Different Volume Fractions Of Martensite Research Articles

Effect of Variation of Martensite with a Constant Carbon Content on Mechanical Behavior and Sliding Wear of Dual Phase Steels

In a dual phase steel, an increase in the martensite volume fraction leads to a decrease in its carbon content, and hence, elucidating the effect of martensite content on the mechanical and tribological behaviors has not been possible. For the first time, the present work dealt with this subject by maintaining a constant amount of carbon in different volume fractions of martensite. Tribological properties was evaluated via a pin-on-disk sliding tribometer. The study of wear tracks and debris revealed oxidative wear as the dominant wear mechanism. Increasing normal load from 10 to 30 N resulted in a decrease in the specific wear rate, which was related to the high work-hardening capacity. This resulted in higher surface hardness, accommodating the formation of a more durable tribofilm on the surface, and a lower coefficient of friction. However, the increase in normal load from 30 to 60 N resulted in an increase in the specific wear rate and reduction of oxide coverage on the sliding surface due to high surface stresses. The worn debris were mainly identified as metallic in the running-in stage and Fe3O4 oxide in the steady-state stage.Graphical Abstract

Low-carbon cast microalloyed steel intercritically heat-treated at different temperatures: microstructure and mechanical properties

In this study, dual-phase (DP, ferrite + martensite) microstructures were obtained by performing intercritical heat treatments (IHT) at 750 and 800 °C followed by quenching. Decreasing the IHT temperature from 800 to 750 °C leads to: (i) a decrease in the volume fraction of austenite (martensite after quenching) from 0.68 to 0.36; (ii) ~ 100 °C decrease in martensite start temperature (Ms), mainly due to the higher carbon content of austenite and its smaller grains at 750 °C; (iii) a reduction in the block size of martensite from 1.9 to 1.2 μm as measured by EBSD. Having a higher carbon content and a finer block size, the localized microhardness of martensite islands increases from 380 HV (800 °C) to 504 HV (750 °C). Moreover, despite the different volume fractions of martensite obtained in DP microstructures, the hardness of the steels remained unchanged by changing the IHT temperature (~ 234 to 238 HV). Applying lower IHT temperature (lower fraction of martensite), the impact energy even decreased from 12 to 9 J due to the brittleness of the martensite phase. The results of the tensile tests indicate that by increasing the IHT temperature, the yield and ultimate tensile strengths of the DP steel increase from 493 to 770 MPa, and from 908 to 1080 MPa, respectively, while the total elongation decreases from 9.8 to 4.5%. In contrast to the normalized sample, formation of martensite in the DP steels could eliminate the yield point phenomenon in the tensile curves, as it generates free dislocations in adjacent ferrite.

Mesoscopic origin of damage nucleation in dual-phase steels

The damage modes in ferrite-martensite dual-phase (DP) steels include ferrite grain boundary (F/F) decohesion, ferrite/martensite interface (F/M) decohesion and martensite cracking. To explore the mesoscopic origin for each damage nucleation mode, we investigated ferrite-martensite dual-phase (DP) steels with different volume fractions of martensite, and characterized mesoscopic strain and stress distributions using microscopic-digital image correlation in SEM (μ-DIC) and finite element (FE) calculations. We performed in-situ tensile testing in a SEM and observed that the dominant damage nucleation mode changed from ferrite grain boundary (F/F) decohesion to ferrite/martensite interface (F/M) decohesion and finally to martensite cracking, with increasing martensite volume fraction (Vm) and varying martensite distribution. Mesoscale stress and strain analysis based on μ-DIC and FE calculations clearly reveal mesoscale origins of these damage modes: 1) F/F decohesion is caused by high strain which promotes accumulation of regular dislocations near the grain boundary and residual dislocations in the grain boundary; 2) F/M decohesion is attributed to high strain gradient which is associated with the accumulation of geometry necessary dislocations; 3) Martensite cracking stems from high stress that is partitioned to martensite. Our work demonstrated the pivot of microstructure engineering to improve the damage resistance of composite-like alloys consisting of soft and hard constituent phases, such as dual-phase steels.

