Duplex Stainless Steel Powders Research Articles

Study on microstructure and properties evolution of duplex stainless steel powders by high-energy ball milling

Powder metallurgy duplex stainless steels have garnered significant attention due to its exceptional mechanical properties. The preparation of raw material powders plays a crucial role in the sintering process of duplex stainless steels. The effects of high-energy ball milling on microstructure and properties of duplex stainless steel powders have been studied. Results show that the particles size are effectively reduced by increasing the rotation speed. After milling at 800 r/min for 15 h, the particles size of duplex stainless steel were reduced from 86.17 μm to 69.46 μm and the α and γ phases were uniformly distributed. Additionally, the microstructure of sintered duplex stainless steel milled at 800 r/min is refined and the microhardness is increased from 267 HV0.3 to 313 HV0.3, with improvement of 17.23%, which is attributed to the grain refinement through high-energy ball milling.

Effect of Ball to Powder Ratios on the Phase Transformation of Austenite and Ferrite During Ball Milling of SAF-2507 Super Duplex Stainless Steel Powders

Effect of Ball to Powder Ratios on the Phase Transformation of Austenite and Ferrite During Ball Milling of SAF-2507 Super Duplex Stainless Steel Powders

Chemistry and microstructure of duplex stainless steel powders from recycled Z100 mixed with 316L steels

Recovered metallic waste can be used in additive manufacturing as a feedstock if the subsequent steps of the waste-to-product process are sufficiently mastered. In this study, impact of recycling of Z100 duplex steel mixed with 316L steel on the resulting powders microstructure and chemical composition was investigated. The utility of the original method of recycling stainless steels into a high-grade powder suitable for additive techniques has been demonstrated. By examining three gradations of powders, namely 20–50 μm, 50–100 μm and 125–250 μm, differences in selected properties in relation to the average particle size are shown. The results suggest that with increasing the particle diameter, fine-crystalline γ-austenite is favoured to precipitate at the boundaries and within the volume of the originally formed large δ-ferrite grains. It is reflected by a decrease of δ/γ fraction ratio from 0.64 in the 20–50 μm powders to 0.20 in the 125–250 μm, respectively. Obtained results indicate non-diffusional, shear or semi-shear character of δ → γ + δ phase transformation. The resulting fine-crystalline austenite is characterised by a significant dislocation density, which induces dislocation strengthening effect, responsible for an increase in Vickers hardness from 145 HV and Young's modulus from 29 GPa in the 20–50 μm group to 310 HV and 146 GPa in the 125–250 μm fraction, respectively.

Electrochemical Determination of Ascorbic Acid by Mechanically Alloyed Super Duplex Stainless Steel Powders

SAF-2507 super duplex stainless steel powders (SDSS) were prepared using a high-energy planetary ball milling process. The X-ray diffraction (XRD) shows peak broadening after 20 h of ball milling and revealed a phase transformation resulting in a two-phase alloy mixture containing nearly equal amounts of ferrite (α) and austenite (γ). After 20 h of ball milling the particle size was reduced to ~201 nm. Scanning electron microscope (SEM) micrographs showed small-size irregular grains with an average particle size ranging from 5–7 µm. The high-resolution transmission microscope (HRTEM) analysis confirmed the presence of nanocrystalline particles with sizes ranging from 10 to 50 nm. The presence of ferrite phase is visible in the corresponding diffraction pattern as well. In this paper, we have discussed the electrochemical sensor application of mechanically alloyed nano-structured duplex stainless steel powders. The fabricated 4 mg duplex stainless steel modified carbon paste electrode (SDSS-MCPE) has shown excellent current sensitivity in comparison with 2, 6, 8, and 10 mg SDSS-MCPEs during the detection of ascorbic acid (AA) in a phosphate buffer solution with a pH of 6.8. The calculated electrode active surface area of SDSS-MCPE was found to be almost two times larger than the surface area of the bare carbon paste electrode (BCPE). The limit of detection (LD) and limit of quantification (LQ) were found to be 0.206 × 10−8 M and 0.688 × 10−8 M, respectively, for the fabricated 4 mg SDSS-MCPE.

