Ferrite Phase Research Articles

Effect of Intercritical Annealing Time on the Microstructure and Mechanical Properties of Dual‐Phase Steel Processed by Large‐Strain Asymmetric Cold‐Rolling

Herein, the effect of intercritical annealing time on the microstructure and mechanical properties of dual‐phase steel processed by large‐strain asymmetric cold‐rolling is studied. It is observed that the martensite islands are uniformly distributed in the ferrite phase in the microstructures of dual‐phase steels due to performing the asymmetric cold‐rolling before intercritical annealing treatment. As the intercritical annealing time increases up to 10 min, the fraction of martensite increases. By increasing the holding time and fraction of martensite, the carbon content of the martensite phase is decreased. The short‐term intercritical annealing eliminates the yield point phenomenon. However, intercritical annealing at 860 °C for 20 min leads to the reoccurrence of a yield point phenomenon. Increasing the intercritical annealing time to 10 min improves the yield strength to 505 MPa and ultimate tensile strength to 834 MPa. However, the strength decreases sharply after the holding time of 20 min. There is a perfect linear relationship between the mechanical properties and the fraction of martensite. Ductile failure is observed at the center of the fracture surfaces of dual‐phase steels while shear failure occurs at the edges of the fracture surfaces.

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

Structural, magnetic, and dielectric properties of yttrium doped cobalt ferrite and their nanocomposites with polythiophene

In the present study, the properties of nanocomposites of yttrium-doped cobalt ferrite and polythiophene (PTh) were investigated. Yttrium-doped cobalt ferrite nanoparticles CoY0.02Fe1.98O4 (CY2), CoY0.04Fe1.96O4 (CY4), and CoY0.06Fe1.94O4 (CY6) were synthesized through sol–gel autocombustion method. Their nanocomposites with polythiophene (CY2/PTh, CY4/PTh, and CY6/PTh) were synthesized via the in-situ polymerization method. XRD, FTIR, Raman spectroscopy, FESEM, VSM, and impedance analyzer characterization techniques were used to analyze the nanocomposites. The polycrystalline nature of ferrites along with the amorphous nature of PTh was observed in the XRD patterns of the nanocomposites. Additionally, FTIR spectra showed vibrational bands corresponding to both ferrite and PTh. The absence of peak at 240 cm−1 in the Raman spectra and XRD analysis confirmed the pure phase of spinel ferrites. Lesser magnetization was observed in the ferrite/PTh nanocomposites (∼27 - 51 emu/g) in comparison to yttrium-doped cobalt ferrite nanoparticles (∼64 - 84 emu/g). The incorporation of PTh in ferrite led to enhanced dielectric behavior of the materials. The saturation magnetization of ∼ 27 emu/g and maximum conductivity of ∼ 0.8 S/m (at 50 MHz) was obtained for CY4/PTh nanocomposite. Thus, the CY4/PTh nanocomposite showed optimum magnetic and dielectric behavior making it a potential material for electromagnetic interference shielding and supercapacitor applications.

Numerical interpretation of thermal desorption spectra of hydrogen from high-carbon ferrite-austenite dual-phase steel

The peak in the thermal desorption spectra of hydrogen from a high-carbon ferrite-austenite dual-phase steel shifts to the high-temperature side as the amount of charged hydrogen increases. However, the hydrogen-trapping state and the peak formation process are unclear. We interpreted the spectra by reproducing them using the previously reported model. Introducing a diffusion barrier from the ferrite phase to the austenite phase and excluding hydrogen entering the austenite phase shows that the peak is attributed to the carbide trap site when the amount of charged hydrogen is comparatively small and hydrogen trapped by the interface trap site increasingly influences the peak as the amount of charged hydrogen increases. We also found that the thickness dependence of the peak for the interface trap site could be attributed to a different reason than the conventional diffusion-determining process.

The Microstructure Evolution and Mechanical Properties of Rotary Friction Welded Duplex Stainless Steel Pipe.

