Increase In Volume Fraction Of Martensite Research Articles

Effect of rare earth elements on microstructure and mechanical properties of bainite/martensite bearing steel

This study aims to investigate the effect of rare earth (RE) elements on the microstructure and mechanical properties of bainite/martensite (B/M) bearing steel. For this purpose, GCr15 bearing steels with different RE contents were prepared and analyzed after experiencing B/M heat treatment. The results show that RE addition significantly delays the bainite transformation by reducing grain boundary energy and carbon diffusion rate, leading to a reduction of bainite volume fraction from 27.3% for bearing steel without RE addition to less than 14.5% for RE-treated steels, and the corresponding increase of martensite volume fraction from 39.1% to more than 50.2%. The volume fraction of retained austenite (RA) remains nearly unchanged. Meanwhile, the width of the bainite needles gets refined in RE-treated steels while martensite and RA blocks become bigger. Compared with bearing steel without RE addition, larger RA blocks in RE-treated steels unexpectedly have higher stability due to higher dislocation density and surrounding phases strength. Accompanied by the change of microstructure, the hardness in RE-Treated steels improves slightly, while the fracture toughness dramatically deteriorates which can be directly explained by two mechanical phenomena during toughness test. The one is that the RA in RE-treated steels is harder to transform into martensite and thus consumes less energy; The other one gives that cracks tend to propagate along grain boundaries or martensite interfaces with less energy consumption easily in RE-treated steels instead of breaking off bainite.

Effect of limited retained austenite on the strength–ductility trade-off in low-alloyed TRIP steel

A comprehensive experimental and numerical investigation of TRIP effect on mechanical behaviors, strain partitioning and fracture is carried out on a low-alloyed TRIP steel with and without subzero treatment. Load responses of individual phases and macroscopic mechanical properties are investigated through nanoindentation and uniaxial tensile tests. The increase of martensite volume fraction caused by subzero treatment improves tensile strength but appears detrimental to ductility. Two types of martensite transformation are observed: a twin-relationship variant selection to accommodate strain interior unstable austenite grains, and a variant selection based on the same crystal orientation as adjacent coarse ferrite grain reducing interfacial energy between ferrite and newly transformed martensite. A microstructure-based simulation model considering martensite transformation is developed with phenomenologically calibrated crystal plasticity parameters. The shift of strain localization from unstable austenite grain to neighboring ferrite grain is clarified to reduce the strain partitioning at interfaces. This effect is argued to be the origin of ductility enhancement in low-alloyed TRIP steel.

Experimental and modelling study of the effect of martensite volume fraction on friction and wear of dual-phase steel at microscopic scale

In this work, the influence of martensite volume fraction, load and attack angle on hardness, friction and wear of ferrite and martensite phases of a dual-phase microstructure is investigated through nanoindentation and scratch tests with conical tips. Friction coefficient increases with the attack angle and is always higher for ferrite than for martensite. For martensite phase, friction coefficient increases and wear resistance decreases as martensite volume fraction increases. As the attack angle increases, wear mechanism changes from ploughing to cutting for martensite. Experimental results correlate to the Axén & al. approach based on the Equal Pressure model.

Fatigue Crack Growth and Fracture Toughness in a Dual Phase Steel: Effect of Increasing Martensite Volume Fraction

Crack growth resistance in dual-phase steel was studied. The dual phase steel microstructure was modified through heat treatments to increase the martensite volume fraction from 10% to 40%. The as-received and heat-treated samples were evaluated using a uniaxial tensile test, fatigue crack growth test, and fracture toughness test. Extended Finite Element Method (XFEM) was used to simulate the crack growth in compact tension test specimens. The results showed that an increase in martensite volume fraction is an effective way to increase the fracture resistance under different load conditions, quasistatics and dynamic, increasing the fracture toughness, tensile strength and fatigue resistance of the heat-treated material. Presence of a highest content of martensite results in formation of an important number of secondary cracks during the fatigue crack growth, which slow down the crack propagation. Moreover, martensite generates a crack closure over the crack tip, making the propagation difficult due to the irregularities caused by the crack growth on the martensite. Finally, the computational load-displacement curves are in good agreement with the experimental data.

