Ferrite Phase Transformation Research Articles

Evolution of inclusions in DH36 grade ship plate steel during high heat input welding

In this paper, the solid solution and precipitation behavior of inclusions on the surface and 1/2 thickness of the tested steel plate under the condition of welding heat input of 400 kJ/cm is investigated by using laser confocal experiments with hot-rolled state DH36 ship plate steel as the research object, and the mechanism of the effect of inclusions on the phase transformation of an acicular ferrite is revealed. The results show that the inclusions of the tested steel are mainly composed of Oxide-MnS, MnS, Oxide, TiN, Spinel, etc. The amount of inclusions on the surface of the tested steel plate is significantly higher than that at the 1/2 thickness position. During the heating stage, the small inclusions on the surface immediately disappeared, and the large inclusions gradually solidified in the matrix; atomic diffusion occurred at the bond between the inclusions and the matrix; while the small inclusions at the 1/2 thickness position gradually disappeared at the beginning of the heating stage, and the inclusions began to precipitate and grow when the temperature was increased to 990 °C. The acicular ferrite preferentially nucleates and grows near the boundary of the inclusions during the post-weld cooling stage, and its growth ends when two acicular ferrites cross.

Effect of final rolling temperature on the microstructure and properties of high-manganese austenitic low-temperature steel

The effect of final rolling temperature ranging from 962 to 696 °C on as-rolled high-Mn austenitic low-temperature steel was investigated using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The microstructure of the experimental steel remained austenite without suffering ferrite phase transformation, even when produced with a final rolling temperature as low as 696 °C. A decrease in the final rolling temperature of the steel was accompanied by a decrease in grain size from 20.2 to 6.0 μm, accompanied by an increase in dislocation density from 3.01 × 1014 to 10.9 × 1014 m−2. In addition, M23C6 carbides precipitated at the grain boundaries when the final rolling temperature was below 782 °C. Grain refinement and increased dislocation density significantly enhanced the strength of the material. However, an increase in the intragranular dislocation density compromised grain deformation ability. Moreover, the precipitation of M23C6 reduced the plasticity and toughness of the material. Considering the balance between yield strength and low-temperature toughness, the optimal final rolling temperature was 898 °C; this produced a material with a yield strength of approximately 507 MPa and a low-temperature impact energy absorption of approximately 143 J.

Comparisons of {100} texture improvement and formability in hot-rolled non-oriented electrical steel by austenite–ferrite phase transformation and shear deformation

Comparisons of {100} texture improvement and formability in hot-rolled non-oriented electrical steel by austenite–ferrite phase transformation and shear deformation

Wire-arc directed energy deposition of oxide-modified H13 steel: Microstructural characterization and mechanical properties

The preparation of H13 steel using wire-arc directed energy deposition has broad application prospects; however, the high hardenability of H13 steel makes it prone to forming cold cracks during deposition, thus limiting its further applications. Fortunately, the oxide metallurgy technology holds promise in addressing this issue. Specifically, nano oxides have the capability to refine the grain and facilitate the formation of intracrystalline ferrite, ultimately resulting in an improvement in steel's toughness and subsequently mitigating its propensity towards cold cracking. In this study, H13 steel wires, incorporating Al-Si-Ti composite oxides, were used to deposit part (W-O-H13) with excellent performance and no cold cracks. Owing to the special thermal cycling during wire-arc directed energy deposition, a periodic build unit was formed that could be roughly divided into three zones, namely, fine grain, columnar grain, and heat-affected zones. The microstructure of the parts contained lath martensite, high-alloy martensite, and ferrite. After arc exposure, the oxide formed a complex core-shell structure, with vanadium carbide (VC) encapsulating the amorphous oxide core. Moreover, the oxide particles refine the grain structure and induce the formation of ferrite, thereby enhancing the toughness of the part and further improving its strength and wear resistance. The low mismatch between the VC thin layer wrapping the oxide and ferrite was a crucial factor in the conductive ferrite phase transformation. Additionally, the elemental segregation caused by the rapid cooling rate ultimately led to the formation of high-alloy martensite having a maximum hardness of 5.32 GPa. The results reveal that the average tensile strength of W-O-H13 steel in the X, Y, and Z directions is improved by 21.0 %, 20.3 %, and 14.3 % respectively, compared to the parts fabricated solely from H13 steel wire without the addition of oxide (W-H13). Additionally, the impact toughness of W-O-H13 steel surpasses W-H13 steel in all three directions, achieving an average of 33, 27, and 28 J/cm2 respectively in the X, Y, and Z axes. In comparison to W-H13, the impact toughness of W-O-H13 has witnessed a substantial increase ranging from 10 % to 30 %. Furthermore, the wear resistance of W-O-H13 steel significantly surpasses that of W-H13 steel, particularly under high-temperature wear conditions. Specifically, the average wear volume of W-O-H13 steel at temperatures ranging from 400 to 600 °C is merely 0.163 and 0.224 mm³, representing only 30 % and 40 % of the wear volume exhibited by W-H13 steel under the same wear conditions.

