Volume Fraction Of Bainite Research Articles

Effect of bainite volume fraction on mechanical properties of a ferrite-bainite-martensite steel

Effect of bainite volume fraction on mechanical properties of a ferrite-bainite-martensite steel

Physical Simulation of Thermo-Mechanical Processing of Ferritic-Bainitic Dual Phase (FBDP) Steel

The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac1= 766 °C, while Ac3 was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σf/ε) was directly proportional to temperature difference from pass to pass.

Formation of abnormal structures and their effects on the ductility of eutectoid steel

The formation of abnormal structures and their effects on reduction of area (RA) were investigated in eutectoid steels transformed at different temperatures ranging from 560 °C-650 °C. The occurrence of abnormal structures, such as upper bainite, degenerate pearlite, free ferrite, and grain boundary cementite, was confirmed. The volume fraction of upper bainite and degenerate pearlite decreased on increasing the transformation temperature, while the amount of free ferrite increased. As the transformation temperature increased, RA increased, reached a maximum, and then decreased, while the tensile strength continuously decreased. The crack formations during the tensile test could be classified into three types: tearing, shear cracking, and void formation/ coalescence. The decrease of the ductility at low transformation temperatures was attributed to the increased amount of upper bainite and degenerate pearlite, since the formation of cracks occurred by tearing interfaces or by void formation at abnormal structures during the tensile test. Meanwhile, the decrease in RA at high transformation temperatures was attributed to the occurrence of shear cracking rather than the presence of abnormal structures.

Effect of Nitrogen Content and Cooling Rate on Transformation Characteristics and Mechanical Properties for 600 MPa High Strength Rebar

AbstractTo obtain appropriate chemical composition and thermo-mechanical schedules for processing the V-N microalloyed 600 MPa grade high strength rebar, the microstructure analysis during dynamic continuous cooling and tensile tests of three experimental steels with different nitrogen contents were conducted. The results show that increasing nitrogen content promotes ferrite transformation and broadens the bainite transformation interval, when the nitrogen content is in the range of 0.019–0.034 mass%. Meanwhile, the martensite start temperatures decrease and the minimal cooling rate to form martensite increases. To achieve a good combination of strength and ductility, the cooling rates should be controlled in the range of 0.5–3°C/s, leading to the microstructure of ferrite, pearlite and less than 10% bainite (volume fraction). Furthermore, all the experimental steels satisfy the performance requirement of 600 MPa grade rebar and the rebar with nitrogen content of 0.034 mass% shows the highest strength through systematically comparative investigation.

Effects of aging treatment on the microstructure and superelasticity of columnar-grained Cu71Al18Mn11 shape memory alloy

The effect of aging treatment on the superelasticity and martensitic transformation critical stress in columnar-grained Cu71Al18Mn11 shape memory alloy (SMA) at the temperature ranging from 250°C to 400°C was investigated. The microstructure evolution during the aging treatment was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that the plate-like bainite precipitates distribute homogeneously within austenitic grains and at grain boundaries. The volume fraction of bainite increases with the increase in aging temperature and aging time, which substantially improves the martensitic transformation critical stress of the alloy, whereas the bainite only slightly affects the superelasticity. This behavior is attributed to a coherent relationship between the bainite and the austenite, as well as to the bainite and the martensite exhibiting the same crystal structure. The variations of the martensitic transformation critical stress and the superelasticity of columnar-grained Cu71Al18Mn11 SMA with aging temperature and aging time are described by the Austin–Rickett equation, where the activation energy of bainite precipitation is 77.2 kJ·mol−1. Finally, a columnar-grained Cu71Al18Mn11 SMA with both excellent superelasticity (5%–9%) and high martensitic transformation critical stress (443–677 MPa) is obtained through the application of the appropriate aging treatments.

Transformation kinetics and microstructural features in Low-Temperature Bainite after ausforming process

Acceleration of the bainitic transformation and modifying the microstructural characteristics are believed to be of major concerns in production of Low-Temperature Bainite. Ausforming prior to isothermal bainitic transformation seems to be an effective procedure in this regard. This article aims to study the influence of different ausforming strains on transformation kinetics and microstructural features of low temperature nano bainite obtained after austempering at 300 °C. Results indicated that bainitic transformation tended to be accelerated by plastic deformation of primary austenite depending on applied strain level. In addition, the volume fraction and morphology of the microstructural constituents have been shown to be affected by ausforming after which higher volume fraction of bainite, finer austenite micro blocks and thinner bainitic ferrites could be achieved. Finally, quantitative analysis demonstrated that ausforming favoured the distribution of bainitic sheaves in limited crystallographic directions whereas they spread randomly in different crystallographic orientations in non-ausformed specimens. This was more evident at higher ausforming strain levels at which bainitic sheaves' orientation distributions were narrower relative to the rolling direction regardless of the isothermal bainitic heat treatment time.

