Ferrite-martensite Dual Phase Research Articles

Noise Removal and Image Segmentation in Micrographs of Ferrite-Martensite Dual-Phase Steel

Present work proposes a new methodology for removal of noise (artifacts) from the micrograph, which normally poses a problem during segmentation of phases and measurement of their volume percentages. Segmentation of the phases is one of the primary steps towards quantification of microstructures. Dual-phase steel micrographs, generated from light microscope, consisting of ferrite (white) and martensite (black) phases are segmented by implementation of Otsu threshold algorithm, a well-known threshold algorithm of digital image processing. After thresholding, the noise removing algorithm is applied in different modes. The obtained results are compared with the findings of a commercial metallographic image processing software (Axio Vision). The proposed scheme is found to segment ferrite-martensite dual-phase micrographs successfully.

Abnormal mechanical behavior of a medium-carbon steel under strong ferrite-pearlite-martensite triple-phase microstructures

Understanding the relationships between the structure and mechanical properties of steel, which can be difficult to determine, is important to design and develop various high-strength, multi-phase steels. In the present work, the structure-property relationships of a medium-carbon AISI 1045 steel with ferrite-pearlite-martensite triple-phase (TP) microstructures are investigated and compared to those with ferrite-martensite dual-phase (DP) microstructures. Various step-quenching heat treatment cycles were carried out at 650°C for different holding times, followed by subsequent water quenching after reaustenitization of the samples at 860°C for 30min. Optical microscopy and field-emission scanning electron microscopy equipped with X-ray energy dispersive spectroscopy were used along with macrohardness, tensile, and nanoindentation tests to perform a detailed investigation of the structure-property relationships of the heat-treated samples. An abnormal mechanical behavior due to pearlite formation was observed in the ferrite-pearlite-martensite TP samples in comparison to the ferrite-martensite DP samples. The ferrite-pearlite-martensite TP samples consisting of more than 71% martensite were associated with a higher level of mechanical properties compared to those of the ferrite-martensite DP samples. The results are rationalized in part by the increased hardening response of 50.7% of the constrained pearlite in the TP microstructures induced by the martensitic phase transformation in conjunction with the lower ferrite hardening response of 22.6% in the DP samples.

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.

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel, in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time, and micro-failures mainly occurred from the locations with typical characteristics of stress concentration (i. e. ferrite interiors, the interfaces of ferrite-martensite grains and the martensite-martensite interfaces). Growth of micro-crack generally experienced the following stages: cracking from martensite boundaries, tiny particles in ferrite interiors, or martensite interiors, propagating in ferrite, bypassing martensite boundaries, or passing through martensite-martensite interfaces, finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Microstructure deformation was inhomogeneous in the stage of plastic deformation.

Mutual mechanical effects of ferrite and martensite in a low alloy ferrite-martensite dual phase steel

Abstract In this paper micromechanical behavior of ferrite and martensite microphases was evaluated by means of comprehensive micro-nanohardness measurements supplemented by light and electron microscopies and wavelength-dispersive spectroscopy (WDS). Experimental results indicated that not only ferrite but martensite hardening behavior was significantly influenced by ferrite and martensite volume fractions in dual phase (DP) microstructures. Ferrite hardness was continuously diminished with increasing ferrite volume fraction whereas the martensite hardening was increased with increasing ferrite volume fraction and then it did not varied in line with further progress in ferrite transformation and relevant carbon enrichment of prior austenite. In addition to these unexpected ferrite and martensite hardening variations, the lower hardness of martensite in DP microstructures in comparison to that of martensite in full martensitic microstructure cannot be completely interpreted by carbon alloying effects. These results are rationalized to the introduction of mobile dislocations into prior austenite due to plastically accommodation of martensitic transformation strains by prior austenite areas during quenching and the influence that formation of ferrite would have on this hardening mechanism.

