Ferrite Matrix Research Articles

Influence of a Nonuniform Microstructure on the Rolling Contact Fatigue Properties of ER8 Steel Wheel

To mitigate the damage caused by rolling contact fatigue (RCF) on wheels, RCF tests are conducted to obtain the RCF limit of the surface area of an ER8 wheel tread that comprised a nonuniform microstructure. The microstructure topography of the RCF area is observed with optical microscopy, a scanning electron microscopy, and a transmission electron microscopy. The hardness, elasticity, and plasticity of different areas of the nonuniform microstructure are analyzed with a nanoindentation instrument. The results show that, besides pearlite and proeutectoid ferrite (matrix), a large amount of upper bainite is present. This component, therefore, interrupts the continuity of the wheel steel matrix, leading to a nonuniform microstructure and an RCF limit of 1074 MPa, which is lower than that of a wheel worn to the limit area and that comprised a normal microstructure. Furthermore, cracks mainly initiate and propagate at the boundary of the upper bainite and matrix. Although the hardness and the elasticity of the upper bainite are higher than those of the matrix, the plasticity is lower. Furthermore, under the effect of the contact stress, the difference between their deformations leads to stress concentration at the boundary, facilitating the initiation and propagation of cracks in steel wheels and accelerating the damage due to RCF.

Influences of Vanadium and Silicon on Case Hardness and Residual Stress of Nitrided Medium Carbon Steels

Medium carbon steels alloyed with two levels each of V and Si were heat treated to form tempered martensite and bainite, followed by nitriding to evaluate their effects on case hardness and residual stress. Microstructures were quantitatively characterized with Vickers microhardness testing, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), conventional transmission electron microscopy (CTEM), and scanning transmission electron microscopy (STEM). Measurements of subgrain size, cementite size, dislocation density, and volume fractions of nitride precipitates were combined in a strength model to predict the contributions of V and Si to maximum case hardness after nitriding. Higher V contents lead to increases in maximum case hardness by increasing the volume fraction of MX precipitates. Increases in Si content lead to increased maximum case hardness by increasing the volume fraction of amorphous (Si and Mn)-containing nitride precipitates; the presence of Mn in these precipitates has not been previously reported. Higher Si contents also lead to increases in solid solution strengthening because the majority of Si remains in solution in the ferrite matrix. Greater volume fractions of MX and (Si and Mn)-containing nitrides both lead to higher magnitudes of compressive residual stress. The results presented here demonstrate that alloying can improve the properties after nitriding, providing insight into advanced nitriding steel grades and potentially enabling a wider range of steel microstructures to be nitrided without a debit in performance.

Investigation of Screw Failed During Installation

Material used for screw production was investigated due to its inconvenient properties which caused the screw breaking on threads or in the head during the installation. Chemical composition of analyzed material corresponded with standard STN 17153 according to technological drawing for specific product. The metallographic analysis showed that failure of screws happened due to improper microstructure resulting from unsuitable thermal treatment of material. Fine inclusions based on aluminum nitride (AlN) and chromium carbonitride (Cr(C,N)) were segregated along the ferritic grain boundaries. Coarse aluminum nitride inclusions (AlN) in ferritic matrix affected the character of present fracture surface characterized by cleavage facettes. The fracture was propagated step by step following the planes with the increased concentration of inclusions across the whole cross-section of the screw.

Crack Resistance Evaluation for 38KhN3MFA-Sh Structural Steel According to Fracture Characteristics and Elastic Wave Velocities

