Search Results (3,220)

This paper studies the effect of welding heat input on microstructure and low-temperature toughness of laser-arc hybrid welded (LAHW) SAF2507 super-duplex stainless-steel (SDSS) joints. Heat input was adjusted from 0.204 to 0.407 kJ/mm by changing the welding speed. Results indicate that low heat input brings high ferrite content and low impact toughness. The medium heat input generates a balanced two-phase microstructure and gains the highest impact energy of 36.0 J at −46 °C. Excessively high heat input results in obvious grain coarsening and degraded impact performance. This study offers an experimental basis and parameter reference for practical welding production. Full article

First-principles calculations based on density functional theory were employed to systematically investigate the atomic structure, stability, and Nb segregation behavior of the bcc-Fe(100)/NbN(100) interface. Convergence tests of surface energy determined that an interface model consisting of 7 bcc-Fe layers and 5 NbN layers is appropriate. The work of adhesion and interfacial energy were calculated for four interface configurations with different terminations. Interface 2 was identified as the most thermodynamically stable configuration, exhibiting a work of adhesion of 0.569 J/m² and an interfacial energy of 3.221 J/m². Nb atoms displayed pronounced site-selective segregation at the interface; the segregation tendency decreases in the order site 1 (−0.74 eV) > site 2 (−0.59 eV) > site 4 (−0.38 eV) > site 3 (−0.24 eV). Electronic structure analysis indicated that strong hybridization between Nb-4d and Fe-3d orbitals near the Fermi level leads to localized charge accumulation, which is the electronic origin of interfacial strengthening. As the Nb concentration increases from 1.6 at.% to 6.3 at.%, the segregation energy continuously drops from −0.74 eV to −1.55 eV, the work of adhesion monotonically increases from 0.569 J/m² to 0.786 J/m², and the interfacial energy decreases from 3.221 J/m² to 2.374 J/m², demonstrating that Nb segregation significantly enhances the interfacial stability. The calculation results provide a theoretical framework for understanding the experimentally observed evolution of Nb(C,N) precipitates and offer insights for the optimization of Nb microalloying in 442D ferritic stainless steel. Full article

Hydrogen trapping is a key mechanism to mitigate hydrogen embrittlement (HE) in pipeline steels, yet how trapping behavior evolves during plastic deformation and which microstructural features are responsible remain poorly understood. In this work, interrupted slow strain rate tensile tests were performed on pipeline girth weld metal (WM) under 6.3 MPa H₂ combined with thermal desorption spectroscopy (TDS), hydrogen microprint technique (HMT), and electron backscatter diffraction (EBSD). The results reveal that hydrogen trapping is highly strain-dependent. In the undeformed stage, only weak traps (Peak A at ~250 °C) and moderate traps (Peak B at ~500 °C) are present, as identified by TDS. Upon tensile straining to 15%, a new strong trap (Peak C at ~700 °C) emerges and intensifies, which is attributed to deformation-induced defects within acicular ferrite (AF). HMT shows that hydrogen distribution becomes homogeneous inside AF grains with increasing strain, while remaining localized at grain boundaries in proeutectoid ferrite (PF). EBSD indicates that AF undergoes more uniform plastic deformation and dislocation accumulation than PF. Most importantly, the WM with a higher AF content (74.5%) exhibits a significantly lower HE index (13.4%) than the WM with a lower AF content (53.9%, HE index 23.5%), confirming the beneficial role of AF. These findings demonstrate that AF is not merely a static hydrogen trap but a dynamic source of strain-induced strong traps, which effectively immobilize hydrogen and reduce embrittlement susceptibility under service-relevant deformation. Full article

