Higher Volume Fraction Of Martensite Research Articles

Development of Ultrahigh Strength TRIP Steel Containing High Volume Fraction of Martensite and Study of the Microstructure and Tensile Behavior

A new transformation induced plasticity (TRIP) steel containing high volume fraction of martensite was produced by austempering heat treatment cycle. Microstructure and tensile properties of this TRIP steel were investigated and compared to those of a dual phase (DP) steel with high martensite volume fraction. Microstructural analysis showed a mixture of ferrite, bainite, retained austenite and about 25–30 vol% of martensite in the TRIP steel. As a result of the strain induced transformation of retained austenite to martensite, the TRIP steel showed a strength elongation balance of 86% higher than that for the DP steel. In comparison to the commercial TRIP780 steel, the current TRIP steel showed a 15% higher ultimate tensile strength value while maintaining the same level of ductility. TRIP steel also had a larger work hardening exponent than DP steel at all strains.

Laser beam welding of dual-phase DP1000 steel

Autogeneous laser beam welding is an efficient process to join ferritic-martensitic dual-phase steels without large dimensional distortions. Localized softening may occur in the heat affected zone, particularly in DP1000 steel with a high martensite volume fraction. DP1000 steel was welded in bead-on-plate configuration varying the welding power between 0.4 and 2.0kW and the welding speed between 20 and 150mm/s. Light optical microscopy, scanning electron microscopy, and X-ray diffraction were used to perform the microstructural characterization of the welded joints. High-quality laser beam welds of thin sheets of DP1000 steel can be produced using appropriate welding parameters. The optimal welding condition was a nominal laser power of 2.0kW and a welding speed of 150mm/s. This condition minimizes the amount of softening of prior martensite and yields a narrow heat affected zone and a small volume fraction of retained austenite in the weld.

Influence of microstructure and pre-straining on the bake hardening response for ferrite-martensite dual-phase steels of different grades

This work aims to investigate the effects of pre-straining and microstructure on bake hardening (BH) response of dual phase (DP) steels. Four kinds of different grades of commercial DP steels, DP340/600, DP420/780, DP500/780 and DP550/980, were pre-strained in tension to 0.5%, 1%, 2%, 4%, 6% and 8%, then baked at 170°C for 20min followed by restraining, or just baked at 170, 190, 210, 230, 250°C for 20min and at 250°C for 40min followed by straining without pre-straining. Electron backscattered diffraction (EBSD) measurements and temperature-dependent internal friction measurements were conducted to characterize geometrically necessary dislocations (GNDs) and analyze solute carbon content and interactions between point defects and dislocations in DP steels. The results show that for all grades of DP steels investigated, BH values increase to peak values at pre-straining ranging from 0.5% to 2% and then decline with further pre-straining. At BH condition of 170°C/20min, peak BH values with pre-straining are 33MPa for DP340/600, 34MPa for DP420/780, 78MPa for DP500/780 and 90MPa for DP550/980 respectively. Pre-straining can cause increase in tensile strength and decrease in total elongation after baking, especially for DP500/780 and DP550/980. DP steels without pre-straining can reach very high BH0 values by applying either higher temperature or longer holding time. Solute carbon content in ferrite controls the speed of BH response in DP steels. Microstructures of higher volume fraction of martensite, smaller martensite islands and smaller ferrite grains can produce higher BH values.

Effect of substitution of Si by Al on the microstructure and mechanical properties of bainitic transformation-induced plasticity steels

The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity (TRIP) steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3rd generation advanced high strength steels (AHSS). It is shown from the dilatometry study, microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than 450°C whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450°C. Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450°C while Si-added steel has a better strength-elongation combination at temperatures lower than 450°C. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite (MA) blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.

Prediction of Stress–Strain Curves of Metastable Austenitic Stainless Steel Considering Deformation-Induced Martensitic Transformation

Transformation-induced plasticity (TRIP) effect is the outstanding mechanism of austenitic stainless steel. It plays an important role in increasing formability of the steel due to higher local strain hardening during deformation. In order to better understand forming behavior of this steel grade, the strain-induced martensitic transformation of the 304 stainless steel was investigated. Uniaxial tensile tests were performed at different temperatures for the steel up to varying strain levels. Stress–strain curves and work hardening rates with typical TRIP effect characteristics were obtained. Metallographic observations in combination with X-ray diffraction method were employed for determining microstructure evolution. Higher volume fraction of martensite was found by increasing deformation level and decreasing forming temperature. Subsequently, micromechanics models based on the Mecking–Kocks approach and Gladman-type mixture law were applied to predict amount of transformed martensite and overall flow stress curves. Hereby, individual constituents of the steel and their developments were taken into account. Additionally, finite element (FE) simulations of two representative volume element (RVE) models were conducted, in which effective stress–strain responses and local stress and strain distributions in the microstructures were described under consideration of the TRIP effect. It was found that flow stress curves calculated by the mixture law and RVE simulations fairly agreed with the experimental results. The RVE models with different morphologies of martensite provided similar effective stress–strain behavior, but unlike local stress and strain distributions, which could in turn affect the strain-induced martensitic transformation.

