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.
POWDERS
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.
PROCESSING
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.
Sintering
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.
STRUCTURAL PARTS
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.
Composition
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 lowdensity 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/cm 3 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 throughhardening 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:
1. A minimum (and possibly a maximum) average apparent hardness should
be specified for acceptance inspection of PM parts.
2. The number of indentations per part, or the number of parts to be
checked, or both, should be specified.
3. The section of the part to be checked should be specified.
4. If parts are thin, a scale should be chosen to ensure that indentation
effects do not penetrate through the part.
5. The hardness scale must be reported.
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
1. Ferrous Powder Metallurgy, Andrej Šalak, published by Cambridge
International Science Publishing, 1995.
2. Powder Metallurgy – Criteria for Design and Inspection, Enrico Mosca,
published by Associazione Industriali Metallurgici Meccanici Affini, 1984.
3. ASM Handbook, Volume 7, Powder Metal Technologies and Applications,
published by ASM International, 1998.
4. Powder Metallurgy Science, second edition, Randall M. German,
published by Metal Powder Industries Federation, 1994.
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John Wiley & Sons, Inc., 1998.
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Wiley & Sons, Inc., 1996.
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by Metal Powder Industries Federation, 1980.
8. Fundamentals of Powder Metallurgy, Leander F. Pease III and William G.
West, published by Metal Powder Industries Federation, 2002.
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Powder Industries Federation, 1998.
10. The Common Cracks in Green Compacts, David C. Zenger and Haimian
Cia, published by Metal Powder Industries Federation, 1997.
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Publishing Co., 1947.
12. Handbook of Powder Metallurgy, Henry Hausner, Chemical Publishing
Company, 1973.
13. Powder Metallurgy and Particulate Materials Processing, Randall German,
Metal Powder Industries Federation (2005).
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Properties, Erhard Klar and Prasan Samal, ASM International, 2007.
15. Particle Packing Characteristics, Randall German, Metal Powder
Industries Federation, 1989.
16. MPIF Standard 35, Material Standards for PM Structural Parts - 2009,
published by Metal Powder Industries Federation, Princeton, NJ.
17. MPIF Standard 35, Material Standards for PM Self-Lubricating Bearings 2010, published by Metal Powder Industries Federation, Princeton, NJ.
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