Improvement of impact toughness of 5Mn-1Al-0.5Ti steel by intercritical annealing

The present study is aimed at improving the impact toughness of 5Mn-1Al-0.5Ti steel by incorporating ferrite-martensite dual phase microstructure by intercritical annealing. Although (8-12)Mn martensitic steels usually show very low impact toughness due to the occurrence of intergranular fracture, the martensitic structure of the present 5Mn-1Al-0.5Ti steel fails by transgranular cleavage fracture due to higher grain boundary strength than matrix strength incurred by reduced Mn content and segregation of Ti along grain boundaries. Nevertheless, it still shows very poor impact toughness at room temperature due to its coarse grain size. The application of intercritical annealing, i.e., formation of dual phase microstructure, is shown to significantly decrease ductile-to-brittle transition temperature (DBTT), with only a small degradation of tensile properties; however, microstructural examinations show that most of ferrite/martensite interfaces have a character of low angle boundaries and therefore such decrease in DBTT is not necessarily due to the formation of ferrite-martensite dual phase structure, but rather to the refinement of grain size by low temperature annealing.

Research on grain feature parameters based on inhomogeneous plastic deformation in microstructure of ferrite/martensite dual phase steels

Abstract To investigate the inhomogeneity of microstructure in ferrite-martensite dual phase steel, microstructure in different regions of DP600 steel was obtained from microscope observation. The volume fraction, grain shape and grain distribution of two individual phases were discussed, respectively. It was found that the inhomogeneity is a typical characteristic of microstructure. The inhomogeneous micromechanical properties of microstructure were discussed and it is shown that the main reason is due to the inhomogeneity of microstructure, especially for martensite distribution form. In describing the inhomogeneous micromechanical properties in ferrite grain precisely, two grain feature parameters were defined and they refer to the percentage of the F-M common boundary p and the minimum distance l between indentation location and the F-M common boundary. Nanoindentation tests were performed on a DP600 sample using spherical indenter to verify the influence of two parameters on yield strength and nanohardness in ferrite grain. The experimental results indicate the ferrite yield strength and nanohardness are linear related to the percentage of F-M common boundary p and the minimum distance l, respectively.

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 martensite volume fraction and abrasive particles size on friction and wear behaviour of a low alloy steel

Friction tests between a ferrite-martensite dual phase steel, with different martensite volume fractions (48–98%), and an abrasive paper with different sizes (15–200µm) under various normal loads (50–110N) are performed.It is highlighted that low normal loads minimize the wear rate whereas high normal loads minimize the friction coefficient. The lowest abrasive particle size minimizes the wear rate whereas an intermediate one (35µm) minimizes the friction coefficient. The wear rate and the friction coefficient decrease with increasing the martensite volume fraction. Conversely to the wear rate, for high normal loads, the friction coefficient becomes insensitive to the martensite volume fraction. It is also noticed that there are combined effects of martensite volume fraction and abrasive particle size on wear mechanisms.

Micromechanisms of deformation in dual phase steels at high strain rates

The effect of strain rate (0.001–800/s) on two commercial ferrite-martensite dual phase steels (DP600 and DP800), having different martensite contents (10.2% and 33.2%, respectively) were investigated. Microstructures of the deformed samples have been studied to understand the influence of strain rate on the micro-mechanisms of deformation in dual phase steels. The observations reveal that the volume fraction and size of the martensite play an important role in the deformation at different strain rates. Dislocation cells formed as the steels were deformed at different strain rates but the size of these cells and the extent of cell formation varied significantly with strain rate and martensite fraction. While deforming at various strain rates, martensite in DP600 was found to remain undeformed where the deformation was mainly prevalent within the ferrite grains. Although at high strain rates, extreme dislocation generation within the matrix of DP800 caused the fragmentation of martensite crystals. Secondary slip system of BCC (〈111〉{112}) was also found to coexist with the primary slip system at higher strain rates which aided the plastic deformation at high strain rates within the ferrite grains. The difference in the martensite volume fraction and distribution in these two steels also affected the void initiation and fracture morphologies when deformed at various strain rates.