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025 K s−1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (fb), and the average energy is about 136.7 kJ mol−1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide fb range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.

A stress-based fracture criteria validated on mixed microstructures of ferrite and bainite over a range of stress triaxialities

Hot stamping is a sequential process for formation and heat-treatment of sheet metal components with superior mechanical properties. By applying different cooling rates, the microstructural composition and thus the material properties of steel can be designed. By controlling the cooling rate in different sections of a blank, the material properties can be tailored depending on the desired toughness. Under continuous cooling, various volume fractions of ferrite and bainite are formed depending on the rate of cooling. This paper focuses on the ductile fracture behavior of a thin sheet metal made of low-alloyed boron steel with varying amounts of ferrite and bainite. An experimental setup was applied in order to produce microstructures with different volume fractions of ferrite and bainite. In total, five different test specimen geometries, representing different stress triaxialities, were heat treated and tensile tested. Through full-field measurements, flow curves extending beyond necking and the equivalent plastic strain to fracture were determined. Experimental results were further investigated using a mean-field homogenization scheme combined with local fracture criteria. The mean-field homogenization scheme comprises the influence of microstructure composition and stress triaxiality with usable accuracy, connoting auspicious possibilities for constitutive modeling of hot-stamped components.

Quantitative analysis of martensite and bainite microstructures using electron backscatter diffraction.

In the present work, ultra-high-strength steels with multiphase microstructures containing martensite and bainite were prepared by controlling the cooling rate. A new approach was proposed for quantitatively statistical phase analysis using electron backscatter diffraction (EBSD) based on the band contrast which correlates to the quality and intensity of the diffraction patterns. This approach takes advantage of the inherently greater lattice imperfections of martensite, such as dislocations and low-angle grain boundaries, relative to that of bainite. These can reduce the intensity and quality of the EBSD patterns of martensite, which decrease the band contrast. Thus, combined with morphological observations, Gaussian two-peak fitting was employed to analyze the band contrast profile and confirm the ranges of band contrast for the two phases. The volume fractions of bainite and martensite in different samples were determined successfully. In addition, the results show that increased cooling rates improve the proportion of martensite and the ratio of martensite to bainite. Microsc. Res. Tech. 79:814-819, 2016. © 2016 Wiley Periodicals, Inc.

Numerical modeling of TRIP steel in axial crashworthiness

With their high strain rate hardening capacity and good combination of strength and ductility, transformation induced plasticity (TRIP) steels offer lightweight solutions for reinforcement components undergoing crash events. The TRIP effect correlates with stress-stimulated martensitic transformation from austenite, which depends on a myriad of kinematic ingredients, including three-dimensional plastic deformation, temperature change, strain-rate, and stress state. The complex interdependency of these kinematic ingredients requires stepwise and time iterative numerical analyses, especially when boundary-value problems are imposed. Typically engineering desired boundary value problems involve simulations designed to control energy absorption under complex crash scenarios. In this paper, we demonstrate a new phenomenological framework for thermo-mechanical modeling of multi-phase TRIP steel that captures the evolution of phase transformation. The model was implemented into the user-defined subroutine of the explicit dynamic version of the commercial finite element software LS-DYNA. Simulations were performed on a top-hat crush tube made of an industrial multiphase TRIP 800 steel. The results were utilized within an optimization simulation framework to pinpoint composition windows, which maximize the energy absorption capacity. When not concerned about peak crush force, larger compositions of austenite (typically ∼70%) will outperform all other compositions. However, lowering the austenite volume fraction with higher volume fractions of bainite (∼80%) with balanced ferrite could produce a top-hat crush rail with a superior crush force and efficiency compared to TRIP 800 steel without increasing the peak crush force.

Microstructural Characterization and Properties of Microalloyed Powder Metallurgy Steels

The aim of this research is to investigate the effect of alloying elements of nickel, molybdenum, carbon and copper on hardness, density and microstructure of low alloy steel produced by powder metallurgy method. The results showed that molybdenum addition could lead to increase the density and hardness. Also, an increase in carbon content up to 1 wt. % could result in an increase in density and hardness. Copper is mainly added to increase strength as a result of solid solution hardening effect. Molybdenum is a ferrite stabilizer while Nickel is a strong austenite stabilizer. Consequently, the nickel and copper rich regions are mainly surrounded by austenite or bainite. Regions with higher amounts of alloying elements appeared to be martensitic islands. Increasing martensite and bainite volume fraction led to increase hardness.

