Large Ferrite Grains Research Articles

The impact of annealing temperature and time on the grain distribution and texture evolution of cold-rolled electric-furnace annealed low-carbon steel

This study investigated the effect of annealing temperature and time on the grain distribution and textural development of commercial-grade low-carbon steel that undergone cold rolling and subsequent electric-furnace annealing at either 700 or 800 °C for 5 and 10 min. Scanning electron microscopy and electron backscatter diffraction analyses of the annealed samples revealed equiaxed microstructures with cementite at the ferrite grain boundaries. The samples annealed at 700 °C for 5 and 10 min exhibited a bimodal grain distribution, while larger ferrite grains formed at 800 °C. The orientation distribution function texture of the sample annealed at 700 °C for 5 min exhibited both a γ-fiber ND//〈111〉 and a cube texture ND//〈001〉 . Raising the annealing temperature and time reduced the intensity of cube texture and strengthened the γ-fiber, increasing tensile elongation from 8 to 38 %.

Structure and mechanical properties anisotropy of a steel product manufactured by layer-by-layer electric arc wire 3D printing

The work presents the study of structure and mechanical properties anisotropy of a metal wall obtained using electric arc wire 3D printing (WAAM) with ER70S-6 wire. The layers were deposited in the protective gases of carbon dioxide and argon. As a result of structural studies, it was found that the internal structure of the model product in form of a wall can be divided into three zones. Repeated heating, cooling cycles and degree of accumulated heat influence the formation of different wall zones. As a result of rapid heat removal to the substrate during deposition of the first layers, the wall base (zone 1) contains large elongated grains with acicular ferrite structure. The wall middle part (zone 2) consists of ferrite-pearlite structure, which was formed as a result of recrystallization under conditions of repeated heating and cooling during 3D printing. The size of ferrite grains in zone 2 varies from 11 to 16.3 µm with increasing the number of layers. The gradual accumulation of heat during 3D printing led to the formation of structures in zone 3 under conditions of overheating and a reduced cooling rate. As a result, the wall upper part (zone 3) consists of large ferrite grains (up to 29.8 μm), sorbite, and a small proportion of Widemanstatten ferrite and acicular ferrite. It is shown that the most uniform level of mechanical characteristics (σ0.2 = 340 MPa, σu = 470 MPa, ε = 28 %) correspond to the samples cut from zone 2 in a direction parallel to 3D prin­ting direction. The samples cut in the vertical direction relative to 3D printing and from zone 3 show the lowest level of microhardness and mechanical characteristics (σ0.2 = 260 MPa, σu = 425 MPa, ε = 20 %).

An in situ investigation of neighborhood effects in a ferrite-containing quenching and partitioning steel: Mechanical stability, strain partitioning, and damage

Understanding the micromechanical consequences of transformation-induced plasticity (TRIP)-assisted steels is challenging, due to the presence of multiple phase constituents and difficulties in spatially and temporally resolving the transformation of sub-micron scale retained austenite. Local stress or strain partitioning among constituents is considered important in determining the stability of retained austenite, which often competes with other effects such as composition or crystal orientation. In this work, we study the neighborhood effects on micromechanical behaviors of multi-phase TRIP steel using a ferrite-containing quenching and partitioning steel as a model system by employing in situ SEM/EBSD and in situ synchrotron X-ray diffraction tensile tests, phase-specific nanoindentation, and atom probe tomography. Our results reveal that retained austenite, bainite, and tempered martensite clusters behave similarly due to underlying strengthening mechanisms, resulting in the early transformation of retained austenite surrounded by bainite and tempered martensite matrices. Conversely, in large ferrite grains, the intragranular strain distribution develops heterogeneously, which retards the transformation of some embedded retained austenite grains. The in situ microstructure-based strain mapping also demonstrates that the local strain increment of the retained austenite inside ferrite grains decreases as the deformation level increases, leading to a non-linear strain partitioning behavior at high strain levels. Another neighborhood-induced effect is observed in ferrite grains. When ferrite grains are neighbored by hard fresh martensite, their lateral contraction is inhibited, causing a plane strain tension path locally in ferrite, even when the globally applied strain path is uniaxial tension.