The results of studies on the effects of tempering temperature on the static strength and crack resistance of 38KhN3MFA-Sh steel are presented. The fractured surface texture has been studied after different heat treatment modes using electron fractography analysis. Relationships between the fracture characteristics and the critical stress intensity factor have been established. The effect of tempering temperature on the velocities of ultrasonic bulk waves has been investigated. A linear relationship between the velocity of elastic waves and the critical stress intensity factor of 38KhN3MFA-Sh steel has been found. The revealed relationship makes it possible to estimate the changes in crack resistance of steel using a nondestructive testing method with variations in the tempering temperature. The method of electron fractography has been used to analyze the fractures in the samples of structural high-quality 38KhN3MFA-Sh steel with a crack. The investigations of the fractured surface texture after different heat-treatment modes have shown that the microrelief is represented by flattened cone-shaped pits. The increase in the tempering temperature is accompanied by an increase in the diameter of flattened pit-cones on the fracture surface. A quadratic relationship between the crack resistance parameter and the diameter of the pits has been revealed. It has been shown that the contribution of the structural condition of the ferrite matrix to crack resistance is much more significant than the contribution of isolated carbides. The elastic wave velocities in steel have been measured; their values exhibit an increase with an increase in the tempering temperature. The strength and crack resistance parameters of the structural steel exposed to high-temperature tempering have been predicted on the basis of the values of transverse wave velocities. The deviations of the predicted values of crack resistance K1c and ultimate strength σu from the experimental values do not exceed 5.4 and 12.6%, respectively.

MICROSTRUCTURAL EVOLUTION AND COARSENING KINETICS OF B2-(Fe,Ni)Al IN FERITIC STEEL OF Fe-14Ni-9Al-7,5Cr-1Mo AT HIGH TEMPERATURES.

MICROSTRUCTURAL EVOLUTION AND COARSENING KINETICS OF B2-(Fe,Ni)Al IN FERRITIC STEEL OF Fe-14Ni-9Al-7,5Cr-1Mo AT HIGH TEMPERATURES. Increasing the efficiency of power plant facilities, such as steam power plant, is demanding, and this drives to the development of materials capable for higher temperature operations. Ferritic steels have been widely used for boiler components due to their relatively economical and higher in thermal conductivity. Nevertheless, the application of ferritic steels for boilers is limited only at 630 ℃ due to their weakness in creep strength and corrosion at higher temperatures. Efforts to increase the performance of ferritic steels at higher temperature applications have been intensively carried out, one of which is by developing B2-(Fe,Ni)Al precipitates in the ferrite matrix. This paper discusses the microstructural evolution in alloy of Fe-14Ni-9Al-7.5Cr-1Mo (in wt.%) strengthened by B2-(Fe,Ni)Al precipitates during heating at 800, 900, and 1000 oC for 6, 20 and 48 hours. Ageing at 800 oC for 6 hours gave rounded cuboidal B2-(Fe,Ni)Al precipitates dispersed homogeneously in the ferrite matrix and changed to rounded shape at longer ageing times. At 1000 oC, however, the precipitates had rounded shape at all times of ageing. Coarsening of precipitates occurred during ageing at 800 and 1000 oC with the rate constants of 5,7 x 104 nm3/h and 2,1 x106 nm3/h respectively, which is considered relatively low. The highest hardness value of 520 HVN was observed for the sample aged at 1000 °C for 6 hours.Keywords: Ferritic steels, B2-(Fe,Ni)Al precipitate, coarsening, microstructure evolution

Effect of filler metal on solidification, microstructure and mechanical properties of dissimilar super duplex/pipeline steel GTA weld

In the present experimental work, an attempt has been made to investigate the effect of filler metal on solidification, microstructure, and mechanical properties of the dissimilar weld between super duplex stainless 2507 and high strength low alloy API X70 pipeline steel. This joint is widely used in offshore hydrocarbon drilling risers and oil-gas transportation pipelines. Welds have been fabricated by the gas tungsten arc welding process using super duplex 2594 and austenitic 309L grade filler. A comparative assessment of both the fillers has been done by investigating multiple aspects of weld's structural integrity. 309L fillers weld has skeletal ferrite morphology, whereas 2594 filler solidifies with precipitation of multiple reformed austenites in the ferrite matrix. Lower width unmixed zone is formed on the API X-70 side in 2594 filler as compared to 309L. The tensile and impact strength of 309L filler weld is superior to 2594 filler. Precipitation of reformed austenite in the weld zone leads to lower hardness in 2594 as compared to skeletal ferrite dominated 309L weld zone. High chromium concentration in the weld zone results in superior pitting corrosion resistance of 2594 filler. • Filler 309L solidifies with skeletal ferrite morphology in weld zone. • Filler 2594 weld zone has multiple reformed austenite formation. • Tensile, impact strength and hardness of 309L filler weld is superior to 2594 weld. • 2594 filler weld has better pitting corrosion resistance than 309L filler weld. • Unmixed zone on X70 side is narrower in 2594 filler weld.

