Ferritic Steel Powders Research Articles

Effect of Nitrogen Atmosphere Annealing of Alloyed Powders on the Microstructure and Properties of ODS Ferritic Steels.

Oxide Dispersion Strengthened (ODS) ferritic steels are promising materials for the nuclear power sector. This paper presents the results of a study on the sintering process using the Spark Plasma Sintering (SPS) technique, focusing on ODS ferritic steel powders with different contents (0.3 and 0.6 vol.%) of Y2O3. The novelty lies in the analysis of the effect of pre-annealing treatment on powders previously prepared by mechanical alloying on the microstructure, mechanical, and thermal properties of the sinters. Using the SPS method, it was possible to obtain well-densified sinters with a relative density above 98%. Pre-annealing the powders resulted in an increase in the relative density of the sinters and a slight increase in their thermal conductivity. The use of low electron energies during SEM analysis allowed for a fairly good visualization of the reinforcing oxides uniformly dispersed in the matrix. Analysis of the Mössbauer spectroscopy results revealed that pre-annealing induces local atomic rearrangements within the solid solution. In addition, there was an additional spectral component, indicating the formation of a Cr-based paramagnetic phase. The ODS material with a higher Y2O3 content showed increased Vickers hardness values, as well as increased Young's modulus and nanohardness, as determined by nanoindentation tests.

Manufacturing of Corrosion-Resistant Surface Layers by Coating Non-Alloy Steels with a Polymer-Powder Slurry and Sintering.

This paper describes the combination of surface engineering and powder metallurgy to create a coating with improved corrosion resistance and wear properties. A new method has been developed to manufacture corrosion-resistant surface layers on steel substrate with additional carbide reinforcement by employing a polymer-powder slurry forming and sintering. The proposed technology is an innovative alternative to anti-corrosion coatings applied by galvanic, welding or thermal spraying techniques. Two different stainless-steel powders were used in the research. Austenitic 316 L and 430 L ferritic steel powders were selected for comparison. In addition, to improve resistance to abrasive wear, coatings containing an additional mixture of tetra carbides (WC, TaC, TiC, NbC) were applied. The study investigates the effects of using multicomponent polymeric binders, sintering temperature, and atmosphere in the sintering process, as well as the presence of reinforcing precipitation, microstructure and selected surface layer properties. Various techniques such as SEM, EDS, hardness and tensile tests and corrosion resistance analysis are employed to evaluate the characteristics of the developed materials. It has been proven that residual carbon content and nitrogen atmosphere cause the release of hard precipitations and thus affect the higher mechanical properties of the obtained coatings. The tensile test shows that both steels have higher strength after sintering in a nitrogen-rich atmosphere. Nitrogen contributes over 50% more to the tensile strength than an argon-containing atmosphere.

Formation and Microstructural Evolution of Ferritic ODS Steel Powders during Mechanical Alloying.

Ferritic ODS steel elemental powder compositions with various Zr content (0.3-1.0 wt.%), ground in a Pulverisette 6 planetary ball mill, were extensively studied by X-ray diffraction line profile analysis, microscopic observations, microhardness testing and particle size measurements. A characteristic three-stage process of flattening the soft powders, formation of convoluted lamellae and, finally, formation of nanocrystalline grains was observed. In order to quantify the microstructural properties, expressed mainly in terms of crystallite size and dislocation density, a methodology for detailed and accurate microstructure analysis of nanosized and severely deformed materials was proposed by the Whole Powder Pattern Modelling (WPPM) approach. In the case of the proposed ODS alloy composition, the overlapping of Fe and Cr Bragg reflections makes the microstructure analysis certainly more complicated. The results showed that the microstructure of powders evolved towards the nanocrystalline state consisting of fine (diameter of ~15 nm) and narrowly dispersed domains, with extensive dislocation density exceeding 1016 m-2.

Laser Powder Bed Fusion of ODS 14YWT from Gas Atomization Reaction Synthesis Precursor Powders

Laser powder bed fusion (LPBF) additive manufacturing (AM) is a promising route for the fabrication of oxide dispersion strengthened (ODS) steels. In this study, 14YWT ferritic steel powders were produced by gas atomization reaction synthesis (GARS). The rapid solidification resulted in the formation of stable, Y-containing intermetallic Y2Fe17 on the interior of the powder and a stable Cr-rich oxide surface. The GARS powders were consolidated with LPBF. Process parameter maps identified a stable process window resulting in a relative density of 99.8%. Transmission electron microscopy and high-energy x-ray diffraction demonstrated that during LPBF, the stable phases in the powder dissociated in the liquid melt pool and reacted to form a high density (1.7 × 1020/m3) of homogeneously distributed Ti2Y2O7 pyrochlore dispersoids ranging from 17 to 57 nm. The use of GARS powder bypasses the mechanical alloying step typically required to produce ODS feedstock. Preliminary mechanical tests demonstrated an ultimate tensile and yield strength of 474 MPa and 312 MPa, respectively.

