Ferrite-martensite Microstructure Research Articles

Assessment of structure-property relationships at the micrometer length scale in dual phase steels by electron microscopy and nanoindentation

This work presents a correlative investigation using nanoindentation and electron microscopy to characterize Advanced High Strength Steels (AHSS) that consist of a ferrite matrix along with a strengthening secondary phase/constituent such as pearlite, bainite and martensite. DP600 grade steel was hot rolled above the austenite non-recrystallization temperature (TnR) followed by coiling at different transformation temperatures to obtain ferrite-pearlite, ferrite-bainite and ferrite-martensite microstructures with identical nominal carbon content. Unlike the macroscale mechanical testing on dual phase steels which provides a composite response, in this work the variation in local mechanical properties as a function of processing was captured by high speed nanoindentation mapping at the length scale of the microstructure. Excellent agreement between the microstructure and hardness was observed for different dual phase microstructures (ferrite-pearlite (FP), ferrite-bainite (FB) and ferrite-martensite (FM)). High speed nanoindentation mapping data was deconvoluted using a clustering algorithm to obtain the hardness of the constituents. The hardness of ferrite in FP, FB and FM samples was found to be 3.26 GPa, 3.33 GPa and 4 GPa, respectively and the corresponding second phase / constituent was found to be 4.6 GPa, 4.43 GPa and 4.93 GPa, respectively. In spite of having identical nominal composition, the three different dual phase microstructures show differences in area fraction and hardness of ferrite and also the secondary constituents. These experimental observations are reconciled based on processing conditions and the length scale of the microstructure.

Interphase precipitation of Cu in dual-phase steel

Two experimental 0.2C steels with 1 wt% and without Cu were studied. A dual ferritic-martensitic microstructure was prepared by controlled rolling and subsequent quenching. The heterogenous microstructure was obtained due to the severe cooling of the surface layer during the rolling process. The surface layer of the sheet had a significantly finer ferritic grain than the sheet’s core. Mechanical properties of the core were not correlated to the Cu content, whereas the surface layer revealed Cu-related strengthening due to Cu precipitation. Rows of particles typical for interphase precipitation were observed in some of the ferrite grains of the Cu-containing sample. Mode of the ferrite growth from austenite has apparently a crucial role in Cu ability to rapidly precipitate in ferrite grain during or shortly after its formation.

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Magnetic After‐Effect Study of Carbon Distribution and Grain Boundary Diffusion in FeCrC Alloys and Steels

Herein, carbon distribution and grain‐boundary diffusion processes are studied in a variety of FeCr alloys and steels with different chromium content and microstructure, based on magnetic after‐effect measurements performed in a broad temperature range, from 100 to 1000 K. The existence of three metastable carbon positions in the lattice is revealed in the FeC alloys. Carbon as interstitial in the lattice, segregated at the dislocations and grain boundaries, is represented by the relaxation peaks at about 269, 432, and 607 K, respectively. The relaxation process due to the grain‐boundary self‐diffusion is clearly distinguished from those mentioned earlier, and is observed to be at about 643 and 681 K in low and high carbon Fe, respectively. Addition of Cr has a twofold effect: a) for Cr concentrations higher than about 3 wt carbon‐related relaxation processes completely disappear from the spectra, most probably due to formation of carbides, b) the solute grain‐boundary diffusion relaxation peak appears in the spectra at about 800 K, with an activation energy that is directly dependent on the Cr content. Activation energy of the solute grain‐boundary diffusion is found to be generally smaller in ferrite–martensite microstructure in comparison with fully ferrite alloys.

REACHING THE HIGH LEVEL OF MECHANICAL PROPERTIES IN THIN-WALLED TUBULAR PRODUCTS AND SHAPES OF HIGH STRENGTH STEELS

To produce thin-walled tubular products and profiles with high level of mechanical properties, it is advisable to use two-phase (DP) steel, which has a ferritic-martensitic microstructure. The main technological element in the production of such products, which provides a combination of high strength and ductility is a special heat treatment in the temperature range from A1 to A3. This heat treatment provides a microstructure consisting of a soft ferrite matrix containing martensite inclusions located at the grain boundaries. Tubular products and profiles made of high-strength steels are obtained in continuous units, in which the areas of heating, deformation and cooling are separated. In this case a water-air sprayer must be integrated into the mill line for controlled cooling of the outgoing product. In the case where the deformation process takes place at temperatures below A1, it is rational to carry out the heat treatment directly in the line with heating by means of a through-inductor followed by cooling.