Correlation of ferrite and martensite micromechanical behavior with mechanical properties of ultrafine grained dual phase steels

Thermomechanical treatment was employed to produce ultrafine grained dual-phase (UFG DP) steels consisting of different volume fractions of martensite and different ferrite grain sizes. The effects of intercritical annealing temperature on the microstructure and deformation behavior of DP steels were investigated. The modified Crussard-Jaoul analysis illustrated four-stage deformation of the investigated DP steels. The ferrite nanoindentation response in the microstructures showed that increasing the intercritical annealing temperature from 780 to 820 °C, resulted in an increase in the ferrite nanohardness by about 46% due to a decrease in the average ferrite grain size by about 34%. The average martensite nanohardness decreased by about 18% with decreasing its carbon content by about 18%. The mechanical properties of UFG DP steels, such as strength, elongation, energy absorption, and strain hardening characteristics were well correlated with the ferrite and martensite micromechanical behavior.

Effect of Heat Treatment Parameters on Microstructure and Mechanical Properties of (AISI1005) Steel

This study aims to investigate tensile properties and work hardening behavior of dual phase (DP) steels. A series of DP steels containing ferrite and martensite with different volume fractions of martensite (Vm) were produced by intercritical heat treatment. Microstructural investigations, hardness test, and tensile test were carried out. The experimental results showed that dual phase steels at 760oc and10s have excellent mechanical properties in terms of tensile strength, ductility and fracture energy. A further increase in Vm was found to increase tensile strengths and ductility. The increasing and then decreasing trend in tensile strength is in contrast to the law of mixture. These unusual behaviors are discussed and explained. Work hardening behavior was analyzed in terms of Holloman analysis.

On the Bauschinger effect in dual phase steel at high levels of strain

The effect of volume fraction and hardness of martensite on the Bauschinger effect in Dual Phase (DP) steel was investigated for strain levels close to those observed in automotive stamping. Five different grades of DP steel were produced by controlled heat treatment allowing the examination of the Bauschinger effect for three different volume fractions of martensite and three levels of martensite hardness. Compression–tension and shear reversal tests were performed to examine the Bauschinger effect at high levels of forming strain. Good correlation between the shear reversal and the compression–tension test was observed suggesting that for DP steel, shear stress strain data, converted to equivalent stress–strain, may be applied directly to characterize kinematic hardening behavior for numerical simulations. Permanent softening was observed following strain reversal and increased with martensite volume fraction and pre-strain level. While the Bauschinger ratio saturates at 3% pre-strain, the Bauschinger strain increases linearly with forming strain without showing saturation. This suggests that to model material behavior accurately in forming processes involving complex loading paths and high levels of strain, test data generated at high strain is required.

Effect of martensite morphology and volume fraction on strain hardening and fracture behavior of martensite–ferrite dual phase steel

Two different morphologies of martensite in dual phase (DP) steel were obtained using two different processing routes. In one case, intermediate quenching (IQ) was adapted, where DP steel was water-quenched to obtain martensite phase, followed by inter-critical annealing. In the second case, the steel was cold rolled, followed by inter-critical annealing (CR-IA). For IQ and CR-IA steels, the inter-critical temperatures varied from 750°C to 850°C to obtain different volume fractions of martensite. An understanding of structure–property was obtained using a combination of scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. It was observed that fibrous martensite presented in IQ samples, gradually transformed to blocky martensite with increase in inter-critical temperature, resembling the CR-IA steels. The fibrous martensite encouraged martensite cracking, however, the martensite cracking was dramatically decreased in the IQ samples with increase in martensite fraction. The strain hardening behavior studied using the differential C–J model indicated multistage depending on the fraction of martensite. The low volume fraction of martensite in the DP steel provided high ductility–toughness combination and improved strain hardening ability due to the presence of soft ferrite phase in DP steel. Fibrous martensite in DP steel resulted in less strain hardening than blocky martensite, prior to exceeding a threshold volume fraction. The threshold value was significantly smaller for DP steel with blocky martensite.