Modeling solute drag during austenite–ferrite transformation with ab initio binding energies

The solute drag effect can have a considerable impact on the austenite to ferrite transformation. Most models describing the solute drag effect require knowledge of solute binding energies, which due to lack of accurate data typically are treated as fit parameters. In this study we use density functional theory (DFT) calculations to estimate the effective binding energies of the substitutional alloying elements in an ultra low carbon steel. For the first time these energies are used to calculate the solute drag effect considering site competition for the austenite to ferrite phase transformation. The depletion of elements in solid solution as a result from precipitation is computed and the solute drag contribution of each element is calculated. Comparison of the binding energies with the resulting solute drag effect to literature data shows reasonable agreement. The outlined approach points the way to future alloy development based on interface-controlled integrated computational materials engineering. Another field of application is given by the circular economy driven transition in the steel industry from the BF/BOF- to the EAF-based production route with increased utilization of scrap material introducing new tramp elements.

Competitive Formation Behavior of Calcium Silicate and Calcium Ferrite in Sintering of Iron Ore Fines

Calcium silicate is the undesirable bonding phase in sinter with relatively high melting point and poor reducibility, while the formation mechanism in sinter is ambiguous. Herein, the phase transformation, phase quantification, and morphology evolution of calcium silicate and calcium ferrite in CaO‐SiO2‐Fe2O3‐Al2O3 systems under different n(Fe2O3)/n(CaO) and Al2O3 content are investigated. It is found that with the increase of n(Fe2O3)/n(CaO), the calcium ferrite phases transform from Ca2Fe2O5 (C2F) to complex calcium ferrite, and calcium silicate phases including β‐Ca2SiO4 (β‐C2S) and γ‐Ca2SiO4 (γ‐C2S) are promoted. Moreover, small amount (2%) of Al2O3 can effectively promote the content of silico‐ferrite of calcium and aluminum (SFCA) increasing from 20.95% to 46.69%, whereas inhibiting the β‐C2S from 51.79% to 42.34%. When the content of Al2O3 is over 4%, the Ca2Al(AlSiO7) phase is formed, where both the SFCA and calcium silicate are inhibited. On this base, the competitive mechanism for the formation of calcium silicate and calcium ferrite is revealed, where the Ca2+ and Si4+ are preferred to react with Fe3+ and Al3+ to form liquid phase SFCA, while the amount for β‐C2S or γ‐C2S can be decreased due to the deprivation of Ca2+ and Si4+.

Arc deposition oxide-modified H13 steel: Investigating the structure of oxygen-containing second-phase particles and their influence on the microstructure

