Volume Fraction Of Bainite Research Articles

Effect of austenitic state before ferrite transformation on the mechanical behavior at an elevated temperature for seismic-resistant and fire-resistant constructional steel

Abstract In this study, steels with different volume fraction of bainite and ferrite were obtained by tuning the rolling process to change the austenitic state before ferrite transformation. The volume fraction of bainite in the steel when the austenite was undeformed was ~83%, and in the steel when the austenite was deformed was ~36%. The yield strength at room temperature of the experimental steel with undeformed austenite was ~63.5 MPa higher than the experimental steel with deformed austenite, while the yield strength at 600 °C of the former was ~104.8 MPa higher than the latter. The main strengthening methods that led to difference in yield strength at elevated temperature are precipitation strengthening, dislocation strengthening and grain boundary strengthening. The experimental steel with higher volume fraction of bainite was characterized by higher Young's modulus and dislocation density at 600 °C, while the volume fraction of precipitates was relatively lower. The higher Young's modulus was mainly because of higher volume fraction of alloyed cementite precipitated from steel with higher volume fraction of bainite at 600 °C. Furthermore, the boundary densities of packet boundaries, block boundaries and sub-block boundaries had an important influence on the yield strength of steel at 600 °C.

Microstructure-Impact Toughness Relationships in Quenched and Tempered Low Carbon Low Alloy Steels

The quantitative microstructure - impact toughness relationships in two batches of the same steel grade subjected to quenching and tempering (Q&T) have been established via characterization using EBSD technique and FIB visualization. The EBSD-based criterion for separation of structural constituents in microstructure of Q&T low carbon low alloy steels is proposed. Impact toughness differences between two steel batches subjected to nominally identical Q&T are caused by the changes in the volume fraction of structural constituents caused by various cooling conditions at quenching stage. High volume fraction of bainite containing more distorted bainitic ferrite and the highest amount of brittle cementite precipitates leads to the increase in strength and to the decrease in impact toughness.

Development of Direct Hot-Rolled Ultralow-Carbon Pre-peritectic Ferrite-Bainite Dual-Phase Steel for a Compact Slab Production Plant

The current work developed a direct hot-rolled ultralow-carbon ferritic-bainitic dual-phase “FBDP” steel with high ductility values in accordance with a compact slab production CSP plant. Four ultralow-carbon alloys with chemical compositions of 0.045% C, 0.28 Si%, 0.91% Mn and different Cr weight contents of 0.59, 1, 1.56 and 2% were determined within the pre-peritectic region from phase diagrams plotted by Thermo-Calc 2020. These alloys were melted and cast into blocks, and then, a certain thermomechanical schedule was applied using the actual data obtained from the log files of a CSP plant. The cooling schedule was designed with the aid of CCT and TTT curves plotted by JMat Pro. The results show that hot-rolled FBDP steel contains a substantial amount of bainite from 10 to 28% and negligible martensite. The increase in Cr led to an increase in the bainite volume fraction and an increase in the yield strength and tensile strength, while the elongation decreased slightly. The strain hardening exponent (n-value) reached values ranging from 0.28 to 0.36, while a value of 0.18 was found in the literature for martensitic DP steel. Stroke-force curves plotted from the Erichsen test achieved high Erichsen index values from 14 to 18 mm.

Effect of bainite fraction on hydrogen embrittlement of bainite/martensite steel

Abstract The paper quantifies how the fraction of bainite in dual-phase (bainite/martensite) steel affects its resistance to hydrogen embrittlement (HE). For this purpose, three steels that had different amounts (0, 0.9, and 1.8 wt%) of nickel were used. Heat treatment was conducted to vary the volume fraction of bainite while maintaining same strength level. Hydrogen charging was achieved using an electrochemical method, and the notch fracture strength was measured using a slow strain-rate test. The steel that had 1.8% Ni had the highest bainite fraction and showed excellent resistance to HE. This resistance was attributed to hydrogen trapping at irreversible trapping sites, which were regarded to be the cementite/ferrite interfaces with interfacial dislocations. They were more pronounced in bainite than in martensite because of higher cementite fraction in bainite than in martensite. To increase HE resistance of high-strength steels, the number of fine cementite particles in the bainite should be increased to provide irreversible trapping sites for migrating hydrogen during plastic deformation.