The effect of cold-rolling prior to the inter-critical heat treatment on microstructure and mechanical properties of 4340 steel with ferrite – Martensite microstructure

This study investigates the effect of cold rolling prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850 °C for 1 h followed by oil quenching, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness. The inter-critical annealing treatment (750 °C, 120min) was performed to generate a ferrite-martensite microstructure. Microstructural studies showed that increasing the applied cold rolling, leading to increase in volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were measured by tensile, impact and hardness tests. Results showed that ultimate tensile strength, yield strength, micro-hardness and toughness (e.g. total elongation, uniform elongation and impact energy) increased with increasing the applied cold rolling. Improvement of mechanical properties were related to increase in martensite volume fraction and ferrite grain refinement. Analysis of strain hardening behavior of DP steels, by Hollomon analysis, showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (nI) increased with increasing volume fraction of martensite. The energy absorption capacity (UE × UTS) increased with increasing cold rolling deformation.

Kinetics of formation of pack aluminized coating on 9Cr[sbnd]1Mo steel and interdiffusional behaviour of iron aluminides at intermediate temperatures

Development of a suitable iron aluminide coating layer on 9Cr1Mo (P91 grade) steel at or below 750 °C finds importance for improving its oxidation and corrosion behaviour while retaining the tempered ferritic-martensitic microstructure. A theoretical model to predict the kinetics has been developed for correlating growth of the aluminide coating on P91 steel substrate with process parameters of pack aluminizing. Experiments were conducted by varying the pack Al (2–10 wt%) and activator NH4Cl (2–8 wt%) content, temperature (600–700 °C), and time (2-16 h). The coating formation results obtained from the experiments authenticated the validity of the proposed model. The coating mainly consisted of single Fe2Al5 phase. The activation energy for Fe2Al5 formation during pack aluminizing was calculated to be 95 ± 7 kJ mol−1. Interdiffusion studies between Fe2Al5/steel in 550–750 °C with varying times of 8–40 h revealed the formations of FeAl2, FeAl, and Fe(Al) layers of different thicknesses. The diffusional growth of the most desired FeAl aluminide found to follow the parabolic kinetics and the activation energy for the growth of FeAl was calculated to be 237.5 kJ mol−1. Scanning electron microscopic and electron back scattered diffraction analysis revealed the formation of columnar grains in B2-FeAl layer.

Improvement of the strength-ductility balance in ultrafast heated steels by combining high-temperature annealing and quenching and partitioning process

The microstructure and mechanical properties of an Fe-0.24C-1.4Mn-1.4Si steel were investigated after combining ultrafast heating (UFH) at a heating rate of 500 °C/s followed by fast cooling to room temperature (DQ) or quenching and partitioning processes (Q&P). Two peak temperatures were studied, annealing into the intercritical range and above the AC3 temperature. After ultrafast heating and quenching, the resulting microstructures revealed that intercritical annealing led to the formation of a banded ferritic-martensitic microstructure. On the other hand, heating above the intercritical range led to an even distribution of allotriomorphic ferrite grains upon fast cooling and a complex phase microstructure, consisting mainly of martensite, was produced. Q&P steel grades exhibit an enhanced mechanical behavior compared to their DQ counterparts, where yield strength, uniform elongation, and total elongation increased after partitioning at 400 °C. The ultimate tensile strength of the Q&P steels decreased compared to the DQ steels annealed at the same peak temperature. However, the final strength-ductility balance of the studied Q&P steels was superior to the DQ steel grades. Moreover, considerable strength and improved ductility were obtained through the combination of peak annealing above the AC3 temperature followed by Q&P. These results are attributed to an interplay between a sustainable TRIP effect and effective strain-stress partitioning among the microconstituents resulted after the Q&P process.