Investigation of the microstructure and toughness of 550 MPa grade pipeline after the hot-bending process

In this work, the effects of the hot-bending process on the microstructure and low temperature (−40°C) toughness of weld metal of pipeline K65 (the highest grade of the Russian natural gas pipeline) were studied. The weld metal from the longitudinally submerged arc welding (LSAW) pipe making process was compared with that subsequently treated by the hot-bending process. It was found that the latter process led to serious deterioration in toughness, which obviously degraded the sound properties achieved in the original weld metal of LSAW pipes. Microstructural characterisation revealed that the weld metal, which originally consisted mainly of acicular ferrite in the as-deposited condition, became predominantly composed of bainitic ferrite after hot bending. It is clear that reaustenisation caused a smaller austenite grain-sized matrix, which brought about a very high volume fraction of bainite. Consequently, the low temperature toughness was deteriorated, in contrast to the excellent toughness achieved in the as-deposited weld metal.

CALPHAD-based alloy design for advanced automotive steels – Part II: Compositional and microstructural modification for advanced carburizing steels

The CALPHAD-based computational techniques established in the first part of this series [1] have been applied for the development of advanced carburizing steels used for gears in vehicle transmissions. To improve the strength and hardenability of a conventional carburizing steel, the alloying composition has been modified based on the calculation of phase equilibria and the prediction of mechanical properties such as the yield and tensile strengths, hardness, and volume fractions of martensite and bainite. The size and density of carbides precipitated with V, Nb or Ti microalloying elements as well as their austenite grain size have been predicted by thermo-kinetic simulation to optimize the microstructure. The reliability of the computational results has been experimentally confirmed by comparing the austenite grain size and the hardness of the newly developed carburizing steels with those of the conventional carburizing steels.

Numerical Predictions for the Thermal History, Microstructure and Hardness Distributions at the HAZ during Welding of Low Alloy Steels

A phenomenological model to predict the multiphase diffusional decomposition of the austenite in low-alloy hypoeutectoid steels was adapted for welding conditions. The kinetics of phase transformations coupled with the heat transfer phenomena was numerically implemented using the Finite Volume Method (FVM) in a computational code. The model was applied to simulate the welding of a commercial type of low-alloy hypoeutectoid steel, making it possible to track the phase formations and to predict the volume fractions of ferrite, pearlite and bainite at the heat-affected zone (HAZ). The volume fraction of martensite was calculated using a novel kinetic model based on the optimization of the well-known Koistinen-Marburger model. Results were confronted with the predictions provided by the continuous cooling transformation (CCT) diagram for the investigated steel, allowing the use of the proposed methodology for the microstructure and hardness predictions at the HAZ of low-alloy hypoeutectoid steels.

The influence of fine ferrite formation on the γ/α interface, fine bainite and retained austenite in a thermomechanically-processed transformation induced plasticity steel

Abstract

Material Characterization and Numerical Simulation of a Dissimilar Metal Weld

A 3D thermal-mechanical-metallurgical finite element (FE) model has been developed to investigate the microstructure and the distribution of residual stress of a muck-up represents a dissimilar metal weld (DMW) joint of a VVER 440 reactor pressure vessel safety-end nozzle. In order to capture the correct microstructure evolution a number of material properties were required for present simulations. The first task of mechanical characterization was the determination, with accurate experimental devices and its numerical interpretation, of the mechanical properties in terms of stress-strain curve of all DMW constitutive materials: austenitic stainless steel, ferrite steel, heat affected ferrite steel zone and buttering layers properties. For characterization and validation purpose microstructure and hardness map were also evaluated. To get addition information for further investigation, fracture properties for base materials, heat affected ferrite steel zone and buttering layers were also measured. The welding was simulated by the 3D finite element model using temperature and phase dependent material properties. The commercial finite element code was used to obtain the numerical results by implementing the Goldak’s double ellipsoidal shaped weld heat source and combined convection radiation boundary conditions. The results of the simulation provide the size of the HAZ and the volume of the molten zone. The volume fraction of bainite and martensite can be quantified and serves as an additional response that can be used to validate this model with experiments and to predict phase volume fractions under new processing conditions. This paper also presents the results of the through-thickness residual stress distributions on the certain DMW. The results of FE analyses were compared with the experimental measurements in the ferritic steel section of the weld.

