Low alloy ferrous materials
Low alloy ferrous Powder Metallurgy materials are predominant in the Press/Sinter structural parts sector. In this sector, these materials are generally based on water atomised or sponge iron powders with elemental alloying additions.
In common with wrought steels, a significant element in increasing strength is carbon, added as graphite. As mentioned in the sintering section, copper is usually added for dimensional control purposes and to provide some solid solution strengthening.
To enhance hardenability, the most commonly used additions have been nickel and molybdenum, as these elements do not form stable oxides at normal sintering temperatures.
Traditionally, more cost-effective hardenability agents, such as chromium and manganese, were not used because of the stability of their oxides at normal sintering temperatures. However, they can be used if high temperature sintering is employed and, increasingly, if the reducing capacity of the sintering atmosphere at normal temperatures can be increased by controlling dew point to low values.
Diffusion alloy grades
Long-established alternatives to elemental mixes are the diffusion alloyed grades. These grades are processed by applying a relatively low temperature treatment to “tack” the fine elemental additions of Ni, Mo and Cu to the surfaces of the iron powder particles.
This ensures that the hardenability effect of these additions can be delivered in a more controlled and predictable manner.
Binder treated mixes
In recent years, a further alloying concept has emerged – binder treated mixes. These grades adhesively attach the alloying additions to the iron powder particles using an organic binder addition that also acts as the pressing lubricant. Grades are available that are binder treated equivalents of the diffusion alloyed grades.
Pre-alloying with molybdenum
It has been found that pre-alloying with molybdenum does not seriously impair the compressibility of iron powder. Consequently, a number of fully pre-alloyed molybdenum grades are now available and some of these have been used as the base for “hybrid” diffusion-alloyed or binder-treated grades.
Fully pre-alloyed powder grades
A range of low alloy steel powder grades is available, where all alloying elements are fully pre-alloyed. These grades have been most commonly used in Powder Forging, where the lower compressibility in compaction is not a significant detriment.
A range of AISI 300 and 400 series stainless steels are available in powder form. These powders are used for the production of Press/Sinter Powder Metallurgy parts (in which case the powder is water-atomised) or of Metal Injection Moulded parts (for which either gas- or water-atomised grades can be used). The precipitation hardening stainless steel grade, AISI 17-4 PH is also frequently used in MIM products.
Stainless steel powders are also used for the production of sintered filter elements.
Copper alloys can be processed as Powder Metallurgy structural parts. These can use either fully pre-alloyed powders or elemental mixes. Bronze powders can be processed into self-lubricating bearings and coarse, spherical bronze powders can be loose sintered to produce filter elements.
A range of aluminium alloy powders are available for Powder Metallurgy processing by Press/Sinter PM, extrusion to semi-products or MIM.
Titanium and titanium alloy powders are available in a number of forms.
The limited use of Press/Sinter titanium Powder Metallurgy has generally used HDH (hydride-dehydride) titanium powder, or even titanium hydride powder, with blended alloying additions in the form of masteralloys.
HIP products generally use titanium alloy powders produced by gas atomisation, the plasma rotating electrode (PREP) process or plasma atomisation, depending on the required quality.
MIM feedstocks most commonly incorporate gas atomised powders, but HDH powders are also now available that have been plasma spheroidised. Standard materials, such as CP-Ti and Ti-6Al-4V, are used, but some higher performance MIM products use specially developed alloys, such as Ti-7Fe-5Zr or Ti-6Al-5Nb.
Hard and heavy materials
These materials all comprise a matrix material and a binder phase.
In hardmetals, the most common matrix material is tungsten carbide (although a range of other carbides, nitrides or carbonitrides can be used) and the most common binder is cobalt (although there are other alternatives).
Diamond tools use diamond grit and a cobalt binder.
Heavy alloys have a tungsten metal powder matrix and a Cu/Ni or Fe/Ni binder.
Magnetic and electrical materials
Soft magnetic Powder Metallurgy grades include plain iron, silicon-iron, cobalt-iron and the more recently developed soft magnetic composites, that are based on a plain iron powder with a suitable binder/insulator addition to separate the iron powder particles after a curing treatment.
Hard magnets are invariably processed from powders and include ferrites and the Al-Ni-Co. Sm-Co and Nd-Fe-B grades.
Electrical materials include metal-graphite carbon brushes and electrical contact materials e.g. copper/tungsten, silver/cadmium oxide.
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