Ferrite Fraction Research Articles

Modeling the microstructure evolution during quenching & partitioning of a conventional CrMo alloy steel

This work presents the simulation approach on the phase transformation and the microstructure evolution of a CrMo steel containing low Si processed with a typical Q&P cycle. The phase transformations at all steps of the Q&P were modelled using kinetics modelling and phase field model in order to predict the austenite grain size, the subsequent fraction and size of microstructural constituents. The initial austenite grain size was predicted considering the solute drag effect due to the presence of Cr obtained from kinetics simulation. The results showed that the austenite grain growth is restricted due to solute drag effect of Cr revealing that the heating stage is important for estimating the austenitic grain size. During quenching, the fraction of initial martensite was predicted. The results were aligned with dilatometry data indicating that the martensite fraction can be predicted via the phase field method. At the partitioning stage, the carbon content in the microstructural constituents determines the fraction of bainitic ferrite and retained austenite at the end of partitioning. In order to predict accurately the carbon content in the microstructure, it is important to simulate the carbon diffusion in the 2nd heating and prior to bainitic transformation. The results showed that the remaining austenite is enriched in carbon. During bainitic transformation, carbon atoms diffuse from bainitic ferrite sheaves towards austenite. By the time C concentration in austenite increases up to para – equilibrium value, austenite is stabilized. Due to fine PAGBs, the enrichment of retained austenite in C can be accelerated; thus smaller partition time can be applied for the stabilization of austenite and to complete the partitioning stage. Phase field results were in agreement with SEM/EBSD results showing that phase transformations can be predicted at all stages of Q&P process.

Structure of the Surface Region of Stainless Steel: Bulk and Thin Films

The surface region of austenitic stainless steel (SS) is investigated by synchrotron X‐ray powder diffraction (XRPD) and X‐ray absorption near edge structure (XANES) measurements, because its composition and structure are crucial for the corrosion resistance of SS. Grazing incidence XRPD of a polished AISI 304 bulk steel sample shows that the near‐surface structure is modified. The concentration of the ferrite phase of Fe, a typical minority phase in AISI 304, increases gradually from 10% to 30% when approaching the surface from 150 nm depth. XANES Fe K‐edge investigations of ultrathin, sputter‐deposited films also reveal much larger ferrite fractions than expected from the austenitic steel composition of the films. Reasons for the increased ferrite fraction in the surface region of bulk steel and thin films are discussed. However, right at the surface, the trend reverses. Analysis of XANES data for an ultrathin, 4 nm SS film shows that 80% of Fe is oxidized and 20% of metallic Fe is present only in austenite structure, suggesting that ferritic iron is preferentially subject to oxidation. The austenitic Fe is located at more than 2–3 nm below the surface where the Ni concentration is >10%.

Deformation behaviour and microscopic investigations of cyclically loaded railway wheels and tyres

Abstract Railway wheel and tyre steels were investigated by specimens from defined volume sections representing the local microstructure of original components. As a consequence of the industrial heat treatment and the size of the components, microstructural gradients exist in the wheel rim and tyre crosssection. The ferrite fraction of the ferritic – pearlitic microstructure and the cementite lamellae spacing vary depending on the distance to the running surface (tread). Stress-controlled load increase and single-step push– pull tests were carried out at ambient temperature with servohydraulic testing systems to study fatigue phenomena with special consideration of microstructural details. Mechanical stress– strain hysteresis, thermometrical, and electrical measurements were used for a comprehensive characterization and evaluation of the cyclic deformation behaviour. The measured physical quantities show a strong interrelation with the underlying fatigue process. Light, scanning, and transmission electron microscopy (TEM), together with digital image processing, allow to interpret the measured data on the basis of microstructural details. In defined fatigue states, TEM investigations were performed to correlate the loading conditions with the appearing dislocation structures.