Influence of Martensite Volume Fraction on Impact Properties of Triple Phase (TP) Steels

Ferrite-bainite-martensite triple phase (TP) microstructures with different volume fractions of martensite were obtained by changing heat treatment time during austempering at 300 °C. Room temperature impact properties of TP steels with different martensite volume fractions (VM) were determined by means of Charpy impact testing. The effects of test temperature on impact properties were also investigated for two selected microstructures containing 0 (the DP steel) and 8.5 vol.% martensite. Test results showed reduction in toughness with increasing VM in TP steels. Fracture toughness values for the DP and TP steels with 8.5 vol.% martensite were obtained from correlation between fracture toughness and the Charpy impact energy. Fractography of Charpy specimens confirmed decrease in TP steels’ toughness with increasing VM by considering and comparing radial marks and crack initiation regions at the fracture surfaces of the studied steels.

Monitoring of the deformation and fracture process of dual phase steels employing acoustic emission techniques

Abstract In this paper, continuing our previous works, a new approach for detection of fracture micro mechanisms of ferrite–martensite dual-phase steels (DPSs) with various microstructures was investigated. For this purpose, dual phase steels with different volume fractions of martensite (VM) were produced by various heat treatment methods on a low carbon steel (0.1% C), and acoustic emission (AE) monitoring was then used during tensile testing of these DPSs. The AE signals from a tensile test using DPS in the range of 12–73% VM and various morphologies, like equiaxed or fibrous martensite phase, were captured. Principally, to understand the AE response and behavior of the martensite or ferrite phase separately, some samples of martensite and heat treated ferrite were tested. After the tests, by utilizing a new function named “sentry function”, we tried to relate the AE signals to various failure mechanisms of these steels. In confirmation of our earlier works, the results show that AE monitoring and sentry function are efficient tools to detect failure micromechanisms, consisting of ferrite–martensite interface decohesion and/or martensite phase fracture, identifying the correlation of failure mechanisms to microstructure in DPS. The results were verified with scanning electron microscopic observations and they indicate that AE monitoring is an efficient tool to detect micromechanisms identifying failure in DPSs.

Work Hardening Behavior of Fe-0.1 C Dual Phase Steel

The strain hardening exponent (n) in the stress strain relationship of metals and alloys is an indicator of their stretchability during press forming operations. The larger the n value, the more the material can deform before instability, and the material can be stretched further before necking starts. This paper aims to investigate work hardening behavior of dual phase steels. A series of dual-phase (DP) steels containing ferrite and martensite with different volume fractions of martensite (Vm) were produced by intercritical heat treatment. Work hardening behavior was analyzed in terms of Holloman analysis. Results showed that in DP steels with less than %50 Vm, the work hardening took place in one stage and the work hardening exponent increased with increasing Vm. By increasing the volume fraction of martensite (Vm>%50) more than one stage will be observed in the Holloman analysis.

Mechanical and corrosion behavior of plain low carbon dual-phase steels

Dual-phase steels are being used in automobile industries for last three decades. The mechanical properties of dual-phase steels can be altered by varying its martensite volume fraction. However, the benefits obtained in mechanical properties have to be viewed in light of other properties such as corrosion resistance. In this work, dual-phase steels with different volume fractions of martensite are obtained after thermal processing using different intercritical soaking times. The mechanical properties of dual-phase steels such as Vickers hardness and tensile properties are measured. Corrosion properties are evaluated using potentiodynamic polarization test and immersion test. It was observed that the tensile strength and hardness increased and ductility decreased with increase in martensite volume fraction. The corrosion rate for dual-phase steels is found to be lower than that for subcritically heat treated ferrite–pearlite steel. The higher corrosion resistance of dual-phase steels is explained on the basis of microstructural features.