A mean-field homogenization approach to predict fracture in as-quenched microstructures of Ductibor® 500-AS steel: characterization and modelling

Ductibor® 500-AS is a hot-stamping steel that has been designed to introduce local ductile regions within hot-stamped tailor-welded blanks (TWBs) used for energy-absorbing sections of automotive structures. This study investigates the flow and fracture behavior of a range of the microstructures of this alloy, corresponding to mostly ferritic (~96% ferrite plus 4% martensite), intermediate ferritic + martensitic (~57% ferrite plus 43% martensite), and mostly martensitic (~10% ferrite plus 90% martensite), obtained by applying various quench rates after austenitization. A series of mechanical tests in various stress states, ranging from simple shear to biaxial tension, was performed to characterize fracture for the developed microstructures. A mean-field homogenization (MFH) technique was then used to predict the flow and fracture response as a function of the microstructure. An MFH scheme designated as the interpolative Samadian-Butcher-Worswick 1 (INSBW1) within the first-order secant-based linearization method was considered to predict the macroscopic hardening behavior of the ferritic-martensitic microstructures. In the MFH modelling, the flow response of the ferritic and martensitic phases was modelled based on a dislocation-based hardening model, taking into account the chemical composition, carbon partitioning between phases, and dislocation generation and annihilation. Damage accumulation and the onset of fracture were predicted using a phenomenological damage indicator defined within each constituent phase given their calculated fracture loci and instantaneous stress and strain states. The experimental results showed that the mostly-martensitic microstructure had the lowest ductility and highest strength, and ductility and strength increased and decreased, respectively, as the martensite volume fraction in the microstructure decreased. The predicted flow curves and fracture strains as well as the fracture-limit curves for all of the microstructures agreed well with the measured data and interpolated modified Mohr-Coulomb (MMC) fracture loci.

Properties of Passive Films Formed on Ferrite-Martensite and Ferrite-Pearlite Steel Microstructures

The effect of ferrite-pearlite and ferrite-martensite phase combinations on the passive layer properties of low carbon steel is investigated in a 0.1 M NaOH solution. Heat treatments were designed to obtain ferrite-pearlite and ferrite-martensite microstructures with similar ferrite volume fractions. Potentiostatic polarisation and electrochemical impedance spectroscopy (EIS) results demonstrated the lower barrier properties of passive films on ferrite-martensite microstructure compared to the ones formed on ferrite-pearlite microstructure. This was attributed to the higher donor density of the passive layer on ferrite-martensite samples, measured with Mott–Schottky analysis. This behaviour was explained by the complex microstructure morphology of the martensite phase that led to the formation of a more defective passive film.

Simultaneous improvement in strength and ductility in low carbon steel via intercritical annealing and quenching treatments

Abstract Hot-rolled low carbon steel sheets offer huge potential to be used in automotive body structures. Hot rolled steel sheets were subjected to intercritical annealing and quenching treatment to obtain high strength and ductility. Optical, scanning electron microscopy and X-ray diffraction were carried out to understand the underlying mechanisms. Microstructure and mechanical property correlation were established. High strength was achieved by increasing hard martensitic phase volume fraction in the soft ferrite matrix. Ductility was imparted by the ferrite matrix and martensite plasticity. Simultaneous improvements in strength and ductility were obtained by optimizing the ferrite-martensite microstructure during intercritical annealing and quenching.

Effect of Intercritical Annealing and Austempering on the Microstructure and Mechanical Properties of a High Silicon Manganese Steel

High Silicon Austempered steels (AHSS) are materials of great interest due to their excellent combination of high strength, ductility, toughness, and limited costs. These steel grades are characterized by a microstructure consisting of ferrite and bainite, accompanied by a high quantity retained austenite (RA). The aim of this study is to analyze the effect of an innovative heat treatment, consisting of intercritical annealing at 780 °C and austempering at 400 °C for 30 min, on the microstructure and mechanical properties of a novel high silicon steel (0.43C-3.26Si-2.72Mn wt.%). The microstructure was characterized by optical and electron microscopy and XRD analysis. Hardness and tensile tests were performed. A multiphase ferritic-martensitic microstructure was obtained. A hardness of 426 HV and a tensile strength of 1650 MPa were measured, with an elongation of 4.5%. The results were compared with those ones obtained with annealing and Q&T treatments.

Evaluation of mechanical properties of ferrite-martensite DP steels produced through intermediate quenching technique

The objective of the present work is to investigate the mechanical properties of the dual phase (DP) steels produced by the intermediate quenching (IQ) technique. To produce the ferrite-martensite microstructure in the dual phase (DP) steel, low carbon micro alloyed steel is intercritically heat treated followed by short intercritical annealing and intermediate quenching using various intercritical temperatures. By varying the intercritical temperature (740, 760, 780, 800, 820 °C) different volume fraction of the ferrite and martensite phases of DP steel samples are produced. These series of dual phase (DP) steel samples were further analyzed through scanning electron and optical microscopy. The experimental investigations have been carried out to find the mechanical properties as per the ASTM testing standards. From the outcomes it is observed that DP780 steel have excellent tensile strength, better hardness and toughness which may be due to the fine distribution and equal volume fraction of ferrite and martensite phases. The enhancement in the mechanical properties of dual phase steel was the effect of intermediate quenching technique. Equal volume fraction of ferrite and martensite phases obtained, at temperature of 780 °C, converted low carbon micro alloyed steel into dual phase steel and also improves its mechanical properties.

