secondary carbide precipitation during tempering.

Microstructure and mechanical properties of simulated unaltered coarse grained heat affected zones of 10CrNi3MoV steel by double-sided double arc welding

This paper aims to reveal the influence of different welding conditions on unaltered coarse grained heat affected zones (UACGHAZs) of backing passes by double-sided double gas tungsten arc welding (DSGTAW) of 10CrNi3MoV steel. UACGHAZs were also compared with coarse grained heat affected zones (GGHAZs) of single gas tungsten arc welding (SGTAW). At lower heat input (21.7kJ/cm for each run without preheating), CGHAZ mainly consisted of lath martensite (LM) while UACGHAZs composed of LM and small amount of lath bainite (LB). At higher heat input (33.2kJ/cm for each run with 150°C preheating), the microstructures of CGHAZ were LM and LB. In UACGHAZs, the microstructures would transform from LB+LM+granular bainite (GB) to LB+GB as arc distance (d) decreased. The toughness of UACGHAZs was related to martensite/bainite packet sizes instead of prior austenite grain sizes. At higher heat input, the formation of granular bainite deteriorated the toughness of UACGHAZs and negated the positive effect of LB on toughness. DSGTAW can significantly improve the toughness of coarse grained zones when lower heat input is adopted.

Thermally Stable Nanocrystalline Steel

Two novel nanocrystalline steels were designed to withstand elevated temperatures without catastrophic microstructural changes. In the most successful alloy, a large quantity of nickel was added to stabilize austenite and allow a reduction in the carbon content. A 50 kg cast of the novel alloy was produced and used to verify the formation of nanocrystalline bainite. Synchrotron X-ray diffractometry using in situ heating showed that austenite was able to survive more than 1 hour at 773 K (500 °C) and subsequent cooling to ambient temperature. This is the first reported nanocrystalline steel with high-temperature capability.