Fundamental aspects of functional grading via powder hot isostatic pressing – Development of microstructure and diffusional processes

Functional grading of a ferritic steel (SA508 grade 3) and austenitic stainless steel (316L) powder via hot isostatic pressing (HIP) consolidation is investigated as an alternative to dissimilar welding. Fundamental aspects of this method such as the development of microstructure and diffusional processes during the HIP process of such mixed powder are studied in this work. A change of the post-HIP α to γ ratio with HIP temperature was observed in mixed powder specimens, combined with the formation of a ferritic lath zone which affects the properties of the specimens (throughout this work the term ferritic laths refers to a ferritic phase of unknown structure which is likely to consist of martensite, bainite or a mixture of both). These effects were attributed to extensive diffusion of alloying elements from 316L into SA508 grade 3. Simulation and experimental quantification of diffusion in diffusion couple experiments were applied to derive an empirical model for estimating α-Fe boundary migration and the formation of the ferritic lath zone and phase fractions for various temperatures and dwell times of the HIP process.

The Characterization of a Gas-Atomized Ferritic Steel Powder Pre-alloyed with 6 Wt Pct Aluminum

The Characterization of a Gas-Atomized Ferritic Steel Powder Pre-alloyed with 6 Wt Pct Aluminum

Optimization of milling speed and time in mechanical alloying of ferritic ODS steel through taguchi technique

The oxide dispersion strengthened (ODS) ferritic steels are one of the most important in fuel cladding materials for 4th Generation nuclear reactors because of their excellent mechanical properties such as irradiation resistance, swelling resistance, and elevated temperature tensile/compressive strength. Mechanical alloying (MA) is one of the most promising routes for developing nanocrystalline ferritic ODS steel materials. For the production of nanocrystalline ferritic ODS steel powders, the most influencing factor is the milling speed and milling time during the mechanical alloying process. With the improper selection of milling time and speed, the final milled powders become an amorphous structure consisting of high impurity inclusions in the microstructure, and strength was also affected. In order to overcome these drawbacks, the present investigation was taken into account for the selection of appropriate mechanical milling speed and time, which was optimized through Taguchi analysis followed by the MA process. The optimized mechanical milling speed and time of milled powders were characterized through X-Ray Diffraction Analysis (XRD) and Scanning Electron Microscope (SEM).

Microstructure formation and mechanical properties of ODS steels built by laser additive manufacturing of nanoparticle coated iron-chromium powders

Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y2O3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y2O3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts.

XRD Line Profile Analysis of Ball Milled ODS Ferritic Steel Powder Consolidated by Hot Isostatic Pressing

In this study, an ODS ferritic steel powder of composition Fe–12Cr–2W–1Zr–0.2Y (in wt %) was mechanically alloyed in a planetary ball mill, until the proper solid solution was established and consolidated by hot isostatic pressing. Basic microstructure features, in terms of the nature, density of the dislocations, lattice parameter and lognormal domain sizes distribution,—were determined by X-ray diffraction profile analysis, using the recent, powerful whole powder pattern modelling approach. XRD analysis revealed that microstructure during ball milling evolved towards the nanocrystalline state made of very fine, narrowly dispersed crystallites with strongly correlated dislocation structure of extensive number density of dislocations reaching almost 1017 m–2. Mean coherent domain size was estimated using both whole powder pattern modelling and modified Williamson–Hall methods, which yielded 9.5 and 7.5 nm, respectively. Powder consolidation induced crystallite growth accompanied with restrained drop in dislocation density. The anisotropic strain broadening of bcc iron diffraction peaks was taken into account by incorporating the dislocation contrast factor, directly related to elastic constants of material and to the dislocation slip system.

