Ferritic Stainless Steel Powders Research Articles

Synthesis of Nano-Structured Duplex and Ferritic Stainless Steel Powders by Dry Milling and Its Comparison with Wet Milling

Synthesis of Nano-Structured Duplex and Ferritic Stainless Steel Powders by Dry Milling and Its Comparison with Wet Milling

Nano-CeO 2 and -LaCrO 3 dispersed ferritic stainless steels as potential interconnect materials for solid oxide fuel cells

Abstract Novel nano-oxide dispersed ferritic stainless steel alloys have been developed as potential interconnect materials for solid oxide fuel cells. Nanopowders of CeO 2 and LaCrO 3 were mixed with ferritic stainless steel powder (SUS430) using conventional ball milling. Rectangular pellets were prepared and sintered at 1400 °C in pure H 2 . The microstructural analysis of the sintered pellets showed a uniform distribution of oxide particles. Long-term area specific resistance (ASR) of the pellets was monitored using the four-probe DC method at 800 °C for 1000 h in air, revealing that an increase in the CeO 2 amount from 0.5 to 3 and 5 wt % increased the oxidation resistance of the steel. ASR values as low 32.2 and 5.5 mΩ cm 2 were observed, respectively, for 3 and 5 wt % nano-CeO 2 dispersed SUS430 steel. However, in the case of LaCrO 3 addition, no such relationship between the oxidation resistance and LaCrO 3 concentration was found. This study showed that the dispersed nano-CeO 2 particles effectively reduced the Cr 2 O 3 scale growth, likely by retarding the inward diffusion of oxygen, hence resulting in a high oxidation resistance at the SOFC operating conditions.

Effect of Sintering Temperature and Atmosphere on Nonlubricated Sliding Wear of Nano-Yttria-Dispersed and Yttria-Free Duplex and Ferritic Stainless Steel Fabricated by Powder Metallurgy

ABSTRACTThe nonlubricated sliding wear behavior of nano-yttria-dispersed and yttria-free duplex and ferritic stainless steel against a diamond tip was studied. The stainless steel samples were fabricated by a conventional powder metallurgy route in which nano-yttria-dispersed and yttria-free duplex and ferritic stainless steel powders were cold compacted and then conventionally sintered at either 1000, 1200, or 1400°C in an argon atmosphere. For comparison, another set of samples was sintered at 1000°C in a nitrogen atmosphere. The wear behavior of sintered stainless steel samples against a diamond indenter was investigated using a pin-on-disc apparatus at 10 and 20 N loads and at a constant speed of 0.0041 m/s. It is proposed that yttria-dispersed stainless steels showed higher wear resistance compared to yttria-free stainless steel due to their improved hardness and density. Stainless steel sintered in a nitrogen atmosphere exhibited better wear resistance than those sintered in an argon atmosphere due to the formation of hard and brittle Cr2N. The wear mechanisms of stainless steels against diamond were found to be mainly abrasive and oxidative. Semiquantitative analysis of the worn surfaces and wear debris confirmed the occurrence of oxidation processes during wear.

Microstructural characterization of ODS ferritic steels at different processing stages

Nanostructured Oxide Dispersion Strengthened Reduced Activation Ferritic Stainless Steels (ODS RAF) are promising structural materials for fusion reactors, due to their ultrafine microstructure and the presence of a dispersion of Y–Ti–O nanoclusters that provide excellent creep strength at high temperatures (up to 750°C). The traditional powder metallurgical route to produce these steels is based on Gas Atomization (GA)+Mechanical Alloying (MA)+HIP+ThermoMechanical Treatments (TMTs). Recently, alternative methods have arisen to avoid the MA step. In line with this new approach, ferritic stainless steel powders were produced by gas atomization and HIPped, after adjusting their oxygen, Y and Ti contents to form Y–Ti–O nanoclusters during subsequent heat treatments. The microstructure of as-HIPped steels mainly consists of ferrite grains, Y–Ti precipitates, carbides and oxides on Prior Particle Boundaries (PPBs). Post-HIP heat treatments performed at high temperatures (1270 and 1300°C) evaluated the feasibility of achieving a complete dissolution of the oxides on PPBs and a precipitation of ultrafine Ti- and Y-rich oxides in the Fe14Cr2W matrix. FEG-SEM with extensive EDS analysis was used to characterize the microstructure of the atomized powders and the ODS-RAF specimens after HIP consolidation and post-HIP heat treatments. A deeper characterization of atomized powder was carried out by TEM.