Effect of martensite phase volume fraction on acoustic emission signals using wavelet packet analysis during tensile loading of dual phase steels

This paper will investigate the application of wavelet-based Acoustic Emission (AE) signal processing on micromechanisms to identify failure in dual phase steels (DPS)s. The AE signals from a tensile test using a range of DPS with different volume fractions of martensite (VM)s, in the range of 11–65% VM, were obtained and their waveforms were decomposed into various wavelet levels, each of which was related to a specific frequency range. Each level includes precise details, or approximations, of the so-called components. The energy percentage of each component was obtained by comparing it with the total energy of the AE signal. The energy distribution criterion in each component indicates that the energy in the AE signals is essentially concentrated on two or three components within a distinct frequency range. Each frequency range is related to a separate micromechanism, identifying failure. The results found for low VM in the contribution of ferrite/martensite interface decohesion figure prominently because 48% of their total energy was related to this micromechanism for a sample with 11% VM. The contribution of martensite phase fracture increased from 12% to 48.3% of total energy with an increase of VM in the range of 11% to 65% VM. The results were verified with microscopic observations and they indicate that wavelet-based signal processing is an efficient tool in the analysis of AE signals to detect micromechanisms identifying failure in DPS.

The effect of intercritical heat treatment temperature on the tensile properties and work hardening behavior of ferrite–martensite dual phase steel sheets

This paper aims to investigate tensile properties and work hardening behavior of dual phase (DP) steels. A series of DP steels containing ferrite and martensite with different volume fractions of martensite ( V m) were produced by intercritical heat treatment. Microstructural investigations, hardness test and tensile test were carried out. Hardness, yield strength, ultimate tensile strength, ductility and fracture energy were correlated to martensite volume fraction. The experimental results showed that dual phase steels with an equal amount of ferrite and martensite have excellent mechanical properties in terms of tensile strength, ductility and fracture energy. A further increase in V m was found to decrease tensile strengths and ductility. The increasing and then decreasing trend in tensile strength is in contrast to the law of mixture. These unusual behaviors are discussed and explained. Work hardening behavior was analyzed in terms of Holloman analysis. Results showed that in DP steels with less than 50% V m, the work hardening took place in one stage and the work hardening exponent increased with increasing V m. By increasing the volume fraction of martensite ( V m > 50%) two work hardening stages were observed in the Hollomon analysis.

Thermomechanical processing in the intercritical region and tensile properties of dual-phase steel

Thermomechanical process of 50% rolling reductions were applied at different intercritical temperatures on a low alloy steel containing 0.09% C, followed by quenching in the iced brine solution. Fibrous (ferrite + martensite) microstructures with variable contents of martensite were obtained. Rolling caused an increase in tensile strength and ductility in the longitudinal than the transverse directions at all levels of martensite, expect 35% of martensite where ductility is little affected. Microvoids were found to nucleate and grow in the necks of tensile specimens by fracture of martensite and by decohesion of the martensite/ferrite interface. The changes in density of the microvoids with distance from the fracture surface defined the straining behavior of the steel with different volume fractions of martensite. The growth of microvoids appeared to be interrupted when a critical stress was reached at which the ferrite cleaved. The martensite fibers in longitudinal directions appeared to be positioned for receiving stresses from ferrite. The increased straining with tensile deformation promoted ductile dimpling on fracture surfaces which were determined by the extent of void growth and coalescence prior to interruption by cleavage.

Characterization of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique

The aim of this work is to nondestructively characterize the dual phase steels using the Magnetic Barkhausen Noise (MBN) method. By quenching of AISI 8620 steel specimens having two different starting microstructures, from various intercritical annealing temperatures (ICAT) in the ferrite-austenite region, the microstructures consisting of different volume fractions of martensite with morphological variations have been obtained. The microstructures were first conventionally characterized by metallographical investigations and hardness tests. Then, the MBN measurements were performed using a μSCAN commercial system. Good correlations between the martensite volume fraction, hardness and MBN emission have been obtained. MBN signal height clearly decreased as the ICAT, therefore the volume fraction of martensite increased. The effect of the initial microstructure prior to intercritical annealing has also been differentiated by the MBN measurements. It has been concluded that MBN method can be used as a useful tool for nondestructive characterization of dual phase steels.

