Ferrite-martensite Dual-phase Microstructures Research Articles

Effect of Carbon Partitioning on Abnormal Martensite Hardening in a Conventional Quench and Temper Medium Silicon Low Alloy Steel under Ferrite-Martensite Dual-Phase Microstructure

Abstract The purpose of this research work was to investigate the effect of carbon partitioning within ferrite and prior austenite (martensite) during progress of ferrite formation and consequently its relation to the associated martensite hardening in a medium silicon low alloy conventional quench and temper steel. For this aim, several ferrite-martensite dual-phase (DP) samples containing various volume fractions of ferrite and martensite microphases were developed. The X-ray diffraction and electron microscopy with spot and line-scan X-ray energy-dispersive spectroscopy (EDS) for carbon analysis were used in conjunction with light microscopy and hardness test to follow the variation of carbon partitioning within ferrite and prior austenite (martensite) regions and consequently the associated martensite hardening in the DP samples.

Improvement in Mechanical Properties of a Surface-Carburized Ferrite–Martensite Dual-Phase Steel by Intercritical Annealing

This study investigated the effects of ferrite volume fraction and morphology on the mechanical properties and rolling contact fatigue life of surface-carburized G20CrNi2Mo-Al steel with ferrite–martensite dual-phase microstructures. Compared with full austenitization treatment, intercritical annealing treatment optimized the properties, and the microstructural evolution of different heat treatment processes was explored. The experimental results indicated that after austenitizing at 900 °C for 30 min and intercritical annealing at 805 °C for 40 min, the steel had 6.9 vol.% continuous grain boundary ferrite in the center, and the mechanical properties reached their maxima. Under the optimal heat treatment conditions, the hardness value, impact toughness and tensile strength increased by 4.0, 11.1 and 2.4%, respectively; the rolling contact fatigue life increased by 76.1% compared with that after conventional full austenitization treatment.

Effect of Dual-Phase Heat Treatment on Microstructure and Mechanical Properties of S135 High-Strength Drill Pipe Steel

Effect of dual-phase heat treatment (DPHT) on microstructure and mechanical properties of S135 high-strength drill pipe steel was studied by means of optical microscope, scanning electron microscope and mechanical property testing. The results show that the ferrite–martensite dual-phase microstructures were obtained at 760-800 °C. With increase in DPHT temperature, the volume fraction of martensite increases and the ferrite decreases. When the DPHT temperature is low, martensite is distributed as an island on the ferrite matrix, and ferrite is distributed as an island on the martensite matrix when the DPHT temperature is high. The hardness and strength of the steel increase, the plasticity and toughness decrease, the steel changes from ductile fracture to brittle fracture and the micro-fracture morphology changes from dimple to cleavage with increase in DPHT temperature. The strain hardening exponent of the as-received steel shows one ‘n’ value, but exhibits two ‘n’ values after DPHT. When the DPHT temperature is 750 °C, the relative content of martensite in the duplex structure is 15-20%, and the impact energy of the steel is similar to that of the as-received S135 drill pipe steel, and the strength is increased by about 15%. Micropores and micro-cracks on the subsurface of tensile fracture surface are mainly formed at the martensite/ferrite phase boundary or in the martensite slit.

Microstructure-based behavior law for globular pearlitic steels

Pearlitic steels are widely used in the industry for their good balance between strength, ductility and machinability. Sometimes, the classical lamellar structure can degenerate in a so-called globular structure which may significantly affect the behavior of the steel. To our knowledge, no size sensitive model is available in the literature to predict their mechanical behaviors and work-hardenings. Inspired by their analogy with ferrite–martensite dual-phase microstructures, we propose a new microstructure-based law for globular pearlite. It describes the evolution of the dislocation density into ferritic grains which is enhanced by the presence of hard particles. The model is as a consequence sensitive to ferritic grain but also to coarse carbide sizes after globularization. The model has been calibrated on a conventional steel grade and appears to be a practical tool for microstructure design.

An automated morphological classification of ferrite-martensite dual-phase microstructures

In this study, a simple classification scheme based on image processing is proposed to categorize optical microstructures of dual-phase steels with varying morphology (i.e., shape, size and distribution) of ferrite-martensite phases. Dual-phase microstructures with varying morphologies have been developed by employing different heat treatment schedules; namely, intercritical annealing (IA), intermediate quenching (IQ) and step quenching (SQ) using a low carbon micro-alloyed steel. The images of optical microstructures of the developed steels have been first segmented using Otsu thresholding technique and then, the ratio of the black (martensite) and white (ferrite) run-lengths for the phases have been calculated. The mean and median values of these ratios have been considered further, and it has been demonstrated that the mean values are more appropriate than the median values for the classification purpose. The mean and standard deviation of the former parameter for IA, IQ and SQ microstructures are 0.679 ± 0.040, 1.540 ± 0.090 and 0.989 ± 0.119, respectively. The simple classification technique is found to be advantageous from different aspects.