Benchmarking a 9Cr-2WVTa Reduced Activation Ferritic Martensitic Steel Fabricated via Additive Manufacturing

Reduced activation ferritic (RAF) martensitic steels are promising candidates for the first wall of fusion reactors. However, current manufacturing capabilities call for these components to be made by welding wrought plates. This limits design freedom and necessitates the use of post-weld heat treatments (PWHT) in accordance with the boiler and pressure vessel code. Additive manufacturing (AM) can offer a unique solution to solve this challenge by leveraging the layer-wise deposition strategy to come up with temper bead deposition techniques to eliminate post-processing heat treatments (PPHT). However, it is necessary to benchmark the properties of RAF steels fabricated by AM with their wrought counterparts to identify the process-structure-property correlation, which is the goal of this study. The study demonstrates that while tensile properties at room temperature and high temperatures are satisfactory, the as fabricated and samples after PPHT have significant heterogeneity in tensile elongation. This has been attributed to the presence of discontinuities in the build. The as-fabricated samples have an average tensile strength of 1190 + 12 MPa and an average elongation of 15 + 5% at room temperature and 658 ± 20 MPa ultimate tensile strength (UTS) and 14 ± 7% at 600 °C. After the post-weld heat treatment, mechanical properties decrease to around 600–650 MPa and an elongation between 20–25% at room temperature to 300 MPa UTS and 25–28% elongation at 600 °C. The characterization of microstructures at various length scales demonstrates that the as-fabricated structure has a significant fraction of delta ferrite in a lath martensitic matrix. No precipitates could be identified in the as-fabricated structure. PPHT led to a decrease in the area fraction of delta ferrite and precipitation of M23C6 and MX. Detailed characterization clearly demonstrates that the lack of precipitates in the as-fabricated structure could be due to the slow tempering response of the alloy. Finally, the needs to develop new alloys to achieve the objectives stated above are articulated.

Modelling the continuous cooling transformation diagram of engineering steels using neural networks

Abstract The neural network model of Van der Wolk et al. [1] describes the effect of composition on the phase regions of the continuous cooling transformation (CCT) diagram, yet does not consider the fractions of microstructural components and the hardness data that are often quoted in CCT diagrams. In the present paper, the construction of two more neural network models, one for the fractions of ferrite, pearlite, bainite and martensite in the microstructure, and one for the hardness after cooling, using the data of 338 and 412 diagrams, respectively. The accuracy of each model was found to be similar to the expected experimental error; moreover, the models were found to be mutually consistent, although they have been constructed independently. Furthermore, the trends in these properties for alloying elements can be quantified with the models, and are largely in line with metallurgical expectations.

Quasi-Non-destructive Characterization of Carburized Case Depth by an Application of Centerless X-ray Diffractometers

A new application of a centerless x-ray diffractometer was proposed that goes beyond residual stress measurements during the regular operation of such equipment. During depth residual stress measurement, it is possible to fit the measured interference function of the ferrite (211) with a different number of curves as a function of depth. The different curves corresponded with the appearance of ferrite fractions with different grain sizes and concentrations in ferrite, perlite, bainite, or martensite. Accordingly, different microstructures were non-destructively detected through the thickness of the carburized layer, and a good correlation was found with results from metallography and hardness testing. This novel method was validated on solid and gas carburized samples, but more work is needed to apply it to other microstructures.

On the influence of microstructure on the neutron irradiation response of HIPed SA508 steel for nuclear applications

The neutron irradiation response of a novel hot isostatically pressed SA508 Grade3 steel was studied, such to deduce any influence this unconventional RPV microstructure has on radiation response. In particular, the role of elevated ferrite fraction was investigated. Neutron irradiation was conducted at 155 ± 10°C to induce 0.1dpa of damage, this corresponded 2.53 ± 0.63 GPa hardening for the ferrite and 1.94 ± 0.57 GPa for the bainite as measured by nanoindentation. Atom probe tomography detected the presence of Mn-Ni-Si type clusters in both microconstituent phases. The ferrite microstructure showed a greater percentage of solute atoms available to form clusters than bainite, but it also contained a lower cluster volume fraction and number density compared to the bainite.