Effect of individual phase properties and volume fractions on the strain partitioning, deformation localization and tensile properties of DP steels

Deformation band localization modes, uniform tensile strength, and uniform elongation of Ferrite-Martensite Dual-Phase (DP) steels are analyzed by finite element (FE) study. Treating the microstructure inhomogeneity as the sole cause of imperfection, failure initiation is predicted as the natural fallout of plastic instability caused by load drop because of localized plastic strain in the Representative Volume Element (RVE) during straining. Strain partitioning between two phases (ferrite matrix and martensite island) are investigated on RVEs, and it reveals that the increase of martensite yield stress decreases the plastic deformation and increases the stress state in martensite. Whereas, a decrease in martensite island volume fraction (Vm) results in the reduction of plastic deformation and stress state in the island. Studies are then carried out to investigate the effects of the ferrite-martensite flow properties and martensite volume fraction on the macroscopic tensile deformation behavior and band localization of DP steels. Micromechanical based FE simulation results emphasize that an increase in initial yield strength and volume fraction of martensite increases the ultimate tensile stress (UTS) with the decrease in uniform elongation. Similarly, as the hardening rate of ferrite increases, it increases the ultimate tensile stress (UTS) and uniform elongation. Additionally, deformation band localization modes alter from inclined to perpendicular to the loading axis with an increase in martensite volume fraction and initial yield strength of martensite. The knowledge of this work can be used to design DP steels with desired mechanical properties.

Mechanical and Electrochemical Behavior of Dual-Phase Steels Having Varying Ferrite–Martensite Volume Fractions

Dual-phase (DP) steels behave differently under corrosive environment depending upon the volume fractions of phases, mainly martensite, ferrite and bainite. The mechanical behavior and corrosion resistance of DP steels with varying martensite (from 32 to 100%) are investigated. The increase in martensite volume fraction of DP steel results in significant improvement in tensile strength and hardness with a subsequent reduction in elongation. Unlike the mechanical behavior, the increase in martensite volume fraction in steel does not exhibit a direct relationship with their corrosion resistance behavior. The corrosion mechanism in DP steels having different martensite and ferrite volume fractions depends on the self-corrosion resistance behavior of both the phases as well as the occurrence of galvanic corrosion between them.

Intercritical Heat Treatment Temperature Dependence of Mechanical Properties and Corrosion Resistance of Dual Phase Steel

Abstract This study investigated the effect of intercritical heat treatment temperature on the tensile properties, work hardening and corrosion resistance of dual phase steel. Ferrite-martensite dual phase steel with different martensite volume fractions were obtained after heat treatment at different intercritical temperatures. Microstructure, mechanical properties of steel were measured and the corrosion resistance was evaluated via polarization test. Tensile strength of the specimens increased by increasing the martensite volume fraction up to 48.2%. Further increase in martensite volume fraction led to decrease in tensile strength. Work hardening behavior analyzing showed that in DP steel with less than 50% martensite volume fraction, the work hardening took place in one stage and by increasing the martensite volume fraction two-stage work hardening behavior was observed in the Holloman analysis. The results of polarization test showed that, the corrosion resistance of dual phase steel is higher than that of plain carbon steel with ferrite-pearlite microstructure.

Effect of Martensite Volume Fraction on Strain Partitioning Behavior of Dual Phase Steel

Monotonic deformation behavior of ferrite-martensite dual phase steels with martensite volume of 13-43% have been analyzed in the current investigation using micromechanics based finite element simulation on representative volume elements. The effects of martensite volume fraction on the strain partitioning behavior between soft ferrite matrix and hard martensite islands in dual phase steels during tensile deformation have been investigated. As a consequence of strain incompatibility between hard martensite and soft ferrite phases, inhomogeneous deformation and finally deformation localization occur during tensile deformation. Restricted local deformation in ferrite phase caused by the adjacent martensite islands triggers the local stress triaxiality development. As the martensite volume fraction increases, the local deformation restrictions in ferrite phase also increases and which results in higher stress triaxiality development. Similarly the strain partitioning behavior between ferrite matrix and martensite island is also influenced by the volume fraction of martensite. The strain partitioning coefficient increases with increasing martensite volume fraction.

