


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. (150 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%
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:





 FATIGUE DESIGN CONCEPTS FOR POWDER METAL PARTS
Powder metal has significantly different stressstrain 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 unnotched 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 
10 6 psi 
16.5 
18.3 
20.3 
22.3 
24.5 
29.9 