Interplay of austenite and ferrite deformation mechanisms to enhance the strength and ductility of a duplex low-density steel

Advanced low-density steels have been highlighted recently due to reduced weight and their superior mechanical properties due to various deformation mechanisms. This study investigates room temperature tensile behavior and the dominant deformation mechanisms of a ferrite-based duplex low-density steel. The present steel was subjected to a thermo-mechanical process including 70% cold rolling at ambient temperature and subsequent annealing at 1200 °C for 3, 5, and 10 min duration times. Room temperature tensile tests were carried out at 0.01s−1 strain rate. The Electron backscatter diffraction (EBSD) analysis on the deformed specimens demonstrates that the deformation induced martensite transformation in austenite, and deformation twinning in ferrite are synergistically responsible for the obtained high strength and ductility in this material. Transformation induced plasticity (TRIP) mechanism in austenite is found as the dominant deformation mechanism in specimens annealed for 3 min, while twinning induced plasticity (TWIP) in ferrite along with the dislocation gliding are characterized for specimens annealed for longer time (i.e., 5 and 10 min).Deformation twins are found to form in large ferrite grains with high Schmid factor, while dislocation gliding was dominant in smaller grains.

Micromechanics driven design of ferritic–austenitic duplex stainless steel microstructures for improved cleavage fracture toughness

Ferritic–austenitic duplex stainless steels are known to offer favorable combinations of good mechanical properties and corrosion resistance to be used for structural purposes. Lean duplex grades have already been introduced for consideration to replace standard 18–8 austenitic stainless steels in various industrial applications. Ferrite and austenite represent different deformation behaviors and contribute to the resulting fracture toughness characteristics. Micromechanical crystal plasticity based assessment of this cleavage fracture behavior is the subject area of current work. The objective is to bridge cleavage fracture models to full field crystal plasticity imaging based modeling of microstructures of ferritic–austenitic duplex stainless steels. The goal is to introduce means to computationally assess the effects of different multi-phase microstructural morphologies to cleavage fracture toughness and develop both the respective constitutive and cleavage fracture modeling capabilities. Such means can be used as an aid to develop better cleavage resistant, while in the current context lean, steel grades. Three different steels are investigated with differing austenite phase morphologies, and their behavior with respect to fracture mechanical response evaluated by micromechanical modeling. The effect of austenite fraction and morphology in terms of improving fracture toughness, a critical parameter concerning these steel grades and improvement of their cleavage fracture properties, is investigated. Deleterious features such as large ferrite grain size and microstructural property mismatching are identified and their implications to fracture toughness and development of applicable modeling capabilities for ferritic–austenitic duplex steels discussed. Simple design task of varying the austenite phase fraction is performed using synthetic microstructural modeling and the results evaluated with respect to their influence on the fracture toughness ductile-to-brittle transition.

Влияние фазовых превращений в процессе электронно-лучевой 3D-печати и последующей термической обработки на закономерности пластической деформации и разрушение образцов высокоазотистой Cr-Mn-стали

