Inspection Considerations

Inspection techniques for finished parts should be based on concurrence between the customer and the supplier. There are many ways to gage most part print specifications, and the specific technique used can have a large effect on the results reported. A great many P/M part attributes may be measured with the same techniques used in other industries. There are, however, significant exceptions as briefly described below.

Hardness. In powder metallurgy there are generally two types of hardness specified--apparent hardness (macrohardness) and microhardness. The microhardness is the hardness of each particle of material, and the apparent hardness is the hardness of the surface--bridging across many particles and the porosity, too.

Apparent hardness is typically measured according to MPIF Standard 43 (Ref 7). The procedure is relatively straightforward and quick. The basics are:

1. Obtain a sample part of adequate thickness and parallel configuration (or, for cylindrical parts, a correction factor may be used).

2. The sample must be large enough so that the indenter marks from the hardness tester are at least three indenter diameters from any edge or previous impression.

3. Sand each face of the sample so that no burrs are present (burrs will cause erroneous readings), or be sure to use a holding fixture that avoids the burrs.

4. Take readings with a properly calibrated hardness tester.

5. Reject obvious outliers and report the average of at least five nonoutliers.

Typically, the outliers are on the low side. The cause of these occasional low readings is a chance happening that the hardness indenter falls right into a pore.

Microhardness is usually measured according to MPIF Standard 51 (Ref 8). The determination of microhardness is significantly more difficult than measuring apparent hardness and requires specialized equipment that many P/M users do not have on-site. The procedure involves:

1. Sectioning the part and making a polished mount for the evaluation.

2. Placing the mount in a special microhardness testing machine.

3. Under magnification, orienting the mount and making a diamond indenter mark precisely over a particle of the material.

4. Measuring the length of the penetration on the particle and converting this length to a hardness reading.

Microfinish. The porosity of P/M causes debate over the proper method of measuring microfinish. When using a standard cone stylus, unmachined P/M can give relatively high microfinish values. The individual particles are very smooth, but the probe path is interrupted by pores of varying sizes (Fig. 9). Because most parts are running against a mate that is much larger than the standard probe, many feel that the chisel probe provides a better gage as to the actual serviceability of the parts. The chisel probe bridges across the porosity and usually produces significantly lower microfinish values.

Fig. 9 Surface measurement with chisel stylus (a). (b) Effect of cone and chisel styli on an as-sized P/M surface. Source: Ref 9, used with permission.

Physical Tests. Performing a physical test on finished parts is a great method of verifying that all processes ran properly, and the final parts will perform adequately. Crush, torque, impact, and tensile tests are commonly called out on P/M part prints. These tests greatly reduce the need for expensive, time-consuming, and sometimes ambiguous, microstructure analysis.

0 0

Post a comment