Effect of Sintering Temperature and Solution Treatment on Phase Changes and Mechanical Properties of High-Nitrogen Stainless Steel Prepared by MIM

High-nitrogen stainless steel (HNSS) has been widely concerned and studied owing to its excellent mechanical, corrosion resistance, and biocompatibility properties. A series of HNSS was prepared by metal injection molding (MIM) using gas atomized Fe-Cr-Mn-Mo-0.3 N duplex stainless steel powders. Both sintering and solution treatments were carried out in an N2 atmosphere. The effects of nitrogen distribution and phase transformation on the mechanical properties of MIM HNSS during sintering and solution were studied. The results show that as the sintering temperature increased, the sample density increased, but the total nitrogen content decreased. Nitrogen and Cr2N concentration gradients along the cross-section of as-sintered samples were formed after cooling. The high nitrogen content promotes the decomposition of γ: γsaturated translated to γ and Cr2N. Meanwhile, the low Mn content in austenite also decomposes γ: γ translated to α and Cr2N. After solution treatment, a single γ phase was obtained for samples sintered at 1200 to 1320 °C. For solution treatment samples sintered at 1320 and 1350 °C, their tensile strength was 988.76 and 1036.12 MPa; yield strength was 615.61 and 636.14 MPa, and elongation was 42.58 and 40.08%, respectively. These values vastly exceeded the published MIM HNSS values.

Gas Atomization of Duplex Stainless Steel Powder for Laser Powder Bed Fusion.

Duplex stainless steel powders for laser additive manufacturing have not been developed extensively. In this study, the melts of a super duplex stainless steel X2CrNiMoCuWN25-7-4 (AISI F55, 1.4501) were atomized with different process gases (Ar or N2) at different atomization gas temperatures. The process gas N2 in the melting chamber leads to a higher nitrogen dissolution in the steel and a higher nitrogen content of the atomized powders. The argon-atomized powders have more gas porosity inside the particles than the nitrogen-atomized powders. In addition, the higher the atomization gas temperature, the finer the powder particles. The duplex stainless steel powders showed good processability during PBF-LB/M (Laser powder bed fusion). The gas entrapment in the powder particles, regardless of the gas chemistry and the gas content, appears to have a negligible effect on the porosity of the as-built parts.

Obtainment of a new metal matrix composite from the recycling of UNS S31803 duplex stainless steel by powder metallurgy

The purpose of circular economy is to maximize resources efficiency. With the increase in consumption of raw materials, energy and waste generation, recycling is necessary for environmental and industrial reasons. Thus, the powder metallurgy, together with mechanical milling, stands as a new method for scraps recycling. This study aimed to produce UNS S31803 duplex stainless steel powders, with vanadium carbide (VC) addition as reinforcement, through mechanical milling. Different uniaxial compaction pressures (700, 800 and 900 MPa) were used, as well as sintering temperatures (1200, 1240 and 1280 °C) and times (1, 1.5 and 2 hours), in order to determine the best conditions. Particle size analysis, scanning electron microscopy and x-ray diffraction were used to characterize the powders. It was observed that the use of VC increased the comminution during the milling process, being obtained an average particle size of 42 μm. The highest density obtained was of 82% and hardness of 72%, in comparison with the steel from the manufacturing process. According to the analysis, it was verified that this method is viable, becoming an alternative route to reuse UNS S31803 duplex stainless steel scraps.

A Study of Multi-Pass Laser-Cladding 2205 Duplex Stainless Steel Coating: Microstructure, Electrochemical Corrosion Behavior, and Wear-Resistance Properties

By applying 2205 duplex stainless steel powders to repair and improve carbon steel, multi-pass laser-cladding tests were conducted on Q235 carbon steel surfaces with different laser powers in the range of 1.9~2.5 kW in order to evaluate the performance of the laser-cladding layers. The phase composition, macro- and microstructure, electrochemical corrosion resistance, friction, and wear resistance of the laser-cladding layers were investigated. Macroscopic observation identified no obvious cracks. The phases that made up the multi-pass laser-cladding layers were γ-Fe and α-Fe. Owing to the optimal laser power at 2.3 kW, a large number of equiaxed crystals and a small number of cellular crystals made up the microstructure of the prepared laser-cladding layer, which contributed to its good corrosion resistance. The wear resistance of the multi-pass laser-cladding layer and the carbon steel was also studied. A combined action of adhesive wear and abrasive wear, accompanied by oxidative wear, was determined through observation of pits and furrows on the friction surface of the multi-pass laser-cladding layer, while a typical mechanism of abrasive wear was confirmed by checking the friction surface of the carbon steel.

