Engineering Properties: Ensuring Performance
While cost is an important factor, the major consideration for part design & development lies around the properties needed for the application. Properties are important to successful performance of your powder metallurgy part.
Density-related, mechanical, and physical properties should be considered when designing a powder metallurgy part.
Most properties of a powder metallurgy (PM) part are closely related to its final density. This density is the mass per unit volume of the part expressed in grams per cubic centimeter (g/cm³). Normally, the density of structural parts is reported on a dry, un-impregnated basis, while the density of bearings is reported on a fully oil-impregnated basis. Density is most commonly determined using the method given in MPIF Standard 42.
Density is also expressed as relative density, which is defined as the ratio of a PM part’s density to that of its pore-free equivalent. In practice, PM parts less than 75% of relative density are low density; those above 90% are high density; and those in between these two points are considered as medium density. In general, structural parts have relative densities ranging from 80% to above 95%. Forgings and hot isostatic pressed products often exceed 99%. Many self-lubricating bearings have relative densities on the order of 75% and filters usually relative
Porosity is the percentage of void volume in a part. Porosity is a controllable function of the raw material and processing techniques. Parts can be produced either with uniform porosity or with variations in porosity (and density) from one section to another to provide different properties. For example, parts can be made self-lubricating in one area and dense and strong in other areas.
Porosity in PM parts can be present as a network of interconnected pores that extend to the surface like a sponge or as a number of closed holes within the part. Interconnected porosity is important to the performance of self-lubricating bearings and is part of the specification for these types of materials.
The ability to pass fluids or gas as, for example, in filters is another unique property that can be designed into PM products. Depending on the forming and sintering techniques, a PM part can provide permeability ranging from highly restricted to open flow.
The part can be produced with permeabilities that will
- separate materials selectively;
- diffuse the flow of gases or liquids;
- regulate flow or pressure drop in supply lines;
- or act as flame arrestors by cooling gases below combustion temperatures.
Filters can be produced in almost any configuration, including sheets and tubes.
Figure 1 shows ultimate tensile and yield strengths of a 2% nickel and 0.8% carbon, pressed-and-sintered PM steel as a function of density. Yield strength, generally 62%–98% of ultimate strength, is closer to the tensile strength than with wrought metals. Also, the yield strength of many PM materials, particularly stainless steels, may be higher than the wrought forms.
Figure 2 shows impact energy of two PM nickel steels as a function of density, with impact energy rising significantly at higher densities and lower carbon contents.
Figure 3 illustrates ductility as a function of density. Ductility, the amount of plastic deformation prior to tensile fracture, is relatively low in PM materials, chiefly due to the presence of pores. Elongation is generally less than 10% for ferrous materials. For some PM brasses and stainless steels, however, elongations range from 15% to 25%. The ductility of most PM materials can be increased considerably by hot or cold re-pressing followed by re-sintering.
Because of differences in structure, gross indentation hardness values of wrought and PM parts cannot be compared directly. Hardness of a PM part, when obtained using standard tester and scale, is referred to as “apparent harness,” a combination of the powder particle hardness and porosity (see MPIF Standard 43).
Figure 4 shows how an indenter can compress the surface between particles or displace powder particles in low-density parts.
Sometimes called matrix hardness, particle hardness is measured by a microhardness test, such as Knoop or Vickers. The purpose in this test is to measure actual metal hardness, unaffected by any porosity. Microhardness tests and evaluation of heat-treated case depths are discussed in detail in MPIF Standards 51 and 52.
Porosity in powder metallurgy (PM) parts significantly affects corrosion resistance due to possible entrapment of corrosive media. Higher density improves corrosion resistance, as it does most other properties. Stainless steel PM parts have relatively good corrosion resistance in the atmosphere and in weak acids. Nonferrous PM materials have corrosion-resistant properties similar to those of wrought forms.
Excellent surface finish is an inherent feature of powder metallurgy parts. The overall smoothness and surface reflectivity depend on density, tool finish, and secondary operations. Conventional profilometer readings (RMS) take into account the peaks and valleys of a machined surfaces, while PM parts have a series of very smooth surfaces which are interrupted with pores of varying sizes. A chisel point stylus should be substituted for the more typical radius stylus when measuring the surface finish of PM parts with a profilometer. Effective surface finish of PM parts compares favorably with ground or ground-and-polished surfaces of wrought and cast components. Surface smoothness can be further improved by secondary operations such re-pressing, honing, burnishing, or grinding.
The porous nature of PM parts provides good sound dampening. Ringing, common with wrought steel gears and other parts, is reduced due to the controllable density in PM products. This is an important benefit in business machines, air-conditioning blowers, and similar products where quiet operation is highly desirable. Dampening characteristics can be further improved by infiltration or impregnation with sound dampening materials. The controllable density of PM parts is also used to dissipate and muffle noise of air-driven tools.
The depth of hardening achievable in steel alloys is known as hardenability. The higher the hardenability rating, the more hardenable the steel. Hardenability data is provided in MPIF Standard 35, Material Standards for PM Structural Parts.