We investigated the phase composition, plastic deformation and fracture micromechanisms of Fe-(25-26)Cr-(5-12)Mn-0.15C-0.55N (wt. %) high-nitrogen chromium-manganese steel. Obtained by the method of electron-beam 3D-printing (additive manufacturing) and subjected to a heat treatment (at a temperature of 1150°C following by quenching). To establish the effect of the electron-beam 3D-printing process on the phase composition, microstructure and mechanical properties of high-nitrogen steel, a comparison was made with the data for Fe-21Cr-22Mn-0.15C-0.53N austenitic steel (wt. %) obtained by traditional methods (casting and heat treatment) and used as a raw material for additive manufacturing. It was experimentally established that in the specimens obtained by additive manufacturing method, depletion of the steel composition by manganese in the electron-beam 3D-printing and post-built heat treatment contributes to the formation of a macroscopically and microscopically inhomogeneous two-phase structure. In the steel specimens, macroscopic regions of irregular shape with large ferrite grains or a two-phase austenite-ferrite structure (microscopic inhomogeneity) were observed. Despite the change in the concentration of the basic elements (chromium and manganese) in additive manufacturing, a high concentration of interstitial atoms (nitrogen and carbon) remains in steel. This contributes to the macroscopically heterogeneous distribution of interstitial atoms in the specimens - the formation of a supersaturated interstitial solid solution in the austenitic regions due to the low solubility of nitrogen and carbon in the ferrite regions. This inhomogeneous heterophase (ferrite-austenite) structure has high strength properties, good ductility and work hardening, which are close to those of the specimens of the initial high-nitrogen austenitic steel used as the raw material for additive manufacturing.

Characterization of Weld Bead Deposited on Low Carbon Steel Plate with TIG Welding Process

The characterization of weld bead deposited on low carbon steel plate with TIG welding is carried out in the present study. Three beads on plate deposits were made on a low carbon steel plate by setting the current at 75, 100 and 125 Amp, voltage at 40 Volt and the weld speed at 0.5 mm/s. The wire is fed at the rate of 2.67 mm/s. A 1.8 mm filler wire made with low carbon steel was used. The macroscopic and microscopic examination of the sample was carried out. The depth of penetration was more with respect to rise in current value for the selected weld speed. At the heat affected zone (HAZ) fine grains were seen, closer to the HAZ recrystalised grains were noted. At the base metal large ferrite grains with fine carbide particles dispersed along the grain boundaries are observed.

Laser surface cladding of mild steel with 316L stainless steel for anti-corrosion applications

Fiber YAG (Yttrium-Aluminum-Garnet) Laser cladding is used to generate a thin layer of 316L stainless steel over low carbon steel substrate alloy to enhance corrosion resistance. In this work, the cladding technique was used at the laser powers of 2800, 2400 and 2000 W with a fixed travelling speed of 6 mm/s. The work aims to investigate the optimal heat-input for the thin clad in terms of microstructure and bonding quality. The microstructure of both formed layers by cladding and substrate was investigated. On the clad cross-section of the specimens, the micro-Vickers hardness was measured. At laser powers of 2800 and 2400 W, the produced layers were well bonded to the substrate without visually noticeable macro-defects. Some defects associated with cracks appeared at a laser power of 2000 W. The stainless steel clad layer consisted mainly from highly refined mixed columnar and cellular dendritic austenite structure elongated perpendicularly to the substrate surface in the direction of heat flow. At higher powers, the structure showed organized parallel columnar dendrites while at lower powers, it shows fine cellular dendrites in random directions. A narrow dilution zone (less than 10 µm) was obtained where the carbon and chromium elements can stay together and form harmful carbides. The depth of the fused part in carbon steel side has a direct relationship with the laser power, and it consisted mainly from feathery bainite inside large ferrite grains. The hardness of the produced layers was improved than the base metal. Sudden hardness increment at the limited area around the interface was obtained after all the laser powers. Decreasing laser power led to hardness increment at the clad and fusion zone.

Microstructures in HAZ after Heat Treatment of Carbon Steel and Austenitic Stainless Steel Welds

The influence of post weld heat treatments (PWHT) at 400°C, 600°C, 900°C on microstructures in heat affected zone (HAZ) of dissimilar welds between carbon steel and austenitic stainless steel was studied. As-welded condition, the fully Martensitic layer along the fusion line, Widmanstatten Ferrite, Bainite, Pearlite phases in the HAZ of carbon side and the fully austenitic zone in the weld metal can be observed. After PWHT, the microstructures of these zones were dramatically modified as a result of carbon diffusion from the carbon steel toward the weld metal. Decarburization of the base metal led to the formation of a zone with large Ferrite grains. Bainite or fine Pearlite were formed by carbon diffused to both the interfacial Martensite and the purely Austenite zone. The lowest hardness value in the decarburization zone was 92HV on average after PWHT at 900°C and the peak hardness value that was documented in the carburize zone with 366HV at 600°C. Carbides precipitation (M23C6, M7C3) were found in both the HAZ of carbon steel and austenitic stainless steel.