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.

Deformation behavior of additively manufactured GP1 stainless steel

In-situ neutron diffraction measurements were performed during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650°C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development.

Evolution of Texture and Microstructure during Cold Rolling and Annealing of Fe-14Mn-6Si-9Cr-5Ni Shape Memory Alloy

Evolution Texture and microstructure has been investigated in a Fe-14Mn-6Si-9Cr-5Ni shape memory alloy during cold rolling and annealing. The starting solution-annealed material show a nearly random texture with microstructure composed of equi-axed austenite grains with some e martensite plates inside. Cold rolling induces a strong alloy type texture with Brass {011}<211> and Goss {011}<100> as major components. Annealing of the cold deformed material produces a nearly random texture. The microstructural investigation reveals that with increasing cold deformation the amount of stress induced e and a’ martensite volume fraction increases. The electron back scattered diffraction (EBSD) phase mapping shows that reversion of the e martensite starts only after commencement of recrystallization.

Effects of cryogenic treatment on the mechanical and dry-sliding tribological behaviour of an Fe75Ni25alloy

An Fe75Ni25alloy was subjected to shallow cryogenic treatment (SCT) at −80°C and deep cryogenic treatment (DCT) at −196°C to induce martensitic transformation to different degrees. The effects of cryogenic treatment on phase composition, microstructure, mechanical properties, and wear resistance of Fe75Ni25alloys were investigated. The results showed that after SCT and DCT processes, the volume fraction of retained austenite approximately reduced from 42 per cent to about 30 per cent and 17 per cent, respectively, and the hardness increased from 174 to 276 and 336 HV accordingly. In the dry-sliding test, the cryogenic-treated samples exhibited better wear resistance compared with the conventional heat (annealing) treated samples, but the friction coefficient of the alloys was independent of cryogenic treatment. The improved hardness and wear resistance were likely to be attributed to the increase of martensite volume fraction.

Influence of microstructural constituents on the hysteresis curves in 0.2%C and 0.45%C steels

Steels with 0.2% and 0.45%C were submitted to different types of heat treatment, leading to different microstructures, with different amounts of the microstructural constituents: ferrite, pearlite and martensite. Measurements of magnetic hysteresis and Barkhausen noise were performed at different frequencies. A relationship was found between coercive force and the volume fraction of the microstructural constituents. The influence of the microstructural constituents on the shape of hysteresis curve is also discussed. The presence of martensite leads to more magnetization reversal by domain rotation, reducing the relevance of domain wall movement as a reversal mechanism. When domain wall movement predominates, permeability increases and coercivity decreases. As a consequence, when the martensite volume fraction increases, coercivity also increases. The results help clarify the relation between microstructure of steels and Barkhausen noise measurements in non-destructive testing.

Effect of martensite volume fraction on work hardening behavior of triple phase (TP) steels

The strain hardening behavior of AISI 4340 steel with dual phase (DP) and triple phase (TP) microstructures with constant ferrite volume fractions of 34% was investigated. The microstructures of specimens were analyzed using an optical microscope. The tensile flow behavior of all the samples was explained using Holloman equation. Results showed that in both DP and TP steels, work hardening takes place in two stages. The onset strain of second stage of work hardening and Holloman equation parameters (work hardening exponent and strength coefficient) in both stages were increased with the increase of martensite volume fraction in TP steels. At first stage of work hardening, ferrite phase deforms plastically and at the second stage, co-deformation of all phases happens. Variations of work hardening behavior with martensite volume fraction were related to internal martensitic transformational stresses and their effects on adjacent ferrite grains. The difference between UTS and YS, and strength ratio as a criterion for work hardening were confirmed Holloman equation parameters’ changes with martensite volume fraction.

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.

