Guidelines for specifying a PM part & Fatigue design concepts

GUIDELINES FOR SPECIFYING A PM PART

Although powder metallurgy industry standards (Refs 1, 2) provide useful physical and mechanical property design data for engineers familiar with the PM process, those less experienced with this manufacturing process may benefit from additional guidance. The following sections offer some guidelines to consider when using the conventional powder metallurgy process [not metal injection molding (MIM) or hot isostatic pressing (HIP)] for a new product design.

Part Size—the size limitation of PM parts is based on powder compressibility and press tonnage. The typical steel PM part will satisfy the following characteristics:

projected surface area—less than 50 in.² (32,000 mm²)

diameter of less than 7 in. (185 mm) or up to 12 in. (300 mm) for parts with a large bore

length of 6 in. (75 mm) maximum, 0.060 in. (1.5 mm) minimum

length:diameter ratio of 5:1 maximum; length:wall thickness ratio of 8:1 maximum
If the product design will use a nonferrous material, the projected area can be increased by 50%.

Part Shape—part geometry must be compatible with a uniaxial compaction motion in the vertical direction (see Figure at end of this document). Undercut, re-entrant, or threaded features are typically formed or machined in a secondary operation. Significant variations in part length in the cross section require different tooling motions in the compaction press. These are typically limited to five different levels, though additional small changes in part length (levels) are possible through selected tool designs [see design manuals (Refs 3, 4) for illustrations of complex PM part designs]. Part lengths greater than 1 in. (25 mm) will result in density variation from the top surface (highest density) to the mid-length position (lower density).

Part Quantity—due to the use of dedicated "hard" tooling, a cost-effective quantity for production orders should be discussed.

Mechanical/Physical Requirements—Industry Standards (Refs 1, 2) provide a wide range of design properties to assist selection of the proper material for most product applications. The medium density steel grades (6.4–6.9 g/cm³) typically replace cast iron or plain carbon steel; the high-density grades (7.0–7.4 g/cm³) typically replace ductile iron and many heat treated steels; powder forged and pore-free material systems typically replace the high-performance steels. If the part design has critical property requirements beyond the typical values listed in the standard, these requirements should be specified on the engineering print.

Effect of Density—the density of PM parts varies according to the pressing pressure used. It is also affected by some of the other processing parameters. It is important to recognize that, in addition to mechanical properties, some key physical properties are also affected by density. For example, elastic modulus and thermal conductivity increase significantly with increasing density; Poisson's ratio and thermal expansion also increase, although more slowly.

Apparent Hardness—because of differences in structure, gross indentation hardness values (Rockwell or Vickers) of wrought metals and PM materials cannot be compared directly. The indenter penetrates further into the porous PM structure, thereby giving a lower hardness value than the intrinsic hardness of the metal itself. Hardness values for PM materials, therefore, when obtained on macro-hardness testers, are referred to as "apparent harness" and represent the combined effect or porosity and the metal particle hardness. For example, a heat-treated PM steel may measure an apparent hardness of 30 Rockwell C while the actual material measures 54–58 Rockwell C.

Part Use —the part specification or drawing should provide as much additional information regarding the part use as possible. The following questions are most important for the material and process selection:

Will the part operate under a pressure? Must it be leak tight?

Must the part be protected from corrosion—how severe?

Will the part be machined—which surfaces, what tolerances?

Will the part require heat treatment—what type?

Will the part be used in a high-impact-loaded application?

Will the part be used in a wear application—which surfaces? Is surface finish an important design feature, where, how to measure?

Will the part be used in a magnetic application?

Will the part be used in a thermally demanding application?

Must the part be burr-free—what type, amount of corner radius?

Will the part be welded?

Is there a region critical to the performance of the part?

Are there any unique packaging requirements?
More detailed product design information is available in the industry design manuals (Refs 3, 4). Material coding systems are explained in the standards (Refs 1, 2)

To select a material optimum in both properties and cost effectiveness, it is essential that the part application be discussed with the PM parts manufacturer. Both the purchaser and the manufacturer should, in order to avoid possible misconceptions or misunderstandings, agree to the following conditions prior to the manufacture of a PM part: minimum strength value, grade selection, chemical composition, proof testing, typical property values and processes that may affect the part application. The Global Powder Metallurgy Database gives raw data on materials currently in production at key component makers worldwide. In many cases the database gives high and low ranges of properties. It is intended as a guide to designers in the selection of materials and processing methods in PM. Figures are not normative and are not to be regarded as guaranteed or minimum values. The database is intended to be the first stop before the designer makes direct contact with individual manufacturers.