Электромагнитные свойства полимерных композитов Li-=SUB=-0.33-=/SUB=-Fe-=SUB=-2.29-=/SUB=-Zn-=SUB=-0.21-=/SUB=-Mn-=SUB=-0.17-=/SUB=-O-=SUB=-4-=/SUB=-/П(ВДФ-ТФЭ) в области частот 100-7000 МГц

The article describes electromagnetic and microwave properties of the polymer composite with the lithium spinel ferrite inclusion of composition Li0.33Fe2.29Zn0.21Mn0.17O4 in the frequency range 100 MHz - 7000 MHz. It is shown that samples with a mass fraction of ferrite 60, 80% have pronounced radio-absorbing properties, measured using the reflection coefficient on a metal plate (return losses). For a composite with 80% ferrite, the minimum return loss was -37.5 dB at 2.71 GHz with an absorption width at -10 dB of 3 GHz. High absorption characteristics are directly related to the use of ferroelectric polymer P(VDF-TFE) as a binder, which is expressed in the combined action of the absorption mechanisms of the magnetic and ferroelectric phases.

Electromagnetic properties of polymer composites Li-=SUB=-0.33-=/SUB=-Fe-=SUB=-2.29-=/SUB=-Zn-=SUB=-0.21-=/SUB=-Mn-=SUB=-0.17-=/SUB=-O-=SUB=-4-=/SUB=-/P(VDF-TFE) in the frequency range 100-7000 MHz

The article describes electromagnetic and microwave properties of the polymer composite with the lithium spinel ferrite inclusion of composition Li0.33Fe2.29Zn0.21Mn0.17O4 in the frequency range 100-7000 MHz. It is shown that samples with a mass fraction of ferrite 60, 80% have pronounced radio-absorbing properties, measured using the reflection coefficient on a metal plate (return losses). For a composite with 80% ferrite, the minimum return loss was -37.5 dB at 2.71 GHz with an absorption width at -10 dB of 3 GHz. High absorption characteristics are directly related to the use of ferroelectric polymer P(VDF-TFE) as a binder, which is expressed in the combined action of the absorption mechanisms of the magnetic and ferroelectric phases. Keywords: lithium ferrite, polymer composite, polyvinylidene fluoride, radio-absorbing material.

Effect of Step Quenching Heat Treatments on the Kinetics of Ferrite Formation and Quenching & Partitioning Modeling for a Commercial C-Mn-Si Steel

Aiming to meet current demands from the automotive industry, a third generation of advanced high strength steel (AHSS) has been developed as an alternative to previous generations. Different alloys and innovative processes have been deeply studied as, for example, quenching and partitioning (Q&P). The published papers highlight that the best heat treatment parameters for a satisfactory Q&P execution are strongly dependent on the austenite conditioning and they can be optimized if thermodynamic and kinetics calculations are performed. In this context, this work evaluated the effect of step quenching (SQ) heat treatments on the kinetics of ferrite formation and Q&P modeling for a commercial C-Mn-Si steel, predicting the microstructural evolution and the final phase fractions as a function of the heat treatment parameters. The JMAK model was optimally fitted to the ferrite fraction, as well as to the microhardness data. The kinetics characterization and the thermodynamic modeling showed that the combination of SQ and Q&P can provide a high retained austenite fraction in a multiphase microstructure. As a conclusion, it is possible to state that a well-planned SQ heat treatment followed by an optimized Q&P cycle has the potential to generate an advanced steel with a final microstructure assisted by the TRIP effect.

Gibbs energy and phase-field modeling of ferromagnetic ferrite (α)→ paramagnetic austenite (γ) transformation in Fe–C alloys under an external magnetic field

Quantitative simulation of microstructure evolution during heat-treatment of alloys under external magnetic field represents a severe challenge. In the present work, the Gibbs free energy contribution from external magnetic field is considered and the experimental Fe–C phase diagram under magnetic field is well reproduced by a thermodynamic model including that contribution. This energy in conjunction with bulk Gibbs energy, demagnetizing field energy and interfacial energy are involved in a developed phase-field model to further bridge the gap between experimental findings and simulative investigations, which is then employed to simulate the microstructure in Fe–C alloys with or without magnetic field. It indicates that the experimental two-phase microstructure (ferromagnetic ferrite α and paramagnetic austenite γ) accompanying the chemically-driven phase transformation in Fe–0.4 and Fe–0.6 wt%C alloys under magnetic field can be quantitatively described by the present phase-field simulation. The predicted phase fraction of ferrite in Fe–C system, which agrees reasonably with the experimental one, is an example to highlight the importance of thermodynamics in tailoring microstructure and properties of realistic alloys with applied magnetic field. This work demonstrates that CALPHAD coupled phase-field model can describe microstructural evolution quantitatively and provides an effective approach for the simulation of corresponding phase transition with external field.

