Ferrite Matrix Research Articles

Phase-field theory based finite element analysis for ratchetting behavior of medium-manganese steel

A phase field model for martensitic transformation considering crystal plasticity was developed to investigate the microscopic mechanism of the transformation-induced plasticity in response to ratchetting of medium-manganese steel with a ferritic matrix containing retained austenite. This model introduced various inelastic deformation mechanisms, i.e., martensitic transformation, the austenite, ferrite and martensite plasticity caused by dislocation slip. Combining phase field model with finite element method, the ratchetting behavior of medium-manganese steel under asymmetric load was simulated by COMSOL software, and the martensitic transformation and its effect on ratchetting was discussed. The simulated results show that the ratchetting strains evolutions of the medium-manganese steel are well described by the developed phase field model. Ratchetting strain increases with increasing stress amplitude, mean stress and stress ratio. The martensite content increases with increasing stress amplitude and decreases with increasing mean stress and stress ratio.When the plastic strain exceeds a certain value, the plastic strain induced by martensitic transformation will increase the ratchetting strain of medium-manganese steel, which indicates that the critical importance of the martensitic transformation for the overall ratchetting behavior of medium-manganese steel. The plastic deformation of medium- manganese steel is mainly composed of dislocation slip of austenite and ferrite, where the former is the main part. Because of martensitic transformation, the non-uniformity of micro strain field in medium-manganese steel is aggravated, and the equivalent plastic strain increases.

Soft magnetic composite based on iron in sintered Mn–Zn ferrite matrix without non-magnetic coating

Low core loss soft magnetic composites based on iron powder particles encapsulated in CoO- and TiO2-doped sintered Mn–Zn spinel ferrite matrix are fabricated with no additional organic nor inorganic non-magnetic insulating layer. As both the iron and ferrite contribute to the entire effective composite properties with their ferromagnetic and ferrimagnetic nature, respectively, the resulting composites possess improved magnetic performance. The absence of any non-magnetic binder ensures higher ability of iron-matrix magnetic coupling. The temperature-dependent properties are tuned to higher level of operating temperatures of about 200 °C. The magnetic losses are shown to be suppressed thanks to the high electrical resistivity of ferrite insulation and permeability is enhanced because of the collectivization of iron particles and ferrite grains in magnetization process. This unique property descends from mediating the paths for magnetic flux between iron particles by ferrite matrix which, at the same time, mitigates the demagnetizing effects on the scale of composite's microstructure.

Mechanical characterization and evaluation of pitting corrosion resistance of a superferritic stainless steel model alloy 25Cr–6Mo–5Ni

Abstract The mechanical properties and corrosion resistance of annealed superferritic stainless steel model alloy 25Cr–6Mo–5Ni were investigated in 0.6 M NaCl solution. The microstructure consisted of a ferrite matrix and eutectoid phase with a lamellar structure distributed at grain boundaries and within the ferrite grains with a spherical morphology. Tensile and impact results suggested brittle behavior of the model alloy. Fractography analysis revealed typical cleavage facets, river patterns and micro-cracks at grain boundaries and across the ferrite grains. Pitting corrosion began within the eutectoid phase, which contains in chrome depletion zones. Electrochemical impedance spectroscopy measurements suggest that the breakdown of passive films was more susceptible in the eutectoid phase.

A 4340 Steel with Superior Strength and Toughness Achieved by Heterostructure via Intercritical Quenching and Tempering