Failure analysis on welded joints of 347H austenitic boiler tubes

Microstructural and thermo-mechanical analyses using the finite element method (FEM) were performed on the welded joint of 347H boiler tubes in a coal-fired power plant after 3600h of operation. The cracks in the failed tube started on the inner surface of the heat-affected zone (HAZ). This area had a high hardness value and volume fraction of strain-induced martensite. The plastic deformation around the crack was concentrated near the grain boundaries. This failure occurred as the stabilizing effect disappeared due to carbide dissolution in the heat-affected zone, and plastic deformation and tensile residual stress were formed at the inner side due to the solidification contraction of the outer bead.

Microlaminated dual phase steel presenting with high strength and ultrahigh toughness

In the present study, a steel with the chemical composition of Fe–0·15 wt-%C–5 wt-%Mn–3 wt-%Al was designed. Its microstructure and mechanical properties were examined by optical microscopy, scanning electron microscopy and mechanical tests. It was found that hot rolling in the dual phase region of ferrite and austenite at high temperature resulted in microlaminated dual phase microstructure. An excellent combination of high strength (∼1200 MPa) and ultrahigh impact toughness (∼400 J) of the microlaminated dual phase steel was obtained. Based on the measurement of the phase volume fraction and the analysis of the fractography, it was shown that the high strength was mainly dependent on the high martensite volume fraction, while the ultrahigh impact toughness was attributed to the microlaminated microstructure.

Realistic microstructural RVE-based simulations of stress–strain behavior of a dual-phase steel having high martensite volume fraction

ABSTRACT Micro-mechanical behavior of a low carbon, high-strength (980 MPa grade) dual-phase (DP) steel of current technological interest that contains high volume fraction of martensite is simulated and uniaxial stress–strain curve is computed. Polygon-based vectorized microstructural images obtained via a new image analysis procedure are used for simulations of the micro-mechanical behavior. Simulations are performed using physics-based constitutive relationships for martensite and ferrite constituents and generalized plane-strain mesh elements. It is shown that there are significant variations in the computed stress–strain curve from one microstructural field of view to another for microstructural windows of 110 µm×110 µm size or less. A random statistical sample of 10 microstructural windows of 110 µm×110 µm size or larger constitutes a RVE of this class of microstructures such that the simulated stress–strain curves of such RVEs are in close agreement with one another, and therefore, represent the global stress–strain response of the simulated material. For the constitutive equations, boundary conditions, and type of mesh elements used in the present work, such average simulated stress–strain curves are also in close agreement with the experimentally measured uniaxial stress–strain curve of the DP980 steel. It is also shown that in this dual-phase microstructure, martensite carries most of the stress whereas most of the strain is contained in ferrite. Consequently, the strain localization and failure is predicted to go preferentially through ferrite compared to martensite, which is in agreement with experimental quantitative fractographic observations reported elsewhere.

New insights to the effects of ausforming on the bainitic transformation

The effects of prior deformation conditions on the bainitic transformation are investigated using dilatometry and optical metallography. The influences of both deformation temperature and strain on bainite transformation kinetics and morphology are examined in superbainitic carbide-free steel. The transformation is retarded by deformation at high temperature due to the retardation of bainite growth by dislocation debris in deformed austenite. At low deformation temperatures, the initial transformation rate is accelerated due to the presence of prolific nucleation sites in deformed austenite. The growth rate is, again, retarded by deformation. The combined effect of accelerated nucleation and retarded growth results in a complex dependence of the phase transformation kinetics on applied strain. For small deformation strains at 300°C, the overall transformation kinetics is faster than that in the non-deformed material. From a practical point of view, this provides an important opportunity to reduce the processing times for carbide-free bainitic steels. Interestingly, both the final transformed bainite and retained austenite fraction under these conditions are much higher than the non-deformed material. The retardation of growth dominates the overall transformation kinetics at large strains and low temperature leading to a high volume fraction of martensite in the final microstructure.

