316L Powder Research Articles

Net-Shape Manufacturing of Micro Porous Metal Components by Powder Injection Molding

A novel production method for porous metal components has been developed by applying powder space holder (PSH) method to metal powder injection molding (MIM) process. The PSH-MIM method has an industrial competitive advantage that is capable of net-shape manufacturing the micro-sized porous metal products with complicated shapes and controlled porosity and pore size. In this study, the small impeller with homogeneous micro-porous structure was manufactured by the PSH-MIM method using porous compounds composed of fine stainless steel 316L powder and polymethylmethacrylate (PMMA) particle. The effects of combinations in size and fraction of PMMA particle on dimensional tolerance and variation of sintered porous specimens were investigated. It was concluded that the PSH-MIM method could manufacture commercially micro-porous metal components with high dimensional accuracy.

In-flight particle velocity measurements with DPV-2000 in cold spray

The measurements of in-flight particle velocities and deposition efficiency at cold spraying of two stainless 316L powders having different morphology and almost the same size distributions were carried out. It was found that the angular particle had faster velocity than spherical one and resulted in greater deposition efficiency. The critical velocity of both powders was almost the same and did not depend on their micro-hardness. Therefore, the deposition efficiency was not affected by the particle micro-hardness. The critical velocity significantly depended on He gas temperature and decreased as the temperature increases, but little depended on N 2 gas temperature. The critical velocity little depended on operating gas pressure. The cause why the angular particle morphology gives faster velocity in supersonic gas flow compared with the spherical one has to be investigated and clarified. The drag coefficient appears to depend on particle morphology. An angular particle seems to have a larger drag coefficient than a spherical particle.

Improvements in microstructure homogenization and mechanical properties of diffusion-alloyed steel compact by the addition of Cr-containing powders

The use of diffusion-alloyed powders for fabricating powder metal parts, despite alleviating the segregation problem of the alloying elements while retaining good compressibility, still cannot attain homogeneous microstructure in as-sintered products. The presence of soft Ni-rich areas and pores causes poor mechanical properties compared to those of wrought steel counterparts. This study investigated the effects of adding 0.5 wt pct Cr, which was introduced in the 316L stainless steel powder form, on the microstructure and mechanical properties of diffusion-alloyed Fe-4Ni-1.5Cu-0.5Mo-0.5C (Metal Powder Industries Federation (MPIF) FD-0405) steels. The results show that weak Ni-rich areas were present in the Cr-free specimen when sintered at 1120 °C and 1250 °C. These areas were lean in carbon because of the strong repelling effect between Ni and C. With the addition of 316L powders, the Cr was uniformly distributed and helped eliminate the soft Ni-rich areas, particularly in specimens sintered at 1250 °C. The distribution of carbon also improved. With a more uniform distribution of Ni and C, and more homogeneous microstructure, which consisted mainly of bainite and martensite, the mechanical properties of the Fe-4Ni-1.5Cu-0.5Mo-0.5C diffusion alloy steels were improved significantly.

Corrosion behavior of powder metallurgical stainless steels in urban and marine environments

This work studies the development of corrosive attack on sintered components manufactured from AISI 316L and AISI 304L powders. The stainless steels were sintered in vacuum and in nitrogen-base atmosphere at 1,120 and 1,250 oC, and their corrosion resistance was then analyzed by electrochemical techniques and by atmospheric corrosion testing (two years) at urban and marine test sites. Images are shown of the morphology of the attack on the surface of the stainless steels and the development of this attack in the interior of the material.