The challenge associated with depositing high-strength steel using the arc deposition method lies in the propensity for hardened microstructures to form during the deposition process. This hardened microstructure can lead to a decrease in material toughness and potentially cause cold cracks. Fortunately, oxide metallurgy technology is expected to address this issue. Specifically, nano oxides can refine the grain and induce the formation of intracrystalline ferrite, thereby enhancing steel's toughness. To gain a deeper understanding of this process, a study was conducted on the morphology and structure of oxygen-containing second-phase particles generated in parts made from H13 steel wire modified with Al2O3, TiO2, and SiO2. Additionally, the impact of these oxygen-containing second-phase particles on microstructure transformation was investigated. The following conclusions were drawn from the study: (1) The arc's high temperature causes complex reactions among the oxides in the molten pool, ultimately resulting in the formation of oxygen-containing second-phase particles with a unique core-shell structure. The size and composition of these particles vary within the interdendritic and intradendritic regions due to element segregation. (2) Through in-situ observations, this study discovered that oxygen-containing second-phase particles promote the nucleation of high-temperature ferrite and austenite. This promotes grain refinement and induces the formation of intracrystalline ferrite, which positively impacts material toughness. (3) The driving force for ferrite phase transformation induced by oxygen-containing second-phase particles may stem from the Mn-depleted zone surrounding the particles and the low mismatch between the VC outer layer of the particles and the ferrite.

The Low‐Temperature Toughness Improvement of High‐Strength Low‐Alloy Steel Due to the Microstructural Changes Caused by the Reheating/Rolling Temperature

Herein, the microstructure, low‐temperature toughness, and fracture characteristics of high‐strength low‐alloy (HSLA) steel prepared by various thermomechanical control processes are systematically studied. Three different reheating/rolling temperatures are designed to obtain microstructures with different prior austenite grain sizes: 20.12 μm (S810), 27.58 μm (S910), and 35.19 μm (S1010). After hot rolling, S810 steel with smaller prior austenite grain size can provide high‐density deformed grain boundaries that promote ferrite phase transformation. The S910 and S1010 steels predominantly undergo bainite transformation as the prior austenite grain size and rolling temperature increase. Also, excellent low‐temperature toughness is obtained by reducing the reheating/rolling temperature (i.e., S810 steel), reflecting that Charpy impact energy of the core of the steel plate is 118.53 J at −120 °C. High performances is mainly attributed to the higher ferrite content, grain refinement, lower dislocation density, and higher proportion of high‐angle grain boundaries. Furthermore, fracture morphology analysis demonstrates that the S810 steel exhibits ductile fracture and noticeable plastic deformation at −120 °C, while S910 and S1010 steels exhibit brittle fracture and rapid crack propagation. These findings manifest the applicability of the suggested technique in preparing HSLA steel with excellent low‐temperature toughness.

The Effect of Cooling Rate on the Banded Structure of 20CrMnTi Gear Steel Based on C Diffusion and Partitioning Analysis

This study, through experimental analysis and numerical calculations, not only identifies the critical cooling rate for eliminating banded structures in 20CrMnTi gear steel but also analyzes the impact of cooling rate on banded structures, including the mechanisms related to element distribution and interface migration. The results indicate that the fundamental cause of banded structure formation is the presence of chemical segregation bands, and increasing the cooling rate leads to a reduction in the grade of ferrite/pearlite banded structures, with a critical cooling rate of 0.55 °C s−1. Bainite and martensite banded structures are observed in samples cooled at rates of 3 and 5.8 °C s−1, respectively. As the cooling rate increases from 0.07 to 1 °C s−1. The ferrite phase transformation time in the solute‐poor zone is shortened from 171 to 12 s within the ferrite phase transformation temperature difference (Δ) between the poor and rich solute zones. The concentration difference of carbon (C) on both sides of the α/γ phase interface increases from 0.350 mass% to 0.431 mass%, and the diffusion distance of C decreases from 34.38 to 7.57 μm. When the diffusion distance of C is less than half of the width of the solute‐rich banding (12.5 μm), the ferrite banded structure disappears.