Effect of Ce Content on Microstructure-Toughness Relationship in the Simulated Coarse-Grained Heat-Affected Zone of High-Strength Low-Alloy Steels

The study aimed to identify a moderate degree of Ce addition to improve the toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels, based on the effect of the Ce content on particle characteristics, microstructure and impact toughness. Three steels with 0.012 wt.%, 0.050 wt.% and 0.086 wt.% Ce content were subjected to 100 kJ/cm heat input in their thermal welding cycles. The particles and microstructures in the simulated CGHAZ of each steel were characterized and the impact-absorbance energy levels were measured at −20 °C. The results indicated that Ce2O2S inclusion compounds were gradually modified to CexSy-CeP and CeP with the increasing of the Ce content. A higher fraction of acicular ferrite was formed in the 0.012 wt.%-Ce-treated steel due to the lower mismatch between Ce2O2S and α-Fe. Furthermore, a lower fraction of M-A constituent was obtained in the 0.012 wt.%-Ce-treated steel. As a result, superior toughness and a typical amount of ductile fracture were detected in the simulated CGHAZ of the 0.012 wt.%-Ce-treated steel. Compared with the 0.012 wt.%-Ce-treated steel, a smaller prior austenite grain was observed in the 0.086 wt.%-Ce-treated steel because of the segregation of CeP at the grain boundary. However, the larger size and density of CeP led to poor toughness in the CGHAZ of the 0.086 wt.%-Ce-treated steel.

The Kinetics of Phase Transition of Austenite to Ferrite in Medium-Carbon Microalloy Steel

To reduce energy and resource consumption, high-strength hot-rolled rebars with yield strengths of ≥400 MPa (HRB500) and ≥500 MPa (HRB600) have been designed and produced in recent years. Optimizing the microstructure in the steel to improve strength necessitates determining the kinetics of the phase transition of austenite to polygonal ferrite. Therefore, in the study, the effect of temperature and holding time on the volume fraction of ferrite is investigated in HRB500 and HRB600 steels. Experimental results show that the ferrite percentage initially increases with an increase in temperature and then decreases as the temperature increases from 600 to 730 °C. The optimum temperature range is 680–700 °C for HRB500 steel and 650–680 °C for HRB600 steel. Based on the Johnson–Mehl–Avrami equation, phase transition kinetic models are established. Model predictions are consistent with the validation data. Thus, this study establishes a reference for studying ferrite formation during cooling.

Effect of Alloying Elements and Microstructure on the Dynamic Strain Aging Behavior of 1.25Cr-0.5Mo and 2.25Cr-1Mo Steels

The effect of alloying elements and microstructure on the dynamic strain aging (DSA) behavior of 1.25Cr-0.5Mo (P11, ASTM 335Gr.P11) and 2.25Cr-1Mo (P22, ASTM 335Gr.P22) steels was investigated. For both steels, different cooling conditions such as air-cooling (AC) and oil-quenching (OQ) were applied. Tensile tests were conducted in the temperature range of 20-450 ℃ and a strain rate in the range of 6 × 10<sup>-5</sup>- 6 × 10<sup>-3</sup> s<sup>-1</sup> for the steels with different cooling conditions. The P11AC steel showed serration behavior over a wider temperature range and exhibited higher ultimate tensile strength (UTS) than for the P22AC steel. This is attributed to the effects of alloying elements (Cr, Mo and Si) due to dissolved C, and the ferrite fraction on mechanical behavior. Meanwhile, the P11AC and P11OQ steels also showed different behaviors for DSA starting temperature, DSA temperature range, and serration type. The AC condition showed higher UTS from the interaction solid solution hardening (ISSH) effect due to substitutional Cr, Mo, and interstitial C elements. The calculated activation energy value (Q) for the P11 steel was around 94-103 kJ/mol<sup>-1</sup>, similar to that of ferritic steels, and it was higher for the P22 steel, with a Q value of 233 kJ/mol<sup>-1</sup> from the ISSH effect.

