Austenitic Stainless Steel Powder Research Articles

Effects of Nano / Meso Harmonic Microstructure on Mechanical Properties in Austenitic Stainless Steel Produced by MM / HRS Process

Mechanically milled austenitic stainless steel powder is applied to hot roll sintering (HRS) process. Microstructure and mechanical properties of the HRS material are investigated in detail. The mechanically milled powder has a bimodal structure with a severely deformed powder surface domain which is named as “Shell”, and an inner domain which is named as “Core”. The shell and core microstructure in the milled powder can be maintained even after sintering. As the result, microstructure of the HRS materials consists of a shell and core bimodal microstructure. Because severe plastic deformation mainly concentrates to the shell domain, a nano grain structure forms in the shell, while a coarse (meso) grain structure forms in the core. Such a nano / meso harmonic structured material demonstrates not only superior strength but also a large elongation. The mechanical properties of the HRS materials are strongly influenced by the nano / meso harmonic microstructure, such as grain size of the shell / core and the shell volume fraction. The shell has role of strength and the core has role of ductility. Thus, the nano / meso harmonic microstructure has been proved to be very effective to improve mechanical properties of structure materials.

Evaluation of Physical Properties on Functionally Graded Piping Joints Made from Cu and Austenitic Stainless Steel Powder

Welding materials, that the principal chemical component is nickel, are used usually for the welding between copper and austenitic stainless steels. But many kinds of mechanical or physical properties of welds between two materials will change largely. In this study, Functionally Graded Piping Joints (FGPJ) have been manufactured as an experiment using copper and austenitic stainless steel (SUS304) powder by a process based on HIP. This composition has been confirmed by absorbed electron and characteristics X-ray images of each mixed layer for FGPJ to be uniform or continuous. The following items have been investigated and compared with the linear law of mixture rule: Vickers hardness, thermal expansion, and thermal conductivity at a one-dimensional non-steady state etc. Those physical properties have been identified to depend on the mixing ratios of copper and austenitic stainless steel (SUS304). Pretty good agreements have been obtained between the experimental data and the calculated values according to the linear law of mixture rule.

Abrasive wear characteristics of coating area of low carbon steel surface alloyed through tungsten inert gas welding process

Low carbon steel surfaces were alloyed with composite powders using the tungsten inert gas welding method. After the alloying process, the effects of welding parameters, such as energy input, progress speed and coating thickness on the microstructural characteristics of the alloyed samples were examined. In the experimental investigation, a low carbon steel surface was alloyed with austenitic stainless steel powder and austenitic stainless steel composites mixed with 4·5% Co, Mo and Ti particles respectively. Following surface alloying, conventional characterisation techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectrograph and X-ray diffraction, were used to study the microstructure of the alloyed zone. Hardness measurements were also performed across the alloyed zone. Examination of the microstructure revealed the presence of M23C6 carbides, solid melt phases, and intermetallic phases, such as Ni3Ti, depending on the alloying element in the composite. As the amount of the reinforcing material increased, the saturation rates for the samples decreased, while their hardness increased. The abrasive wear tests conducted revealed that temperature input plays a significant role on the microstructure characteristics, which positively affected the abrasive wear values of the samples. Consequently, the tungsten inert gas welding method was successfully used for the surface alloying of low carbon steels.

Experimental investigation into selective laser melting of austenitic and martensitic stainless steel powder mixtures

Selective laser melting (SLM) is a novel additive layer manufacturing method that has been used to directly consolidate a wide range of metallic powders to form net-shape metallic parts for various engineering applications. It is capable of processing steel powders in a range of grades and also allowing the creation of custom alloys by consolidating the mixed powders of two or more grades. This study investigates the effects of varying the ratio of austenitic and martensitic powder mixtures on the laser consolidation of steel parts. Specimens were fabricated by SLM processing from 316L austenitic and 17-4PH martensitic stainless steel powder mixtures with varying composition ratios. Mechanical and magnetic properties of the specimens were assessed by micro-hardness testing and comparison of ‘magnetic adherence’ forces. The composition ratios of the mixed powders have been found to influence the laser consolidation mechanisms and the resulting microstructures in the specimens. The microstructures play an important role in altering the micro-hardness and the magnetic properties of the specimens and determining a definite relationship between these two properties. The results of this study prove that a new stainless steel grade can be produced via the SLM process to have tailored mechanical and magnetic properties.