This study investigated the temperature-dependent σ phase transformation behavior of 24Cr–14Ni stainless steel subjected to aging at 700–1000 °C for 1–8 h. At 700 °C, the σ phase mainly retained the original dendritic morphology, whereas at 800 °C, the δ → σ + γ₂ eutectoid decomposition became most pronounced. The σ phase begins to decompose from the dendrite arms. XRD analysis confirmed that σ phase precipitation was most significant at 800 °C, while only weak σ peaks were detected at 900 and 1000 °C, indicating suppressed precipitation and partial σ phase dissolution at higher temperatures. EPMA/WDS analysis showed that σ phase preferentially formed in Cr-rich δ-ferrite regions, while Ni was mainly enriched in the γ-phase. At 1000 °C, the more homogeneous Ni distribution suggested that δ-ferrite dissolution and δ → γ transformation became dominant. JMAK analysis revealed two distinct kinetic regimes: diffusion-controlled σ phase precipitation at 700–800 °C, with Avrami exponents of 0.4327–0.4606, and σ phase dissolution at 900–1000 °C, with higher Avrami exponents of 0.7932–0.8671. The apparent activation energies for precipitation and dissolution were 37.18 and 122.95 kJ·mol⁻¹, respectively. These findings indicate that σ phase transformation in 24Cr–14Ni stainless steel changes from precipitation-dominated behavior at 700–800 °C to dissolution-dominated behavior at 900–1000 °C. Full article

Multicomponent ferrite oxides with mixed valence states and tunable oxygen-defect chemistry are promising active materials for low-power memristive synapses. In this work, Ag/Ni_0.3Cu_0.2Zn_0.5Co_0.005Mn_0.005Fe_1.99O/Ag memristors were fabricated by pulsed laser deposition, and the effects of post-deposition annealing at 700–900 °C on film structure, chemical states, magnetic behavior, resistive switching, and synaptic performance were investigated. The film annealed at 800 °C exhibited a dense surface morphology, improved crystallinity, and uniform elemental distribution. X-ray photoelectron spectroscopy confirmed the coexistence of Fe²⁺/Fe³⁺ states and oxygen-related defect components, indicating the presence of oxygen vacancies. Room-temperature magnetic hysteresis measurements revealed ferrite-type magnetic behavior in the annealed films, with the 800-annealed sample showing a relatively well-defined normalized hysteresis response. The optimized device exhibited representative bipolar resistive switching within ±0.5 V, distinguishable high- and low-resistance states, Ohmic conduction in the low-resistance state, and Schottky-emission-dominated transport in the high-resistance state. These results suggest that reversible oxygen-vacancy migration and interfacial barrier modulation govern the switching process. The device showed preliminary synaptic-like transient current responses. Further systematic reliability and conductance-modulation measurements are still required to fully evaluate endurance, reproducibility, and synaptic weight-update behavior. This study demonstrates that annealing-controlled multicomponent ferrite oxides offer a feasible route for energy-efficient memristive synaptic devices. Full article

Friction stir processing (FSP) was applied to rolled 65Mn steel to improve its surface wear resistance, with untreated and conventionally water-quenched specimens used as reference conditions. A 3 × 3 full-factorial combination of rotational speeds of 600, 700, and 800 rpm and traverse speeds of 200, 250, and 300 mm/min was investigated. Continuous modified layers without obvious macroscopic defects were obtained under all processing conditions. FSP substantially fragmented and reconstructed the original ferrite–pearlite microstructure, producing a fine and relatively homogeneous acicular/lath-like transformed microstructure. The average grain size decreased from 27.1 μm to 11.6–17.2 μm, corresponding to reductions of 36.5–57.2%, while the high-angle grain-boundary fraction increased from 76.7% to a maximum of 86.2%. Although the hardness of the FSP-treated regions was comparable to that of the quenched specimen, FSP produced a more continuous and spatially uniform gradient-hardened layer. The minimum wear rate was obtained at 800 rpm and 200 mm/min, representing reductions of 97.34% and 96.14% relative to the base material and quenched specimen, respectively. The condition of 800 rpm and 300 mm/min was identified as the optimum overall condition because it provided a favorable balance among wear rate, mass loss, hardened-layer continuity, and wear-track morphology, with wear-rate reductions of 96.57% and 95.02% and mass-loss reductions of 66.67% and 57.14% relative to the two reference conditions. The improved wear resistance is primarily associated with grain refinement, grain-boundary reconstruction, microstructural homogenization, and the formation of a continuous gradient-hardened layer. Full article

