Introduction

POWDER METALLURGY MATERIALS encompass enough differences to necessitate providing specific specimen preparation procedures in addition to those given in the Section "Metallographic Techniques" in Metallography and Microstructures, Volume 9 of the ASM Handbook. The major difference between parts made of metal powders and those made of wrought metal is the amount of porosity. Sintered materials generally exhibit 0 to 50% porosity, which affects mechanical properties and strongly interferes with metallographic preparation and interpretation of the structure. When examining micrographs, it is important to understand how the specimens were prepared. Careful metallographic preparation is significant in the analysis of sintered structures because the shape of the porosity is as important as the amount in judging sintered strength and degree of sintering.

In the metallographic preparation of most sintered specimens, the pores are smeared during sectioning, grinding, and rough polishing. This occurs to some degree even in materials whose pores have been filled with plastic resins. Proper polishing should open the smeared pores, then reveal their true shapes and area fractions. Routine metallography of the type used on a medium-carbon, ingot-base steel will not suffice. Because test method ASTM B 328 (Ref 1) cannot be used to measure density differences over short distances such as 0.25 to 6.25 mm, (0.01 to 0.25 in.), the true amount of porosity must be determined by image analysis to facilitate local density measurement. When the specimen is properly prepared, the area fraction of porosity will equal the volume fraction of porosity, and these must equal the porosity calculated from the actual measured density and pore-free density of a uniformly dense part:

Vp = (Pd - Md)/Pd where vp is the volume fraction porosity, pd is the pore-free density, and md is the measured density, for example, by ASTM B 328. Detailed information on density and porosity measurements may be found in the article "Surface Area, Density and Porosity of Powders" in this Volume.

Assuming that a part is uniformly 80% dense, in a properly prepared specimen, 20% of the area should appear as porosity. The surface of cold pressed and sintered parts will always be somewhat denser than the interior because of pressure losses caused by interparticle friction. However, parts that are P/M forged in tools at approximately 370 °C (700 °F) can have a chilled surface lower in density than the hotter, softer interior.

During sintering of cold-pressed compacts, the original particle boundaries disappear and result in a plane of fine pores that then grow into larger pores. In as-pressed parts, particle boundaries appear as thin gray lines in the metallographically mounted material. The progress of sintering can be judged by the disappearance of these boundaries. The original particle boundaries are similar to elongated, disk-shaped pores and have very sharp corners. These are extreme stress raisers. There is virtually no bonding across the original particle boundaries. Proper specimen preparation is required to distinguish residual original particle boundaries from the thin, gray boundaries that often appear between the edges of a particle and pores that were smeared during grinding and polishing. Therefore, an improperly prepared specimen with smeared porosity is often erroneously judged to be undersintered. If microhardness testing is to be performed, proper presentation of the porosity will result in fewer Knoop diamond indentations falling into hidden pores and thus in fewer wasted or incorrect readings. For additional information on microhardness testing of P/M materials, see the article "Testing and Evaluation of Powder Metallurgy Parts" in this Volume.

The open porosity in a mounted sintered part may trap water (moisture). During etching, this water may bleed out, resulting in staining. Water also corrodes some sintered materials. It may evaporate and then condense on the objective lens of the microscope, resulting in a foggy image. Etchants cause similar problems. Open porosity may trap abrasives and carry them onto subsequent cloths, which should hold only fine abrasives. The result is an increased tendency toward scratching of specimen surfaces. Filling the pores with epoxy resins alleviates these difficulties, but requires considerable technique.

Many of the interesting structures seen in P/M parts are caused by porosity and by the mixtures of elemental powders that constitute many alloys. These mixtures do not always result in homogeneous, well-diffused structures. Such heterogeneity is not necessarily detrimental and, in certain nickel steels and diffusion-alloyed steels, may be advantageous because the softer nickel-rich phases increase impact resistance and reduce the tendency toward ductility-limited tensile strength. It is therefore important to recognize when the observed heterogeneity is beneficial. The pores allow carburizing and nitriding gases to penetrate the interior of a sintered steel part, resulting in less-well-defined cases on carbon steel and the nitriding of 300 series stainless steels. The P/M steels are generally low in manganese. When the low alloys are prepared as elemental mixes with nickel and carbon, hardenability is lower than for fully dense, homogeneous low-alloy steels. Hardenability is not a problem in the fully dense prealloyed steels fabricated by forging or metal injection molding.

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