Effects of Alloy Composition on Microstructure and Mechanical Properties of Iron-Based Materials Fabricated by Ball Milling and Spark Plasma Sintering

Three PM steels, Fe-0.8C, Fe-2Cu-1.5Ni-0.5Mo-0.8C, and Fe-2Cu-2Ni-1Mo-1C, were fabricated by mechanical milling and spark plasma sintering. Dense sintered samples with fine and homogeneous microstructure were obtained. According to the results of X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy, it is suggested that the temperature of the sample can be ~50 K (50 � C) greater than that recorded. The microstructures of the as-sintered samples are divided into two groups. One consists of both ferritic and martensitic structures, and the others are of a ferritic structure. A considerable amount of martensite exists only in those high alloy Fe-2Cu-2Ni-1Mo-1C samples. The hardness of the sintered samples mainly depends on microstructure and composition. It shows that the hardness enhances with the volume fraction of martensite. However, a lower compressive strength is observed in the samples with higher volume fraction of martensite. The analysis of the deformation behavior demonstrates that the yield strength and ultimate strength are solely correlated to the properties of ferritic structure. Discontinuously yielding phenomenon, initial work hardening exponent, and decreasing rate of strain hardening exponent with strain are considered to be sensitive to the morphology of carbides formed in the ferritic structure.

Using Martensite Formation during Tube‐Forming to Optimize Fatigue Strength

Metastable austenitic stainless steels can undergo a deformation‐induced phase transformation from austenite (fcc) into ϵ‐martensite (hcp) and the harder α′‐martensite (bcc). This class of materials is widely used in sheet metal forming processes because of a beneficial combination of strength, ductility, and corrosion resistance. In such manufacturing processes phase transformation can be exploited in order to optimize the monotonic and cyclic strength of the components locally. In this paper it is shown that the amount of transformation can be controlled by a variation of process parameters such as deformation rate, temperature, and amount of deformation. An increase in initial temperature and strain rate reduces the rate of martensite formation. The effect of martensite volume fraction on the fatigue behavior is systematically investigated and the damage mechanisms in the VHCF regime are analyzed. For a very high number of cycles (beyond 107) a high martensite volume fraction (of more than 50%) leads to internal crack initiation from oxidic inclusions. Form and dimension of the inclusion are found to determine the number of cycles up to failure. An optimum volume fraction of the martensite phase is proposed for both the HCF as well as for the VHCF regime.

Effect of Heat Treatment on Microstructures and Mechanical Properties of Hot-Dip Galvanized DP Steels

In order to simulate the hot-dipped galvanizing of dual-phase (DP) steel (wt%) 0.15C–0.1Si–1.7Mn, the DP steels were obtained by different annealing schedules. The effects of soaking temperature, time, and cooling rate on ferrite grain, volume fraction of martensite, and the fine structure of martensite were studied. Results showed that the yield strength (YS) of DP steel is sensitive to annealing schedule, while total elongation has no noticeable dependence on annealing schedule. Increasing soaking temperature from 790 to 850 °C, the YS is the lowest at soaking temperature of 850 °C. Changing CR1 from 6 to 24 °C/s, the YS is the highest when CR1 is 12 °C/s. Increasing soaking time from 30 to 100 s, the YS is the lowest at soaking time of 100 s. Besides, it was found that sufficient movable dislocations within ferrite grains and high martensite volume fraction can eliminate yield point elongation, decrease the YS, and increase ultimate tensile strength. Through TEM observations, it was also found that increasing annealing temperature promotes austenite transformation into twin martensite, and increases volume fraction of martensite at sufficient cooling rate. With increasing the martensite volume fraction, the deformation substructure in the ferrite is well developed.

Improvement of mechanical properties in a dual-phase ferrite–martensite AISI4140 steel under tough-strong ferrite formation

This paper has been concerned to investigate in details the mechanical properties of AISI4140 heat treatable steel under ferrite–martensite dual-phase (DP) microstructures in conjunction with that of conventional quench-tempered (CQT) full martensitic condition. For this purpose, a wide variety of ferrite–martensite DP samples containing different volume fractions of ferrite and martensite microphases have been developed using step quenching heat treatment processes at 600°C for 20–55s holding times with the subsequent hot oil quenching after being austenitized at 860°C for 60min in the same situation as to the CQT condition. The finalized tempering heat treatment has been carried out at 600°C for 30min for both of direct quenched full martensitic and DP samples in order to optimize the strength–ductility combination. Light and electron microscopes have been used in conjunction with mechanical tests to assess the structure–property relationships in the DP and CQT heat treated samples. The experimental results indicate that the DP microstructures consisting about 7% volume fraction of fine grain boundary ferrite in the vicinity of martensite are associated with excellent mechanical properties in comparison to that of CQT condition. These observations are rationalized in terms of higher carbon concentration of the remaining metastable austenite leading to the harder martensite formation on the subsequent hot oil quenching, and so developing much harder ferrite grains as a consequence of more constraints induced in the ferrite grains during martensitic phase transformation in the remaining austenite adjacent to the ferrite area. The higher martensite volume fraction in the vicinity of thin continuous grain boundary ferrite network has been associated with the harder ferrite formation, causing higher work hardening behavior in the short time treated DP samples. Moreover, it has been found that in order to optimize the mechanical properties of ferrite–martensite DP samples, two independently parameters should be simultaneously controlled: one is the ferrite volume fraction and the other is ferrite morphology.

