General Description of Production, Properties, and Uses of Sintered Powder Metallurgy Bearings and Structural Parts
This information serves as a guide to the specifications for sintered PM bearings and structural parts
(ASTM designations: B595, B783, B817, B823, B848, B853, B883)
Powder metallurgy (PM) components can be made by pressing and sintering metal powders. These parts may be used as bearings or in structural applications. The powder metallurgy process is used for economic advantage and for unique properties.
Metal powders are made by mechanical, chemical, and physical processes such as milling, solid-state reduction of an oxide, or atomization. The method of choice depends on the particular metal.
The major processing steps are compaction of a metal powder and sintering of the PM compact.
Compaction
The initial step is the application of pressure to the desired quantity of metal powder that is contained in a rigid die. To a large degree, the initial compaction dictates the density, shape, dimensions, and mechanical properties of the finished part.
Under properly controlled conditions of temperature, heating rate, time, and furnace atmosphere, the compacted metal powder particles are metallurgically bonded and the green compact becomes a useful part. The sintering temperature has the greatest influence on final properties. Nevertheless, sintering time and furnace atmosphere can be selected to provide optimum properties along with cost-effective processing.
Post-Sinter Processing (Secondary Operations)
Additional processing steps include sizing, coining, restriking, double pressing double sintering (DPDS or 2P2S), impregnation, and infiltration (see Terminology B243). Of course, PM parts can also be heat treated, plated, brazed, welded, or machined as required. Because PM parts are less dense than cast or wrought materials (they contain pores), these processing steps may have to be modified from the typical practice used for fully dense parts.
BEARINGS
PM bearings (Specifications B438 and B439) are self-lubricating because their porosity is impregnated with lubricants during the manufacturing process. In use, heat causes the lubricant to expand out of the pores forming a film between mating parts. When operation is suspended, the lubricant cools and is drawn back into the pores for subsequent reuse. Low coefficients of friction, minimal maintenance and trouble-free service life, low cost, and simple installation are the chief advantages of PM bearings.
Both copper-base and iron-base PM bearings are manufactured and in common use. The copper-base bearings have bronze (Cu-10 Sn) compositions when considered for general-purpose utilization. These alloys are also made with graphite additions when the added solid lubrication is deemed necessary. Bronze may be diluted with 40 to 60% iron to lower the material cost. These bearings usually contain 0.5 to 1.3% graphite for self-lubrication. Iron-base sintered metal bearings are supplied in a variety of compositions such as plain iron, iron-carbon, iron-copper, and iron-copper-carbon. They are used where ample lubrication is provided, where higher loads and slower speeds are prevalent, and where cost factors are significant.
PM structural parts are used wherever they offer an economic advantage. This advantage can be realized in the manufacture of unusual shapes, the elimination of post-sintering (secondary) machining operations, and the minimization of scrap losses. Structural parts can be processed to various levels of mechanical properties, which make them desirable in upgrading or improving products at little or no extra cost. The properties of PM parts depend upon a number of factors.
As in most metallic materials, chemical composition has a major effect on properties. For ferrous PM materials, the manner in which the chemical composition is achieved (the alloying method) has a significant effect on the resulting microstructure and hence the mechanical properties.
There are four commercial alloy types in ferrous PM materials: admixed, diffusion-alloyed, prealloyed, and hybrid-alloyed alloyed. Admixed materials consist of elemental or ferroalloy additions to a base iron. With diffusion-alloyed materials, alloy additions are thermally bonded via diffusion to either an iron or a prealloyed powder base. Adding elemental or ferroalloy powders to either a prealloyed or a diffusion-alloyed base powder produces hybrid alloys.
For ferrous parts, those made from unalloyed iron powder have the lowest strength and apparent hardness. For stronger parts, iron-carbon, iron-copper, and iron-copper-carbon alloys are used. Iron-nickel-carbon alloys are used for heat-treated applications (Examples of these variations may be found in Specification B783).
PM parts with combined carbon may be heat treated by methods appropriate to cast and wrought steel parts. For increased hardenability, prealloyed,
diffusion-alloyed, or hybrid alloys may be used. It is common practice to quench ferrous PM parts in oil; salt bath treatments are generally to be avoided due to potential corrosion problems as a result of salt entrapment within the pores. Careful control of sintering and heat-treatment atmospheres is necessary to maintain the desired combined carbon level at the part surface. Carburizing and other gaseous surface treatments are applicable to ferrous PM parts. Depending on the level of porosity, the depth of surface reaction will be greater for a fixed time compared with fully dense parts.
Density and Mechanical Properties
An important factor affecting the properties of structural parts is their porosity. (The relative density is equal to one minus the volume fraction of porosity). In general, increasing the density of a PM part will increase its tensile strength, compressive strength, and ductility. The indentation hardness of a sintered material is also strongly affected by its porosity, because the pores in the structure of a material do not contribute to the support of the indenter. The indentation of a porous material results in an apparent hardness, which is lower than the indentation hardness of a pore-free material with identical composition and microstructure. For a material of a given composition and microstructure, there is a direct relationship between density and hardness; the lower the density, the lower will be its apparent hardness. Microindentation hardness techniques are recommended for determining the hardness of the metallic matrix, independent of the effects of the porosity. The use of low indenter loads and the relatively small volume of the resulting hardness impression make this possible.