Machine learning interphase precipitation behavior of Ti micro-alloyed steel guided by physical metallurgy principle

Nano-sized interphase precipitates, which form in ordered rows, are critical for high strength low alloy (HSLA) steels, in achieving the desired strength needed for downgauging for light-weighting automotive structures. The occurrence of interphase precipitation depends on many interrelated parameters, such as alloy chemical composition, temperature, processing parameters and crystallography. In this paper, we use data analysis based on machine learning algorithm: decision tree to predict whether interphase precipitation can occur. Due to the high strength of interphase precipitation and the minimum alloy content is one of the important goals pursued of iron and steel enterprises. Therefore, under the condition that interphase precipitation occurs, the chemical composition of the alloy can be reduced by using interphase precipitation. Guided by the physical metallurgy principle, this paper transforms the processing parameters into physical metallurgy parameters such as grain size, stored deformation energy, ferrite phase transformation temperature, and decision tree (DT) algorithm was used to model and verify whether interphase precipitation can occur. Under the condition of interphase precipitation, the support vector machine (SVM) models of interphase precipitation characteristic values were established. Based on the DT model and the SVM model, the particle swarm optimization (PSO) algorithm was used for the reduction design of the alloy, and the chemical composition can be reduced according to the target strengthening strength. The results of alloy reduction have been verified by experiments, which shows that the alloy can be reduced by using interphase precipitation.

The austenite to polygonal ferrite transformation in low-alloy steel: multi-phase-field simulation

The austenite to ferrite phase transformation is a critical structural transformation in steel production, where the morphology and grain size of ferrite substantially influence the mechanical properties of steel materials. In this work, the influences of cooling rate, prior austenite grain size (PAGS), and Mn content on the microstructure evolution and component distribution of austenite-to-polygonal ferrite phase transformation are investigated by a multi-phase-field model. It is found that higher cooling rates intensify the driving force for austenite to polygonal ferrite phase transformations and delay the phase transformation process. As PAGS decrease, the increased proportion of austenite grain boundary offers more nucleation sites for polygonal ferrite and thus refines the polygonal ferrite grain. Additionally, increased Mn content results in significant grain refinement due to a reduction in the transformation temperature of austenite to polygonal ferrite. This work provides valuable insights into adjusting and designing desired microstructures of polygonal ferrite for enhancing the mechanical performance of steel.

Austenite grain boundary segregation and precipitation of boron in low-C steels and their role on the heterogeneous nucleation of ferrite

The addition of boron (B) to steels suppresses the austenite to ferrite phase transformation dramatically, thus increasing their hardenability. It achieves this through grain boundary (GB) segregation at the austenitic GBs that delays the ferrite nucleation. Though the effects of B segregation on hardenability have long been known, the mechanisms of B segregation and how exactly B suppresses the ferrite nucleation remain elusive. We designed a B-containing low-C steel to study the B segregation and precipitation behavior. We conducted heat treatments with different austenitization temperatures and cooling rates. Site-specific atom probe tomography, complemented by high-resolution secondary ion mass spectrometry reveals B segregation at prior austenite grain boundaries (PAGBs) along with carbo-boride precipitation. The B segregation mechanisms are discussed in detail based on these observations considering their dependence on austenitization temperature and cooling rate. Furthermore, we analyzed the impact of GB energy reduction through nucleation kinetics calculations. From our analysis, we conclude that the reduction in GB energy affects the grain corner and grain edge nucleation substantially. However, the retarding effects of carbo-boride precipitation on ferrite nucleation cannot be completely excluded. We provide a detailed account of the possibilities of how B-containing precipitates may be suppressing ferrite nucleation.

Effect of Complex Strengthening on the Continuous Cooling Transformation Behavior of High-Strength Rebar.

The effects of niobium and composite strengthening on the phase transformation characteristics and precipitation behavior of continuous cooling transformation of high-strength rebar during thermal deformation and subsequent cooling were investigated. The results show that when the cooling rate was within 0.3-5 °C/s, ferrite transformation and pearlite transformation occurred in the experimental steels. The Nb content increased to 0.062 wt.%, and the starting temperature of the ferrite transformation decreased. Meanwhile, the ferrite phase transformation zone gradually expanded, and the pearlite phase transformation zone gradually narrowed with the increase in the cooling rate. When the cooling rate was 1 °C/s, bainite transformation began to occur, and the amount of transformation increased with the increase in the cooling rate. It was found that the main precipitates in the experimental steels were (Nb, Ti, V)C, with an average particle size of about 10-50 nm. When the Nb content was increased to 0.062 wt.% and the cooling rate was increased to 5 °C/s, the ferrite grain size was reduced from 19.5 to 7.5 μm, and the particle size of the precipitate (Nb, Ti, V)C could be effectively reduced. The strength of the steel was significantly improved, but the elongation of the steel was reduced. However, the comprehensive mechanical properties of 0.062 wt.% Nb experimental steel was significantly improved at a cooling rate of 5 °C/s.