Optimization of milling parameters for the synthesis of nano-structured duplex and ferritic stainless steel powders by high energy planetary milling

Abstract Nano-structured duplex and ferritic stainless steel powders were prepared by high energy planetary milling of elemental Fe, Cr and Ni powders. Here, we have studied the effect of process control agent (PCA) such as stearic acid (SA), effect of ball to powder weight ratio (BPR 6:1and 12:1) and milling speed (64% and 75% critical speed) during planetary milling of elemental Fe–18Cr–13Ni (duplex) and Fe–17Cr–1Ni (ferritic) powders for 10 h in a dual drive planetary mill (DDPM). We have found that all these milling parameters have great influence in tuning the final particle morphology, size and phase evolution during milling. It was found that addition of PCA, a BPR of 12:1 and 75% critical speed is more effective in reducing particle size and formation of duplex and ferritic stainless steel after 10 h milling of elemental powder compositions than their counterparts. The particle size of duplex and ferritic stainless steel milled in the presence of SA for 10 h is found to be 13 and 14 μm, whereas the particle size is 20 and 16 μm without SA respectively. The particle size of powder milled at 12:1 BPR is lower than powder milled at 6:1 BPR because of more impact energy induced by balls on powders and it is found to be 10 and 12 μm respectively for duplex and ferrite in presence of SA. The median particle size of duplex and ferritic stainless steel milled at 75% critical speed is found to be 3.5 and 2.4 μm respectively.

Synthesis of nano-structured duplex and ferritic stainless steel powders by planetary milling: An experimental and simulation study

Nano-structured duplex and ferritic stainless steel powders were prepared by planetary milling of elemental Fe, Ni and Cr powder compositions (duplex: 69Fe-18Cr-13Ni and ferritic:82Fe-17Cr-1Ni) in a dual drive planetary mill for 10 hours. The feasibility for solid solution formation of Cr and Ni in Fe matrix were studied. The samples were collected at regular time intervals and characterized for their morphological and phase analysis using X- Ray diffraction (XRD) and scanning electron microscopy (SEM). It has been observed that as the milling time increases crystallite size decreases and lattice strain increases. Thermodynamic aspects of milled stainless steel powders were performed using Miedema model. The theoretical values ΔH,ΔS and ΔG of and were calculated and were found to be -14.32kJ/mol, 6.93J/mol and -16.39 kJ/mol for duplex stainless steel and -2.81kJmol, 4.35Jmol and -4.11 kJ/mol for ferritic stainless steel respectively.

Development of nano-structured duplex and ferritic stainless steels by pulverisette planetary milling followed by pressureless sintering

Nano-structured duplex and ferritic stainless steel powders are prepared by planetary milling of elemental Fe, Cr and Ni powder for 40h and then consolidated by conventional pressureless sintering. The progress of milling and the continuous refinement of stainless steel powders have been confirmed by means of X-ray diffraction and scanning electron microscopy. Activation energy for the formation of duplex and ferritic stainless steels is calculated by Kissinger method using differential scanning calorimetry and is found to be 159.24 and 90.17KJ/mol respectively. Both duplex and ferritic stainless steel powders are consolidated at 1000, 1200 and 1400°C in argon atmosphere to study microstructure, density and hardness. Maximum sintered density of 90% and Vickers microhardness of 550HV are achieved for duplex stainless steel sintered at 1400°C for 1h. Similarly, 92% sintered density and 263HV microhardness are achieved for ferritic stainless steel sintered at 1400°C.

Phase transformation and microstructure study of nano-structured austenitic and ferritic stainless steel powders prepared by planetary milling

In the present work, nano-structured austenitic and ferritic stainless steel powders were prepared in bulk by milling elemental powders in a specially designed dual-drive planetary mill (DDPM) and Fritsch pulverisette planetary mill (P5 mill) for 10 and 40h respectively. The progress of milling and phase transition of stainless steel have been studied by means of X-ray diffraction. The crystallite size and the lattice strain of the austenitic stainless steel after 10h of milling are 9nm and 5.59×10−3 respectively. Similarly, the crystallite size and the lattice strain of the ferritic stainless steel after 10h of milling are 8nm and 9.05×10−3 respectively. Annealing of milled powder at 750°C promotes ferritic to austenitic transformation in both argon and nitrogen atmospheres as limited transformation takes place after milling. However, nitrogen favors the transformation to a greater extent than argon. Lattice parameters calculated from both high resolution transmission electron micrographs (HRTEM) and Nelson–Riley method match with austenitic and ferritic stainless steels. It has been found that initially particles are flattened and finally become almost spherical of size around 10–15μm in both cases.