Influence of particle size on powder rheology and effects on mass flow during directed energy deposition additive manufacturing

Metal powders used in directed energy deposition additive manufacturing are subjected to particle-particle and particle-gas interactions not captured through traditional powder characterization. Variations in powder performance were identified through mass flow rate measurements of lean (UNS S32101), standard (UNS S32205), and super (UNS S32750) duplex stainless steel powders specified for directed energy deposition on three commercial powder feeders. Across these different systems, the lean grade consistently displayed the lowest mass flow rates. Differences between the three powders were only identified with advanced powder and rheological characterization tools, which linked particle characteristics and rheological properties to powder flow and performance. A slight shift to a larger size distribution in the lean grade produced a higher percentage of larger particles with higher masses that resisted motion and increased flow resistance. Higher flow resistance was captured through higher cohesion strength and fluidization measurements during rheological testing.

Effect of sintering temperature and solution treatment on microstructure and mechanical properties of high-N Ni-free austenitic stainless steel prepared by metal injection molding

High-N austenitic stainless steel was prepared by injection molding and sintering under nitrogen atmosphere by using gas atomized Cr-Mn-Mo-N duplex stainless steel powders. The powders and binder were mixed, injected, debinded and sintered. Effect of sintering temperature and solution treatment on microstructure and mechanical properties were studied. It shows that sintering temperature has significant effect on N content and density. With the increase of sintering temperature, the sample density was increased, but the N content was decreased. Samples sintered at 1200 °C was composed of austenite and Cr2N, while only austenite can be detected by XRD for samples sintered at 1250 and 1300 °C. The fracture mode of the samples sintered at 1200 and 1250 °C is brittle fracture, along the 45° direction and stress axis, but samples sintered at 1300 °C did not fractured at the maximum load (21 kN) and showed ductility during compression. After solution treatment, the Cr2N was disappeared and samples sintered at 1250 °C did not fractured during compression.

Characterization of high-strength high‑nitrogen austenitic stainless steel synthesized from nitrided powders by spark plasma sintering

A high-N austenitic stainless steel (HNASS) was fabricated using a novel approach, in which duplex stainless steel powders were firstly gas nitrided in the temperature range of 700–900 °C and then consolidated using spark plasma sintering, resulting in an FCC matrix with average grain sizes of 1.6–8.2 μm. The Cr2N precipitates transformed from cellular precipitates (lamellar within the grains) to intergranular precipitation (equiaxed along the grain boundaries) as the nitriding temperature rose from 700 °C to 750 °C, and the N content rose from 0.68 wt% to 1.71 wt%. The 700 °C-treated alloy exhibits a yield strength of 783 MPa, a high tensile strength of 1091 MPa, and an elongation of 41% while the 900 °C-treated alloy exhibits an ultra-high compressive strength of 3017 MPa, and hardness of 520 HV. These values dramatically exceed the published values of similar HNASSs. Quantitative calculations showed that these superior mechanical properties could be attributed to increased solid solution and grain boundary strengthening. Furthermore, it can be inferred that while the equiaxed Cr2N phase increased the yield strength, it greatly degraded the fracture toughness.