Microstructure evolution and parameters optimization of follow-up hammering-assisted hybrid wire arc additive manufacturing

Coarse columnar grains are the typical microstructure of wire arc additive manufacturing, resulting in the poor mechanical properties. A follow-up hammering-assisted (FH) hybrid wire arc additive manufacturing process is proposed to improve the microstructure of deposited weld bead. The effects of different hammering temperatures (higher than Ae3 line) and different deformation amount on the microstructure are systematically studied, and the effect on mechanical properties is investigated correspondingly by fabricating a deposited block. The Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscope (SEM), and Optical Microscopy (OM) are used for characterization. The results show that the FH process can significantly refine the grain size of deposited weld bead, and meanwhile weaken the texture strength. The grain refinement mechanism for hammering higher than the austenite recrystallization temperature is austenite dynamic recrystallization. Instead, for hammering lower than the austenite recrystallization temperature, the more significant grain refinement is attributed to the high-temperature deformation increasing the ferrite phase transformation nucleation site, nucleation rate and the deformation-induced ferrite phase transformation. Hammering temperature 850 °C and hammering deformation 40% are determined to be the optimal process parameters to refine the grains. Using the optimal process parameters, the strength and hardness of the deposited layers are greatly promoted, and the anisotropy is also improved, while the ductility is reduced to some degree, but improved obviously compared to the low-temperature hammering.

Thermomechanical and Microstructural Analysis of the Influence of B- and Ti-Content on the Hot Ductility Behavior of Microalloyed Steels

The effects of the combined addition of B and Ti, as well as the influence of different strain rates on the hot ductility behavior of low carbon, continuously cast, microalloyed steels were investigated in this work. Tensile tests, microstructure analyses, and thermokinetic simulations were performed with in situ melted samples. Furthermore, prior austenite grain evaluations were carried out for the two different microalloyed steels. Increasing the strain rate brought improvements to the ductility, which was more significant in the steel with the leanest composition. The steel containing more B and Ti presented a better hot ductility behavior under all conditions tested. The main causes for the improvements rely on the precipitation behavior and the austenite–ferrite phase transformation. The preferential formation of TiN instead of fine BN and AlN was seen to be beneficial to the ductility, as well as the absence of MnS. Grain boundary segregation of free B that did not form BN retarded the ferrite formation, avoiding the brittleness brought by the thin ferrite films at the austenite grain boundaries. Furthermore, it was revealed that for the steels in question, the prior austenite grains have less influence on the hot ductility behavior than the precipitates and ferrite formation.

Effect of Tungsten Addition on Continuous Cooling Transformation and Precipitation Behavior of a High Titanium Microalloyed Steel

Effects of tungsten addition on the continuous cooling transformation (CCT) characteristics and precipitation behavior of a high titanium microalloyed steel were investigated by dilatometry, optical microscopy, scanning and transmission electron microscopy, and hardness measurements. The results showed that the ranges of transformation products were moved to the right side of the CCT diagram when the 0.4% W was added. Accordingly, the following observations were made: (i) the ferrite phase transformation was shifted to the side of lower cooling rates and reduced temperatures; (ii) the bainite phase transformation region ran throughout the whole cooling rate range studied. Addition of W had a positive effect on the particle size refinement and number density increase of the precipitates. At the low cooling rates, in the range of <14 °C/s, W addition shifted the precipitation hardening peak to the low cooling rate side as the ferrite transformation induced stronger precipitation strengthening than the bainite one. Furthermore, the effect of W addition on phase transformation strengthening was obvious (increase in hardness: ~40Hv) at the high cooling rate range, over 14 °C/s.

