PM Structural Parts
MPIF Standard 35-SP Materials Standards for PM Structural Parts
Definitions and Notes
Minimum Value Concept
The Metal Powder Industries Federation (MPIF) uses minimum strength values for Powder Metallurgy (PM) materials in structural applications. These values guide PM part design. The PM process can achieve equivalent tensile strengths by varying chemical composition, particle configuration, density, and processing techniques. Along with minimum strength values, typical values for other properties help users select suitable PM materials for specific applications.
Typical Values
Typical values for PM materials include density, hardness, and elongation. These values are for general guidance and are not minimum values. They vary based on component area and manufacturing process. Required property values should be discussed with the PM parts manufacturer before specification.
Heat Treatment
Ferrous PM parts with ≥0.3 wt.% carbon can be quench-hardened and tempered for increased strength and wear resistance. The degree of hardening depends on carbon content, alloying elements, and density. Tempering after quenching is required for maximum strength.
Microstructure
Microstructural analysis of PM parts reveals sintering degree and other critical metallurgical information. Preferred sections for analysis are interior planes parallel to the pressing direction. Various etchants and procedures are recommended for preparing PM specimens.
Minimum Value
For as-sintered PM materials, the minimum value is the yield strength (0.2% offset) in psi or MPa. For heat-treated PM materials, the minimum value is the ultimate tensile strength. Soft-magnetic materials use coercive field strength in oersteds x 10. Tensile properties are obtained from test specimens prepared for evaluating PM materials and may differ from those of commercial parts.
Chemical Composition
The chemical composition table lists principal elements by minimum and maximum mass percentage and includes other minor elements. It specifies the material before any additional processes like oil impregnation.
Sinter Hardening
Some PM materials can be quench hardened during the sintering cooling cycle, especially certain prealloyed steels and martensitic stainless steels. Tempering after sinter hardening is required for durability.
PM Material Code Designation
The PM material code specifies chemical composition and minimum strength. For example, FC-0208-60 indicates a PM copper steel with 2 wt.% copper, 0.8 wt.% combined carbon, and a minimum yield strength of 60,000 psi (410 MPa) in the as-sintered condition.
Specifying a PM Material
To demonstrate minimum strength values, a static or dynamic proof test is used by the manufacturer and purchaser on the first production lot. Strength can also be verified using test specimens from the same material lot and processing as the parts. Machining test specimens from the parts themselves is less reliable and should be agreed upon if used.
Mechanical Properties
Mechanical property data, indicating minimum and typical values, are based on test specimens prepared under commercial conditions. Hardness values are given as apparent and microindentation hardness values.
Surface Finish
PM material surface finish depends on density, tool condition, and secondary operations. Profilometer readings may not accurately reflect the smoothness due to the unique PM surface characteristics. Surface finish requirements should be agreed upon by purchaser and producer.
Soft-Magnetic Alloys
For soft-magnetic alloys, specific properties are measured under DC magnetization conditions. The suffix for these materials does not indicate yield or tensile strength but rather the maximum coercive field strength and an alphabetic density designator.
PM Structural Parts
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Iron and Carbon Steel PM Materials
Prealloyed Steel
Diffusion-Alloyed Steel PM Materials
Stainless Steel – 400 Series Alloy
Iron-Copper and Copper Steel PM Materials
Hybrid Low-Alloy Steel PM Materials
Copper-Infiltrated Iron and Steel
Copper and Copper Alloys
Aluminum Alloys
Iron-Nickel and Nickel Steel PM Materials
Sinter-Hardened Steel PM Materials
Stainless Steel – 300 Series Alloy
Soft-Magnetic Alloys