Effects of Prestrain and Inner Stress on the Susceptibility of Hot Stamped Boron Steel to Hydrogen Embrittlement

Hot stamped boron steel parts with martensite and bainite combined microstructures are widely used in lightweight manufacturing of automotive bodies. The final products usually experience the manufacturing process, such as trimming or punching, which inevitably introduces inner stress and plastic strain. Hot stamped parts with complex microstructures and manufacturing effects exhibit some special characteristics of hydrogen embrittlement (HE) in service. In this paper, hardened boron steels with a total martensite content and different martensite-bainite combinations, called martensite or bainite dominant microstructures (MDM/BDM), are experimentally obtained by a flexible quenching tool, and the HE susceptibility considering the effects of prestrain and inner (tensile/compressive) stress is investigated based on slow strain rate tensile tests by developing hydrogen charging equipment under constant loads. SEM and EBSD analyses on hydrogen-inducted cracks (HICs) and thermal desorption analysis of hydrogen for different microstructures and strain/stress states are carried out to illustrate the HE mechanism. The HE susceptibility of the as-quenched specimen decreases with increasing volume fraction of bainite, with the fractograph exhibiting intergranular fracture in the MDM and the dimples feature in the BDM. Prestrain weakens the HE susceptibility in the MDM but raises it in the BDM due to the enhancement of hydrogen adsorption, promoting local plasticity in the evolution of the HICs. Inner stresses increase the HE susceptibility in the MDM, which lifts the hydrogen segregation at the grain boundaries to intensify the hydrogen-enhanced decohesion effect. Tensile stress intensifies the sensitivity to HE in the BDM. The promotion of hydrogen adsorption facilitates crack initiation. However, compressive stress has opposite effects on the HE susceptibility due to the improvement of the material ductility by reducing the hydrogen content in the specimen.

A Coupled Thermo-Mechanical-Metallurgical Model Capturing Essential Transformations in the Welding of High Strength Low-Alloy Steels

Abstract: A well-known challenge is to predict the transformations occurring during the metal alloys welding aiming to control the weldment properties. Thus, this study presents a Thermo-Mechanical-Metallurgical model to numerically predict the thermal history, the solid-state phase transformations, the solidification microstructure and the hardness distribution during and after the welding of high strength low-alloy steels. The model was numerically implemented in an in-house computational code based on the Finite Volume Method, which allowed to dynamically track and calculate the volume fractions of ferrite, pearlite, bainite and martensite at the heat-affected zone, besides the formation and determination of dendrite arm spacing at the fusion zone, whereas the hardness distribution at the heat-affected zone was calculated by applying the phase mixture rule. For this, single-pass autogenous Gas Tungsten Arc Welding welds were numerically simulated and experimentally carried out on high strength low-alloy AISI 4130 steel samples, including their preheating to evaluate the effectiveness of the proposed model to simulate the workpieces welding in different initial thermal conditions and a close agreement between the calculated and experimental results were obtained.

Characteristics of bainitic transformation and its effects on the mechanical properties in quenching and partitioning steels

Abstract Steel treated by quenching and partitioning exhibits an excellent combination of strength and ductility by taking advantage of the transformation-induced plasticity effect of retained austenite. Therefore, the precise tailoring of retained austenite is crucial for quenching and partitioning steels. However, austenite decomposition including bainitic transformation during partitioning is usually neglected, and consequently, its effects on the mechanical properties are not clear. Here, we elucidate the significant role of bainitic transformation during the partitioning process in the microstructure and mechanical properties of 0.2C–2.82Mn-1.58Si steel subjected to one-step quenching and partitioning process and two-step quenching and partitioning process. The results show that the occurrence of bainitic transformation during the QP as a result, the amount of retained austenite is increased, and more blocky austenite is retained due to the optimization of carbon redistribution during bainitic transformation. Moreover, an increase in the bainite volume fraction and the various morphologies of retained austenite lead to excellent work hardening performance, thereby significantly improving the elongation.