For more information online consult industry Web sites as follows:

Designing with PM:
> www.epma.com—enter—about PM—PM Design
> www.mpif.org—PM Design Center

Hard-copy Publications on PM:
> www.epma.com—enter—publications
> www.mpif.org—publications

References
1—MPIF Standard 35 "Materials Standards for PM Structural Parts"
2—ISO5755:2001 "Sintered metal materials—specifications"
3—"Powder Metallurgy Design Manual" (published by MPIF)
4—"Introduction to Powder Metallurgy: The Process and its Products" (published by EPMA)

Designing Sintered Components: Some Dos and Don'ts

FATIGUE DESIGN CONCEPTS FOR POWDER METAL PARTS

Powder metal has significantly different stress-strain and notch sensitivity from wrought steels.Therefore to obtain the most cost effective solution using PM parts it is essential that these are well understood by the design engineer, and that PM data is used correctly in design calculations.

In structural fatigue analysis of Powder Metal (PM) components, there are strong arguments for adopting a local stress design concept.

The analysis of PM materials differs significantly from that of, for instance, wrought materials principally in terms of two issues: notch sensitivity and the correction for the effect of mean stress.

Notch Sensitivity
Porous PM materials are relatively insensitive to the influence of external notches. The following figure summarizes reported information on the notch sensitivity (i.e., the ratio of fatigue strength in notched and un-notched states) of PM materials in axial and bending modes as a function of the stress concentration factor, Kt, of the external notch. It can be seen that notch sensitivity of PM materials is particularly low in the bending mode.

The low notch sensitivity of fatigue endurance limit in bending mode of PM steels, compared with those of both wrought steels and nodular cast irons, is demonstrated, as follows:

Mean stress correction
To assess the influence of mean stress on fatigue endurance limit of a PM material, it is necessary to have the results of fatigue tests carried out for at least two different values of stress ratio, R ( = smin/smax). The database contains information, for certain material grades, on fatigue endurance limit at values of R other than -1. On the basis of these data, Haigh diagrams can be contructed to interpolate or extrapolate to the mean stress or R ratio of interest. The following normalized Haigh diagrams have been constructed to summarize the reported data for influence of mean stress on fatigue endurance limit, in axial and bending loading modes. All reported relationships for PM materials fall within the limits of the pairs of lines on these diagrams.

Compared with wrought steels, PM materials are relatively sensitive to changes in mean stress; their response is much more comparable with that of cast irons. PM materials are therefore particularly suited to fatigue loading regimes with negative values of R, i.e., with loading being predominantly compressive.

Further supporting detail on fatigue design of PM parts is available from the following document (right click and choose 'Save target as...'):

"Concepts and Required Material Data for Fatigue Design of PM Components" by C.M. Sonsino. Proceedings of EuroPM 2001 Conference, Nice.

or from publications available from the following Web sites:

www.epma.com—enter—publications

www.mpif.org—publications

Rolling Contact Fatigue (RCF) behavior of PM materials
The controlling factor in rolling contact fatigue is the Hertzian contact stress. The Hertzian contact stress, S, between two parallel cylindrical rollers, for instance, is given by the relationship:- S = [0.35F (1/r1 + 1/r2)]/[b (1/E1 + 1/E2)] where F is the applied load, r1 and r2 are the radii of the two rollers respectively and E1 and E2 are the moduli of elasticity for the two roller materials respectively.

PM materials below full density have an elastic modulus lower than that of conventional steels. So, for a given load, these materials operate at a lower Hertzian stress. It is therefore important to use the correct elastic modulus when carrying out design calculations on PM materials.

The following table shows the relationship between E and density level for ferrous PM materials:

	Units
Density	gm/cm³	6.6	6.8	7.0	7.2	7.4	7.86
Young's Modulus	Gpa	114	126	140	154	169	206
Young's Modulus	10⁶ psi	16.5	18.3	20.3	22.3	24.5	29.9

This issue is discussed in more detail in the following downloadable document (right click and choose 'Save target as...'):

"Wear performance of compositions made by low alloy iron/high alloy powder mixtures". P.D. Nurthen, T. Davis, J.V. Wood, T. Cadle, C. Landgraf; Advances in Powder Metallurgy—1991; Volume 5, pp. 135–149; MPIF/APMI.