Creep and fracture behavior of long-annealed weld HAZ in CB2 steel

CB2 steel for thick castings has been designed and investigated in the frame of COST-522 and C0ST-536 R&D actions of EU to work at supercritical conditions of the steam main lines. The steel exhibits loss of impact strength and hardness after long annealing and also of strength and life when creep tested after the long annealing. The most serious decrease of properties appears in weld’s heat-affected zones during the creep tests. Differences in microstructures of base metal (BM) and weld’s heat-affected zone (HAZ) have been studied in initial state and after long-term annealing/aging for 10 kh and 30 kh at 625 °C, followed by creep testing. The initial as-cast microstructure of CB2 is inhomogeneous, and the welding thermal cycle causes additional separation of phases in the intercritical region. TEM observations revealed appearance in HAZ of large ferrite grains between compact colonies of spheroidal carbides, associated with the decrease of creep strength and life in the HAZ.

Three-Dimensional (3D) Microstructure-Based Modeling of a Thermally-Aged Cast Duplex Stainless Steel Based on X-ray Microtomography, Nanoindentation and Micropillar Compression

Finite element analysis was conducted on a thermally-aged cast duplex stainless steel based on the true three-dimensional (3D) microstructure obtained from X-ray microtomography experiments and using the constitutive behavior of each individual phase extracted from nanoindentation on single-crystal and bicrystal micropillar compression tests. The evolution of the phase morphology, the mechanical properties and the boundary deformation behavior during the aging process are highlighted. Quantitative analysis in terms of the distribution and evolution of the stress and strain in both the as received and aged conditions was performed. The experimental results show that aging at an intermediate temperature has a negligible influence on the morphology of the two phases in cast duplex stainless steel (CDSS). Results from simulations reveal that the mechanical behavior of this material were seriously affected by the microstructure and the mechanical properties of the individual phase and the necking deformation tend to form in the area with less large ferrite grains after aging. In addition, stress localization tends to form at the austenite/ferrite interface, in the narrow region of ferrite grains and in the small ferrite grains.

Fine tuning the mechanical properties of dual phase steel via thermomechanical processing of cold rolling and intercritical annealing

Abstract The simultaneous effects of cold rolling reduction and intercritical annealing time on the microstructure and tensile properties of St12 steel were studied. It was revealed that by increasing the rolling reduction, the ferrite grain size of the dual phase (DP) microstructure is refined and a chain-like morphology of martensite develops, which results in improvement of work-hardening capacity and the tensile properties. Such a microstructure can be obtained via applying an optimum holding time at the intercritical annealing temperature. Beyond that optimum, grain coarsening occurs with the resulting deterioration of tensile properties. It was also shown that by consideration of quenched sheet or by applying low reductions in thickness, secondary recrystallization or abnormal grain growth (AGG) takes place during intercritical annealing, which is characterized by small grains and some large ferrite grains containing martensite islands. This study clarified that applying high reduction in thickness for grain refinement and also controlling the holding time in the two-phase austenite-ferrite region are the essential prerequisites to achieve the desired tensile strength, ductility, and toughness.

Nanoindentation investigation on the initiation of yield point phenomenon in a medium Mn steel

In general, the medium Mn steel with a dual phase microstructure of austenite and ferrite has a yield point phenomenon. It is still not clear which phase firstly initiates the plasticity and therefore governs the yield point phenomenon. In this paper, the mechanical behaviours of ferrite and austenite phases are investigated by nanoindentation. It is found that the large ferrite grains in which dislocations are present have a lower resistance to dislocation plasticity as compared to the austenite phase and the small ferrite grains, suggesting that the large ferrite grains shall be responsible for the initiation of yield point phenomenon.