Influence of the formulation of a flux-cored wire on the microstructure and hardness of welded metal

For this paper, the microstructure and hardness of the weld metal were investigated by conducting experiments with the flux cored arc welding process in underwater and air conditions. A rutile/oxidizing tubular wire was used, manufactured by the Robotics, Welding and Simulation Laboratory at Minas Gerais Federal University, especially for underwater wet welding. Underwater welds had a lower volumetric fraction of acicular ferrite in the weld metal compared to air welds. In the thermally affected zone, for both welds, there was a predominant formation of martensite. However, the grain size and width of the thermally affected zone of underwater welds are smaller. The hardness values shown correspond to the microstructure formed in the weld metal. On the other hand, in the region of the thermally affected zone, the hardness values were higher underwater welds, due to the smaller martensite grains presented.

Influence of Cooling Rate on Microstructure Formation of Si–Mo Ductile Iron Castings

The present study highlights the effect of the cooling rate on the microstructure formation of Si–Mo ductile iron. In this study, experiments were carried out for castings with different wall thicknesses (i.e., 3, 5, 13, and 25 mm) to achieve various cooling rates. The simulation of the cooling and solidification was performed through MAGMASOFT to correlate the cooling conditions with the microstructure. The phase diagram of the investigated alloy was calculated using Thermo-Calc, whereas the quantitative metallography analyses using scanning electron microscopy and optical microscopy were performed to describe the graphite nodules and metallic matrix morphologies. The present study provides insights into the effect of the cooling rate on the graphite nodule count, nodularity, and volumetric fractions of graphite and ferrite as well as the average ferritic grain size of thin-walled and reference Si–Mo ductile iron castings. The study shows that the cooling rates of castings vary within a wide range (27 °C–1.5 °C/s) when considering wall thicknesses of 3 to 25 mm. The results also suggest that the occurrence of pearlite and carbides are related to segregations during solidification rather than to cooling rates at the eutectoid temperature. Finally, the present study shows that the longitudinal ultrasonic wave velocity is in linear dependence with the number of graphite nodules of EN-GJS-SiMo45-6 ductile iron.

Synthesis and Magnetic Properties of Na-La-Co System M-type Ferrite

In this study, we investigated the synthesis of Na-La system M-type ferrite with different mole ratios. Some part of Fe in the M-type ferrite was substituted by Co, and variation of fraction of the M-type ferrite as function of mole ratio was investigated. Sample with/without Co-substitution composed of multiphases of M-type ferrite and other phases. Fraction of M-type ferrite in the samples was depending on mole ratio in the raw material. Curie temperatures of Co-free and Co-substituted M-type ferrite were approximately 432℃ and 428 ℃, respectively. Saturation magnetisation of the Co-free NaSUB0.5/SUBLaSUB0.5/SUB system M-type ferrite was approximately 72.5 emu/g.

Inter-relationship between microstructure evolution and mechanical properties in inertia friction welded 8630 low-alloy steel

