Bainite Dual Phase Research Articles

Effect of phase content on deformation compatibility in ferrite and bainite dual-phase steel: experimental and crystal plasticity finite element analysis

Effect of phase content on deformation compatibility in ferrite and bainite dual-phase steel: experimental and crystal plasticity finite element analysis

Modeling Bainite Dual-Phase Steels: A High-Resolution Crystal Plasticity Simulation Study

A bainite dual-phase (FB) steel containing polygonal ferrite and granular bainite is thermo-mechanically rolled, followed by an accelerated cooling. Two different cooling rates are applied to obtain two different materials. The aim of the study is to explore the reasons for the differences in the mechanical response experimentally observed for these two materials which are modeled by means of high-resolution crystal plasticity simulations with a phenomenological constitutive description. First, the CP parameters of the individual constituents are determined. Second, different three-dimensional (3D) representative volume elements (RVEs)—one of which includes the substructure of bainite—are used to study the mechanical properties of both FB microstructures. It is shown that, in contrast to the macroscopic response, the microscopic response differs among the RVEs. Third, a comparison of both materials is performed by analyzing their stress–strain response. The onset of plasticity in granular bainite is found to be different for both materials in addition to the strain partitioning, although they both obeyed the iso-work assumption. Finally, a parameter study is carried out in order to investigate the correlation between different microstructures and damage initiation that can be seen experimentally in this steel. It is shown that the difference in ultimate elongation may depend on whether the first voids appear within polygonal ferrite or at the phase boundary.

Effect of distribution characters of ferrite/bainite on deformation compatibility in dual-phase pipeline steel: Experimental and numerical study

The morphology distribution of microstructure has an important influence on the match of the strength and plasticity of dual-phase pipeline steel. Bainite and PF dual-phase steels with layered and network morphology were obtained by combining the Thermomechanical Control Process (TMCP) and inter-critical isothermal heat treatment. A novel experimental-numerical methodology studied the effect of phase distribution characters on the deformability of PF and bainite dual-phase pipeline steel. The plastic damage features and microscopic strain and stress distribution in both network and layered microstructure were clarified. The obvious differences in the microstructure evolution for both samples were captured by a quasi-situ tensile test. The PF phase in the network morphology sample was believed the main deformation phase, but the deformation of the PF phase in the layered morphology sample was restrained. Two three-dimensional (3D) crystal plasticity finite element models (CPFEMs) were established based on the electron backscatter diffraction (EBSD) data. The simulation results indicated that the strain field in layered microstructure was more uniform than that in network microstructure, and the continuous internal stress relaxation (ISR) in the bainite phase led to plastic damage at low tensile strain level; The strain field in network microstructure mainly existed in the PF phase, the deformation of the PF phase can be fully utilized. When changing the tensile x-direction of the layered microstructure to the z-direction, the strain field showed an obvious anisotropy, and loading in the z-direction increased the risk of interlaminar crack formation. • Quasi-situ tensile test was used to study the deformation behavior • Strain/stress distribution characteristics were clarified using CPFEM method • Uniform strain distribution and ISR in layered microstructure decreased its plasticity • z-axis loading increased the risk of interlaminar crack for layered microstructure

The Effect of Bainite Volume Fraction on Wear Behavior of AISI 4340 Ferrite–Bainite Dual-Phase Steel

The Effect of Bainite Volume Fraction on Wear Behavior of AISI 4340 Ferrite–Bainite Dual-Phase Steel

Machinability study and optimization of tool life and surface roughness of ferrite: Bainite dual phase steel

Aim of this present research work is to obtain the machining parameters to optimize the tool life and surface roughness for ferrite-bainite dual phase steel. Machinability tests are carried out using orthogonal array of 27, the Taguchi method, in which the machining parameters are considered as control factors. The effect of speed, feed and depth of cut on tool life and surface roughness of dual phase structure steel is analysed using ANOVA. Regression analysis is used to obtain the equations for predicting the tool life and surface roughness. Experiment is conducted using uncoated carbide insert tool by varying the process parameters. Optimum tool life and surface is analysed using Response Surface Methodology. Hardness and microstructure revealed the dual phase condition in different intercritical zones. It is found that hardness improves as the intercritical temperature is increased from 750 to 770°C. Experimental results prove that dual phase structure has better machining characteristics at an intercritical temperature of 750°C.