Effect of Martensite Morphology on Tribological Behaviour of a Low-Alloy Steel

Few works are devoted to study the microstructure effect on tribological behaviors of contacting materials. The most of them are only focused on wear and without fixing the hardness and/or the chemical composition. A contribution is proposed by investigating the combined effects of microstructure and abrasive particle size. Friction tests are performed for steel pins characterized by various microstructures with similar hardness level (around 410 \( {H_{\text{V}}} \)) and chemical composition. The microstructures are composed of a quenched martensitic microstructure, a tempered martensitic microstructure, and ferrite–martensite dual-phase microstructures with various martensite colony morphologies. These pins slide against abrasive papers with various particle sizes, from 15 to 200 μm, under different normal loads from 50 to 110 N. Dual-phase microstructures enhance friction and wear behaviors. Among these microstructures, compared to fine and fibrous martensite colony morphologies, coarse and equiaxed martensite colonies minimize friction coefficient and wear rate. It is worth noting that for a given load, a transition in friction behavior is observed for a critical particle size (CPS) which depends on microstructure and normal load. This study also showed that whatever the microstructure and the abrasive particle sizes, friction coefficients decrease with increasing normal load. However, for wear rate, a reverse trend is observed.

Experimental study and modelling of the effect of microstructure on friction and wear mechanisms of low alloy steel

Few models are focused on the combined effects of microstructure and roughness on the tribological behavior of materials. Hardness is the material property mainly used in the tribological models which are usually at a macroscopic scale. For a dual-phase steel, experimental and predicted values of friction coefficients and specific wear resistances are compared. The investigated models are declined into two pressure distribution modes between the phases. Friction tests are performed between steel pins composed of a ferrite-martensite dual-phase microstructure against abrasive papers with various abrasive particle sizes ranging from 15 µm to 200 µm. By using heat treatments on a low alloy steel, dual-phase microstructures with various martensite volume fractions, ranging from 45% to 100%, are generated.As martensite volume fraction increases, the experimental and predicted results show that the specific wear resistance increases whereas the friction coefficient decreases. Furthermore, the latter evolutions depend on roughness. For a predominance of abrasive wear mechanisms generated by coarse abrasive particles (~200 µm), the experimental tribological parameters tend to follow the predicted ones associated to the mode characterized by equal pressures between the phases. Then, as the abrasive particle size decreases, abrasive wear mechanisms reduce whereas adhesive wear mechanisms increase and the experimental tribological parameters tend to follow the predicted ones associated to the mode characterized by equal wear rate between the phases.

Microstructure–mechanical property relationship in hot-rolled high-Al-low-Si dual-phase steel

ABSTRACTIn this study, the effect of finish rolling temperature and coiling temperature on the microstructure and mechanical properties of high-Al-low-Si dual-phase (DP) steels is explored. Two different finish rolling temperatures (850 and 790°C) and three different coiling temperatures (200, 250 and 300°C) were studied. The results indicated that all the different processing conditions led to ferrite-martensite DP microstructure. With the decrease in finish rolling temperature, the volume fraction of ferrite was increased and martensite content was decreased. When the coiling temperature was increased to 300°C, autotempered martensite was obtained, which led to the softening of martensite and decrease in tensile strength and strain hardening ability, but higher post-necking elongation. Moreover, the nanoscale Nb-based carbides played a crucial role in refining the microstructure of hot-rolled high-Al-low-Si DP steel. EBSD (Electron Backscattered Diffraction) analysis revealed that the ferrite grains were fine, and decrease in finish rolling temperature and coiling temperature led to an increase in low-angle boundaries. When the finish rolling temperature was decreased to 790°C and coiling temperature was decreased to 200°C, the steel had excellent mechanical properties with tensile strength of 885 MPa, uniform and total elongation of 16.0 and 25.94%, respectively, and the product of tensile strength and total elongation was 20 264 MPa%. The improvement of strength and plasticity can be attributed to the fraction of ferrite and martensite, precipitation of NbC, fine microstructure.

