Porous Sintered Stainless Steel

To view Gas flow graphs for sintered stainless steel click this link

This material is made from tiny metal particles each having a rather irregular shape, made from 316L stainless steel

There are two methods of producing sintered stainless components. These are gravity sintering and pressing & sintering.

Gravity sintering is a similar technique to that used for tin-bronze & nickel-bronze powders. A heatproof mould is made and filled with powder of the appropriate grain size. This is then sintered in a special atmosphere furnace, and the particles bond themselves together where they touch. The sinters made by this process are relatively open in structure with pore percentages of 40-50%. They are only around one third of the strength of stainless sinters made by pressing before sintering. However, it is a useful technique for making large parts or parts of uneven thickness or irregular shape which would be difficult to press. The tools are of course relatively cheap for this process.

Most stainless sinters are made by pressing and then sintering. The metal particles are first mechanically pressed together at extremely high pressures of around 30 tons/square inch and the resulting compacts, with mechanically interlocked particles, are sintered close to the melting point of the metal. What results is a porous product (up to 35% porosity) with strong bonds between the constituent particles. The moulds can be flat to make discs or plates, cylindrical to make tubes, or can also have more complex shapes to make near-nett-shape sinters for special applications.

The tooling for axially pressed parts is made of tool steel and has to be very strong to withstand the pressing pressures. For small quantities therefore, the toolcost is an important factor in determining the cost of the parts and, if it is possible to use something from the already tooled range, it will certainly be quicker to obtain and much cheaper.

The axial pressing process can only make tubes of a relatively short length, due to friction in the powder. For longer tubes with thin walls, the powder has to be radially pressed onto a mandrel using a flexible rubber bag as the outer part of the tool. This process is known as isostatic pressing.

A third pressing method prior to sintering is roller-forming. This technique is a bit like making pastry, and results in thin sheets whose width is determined by the size of the plant (210mm / 8.5" at present) and whose length can typically be up to 1m (40"). The finer grades of powder make thinner sheets while coarser grades make thicker sheets. Click the link for more details.

Applications usually involve controlled gas or liquid flow through the porous metal, and the particle size of the metal is chosen to give the desired pore size in the resulting sinter. Pores can be from 2 microns mean size up to 200 microns. Filtration ratings are from 0.2 microns up to 100 microns. The particles of the metal matrix tend to be about 20 times the size of the mean size of the holes.

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