A study on homogenization methods for steels with varying content of ferrite, bainite and martensite

The demand of ultra high strength steel (UHSS) components increased in the last decade due to their high strength to weight ratio. The driving force in this development is the automotive industry and regulations concerning passenger safety and fuel consumption. The use of ultra high strength steel enables design of lighter car bodies with equal or better passenger safety compared to earlier car generations. The automotive industry and their suppliers need predictive tools in the development of components with tailored material properties. Components with tailored material properties are produced by hot stamping, in this process a blank is austenitized before it is formed and quenched in one step. By use of sequential heated or cooled tools, different mechanical properties distributed within the same component are achieved.In order to develop a constitutive model for components consisting of regions with varying phase content, a suitable method to describe the elasto-plastic part of the yield curve is needed. The focus of this work is on the description of the elasto-plastic constitutive model of an ultra high strength steel depending on the phase content in the material. Different volume fractions of ferrite, bainite and martensite are experimentally formed. In this study the capability of different homogenization methods on the prediction of the material response of a multi-phase steel depending on the volume fraction of formed phases is investigated. The modeling results are compared to experimental results.The prediction of the composite response using the micromechanical based double-inclusion model and pure phase measured data as well as experimentally obtained phase volume fractions of present phases showed good agreement throughout all samples tested in this study.

Microscopic deformation and strain hardening analysis of ferrite–bainite dual-phase steels using micro-grid method

The local strain measurement method using nanometer-scaled micro grids printed on the surface of a specimen by an electron lithography technique (the micro-grid method) has been established. Microscopic deformation behavior of the ferrite–bainite steels with different bainite volume fraction, 16% and 40% of bainite, was evaluated. Strain localization in the ferrite phase adjacent to the ferrite/bainite boundary was clearly observed and visualized. Highly strained regions expanded toward the inner region of the ferrite phase and connected each other with an increase of macroscopic strain. The existence of hard bainite phase plays an important role for inducing strain localization in the ferrite phase by plastic constraint in the boundary parallel to the tensile direction. In order to obtain further understanding of microscopic deformation behavior, finite element analysis using the representative volume element, which is expressed by the axisymmetric unit cell containing a hard phase surrounded by a soft phase matrix, was conducted. It was found that the macroscopic stress–strain behavior of ferrite–bainite steels was well simulated by the unit cell models. Strain concentration in the ferrite phase was highly enhanced for the ferrite-40% bainite steel, and this imposed higher internal stress in the bainite phase, resulting in higher strain hardening rate in the early stage of the deformation. However, smaller ferrite volume fraction of ferrite-40% bainite steel induced bainite plastic deformation in order to fulfill the macroscopic strain of the steel. Accordingly, strain hardening capacity of the ferrite-40% bainite steel was reduced to a significant degree, resulting in a smaller uniform elongation than the ferrite-16% bainite steel.

Effects of Cooling Rate and Carbon Content on Mechanical Property of Sintered Fe-Cr-Mo Alloys

Advanced high strength steel (AHSS) was prepared using the conventional ‘press and sinter’ process.The compacts of ultralow carbon Fe-Cr-Mo powder with carbon additions (base metal powder admixed with 0.1,0.2 and 0.3 wt.% graphite) and without carbon addition (plain base powder) were sintered in a vacuum furnace at pressure of 1.28 x 10-5MPa at 1280 °C for 45 min. After sintering, the sintered specimens were continuously cooled with different nitrogen gas pressures of 0, 2500 and 5000 mbars (or 0, 0.25, 0.5 MPa). Mechanical properties of the sintered alloys were strongly controlled by carbon contents and cooling rates after sintering. The sintered specimens, with 0.3 wt.% carbon and cooled by nitrogen of 5000 mbars, showing superior tensile strengths and good ductility, had microstructures dominated by carbide-free bainitic structures and some retained austenite. The sintered specimens with lower carbon contents and cooled under slower cooling rates, having lower tensile strengths but slightly higher ductility, had microstructures with lower bainite volume fractions and even without bainitic structures. The dominant phase in the sintered specimens with low strength but high ductility was ferrite.

A bainite transformation kinetics model and its application to X70 pipeline steel

A new continuous-cooling kinetics model for bainite transformation based on diffusionless-displacive mechanism is proposed in this work to predict bainite transformation in the coarse-grained heat-affected zone (CGHAZ) of X70 pipeline steel. The Gleeble-3500 thermal–mechanical simulator was used to analyze the bainite transformation process in CGHAZ of X70 pipeline steel. The features of bainite transformation predicted from the proposed model were compared with the values measured from experiment, and the results show that the predicted bainite volume fraction and transformation velocity are in good agreement with the measured values. This indicates that the proposed kinetics model used to predict the bainite transformation process in the CGHAZ of high-strength low-alloy steel is reasonable. The model could be widely used in prediction of bainite transformation with high cooling rate, such as in welding CGHAZs.