Effect of abrasive particle size on friction and wear behaviour of various microstructures of 25CD4 steel

Many parameters, such as normal load and material bulk hardness, control the wear and friction behaviours of materials. Nonetheless, the investigation of the coupled contributions of microstructure and abrasive particle size are still lacking. A contribution is proposed by using steel pins with various microstructures with a similar macro-hardness (around 410HV) and chemical composition. A quenched martensitic microstructure, a tempered martensitic microstructure and three ferrite-martensite dual-phase microstructures, with a similar martensite volume fraction (around 67%) and different martensite colony morphologies, are established. Friction tests are performed between these pins and abrasive papers with different sizes ranging from 15μm to 200μm. Compared to single-phase microstructures (quenched and tempered martensitic microstructures) and whatever the abrasive particle size, ferrite-martensite dual-phase microstructures reduce the friction coefficient and provide better wear resistance. For the ferrite-martensite dual-phase microstructures and unlike fine and fibrous martensite colonies, coarse and granular martensite colonies minimize the friction coefficient. In addition, characterized by a change of wear mechanisms between abrasion and adhesion, an intermediate abrasive particle around 35 μm minimizes the friction coefficient. This study also reveals that the wear rate increases with the abrasive particle size which is associated to an increase of the attack angle of abrasive grains.

Effect of Coiling Conditions on Microstructure and Mechanical Property Characteristic of a C–Mn–Si–Cr Steel

Coiling condition in thermal mechanical control process can affect the final microstructure and mechanical property significantly. To achieve the ferrite–martensite dual-phase steel with less alloy content and lower cost, a C–Si–Mn–Cr steel was designed and air-cooled to 750 °C after hot rolling and then coiled on four conditions: (i) water cooling to 300 °C and coiling, (ii) quench to room temperature, (iii) water cooling to 90 °C and coiling at 250 °C, and (iv) water cooling to 90 °C and coiling at 170 °C. After uniaxial tensile test, specimens 2# and 4# present better formability than the other two specimens. By using optical microscopy, SEM and TEM, 2#, 3#, and 4# acquired ferrite–martensite dual-phase microstructure. 1# is composed of ordinary ferrite–pearlite microstructure.

Effect of inter-critical annealing parameters on ferrite recrystallization and austenite formation in DP 590 steel

ABSTRACTThis research investigates the effect of inter-critical annealing parameters on ferrite recrystallization and austenite formation during processing of a dual phase microstructure from a cold rolled low carbon steel. The main effort was to determine optimum annealing parameters for producing a desired ferrite-martensite dual phase microstructure in the steel for improved strength–ductility combination. A 57% cold rolled steel sheet was subjected to inter-critical annealing under different temperature–time conditions. Annealing temperatures were determined using Thermo-Calc. After annealing experiments, the resulting microstructures and corresponding hardness values were evaluated to determine ferrite recrystallization and austenite fraction under different conditions. The activation energy for ferrite recrystallization was 235.6 kJ/mol using standard Johnson–Mehl–Avrami–Kolmogorov analysis. Experiments showed that inter-critical annealing parameters affect the phenomenon of ferrite recrystallization and austenite formation. It was observed that both the rate of ferrite recrystallization and austenite formation increase with an increase in annealing temperature. Finally, steel was annealed under conditions similar to industrial processing in an annealing simulator with the selected annealing parameters to obtain improved strength–percentage elongation combinations. The steel under these conditions showed significant improvements in strength–ductility combination (610 MPa–26%; 680 MPa–15%) with an ideal yield strength to an ultimate tensile strength ratio of 0.5.

Effect of Carbon Distribution During the Microstructure Evolution of Dual-Phase Steels Studied Using Cellular Automata, Genetic Algorithms, and Experimental Strategies

The development of ferrite-martensite dual-phase microstructures by cold-rolling and intercritical annealing of 0.06 wt pct carbon steel was systematically studied using a dilatometer for two different heating rates (1 and 10 K/s). A step quenching treatment has been designed to develop dual-phase structures having a similar martensite fraction for two different heating rates. An increase in heating rate seemed to refine the ferrite grain size, but it increased the size and spacing of the martensitic regions. As a result, the strength of the steel increased with heating rate; however, the formability was affected. It has been concluded that the distribution of C during the annealing treatment of cold-rolled steel determines the size, distribution, and morphology of martensite, which ultimately influences the mechanical properties. Experimental detection of carbon distribution in austenite is difficult during annealing of the cold-rolled steel as the phase transformation occurs at a high temperature and C is an interstitial solute, which diffuses fast at that temperature. Therefore, a cellular automata (CA)-based phase transformation model is proposed in the present study for the prediction of C distribution in austenite during annealing of steel as the function of C content and heating rate. The CA model predicts that the carbon distribution in austenite becomes more inhomogeneous when the heating rate increases. In the CA model, the extent of carbon inhomogeneity is measured using a kernel averaging method for different orders of neighbors, which accounts for the different physical space during calculation. The obtained results reveal that the 10th order (covering 10-µm physical spaces around the cell of interest) is showing the maximum inhomogeneity of carbon and the same effect has been investigated and confirmed using auger electron spectroscopy (AES) for 0.06 wt pct carbon steel. Furthermore, the optimization of carbon homogeneity with respect to heating rate has been performed using a bi-objective genetic programming (BioGP) strategy for the steel composition varying from 0.06 to 0.12 wt pct carbon along with other parameters like average austenite grain size and time of heating as the input variables. The analysis of the results obtained from BioGP suggests that the homogeneity of carbon increases with the increasing carbon concentration of steel. This is corroborated by analyzing the AES results obtained for 0.28 wt pct carbon steel using the same technique as that used for 0.06 wt pct carbon steel.