Mesoscopic Analysis of Deformation Heterogeneity and Recrystallization Microstructures of a Dual-Phase Steel Using a Coupled Simulation Approach

Microstructure-based numerical modeling of the deformation heterogeneity and ferrite recrystallization in a cold-rolled dual-phase (DP) steel has been performed by using the crystal plasticity finite element method (CPFEM) coupled with a mesoscale cellular automaton (CA) model. The microstructural response of subsequent primary recrystallization with the deformation heterogeneity in two-phase microstructures is studied. The simulations demonstrate that the deformation of multi-phase structures leads to highly strained shear bands formed in the soft ferrite matrix, which produces grain clusters in subsequent primary recrystallization. The early impingement of recrystallization fronts among the clustered grains causes mode conversions in the recrystallization kinetics. Reliable predictions regarding the grain size, microstructure morphology and kinetics can be made by comparison with the experimental results. The influence of initial strains on the recrystallization is also obtained by the simulation approach.

Effect of trace solute titanium on plastic deformation of α-(Fe, Cr) single-crystal micropillars fabricated from 18Cr ferritic stainless steel

Abstract The effect of a trace amount (~0.1 at.%) of solute Ti on the compression response of cylindrical single-crystal micropillars fabricated from a 18Cr ferritic stainless steel was investigated. The micropillars of the solution-treated steel samples were fabricated on a special grain with different diameter ranges (d = 2–3 μm or 5–6 μm). By this process, the flow stress required for slip initiation (corresponding to the yield stress) and its strain rate dependence could be determined. The compression tests on the micropillars revealed that Ti addition (solute Ti in the α-(Fe, Cr) ferrite matrix) has a slight effect on the activated slip system and stress level required for the slip initiation of 18Cr steel specimens with two different sizes. In addition, the strain rate sensitivity (m) of the yield stress at an offset strain of 0.2% (0.2% proof stress) in the small micropillars (d = 2–3 μm) of the micron-sized 18Cr steel specimens decreased slightly (from 0.12 to 0.08), owing to the Ti addition. However, the m value (0.04) of the large micropillars (d = 5–6 μm) of the 18Cr steel is unaffected by Ti addition and is equivalent to the m value measured for millimeter-sized specimens (conventional bulk specimens). The low activation volume (v*) values (18–19 b3) were obtained for the small micropillars of both the 18Cr and Ti-added steels. For the large specimens, the v* value of the 18Cr steel decreased slightly from 50 b3 to 46 b3 owing to the Ti addition and is close to the value obtained from the millimeter-sized specimens. These results indicate that the trace amount of solute Ti has a slight effect on the thermal activation process for controlling the dislocation motion in the 18Cr ferritic stainless steels. The intriguing low activation volume in the small micropillars is discussed in comparison with the results of micron-scale specimens of various bcc metals and alloys.

Role of Martensite Structural Characteristics on Corrosion Features in Ni-Advanced Dual-Phase Low-Alloy Steels

The relationship between the corrosion resistance and martensite structure of Ni-advanced dual-phase weathering steel was studied using transmission electron microscopy, scanning electron microscopy, electrochemical analysis, and atomic force microscopy. The investigations indicate that the final microstructure of the dual-phase weathering steel was composed of a large amount of low-carbon lath martensite distributed in the ferrite matrix. The potential of the martensite phase is higher than that of ferrite, which acts as a microcathode. As the martensite volume fraction in the Ni-advanced dual-phase weathering steel increased, the corrosion rate increased owing to the greater galvanic couple formed between the ferrite and martensite from the increasing ratio of the cathode area to the anode area. In addition, this work provides a method to obtain advanced weathering steel with improved mechanical properties and corrosion resistance.