The conventional 4340 steel was used after quenching and tempering, strengthened by the classical pearlitic structure where cementite particles are dispersed through the ferrite matrix. In the present study, a heterostructure microstructure consisting of micro-sized residual ferrite zones and pearlitic zones was introduced by an optimized process of intercritical quenching and tempering, resulting in a steel with higher strength and better toughness. The pearlite steel has a tensile strength of 1233 MPa, yield strength of 1156 MPa, and toughness of 121.5 MJ/m3. Compared with the pearlite steel, the tensile strength and yield strength of the heterostructure steel have been improved by 67 MPa and 74 MPa, respectively, while the toughness has been increased by 52.5 MJ/m3. In this heterostructure, the micro-sized ferrite bulks serve as the soft zones surrounded by the hard zones of the pearlite structure to achieve a remarkable work-hardening capacity. Statistical analysis shows that the heterostructure has the best hetero-deformation-induced (HDI) hardening capability when the residual ferrite bulk contributes ~31% by volume fraction, and the quenching temperature is around 780 °C. This study opens new ways of thinking about the strengthening and toughening mechanism of heat treatment of medium carbon steels.

Atomic-scale deformation mechanism of nano Cu precipitate along the rolling process of ferritic steel

The deformation behavior of nanoscale precipitates and their interaction with the matrix in the alloys have been rarely explored. In this study, the deformation mechanism of nanoscale soft Cu precipitates and its interaction with ferritic matrix during rolling deformation were studied at the atomic scale. The ellipsoidal Cu precipitates with an average diameter of about 34 nm are flattened into slender rods under rolling reduction of about 70% and are elongated along the rolling direction. The initial K-S orientation relationship was transformed into N-W orientation relationship after rolling deformation. The shear deformations are induced in Cu precipitate near the interface and the serrated boundary are formed combined with the plastic flow of the matrix. A successive lattice torsion band with a width of 1.5 ∼ 10 nm was formed at the serrated interface, while there is only local lattice distortion at the smooth interface. The Cu precipitates maintained good deformability and excellent strain coordination ability under severe deformations. These results have important value for understanding the deformation mechanism of Cu precipitates.

Solidification Behavior of Fe-6.5Si Alloy Powder for AM-SLM Processing, as Assessed by Differential Scanning Calorimetry.

Lab-scale investigations on the processing of small powder volumes are of special importance for applications in additive manufacturing (AM) techniques. Due to the technological importance of high-silicon electrical steel, and the increasing need for optimal near-net-shape AM processing, the aim of this study was to investigate the thermal behavior of a high-alloy Fe-Si powder for AM. An Fe-6.5wt%Si spherical powder was characterized using chemical, metallographic, and thermal analyses. Before thermal processing, the surface oxidation of the as-received powder particles was observed by metallography and confirmed by microanalysis (FE-SEM/EDS). The melting, as well as the solidification behavior of the powder, was evaluated using differential scanning calorimetry (DSC). Due to the remelting of the powder, a significant loss of silicon occurred. The morphology and microstructure analyses of the solidified Fe-6.5wt%Si revealed the formation of needle-shaped eutectics in a ferrite matrix. The presence of a high-temperature phase of silica was confirmed by the Scheil-Gulliver solidification model for the ternary model Fe-6.5wt%Si-1.0wt%O alloy. In contrast, for the binary model Fe-6.5wt%Si alloy, thermodynamic calculations predict the solidification exclusively with the precipitation of b.c.c. ferrite. The presence of high-temperature eutectics of silica in the microstructure is a significant weakness for the efficiency of the magnetization processes of soft magnetic materials from the Fe-Si alloy system.

Strengthening Mechanisms of Tempered Martensite in Vanadium-Added Steels

To clarify the effects of vanadium additions on the strengthening mechanisms of tempered martensitic steel, the microstructures, precipitates, dislocation densities, and tensile properties of water-quenched and tempered Fe–0.2C–0.5Si–2.5Mn–xV (mass%; x = 0–0.82) steels were analyzed. The vanadium carbide precipitates were plate-shaped and had a Baker–Nutting orientation relationship with the ferrite matrix. The size and shape of the vanadium carbide precipitates on the slip plane were considered when evaluating the contribution of precipitation strengthening. The increase in yield strength upon adding 0.82 mass% vanadium to the tempered steel was mainly caused by precipitation strengthening owing to the vanadium carbide precipitates and dislocation strengthening owing to the high dislocation density. This study demonstrates that the contribution of precipitation strengthening might be overestimated if it is assumed that the precipitates all hinder dislocation motion by the Orowan mechanism.