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND//<111> preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} <110> orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

Microstructure and Mechanical Properties of ER70-Ti Steel for Gas Shielded Welding Wire

In this paper, ER70-Ti welding wire steel produced by an enterprise was used as the test material. The final rolling temperature was set at 960 °C, 930 °C and 900 °C, and the spinning temperature was set at 880 °C, 860 °C and 840 °C. The results showed that the microhardness of the steel decreased from 303HV to 248HV and from 317HV to 276HV as the spinning temperature decreased from 880 °C to 840 °C. The microstructure and mechanical properties of the wires with the diameters of 5.5mm, 4mm, 2.5mm, 1.4 mm and 1.2mm were examined. It was observed that the microstructure of each sample had bainite and ferrite dual phase structure. With the decrease of wire diameter, the strength gradually increased and the ductility decreased. The experimental results show that the existence of bainite structure in the welding wire is the main reason for the high strength of the welding wire and easy fracture in drawing. Based on this, the final rolling temperature of 900 °C and the spinning temperature of 840 °C should be adopted in the production of ER70-Ti welding wire steel.

Study on Fracture Mechanism and Microstructural Evolution in 780 MPa Grade FB Steel with a High Hole Expanding Ratio

FB780 steel with a high hole-expanding ratio, as ferrite–bainite dual-phase steel, is an indispensable material to manufacture different automobile parts. In the present study, uniaxial tension tests and metallographic analyses were conducted to reveal the fracture mechanism in FB780 steel. TiN particles ruptured and formed micron-sized crack sources due to stress concentrations, and voids were generated by dislocation pile-ups in Fe3C particles. Intragranular dislocations mainly accumulated around second-phase particles and grain boundaries. And these dislocations started to manifest intertwining characteristics with the prolonged deformation and formed serious stress concentrations, thus finally, turned into the crack sources. Hence, the aggregation and growth of crack sources and voids resulted in ductile fracture. In addition, the fraction of low-angle grain boundaries in ferrite grains increased with the prolonged deformation. Moreover, the γ fiber texture and the {001} texture in the tensile deformation area facilitated the propagation of cracks. Small-sized newborn grains appearing in the tensile fracture mainly accumulated at low-angle grain boundaries and facilitated center separation during fracture.

Tensile deformation damage behavior of a high deformability pipeline steel with a ferrite and bainite microstructure

The work-hardening characteristics and deformation behavior of high deformability pipeline steel with a polygonal ferrite and bainite microstructure were studied in this paper. The high work-hardening ability of polygonal ferrite with 35% bainite (PF+35%B) dual phase sample contributed to its high tensile properties. Electron backscattered diffraction was conducted to analyze the mechanism of the micro-damage behavior. The results of kernel average misorientation and grain reference orientation deviation of the deformation microstructure revealed that the main deformation mechanism of the polygonal ferrite grain was slip with a number of slip systems. Dislocation rearrangement and a local orientation gradient caused by crystal rotation during sliding to cause the formation of micro-voids. The strain dispersion ability of the PF+35%B sample was better than that of the polygonal ferrite with 27% bainite (PF+27%B) dual phase sample. The risk of micro-voids nucleation and crack propagation were reduced to an improved plastic deformation ability for PF+35%B sample. The average Taylor factor (M) of the PF+35%B sample was higher than that of the PF+27%B sample, which indicated the grains of the PF+27%B sample was more liable to slip during deformation and yield under low stress. The evolution characterization of the micro-texture showed that the γ-fiber was more stable compared with the α-fiber, and the stronger γ-fiber may contribute to the improved plasticity.

Comparison of the fatigue performance of ferrite–pearlite and ferrite–bainite dual-phase steels