Microstructure characterization and tensile properties of hot isostatic pressed China ultrahigh strength steel

Powder metallurgy hot isostatic pressing (HIP) is a promising technology for producing complex ultrahigh strength steel (UHSS) components. In the present study, a kind of China UHSS, 30CrMnSiNi2A steel, was prepared by HIP with different processing temperatures. The microstructure and tensile properties of HIPed compacts were analyzed in depth via scanning electron microscopy, X–ray diffraction, electron back–scattered diffraction, transmission electron microscopy, tensile testing and nanoindentation experiment. Results show that all HIPed compacts are granular bainitic structure composed of bainitic ferrite matrix and M–A islands. With the HIP temperature increasing, the volume fraction of M–A islands decreases, and the average grain size increases. Besides, a harmful structure (that is prior particle boundaries, PPBs), which is composed of a great number of Mn–rich and Si–rich oxide inclusions, is formed in the HIPed compacts. Notably, increasing HIP temperature is beneficial to eliminate PPBs. The volume fraction of M–A islands, grain size and the state of PPBs together determine the tensile properties of HIPed compacts, in which the state of PPBs has a most significant impact. With the HIP temperature rising, the tensile fracture mode changes from inter–particle debonding to transgranular quasi–cleavage, which benefits from the elimination of PPBs.

The effects of a ferritic or martensitic matrix on the tensile behavior of a nano-precipitation strengthened ultra-low carbon Ti–Mo–Nb steel

This work investigates the correlation between microstructure and mechanical properties of a nano-precipitation strengthened ultra-low carbon (NPS- ULC) Ti–Mo–Nb steel. Two types of matrix microstructures (ferrite and martensite) with nano-precipitates were obtained through hot rolling and isothermal transformation in the case of ferrite, or by quenching and tempering, for martensite. The martensitic microstructure showed increases in both YS and UTS by ~110 and ~100 MPa, respectively, over the ferritic microstructure without sacrificing tensile ductility. All ferritic and martensitic specimens exhibited a two-stage work hardening behavior with different work hardening rates at high strain levels. Quantitative analysis of the strengthening contributions confirms that the increase in yield stress of the NPS-ULC specimens with a martensitic matrix was a result of the fine martensitic laths as well as the higher dislocation density and nano-precipitates.

Application of advanced experimental techniques to elucidate the strengthening mechanisms operating in microalloyed ferritic steels with interphase precipitation

The microstructure and tensile properties of thermo-mechanically processed ferritic steels alloyed with Mo, Cr, V and Nb were investigated using scanning transmission electron microscopy, atom probe tomography and uniaxial tensile loading. The microstructures consisted of predominantly polygonal ferrite (~4 ± 3 µm average grain size) and a high number density of nano-sized, disc-shaped interphase precipitates (~2-4 nm diameter and 1 ± 0.4 nm thick). Interphase fiber carbides were also present in V, Cr and Nb -alloyed steel. Cluster formation was observed both between the interphase precipitates in rows and in ferrite matrix between the rows. The individual contributions to yield strength arising from different operating strengthening mechanisms were estimated. For the first time, the operation of coherency and modulus strengthening mechanisms for nano-sized interphase precipitates in this class of steels was considered. The major contributions were due to grain boundary strengthening and nano-sized precipitation, in the order of ~280 MPa and ~395 MPa, respectively. Significant cluster strengthening (~150-170 MPa) was also evident.

In Situ Determination of Phase Stress States in an Unstable Medium Manganese Duplex Steel Studied by High-Energy X-ray Diffraction

Duplex medium Mn steels are high-potential advanced high-strength steels (AHSS) for automotive construction. Their excellent forming properties stem from the specific stress partitioning between their constituting phases during deformation, namely the ferritic matrix, unstable retained austenite, and strain-induced fresh martensite. The stability of the retained austenite and the 3D stress tensors of each phase are determined simultaneously in this work by in situ high energy X-ray diffraction on synchrotron beamline during a tensile test. The role of internal stresses inherited from the manufacturing stage are highlighted for the first time as well as new insights to understand the origin of the serrations shown by these alloys.

The Initial Precipitation Behavior of Copper in Ferritic Stainless Steel

The precipitation of a suitable amount of Cu phase is the key to the antibacterial performance of Cu-containing antibacterial stainless steel. In order to study the precipitation behavior of Cu in ferritic stainless steel, the nucleation rate–temperature curve and phase transition kinetics curve of the Cu-rich phase were calculated theoretically. The distribution, size, and composition of Cu precipitates were characterized, and the distributions of Ni and Mn in the Cu precipitates at different annealing times were also analyzed by atom probe tomography and transmission electron microscopy. The crystal structure of the Cu precipitates was observed by high-resolution electron microscopy. With an increase in annealing temperature, the nucleation rate decreases and the time for nucleation of Cu precipitates increases monotonously. Accordingly, the size of the Cu-rich phase increases, and the number density decreases with increasing annealing time. Moreover, the contents of Ni and Mn gradually increase, and the Ni and Mn atoms aggregate on the surface of the Cu-rich phase. When the steel is annealed for 60 s, the Cu precipitate in the ferrite matrix exhibits a 9R structure with multiple twins.