Corrosion-Resistant Stainless-Steel Strands for Prestressed Bridge Piles in Marine Atmospheric Environments

This study demonstrated that stranded highly cold-worked 2205 stainless steel (SCW2205) exhibits superior corrosion resistance when compared to stranded cold-worked AISI 1080 (ASTM A416) steel and cold-worked modified austenitic stainless steel 201, making it a viable candidate for prestressing applications with extended service life. Laboratory and field testing were performed. The material microstructure was characterized using scanning electron microscopy/energy dispersive x-ray spectroscopy, transmission electron microscopy, and x-ray diffraction. Then, laboratory corrosion testing, including linear sweep voltammetry, pitting resistance exposure, and four-point bend stress corrosion cracking (SCC) testing, was performed under different conditions. These conditions included concrete pore solution saturated with chlorides, direct exposure to artificial seawater, various concentrations of NaCl and MgCl2, representative inland salt deposition conditions, and oxidizing conditions with high chloride concentrations. The laboratory studies were augmented with field testing (259 d), comprised of four-point bend SCC, U-bend SCC testing (ASTM G30), and atmospheric contaminant measurements. The pitting resistance results, corrosion morphology, stable pit safe range, SCC results in the lab and in the field, and hydrogen embrittlement (HE) testing by slow strain rate testing (SSRT) under cathodic polarization as a diagnostic showed that SCW2205 outperformed the other steels tested, in the case of marine atmospheric corrosive conditions. SCC in SCW2205 was characterized by a selective localized anodic dissolution of the ferrite matrix and environmentally assisted cracking in the austenite phase. However, SCC was only found in SCW2205 at or above 65°C. SSRTs confirmed susceptibility to hydrogen uptake and a hydrogen-assisted mechanism of HE given sufficient hydrogen. It is speculated that hydrogen uptake in pits or crevice sites might be a route to hydrogen absorption worth exploring further since the absence of cathodic polarization in application precludes hydrogen production and uptake.

Microstructural analysis of some reinforcement steel rods used for concrete reinforcement and their effect on the mechanical properties, and carbon equivalent values of the reinforcement steel rods

The microstructural analysis of some reinforcement steel rods used for concrete reinforcement and their effect on the mechanical properties, and carbon equivalent values of the reinforcement steel rods have been studied. The work subjected the samples to test specimen preparation as required by standard specification for each test. Samples from five different mini mills were investigated for hardness, impact strength, composition, and microstructure. The result obtain from the work, has clearly shown that the microstructure of all the samples investigated reflected the hardness, impact strength, and the CEV of the samples. The microstructure of sample D with spheroidal pearlite dark areas, and a ferrite matrix had the highest hardness value of 34.61BHN, impact strength of 274.7J and CEV of 0.545. These values correlate with the microstructure. The work showed that as the CEV is increased the hardness and the impact strength of the material also increased. The work also show that as the total carbon (TC) % in the samples increased the microstructure becomes darker and the hardness and the impact strength also increased. In conclusion the microstructure of the reinforcement steel rods had effect on the hardness, impact strength properties, and carbon equivalent value of the reinforcement steel rods.

Study of Surface Modifications of Textile Card Clothing (AISI 1065 Alloy) by Laser Shock Peening.

AISI 1065 is a carbon steels that is widely used in manufacturing industrial components owing to its high tensile strength and wear resistance. One of the major applications of such high-carbon steels is the manufacturing of multipoint cutting tools for materials such as metallic card clothing. The quality of the yarn is determined by the transfer efficiency of the doffer wire, which depends on its saw tooth geometry. The life and efficiency of the doffer wire depends on its hardness, sharpness, and wear resistance. This study focuses on the output of laser shock peening on the surface of the cutting edge of samples without an ablative layer. The obtained microstructure is bainite, which is composed of finely dispersed carbides in the ferrite matrix. The ablative layer induces 11.2 MPa more surface compressive residual stress. The sacrificial layer acts as a thermal protectant by decreasing surface roughness to 30.5%. The sample with a protective layer has a value of 216 HV, which is 11.2% greater than that of the unpeened sample.