The evolution of microstructure and mechanical properties in AISI 8630 low-alloy steel subjected to inertia friction welding (IFW) have been investigated. The effects of three critical process parameters, viz. rotational speed, friction and forge forces, during welding of tubular specimens were explored. The mechanical properties of these weld joints, including tensile and Charpy V-notch impact were studied for determining the optimum welding parameters. The weld joints exhibited higher yield strength, lower hardening capacity and ultimate tensile strength compared to base metal (BM). The maximum strength and ductility combination was achieved for the welds produced under a nominal weld speed of ~ 2900–3100 rpm, the highest friction force of ~ 680–720 kN, and the lowest axial forging load of ~ 560–600 kN. The measured hardness distribution depicted higher values for the weld zone (WZ) compared to the thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ) and BM, irrespective of the applied welding parameters. The substantial increase in the hardness of the WZ is due to the formation of microstructures that were dominated by martensite. The observed microstructural features, i.e. the fractions of martensite, bainite and ferrite, show that the temperature in the WZ and TMAZ was above Ac3, whereas that of the HAZ was below Ac1 during the IFW. The fracture surface of the tensile and impact-tested specimens exhibited the presence of dimples nucleating from the voids, thus indicating a ductile failure. EBSD maps of the WZ revealed the formation of subgrains inside the prior austenite grains, indicating the occurrence of continuous dynamic recrystallisation during the weld. Analysis of crystallographic texture indicated that the austenite microstructure (i.e. FCC) in both the WZ and TMAZ undergoes simple shear deformation during IFW.

Investigation of Process Stability and Weld Quality of Underwater Wet Flux-Cored Arc Welding of Low-Alloy High-Strength Steel with Oxy-Rutile Wire

Abstract The paper described the experimental findings of underwater wet welding of E40 steel using self-shielded flux-cored wire with a TiO2-FeO-MnO slag system. The arc stability, weld quality and corrosion resistance with different heat inputs were studied. The results showed that the wet welding process of the designed wire displayed good operability in the range of investigated parameters. The microstructure and mechanical properties of the weld metal depended on the heat input. Due to the high fraction of acicular ferrite in the weld metal, the mechanical properties of the weld metal under low heat input had better tensile strength and impact toughness. Fracture morphologies at low heat input had uniform and small dimples, which exhibited a ductile characteristic. The diffusible hydrogen content in the deposited metal obtained at a heat input of 26 kJ/cm significantly reduced to 14.6 ml/100g due to the combined effects of Fe2O3 addition and the slow solidification rate of molten metal. The microstructure also had a significant effect on the corrosion resistance of the weld metal. The weld metal with high proportions of acicular ferrite at low heat input exhibited the lowest corrosion rate, while the base metal possessed a reduced corrosion resistance. These results were helpful to promote the application of low alloy high strength steel in the marine fields.

Microstructural and crystallographic characterization of 455 MPa sour service plates with low Mn high temperature processed alloy concept and its association with drop weight tear test performance

The aim of this study was to analyze the mechanical properties of a X65 low Mn and high Nb steel for sour service application, particularly the drop-weight-tear test (DWTT) performance in order to establish correlation with the microstructural and crystallographic features of a steel with complex microstructure. Samples were analyzed at 55%, 65% and 75% of deformation during the finishing stage of rolling. The characterization of the samples involved electron backscatter diffraction (EBSD) technique for the determination and volume fraction quantification of the microconstituents and crystallographic texture analyses. The results showed that the plate with 55% of deformation displayed the lowest level of strength and the lowest DWTT shear area, while the plate with 65% of deformation exhibited the highest strength and DWTT toughness levels. EBSD results indicated that the plate produced with 65% of deformation had the largest dislocation density among all three samples and showed the highest intensity of the texture components concentrated along the α-fiber, particularly in the vicinity of the {113}<110> and {112}<110> components, responsible for activating ductile fracture, while samples 55% and 75% had a broader spread towards {001}<110>, and an overall higher intensity of the rotated cube components that favor cleavage fracture. For the plate with 75% of deformation, the mean flow stress (MFS) result suggested that partial recrystallization occurred during the finishing stage of hot rolling which was also proved by EBSD.These findings have proven that low Mn steels processed at high temperatures have a crucial dependency on Nb for the development of deformation textures that will help sustain high levels of fracture toughness, particularly when dealing with heavy plates for offshore applications. It has been observed that this steel concept is inclined to experience loss of toughness due to the softening of austenite during the finishing stage of rolling, even though the temperatures were below the no-recrystallization temperature, and a proper rolling schedule must be selected to mitigate this effect.The overall results showed that a rolling scheduled aiming at increasing the volume fraction of acicular ferrite and reducing the polygonal ferrite together with a higher level of strain and crystallographic texture consisting of high intensity deformation components, will result in a better combination of strength and toughness.