An investigation on the sintering behavior of 316L and 17-4PH stainless steel powders for graded composites

This paper presents the densification and microstructure of bilayer structures made from 316L and 17-4PH stainless steels powders during sintering. The requirements for such objects could be magnetic properties at one area of the part and non-magnetic properties at another area of the object. A pressureless solid state sintering method in conjunction with a powder layering technique was used. The sintering was carried out at temperatures ranging from 1100 to 1340 °C for 120 min in hydrogen and vacuum atmospheres. Non-isothermal sintering behavior was also examined by dilatometric analysis. It was found that the strain rate of 17-4PH stainless steel powder is higher than that of 316L during the course of sintering. This induces mismatch strain between two layers that can be harmful for the co-sintering process. The amount of strain incompatibility is noticeably higher in vacuum sintering compared to hydrogen sintering. It has also been shown that the mismatch strain induces biaxial stresses during sintering, affecting the densification rate of the bilayer. An enhanced densification rate compared to the individual layers was noticed, particularly during vacuum sintering. A narrow interlayer diffusion boundary with high volume fraction of ferrite phase was realized.

Microstructure and Mechanical Properties of the Stainless Steels Obtained by Sintering Mixtures of AISI 316L and Silicon Powders

The aim of the study was to obtain a ferritic-austenitic stainless steel through sintering of the mixture of austenitic steel AISI 316L powders with silicon in the amount ranging from 1 to 7%. The pressed mixtures were sintered at 1240oC for 60 minutes under hydrogen atmosphere. The results of the silicon admixture on the density, porosity, microstructure and mechanical properties of the sintered specimens are discussed.

Elaboration of a test method for the study of metal release from stainless steel particles in artificial biological media

Data on metal release from stainless steel particles can be used in the assessment and quantification of the potential impact of stainless steel on health and the environment. To elaborate a test method suitable for the study of metal release from particles, the experimental parameters particle loading, agitation and separation of particles from the solution were investigated through exposure of 316L powder particles in two artificial biological media. The results suggest that a small particle loading, bi-linear shaking and centrifugation for separation of particles from the solution give the most reproducible results. They also show that metal release rates are strongly influenced by the physico-chemical properties of the test medium and the effective surface area of particles during exposure.

Solvent debinding kinetics and sintered properties of injection moulded 316L stainless steel powder

A stainless steel (316L) powder having a median particle size of ∼25 μm was injection moulded using binders composed of polyethylene glycols (PEG) and polymethylmethacrylate (PMMA). The PEG content was leached in water and debinding kinetics studied as a function temperature. At 40°C, the removal of PEG was controlled by dissolution of the polymer in water and by diffusion of water and PEG for leaching time <60 min and >60 min, respectively. Water soluble PEG was completely removed within 180 min and the diffusion mechanism controlled the de-binding process at 60 and 80°C. Sintered properties improved with increased sintering temperature and time. The improvement in mechanical properties was less pronounced for a longer soaking time owing to more grain coarsening.

Corrosion fatigue characterisation of sintered multiphase stainless steels

Multiphase stainless steels are produced for their attractive properties of mechanical strength and corrosion resistance relative to their austenite–ferrite structure. The manufacture of these steel by powder metallurgy technology presents some advantages in terms of low cost and formability of complex shapes. Mechanical and corrosion resistances are not at the level of the wrought steels due to their porous nature. In this work the fatigue and corrosion fatigue behaviour of some sintered steels obtained by sintering from 316L and 434L base powders has been studied for characterisation and comparison. The sintered steels were fatigue tested in two different environments: air and NaCl aqueous solution. The tests performed indicate that the chemical and microstructral composition has no great influence on fatigue behaviour in comparison with the manufacturing technology (sintering). This is most evident in the more aggressive environment, like seawater, in which these steels could be advantageously used. The analysis of fracture surfaces using SEM microscopy shows a peculiar crack propagation characterised by cleavage, stress intensification due to porosity, and features of localised ductility on sintering necks and base powder particles.