The paper presents a critical review of the key work published to date on the hot ductility of steels in relation to the problem of cracking during continuous casting, including recent publications in the field. Laboratory testing methods that are most appropriate for evaluating cracking susceptibility are examined, with particular emphasis on the hot tensile test. The discussion covers both conventional carbon–manganese (C–Mn) and high-strength low-alloy (HSLA) steels, as well as the more complex advanced high-strength steels. Special attention is given to the influence of strain rate and the role of ferrite, both transformation-induced and deformation-induced, in controlling ductility. Increasing the strain rate invariably improves the ductility of steels containing a thin film of ferrite when it is present. This improvement is attributed to the work hardening of the ferrite, which promotes a more uniform distribution of strain, rather than localisation within the thin ferrite layer, thereby reducing the likelihood of fracture. The difficulties in increasing the strain rate in continuous casters are cited. Finally, based on insights from tensile testing, the paper considers practical approaches to preventing cracking in conventional curved-mould and vertical-mould arc continuous casting machines. Newly designed chamfered moulds have also recently been introduced, and these are claimed to reduce the incidence of corner cracking; their role is also discussed. Full article

The novel martensitic heat-resistant SIMP steel is considered a primary candidate structural material for Accelerator Driven Sub-critical Systems (ADSs) due to its excellent comprehensive properties. However, welding difficulties arising from its high alloy content represent a major bottleneck for its engineering application. This study explored and established optimized manual Tungsten Inert Gas (TIG) welding process parameters suitable for 20 mm thick SIMP steel plates, involving 24 layers and 110 passes, with an average heat input of approximately 4 kJ/cm, successfully achieving well-formed and defect-free welds. The welded joints were investigated using optical microscopy, scanning electron microscopy, and other performance tests. The results revealed the presence of δ-ferrite and a small amount of brittle Laves phase in the weld metal, leading to uneven hardness distribution. Furthermore, poor interfacial coordination between δ-ferrite and tempered martensite significantly reduced the ductility of the joint. Through optimized process parameters, the impact energy of the weld metal reached 183.9 J, while that of the heat-affected zone was 77.9 J. The room temperature ultimate tensile strength and yield strength of the joint were 802.63 MPa and 596.33 MPa, respectively, meeting the weld performance requirements stipulated in GB/T 38875-2020. This research provides data support for promoting the engineering application of SIMP steel in fields such as nuclear power and for enhancing the long-term service reliability of welded structures. Full article

Carbonation-induced pH reduction is a key factor triggering steel depassivation and corrosion initiation in reinforced concrete. However, the influence of pore solution chemistry on passive film (PF) stability remains unclear. In this study, ordinary Portland cement simulated pore solution (OPC-SCP) and ferrite-aluminate cement simulated pore solution (FAC-SCP) were used to investigate the evolution of PF formed at pH 12.5 and subsequently exposed to pH 11.0 and 9.5 environments. Electrochemical and microscopic techniques were employed to investigate the degradation behavior of PF under reduced alkalinity. The results show that PF in both systems degraded with decreasing pH, but exhibited markedly different stability. In the OPC-SCP system, the PF resistance decreased slightly from 4.24 × 10⁶ to 2.85 × 10⁵ Ω·cm², indicating that the steel remained in a highly passive state. In contrast, the PF resistance in the FAC-SCP system dropped significantly from 1.13 × 10⁶ to 5.57 × 10³ Ω·cm². AFM and SEM observations further revealed greater surface roughness and more severe local damage in the FAC-SCP system. The superior stability of PF in OPC-SCP may be attributed to the higher Ca²⁺ concentration, which is likely beneficial for the formation of a relatively dense and protective film. Conversely, the higher SO₄²⁻ concentration and lower Ca²⁺ content in FAC-SCP may facilitate defect growth and local dissolution, thereby contributing to depassivation. These findings highlight the critical role of pore solution chemistry in regulating PF stability under reduced alkalinity conditions. Full article

Models can help to obtain the desired properties of steel by predicting when different microstructures form during phase transformations in manufacturing processes. One prominent model for low-alloy steel is the Kirkaldy–Venugopalan model but it has not been evaluated for wear-resistant ultra-high-strength steels (UHSS). A modified Kirkaldy-type model was developed in this work for the phase transformation kinetics in a wear-resistant UHSS. A modified incremental Koistinen–Marburger model was used for the martensite transformation which considers the gradual start of the transformation. The framework was validated by simulating the dilatometry experiments in a finite element model. Good agreement was obtained for the low cooling rates 2.5 to $15$ °C/s yielding ferrite, pearlite, and bainite, as well as for the high cooling rates 20 to $50$ °C/s yielding bainite and martensite. The model was also applied to the steel Hardox 450 where it predicted the formation of 99.7% martensite at the experimental critical cooling rate for full martensite formation of $12$ °C/s found in the literature, which demonstrates the model’s capability to be used more generally on wear-resistant UHSS. The predicted hardness also captured the general trend seen in the hardness measurements. Full article