Magnetically assisted directed energy deposition

Poor powder catchment efficiency, is a limiting factor in the adoption of Directed Energy Deposition for the processing of expensive materials, complex shapes and for the sake of energy efficiency. This study demonstrates a novel approach to improve powder catchment efficiency using a magnetic field to direct ferritic steel powder during deposition. Powder catchment efficiency increased to a value of 56% on a flat plane, compared to 26% under the same conditions without magnetic assistance. The mild steel substrate, being ferromagnetic, permitted greater magnetic manipulation of powder and higher powder catchment, compared to the austenitic stainless steel, which is essentially non-magnetic. It is proposed that the increase in powder catchment comes about by both the increased concentration of powder directly beneath the nozzle when passing over a magnet, as well as pre-loading of powder over the positions of magnets, due to rebounding particles, prior to the laser beam reaching them. Magnetic assistance was also shown to significantly improve catchment efficiency at substrate inclination angles of up to 45°. In addition, evidence of compression of deposited tracks was also seen on mild steel when magnets were used. Magnetic assistance therefore has significant potential as a process augmentation for improving material and energy efficiency in directed energy deposition, with the benefit of being able to be retrofitted to existing setups.

Investigation of high-temperature oxidation behavior of silicon added 14Cr nanostructured ferritic alloys synthesized via mechanical alloying and spark plasma sintering

In the present study, the effect of Si addition on the microstructure and subsequently, on the oxidation behavior of nanostructured oxide dispersion strengthened (ODS, 14YWT) ferritic steel was investigated. Two types of nanostructured ODS ferritic steel powders viz. Fe-14Cr-2W-0.3Ti-0.3Y2O3 (Si-free) and Fe-14Cr-2W-0.3Ti-1Si-0.3Y2O3 (Si-containing) were mechanically alloyed up to 50 h and consolidated via spark plasma sintering. The Si-containing sintered ODS steel contained single phase ferritic microstructure with finer grains and nanoparticles of Y2Ti2O7, SiO2, and Cr2TiO4; whereas, the Si-free one contained different phases such as ferrite, austenite, and martensite with nanoparticles of Cr2O3 and Y2Ti2O7. After performing oxidation at 850 °C for 100 h, the weight gain in the Si-containing sample was ∼18 times lower than that of the Si-free sample. Analyzing the surface and cross-section of the oxide layers via SEM-EDS, XRD, Rietveld refinement of XRD patterns and Raman spectroscopy, it was revealed that Si-free ODS steel consisted of outer Fe2O3 and inner FeCr2O4 layers which were porous and possessed whisker-like morphology. However, the oxidized Si-containing ODS steel possessed thin, dense, adherent and strong protective outer layer of (Fe, Cr)2O3 and the inner layer consisted mixture of Cr2O3 and Fe2SiO4 oxides. Synergistic effect of the single-crystal structure matrix and Si addition in the Si-containing ODS steel played a crucial role in forming thin, dense and protective oxide layers during the oxidation process, resulting in improved oxidation resistance.

Magnetic properties of oxide dispersion strengthened (ODS) ferritic steel

The purpose of this study is to investigate the magnetic properties of oxide dispersion strengthened (ODS) ferritic steel. In this study, two samples of ODS ferritic steel powder were prepared by mechanical alloying method. The crystal structure and morphology of the samples were identified by X-ray Diffraction (XRD) analysis and examined by Field Emission Scanning Electron Microscopy (FESEM) respectively. The magnetic measurement including saturation magnetization (Ms), retentivity (Mr) and coercivity (Hc) of the samples were carried out at room temperature by using a Vibrating Sample Magnetometer (VSM). The magnetization curve of the samples (12Y and 14Y) approached the soft ferromagnetic behavior which is similar to its iron-based material due to the high content of iron (Fe) within the matrix. 12Y sample exhibit higher saturation magnetization, Ms value which is 295 emu/g due to the lower content of chromium (Cr) compared to 14Y sample which saturate at 162 emu/g.

Effect of interlayers and scanning strategies on through-thickness residual stress distributions in additive manufactured ferritic-austenitic steel structure

A total of five different types of specimens were additively manufactured by directed energy deposition (DED) process. The specimens have a functionally graded material (FGM) structure, which has been deposited with variation of chemical composition of ferritic and austenitic steel powders in each interlayer on a steel substrate. Residual stress distributions were experimentally measured through the thickness of the specimens by the contour method, neutron diffraction, and deep/incremental center hole drilling. Neutron diffraction provided three orthogonal stress components in each FGM part and the results were compared to the two-dimensional stress map obtained by the contour method and confirmed its criticalities from the highly spatial resolved depth profile by the hole drilling method. Significant variations from tension to compression (up to 950 MPa) in the sine-wave stress profile were alleviated to about 430 MPa when the FGM were deposited with orthogonal or island DED scanning strategies with interlayers. Gradual changes (16.3–12.1 × 10−6/°C) of the thermal expansion coefficient were measured among the inserted DED FGM parts and grain structure with defects along the interface was three dimensionally examined by neutron tomography.