Investigation of α to γ transformation in the production of a nanostructured high-nitrogen austenitic stainless steel powder via mechanical alloying

In this study, phase transformation of α to γ in a nanostructured high-nitrogen Fe–18Cr–11Mn austenitic stainless steel powder produced by mechanical alloying (MA) was investigated. MA was performed under nitrogen and argon atmospheres using a high-energy planetary ball mill. The obtained results from X-ray diffraction (XRD) and Oxygen/Nitrogen analyser revealed that nitrogen solubility in powders significantly increases by increasing the milling time under nitrogen atmosphere. Nitrogen solubility reaches to 0.65 wt% in samples milled for up to 100 h. The obtained results also indicated that MA in argon (Ar) atmosphere produces a ferrite structure after 120 h of milling. In contrast, a completely austenitic structure was obtained by performing MA for 100 h in nitrogen (N 2) atmosphere. Nanostructured powders of below 10 nm grain sizes were obtained by MA under both Ar and N 2 atmospheres.

Effects of different metals on adhesive wear behaviour of alloy surface produced by TIG process

Low carbon steel surfaces were alloyed with composite powders using the tungsten inert gas welding method. After the alloying process, the effects of cladding surface on the microstructural characteristics and adhesive wear of the alloyed samples were examined. The sliding wear behavior of samples was investigated in a block on ring apparatus under the loads of 20, 40, 60 and 80 N respectively. In the experimental investigation, a low carbon steel surface was alloyed with austenitic stainless steel powder and austenitic stainless steel powder mixed with 4·5% Co, Mo and Ti particles respectively. Following surface alloying, conventional characterisation techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectrograph and X-ray diffraction, were used to study the microstructure of the alloyed zone. Examination of the microstructure revealed the presence of M23C6 carbides, solid melt phases and intermetallic phases, such as Ni3Ti, depending on the alloying element in the composite. As the amount of the reinforcing material increased, the saturation rates for the samples decreased, while their hardness increased. The adhesive abrasion tests conducted revealed that temperature input plays a significant role on the microstructure characteristics, which positively affected the adhesive abrasion values of the samples. Consequently, the tungsten inert gas welding method was successfully used for the surface alloying of low carbon steels.

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.

Specific Phenomena in Severe Plastic Deformation Processed SUS310S Austenitic Stainless Steel Powder

Mechanical Milling (MM) is a Severe Plastic Deformation - Powder Metallurgy (SPD-PM) process which enables to produce a nano grain structure. A BCC layer with a nano grain structure appeared in the vicinity of the MM powder surface. Conventional cold work at room temperature never induces a strain-induced-martensitic-transformation in the SUS310S stainless steel. Therefore, a BCC layer formation from austenitic matrix is a specific phase transformation, and is attributed to the rise of the grain boundary energy by the nano grain formation. The hardness of this surface layer has approximately 540Hv, while that of the inner area has about 290Hv. As the MM powder anneals at 333K for 300s, the hardness of surface and inner area decreases to approximately 470Hv and 280Hv, respectively. Result of such a large hardness decrease in the surface of MM powder after annealing at near the room temperature indicates an existence of a huge number of defects, such as vacancy and interstitial atom, by the SPD-PM Process.

SUS316Lステンレス鋼における超強ひずみ加工による(α+γ)ナノ2相組織の形成

Mechanical milling (MM) process is applied to SUS316L austenitic stainless steel powder and a nano-duplex structure formation behavior is investigated. A nano grain structure with BCC phase forms in surface region of the powder milled for 720 ks. Those nano grains have the grain size of less than 20 nm. The (α+γ) nano-duplex structure, whose grain size is approximately 100 nm, forms in the inner region of the milling powder. The α grain in the surface region is an aggregated structure and consists of equiaxed grains, while those α grains in the inner region disperse homogeneously and indicate more spherical and smooth shape compared with the γ grains around them. TEM/EDS examination revealed that there occurs two kinds of BCC transformation. Those α nano grains in the surface region are considered to form by increase of grain boundary energy. On the other hand, the spherical α nano grains in the inner region form by increase of the grain boundary energy as well as the chemical free energy. Such an increase of chemical free energy is assumed to be caused by the existence of a huge number of vacancies which are introduced by the MM treatment. Although the austenite phase in the SUS316L steel is meta-stable at the room temperature, it hardly transforms to martensite phase even by a heavy cold rolling. Thus, these two kinds of α transformation are extraordinary phenomena and are considered quite different from these conventional strain induced martensitic transformation. A (γ+σ) nano-duplex structure material obtained by sintering (α+γ) nano-duplex powder has a good mechanical property.