The use of duplex stainless steel (DSS) in various fields is promising due to its excellent anti-corrosion properties, but traditional welding can lead to the formation of unfavorable phases that deteriorate its quality. This study aimed to use the rotary friction weld (RFW) technique to prevent the formation of harmful phases in the welding of an S32205 alloy pipe. The welding parameters used included a rotating speed of 20 m/s, a friction pressure of 10 MPa, a friction time of 30 s, and a forging pressure of 30 MPa. The microstructure and mechanical properties of the resulting RFWed joint were investigated. The results revealed that the weld zone exhibited a microstructure consisting of ferrite and austenite phases, with no deleterious phase detected. The ferrite content was measured to be 53.3%, 54.5%, and 68.7% in the base metal, thermomechanical affected zone (TMAZ), and weld, respectively, owing to the rapid cooling rate in the RFW process, which prevented any harmful phase formation in the weld zone. Furthermore, the RFW process successfully produced an ultrafine grain with a ferrite/austenite grain size of 0.40 μm and 0.41 μm, respectively. The weld zone and TMAZ contained more low-angle grain boundaries (LAGBs) compared to the base metal, which was attributed to the dynamic recovery (DRV) within a grain. The high heating and cooling rates and short welding time of the RFW process did not allow sufficient time for the dynamic recrystallization of the microstructure in the weld zone. However, a slight increase in the ferrite content in the weld zone resulted in grain refinement and an increase in the dislocation density, resulting in a slight increase in the 358 HV0.2 hardness and 823 MPa tensile strength of the weld zone. This study offers a novel approach for obtaining ultrafine grain duplex stainless steel pipes with exceptional mechanical properties through the application of RFW.

Investigation of Structural and Dielectric Behavior of Ba0.7Gd0.3MoxFe12-xO19 (Where x = 0.02) Hexaferrite Ceramics

Multiferroic ceramic composites of (Ba0.7Gd0.3MoxFe12-xO19 were prepared by using the solid-state reaction method with mol% fractions of x = 0.02, The preliminary structural studies carried out by X-ray diffraction at room temperature. It reveals that the samples have a hexagonal structure ferrite phase. The impurity phase of x-Fe2O3 appear at 32 ∼ 34 degree. Debye–Scherer and Williamson–Hall method were used to analysis for peak broadening Ba0.7Gd0.3MoxFe12-xO19 (where x = 0.02). Dielectric graphs show that curie temperature shifted towards the high temperature with increasing the frequency. the dielectric transitions at 200–300 °C.

Magnetic exchange coupling and effect of grain and grain boundaries on conduction mechanism of (MgFe2O4)100-x /(BaFe12O19)x nanocomposites

(MgFe2O4)100-x/(BaFe12O19)x, soft/hard nanocomposites (NCs) having different hard ferrite ratio i.e. x = 0, 25, 50, 75 and 100 have been synthesized via in-situ auto-combustion sol-gel method. XRD patterns ensured the co-existence of both hexaferrite and spinel ferrite phases without any impure phase and no significant difference in average crystallite size was observed. The FTIR and Raman spectra confirmed the fabrication of desired material without any impurity. The absence of blocking peak in the ZFC measurements revealed that the blocking temperature of these NCs lie above room temperature. The M-H loops taken at different temperatures resulted into single smooth curve indicating strong interphase exchange-coupling among hard-soft ferrite phases. The occurrence of shoulder peak in dM/dH versus H curve for molar ratio x = 50 is indication of weak exchange coupling between the two phases. From modified Block's law fitting, the Block's constant (B) exponentially declined with the addition of hard phase that revealed higher value of exchange-coupling constant J. The complex impedance analysis (Nyquist/Cole-Cole plots) and its simulation using an equivalent model circuit revealed the major contribution of both grains and grin-boundaries toward the total conduction mechanism of the NCs. These findings indicate that the higher value of impedance makes these NCs favorable candidates for numerous technological areas like electromagnetic shielding and high band-pass filters.

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.