Recent demands for large vessels and deep water exploration have increased the requirements for thicker plates to ensure the sufficient strength of such structures. The use of thicker plates, however, has an adverse influence on the fatigue performance. To enhance the fatigue life, post-weld treatments, such as weld-toe grinding, TIG (tungsten inert gas) dressing, hammer peening, and high-frequency mechanical impact (HFMI) treatments are commonly employed. On the other hand, these techniques require additional fabrication time and production cost. Therefore, there is keen interest with respect to thick steel plates with improved fatigue strength. This study examined the fatigue characteristics of conventional steel (Ferrite–Pearlite, F–P steel) and Ferrite–Bainite dual-phase steel (F–B steel). The fatigue behavior of both steels was investigated by a series of fatigue tests with three different types of welded joints. In case of gusset and cruciform-welded joints, fatigue strength of F–B steel is higher than that of F–P steel. F–B steel is known to exhibit improved fatigue performance associated with the existence of bainite microstructure. In addition, the fatigue data in this study are validated against the existing literatures. Based on the fatigue test results, a modified fatigue design curve is suggested from this study. In order to discuss the effects of fatigue crack path, the fatigue crack growth rate (FCGR) of both steels was investigated in terms of Paris’ law. Finally, microstructural analysis was conducted to discuss the fatigue performance.

Ferrite-Bainite dual phase 강의 피로균열진전 특성 평가

With the recent increase in size of ships and offshore structures, there are more demand for thicker plates. As the thickness increases, it is known that fatigue life of the structures decrease. To improve the fatigue life, post weld treatments techniques, such as toe grinding, TIG dressing and hammer peening, are typically employed. However, these techniques require additional construction time and production cost. Therefore, it is of crucial interest steels with longer fatigue crack growth life compared to conventional steels. This study investigates fatigue crack growth rate (FCGR) behaviours of conventional EH36 steel and Ferrite- Bainite dual phase EH36 steel (F-B steel). F-B steel is known to have improved fatigue performance associated with the existence of two different phases. Ferrite-Bainite dual phase microstructures are obtained by special thermo mechanical control process (TMCP). FCGR behaviours are investigated by a series of constant stress-controlled FCGR tests. Considering all test conditions (ambient, low temperature, high stress ratio), it is shown that FCGR of F-B steel is slower than that of conventional EH36 steel. From the tensile tests and impact tests, F-B steel exhibits higher values of strength and impact energy leading to slower FCGR.

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.

Corrosion Resistance of the Welded Joint of Submarine Pipeline Steel with Ferrite Plus Bainite Dual-Phase Microstructure

In this work, the corrosion resistance of the welded joint and base metal of submarine X65 pipeline steel with ferrite plus bainite dual-phase microstructure was investigated by electrochemical and immersion tests. The corrosion product films were characterized using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and optical microscopy. To research the development of pitting corrosion in each zone, pitting diameter statistics were finished under optical microscopy and the forming mechanism of pitting induced by inclusion was analyzed. The experimental results proved that the corrosion resistance of the welded joint was better than that of the base metal, both from the general corrosion and pitting corrosion standpoint. In the welded joint, the weld metal has the best corrosion resistance, and then was the heat affected zone (HAZ). However, there was little difference between parent metal (near HAZ) and base metal (far HAZ) on the corrosion resistance. XRD and EDS analysis showed that the composition of the outer rust layer was mainly composed of Fe2O3, Fe3O4, FeOOH, and Fe(OH)3, and the inner layer was consisted of Fe2O3, Fe3O4, and a little FeOOH. Combined with the microstructure of the each zones, it can be concluded that the best corrosion resistance of the weld metal attributes to the refined grain size. Due to a creation of a galvanic effect between the ferrite and bainite, the dual-phase microstructure of the base metal and parent metal displayed weak corrosion resistance.

Development of Ferrite–Bainite Dual Phase (FBDP) High Strength Steel for Automotive Industries

The proposed Ferrite-Bainite Dual Phase (FBDP) steel is suitable for automotive industries. The steel satisfies high specific strength (strength/weight ratio), which is positively reflected on both fuel consumption and crashing resistance.

Fatigue crack growth behaviour of ferrite–bainite dual phase steels

The effect of bainite content on the fatigue threshold characteristics of ferrite–bainite dual phase steels is reported. Five different ferrite–bainite dual phase microstructures are prepared through suitable heat treatment and are subjected to hardness, tensile and fatigue tests apart from microstructural characterisations. Fatigue threshold ΔKth values of the steels are estimated using an earlier reported non-conventional method and the data are analysed to study the role of bainite content on ΔKth. The ΔKth values first decrease and then increase with increasing bainite content in the investigated steels. The nature of variation of ΔKth with bainite content has been explained using the characteristics of the constituent phases.