Characteristics of water- and gas-atomised SS powders for MIM

Given the complexity of the laser-material interactions prevalent in powder bed fusion additive manufacturing, the need for tight control of alloy composition in the metal powder feedstock is leading to the use of specific gases during atomization. These changes in atomization gas also impact powder flow and packing. In order to identify the magnitude of this impact, two nitrogen atomized and one argon atomized 316 L austenitic stainless steel powders with similar size distributions were characterized using traditional powder characterization tools and rotating drum and annular shear rheological tools. While the traditional characterization tools and particle size measurements did not differentiate between the powders, these rheological tools allowed connections between the particle morphologies and rheological properties and powder performance to be identified. In particular, the argon atomized powders displayed higher aspect ratios, which translate into more spherical morphologies, and improved flow, defined by lower avalanche angles, when tested in the rotating drum. On the other hand, these differences in flow properties were not captured in the corresponding basic flow and specific energy measurements made with the annular shear tools. Variations in the powder packing properties and internal powder porosity in the different powder lots decreased the powder mass and introduced increased uncertainty in the force and torque measurements. The void spacing between loosely packed powders was also measured and showed how changes in particle size distribution and morphology impacted both packing density and permeability. Larger void spacing produced higher permeability values, indicating greater gas flow through the loosely packed powder.

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.

微粉添加による304Lステンレス鋼焼結体の高密度化

Granulated type 304L austenitic stainless steel powders were made by adhering fine powders to conventional water-atomized powder of -100 mesh. Our study was carried out to investigate the effect of the fine powder addition on the densification behavior and gas-tightness of the sintered bodies made of these powders. These powders were compacted at 590 MPa, then vacuum-sintered at 1523 K for 3.6 ks. Gas-tightness test was performed by measuring the pressure reduction of compressed air(400 kPa) in the container.

Hot-Press Compaction of High Nitrogen Containing Austenitic Stainless Steel Powders Mechanically Alloyed

High nitrogen containing MA- processed powder materials with the atomic compositions (at.%) of Fe 68 Cr 20 Ni 8 N 4 (I), Fe 62 Cr 20 Ni 11 N 7 (II), Fe 60 Cr 20 Ni 11 Ta 2 N (III), Fe 58.2 Cr 18.8 Mn 17.8 Mo 1.7 N 3.5 (i.e., Fe 60.1 Cr 18 Mn 18 Mo 3 N 0.5 (mass%))(IV), Fe 96 N 4 (V), and Fe 88 Ni 8 N 4 (VI) were consolidated into disc samples using uniaxial hot die pressing in vacuum. Nitrogen contained in the MA materials I - IV with austenitic stainless steel type compositions is retained at temperatures up to 1273K during hot-press compaction, in contrast to the MA materials V and VI with Cr-free compositions. The presence of Cr in the MA materials I - IV retards their grain growth during hot compaction, retaining nanostructures developed by the MA processing in the compact samples.

Stainless Steel Matrix Composites Reinforced with AlCr<sub>2</sub>

The effect of AlCr 2 intermetallic addition to austenitic stainless steel powder on the mechanical properties and corrosion resistance of sintered specimens has been investigated. It has been recognised that loss of chromium at the grain boundaries of sintered stainless steel decreases its corrosion resistance. With the objective of decreasing this effect and to keep a stable passivity, the intermetallic AlCr 2 in the form of powder was mixed with AISI 316L in different proportions. Samples of these materials were compacted at 700 MPa and sintered at 1120°C for 30 minutes in dissociated ammonia and vacuum. The mechanical properties, the corrosion resistance and the microstructure were studied. Composite materials sintered in dissociated ammonia achieve higher mechanical properties than sintered in vacuum. Studies realised by SEM suggest that metal matrix composites present an increase in the content of chromium, but in the grain boundaries exhibit a transient liquid phase produced by diffusion of aluminium from AlCr 2 .

Persistent liquid phase sintering of 316l stainless steel: J.Kazior et al. (Crakow Technical University, Crakow, Poland.) Int. J. Powder Metall., vol 34, no 2, 1998, 21–28

Boron is known to induce persistent liquid phase sintering when added to austenitic stainless steel powders. In the present investigation additions of boron to 316L stainless steel and their influence on tensile and impact properties were invesigated Boron induces two main microstructural modifications which strongly influence the mechanical behaviour of the material. One, it increases density, and two, it gives rise to the formation of grain boundary eutectic borides. Whereas an increase in density is positive, the influence of the borides is negative because they fracture during plastic straining, reducing both ductility and strength. The influence of density and borides on the plastic behaviour of the materials was then analysed in detail with reference to a simple damage model. It is further observed that with the addition of 0.2w/oB, density shows a noticeable increase and the negative effect of borides on the mechanical properties is reduced because of their lower amount and discontinuous morphology. This material may therefore be suitable for technological applications, because of its improved mechanical properties and because of the presence on the surface of the specimen of a homogeneous layer in which pores are practically absent; this promotes good corrosion resistance.