The Effect of Annealing Temperature and Temper‐Rolling on Microstructure, Mechanical Properties, and Lüders Strain of Fe–0.25C–5.9Mn–1.0Al–1.57Si Medium Mn Steel

This study investigates the effect of austenite reverted transformation (ART) annealing temperature and temper‐rolling on the microstructure, mechanical properties, and deformation behaviors of cold‐rolled Fe–0.25C–5.9Mn–1.0Al–1.57Si transformation‐induced plasticity (TRIP) steel. The cold‐rolled steel annealed at 700 °C demonstrates excellent mechanical properties. The ultimate tensile strength, total elongation, and product of strength and elongation are observed as 1212 MPa, 31.8%, and 38.6 GPa%, respectively. The excellent combination of strength and ductility is related to the discontinuous TRIP effect; still, an inhomogeneous deformation is observed during tensile deformation, known as the Lüders strain. Temper‐rolling is used for the ART‐annealed specimens at 700 and 720 °C, and yield point elongation decreases when temper‐rolling reduction increases. When the temper‐rolling reduction increases by 8%, the yield point elongation of the specimen annealed at 700 °C is noted at 1%, while the specimen annealed at 720 °C exhibits continuous yielding. The strain‐induced martensite transformation and increased dislocation density in the ferritic matrix improve the early‐stage strain hardening rate, thus suppressing the Lüders band's formation.

Microstructure‐Based Modeling and Estimation of Mechanical Properties of High‐Carbon Steel

Herein, attempts are made to estimate the mechanical properties using microstructure‐based finite element (FE) modeling and validate these results with the experimental results. The two high‐carbon steel specimens are hot‐rolled and air‐cooled to develop ferrite–pearlite microstructures. Different characterizations are utilized to observe microstructures as well as Vickers hardness and tensile tests are carried out to determine the mechanical properties. Two high‐resolution scanning electron micrographs are chosen for representative volume element‐based FE analyses for modeling the mechanical behavior of ferrite–cementite microstructure. Object‐oriented finite elements (OOF2) and Abaqus FEA 6.14 software are used to estimate the elastic and elastoplastic behavior assuming plane stress conditions. The correlation between cementite lamellae orientation and the predicted elastoplastic properties is investigated and compared with the experimental results. The influence of image size and mesh size on the predicted true stress–true strain behavior is discussed. The hard and brittle cementite lamellae face maximum stress while the softer ferrite matrix experiences maximum strain. It is found that strain accumulation is maximum at the interfaces of ferrite and cementite. These findings are further validated by the microvoid and crack initiation spots in the fracture surface and subsurface micrographs of broken tensile specimens.

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident‐tolerant‐fuel (ATF)‐cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body‐centered‐cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high‐temperature deformation, from only one type of Fe2M (M = Nb, Mo, Ta) Laves phase to Fe2Zr combining Fe2M‐type Laves phase. The Fe2Zr‐type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe2M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.

Atomic-scale investigation of H-trapping by fine NbC precipitates in a low C ferritic steel

The origin of hydrogen trapping by the Nb-carbide precipitates in a ferritic steel was studied at the atomic scale with the help of HRTEM (high resolution transmission electron microscopy) and 3D-APT (atom probe tomography). A Nb-added ferritic steel was prepared in the laboratory and subjected to aging after solutionizing treatment to have fine Nb-carbide precipitates in ferrite matrix. The precipitation kinetics were simulated beforehand using TC-Prisma in Thermocalc software. A large number of nanometer-sized Nb-carbide precipitates were observed in the ferrite matrix using TEM. 3D-APT studies on the steel sample electrolytically charged with D (deuterium) showed that the D atoms are trapped more within the precipitates than at the precipitate-matrix interfaces. It has been argued that H atoms occupy the vacant lattice sites on carbon sub-lattice sites in non-stoichiometric NbC0.83.

Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb

In this study, the effects of Nb on the microstructural characteristics, hardness, and corrosion resistance of high-silicon ductile cast iron (HSDI)-3.6 wt.% Si were investigated. Samples from different castings with 0–0.9 wt.% Nb were obtained and compared to a commercial ductile iron. Microstructures showed that the amount of ferrite in the matrix increased with increasing Nb content, from 34% for unalloyed HSDI to 88% for HSDI-0.9 wt.% Nb. The presence of randomly distributed NbC carbides was identified by EDX for all the samples alloyed with Nb, and the hardness of the HSDI increased with the Nb content. To evaluate the influence of the Nb content on the corrosion resistance of HSDI, potentiodynamic tests were carried out in a solution of H2SO4. The highest corrosion rate on HSDI was obtained for the HSDI-0.3 wt.% Nb sample, with 2802 mills per year, due to the amount of pearlite present and the lowest presence of NbC carbides, compared to the HSDI-0.9 wt.% Nb, with 986 mills per year. This behavior was attributed to the ferrite matrix obtained because of a high Si content in the DI, which delayed the anodic dissolution of the alloy and suppressed the pearlitizing effect of Nb for contents greater than 0.3 wt.%, as well as to the effect of NbC carbides, which acted as inhibitors.

Isothermal low-cycle fatigue, fatigue–creep and thermo-mechanical fatigue of SiMo 4.06 cast iron: Damage mechanisms and life prediction

A series of strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on silicon-molybdenum ductile cast iron SiMo 4.06. LCF tests were performed for various mechanical strain amplitudes at a high strain rate of 3×10−3/s for various temperatures between 20 °C and 750 °C. In addition, several LCF tests were performed for a low strain rate of 1×10−4/s and as fatigue–creep tests with a tensile strain dwell. TMF tests were performed as out-of-phase and fully constrained between 100 °C and 650 °C. The investigation of the damage mechanisms revealed that the predominant failure mode is a combination of graphite-matrix debonding and transgranular crack propagation through ferritic matrix. Finally, a novel damage model based on the dissipated hysteresis energy is proposed in order to effectively predict the lifetime of SiMo 4.06 under LCF and TMF loading. Creep and fatigue damage are each calculated separately and the oxidation effect is taken into account indirectly.

Effect of Molybdenum Contents on Microstructure and High-Temperature Wear Behavior of SiMo Ductile Iron

High-silicon and molybdenum (SiMo) ductile iron is a common heat-resistant alloy that may be exposed to high-temperature wear during service in many of its applications. The wear behavior of four SiMo ductile iron alloys was evaluated at different temperatures up to 750 °C. This research focuses on the influence of various Mo contents on the microstructure, structural stability, and hence, the wear performance of such alloys. Thermodynamic calculations proposed the phase diagrams, critical transformation temperatures, and phase volume fractions in all samples by means of Thermo-Calc software. The dilatometry measurements were carried for confirming the theoretical results of Thermo-Calc thermodynamic calculations. The results revealed that the microstructure of SiMo ductile cast iron consists of nodular graphite and a ferrite matrix with carbides embedded in the fine precipitates at the grain boundary regions. The type of carbides and the nature of these fine precipitates are discussed according to EDX and SEM results. Adding molybdenum enhanced the wear performance of SiMo by decreasing the weight loss by about 40–70% compared to a Mo-free alloy. This is due to the increased molybdenum carbides, which increase hardness and improve wear resistance in SiMo alloys. The high temperatures have a negative effect on reducing the wear resistance at 250 °C. On the other hand, the wear resistance unexpectedly started to increase at higher temperatures of 500 °C and 750 °C because of the contribution of oxidative wear with abrasive wear by forming a protective oxide layer. Furthermore, the obtained results supported the idea that adding molybdenum improves wear resistance at high temperatures. Hence, SiMo has the potential to be wear-resistant material in wider applications requiring high-temperature wear resistance.