Effect of the Austenitizing Temperature on Microstructure Evolution and Impact Toughness of a Novel Bainite Ductile Iron

The effect of austenitizing temperature on microstructure evolution and impact toughness of a newly developed Fe–3.0C–2.8Si–2.0Mn–0.9V–0.2Cr bainite ductile iron was investigated in this research. The ductile iron specimens were heat treated under different continuous cooling process, involving austenitizing between 900 and 980 °C and followed tempering at 200 °C. Optical microscopy, X-ray diffraction, scanning electron microscope and transmission electron microscope tests were conducted to investigate the microstructure evolution. Impact toughness and Rockwell hardness were measured. The results showed that the microstructure of the ductile iron mainly consisted of graphite, acicular bainite and retained austenite after continuous cooling process. The austenitizing temperature could change the volume fraction and size of bainite and retained austenite. There existed a C-area, where retained austenite accumulated near the graphite, except for specimen austenitized at 920 °C. The impact toughness of specimens increased first and then get worse with the increasing of austenitizing temperature. The impact toughness was related with the volume fraction of bainite and the morphology of retained austenite. The fracture mechanism of the bainite ductile iron belonged to cleavage fracture. Chunky graphite acted as the source of microcrack during the impact process. The bulky retained austenite behaved as a prior path for the microcrack propagation, while the bainite and thin filmy retained austenite limited its propagation.

Microstructure and deformation behavior of a novel steel rebar: Effect of the heterogeneous microstructure of soft ferrite and Hard bainite

This study reports a detailed investigation of the deformation behavior and microstructural evolution of a novel ultra-low carbon Cr-Mo alloyed dual-phase steel rebar aimed for marine applications. The rebar matrix consists of the lamellar ferrite/bainite dual phases with the lamellar interfaces along the rolling direction. The soft ferrite phase is composed of larger grains with a low dislocation density, while the hard bainite phase is composed of finer grains with a much higher dislocation density. By comparing the strain hardening behavior from different sites at the rebar with varying ferrite and bainite volume fractions, it shows that the hetero-deformation induced (HDI) hardening is strong and dominates the overall work hardening behavior in the early stage of plastic deformation by prevailing over the conventional sum-up of contribution of each phase alone. In this stage, the plastic deformation of ferrite was constrained by the disproportionally-strained, neighboring bainite, creating the accumulation of geometrically necessary dislocations (GNDs) at the phase interface and the long-range HDI stress. The results also reinforce the understanding of deformation behavior of dual-phase steels, especially around the role of HDI stress and hardening.

Effect of intercritical temperature on the strain hardening of dual-phase bainite/ferrite steel

Medium-carbon dual-phase bainite/ferrite steels with different bainite and carbide volume fractions were prepared via intercritical annealing and austempering. Bainite dominated the microstructure at high intercritical temperatures, and carbides and ferrite phase dominated at low temperatures. Carbides existed in two types, undissolved alloy–cementite and vanadium carbide. The ferrite grain size was reduced to 1.22 ± 0.7 µm at 790°C. Modified Crussard–Jaoul (MC–J) model was used to analyse the strain hardening process, and the highest strain hardening rate was obtained at 755°C. The high carbide fraction at 755°C increased the ferrite dislocation density and enhanced the ferrite phase strength. The high dislocation density in ferrite and the small strength difference in bainite and ferrite enhanced the strain hardening rate at 755°C.