Stress-strain partitioning analysis of constituent phases in dual phase steel based on the modified law of mixture

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase (DP) steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio of martensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

Ferromagnetic properties of deformation-induced martensite transformation in AISI 304 stainless steel

Ferromagnetic properties of plastically deformed AISI 304ss have been studied using magnetic hysteresis and Barkhausen emissions methods. The present study has been concentrated on low volume fraction of martensite, i.e., below 58 pct, as compared to the available literature for a higher percentage of martensite. In measured materials, the coercivity increased with deformation and had a tendency to go toward saturation value. A linear increase in remanence with the deformation was observed. A large number of small amplitude of Barkhausen emissions were found at low percentage of martensite, indicating that magnetization rotation took place within a small region. However, large amplitude Barkhausen emissions were observed with the increase of deformations. Angular variation of Barkhausen emissions indicated the formation of rolling texture within the materials. A model has been proposed to explain the results. At the initial stage, small martensite clusters are formed, which grow with the deformation, and the intracluster exchange interaction becomes predominant. With the increase of deformation, martensite volume fraction increases. In this process, existing clusters grow and new clusters are formed. As a result, martensite clusters come closer and intercluster exchange interaction becomes important.

Strain-induced martensite formation in metastable austenitic steels with varying carbon content

Abstract Fatigue tests are performed on cylindrical, sheet and foil specimens to investigate the strain-induced martensitic transformation of austenatic steels. The tests set a variety of strain amplitudes and temperatures as well as various carbon concentrations for the investigated materaals in order to broaden the scope of the martensitic transformation and thereby its characterization. It is confirmed that there is a threshold value of plastic strain amplitude for the investigated materials above which the incremental increase in martensite volume fraction in the specimen under continued cyclic loading is of significance. Block-loading tests indicate that cycles at amplitudes below this threshold do not contribute to the austenite transformation. Thermodynamic calculations are applied in order to provide information on the stability of the material with regard to carbon content. It is shown that carbon concentration can be established by gas charging of the materials so that the driving force for tran...

Influence of grain boundary films of martensite on strength and impact behaviour of ferrite

The influence of thin films of martensite at the ferrite grain boundaries on the impact behaviour and strength of a high manganese–niobium containing steel has been examined. Normalising followed by intercritical annealing, at 730°C for various times from 0 to 900 min, and quenching introduced these martensite films. The percentage of martensite increased with increased holding time at 730°C from 15 to 33 vol.-%. The introduction of martensite removed the yield point but the yield strength was little affected, increasing only slightly with an increase in martensite volume fraction. In contrast, the impact behaviour markedly deteriorated when martensite was present, but when the amount of martensite further increased above 15 vol.-% it only had a small detrimental influence. The changes in impact behaviour could be explained by regarding the thin films of martensite as being similar to the brittle grain boundary carbides present in ferrite–pearlite steels. These martensite films readily crack yet are always thick enough to produce a sufficiently wide crack to render crack propagation easy and the critical event in fracture then becomes the ability to propagate the cracks through the grain boundaries.

Void formation during tensile testing of dual phase steels

The effects of martensite volume fraction (MVF) and strain state on necking behavior, post-uniform elongation, and the nucleation and growth of voids in thin sheet dual phase steel, strained in tension, were investigated. Steel containing, in weight percent, 0.08C, 1.45Mn, and 0.21Si, was cold rolled 50 pct and intercritically annealed to produce dual phase microstructures. The effects of MVF were evaluated with a series of constant geometry tensile samples with martensite volume fractions between 5 and 40 pct. The effects of strain state within the neck were evaluated with a series of constant thickness samples with 20 pct MVF and with width variations between 3 and 25 mm. A transition from diffuse to localized necking, as well as a decrease in post-uniform elongation, occurred with both an increase in MVF and sample width. Metallographic analysis of deformed samples revealed that the void nucleation occurs primarily at martensite particles by three distinct mechanisms. The void size and density in the necked region increased toward the fracture surface in all samples and the void density was significantly higher for the samples which exhibited localized necking. However, independent of neck geometry, voids were nucleated uniformly throughout the samples, and were associated with the martensite. The difference in void size and density between the samples with different necking behavior indicates that void growth is a consequence of the strain gradient while the shape of the voids depends on both the strain state and strain gradient. The implications of the void structure analysis are interpreted based on the dual phase microstructure.