Pengaruh Variasi Holding Time pada Proses Pack Carburizing dengan Arang Tempurung Kelapa Barium Carbonat Terhadap Sifat Fisik dan Mekanik Baja ST 42

Iron-based production machine components are chosen to support productivity. Steel is a metal material that is easily found on the market, but it has not fulfilled the desires of consumers in the market. This steel can be hardened by heat treatment (heat treatment). The surface treatment process is carried out to obtain the desired hardness, this can be done by increasing the carbon (C) element in the steel material, the carbon content in the standard steel structure is 0.3% - 0.59% the carbon content will have an effect. on hardness, one way to increase the element carbon (C) in steel and increase the hardness on its surface is by carburizing. This research will examine the hardness level of low carbon steel by using a pack carburizing process with different times of 2; 2.5 and 3 hours and the quenching process with pure water media.. The purpose of this study was to determine the effect of holding time on the pack carburizing process with coconut shell charcoal - barium carbonat on the hardness value of ST 42 steel. Besides that for effect of holding time on the pack carburizing process with coconut shell charcoal on the microstructure. There are differences in the surface hardness and microstructure values ​​ before and after the carburizing process. Where the highest hardness value occurs at a holding time of 3 hours with a hardness value of 872.9 HVN or an increase of 382% of the raw material and is supported by the dominance of the martensite phase which is more and more evenly distributed. Compared to the ST 42 steel raw material before the pack carburizing process was carried out, the hardness value was 174.8 HVN. A dominant microstructure consisting of a ferrite phase and a little bit of pearlite dominance.

Mechanical Characterization at Nanoscale of Austenite, Ferrite, and Sigma Phases via Hardness Measurement and Fretting Wear Behavior of a Duplex Stainless Steel

This study aimed at the mechanical characterization, on a nanometric scale, of the constituents obtained for different fractions in duplex stainless-steel plates subjected to 850, 950, 1000, and 1150 °C heating treatments via hardness measurements and determining their influences on the fretting wear behavior of the studied steel. The obtained ferrite (α)-, austenite (γ)-, and sigma (σ)-phase fractions were determined using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The mechanical characterization was carried out using hardness measurement and fretting wear techniques via nanoindentation. For comparison purposes, the Vickers microhardness was also characterized to determine the effect of the σ phase, which eventually formed, on the obtained microstructure properties as a whole. Two distinct behaviors were observed, depending on the eventual formation of σ phase as a function of the treatment temperature: (i) specimens treated at 850 and 950 °C showed a hardening effect (HV0.5 values of 333 ± 15 and 264 ± 13, respectively) due to σ-phase precipitation (hereafter termed ‘as-aged’), and (ii) specimens treated at 1000 and 1150 °C (with HV0.5 values of 240 ± 13 and 249 ± 4, respectively) showed no σ-phase precipitation (hereafter termed ‘as-solubilized’). The increases in the microhardness values for the as-aged specimens were attributed to the hardness of the σ-phase precipitates (which showed nanohardness values varying in the 8.0–8.5 GPa range), which was approximately twice that of the austenite and ferrite grains (both phases showed nanohardness values in the 3.6–4.1 GPa range, on average). When formed (for fractions on the order of 8% and 3% at 850 and 950 °C, respectively), σ phase was mainly observed at the α/γ grain interfaces or boundaries. Fretting wear tests, using a diamond sphere with a radius of 10 μm as the counter body and a load of 20 mN, revealed the same wear mechanisms in the α/γ matrix for all studied conditions. However, as-solubilized specimens (heat-treated at 1000 and 1150 °C) displayed higher resistance to fretting micro-wear in the austenitic grains compared to the ferritic grains, indicating lower plastic deformation in the respective wear scars on the obtained tracks. In particular, as-aged specimens (heat-treated at 850 and 950 °C) exhibited lower coefficients of friction due to their higher surface resistances. The localized wear at σ-phase grains was much less pronounced than at ferrite and austenite grains. Overall, this study provides valuable insights into the mechanical behavior of microstructural changes in duplex steel at the nanometric scale.