The Expanded Lever Rule on the Phase Transformation Rate Curve for Phase Volume Fraction Calculation in Thermal Simulations

The expanded lever rule is more efficient for calculating phase volume fraction when two phases are partly overlapped. It can make up the deficiency of metallography analysis when phases are similar in morphology. In this study, the expanded lever rule on the phase transformation rate curve is introduced, the approximate symmetry principle in applying the expanded lever rule is modified, and the resulting measurement uncertainty is discussed. Results show that (i) the calculated volume fraction of pearlite by the expanded lever rule in an overlapped two-phase process of a high strength low alloy (HSLA) weld metal is in accordance with the result by metallography analysis, and (ii) the expanded lever rule has an advantage over metallography analysis as it can calculate upper bainite, lower bainite, and lath-like martensite volume fractions in a three-phase process with two overlaps of a HSLA steel. The expanded lever rule is applicable to overlapped two-phase processes of any material theoretically since each phase process can be described by Johnson–Mehl–Avrami–Kolmogorov equation.

Effect of prior austenite grain size on the formation of carbide-free bainite in low-alloy steel

ABSTRACT Inconsistencies exist in literature regarding the effect of prior austenite grain size (PAGS) on the extent and kinetics of bainite transformation. Attempts have been made in the present work to address these issues in a low-alloy carbide-free bainitic steel using dilatometry, over a range of PAGS. The bainite transformation kinetics in the above-mentioned conditions have been analysed quantitatively using established kinetic model to extract information related to the transformation mechanisms in such conditions. Greater obstruction from grain boundaries in fine-grained austenite restricts sheaves of bainite to develop completely and thereby reduces the volume fraction of bainite in comparison with coarse-grained austenite. Initial nucleation rate of bainite transformation increases with decreasing PAGS due to an increase in the nucleation site density. However, the maximum nucleation rate decreases consistently with decreasing PAGS due to gradual reduction in the autocatalytic factor.

Elongation improvement in nano bainite steel obtained from plastically deformed primary austenite

ABSTRACT Ausforming is being widely used to overcome the kinetics barrier in nanostructured bainitic steels and to enhance its practical industrial applications. It would also affect the microstructural characteristics and consequently the resultant mechanical performance. This article aims to investigate the tensile behaviour of nano bainite obtained from plastically deformed primary austenite at three different heat treatment stages. 10% of ausforming has been implemented and nanostructured bainite obtained after austempering at 300°C for 2, 4 and 6 h in salt bath furnaces. Results indicated that ausforming could successfully increase the volume fraction of bainite within the microstructure and refine the bainite packets sizes. Moreover, it has been demonstrated that higher mechanical stability of retained austenite and therefore more effective TRIP effect could be attained during the tensile test which consequently resulted in elongation improvement without deteriorating the strength properties. This claimed to be beneficial for materials performance during practical applications.

Correlation between Microstructure and Mechanical Properties of Base Metal and HAZ of 500 MPa Steel Plates for Offshore Platforms

In this study, two types of thick steel plates are prepared by controlling carbon equivalent and nickel content, and their microstructures are analyzed. Tensile tests, Vickers hardness tests, and Charpy impact tests are conducted to investigate the correlation between microstructure and mechanical properties of the steels. The H steel, which has high carbon equivalent and nickel content, has lower volume fraction of granular bainite (GB) and smaller GB packet size than those of L steel, which has low carbon equivalent and nickel content. However, the volume fraction of secondary phases is higher in the H steel than in the L steel. As a result, the strength of the L steel is higher than that of the H steel, while the Charpy absorbed energy at -40 °C is higher than that of the L steel. The heat affected zone (HAZ) simulated H-H specimen has higher volume fraction of acicular ferrite (AF) and lower volume fraction of GB than the HAZ simulated L-H specimen. In addition, the grain size of AF and the packet sizes of GB and BF are smaller in the H-H specimen than in the L-H specimen. For this reason, the Charpy absorbed energy at -20 °C is higher for the H-H specimen than for the L-H specimen.