Electromagnetic and microwave absorption properties of magnetic stainless steel powder in 2–18 GHz

Ferritic stainless steel (FSS) powder exhibits magnetic loss, dielectric loss, and excellent resistance to corrosion and oxidation. In this study, four types of ferritic grades 409L, 410L, 430L, and 434L powders are used as magnetic fillers of microwave absorbers. Using the transmission/reflection method, the complex permittivity and permeability of the FSS-epoxy composites can be measured in 2-18 GHz. The real part of complex permittivity ɛ′ does not exhibit significant variation (∼4.0 to 6.0%) while the corresponding imaginary part ɛ″, however, exhibits relatively large variation. The magnetic susceptibility (μ′−1) of FSS-epoxy absorber is negative above 10 GHz due to the natural magnetic resonance. For FSS 430L with 40 wt.% for a thickness of 3 mm, the calculated reflection loss at 9.7 GHz reaches −11.4 dB and the corresponding 409L absorber has minimum reflection loss −5.2 dB at 11.1 GHz.

Powder injection moulding of premixed ferritic and austenitic stainless steel powders

In this experimental work, powder injection moulding (PIM) of premixed 316L and 430L gas-atomized powders was developed to obtain duplex stainless steels. A multicomponent binder constituted of high density polyethylene (HDPE) and paraffin wax (PW) in a volume ratio 50/50 was selected for the process. Feedstocks with powder loadings of 50, 65, 68 and 70vol.% were prepared. Mixing experiments were carried out at 170°C according to differential scanning calorimetry (DSC) results. The rheological characterization of feedstocks allowed establishing different rheological parameters as power flow index (n) and activation energy (Ea) in order to know their suitability for injection moulding. Critical powder volume concentration (CPVC) was determined by means of oil absorption method and a rheological model. The feedstock was injected at 170°C and three-point bending and tensile parts were obtained. Thermogravimetrical analysis (TGA) of binder allowed us to design the thermal debinding cycle and sintering was carried out in low vacuum at different temperatures. Finally, mechanical properties such as hardness and tensile strength were evaluated.

Effect of the Si Additions on the Mechanical Behaviour and High-Temperature Performance of Hydrogen Sintered 434L Stainless Steels

Non-commercial, 434L ferritic stainless steel powders prealloyed with 2% of Si were uniaxially compacted at 700 MPa and sintered in 100% H2 atmosphere for 30 min at three different temperatures (1225, 1250 and 1275 °C). For comparison, standard, commercial 434L powders were sintered in the same conditions, and 434L+2%Si powders were also sintered in vacuum. The porosity and grain size of the 9 sintered stainless steels were measured by image analysis methods. Tensile strength and Vickers hardness tests show that the steels with 2% of Si exhibit higher mechanical properties. Oxidation tests were carried out in air at 800 and 900 °C. H2-sintered 434L steels prealloyed with Si exhibit higher weight gains per unit of apparent surface than 434L steels sintered in the same conditions, due to the higher porosity of the former. The sintering atmosphere affects the nature and morphology of the scales observed by XRD and SEM. Vacuum sintering of 434L+2%Si stainless steels promotes the formation of more protective scales and lower mass gains (for identical porosity volume) than H2 sintering.

Effect of Ball-Milling Method on the Formation of ODS Fe-14Cr-2Al-1Si-0.3Ta-1Y<sub>2</sub>O<sub>3</sub> Powders

Mechanical alloying (MA) process has been examined to synthesize ferritic stainless steel powder dispersed with nano-sized Y2O3 particles. A pilot-scale horizontal mill was fabricated and compared with laboratory-scale ball mills and an attrition mill. Horizontal milling resulted in a much better distribution of particle size and dispersoids than other milling methods. Although horizontal milling is considered as a low-energy process requiring long times, processing time could be markedly reduced with increasing the diameter of horizontal mill with a proper control of milling parameters.

Effect of temperature on sintered austeno-ferritic stainless steel microstructure

The influence of temperature on microstructural changes of sintered austeno-ferritic steels has been investigated. PM stainless steels have been obtained by sintering mixtures of austenitic and ferritic stainless steel powders. Only temperature-induced phase transformation was observed in austenite, as a result of elements interdiffusion between both phases. Microstructural characterization was completed with atomic force microscopy (AFM) and micro- and nano-indentation test, it is revealed an increase in the hardness with respect to the solutionized materials.

Application of EIS to the study of corrosion behaviour of sintered ferritic stainless steels before and after high-temperature exposure

Two types of ferritic stainless steel powders (AISI 409LNb and 434L) have been sintered in vacuum at three different temperatures (1225, 1250 and 1275 °C). The corrosion behaviour of the six materials produced has been studied through polarization curves and electrochemical impedance spectroscopy (EIS) measurements. The electrochemical tests have been carried out for the six materials before and after a high-temperature exposure at 800 °C. A new parameter, the active area factor (AAF), deduced from EIS measurements, has been proposed. This AAF quantifies approximately the relationship between the apparent surface of the powder metallurgical (PM) stainless steel specimens and the real surface exposed to corrosion.