Density analysis of direct metal laser re-melted 316L stainless steel cubic primitives

This paper reports on the density of cubic primitives made by Direct Metal Laser Re-Melting, a process variant of selective laser sintering (SLS). Here, stainless steel 316L powder fractions are scanned and fused by a 90 W Nd:YAG laser in consecutive 100 μm layers in order to build a 3-Dimensional object. The effects of Q-Switch pulsing frequency, scanning speed and scan spacing on sample density are described. The samples are measured by two methods: a weight/volume analysis and a xylene impregnation technique. The results are supported by microscopy analysis for qualitative arguments. The results show the significant influence of pulsing the beam on the density of the fabricated material. Also reported is the relationship of material density with energy density (as a function of the process parameters; power, scan speed and scan spacing). Optical analysis of material cross sections shows a periodic occurrence of porosity across the whole range of samples. Causes for this are discussed.

Pressureless moulding of 316L and D2 steel powders using a hybrid binder

A forming process for 316L and D2 steel powders has been studied. It consists of pressureless metal injection moulding of slurries containing, in addition to steel powder, a hybrid inorganic - organic binder. The slurries were prepared at room temperature and poured into polymeric moulds, where the binder polymerised within a few minutes. Rigid and resistant greens have been obtained. A curing process at 80 °C has been performed in order to complete the crosslinking of the polymer. Different debinding cycles have been studied by thermogravimetric analyses, as a function of experimental parameters, such as heating rates and atmospheres. A number of bulk samples with different shapes have been debinded, using the optimised cycle and sintered under vacuum, reaching high densities (97% of theoretical). Microstructural characterisation has been performed on the sintered samples by scanning electron microscopy, along with microhardness measurements.

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.

Effect of nickel boride and boron additions on sintering characteristics of injection moulded 316L powder using water soluble binder system

A new metal injection moulding system for 316L stainless steels has successfully been derived and tested. A mixture of small water atomised powder (average size 15 µm), larger gas atomised powder (average size 75 µm), and sintering additives has been coupled with a new water soluble binder system for economical powder injection moulding. The details for each process step and the effect of sintering additives are described. The binder system consists of poly (2-ethyl-2-oxazoline) as the leachable polymer, polyethylene as the backbone, and stearic acid as a surfactant and plasticiser. This binder system provides satisfactory mixture stability, excellent mouldability, and reasonably fast water leaching and thermal debinding rates. The optimum powder/binder compositions were determined using torque and capillary rheometry. Densification was by persistent liquid phase sintering through additives, such as nickel boride and boron. This 316L powder system was sintered to 7·9 g cm -3 (98·75% of theoretical) at 1285°C using nickel boride addition and at 1245°using boron addition. nickel boride additions are particularly effective at increasing the tensile strength and ductility. In contrast, the boron additions only increase the tensile strength and decrease ductility. Based on microstructure evaluations, this effect is traced to a continuous boride phase on the grain boundaries of the boron doped samples and a discontinuous boride phase on the grain boundaries of the nickel boride samples.

Sintered duplex stainless steels from premixes of 316L and 434L powders

Abstract Duplex austenite–ferrite stainless steels were prepared from the premixes of 316L and 434L stainless steel atomized powders. Pronounced densification was observed after 1350°C sintering in hydrogen. 316L-60w/o 434L steel composition exhibited maximum transverse rupture strength, while 40 and 60w/o 434L containing compositions showed total immunity in 1N H 2 SO 4 even after a exposure time of 360 h. Anodic polarization curves also suggest high-corrosion resistance of those two compositions. Magnetic coercivity decreased with increase in sintering temperature while magnetic saturation follows the reverse trend. Wear resistance of the duplex stainless steels under sliding condition was in between the straight steels.

Modified metal injection moulding process of 316L stainless steel powders using thermosetting binder

A modified metal injection moulding (MIM) process of 316L stainless steels powders using an acrylic thermosetting resin has been developed. Gas and water atomised 316L powders were used. In order to optimise the mixing and moulding steps, different volume fractions of the two components were investigated. Mixing of metal powder and binder was carried out at room temperature and immediately moulding was performed by pouring the slurry in the moulds. It was then heated at 90°C to permit the polymerisation and cross linking of the resin. Different heating cycles, rates, and atmospheres were studied by means of thermogravimetrical analysis. The data obtained were used to establish the best debinding cycle. The debound samples were sintered at different temperatures and high densities (98% of theoretical) were obtained. Materials in as moulded (green part), debound (brown part), and sintered conditions were examined by means of SEM.

