Macroexamination

Macroexamination of sintered materials is not commonly practiced. In wrought or ingot-base materials, forging flow lines, oxide segregation, and stringers are studied extensively. These features are not usually found in P/M materials, but there are certain other uses for macroexamination.

In sectioning a heat treated P/M steel, care must be taken not to overheat the specimen and temper or reharden it locally. The etching performed during grinding and polishing to help open the pores will indicate any macroscopic striated darkening from overheating, as shown in Fig. 10 and 11. By revealing a lighter or darker surface than the interior, this same intermediate etching using 2% nital on steels will show if a specimen ground through 600 grit SiC paper is likely to have a decarburized or carburized surface layer. This effect is apparent to the unaided eye.

Fig. 10 Ferrous P/M specimen cut with the use of a coolant. No evidence of overheating. Nital. 12x

Fig. 11 Ferrous P/M specimen cut without the use of a coolant. Evidence of overheating (dark area at right edge of specimen). Nital. 12x

During manual polishing with 1 /' m A1;0,. a porous appearing layer similar to orange peel develops near the outer edges of the specimen adjacent to the mounting medium. This layer spreads inward during subsequent polishing until the entire cross section appears uniformly porous to the unaided eye. If the surface is shiny or specular in certain regions, it almost surely will be found to contain pores that polishing has failed to open (Fig. 6 and 9). The penetration depth of the epoxy resin through the side surfaces of the specimen can be seen during polishing, because the regions where the epoxy is in the pores displays the orange peel appearance. The regions with unfilled pores are temporarily smeared over and shiny. After final polishing, the surface roughness from the pores is much diminished, but variations in density in a part are visible to the unaided eye.

Nital etching of elemental nickel steels reveals the nickel-rich areas as light-reflecting sparkles. Unsintered (green) or sintered, unprepared parts may be examined to 25* for cracks or the presence of added copper in iron. The fracture surface of a heat treated part shows varying degrees of discoloration in bands parallel to the outer surfaces. These dark bands are probably caused by oil impregnation during oil quenching, and the color is caused by in situ partial decomposition of the oil during quenching or tempering. The Metal Powder Industries Federation (MPIF) test method 37 for case depth (now obsolete) used the difference in the fracture appearance of the case region and the interior to measure case depth (Ref 9).

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