A New Method to Recycle Stainless–Steel Duplex UNS S31803 Chips

Due to the increased consumption of raw materials, energy, and the waste it generates, recycling has become very important and fundamental for the environment and the industrial sector. The production of duplex stainless–steel powders with the addition of vanadium carbide in the high energy mechanical milling process is a new method for recycling materials for the manufacture of components in the industrial sector. This study aims to reuse the chips from the duplex stainless–steel UNS S31803 by powder metallurgy with the addition of Vanadium carbide (VC). The mechanical milling was performed using a planetary ball mill for 50 h at a milling speed of 350 rpm and a ball-to-powder weight ratio of 20:1, and the addition of 3 wt % of VC. The material submitted to milling with an addition of carbide has a particle size of less than 140 μm. After milling, the sample went through a stress relief treatment performed at 1050 °C for 1 h and the isostatic compaction process loaded with 300 MPa. The sintered powders and material was characterized by scanning electron microscopy, X-ray diffraction, and micro-hardness tests. The milling process with an addition of 3% VC produced a particle size smaller than the initial chip size. The measurement of micrometric sizes obtained was between 26 and 132 μm. The sintered material had a measurement of porosity evaluated at 15%. The obtained density of the material was 84% compared to the initial density of the material as stainless–steel duplex UNS S31803. The value of the microhardness measurement was 232 HV. The material submitted for grinding presented the formation of a martensitic structure and after the thermal treatment, the presence of ferrite and austenite phases was observed. Thus, in conclusion, this study demonstrates the efficacy in the production of a metal-ceramic composite using a new method to recycle stainless–steel duplex UNS S31803 chips.

Evaluation of high-energy milling efficiency in stainless steel with addition of vanadium carbides

The production of duplex stainless steel powders with the addition of carbides by high-energy mechanical milling is a novel method for recycling chips. With the increase in the consumption of raw material and energy and of the generation of residues, recycling is necessary due to environmental and industrial reasons. In this study, the effect of the addition of vanadium carbide on the morphology, particle size, and magnetic properties of the powders was investigated. The milling was realized using a planetary ball mill for 50 h at a milling speed of 350 rpm. The ball-to-powder weight ratio used was 20:1 and the 0, 1, and 3% wt. vanadium carbide addition. Produced duplex stainless steel powders from recycling chips were characterized by a scanning electron microscope (SEM), a laser particle size analyzer, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and magnetic characterization. The milling process led to the formation of martensite induced by deformation phase. It was verified that the addition of 3% carbide was the most effective in reduction of the particle size when compared to milling without carbide. The particle size of fabricated powders after 50 h of milling with 3% vanadium carbide addition was about 174 times lower than that of initial chips.

Structural and Magnetic Properties of Duplex Stainless steel (UNS S31803) Powders Obtained by high Energy Milling of Chips with Additions of NbC

In this study, the high energy mechanical milling was utilized to the production of duplex stainless steel powders from the recycling chips with and without the addition niobium carbide. The effect of milling time and addition carbide on the morphology, particle size and magnetic properties of the powders was investigated. The utilization of the powder metallurgy constitute an important alternative for the reuse of waste in the industrial sector. The milling was realized using a planetary ball mill for 20 hours at a milling speed of 350 rpm and ball-to-powder weight ratio of 15:1were used, and the (0% and 3 wt. %) niobium carbide (NbC) addition. Scanning electron microscopy and particle distribution analysis were used for characterization of morphology and measure of particle size. Also, to analyze the transformations of phases were investigated using the magnetic characterization and x-ray diffraction. The results indicated that the addition of niobium carbide results in the reduction of the particle size of steel of the order of 20%, besides the reduces the transformation of austenite into strain induced martensitic phase.

Metallographic evaluation of duplex stainless steel powders processed by selective laser melting

PurposeThis paper aims to establish the microstructures and the process-structure relationships in duplex stainless steel powders consolidated by selective laser melting (SLM).Design/methodology/approachA priori data on energy density levels most appropriate to consolidation of duplex stainless steel powders through SLM served as the basis to converge on the laser settings. Experimental designs with varying laser power and scan speeds and test pieces generated allowed metallographic evaluations based on optical and scanning electron microscopy and electro backscatter diffraction analyses.FindingsDuplex stainless steel powders are established for processing by SLM. However, the dynamic point heat source and associated transient thermal fields affect the microstructures to be predominantly ferritic, with grains elongated in the build direction. Austenite precipitated either at the grain boundaries or as Widmanstätten laths, whereas the crystallographic orientations and the grain growth are affected around the cavities. Considerable CrN precipitation is also evidenced.Originality/valueDuplex stainless steels are relatively new candidates to be brought into the additive manufacturing realm. Considering the poor machinability and other difficulties, the overarching result indicating suitability of duplex powders by SLM is of considerable value to the industry. More significantly, the metallographic evaluation and results of the current research allowed further understanding of the material consolidation aspects and pave ways for fine tuning and establishment of the process-structure-property relationships for this important process-material combination.