Effect of alloying element partitioning on ferrite hardening in a low alloy ferrite-martensite dual phase steel

In this paper, the effect of carbon and other alloying elements partitioning on ferrite hardening behavior were studied in details using a low alloy AISI4340 ferrite-martensite dual phase (DP) steel. To do so, various re-austenitised samples at 860°C for 60min were isothermally heated at 650°C from 3 to 60min and then water–quenched to obtain the final ferrite-martensite DP microstructures containing different ferrite and martensite volume fractions. Light and electron microscopic observations were supplemented with electron dispersive spectroscopy (EDS) and nanoindentation tests to explore the localized compositional and hardening variations within ferrite grains in DP samples. The experimental results showed that the ferrite hardness was varied with progress of austenite to ferrite phase transformation in DP samples. In the case of a particular ferrite grain in a particular DP sample, despite a homogeneous distribution of carbon concentration, the ferrite hardness was significantly increased by increasing distance from the central location toward the interfacial α/γ areas. Beside a considerable influence of martensitic phase transformation on adjacent ferrite hardness, these results were rationalized in part to the significant level of Cr and Mo pile-up at α/γ interfaces leading to higher solid solution hardening effect of these regions. The reduction of potential energy developed by attractive interaction between C-Cr and C-Mo couples toward the carbon enriched prior austenite areas were the dominating driving force for pile-up segregation.

Development of carbon–manganese–chromium steels for automotive hot stamping technologies

The hot stamping potential of three commercial carbon–manganese–chromium steels was demonstrated. The steels were selected with the intention of producing ultrahigh strength ferritic–martensitic dual phase microstructures from hot stamping heat treatment, exhibiting superior tensile properties for ‘impact energy absorptive’ crash performance compared to the conventional carbon–manganese–boron steel for automotive hot stamping technologies, 22MnB5. Compared to 22MnB5, each of the carbon–manganese–chromium steels provided at least twice the total elongation, yet with a relatively small loss to ultimate tensile strength. It was concluded that the carbon–manganese–chromium steels provide superior tensile properties for impact energy absorptive crash performance, while exhibiting ‘lean’ chemical compositions compared to previously investigated steels with similar aims and offer opportunities for use in the ‘tailor welded blank’ concept.

The effect of ferrite–martensite morphology on the scratch and abrasive wear behaviour of a dual phase construction steel

A systematic experimental investigation concerning the effect of ferrite–martensite morphology on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels is reported. A hot rolled 22MnB5 steel was subjected to different heat treatments to generate dual phase microstructures with different ferrite–martensite morphologies. The effects of morphology on the scratch resistance and the corresponding failure mechanisms were unravelled using a multi-pass dual-indenter (MPDI) scratch test applying different load combinations. Results show that the ferrite–martensite morphology has a significant influence on scratch resistance and that the effect is contact load dependent. The scratch behaviour is linked to the strength coefficient K in the Hollomon model (σ=Kεn) as well as the initial indentation hardness. Results suggest that the strength coefficient K corresponds well with the scratch resistance. The optimal microstructure to yield the best combination of abrasion resistance and hardness depends on the working conditions. At a low loading condition the relative ranking of the scratch resistance of the various microstructures created is in good agreement with that of the ASTM G65 abrasion test.