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

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

Enhanced shape recovery in cryogenically treated martensitic Ti–Ni alloys

The present investigation concerns the effect of cryogenic treatment on mechanical properties and shape recovery behavior of the Ti-rich Ti–Ni alloy. The sample quenched in liquid nitrogen (CQ) has resulted in higher volume fraction of martensite (B19′) phases and exhibited higher amount of recoverable strain (∼10%) in the region of iso-stress tensile deformation in comparison to the samples quenched in ice water. Cold rolling of the sample at near liquid nitrogen temperature (CR), with and without subsequent aging treatment, has resulted in the bimodal distribution of martensite plate size and negligible amount of recoverable strain under iso-stress tensile deformation.

Effect of Nd:YAG Laser Welding Parameters on the Hardness of lap joint: experimental and numerical approach

The observation on microstructure and microhardness has been evaluated by experimental methods and also approached to simulation work. One found out higher volume fraction of martensite inside the HAZ and the FZ that is the main cause of higher hardness insides these zones and no softening zone created by laser Nd: YAG on sheet metal of dual phase steel DP600. So the precaution during laser welding Nd: YAG at any welding speed, focal point position and power should be considered in order to prevent a crack and fracture of laser lap joint DP600 during service.

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in greater fatigue performance improvement of the CLSP samples compared to that of the RT-LSP samples.

Influence of martensite volume fraction and cooling rate on the properties of thermomechanically processed dual phase steel

Abstract Thermomechanical controlled processing (TMCP) simulations were preformed using a flat compression test, in order to develop a comprehensive understanding of the effect of the martensite volume fraction (MVF) and the cooling rate on the phase transformation behavior, microstructural evolution, mechanical properties and bake hardening (BH) ability of thermomechanically processed dual phase (DP) steel. The cooling schedule affected the microstructural constituents (polygonal ferrite and martensite) and phase transformation behavior of the DP steel. The relationship between the volume fraction of the martensite, the morphology and distribution of phases present in the microstructure, and the mechanical properties of DP steel was studied in detail. A higher MVF and an increasing cooling rate after the last deformation to the fast cooling start temperature (TFC) raised both the strength and the bake hardenability of the DP steel. Moreover, a correlation between MVF and bake hardening effect was found. A larger MVF led to an increase of BH levels.

Study of Bake Hardening Effect on Laser Welded Hot Rolled Ferrite-Bainite Dual Phase Steels

The influence of bake hardening (BH) effect on laser welded hot rolled ferrite-bainite dual phase steel (FBS) with respect to the process conditions was investigated. The samples were firstly laser welded and then pre-strained. Pre-strained the samples with defined degrees of deformation and a subsequent aging treatment leads to enhanced hardness and strengthening for both conditions. The microstructure of the fusion zone (FZ) and heat affected zone (HAZ) was studied. A high volume fraction of martensite could be observed in the FZ as well as in the HAZ. Both steels exhibited a clear BH effect in both, the as-received and the laser welded conditions. The BH effect is more pronounced in the pre-strained laser welded condition.

Repetitive Thermomechanical Processing towards Ultra Fine Grain Structure in 301, 304 and 304L Stainless Steels

Thermomechanical processing as a combination of cold rolling and annealing was performed on austenitic stainless steels 301, 304 and 304L. Two cold rolling steps each one up to a reduction of 75% were combined with an intermediate annealing at 800°C for 20 min. The final annealing was performed at the same temperature and time. Cold rolling contributed to martensite formation at the expense of metastable austenite in the studied materials. Austenite in 30l was found to be less stable than that in 304 and 304L. Hence, higher strength characteristics in the as-quenched 301 stainless steels were attributed to the higher volume fraction of martensite. Both α′-martensite and ɛ-martensite were found to form as induced by deformation. However, the intensity of ɛ-martensite increased as the stability of austenite decreased. Annealing after cold rolling led to the reversion of austenite with an ultra fine grained structure in the order of 0.5–1 μm from the strain induced martensite. The final grain size was found to be an inverse function of the amount of strain induced martensite. The thermomechanical processing considerably improved the strength characteristics while the simultaneous decrease of elongation was rather low.