New materials based on higher-quality stainless steel powder
A significant breakthrough in the technology of powder stainless Swedish researchers have carried out HEGANES AB. The new powder is based on obtaining a powder of stainless steel sheet by spraying water and has improved compressibility. The products obtained from this powder have improved mechanical properties.
It is known that the sputtering process is the most common technique for producing a metallic powder. This spray can be described as splitting the flow of liquid (superheated) metal fine droplets and their subsequent freezing into solid particles of a size typically less than 150 microns.
Spraying water became commercially viable since the early 1950s, when it was applied to obtain powders of iron and stainless steels. Currently, water atomisation is the dominant technique for high-volume metal powders with low production costs. The main advantages of this technology are dominated production costs low, sufficient sintered strength is not able due to irregular shape of powder particles, microcrystalline structure, high degree of supersaturation, the ability of forming metastable phases, no macrosegregation and that the particle microstructure and shape can be adjusted by changing the modes sputtering.
Its essence lies in the fact that during the water atomisation process a vertical stream of liquid metal cross sprayed jets of high pressure water. Liquid metal droplets solidify without the formation of a second fraction and settle at the bottom of the spray tank. The tank is often purged with an inert gas such as nitrogen or argon, to minimize oxidation of the powder surface.
After dewatering the powders are dried and in some cases annealed, whereby occurs at least partial reduction of oxides formed on the surface.
However, have not been overcome until now is the lack spraying water in the oxidation of the powder surfaces, wherein this disadvantage is even more pronounced when the powder contains easily oxidisable elements such as Cr, Mn, V, Nb, B, Si, etc.
As a result, the possibilities of subsequent refining of water-atomised powders are very limited, and the conventional way of producing stainless material (% Sr12 content) of a water-atomised steel powder usually requires very pure and accordingly very expensive raw materials e.g. pure scrap or selected scrap.
On this line was permanently stopped the progress of the development of this area.
There have been attempts to use instead of spraying water spraying gas. However, powders obtained by spraying gas require hot isostatic pressing (HIP), whereby the products obtained from this type of powders are very expensive.
Later transferred to the oil atomisation process for producing steel powders, wherein oil is used as atomization medium. This method is superior to conventional spray water in that it does not occur oxidation of the steel powder, i.e. there is no oxidation of the alloying elements. But it had the drawback consisting in that during spraying the resulting powder is carburized, that is, the diffusion of carbon from the oil to the powder and the subsequent step is to perform decarburization.
As a result, the process of spraying oil was less acceptable than the process of spraying water from the point of view of ecology.
And, recently surprisingly found that stainless steel powders can be obtained from a water-atomised powder from a wide variety of inexpensive raw materials, such as ferrochrome sarbure,
ferrochrome suraffine,
pig iron etc. In comparison with the stainless steel powders obtained by conventional water atomization method, the new powder has a much less impurities, particularly in regard to oxygen and to some extent sulfur after sintering. A low oxygen content allows to obtain a powder with a metallic gloss instead of the brown green color, which is inherent in stainless steel powder obtained by conventional water atomization method.
Furthermore, the density of green bodies prepared from the new powder, was significantly higher than the density of green bodies prepared from conventional powders obtained by spraying with water.
It succeeded in obtaining new important properties such as strength and elongation, the final sintered components prepared from the new powders. Another advantage was that the sintering process can be performed at lower temperatures than conventional temperature, in practice, allowing increased flexibility in choosing furnaces. Reduced cost and energy consumption and reduced wear on the furnace lining and atomising nozzles.
Thus it is possible to use less expensive chromium containing raw materials, the number of species which can be applied significantly increased.
Last major breakthrough, researchers carried out the recent Swedish HEGANES AB.
They managed during the spraying process water to adjust the carbon content to a value which is determined by the expected oxygen content after the spraying process. Thus, the expected oxygen content after spray determined either empirically or by taking a sample of the melt before the atomisation.
Swedish researchers were able to obtain a powder having improved properties are all similar to those mentioned above. The essence of these technologies lies in that the resulting carbon-containing powder, water-atomised, annealed at a temperature of at least from 1120 | C to 1260 | C under optimal 1160 | C.
Obtained based on this technology annealed powder as well as the water-atomised powder comprises 10-30% weight percent chromium, 0-5% molybdenum, 0-15% of nickel, 0-1.5% of silicon, 0-1.5% manganese, 0-2% of niobium, 0-2% titanium, 0-2% of vanadium and at most 0.3% of inevitable impurities and the optimum version - 10-20% chromium, 0-3% molybdenum, 0.1 - 0.3% silicon, 0.1-0.4% manganese, 0-0.5% of niobium, 0-0.5% titanium, 0-0.5% of vanadium and essentially no nickel or alternatively 7-10% of nickel.
New powder showed the best quality, in comparison with all existing analogues. The oxygen content in it substantially reduced during the annealing process to influence the content of nitrogen was positive. It contains less slag inclusions than conventional powders and the mechanical properties of the new materials showed a high density, strength before sintering and springback.
Moreover, the annealed powder incorporates fewer small particles (45 microns) that constitute only 10% of the total weight as compared to 30-35% of the total weight of conventional powders grade 410Ref. And then the powder particles have a high gloss.