Ultrafine ferrite formation through cold-rolling and annealing of low-carbon dual-phase steel

Formation process of ultrafine grained ferrite through a simple thermomechanical route composed of cold-rolling and annealing of dual-phase starting microstructures was investigated. A 0·1%C steel having a ferrite–martensite dual-phase microstructure was cold-rolled by 91% and subsequently annealed below the eutectoid (A1) temperature. During the annealing, the cold-rolled microstructure gradually changed to be equiaxed ultrafine ferrite, without preferential growth of particular ferrite grains. Hardness of the cold-rolled specimen continuously decreased without a significant drop. The main components of texture in the cold-rolled specimen, α-fibre and γ-fibre, did not change greatly after the formation of ultrafine grains. It was suggested that finely subdivided region having large misorientations in the cold-rolled state grew with recovery to form the ultrafine ferrite.

Effects of microstructure on fatigue crack growth behavior in cold-rolled dual phase steels

Fatigue crack growth behaviors of cold-rolled dual phase steels with different microstructures were investigated at room temperature. The ferrite–martensite dual-phase microstructure was obtained by intercritical annealing. Fatigue crack growth (FCG) behaviors were described by both the Paris model and a new exponential model; fatigue fractography and surface morphology near the fracture were arrested by scanning electron microscopy (SEM); the relationship between macroscopic and microcosmic FCG rate was analyzed quantificationally. The results showed that both the models can be used to describe the fatigue crack growth rate of the samples rather well; fatigue striations and secondary cracks were observed in the fracture surface at stable expanding region (II), while the fracture at rapid expanding region (III) combined dimple and quasi-cleavage morphology; the roughness of fracture surface and the degree of secondary cracking increased with an increase in martensite content, leading to a higher threshold value. Moreover, the changes of microcosmic FCG rate were smoother than that of the macroscopic FCG rate.

Tensile Behavior of Ferrite-Carbide and Ferrite-Martensite Steels with Different Ferrite Grain Structures

Ferrite-carbide and ferrite-martensite dual-phase microstructures have been produced in a low-carbon steel with different ferrite grain structures such as, uniform distribution of coarse- and very fine-ferrite grains, and bimodal distribution of ferrite grain sizes comprising of coarse grains (~12 μm) and very fine grains (<2 μm). Very fine-grained dual-phase structure offered the best combination of tensile-strength and ductility among all the samples. The above microstructures have been compared in terms of their strain-hardening rate and the mechanism of plastic deformation.

Phase Analysis on Dual-Phase Steel Using Band Slope of Electron Backscatter Diffraction Pattern

A quantitative and automated phase analysis of dual-phase (DP) steel using electron backscatter diffraction (EBSD) was attempted. A ferrite-martensite DP microstructure was produced by intercritical annealing and quenching. An EBSD map of the microstructure was obtained and post-processed for phase discrimination. Band slope (BS), which was a measure of pattern quality, exhibited much stronger phase contrast than another conventional one, band contrast. Owing to high sensitivity to lattice defect and little orientation dependence, BS provided handiness in finding a threshold for phase discrimination. Its grain average gave a superior result on the discrimination and volume fraction measurement of the constituent phases in the DP steel.