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

High performance stainless steel via powder metallurgy hot isostatic pressing

Argon atomised austenitic stainless steel (AISI 304) powder was characterized for its physical properties such as particle shape, microstructure, median particle size, particle size distribution, apparent density, tap density, and jlowrate. Subsequently, the tap density of the as received powder was improved to the desired level by adjusting the powder distribution followed by mixing and blending. This powder was subjected to hot isostatic pressing (hipping) at two different combinations of temperature and pressure to optimise the microstructure and mechanical properties. Mechanical properties of the stainless steel obtained by the powder metallurgy (PM) hipping route were found to be superior to those of conventionally processed wrought steel. The superior performance of PM hipped steel is attributed to its low oxygen content, fine grain size, and high degree of chemical homogeneity. Although the production of billets by the hipping route does not appear to be economical owing to the high capital cost of the hot isostatic press, the added advantage of obtaining a nearnet shape makes the process economically viable for production of intricate shapes.

Surface reactions during water atomizing and sintering of austenitic stainless steel : T. Tunberg, L. Nyborg (Chalmers University of Technology, Gothenburg, Sweden). PowderMetall., vol 38, no 2, 1995, 120–130

Given the complexity of the laser-material interactions prevalent in powder bed fusion additive manufacturing, the need for tight control of alloy composition in the metal powder feedstock is leading to the use of specific gases during atomization. These changes in atomization gas also impact powder flow and packing. In order to identify the magnitude of this impact, two nitrogen atomized and one argon atomized 316 L austenitic stainless steel powders with similar size distributions were characterized using traditional powder characterization tools and rotating drum and annular shear rheological tools. While the traditional characterization tools and particle size measurements did not differentiate between the powders, these rheological tools allowed connections between the particle morphologies and rheological properties and powder performance to be identified. In particular, the argon atomized powders displayed higher aspect ratios, which translate into more spherical morphologies, and improved flow, defined by lower avalanche angles, when tested in the rotating drum. On the other hand, these differences in flow properties were not captured in the corresponding basic flow and specific energy measurements made with the annular shear tools. Variations in the powder packing properties and internal powder porosity in the different powder lots decreased the powder mass and introduced increased uncertainty in the force and torque measurements. The void spacing between loosely packed powders was also measured and showed how changes in particle size distribution and morphology impacted both packing density and permeability. Larger void spacing produced higher permeability values, indicating greater gas flow through the loosely packed powder.

Surface Reactions During Water Atomisation and Sintering of Austenitic Stainless Steel Powder

The surface oxides formed during water atomisation and sintering of austenitic stainless steel were determined using electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). Optical microscopy and electron microscopy (SEM and TEM) were used for structural analysis of powder and sintered material. The materials studied were 304L, 304L + Si, 304L + Al, and 304L + C. All powders were prealloyed except for 304L + C, which was obtained by admixture with graphite. Sintering was carried out in dissociated ammonia and in a vacuum. It is shown that the surface oxidation is strongly affected by the change in cooling rate with particle size. The average oxide thickness increases significantly with increasing particle size, while the surface oxide changes from a silicon rich oxide to an oxide containing more iron and chromium. A strong correlation between the average oxide thickness and the secondary dendrite arm spacing (i.e. the cooling rate) is observed. It was not possible to distinguish any clear effect of increasing the silicon content above 1 % on the surface oxidation. During sintering, the iron and chromium oxides formed during water atomisation are reduced. The silicon oxide forms a continuous layer at 1120°C, while it is broken up into discrete particles at 1250°C. The reduction favours neck growth resulting in improved mechanical properties. For equal final density, the impact strength can be correlated to the relative neck radius. Admixture with carbon before sintering can further enhance the oxide reduction. As a result of a higher sintering temperature or the addition of carbon, sintering is enhanced and improved mechanical properties are obtained. Prealloying with aluminium leads to a highly inferior mechanical strength due to the formation of aluminium oxide on the powder surfaces