Low-Density Parts
Structural components that have relatively low density and high porosity have the added advantage of self-lubrication as in sintered metal bearings. This
self-lubrication provides a lower coefficient of friction and lower wear in applications in which sliding movement is involved such as in gears and cams. In plating low-density ferrous parts, special precautions must be taken so the plating salts are not entrapped in the surface-connected pores of the parts. This could cause eventual discoloration of the finish, internal corrosion, and “flowering” of the plated surface. Techniques of burnishing, carefully controlled shot peening, and resin impregnation have been developed to either close the surface pores or fill all surface-connected pores and thereby avoid these defects. Ferrous PM parts are being plated successfully with copper, nickel, chromium, cadmium, zinc, and other metals; non-ferrous PM parts may also be plated.
Ferrous-base PM parts may be steam treated by heating them in an atmosphere of superheated steam (Guide B935). Such a process results in a black magnetite (iron oxide) finish that not only improves corrosion resistance, but also makes the parts harder and more wear resistant. Corrosion resistance can be enhanced further by immersing the parts in light oil that will remain in the pores providing a barrier to corrosive media.
As mentioned previously, low-carbon sintered ferrous PM parts may be carburized and surface hardened by gas carburizing and quenching. For low-density parts, however, no sharply defined case is formed since the carbonaceous gas readily penetrates into the core of the parts via their
surface-connected porosity.
Higher-Density Parts
Increased density in structural parts is frequently attainable by additional manufacturing operations. For this reason, higher density parts are generally more costly. Repressing of sintered parts is used to increase part density and introduce cold work. Depending on the degree of densification and cold working, repressing will increase the apparent hardness and strength of parts, while decreasing ductility. In brass parts, for instance, the apparent hardness of the sintered part may be of the order of wrought annealed brass, while repressing may increase the apparent hardness to that of one-quarter hard brass. A second sintering operation after repressing will remove the effect of cold work and result in higher ductility.
Ferrous PM parts with densities up to 7.1 g/cm3 can be produced by using a suitable choice of powders, compacting, and sintering conditions. The four-step process of double pressing, double sintering can reach higher densities. Higher densities may also be reached using warm compaction processing or warm-die compaction processing. In the former, the powder and the compacting tools are heated and high density is achieved in a single compaction stroke. With
warm-die compaction only the die is heated. Warm compaction with 0.6% lubricant additions can achieve green densities of 7.3 g/cm3 in ferrous PM parts (about 98% of the pore-free density of the powder mix). Warm-die compaction with only 0.4% lubricant has permitted green densities of 7.4 g/cm3 to be reached in PM parts.
When the density of ferrous PM parts exceeds 7.3 g/cm3, the amount of
surface-connected internal porosity is negligible. Such parts can therefore be carburized and surface hardened in a manner that is very similar to wrought steel, and they can be plated similarly to wrought parts without any special precautions. High density processed prealloyed, diffusion-alloyed, and hybrid alloy materials are particularly well suited for either surface hardening or through-hardening heat treatments.
Another method of increasing the density of ferrous PM parts, which is widely used for highly stressed parts, is by infiltration with copper or a copper alloy (Specification B783). Infiltration improves the strength, ductility, and impact energy toughness of ferrous parts. Infiltrated parts can also be plated without special precautions and can be joined by brazing. The strength of carbon-free copper infiltrated parts may be increased by a solution and precipitation heat treatment. Carbon-containing infiltrated parts may be heat treated by quenching and tempering.
Qualification Tests
Generally, standard test specimens cannot be machined from many structural PM parts for mechanical property testing. For this reason, the specifications for mechanical properties of structural parts provide for qualification tests. These tests should be agreed upon by the manufacturer and the purchaser and be based on the function the part is to perform.
Compressive strength and ductility can be determined by deforming the part under compression, or by bending, and observing whether fracture occurs before the part has exhibited sufficient strength or ductility for the intended purpose. Impact toughness can be determined by dropping a mass on the part to be tested. The mass, the height from which it is dropped, the point of impact, and the method of supporting the part should be specified.
Hardness may, for certain applications, be used as a qualification test. All normal precautions in hardness testing, such as avoiding an anvil effect in thin parts, shall be observed. Nevertheless, the best manufacturing controls and test procedures cannot prevent occasional stray hardness readings considerably below, or above, the mean hardness value for a sample lot of production sintered PM parts. The inspection of production parts for apparent hardness shall be done in a prescribed manner to avoid a question of acceptability.
Where microindentation hardness test equipment is available, the universality of application of the hardness scale provides for a convenient hardness number series. Conversion of microindentation hardness values to a familiar Rockwell scale may be done using Method B933. Microindentation hardness testing is a favorable method for determining the effective case depth of surface-hardened PM components.
One useful concept for developing an apparent hardness specification for acceptance inspection is “minimum average hardness”. The average of a specified number of indentations shall not be less than a given minimum value. There may be an occasion when the specified hardness may be no less than a stated minimum average and no more than a stated maximum average. Thus, an apparent hardness specification may include details as follows:
Recommendation
The mechanical properties listed in the appendices to the specifications for structural parts should be considered as typical for parts made by normal production methods. Superior mechanical properties can be achieved by special processing methods. It is suggested that the purchaser work closely with the parts maker in the design and development of new parts or in the conversion to powder metallurgy fabrication of existing parts.
BIBLIOGRAPHY
Wbj 9/6/12
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