Effect of Si addition on the microstructure and mechanical property of nanostructured oxide dispersion strengthened ferritic steel synthesized via mechanical alloying and spark plasma sintering

Nanostructured oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel powders of two different nominal compositions, Fe-14Cr-2W-0.3Ti-0.3Y2O3 and Fe-14Cr-2W-0.3Ti-1Si-0.3Y2O3, wt.%, were synthesized by mechanical alloying and consolidated via spark plasma (SPS) sintering at different sintering temperatures (900, 1000 and 1050 °C). The effect of Si addition on the sinterability, phase composition, microstructure, and mechanical properties viz. microhardness, nanoscale hardness, and elastic modulus were investigated. The addition of 1 wt.% Si caused reduction in milling time and resulted in finer (5 ± 1.2 μm) and uniform particles. The addition of Si also led to faster reduction in crystallite size as compared to the Si-free sample. TEM analysis revealed formation of Y2Ti2O7 and Cr2O3 nano-sized particles in the Si-free ODS steel. However, formation of Cr2TiO4 and SiO2 along with Y2Ti2O7 particles were observed in Si-containing ODS steel. Hardness and Elastic modulus of Si-containing ODS steel sample sintered at 900 °C was higher than that of the samples sintered at higher sintering temperature because of fine microstructure and absence of α-Fe to γ-Fe phase transformation in 900 °C sintered sample.

Microstructure and mechanical properties of oxide dispersion strengthened 18Cr-ferritic steel consolidated by spark plasma sintering

This paper presents the results of an experimental study on evolution of the nanocrystalline microstructure in a mechanically alloyed Oxide dispersion strengthened (ODS) 18Cr ferritic steel powder during densification by spark plasma sintering (SPS) in the temperature range of 1273 K (1000 °C) to 1423 K (1150 °C). Systematic Electron Back-Scatter Diffraction analysis has been carried out to study the grain size distribution and texture as a function of consolidation temperatures. Based on the kinetics of the densification process and resultant microstructure/microtexture, a sintering temperature slightly above 1323 K (1050 °C) within a range of 50 K was found to be optimum. The 18Cr-ferritic steel powder consolidated at 1323 K (1050 °C) was also studied to understand the role of dispersoids on microstructure. The dispersoids exerted a profound influence on the strength as well as toughness of the steel by restricting the grain growth at high temperatures. Further, a signature of (1 1 0) grain cluster is observed during consolidation and its preferential growth with increase in sintering temperature is noticed which lead to the alignment of the (1 1 0) plane in the direction of applied pressure. The minimum creep rate of the consolidated steel under a load of 300 MPa was found to be 5E-7 h−1 and 1E-4 h−1 at 873 and 973 K (600 and 700 °C) respectively. The apparent activation energy for creep deformation was estimated as ~ 402 kJ/mol, which is typical of lattice diffusion assisted general climb mechanism of dislocations over the barriers such as present dispersoids.

Thermal analysis, microstructure and impurity phases evolution in Fe14Cr ferritic steel powders ball-milled in air and under an argon atmosphere

Refined structure of the ferritic phase induced by mechanical milling (under reducing atmosphere) and its thermal stability are required in various applications of nanostructured ferritic alloys. The impurification with nitrogen and oxygen uptaken from the air is very probable during ball-milling, especially at the long-time high-energy milling conditions. As a rule, these interstitial impurities in as-milled powders are in quantities under the sensibility limit of conventional measurement techniques, such as XRD and SEM/EDS. To evidence the tendency for microstructure modification by impurities introduced during milling, the Fe–14Cr–3W–0.4Ti–0.25Y2O3 (Fe14Cr) ferritic steel powders (re)loaded in air and milled up to 170 h with interruption of the milling process, and heated up to 1373 K were investigated by thermal analysis in correlation with X-ray diffraction and scanning electron microscopy. XRD failed to detect the impurities in powders milled up to 38 h in air although a consistent mass loss related to the degassing of N2 was registered in thermogravimetric, TG, curves. (Fe,Cr)4N, fcc-γ, (Fe,Cr)2O3 impurity phases in powders milled over 38 h in air and (Fe,Cr)2O4 formed upon heating were readily detected by XRD. The analysis of these results allowed to better understand the impurification process and to generalise it for any as-milled Fe–Cr-based alloy powder processed in any milling conditions irrespective of the milling atmosphere, duration and thus, of amounts of contaminants. The quality of three powders milled for 170 h in three different conditions was compared: in air, under an argon atmosphere with interruptions of the milling process and under an argon atmosphere without interruption of the milling process. The contamination of powder milled for 170 h under an argon atmosphere without interruption of the milling process is insignificant (corresponding to less than 0.5 mass% mass loss in TG) as compared to powders obtained in the other two milling conditions. New approaches for minimising the contamination from air are suggested.