Influence of Microstructure on Low-Cycle and Extremely-Low-Cycle Fatigue Resistance of Low-Carbon Steels

The goals of this study were to quantify and explain the effects of microstructure on the resistance of low-carbon steels to low-cycle fatigue and to extremely low-cycle fatigue (ELCF). Three different microstructures (ferrite–pearlite, ferrite–martensite, and ferrite–bainite–martensite) were tested, and their fatigue properties were analyzed using the strain-based Coffin–Manson model and an energy-based model. According to the Coffin–Manson model, ferrite–pearlite showed the best ELCF resistance, whereas in the energy-based model that considers the effect of tensile strength ferrite–bainite–martensite revealed the highest ELCF resistance. At similar tensile strength, ferrite–bainite–martensite had longer ELCF life than ferrite–martensite; the difference may be a result of the smaller strain incompatibility between bainite and ferrite than between ferrite and martensite. In all three microstructures, cracks initiated at the surface and propagated into the interior; this result indicates that fracture mode was not altered during cyclic loading at high strain amplitudes. Ferrite–martensite microstructure developed many sub-cracks surrounding a main crack; they could facilitate propagation of a main crack, and thereby degrade fatigue life at high strain amplitudes.

A review of recent progress in mechanical and corrosion properties of dual phase steels

New concepts proposed for processing of dual phase (DP) steels as one of the main classes of advanced high-strength steels (AHSSs) to enhance their mechanical properties (strength–ductility combination) and corrosion resistance were introduced. The current review covers (I) the processing routes to obtain the ferritic–martensitic microstructures, (II) parameters of intercritical annealing (IA) treatment, (III) primary thermomechanical treatments, and (IV) post processing. First, the principal heat treatment methods, i.e., step quenching, intermediate quenching, and intercritical annealing of ferritic–pearlitic steel, as well as the partitioning of manganese were critically discussed. Then, the effects of holding time at the intercritical annealing temperature on the austenitization, grain coarsening kinetics, abnormal grain growth, and volume fraction of martensite were summarized. Next, the importance of cold deformation (notably rolling) and heating rate for the development of fine-grained DP microstructures (with chain-networked martensitic islands) through recrystallization and modification of the preferred nucleation sites for the austenite phase was discussed. Moreover, the applications of severe plastic deformation techniques (such as constrained groove pressing), thermal cycling (multi-step or repetitive intercritical annealing), and spheroidization heat treatment were discussed. Finally, the impacts of tempering, quench aging, and bake hardening on the properties of DP steel were reviewed. This short overview shows the opportunities that the conventional and innovative processing routes can offer for the potential industrial applications of DP steels, especially in the lightweight car body for the automotive industry to address the safety, fuel consumption, and air pollution issues.

The effect of metal transfer modes on mechanical properties of 3CR12 stainless steel

The effect of metal transfer modes on mechanical properties of welded 3CR12 stainless steel was investigated. This was achieved by butt welding 10 mm thick plates of 3CR12. The effect of the metal transfer modes on the microstructure and the mechanical properties of the 3CR12 steel was then investigated as it was hypothesized that the change in welding positions will affect the transfer modes partly because of gravity. The microscopic examination revealed that the substrate was characterized by dual phase microstructure. Using the spectroscopic examination results, the ferritic factor calculation had shown that the microstructure was expected to be ferritic–martensitic during air cooling process. The tensile strength and Charpy impact energy were measured to be 498 MPa and 102 J, respectively. The heat input in the material was observed to be greater than 1 kJ/mm, which is the limiting factor for grain growth. Grain growths were observed in the heat affected zone of the welded materials. Ferritic–martensitic microstructure was observed in the microstructure. The grain growth altered the mechanical properties of the test material. Globular down hand had higher mechanical properties than spray down hand. Globular vertical up had better mechanical properties than globular vertical down.