Microstructure and Properties of the Ferroelectric-Ferromagnetic PLZT-Ferrite Composites

The paper presents the technology of ferroelectric-ferromagnetic ceramic composites obtained from PLZT powder (the chemical formula Pb0.98La0.02(Zr0.90Ti0.10)0.995O3) and ferrite powder (Ni0.64Zn0.36Fe2O4), as well as the results of X-ray powder-diffraction data (XRD) measurement, microstructure, dielectric, ferroelectric, and magnetic properties of the composite samples. The ferroelectric-ferromagnetic composite (P-F) was obtained by mixing and the synthesis of 90% of PLZT and 10% of ferrite powders. The XRD test of the P-F composite shows a two-phase structure derived from the PLZT component (strong peaks) and the ferrite component (weak peaks). The symmetry of PLZT was identified as a rhombohedral ferroelectric phase, while the ferrite was identified as a spinel structure. Scanning electron microscope (SEM) microstructure analysis of the P-F ceramic composites showed that fine grains of the PLZT component surrounded large ferrite grains. At room temperature P-F composites exhibit both ferroelectric and ferromagnetic properties. The P-F composite samples have lower values of the maximum dielectric permittivity at the Curie temperature and a higher dielectric loss compared to the PLZT ceramics, however, the exhibit overall good multiferroic properties.

Influence of Heating Rate on Ferrite Recrystallization and Austenite Formation in Cold-Rolled Microalloyed Dual-Phase Steels

Compared with other dual-phase (DP) steels, initial microstructures of cold-rolled martensite-ferrite have scarcely been investigated, even though they represent a promising industrial alternative to conventional ferrite-pearlite cold-rolled microstructures. In this study, the influence of the heating rate (over the range of 1 to 10 K/s) on the development of microstructures in a microalloyed DP steel is investigated; this includes the tempering of martensite, precipitation of microalloying elements, recrystallization, and austenite formation. This study points out the influence of the degree of ferrite recrystallization prior to the austenite formation, as well as the importance of the cementite distribution. A low heating rate giving a high degree of recrystallization, leads to the formation of coarse austenite grains that are homogenously distributed in the ferrite matrix. However, a high heating rate leading to a low recrystallization degree, results in a banded-like structure with small austenite grains surrounded by large ferrite grains. A combined approach, involving relevant multiscale microstructural characterization and modeling to rationalize the effect of the coupled processes, highlights the role of the cold-worked initial microstructure, here a martensite-ferrite mixture: recrystallization and austenite formation commence in the former martensite islands before extending in the rest of the material.

Microstructure and tensile behavior of Fe-8Mn-6Al-0.2C low density steel

The microstructure and tensile behavior of Fe-8Mn-6Al-0.2C low density steel were investigated in the present study. The results show that, after being solution treated at 900°C for 1h, the steel has a duplex microstructure consisting of ferrite and ~ 30% austenite, and it exhibits an excellent combination in mechanical properties, with a tensile strength of 846MPa and a total elongation of 32%, which were attributed to significant contribution of TRIP effect and three stages work hardening behavior. During tensile test, the work hardening behavior can be defined as three-stage characteristics, in which the stage 2(S2) was linked to the TRIP effect. In S2, with the increase of tensile strain, austenite volume fraction decreased while martensite volume fraction increased gradually. During this process, some austenite grains, positioned at grain boundaries among multiple ferrite grains, were the first ones to transform, while, some austenite grains, located between ferrite laths or embedded in large ferrite grains were often found to undergo rotations. The TRIP effect and the high dislocation density in ferrite and martensite significantly contributed to the excellent mechanical property. In addition, the volume fraction of small angle grain boundaries increased greatly, with a value of over 51% being observed at the true strain of 0.27. The steel possessed a density of 6.95g/cm3, which is about 11% lower than pure iron.