Short Crack Growth Behavior in Ferrite-Bainite Dual Phase Steels

The effect of bainite content on the short crack growth behavior of three ferrite bainite dual phase steels has been examined using a rotating bending machine together with examinations of the microstructural constituents associated with the fatigue crack path. The magnitude of ΔKthsc for ferrite bainite dual phase steels, in general, is found to increase with increasing amount of bainite. Critical examinations of the crack paths indicate that it is inter-granular for steels with &lt; 70% bainite while it is intra-granular for steels containing &gt; 70% bainite.

Ferrite Evolution During Isothermal Process in a High Deformability Pipeline Steel

In order to ensure the safety of long-distance oil and natural gas transmission pipeline installed in seismic and/or permafrost region, high strength pipeline steel with excellent deformability has been developed. The ferrite and bainite dual phase pipeline steel is a very important kind of high deformability pipeline steel. Polygonal ferrite is a key microstructure in ferrite and bainite dual phase deformability pipeline steel. Ferrite evolution during isothermal process at 700 °C after 50% deformation at 800 °C was conducted by using a Gleeble-3800 thermal simulator, and microstructure was characterized by using an optical microscope, a scanning electron microscope and a transmission electron microscope. There are two types of ferrite, ferrite with high density dislocation and ferrite with a little dislocation. There is about 7% (volume percent) deformation induced ferrite (DIF) for compression of 50% at 800 °C and strain rate of 1 s−1. During the isothermal process at 700 °C, with the holding time increasing, ferrite volume percent, ferrite grain number and average ferrite grain size increase. As the holding time is prolonged, dislocation recovery occurs in DIF. There are secondary phases in ferrite when the holding time is too long, and secondary phases and dislocation formation in dislocation pinning.

Corrosion behavior of each phase in low carbon microalloyed ferrite–bainite dual-phase steel: Experiments and modeling

In situ observation of the initial corrosion behavior of a low carbon microalloyed ferrite–bainite dual-phase steel showed that the corrosion originated from the inside of ferrite and ferrite boundary. In addition, a model for describing the corrosion behavior of each phase in multiphase steel was established. Based on this model, a method to quantitatively assess the corrosion rate of each phase was presented by white light interference, and the relationship between the surface roughness and corrosion morphology was also established. Meanwhile, the galvanic corrosion at phase-scale and the influence of phase distribution on service safety of multiphase steel were discussed.

Fatigue crack growth behaviors in hot-rolled low carbon steels: A comparison between ferrite–pearlite and ferrite–bainite microstructures

The roles of microstructure types in fatigue crack growth behaviors in ferrite–pearlite steel and ferrite–bainite steel were investigated. The ferrite–bainite dual-phase microstructure was obtained by intermediate heat treatment, conducted on ferrite–pearlite hot-rolled low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), fatigue crack growth (FCG) rate tests, and fatigue fractography analysis. Microscopy images arrested by in-situ SEM showed that the fatigue crack propagation in F–P steel could become unstable more ealier compared with that in F–B steel. The fatigue cracks in ferrite–pearlite were more tortuous and could propagate more freely than that in ferrite–bainite microstructures. However, frequent crack branching were observed in ferrite–bainite steel and it indicated that the second hard bainite phase effectively retarded the crack propagation. The variation of FCG rate (da/dN) with stress intensity factor range (ΔK) for F–P and F–B steels was discussed within the Paris region. It was shown that FCG rate of F–P steel was higher than that of F–B steel. Moreover, the fatigue fracture surface analysis proved that grain boundaries could also play a role in the resistance of crack propagation.

Influence of temperature field on the microstructure of low carbon microalloyed ferrite–bainite dual-phase steel during heat treatment

Abstract To investigate the quantitative relation between the temperature and the microstructure of low carbon steel, the low carbon microalloyed ferrite–bainite dual-phase (FBDP) steel was prepared by heat treatment. On a series of positions chosen in the sample which was cut from the treated FBDP steel, the volume fractions of bainite in the microstructures were measured and the corresponding temperatures were also obtained through the finite element method. The results show that not only the volume fraction of bainite increases linearly, but also the size of bainite increases with the increase of the outset water-cooling temperature. In addition, by comprehensively evaluating the microstructure and the temperature, it can be estimated that the lowest bainite formation temperature for this steel is about 550 °C, which had been proved by the experiment.