Microstructure and mechanical properties of pure Cu interlayer TLP joints of 304 stainless steel to dual phase steel

Bonding was carried out at 1150 °C for holding times of 60 and 90 min followed by water quenching. At the bonding time of 60 min, Cu remained at the bond region resulting in an unstable liquid/solid interface with a sinusoidal microstructure. Isothermal solidification completed at the holding time of 90 min. Unidirectional solidification was observed from the stainless steel side toward the low carbon manganese steel and abnormal grain growth of stainless steel was seen at the bond region due to temperature gradient which originated from different electrical properties of the parent alloys. Joints made at the bonding time of 90 min were homogenized at 730 °C for 75 min and quenched in water to produce dual phase ferrite-martensite microstructure in the low C-Mn steel. Shear strength of the homogenized joint was greater than that of the low C-Mn steel and was about 80% of that of the 304 stainless steel under the same treatment condition.

Finite element and experimental method for analyzing the effects of martensite morphologies on the formability of DP steels

In this article, we investigated the effect of martensite morphology on the mechanical properties and formability of dual phase steels. At first, three heat treatment cycles were subjected to a low-carbon steel to produce ferrite–martensite microstructure with martensite morphology of blocky-shaped, continuous, and fibrous. Tensile tests were then carried out so as to study mechanical properties, particularly the strength and strain hardening behavior of dual phase steels. In order to study the formability of dual phase samples, Forming Limit Diagram was obtained experimentally and numerically. Experimental forming limit diagram was obtained using Nakazima forming test, while Finite Element Method was utilized to numerically predict the forming limit diagram. The results indicated that the dual phase samples with fibrous martensite morphology had the highest tensile properties and strain rate hardening out of the three different microstructures. Blocky-shaped martensite morphology, on the other hand, had the worst mechanical properties. The study of the strain hardening behavior of dual phase sample by Kocks–Mecking-type plots, evinced two stages of strain hardening for all specimens with different microstructures: stages III and IV. The forming limit diagram of dual phase steels also proved that samples with fibrous martensite morphology had the best formability compared to other two microstructures. The simulated forming limit diagram manifested that there is a good agreement between experimental results and those obtained by FEM.

Microstructure and Texture Control in Cold Rolled High Strength Steels

Formability had been important property of metals which is attributed to the texture development during thermomechanical processing particularly during hot rolling and cold rolling. In the present paper, the high strength steels with different carbon and manganese composition have been hot rolled above and below of austenite recrystallization temperature and followed by fast cooling up to different coiling temperature to get hot bands with different texture and two phase microstructure consisting ferrite with pearlite, bainite and martensite. Subsequently, these hot bands were cold rolled with 80 percent under plain strain condition. The microstructure of cold rolled sheets samples were analyzed using scanning electron microscope and showed the cold rolled microstructure with strong pancaked of two phase which was carried from the hot rolling. Cold rolled texture in ferrite pearlite microstructure is completely replaced by new texture components from hot rolled condition without the effect of Tnr. Hot rolled texture was retained in ferrite-bainite and martensite microstructure with the effect of Tnr. Increase in alloy chemistry weakens the texture intensity in ferrite pearlite/bainite microstructure. Whereas increase in alloy chemistry strengthens the texture intensity in ferrite martensite microstructure.

On significance of initial microstructure in governing mechanical behavior and fracture of dual-phase steels

Different initial microstructures were obtained through combination of intercritical annealing and cold-rolling. Subsequently, steels with different microstructures of ferrite–pearlite (FP), ferrite–martensite (FM) and complete martensite (M) were intercritically annealed at 780 °C for 5 min and water quenched to obtain ferrite–martensite microstructure. The significance of initial microstructures on ultimate microstructure, mechanical properties, strain-hardening ability and fracture behavior in dual-phase steels has been elucidated. Initial microstructures of FP, FM and M yielded different martensite morphologies, notably chain-like network structure, fine and fibrous martensite structure, respectively. Furthermore, with increasing martensite content in the initial microstructure, the average grain size of ferrite was significantly refined from about 12.3 to 2.1 μm, which results in that the ultimate tensile strength (UTS) and yield strength were increased, total elongation remained unaffected, and uniform elongation (UE) and strain-hardening ability were increased. A comparison of mechanical properties for different initial microstructures suggested that when the initial microstructure was complete martensite, the steel had excellent mechanical properties, with UTS × UE of 122.5 J cm−3, which was 24% greater than the conventional continuously annealed steels with ferrite–pearlite initial microstructure (98.8 J cm−3). The variation in tensile properties, strain-hardening ability and fracture mechanism of steels with different initial microstructures were discussed in relation to the ultimate microstructures.