Effect of Bainite Volume Fraction on Deformability of Mesostructures for Ferrite/Bainite Dual‐Phase Steel

To obtain high strength and excellent deformability for ferrite/bainite dual‐phase (F/B DP) pipeline steel for gas pipelines based on strain‐based design, the volume fractions of ferrite and bainite should be considered first. In this work, abstract representative volume elements (RVE) of finite element models (FEMs) of mesostructure for F/B DP pipeline steel with volume fractions of bainite between 30% and 58% were established, and the effects of volume fraction of bainite on the tensile properties and deformation compatibility were studied. Results show that the stress and strain in the mesostructure were primarily distributed in the bainite and ferrite, respectively, and strain concentration occurs at the ferrite/bainite interface. With increasing volume fractions of bainite, the strain localization factor (SLF) and strain ratio of ferrite with bainite (εF/εB) decrease, which can improve the deformation compatibility of the F/B DP pipeline steel. However, the stress ratio of bainite with ferrite (σB/σF) and the contributions of bainite to stress and strain sequentially increase, and, as a result, the strength increases and the ductility decreases. Therefore, a balance of strength and deformability can be obtained when the optimal volume fraction of bainite is in the range of 40% to 48%.

A Model for Bainite Formation at Isothermal Heat Treatment Conditions

Abstract An improved model is presented for the formation of bainitic structures during isothermal heat treatment conditions. The model based on displacive mechanism consists of a new expression for the volume fraction of bainite as a function of time, incorporating a temperature and chemical composition-based expression for the number density of initial nucleation sites and limiting the volume fraction of bainite. The model has been validated with respect to experimental data of high- as well as low-carbon steels. It has been found that the isothermal transformation kinetics is well predicted for all steels.

Effect of cerium addition on the bainite transformation behavior of medium carbon microalloyed steel

The bainite transformation kinetics and microstructure evolution of medium carbon microalloyed steels containing 0, 0.0367%, 0.0559%, and 0.0792% of the rare earth element cerium (Ce) were investigated. Optical microscopy and scanning electron microscopy characterizations showed that increasing Ce concentration refines the transformed microstructures, increases the volume fractions of bainite, and reduces the average size of the martensite islands by increasing the carbon content of the bainite. Dilatometry data indicated that Ce promotes the bainite transformation of medium carbon microalloyed steel, increases the bainite transformation rate, and prolongs the total time that the reaction takes to reach completion. Based on the displacive mechanism of bainite formation, the effect of Ce on the transformation kinetics of bainite results from Ce refining the austenite grain and increasing the carbon content in bainite. A Ce content of 0.0792 wt% is also not excessive for the bainite transformation of medium carbon microalloyed steel.

Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.

The Effect of Stress on Bainite Transformation, Microstructure, and Properties of a Low‐Carbon Bainitic Steel

The effects of applying external stress on bainite transformation, microstructure, and mechanical properties are investigated by metallography, dilatometry, etc. Results show that compressive stress during bainite transformation accelerates the bainite transformation and increases the final bainite amount due to the additional mechanical driving force. The lath‐like bainite becomes shorter, and smaller blocky martensite/austenite islands form under stress during the bainite reaction. Different from the results with stress during bainite transformation, applying stress after bainite transformation during isothermal holding cannot increase the volume fraction of bainite, and no obvious change in bainite morphology is observed. In addition, the subsequent martensite transformation is promoted during the cooling process. Moreover, the less amount of retained austenite (RA) but more carbon content in RA is obtained with applying stress after the bainite reaction. Finally, the tensile strength increases, while the elongation decreases when the external stress is imposed after bainite transformation.

Mechanical and Electrochemical Behavior of Dual-Phase Steels Having Varying Ferrite–Martensite Volume Fractions

Dual-phase (DP) steels behave differently under corrosive environment depending upon the volume fractions of phases, mainly martensite, ferrite and bainite. The mechanical behavior and corrosion resistance of DP steels with varying martensite (from 32 to 100%) are investigated. The increase in martensite volume fraction of DP steel results in significant improvement in tensile strength and hardness with a subsequent reduction in elongation. Unlike the mechanical behavior, the increase in martensite volume fraction in steel does not exhibit a direct relationship with their corrosion resistance behavior. The corrosion mechanism in DP steels having different martensite and ferrite volume fractions depends on the self-corrosion resistance behavior of both the phases as well as the occurrence of galvanic corrosion between them.