Irradiation testing of 316L(N)-IG austenitic stainless steel for ITER

In the frame work of the European Fusion Technology Programme and the International Thermonuclear Experimental Reactor (ITER), ECN is investigating the irradiation behaviour of the structural materials for ITER. The main structural material for ITER is austenitic stainless steel Type 316L(N)-IG. The operating temperatures of (parts of) the components are envisaged to range between 350 and 700 K. A significant part of the dose–temperature domain of irradiation conditions relevant for ITER has already been explored, there is, however, very little data at about 600 K. Available data tend to indicate a maximum in the degradation of the mechanical properties after irradiation at this temperature, e.g. a minimum in ductility and a maximum of hardening. Therefore an irradiation program for plate material 316L(N)-IG, its Electron Beam (EB) weld and Tungsten Inert Gas (TIG) weld metal, and also including Hot Isostatically Pressed (HIP) 316L(N) powder and solid–solid joints, was set up in 1995. Irradiations have been carried out in the High Flux Reactor (HFR) in Petten at a temperature of 600 K, at dose levels from 1 to 10 dpa. The paper presents the currently available post-irradiation test results. Next to tensile and fracture toughness data on plate, EB and TIG welds, first results of powder HIP material are included.

Modelling of the in-flight behaviour of stainless steel powder particles in high velocity oxy-fuel spraying

Modelling of the mechanical and thermal behaviour of stainless steel 316L powder particles during high velocity oxy-fuel (HVOF) spraying is presented. This modelling accounts for the combustion process, the gas dynamics inside and outside of the spray gun, gas-particle interactions, acceleration and deceleration of the gas flow, internal heat conduction in the powder particles and particle heating, melting, cooling and solidification. Variations of the times of flight and melting of the particles are studied. Optimal conditions of spraying are predicted. The results agree with experimentally-established HVOF spraying practice.

Algunas observaciones sobre la sinterización del acero austenítico 316L en atmósfera de argón

PM high speed steels are prone to higher corrosion rates due to residual porosity as well as chromium depletion of the matrix during sintering AISI 316L powders have been cold compacted (100-1,000 MPa) and sintered (1,000-1,250°C, 15-240 min) under several argon containing or vacuum atmospheres. Better densification rates can be achieved as sintering time or temperature increase in the presence of argon. No surface oxidation has been observed with the use of low pressure argon atmospheres, as long as samples are protected in a stainless steel partially sealed container.

Numerical modelling of HIP: Influence of operating parameters and rheology of the porous material: C. Dellis, et al, (Institut Mecanique de Grenoble, Grenoble, France)

Hot isostatic pressing (HIPing) is a promising near net-shape manufacturing technology that can be employed for fabrication of complex parts out of metal powders. Design of tooling/canister that allows net-shape HIPing is still based on expensive experimental try-outs and subsequent iterations to modify the initial canister geometry. An auspicious alternative approach is finite element (FE) simulation. However, the FE results are strongly dependant on the implemented powder metal constitutive model. The current research shed the light on finite element analysis of HIPing process, based on steel 316L powder, using three different constitutive models namely; CAM-Clay, modified Drucker-Prager and modified Drucker-Prager with creep. Comparison with experimentally deformed final geometry and densification history of the HIPed material were carried out. Discrepancies in predicted final geometry dimensions were ranging from 1% to 6.34% compared to experimental trials. Drucker-Prager with creep constitutive model showed the highest accuracy in final geometry predictions with relative error of 1.5∼4.8%.