Reliability/unreliability of mixture rule in a low alloy ferrite–martensite dual phase steel

Abstract The aim of this paper is to investigate in details the relationship between the volume fractions of ferrite and martensite with the variation of hardness in a low alloy ferrite–martensite dual phase (DP) steel. For this purpose, a wide variety of ferrite–martensite DP samples consisting different volume fractions of ferrite and martensite have been developed using step quenching heat treatment cycle involving reheating at 860 °C for 60 min, soaking at 600 °C salt bath for various holding times followed by 70 °C hot oil quenching. Optical microscopy has been supplemented by electron microscopy and hardness measurements to follow microstructural changes and their relation to the variation in hardness. The results showed that there is a non-linear relationship between the hardness of DP samples with the volume fraction of phase constituents indicating that the mixture rule is not reliable in the ferrite–martensite DP microstructures. The unreliability of mixture rule is related to the variation of ferrite and martensite hardening responses developed in the DP samples. The DP microstructure consisting 6–7% volume fraction of continuous grain boundary ferrite in the vicinity of martensite has been associated with a remarkable higher hardness for both ferrite and martensite in comparison with the other DP microstructures. The higher martensite hardness is due to the higher carbon content of the remaining metastable austenite developed in the ferrite–austenite two phase field area, leading to the harder martensite formation on the subsequent 70 °C hot oil quenching. The harder ferrite grains have been developed as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation. The higher martensite volume fraction in the vicinity of thinner continuous grain boundary ferrite networks has been associated with the harder ferrite formation.

Effect of initial microstructures on the properties of Ferrite-Martensite Dual-Phase pipeline steels with Strain-Based design

This study aims to investigate the effect of initial microstructures on the properties of ferrite-martensite dual-phase pipeline steels with strain-based design. For this purpose, the as-received acicular ferrite steels were first austenitized at 920 ºC for 15 minutes followed by air cooling and water quenching to produce ferrite-pearlite and ferrite-martensite microstructure, respectively. Subsequently, the steels with ferrite-pearlite, ferrite-martensite and as-received acicular ferrite microstructure were intercritically annealed at 820 ºC for 10 minutes followed by water quenching to produce three different ferrite-martensite dual-phase microstructures. Tensile tests, Vickers hardness and Charpy impact tests were carried out to investigate the mechanical properties. Scanning electron microscope was used to analyze the microstructures and tensile fractographs. The results showed that all the tensile specimens of these three different ferrite-martensite dual-phase steels fractured in ductile mode, however, their microstructures and mechanical properties varied significantly. By contrast, the ferrite-martensite dual-phase steel derived from acicular ferrite initial microstructure had optimal combination of the strength, toughness and deformability, which provided a good candidate for the pipeline steels with strain-based design used in severe geological environments.

Microstructure and mechanical properties of laser welded dissimilar DP600/DP980 dual-phase steel joints

The use of dual phase (DP) steels in the automobile industry unavoidably involves welding and dynamic loading. The aim of this investigation was to evaluate the microstructural change and mechanical properties of laser welded dissimilar DP600/DP980 steel joints. The dissimilar joints showed a significant microstructural change from nearly full martensite in the fusion zone (FZ) to the unchanged ferrite–martensite dual-phase microstructure in the base metal. The welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat-affected zone (HAZ). The dissimilar welded joints were observed to exhibit a distinctive unsymmetrical hardness profile, yield-point-like phenomenon, and single-stage work hardening characteristic, with yield strength and work hardening rate lying in-between those of DP600 and DP980 base metals, and ultimate tensile strength equivalent to that of DP600 base metal. Although the welded joints showed a lower fatigue limit than the base metals, the fatigue life of the welded joints at higher stress amplitudes was almost the same as that of the DP600 base metal. The welded joints failed in the soft zone at the DP600 side under tensile loading and fatigue loading at the higher stress amplitudes. Fatigue crack initiation occurred from the specimen surface and crack propagation was characterized by typical fatigue striation together with secondary cracks.

Some aspects of fatigue crack growth retardation behaviour following tensile overloads in a strucutural steel : Shuter, D.M. and Geary, W. Fatigue Fract. Eng. Mater. Struct. (1996) 19 (2–3), 185–199

Fatigue crack growth behaviors of cold-rolled dual phase steels with different microstructures were investigated at room temperature. The ferrite–martensite dual-phase microstructure was obtained by intercritical annealing. Fatigue crack growth (FCG) behaviors were described by both the Paris model and a new exponential model; fatigue fractography and surface morphology near the fracture were arrested by scanning electron microscopy (SEM); the relationship between macroscopic and microcosmic FCG rate was analyzed quantificationally. The results showed that both the models can be used to describe the fatigue crack growth rate of the samples rather well; fatigue striations and secondary cracks were observed in the fracture surface at stable expanding region (II), while the fracture at rapid expanding region (III) combined dimple and quasi-cleavage morphology; the roughness of fracture surface and the degree of secondary cracking increased with an increase in martensite content, leading to a higher threshold value. Moreover, the changes of microcosmic FCG rate were smoother than that of the macroscopic FCG rate.