Selective Laser Melting of Duplex Stainless Steel Powders: An Investigation

Selective laser melting gained substantial momentum in the recent past and quite a few alloy systems have been researched and made available for commercial use; titanium, aluminum, stainless and tool steels, cobalt chrome, and Inconel being the most popular examples. Despite the application potential, and the successful processing of powder forms by traditional powder metallurgy methods, selective laser melting of duplex stainless steels was not attempted so far. The response of a duplex stainless steel alloy to processing by selective laser melting with varying process conditions is evaluated in the current research. Experimental results ascertained that the complete cycle starting from duplex powders, consolidating into 3D forms by selective laser melting and then post-process heat treatment to bring the microstructures back to duplex forms is feasible. Within the current experimental domain, the multi-layer samples are close to 90% density and showed a maximum dimensional variation of 2–3%, while the austenite to ferrite ratio is 45:55 after the post-process heat treatment.

Effect of Nanosize Yittria and Tungsten Addition to Duplex Stainless Steel During High Energy Planetary Milling

In this present investigation, elemental powders of duplex stainless steel composition (Fe-18Cr-13Ni) with 1 wt. % nano yittria and tungsten were milled separately in dual drive planetary mill (DDPM) for 10 h to fabricate yittria dispersed and tungsten dispersed duplex stainless steel powders. The milled powder samples were characterized by X-Ray diffraction and scanning electron microscopy (SEM) to study the size, morphology and phase evolution during milling. The gradual transformation from ferrite to austenite is evident from XRD spectra during milling. The crystallite size and lattice strain of yittria dispersed duplex stainless steel after 10 h milling were found to be 7 nm and 1.1% respectively. The crystallite size of tungsten dispersed duplex stainless steel was 5 nm. It has been observed from SEM analysis that particles size has been reduced from 40 to 5 μm in both cases. Annealing of 10 h milled powder was performed at 750°C for 1 h under argon atmosphere to study phase transformation in both yittria and tungsten dispersed duplex stainless steel. The XRD analysis of annealed stainless steel depicts the phase transformation from α-Fe to γ-Fe with the formation of oxides of Y,Fe and Cr. The differential scanning calorimetry analysis was conducted by heating the milled powder from room temperature to 1200°C under argon atmosphere to investigate the thermal analysis of both the stainless steel powders.

Characterization of functional gradient structures in duplex stainless steel castings

Titanium carbide particle reinforced metal-matrix composite coatings, formed on duplex (austenitic/ferritic) stainless steel castings, were studied. These coatings, developed for wear applications, develop a functional gradient structure through a self-propagating high-temperature synthesis (SHS) of titanium carbide (TiC). This SHS-reaction was ignited in reactive inserts by the heat of liquid steel. The reactive inserts were compressed from mixtures of titanium, carbon and molybdenum powders together with iron or duplex stainless steel powders. The coating micro- structure depends on the reactive insert composition. Molybdenum reduced the TiC particle diameter, and reducing the binder content reduced the areal fraction of TiC.

Activated sintering of P/M duplex stainless steel powders

Boron alloyed duplex stainless steels have been obtained by sintering the mixture of austenitic and ferritic stainless steels powders, and their mechanical properties have been studied in comparison to the boron free sintered duplex stainless steels. Boron activates sintering and enhances densification, closing the residual porosity and increasing ductility with respect to the boron free sintered duplex stainless steels.

Sintering of prealloyed duplex stainless steel powders at atmospheric pressure

In this paper an extension of the sintering method at atmospheric pressure, already used for tool steels, is proposed for prealloyed duplex stainless steel powders. A wide temperature range is tested and the effectiveness of boron as activator of the process is investigated. In all the samples no evidence of liquid phase is observed ; the sintering process allows full density in case of no boron addition and it occurs by solid phase diffusion, enhanced by the presence of two phases, austenite and ferrite : this makes needless to add boron as activator. We obtain density and hardness values comparable with the “as-cast” ones, isotropic phases structure and γ/δ ratio of about 50/50 for the best sintering temperature (1150 °C).