A New Heat Treatment Cycle Design for High Wear Resistant in PM Steels

In this study, 1.5% by weight of natural graphite powders were added to Atomet 1001 pure iron powders and unalloyed high carbon powder metallurgy steel specimens were obtained. After the mixture was uniaxial pressed at room temper-ature and 700 Mpa pressing pressure, it was sintered in Ar protected atmosphere controlled oven at 1150 ° C for 20 minutes. The densities of the specimens before and after sintering were measured using precision scales and electronic calipers and presented graphically. After sintering, microstructure specimens containing primary cementite and typical dense perlite colonies were produced. The sintered specimens having primary cementite plus lamellar pearlitic structures were fully quenched from 950 °C temperature and then over-tempered at 705 °C temperature for 60 minutes to produce spherical-fine cementite particles in the ferritic matrix. After by this treatment, these specimens annealed at 735 °C temperature for 3 minutes were austempered at 300 °C salt bath for a period of 1 to 5 hours. After the heat treatment, dry sliding wear tests of specimens were carried out under 10N constant load and 500-1500m wear distance. At the end of the dry sliding wear test, the weight losses of the specimens were measured and it was determined that the dry sliding wear resistance increased with increasing austempering time after spheroidization annealing.

Influence of Cobalt in the Tensile Properties of ½ Inch Ductile Iron Y-blocks

The ferrous industry keeps evolving, and the demand for castings with complex geometries is increasing. Due to this, some foundries are facing several challenges when it comes to producing highly complex parts with a specific microstructure. Normally, heat treatment is performed when a ferritic matrix is desired. However, distortion and cracking can become a problem too. Because of this, it is important to explore alternative methods that can potentially help with these problems. In ductile iron, cobalt additions are known to increase the nodule count, which favors higher ferrite fractions. Hence, the addition of cobalt was studied to investigate its effects on the microstructure and tensile properties of ductile iron. Five heats were produced and cast into ½ inch ASTM A536 Y-blocks: 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt% Co. Metallography was performed to evaluate the percent nodularity, nodule count (N/mm2), and ferrite/pearlite percentages. Tensile testing was executed using sub-size round samples. Brinell hardness and micro-Vickers were conducted on each Y-block to assess the macro and microscopic behavior of the cobalt bearing ductile iron. The addition of 4 wt% Co was found to decrease the nodule size and increase the percent nodularity and nodule count resulting in higher ferrite contents. Cobalt did not have a statistically significant effect in tensile strength and percent elongation. However, cobalt was found to increase the yield strength due to the solid solution strengthening effect in ferrite.

Sequential ion irradiations on Fe-Cr and ODS Fe-Cr alloys

• Sequential dual He/Fe irradiation at RT of a model Fe14Cr alloy and an ODS steel. • Both show small (<2nm) irradiation induced bubbles, mostly in the ferritic matrix. • TEM investigations of the ODS steel suggest that nanoparticles remain quite stable. • DB PAS suggest that He associates with vacancies introduced by the Fe+ irradiation. Oxide dispersion strengthened steels are candidate materials for nuclear reactor applications due to a powerful combination of properties, such as reduced activation, high-temperature strength and increased creep resistance. The dispersion of nanometric oxide particles in the steel matrix may also enhance radiation resistance by acting as trapping sites for irradiation induced defects. In this work, an Fe–14Cr–2 W–0.3-Ti–0.3Y 2 O 3 (wt%) steel and a model Fe-14Cr (wt%) alloy were sequentially irradiated with He + and Fe + ions up to 15 dpa and 8000 appm to simulate fusion radiation damage. Their microstructural stability was investigated by positron annihilation spectroscopy and transmission electron microscopy. Transmission electron microscopy studies show that under these irradiation conditions there are no significant changes in the mean size, qualitative chemical composition and number density of nanoparticles, although the irradiation appears to induce a slight coarsening of the smaller nanoparticles. Both materials exhibit very small (<2 nm) irradiation-induced bubbles, with similar sizes but lower number density in the ODS steel. Positron annihilation spectroscopy results show the presence of irradiation induced open volume defects, much more noticeable in the model alloy. In both alloys, helium appears to associate with the newly formed vacancy-type defects introduced by the subsequent Fe + irradiation.