A fundamental study of chromium deposition on solid oxide fuel cell cathode materials

Chromium contamination of metal oxides and SOFC cathode catalysts is studied in the range 700–1000 °C. Samples are exposed to a moist air atmosphere saturated with volatile Cr species in the presence and absence of direct contact between the sample and ferritic stainless steel powder. Chromium contamination of the samples is observed to occur via two separate pathways: surface diffusion from the stainless steel surface and vapor deposition from the atmosphere. Surface diffusion dominates in all cases. Surface diffusion is found to be a significant source of Cr contamination for LSM and LSCF at 700, 800, and 1000 °C. Vapor deposition of Cr onto LSCF was observed at each of these temperatures, but was not observed for LSM at 700 or 800 °C. Comparison of the behavior for LSM, LSCF, and single metal oxides suggests that Mn and Co, respectively, are responsible for the Cr contamination of these catalysts.

Ferritic stainless steel powders for high-density applications : A. Bergkvist, O. Andersson. (Höganäs AB, Höganäs, Sweden.)

Ferritic stainless steel powders for high-density applications : A. Bergkvist, O. Andersson. (Höganäs AB, Höganäs, Sweden.)

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

The microstructural transformations and the dimensional evolution of green specimens obtained by pressing mixtures of austenitic and ferritic stainless steel powders have been investigated by sintering at 1120 and 1240°C. Dilatometry experiments show that the linear shrinkage is influenced by the amount of ferritic powder. Moreover, during sintering Ni diffuses into the ferritic grains causing austenite destabilisation and the formation of a mixed constituent, whose constitution has been investigated by means of EDXS and interpreted on the basis of the Schaeffler diagram. Sigma phase also forms during sintering of the duplex mixtures.

Ultra Grain Refining and Decomposition of Oxide during Super-heavy Deformation in Oxide Dispersion Ferritic Stainless Steel Powder.

Mechanical milling using a high energy planetary ball mill was applied to the powder mixtures of iron, chromium and yttria (Y2O3) (Fe-24mass%Cr-0-15mass%Y2O3) to introduce a very large strain into the iron-base matrix, and microstructural changes during mechanical milling were investigated in relation to decomposition behavior of Y2O3 particles. Mechanical milling of more than 36 ks was long enough to allow the mechanical alloying of iron and chromium powders. After the milling of 36 ks, ultrafine bcc crystalline grains of 10 to 20 nm were formed within Fe-24mass%Cr-15 mass%Y2O3 powder mixture and 15 mass% of Y2O3 particles were almost decomposed. The resultant powder mixture markedly hardened to about 1000 Hv. The decomposition of Y2O3 particles can be explained as being due to the formation of an amorphous grain boundary layer where yttrium and oxygen atoms are enriched. As a result, it is proposed that, for the dissolution of Y2O3, bcc crystalline grains should be refined to a nanometric size to provide a sufficient volume fraction of the grain boundary layer, and that Y2O3 particles should be crushed to several nanometers to produce the driving force for the decomposition of Y2O3 particles.

粉末材料 焼結ステンレス鋼の耐酸化性

The high temperature oxidation resistance of the sintered stainless steels was fundamentally investigated. Type 410L, 434L and 444L ferritic stainless steel powders, and type 310L and 316L austenitic stainless steel powders, moreover the pre-alloyed powders with addition of Si and Nb in type 310L and 410L stainless steels, were used. And these powders were compacted to the same sintered density by controlling the compacting pressure, then vacuum-sintered at 1473K for 3.6ks. The oxidation tests at 1123K in air and the cyclic oxidation tests at room temperature-1123K in the exhaust gas were performed. The results obtained are as follows: (1) As for the high temperature oxidation tests in air, Si and Nb addition little affected oxidation resistance in both of the sintered 310L austenitic and sintered 410L ferritic stainless steels. (2)And it is obviously shown that the oxidation behaviors were influenced by sintered densities, especially by the open porosities. The oxidation resistance depended on the sintered density and more strongly on the open porosity than on the chemical compositions of sintered materials. (3) On the same manufacturing condition, the oxidation resistance of sintered ferritic stainless steels was better than that of sintered austenitic stainless steels, because their powders densified easier than austenitic stainless steel powders. (4) As for the cyclic oxidation test in exhaust gas, the spalling of the oxide scale occurred to the sintered 410L . But the sintered 410L-2Si was