Chemical Redistribution and Microstructural Evolution of ODS Fe-Cr Powders During Mechanical Alloying

The oxide dispersion strengthened ferritic steel powders with chemical composition of Fe-14Cr-3W-0.3Y2O3 were mechanically alloyed from elemental powders in a planetary ball mill. Microstructural and chemical changes at different milling times were investigated by electron microscopy (SEM-EDS) and X-ray diffraction analysis (XRD). It was observed that morphology and structure of powders have experienced many stages during milling, and a quantitative mechanism was proposed. The initiation and evolution of the alloy formation started somewhere around 32 h of mechanical alloying (MA). According to microscopy and XRD analysis, in the first MA stages, milling chiefly has resulted in severe plastic deformation and grain refinement of powders, while in the later stages, alloying was progressed. It seems that 32 hours of milling are necessary to initiate the alloying process of Fe with Cr, but 78 h are not sufficient for completely dissolving W into �-Fe matrix retarding the Fe-Cr-W solid solution formation.

Effect of milling time and annealing temperature on nanoparticles evolution for 13.5% Cr ODS ferritic steel powders by joint application of XAFS and TEM

The characteristics of strengthening nanoparticles have a major influence on the mechanical property and irradiation resistance of oxide dispersion strengthened (ODS) steels. To determine how to control nanoparticles evolution, 0.3% Ti with 0.3% Y2O3 were added in 13.5%Cr pre-alloyed steel powders via different milling and consolidation conditions, then characterized by transmission electron microscopy (TEM) and X-ray absorption fine structure (XAFS) at synchrotron irradiation facility. The dissolution of Y2O3 is greatly dependent on the milling time at fixed milling speeds. After 24 h of milling, only minor amounts of the initially added Y2O3 dissolve into the steel matrix whereas TEM results reveal nearly complete dissolution of Y2O3 in 80-h-milled powder. The annealed powder FT-A800 (at 800 °C for 1 h) exhibits a structure near to the initially added Y2O3. The slightly deviation may be accounted for considerable lattice distortion related to the presence of atomic vacancies or formation of Y-Ti-O nucleus. The annealed powders FT-A1000 and FT-A1100 contain complex mixtures of Y-O/Y-Ti-O oxides, which cannot be fitted by any single thermally stable compounds. The coordination numbers of these first two shells in the annealed powders significantly raise as a function of the annealing temperature, indicating the formation of more ordered Y-O or Y-Ti-O particles. The extended X-ray absorption fine structure (EXAFS) spectrum could not necessarily distinguish the dominant oxide species.

ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation, HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y2O3 powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y–Ti–O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe–14Cr–2W–0.3Ti–0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y–Ti–O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300°C. Specimens HIPped at 900 and 1220°C were heat treated (HT) at temperatures ranging from 1200 to 1320°C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.

Effect of Process Parameters on Microstructure and Hardness of Oxide Dispersion Strengthened 18Cr Ferritic Steel

Pre-alloyed ferritic 18Cr steel (Fe-18Cr-2.3W-0.3Ti) powder was milled with and without nano-yttria in high-energy ball mill for varying times until steady-state is reached. The milled powders were consolidated by upset forging followed by hot extrusion. Microstructural changes were examined at all stages of processing (milling, upset forging, and extrusion). In milled powders, crystallite size decreases and hardness increases with increasing milling time reaching a steady-state beyond 5 hours. The size of Y2O3 particles in powders decreases with milling time and under steady-state milling conditions; the particles either dissolve in matrix or form atomic clusters. Upset forged sample consists of unrecrystallized grain structure with few pockets of fine recrystallized grains and dispersoids of 2 to 4 nm. In extruded and annealed rods, the particles are of cuboidal Y2Ti2O7 at all sizes and their size decreased from 15 nm to 5 nm along with significant increase in number density. The oxide particles in ODS6 are of cuboidal Y2Ti2O7 with diamond cubic crystal structure (Fd \( \bar{3} \) m) having a lattice parameter of 10.1 A and are semicoherent with the matrix. The hardness values of extruded and annealed samples predicted by linear summation model compare well with measured values.