STUDY OF MECHANICAL PROPERTIES, MICROSTRUCTURE AND RESIDUAL STRESS BY X-RAY DIFFRACTION IN WELDED JOINTS OF DUPLEX STEEL UNS S 31803 BY COATED ELECTRODE

Duplex stainless steels are widely used in the chemical, nuclear and oil and gas industries. They have an austenitic-ferritic microstructure, in equal parts. In this work, the impacts on the mechanical properties, on the metallographic behavior and the analysis of the residual stresses caused after the welding process by coated electrode were analyzed. In these analyses, a specimen produced from a 10 mm thick duplex sheet was welded, and tensile tests, micrography and residual stress analysis by x-ray diffraction were carried out. The tensile test results showed values higher than those required by the material's manufacturing standard. The specimens broke in the base metal during the tests, indicating that the welding parameters were properly specified and applied to the specimen. The metallographic analysis showed an increase in the ferritic phase in the weld region, which makes this region susceptible to pitting corrosion in environments containing chlorides. Residual stress analysis showed tensile stresses in the weld bead, in the longitudinal and transverse directions, and in the HAZ and in the root compressive stresses in the transverse direction and tensile stresses in the longitudinal direction. These stresses must be considered in the design of equipment and structures manufactured from this steel and when using coated electrode welding, to avoid premature failure, especially fatigue failure.

Investigation of mechanical and microstructural properties in joining dissimilar P355GH and stainless 316L steels by TIG welding process

In this study, the joining of high-temperature and pressure-resistant P355GH and austenitic stainless 316L steels by the Tungsten Inert Gas (TIG) welding method was investigated in terms of microstructure, mechanical properties, and weld efficiency. Comprehensive tests were carried out for the weld zone, heat-affected zones (HAZ), and base metals. In metallographic characterization research, optical microscope (OM), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDX/EDS), X-ray diffraction (XRD), and EBSD analyses were performed. Average grain size and IPF (Inverse Pole Figure) map were determined from the EBSD analysis of P355GH steel, welded zone, heat-affected zones (HAZ), and 316L steel. Vickers microhardness, bending, Charpy V-Notch impact, and tensile tests were performed to determine the mechanical properties of the welded joint and the base materials. According to the EBSD results, the average grain size of the P355GH base metal was 5.13 ± 1 μm, the average grain size of the 316L base metal was 16.33 ± 5 μm, and the average grain size of the weld center was 10.09 ± 6 μm. As a result of XRD analysis, the delta ferrite phase was determined in the microstructure of the welding region. The highest Vickers microhardness value was determined as 268 ± 15 HV at the weld center. In the tensile test results, the highest tensile strength observed in the transverse weld joint was 593.92 ± 06 MPa and rupture occurred in the 316L region. Bending forces were measured as 34.331 ± 10 kN and 32.966 ± 11 kN for transverse and longitudinal welded samples, respectively. The TIG welding process showed superior properties such as toughness, high plastic deformability, and high weld efficiency of 101.89% in the joining of 316L and P355GH materials.

Understanding passive film degradation and its effect on hydrogen embrittlement of super duplex stainless steel – Synchrotron X-ray and electrochemical measurements combined with CalPhaD and ab-initio computational studies

The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic polarization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polarization, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.

Effect of titanium substitution on magnetic and dielectric properties of BaFe12O19 ceramics prepared by solid-state and co-precipitation methods

Barium ferrite has been modified by addition of titanium ions according to the formula BaTixFe(12-x)O19, with values of x as 0.00, 0.07, 0.14, 0.42, and 0.70. Samples were synthesized via solid-state and co-precipitation methods and then characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) as well as thermal analysis of dielectric constant and magnetization. The majority phase of barium ferrite with a hexagonal structure is observed in all samples, while the presence of Ti 2p was confirmed in the titanium-substituted samples. The saturation magnetization values for samples with x = 0.00, 0.07, and 0.14 are near the 50 emu/g. In contrast, samples with higher x values as 0.42 and 0.70 show significant magnetization loss with some low-temperature magnetic transition observed for x = 0.42. Dielectric characterization performed by thermal scanning of dielectric permittivity shows a substantial increase in the dielectric constant. It reveals the presence of a paraelectric-like transition at high temperatures induced by the addition of titanium.