The effect of martensite volume fraction on the scratch and abrasion resistance of a ferrite–martensite dual phase steel

A systematic experimental investigation was conducted to study the effect of the martensite volume fraction on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels. Dual phase microstructures with relatively high martensite fractions were generated by tailored intercritical annealing treatments. The scratch resistance and corresponding failure mechanisms were evaluated using a multi-pass dual-indenter (MPDI) scratch test for different loading conditions. Results show that the scratch resistance depends on not only the volume fractions of each phase, but also the loading conditions employed. Under low load conditions, the scratch depth decreases with increasing martensite fraction, in line with the increasing hardness evolution. However, under high load conditions, the scratch depth initially decreases with increasing martensite fraction up to a critical value (optimal fraction), beyond which the scratch depth starts to rise. This optimal fraction displays the best scratch resistance despite of a lower hardness compared to that of the full martensite. The scratch behaviour was correlated to the tensile behaviour and the corresponding work hardening characteristics. A two-stage strain hardening model was applied to interpret the scratch behaviour at different loads. The scratch behaviour at low load was found to correlate well with the standard ASTM G65 abrasion test.

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase (DP) steels with almost constant martensite contents of 20vol% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature (1043 K, FH1) with fast-heating (300 K/s) show banded ferrite/martensite structure, whereas those soaked at high temperature (1103 K, FH2) with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated (5 K/s) reference material (SH0). The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard–Jaoul technique demonstrate two stages of work hardening for all samples.

Retardation of plastic instability via damage-enabled microstrain delocalization

Multi-phase microstructures with high mechanical contrast phases are prone to microscopic damage mechanisms. For ferrite–martensite dual-phase steel, for example, damage mechanisms such as martensite cracking or martensite–ferrite decohesion are activated with deformation, and discussed often in literature in relation to their detrimental role in triggering early failure in specific dual-phase steel grades. However, both the micromechanical processes involved and their direct influence on the macroscopic behavior are quite complex, and a deeper understanding thereof requires systematic analyses. To this end, an experimental–theoretical approach is employed here, focusing on three model dual-phase steel microstructures each deformed in three different strain paths. The micromechanical role of the observed damage mechanisms is investigated in detail by in-situ scanning electron microscopy tests, quantitative damage analyses, and finite element simulations. The comparative analysis reveals the unforeseen conclusion that damage nucleation may have a beneficial mechanical effect in ideally designed dual-phase steel microstructures (with effective crack-arrest mechanisms) through microscopic strain delocalization.

Nanoindentation study of ferrite–martensite dual phase steels developed by a new thermomechanical processing

Dual phase (DP) steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced by a new route utilizing cold-rolling and subsequent intercritical annealing of ferrite/martensite duplex starting structure at 770°C for different times. Scanning electron microscopy has been supplemented by nanoindentation and tensile test to follow microstructural changes and their correlations to the variation in phase's hardness and mechanical properties. The results showed that longer holding times resulted in coarser and softer ferrite grains in DP microstructures. Martensite nanohardness variation with holding time is related to change in its carbon content. Mechanical properties such as strength, elongation and toughness are well correlated with the martensite/ferrite hardness ratio.

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.

Computational modeling of the effect of equiaxed heterogeneous microstructures on strength and ductility of dual phase steels

In this study, a code was developed to create virtual random representative volume elements (RVEs) depicting the actual and highly equiaxed heterogeneous microstructure of ferrite–martensite dual phase (DP) steels. Within this approach it was possible to perform a parametric study of the effects of DP microstructure (e.g., volume fraction, size, and distribution of the martensite; grain size and boundaries of the ferrite; martensite–ferrite interphase) and mechanical properties of the ferrite and martensite phases on the overall stress–strain behavior. A finite strain elastic–viscoplastic constitutive model has been used in conducting these microstructural-based simulations. It is shown that plastic strain localization in the form of localized narrow bands significantly control the ductility and ultimate fracture of DP steels. It was also noticed that by adding viscosity into the material property, the ductility increased significantly without compromising the strength of DP steels. It is shown that by decreasing the size of martensite phase and considering the ferrite–martensite interphase the overall response yields a simultaneous increase in strength and ductility.

Tensile Behavior of Ferrite-martensite Dual Phase Steels with Nano-precipitation of Vanadium Carbides

Tensile Behavior of Ferrite-martensite Dual Phase Steels with Nano-precipitation of Vanadium Carbides