In the present work, four grades of austenitic stainless steels, namely AISI 321, AISI 316Ti, AISI 309, and AISI 310S, are investigated in terms of electrochemical hydrogenation effect on their room-temperature impact toughness. All the materials were studied in their as-received (AR), i.e., industrially manufactured, material condition. LOM and SEM microstructural analyses combined with phase XRD and EBSD phase analyses revealed in all steels the polygonal-grain austenitic matrix and varying minor amounts of elongated δ-ferrite grains. Moreover, the metastable AISI 321 and AISI 316Ti steels exhibited noticeable occurrence (16% and 10%, respectively) of the BCC-structured phases (i.e., the strain-induced α′-martensite and non-equilibrium δ-ferrite) and little occurrence of primary TiN nitrides (below 1%). The AISI 321 and AISI 316Ti steels exhibited average amounts of 2.95% and 6.32% of δ-ferrite, respectively. The stable AISI 309 steel exhibited the occurrence of intergranular (Cr,Fe)₂₃(C,N)₆ precipitates (below 3%), indicative of prolonged (slow) cooling from the warm working temperature during the material manufacturing. The individual steel grades exhibited variable values of hardness and impact toughness depending strongly on their solid solution alloying and the amounts of individual minor phases in their microstructures. The AISI 316Ti steel exhibited the highest average hardness (273 HV) and lowest impact toughness (160 J/cm²) due to Mo-alloying and having the highest amount of δ-ferrite. The AISI 310S steel showed the highest impact toughness (210 J/cm²) and the second highest hardness (245 HV) thanks to having the most stable austenitic microstructure with the highest Ni- and Cr-alloying. The AISI 321 and AISI 309 steels show similarly low hardness (195 HV vs. 196 HV) and medium values of impact toughness (202 J/cm² vs. 193 J/cm²). More importantly, all the steels under investigation exhibited detectable hydrogen-induced toughening effects, indicated by the negative HEI values. The metastable steels showed the lowest toughening effects (HEI: −2.0% and −3.8% for AISI 321 and AISI 316Ti, respectively), likely due to the adverse effect of α′-martensite. In contrast, the stable steels exhibited much higher toughening (HEI: −5.2% and −7.6% for AISI 309 and AISI 310S, respectively). Microstructural observations indicated that such toughening behavior might be related to the hydrogen-enhanced deformation banding and hydrogen-enhanced deformation twinning mechanisms, dividing the grains into smaller deformation zones, increasing the overall dissipation of deformation energy and consequently the materials’ impact toughness. Full article

The corrosion behavior of ferritic–martensitic 17Cr stainless steel in CO₂-saturated brine was investigated using static autoclave immersion tests in 4.12 wt% NaCl solution at 230 °C under CO₂ partial pressures of 6.36, 18.28, and 24.57 MPa. The calculated in situ pH values obtained using the OLI System were 3.79, 3.55, and 3.49, respectively. Corrosion morphology, microstructural evolution, and corrosion products were characterized by SEM, EDS, EBSD, and Raman spectroscopy. The average mass-loss corrosion rate increased from 0.138 ± 0.0221 mm/year at 6.36 MPa pCO₂ to 0.326 ± 0.0142 mm/year at 24.57 MPa pCO₂. Although the specimens did not show severe macroscopic pitting, localized attack preferentially occurred in fine-grained martensitic banded regions. EBSD analysis revealed that these regions exhibited higher local misorientation and defect density, which may reduce the stability of Cr-rich surface films. Raman spectra identified Cr(OH)₃ in the corrosion products, and the Cr(OH)₃ signal became more evident with increasing CO₂ partial pressure. The results indicate that, under fixed temperature and salinity, the corrosion behavior of 17Cr stainless steel is governed by CO₂ partial pressure and microstructural heterogeneity. Full article