Influence of thermo-mechanical treatment in austenitic and ferritic field condition on microstructural and mechanical properties of reduced activation ferritic-martensitic steel

Abstract 9Cr-1W-0.06Ta reduced activation ferritic-martensitic (RAFM) steel has been investigated in normalized and tempered (N + T) condition and in thermo-mechanically treated (TMT) in austenitic and ferritic field conditions. RAFM steel in N + T condition has a tempered martensitic structure (body-centred tetragonal, bct). It is unstable and will transfer into a stable body-centered cubic (bcc) in the structure during a long period subjected to high temperatures. To make it stable structure, the steel was subjected to austenitization at 1423 K for 10 min, and then cooled in still air (≈−1 K s−1) to 973 K and this temperature was maintained for 2 h in furnace to fully convert austenite into ferritic phase then the steel was subjected to 25% reduction in thickness by hot rolling. Optical, scanning and transmission electron microscopic investigations have been carried out to assess the microstructural changes of the steel N + T and TMT conditions. Hardness, tensile and creep studies are carried out and the results were correlated with the microstructural studies. TMT processed steel resulted in coarser prior austenite grains and exhibited ferrite in phase with fine distribution of M 23C6 and MX precipitates whereas N + T condition is subjected to tempered martensitic structure with coarser M 23C6 and MX precipitates. Even though ferrite phase present in TMT processed steel it exhibits higher tensile and creep strengths due to the presence of high dislocations and finer distribution of precipitates than the N + T condition.

Phase Field Study of Cr-Oxide Growth Kinetics in the Crofer 22 APU Alloy Supported by Wagner’s Theory

The Crofer 22 APU alloy is a frequently used metallic material to manufacture interconnects in solid oxide fuel cells. However, the formation and evaporation of Cr2O3 not only increases the electrical resistance but also leads to the Cr-related degradation over the service time. In order to investigate the growth kinetics of Cr-oxide, i.e., Cr2O3, the multi-phase field model coupled with reliable CALPHAD databases is employed. The phase field simulation results are benchmarked with the predictions of Wagner’s theory. Moreover, we evidence the influence of the temperature and Cr concentration on the ferritic matrix phase and the oxygen concentration at the Cr2O3/gas interface on the growth kinetics of Cr-oxide, paving the way for further investigations of Cr-related solid oxide fuel cell degradation processes.

Modifying Barium Hexaferrite Magnets by Adding Sol-Gel Synthesized Cobalt Ferrite Phase

Combining various types of ferrites brings about magnetic properties desirable for different applications. This study aims to modify barium hexaferrite (BaFe12O19) by physically mixing it with cobalt ferrite (CoFe2O4). BaFe12O19/CoFe2O4 magnets were produced by ball-milling and pressing sol-gel-derived ferrite powders. The ferrite composites showed variations in magnetic properties from BaFe12O19 magnets with a saturation magnetization of 69.46 emu/g and a maximum energy product of 0.4529 MGOe. For the BaFe12O19:CoFe2O4 weight ratio of 4:1, both satura-tion and remanent magnetizations were increased due to the addition of CoFe2O4 with high magnetizations. However, the magnetizations were reduced when the BaFe12O19:CoFe2O4 ratio was reduced to 2:1. On the other hand, the coercivity was monotonously decreased with increasing CoFe2O4. Interestingly, the maximum energy product in this study was linearly decreased with the bulk density of the magnets from 3.59 to 3.15 g/cm3. It is concluded that magnetic properties could be modified from a facile physical mixing of ferrites.