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Effect of tin microalloying on the microstructure of low-carbon free-machining steels

Sn is a well-known grain boundary segregation element that improves the machinability of steel. Sn has been considered as a replacement for Pb in super-free-machining steels. The effect of Sn on the microstructure of the Fe-0.05C-0.03Si-1.28Mn-0.36S-0.05P base composition containing 0.0002 (without addition), 0.062, 0.12, and 0.18 Sn (wt.%) low-carbon free-machining steels was investigated though thermodynamic calculations, optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and high-temperature laser scanning confocal microscopy. The microstructure of the free-machining steels was composed of α-ferrite, pearlite, and manganese sulfide. Sn significantly decreased the pearlite content of the steels. Most of the Sn was dissolved in the matrix, and the remainder was dissolved in manganese sulfide. No FeSn intermetallic compound precipitation was observed through transmission electron microscopy, but a significant strengthening effect was observed in α-ferrite. Sn had little effect on the solidification behavior or sulfide precipitation behavior of the steels when its content was lower than 0.2 wt.%. Similar to Al and Si, Sn is a ferrite-stabilizing element that expands both the δ-ferrite and α-ferrite phase regions, promotes α-ferrite formation, and inhibits carbide precipitation. Sn segregates at the interface, decreasing the interfacial energy and promoting the Widmanstätten ferrite phase transformation. A much lower cooling rate than that of Sn-free free-machining steels should be adopted to restrain the formation of Widmanstätten ferrite after hot rolling. Sn addition greatly improved the machinability of the experimental steels.

Phase Transformation and Zinc Extraction from Zinc Ferrite by Calcium Roasting and Ammonia Leaching Process

Zinc ferrite (ZnFe2O4), one of the major forms of zinc in electric arc furnace dust and zinc leaching residue, is very difficult to chemically dissolve in either acidic or alkaline solutions. It is therefore necessary to study the extraction of zinc from zinc ferrite to achieve the full recovery of the zinc resources. In this paper, CaO was used to degrade the structure of zinc ferrite and then the ammonia leaching method was applied to recover zinc. The thermodynamic analysis indicated that the zinc ferrite could react with CaO to form zinc oxide and dicalcium ferrite. The CaO–Fe2O3–ZnO phase diagram shows that Ca2Fe2O5 can be in equilibrium with ZnO rather than ZnFe2O4 at a higher CaO area. The experimental results indicated that the phase transformation of zinc ferrite, reacted with CaO, to zinc oxide and dicalcium ferrite could be completely achieved with a CaO addition of 2.4:1 (mole ratio). The peaks of zinc ferrite in the X-ray diffraction (XRD) patterns cannot be clearly observed after calcium roasting at 1220 °C for 2 h. The ammonia leaching tests showed that the zinc leaching rate was about 90% under optimal roasting conditions.

Predicting kinetic interface condition for austenite to ferrite transformation by multi-component continuous growth model

A critical issue restricting the application of Hillert-Agren-Liu solute-drag based model to predict interfacial conditions at a migrating ferrite-austenite interface is the lack of value for trans-interface diffusion parameter, i.e., the L parameter. Even an estimation of the parameter's order of magnitude is difficult due to its ambiguous physical interpretation. In this paper we extend a different solute-drag based model, namely the binary continuous growth model originally developed for rapid solidification, toward austenite to ferrite phase transformation to avoid this long-standing issue. The extensions consist of the treatments of multi-alloying components including interstitial carbon element and Gibbs-Thomson effect. The extended multi-component continuous growth model employs a physical parameter with clear physical meanings, i.e., interface diffusive speed in describing trans-interface energy dissipation, and can predict kinetic interface conditions and spontaneous diffusion-controlled to diffusion-less transition without using the ambiguous L parameter. The model is verified by the good agreement of its calculation results with those predicted by Hillert-Agren-Liu model for Fe–C alloys and Fe–C–Mn alloys. Further the model's advantages over Hillert-Agren-Liu model are demonstrated by calculating the kinetic interface condition phase diagrams of Fe–C–Mn–Ni alloys. It is concluded that the extended multi-component continuous growth model is valuable in unifying the efforts in addressing the common question of predicting deviations from local equilibrium at a fast-migrating interface during solidification and solid-state phase transformation.