Synthesis and Characterization of Innovative Fe–Cr–Mn Ferritic Stainless Steel by Laser Powder Bed Fusion

Crack development for laser‐fused metallic materials is one major challenge for laser‐based powder bed fusion (LPBF). The cracks are manifested in two forms, i.e., macrocracks along building layers due to spalling and microcracks along the high‐angle grain boundaries. To mitigate the crack formation, a new type of ferritic stainless steel Fe–Cr–Mn are synthesized by LPBF. The well‐developed columnar ferrite grains with an enhanced fine acicular austenite precipitation, which possess Nishiyama–Wassermann (N–W) orientation relationship with the ferrite matrix, can be readily produced when the laser energy density is beyond 100 J mm−3. Porosity can be further reduced below 0.5% by optimizing the laser power and scanning speed. It is found that laser rescanning may eliminate the cracks without modifying the grain morphology, crystallographic texture, and phase distribution.

Role of silicon on formation and growth of intermetallic phases during rapid Fe–Zn alloying reaction

The study of liquid metal embrittlement in Fe–Zn systems is challenging because of the high temperature and vapor pressure of Zn, which hinders in-situ investigations with sufficiently high spatial resolution. This is typically associated with subsecond processing steps and the coexistence of a solid substrate and a liquid Zn phase, which renders direct observations at the microstructural scale difficult. In this study, we comprehensively investigate the reactions occurring during the rapid heating and cooling stages of Fe–Zn systems using synchrotron X-ray diffraction. The phase transformation is analyzed for specimens with different interfacial structures, with focus on changes in the coating microstructure above and below the peritectic temperature (782 °C) of the Fe–Zn system. Advanced high-strength steel (AHSS) variants with 1.5 wt% Si content show a prominent destabilizing effect at the onset of Fe–Zn intermetallic compound formation and a simultaneous deceleration of the liquid Zn depletion rate compared with other AHSS alloys containing 0 wt% Si. Furthermore, the addition of Si suppresses the formation of the Γ phase, which is due to turbulence in the outburst Zn at temperatures above 773 K. Consequently, the fraction of Γ phase compounds with accumulated Zn deceases as the exposure time of the liquid Zn to the ferritic matrix increases owing to the solute redistribution of Si in the liquid Zn during rapid heating. Meanwhile, the absence of silicon in the substrate causes the formation of a ζ phase with a low melting point, which delays the formation of liquid Zn via an increase in the melting point of the Zn layer in the early stage of heating. Additionally, the amount of residual liquid Zn decreases due to the rapid depletion of Zn during the liquid initiation stage via an equilibrium Fe/Zn binary phase transformation. The results of this study provide a deeper understanding of the Fe–Zn intermetallic phase transformation and the sensitivity to liquid metal embrittlement of third-generation AHSSs based on silicon content.

Effect of Intercritical Annealing on the Properties of Dual Phase Steel via Finite Element Method

Dual phase (DP) steels are rapidly becoming more and more popular for automotive applications. They offer a weight reduction with a combination of energy absorption for crash zones. Rails, reinforcements, back panels, cross members, and pillars can be given as application examples. DP steels microstructure consists of a soft ferrite matrix with hard martensite islands. The hard martensite islands provide strength while the ductile ferrite provides formability. The strength level of DP steel is related to the amount of martensite in the microstructure, and the martensite amount can be arranged via intercritical annealing. In this work, thermodynamic analysis of St52 steel was carried out with Thermo-Calc software. A1 and A3 temperatures were determined by calculating the temperature-dependent phase fractions. Intercritical annealing temperatures were determined according to the calculated critical temperatures (A1 and A3). The intercritical annealing process was modelled by using Simheat NxT software. In this modelling and simulation work, the effect of intercritical annealing temperature on the final microstructure and hardness of DP steel was investigated.