Effect of U-bending deformation on pitting corrosion of 2205 duplex stainless steel under wet-dry cycling of chloride salt droplets

The pitting behavior of 2205 duplex stainless steel after U-bending deformation was investigated under wet-dry cycling of droplets. At the tensile side, the average critical relative humidity for pitting (RHpit) was increased from 44.5% to 55.6%, the critical relative humidity (RHrep) for repassivation was correspondingly increased from 59.9% to 76.6%. The compressive strain increased the critical relative humidity to 52.9% (RHpit) and 73.3% (RHrep), respectively. Compared to the hemispherical pits without deformation, selective dissolution of the ferrite phase with irregular pits appeared on the tensile side. The decrease in pitting resistance and the formation of irregular pit morphology were discussed.

Process Parameter Optimization of 2507 Super Duplex Stainless Steel Additively Manufactured by the Laser Powder Bed Fusion Technique

Laser powder bed fusion is an attractive technology for producing high-strength stainless steel alloys. Among the stainless steels, 2507 super duplex stainless steel (2507 SDSS) is known for its excellent combination of corrosion resistance and high strength. Although there are some studies that aimed at optimizing the laser powder bed fusion (LPBF) printing parameters to print highly dense 2507 SDSS parts; However, a full optimization study is not reported yet. This study aims at optimizing the printing parameters for 2507 SDSS, namely: laser power, scan speed, and hatch distance. The response surface methodology was used in generating a detailed design of experiment to investigate the different pore formation types over a wide energy density range (22.22–428.87 J/mm3), examine the effects of each process parameter and their interactions on the resulting porosity, and identify an optimized parameter set for producing highly dense parts. Different process parameters showed different pore formation mechanisms, with lack-of-fusion, metallurgical or gas, and keyhole regimes being the most prevalent pore types identified. The lack-of-fusion pores are observed to decrease significantly with increasing the energy density at low values. However, a gradual increase in the keyhole pores was observed at higher energy densities. An optimal energy density process window from 68.24 to 126.67 J/mm3 is identified for manufacturing highly dense (≥99.6%) 2507 SDSS parts. Furthermore, an optimized printing parameter set at a laser power of 217.4 W, a scan speed of 1735.7 mm/s, and a hatch distance of 51.3 µm was identified, which was able to produce samples with 99.961% relative density. Using the optimized parameter set, the as-built 2507 SDSS sample had a ferrite phase fraction of 89.3% with a yield and ultimate tensile strength of 1115.4 ± 120.7 MPa and 1256.7 ± 181.9 MPa, respectively.

Age-hardening behaviors of the weld metals of 22% Cr and 25% Cr duplex stainless steels at 400 °C

Duplex stainless steel (DSS) alloys are attractive materials for pressure components in nuclear power plants as they offer a good combination of high strength, corrosion resistance, and weldability. However, DSS suffers from embrittlement and hardening upon exposure to temperatures of 250–500 °C. Most previous studies addressed only solution-treated materials, and limited research has been conducted on weld metals. In this study, the relationship between the nanostructural evolution and hardening of 22% Cr and 25% Cr DSS weld metals (fabricated with grade-2209 and 2594 welding materials, respectively) during aging at 400 °C was investigated. The results revealed that the degree of hardening of ferrite after aging at 400 °C for 1000 h was greater in the 22% Cr DSS weld metal than in the 25% Cr DSS weld metal. The ferrite phases of both samples underwent two different phase decomposition processes during aging, (1) phase separation (α → Fe-rich α + Cr-rich α’) and (2) solute clustering to form G-phase precursors, both of which were identified within the body-centered cubic structure of ferrite in the weld metals. Phase separation was less pronounced, whereas formation of G-phase precursors was more pronounced in the 22% Cr DSS weld metal than in the 25% Cr DSS. These results indicate that the extent of formation of G-phase precursors has a significant influence on predicting the degree of hardening of ferrite. The enhanced formation of G-phase precursors in the 22% Cr DSS weld metal was elucidated using thermodynamic simulations; the formation of the G-phase precursor was predominantly facilitated by the higher Mn fraction in the 22% Cr DSS weld metal. The findings of the present study are expected to aid the selection and/or further improvement of the compositional design of DSS welding materials, thereby increasing the long-term safety of DSS welded components in nuclear applications.