Tensile shear stress and microstructure of low-carbon dual-phase Mn-Ni steels after spot resistance welding

Tensile shear stresses and microstructure of dual steels with different volume fractions of martensite are studied after spot resistance welding. A Mn-Ni low-carbon steel heated in the intercritical temperature range and cooled in water and in air is studied. Spot resistance welding of test pieces is performed for different times at constant welding pressure and peak value of the welding current. The microstructure of the welded specimens is investigated and their permissible tensile shear stresses are determined.

Characterizing DP-steels using micromechanical modeling of cells

Dual phase (DP) steels having a microstructure consisting of martensite dispersed in a softer matrix called ferrite have received colossal attention due to their appealing combination of high strength, high work hardening rate and good ductility, which are favorable properties especially for forming processes. A model based on micromechanical modeling of cells is developed to capture the mechanical behavior of the DP-steels. The single-phase material behaviors of the constituents, which are the only material description required in micromechanical modeling, are produced in this work. Different material considerations, which are shown to have significant influence on the material behavior, are employed in the model. These include ferrite grain size, martensite softening by carbon dilution and particle size distribution effects. DP-steels of different volume fractions of martensite are produced in this work from heat treatment of a low carbon steel. Results of tensile tests are compared to the model predictions, which show good agreement between the experiment and model in terms of stress strain trends, in general, and specifically the UTS and the uniform strain and in terms of the mechanics and mechanisms of deformation, which takes place in DP-steels.

Computational modeling of phase transformations and mechanical properties during the cooling of hot rolled rod

Recently, great emphasis has been placed on controlled the cooling of hot rolled steel rods to achieve the desired microstructure and final mechanical properties. The approach is to develop the desired microstructure and temperature in the hot rolled rod and then transform it from the austenite phase to different volume fractions of martensite, bainite, pearlite and ferrite phases. This paper presents a computational approach to grain size evolution and finishing temperature predictions during the rolling process using Sellar’s evolution equations. This is followed by digitization of the continuous cooling curves to determine the volume fraction of various phases as the hot rolled rod transforms during the post rolling cooling. The mechanical properties are evaluated using the volume fractions of transformed phases using standard relationships for steels. These evolution—transformation—property relations are implemented in a 3D finite element program that uses a rigid–viscoplastic materials model and has a microstructure-based flow stress module. The efficacy of this approach is demonstrated by applying it to an eight pass square-to-round bar rolling sequence. Such an approach has been shown to be a powerful analysis tool that can be used as an inexpensive alternative to the traditional trial-and-error approach in roll pass design.

Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (Vm) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the intercritical annealing temperature. The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures. The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼0.55 Vm. A further increase in Vm was found to decrease the yield and tensile strengths as well as the impact properties. It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with Vm>0.55. Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation.

Low cycle fatigue behaviour of dual-phase steel with different volume fractions of martensite

Completely reversed low cycle fatigue tests were carried out at a constant strain rate of 10 −2s −1 on 3 mm thick sheet specimens of a dual-phase steel treated to give 1.5 to 28% martensite without changing the carbon content. Hysteresis loop shape, stress/strain response and plastic strain energy as a function of applied cycles are analysed for different microstructures. Strain/life and plastic strain energy per cycle ( Δ W ̄ p )/life (2 N f) plots are discussed in terms of microstructure. It is shown that during cycling the shape of the hysteresis loop continuously changes at lower volume fractions of martensite whereas it remains more or less constant for microstructures with a higher percentage of martensite. A Coffin-Manson type of plot between log ( Δ W ̄ p ) and log (2 N f) is found to be applicable to test results of dual-phase steel with a wide range of martensite contents and is thus more versatile than the plot between log ( Δϵ p/2) and log (2 N f) for predicting the fatigue life.