This study investigates the synthesis–structure relationships governing sol–gel-derived nanostructured MgFe₂O₄ ferrite powders for high-temperature thermochemical energy conversion applications. The effects of key processing parameters, including propylene oxide (PO) concentration, gel aging time, calcination temperature, and calcination duration, were systematically examined to tune the phase composition, specific surface area (SSA), pore volume, crystallite size, and nanoparticle morphology of MgFe₂O₄. Increasing the PO concentration from 5 to 20 mL shortened the gelation time from 585 to 323 s and increased the SSA from 5.30 to 17.88 m²/g, while the pore volume increased from 0.0074 to 0.0210 cm³/g. In contrast, gel aging time between 24 and 120 h produced negligible changes in SSA, pore volume, and crystallite size, indicating that extended aging is not required for microstructural control. Calcination temperature strongly influenced the nanostructure: increasing the temperature from 600 to 1000 °C decreased SSA and pore volume while increasing crystallite size from 21.33 to 48.76 nm. Longer calcination times produced a similar but less pronounced effect, decreasing SSA from 18.83 to 14.89 m²/g and increasing crystallite size from 17.55 to 30.12 nm. Overall, phase-pure MgFe₂O₄ with favorable textural properties was obtained using 20 mL of PO, 24 h of aging, and calcination in the 700–800 °C range. Under the identified synthesis conditions, namely 20 mL of PO, 24 h of aging, and calcination in the range of 700–800 °C for 2 h, phase-pure MgFe₂O₄ nanoparticles with particle sizes of approximately 10–50 nm were obtained. These results establish a processing–microstructure framework for engineering MgFe₂O₄ nanomaterials with tunable textural properties for solar thermochemical redox cycles and related high-temperature energy applications. Full article

Ferrite is an essential functional material for high-power nonreciprocal microwave components, such as circulators and isolators, and its secondary electron emission (SEE) property is critical for suppressing the multipactor effect under vacuum conditions. In this work, we propose a surface engineering strategy based on periodic raised microstructures to regulate the secondary electron yield (SEY) of ferrite coatings/substrates. A Monte Carlo-based numerical method is developed to calculate the SEY of ferrite with hexagonal and annular microprotrusions of varying heights and geometric parameters. The dependence of SEY on microstructure dimensions is systematically analyzed. Spinel ferrite samples with designed microstructures are fabricated via mechanical processing. For hexagonal column arrays with a height of 1.5 mm, a side length of 0.5 mm, and a pitch of 1.67 mm, the maximum SEY is reduced from 2.4 (smooth surface) to 1.7 while the annular concentric protrusion arrays decrease the maximum SEY to 2.1. Effective suppression is achieved over the entire incident energy range for both microstructural designs. Despite a lower reduction, the annular arrays offer geometrically isotropic electron trapping, which may be advantageous for devices with circular or coaxial cavity geometries where directional dependence of the surface texture is undesirable. The results demonstrate that tailored surface microstructures can significantly mitigate SEE of ferrite, providing a promising route for developing high-performance ferrite coatings toward multipactor suppression in space microwave devices. Full article

Pearlitic wheel steel subjected to thermal–mechanical fatigue (TMF) during braking can undergo catastrophic fracture. This study clarifies the microstructural evolution governing the macroscopic cyclic hardening/softening behavior of pearlitic wheel steel under thermal–mechanical fatigue (TMF) with a constant mechanical strain range of −0.4% to +0.2%. At lower temperature amplitudes (200–500 °C), the geometrically necessary dislocation (GND) density reaches 20.4 × 10¹⁴/m² during initial cycles, corresponding to cyclic hardening due to dislocation pile-ups at cementite lamellae interfaces. With increasing cycles, the GND density decreases to 12.3 × 10¹⁴/m², concurrent with softening arising from lamellar bending/fracture, partial spheroidization, and dynamic recrystallization of ferrite. At higher temperature amplitudes (200–730 °C), the GND density decreases from 8.8 × 10¹⁴/m² to 3.5 × 10¹⁴/m², reflecting sustained cyclic softening dominated by thermally activated mechanisms, including cementite spheroidization and dislocation annihilation. The resulting softened microstructure consists of ferrite grains, intragranular dispersed cementite, and chain-like coarse cementite at boundaries. Unlike previous studies that focused on single loading conditions (e.g., thermal fatigue, rolling contact fatigue, or wear), the present work addresses the more complex TMF scenario and quantitatively elucidates the interplay between mechanical response and microstructural evolution in pearlitic steel. This work provides theoretical guidance for the development of a fatigue life prediction model for pearlitic wheels under braking. Full article