Precipitation behavior in bimodal ferrite grains in a low carbon Ti-V-bearing steel

With austenite under 50% compression using a dilatometer, ferrite transformation in a low-carbon Ti-V-bearing steel was sequenced through the isothermal transformation at 650°C for various holding times (10s, 20s, 30s). Bimodal grain size distribution was observed in the prolonged holding conditions. Corresponding measurements of Vickers and nano-indentation hardness were carried out; the tiny ferrite grains possessed lower hardness (303HV and 4.8GPa) as compared to larger ferrite grains (336HV and 5.6GPa). Transmission electron microscopy provided strong evidence attributing this discrepancy to interphase precipitation taking place in the larger-grained ferrite, instead of in the tiny-grained ferrite.

Effect of microstructure on mechanical behavior for eutectoid steel with ultrafine- or fine-grained ferrite+cementite structure

A eutectoid steel with various ultrafine- or fine-grained ferrite matrix (α)+cementite particle (θ) structures was fabricated to explore the effects of the microstructural features on the mechanical behavior of hard particle-strengthened two-phase alloys. The effect of microstructure on the parameters of an analytical model and the mechanical behavior for the eutectoid steel with ultrafine- or fine-grained α+θ structure were analyzed basing on statistical data and physical metallurgy. The results showed that the rate of dislocation-storage caused by ferrite grain boundaries and cementite particles is approximately a microstructural constant and is proportional to the dislocation mean free path. The larger ferrite grains and the larger volume fraction of intragranular cementite particles are beneficial to obtaining a lower rate of dynamic recovery when ultrafine- or fine-grained α+θ structures with an equal dislocation mean free path, and the uniform elongation increases with the decrease in the rate of dynamic recovery. Moreover, the ultimate strength is closely related to the effective dislocation mean free path including both roles of the storage and the recovery of dislocations. It is feasible to design a microstructure consisting of ultrafine- or fine-grained ferrite matrix and tiny cementite particles mainly within grain interior to possess an enhanced strength-plasticity synergy for the eutectoid steel.

Features of Structure and Property Formation of Rolled Coil for Wire Manufacture

Results are given for research to develop scientifically based process solutions to improve quality characteristics of rolled coils cooled in a two-stage Stelmor line ensuring effective metal microstructure and processing properties for wire and wire products. An increase in welding rod ductility is achieved by limiting the steel’s strengthening element content, effective application of the boron to nitrogen ratio, and optimization of two-stage cooling, ensuring the formation of large ferrite grain size with rolling heat during isothermal soaking, thereby minimizing the formation of non-ductile structural components (bainite and martensite). For high-carbon wire rod used in the manufacture of concrete sleepers, it is necessary to achieve the fine pearlite structure of point 1 (GOST 8233–56) that is provided by alloying steel with boron, vanadium, and/or chromium, and also use of fan cooling. This structure is the optimum from the point of view of metal ductility and strength.

Characterization of twin-like structure in a ferrite-based lightweight steel

The present study examined cold to warm compressive deformation behavior of a ferrite- based lightweight steel through characterization of the banded structures. Compression tests were carried out at 25 to 500 °C at a strain rate of 0.01 s-1 up to true strain of 0.6. Analysis of the microstructural evolution using electron back scatter diffraction indicated that the twin-like bands in the large ferrite grains occurred with the {112}[111] system at a 60° misorientation. Density of the twin-like bands is increased by raising the deformation temperature. EBSD results showed that the primary and secondary twins occurred in the [-11-1] and [1-1-1] directions. In addition, the strain at 500 °C distorted the twin-like bands and resulted in wavy boundaries. The strain hardening behavior was also examined using the Crussard-Jaoul (C-J) model and the n-values were calculated for each stage of imposing strain. The results showed high dislocation density in the adjacent of twin-like boundaries intersections which resulted in the n-value increment.