Microstructure and Mechanical Properties of Forged High Yttria 18Cr-ODS Steels

Oxide dispersion strengthened (ODS) ferritic steels are candidate materials for clad tubes in the upcoming Generation IV nuclear reactors. In the present work, a powder forging consolidation technique has been used for fabrication of ODS steels. Two alloys having nominal compositions (in weight %) of Fe-18Cr-2W-0.285Ti-0.5Y2O3 and Fe-18Cr-2W-0.571Ti-1Y2O3, respectively, have been studied in this work. The alloys were prepared by mechanical alloying of elemental powders with yttria in a Simoloyer high energy horizontal attritor. The milled powders were consolidated at 1473 K by powder forging in a flowing hydrogen gas atmosphere. Yttria to titanium ratio was kept constant at ~ 1.75 for both the alloys. TEM micrographs of the forged alloys showed fine recrystallized grains with a dispersion of nano-size Y-Ti-O oxide particles. High-resolution transmission electron microscope fringes and the corresponding fast Fourier transformation confirmed the presence of orthorhombic Y2TiO5 oxide particles in a ferrite matrix. These were the predominant oxide particles in the forged alloys. The Y2TiO5 particles were incoherent with the matrix and exhibited a cuboidal morphology. Despite their high yttria content, both the alloys showed high tensile strength and ductility at room temperature and 973 K. Reasons for this are discussed.

Use of the Correlation between Grain Size and Crystallographic Orientation in Crystal Plasticity Simulations: Application to AISI 420 Stainless Steel

Crystal plasticity models attempt to reproduce the complex deformation processes of polycrystalline metals based on a virtual representation of the real microstructure. When choosing this representation, a compromise must be made between level of detail at the local level and statistical significance of the aggregate properties, also taking into account the computational cost of each solution. In this work, the correlation between crystallographic orientation and grain size is considered in the definition of virtual microstructures for the simulation of the mechanical behavior of AISI 420 stainless steel (consisting of a ferrite matrix with large carbide precipitates), in order to improve the accuracy of the solution without increasing model complexity or computation time. Both full-field (DAMASK) and mean-field models (Visco Plastic Self Consistent (VPSC)) are used together in combination with experimental results to study the validity of the assumptions done in each of the models.

Effect of creep temperature on Z-phase formation in heat-resistant 9% Cr–3% Co martensitic steel

The Z-phase (CrVN) precipitation in a 9% Cr–3% Co martensitic steel during creep at 923 K and 948 K has been investigated with aim to establish the effect of temperature on the nucleation mechanism of these particles and their coarsening behavior. Ostwald ripening of VX carbonitrides strongly affects these processes. An increase in creep temperature significantly accelerates the transformation of the nanoscale MX carbonitrides into the Z-phase particles causing the Z-phase nucleation in a shorter time. Two different mechanisms of the Z-phase precipitation have been observed. Firstly, for creep tests at 923 K and 948 K with creep rupture time longer than 2000 h, the formation of the stable Z-phase particles with a tetragonal lattice occurs through in-situ transformation of a cubic lattice of the MX carbonitrides leading to a continuous flux of Cr atoms from the ferritic matrix into these particles. The coarsening Z-phase occurs at the expense of dissolution of VX carbonitrides. Secondly, for creep test at 948 K with duration of 773 h, the strain-induced metastable Z-phase with the cubic lattice nucleates on the V-rich MX/ferrite interfaces with following transformation into the stable Z-phase with the tetragonal lattice under creep testing with longer duration. Concurrently, extensive coarsening Cr-rich VX carbonitrides occurs independently. As a result, coarsening Z-phase leads to insignificant dissolution of VX carbonitrides. The creep strength breakdown appearance is not related to the formation and/or coarsening of the Z-phase at both temperatures.