Microstructure

The examination of the microstructure of a PM part can serve as a diagnostic tool and reveal the degree of sintering and other metallurgical information critical to the powder metallurgy process. There are several observations common to most sintered materials, as described briefly below. Comments on specific materials will be found in the subsections devoted to those particular materials.

In selecting a section of a PM part for microstructural analysis, an interior plane, parallel to the pressing direction is preferred for mounting and polishing. Coarse and fine polishing should be continued until all of the pores are opened to view and the area fraction of porosity represents the density of the part. For example, an 80% dense part should show 20% of its area as pores. (See MPIF Standard 69 for additional details.)

Parts with interconnected porosity can be impregnated with liquid epoxy during preparation of the specimen for microstructural examination. This will help prevent distortion of the voids during grinding or polishing.

Sintered parts are always examined first in the unetched condition. In an average sinter there will be very few or no original particle boundaries seen at 200X. The more rounded the pores, the higher the strength, ductility and impact resistance.

For mixes of iron and carbon with low nickel and copper content, the approximate carbon content can be estimated by the area fraction of pearlite. For an iron-copper carbon alloy with less than 5% copper, one hundred percent pearlite corresponds to approximately 0.8% combined carbon. Lesser amounts of pearlite mean proportionally less carbon. In nickel steels, even with only 2 to 4% by mass of nickel, the nickel-rich areas occupy a substantial area fraction. These should be discounted in estimating the area fraction of pearlite. The nickel-rich areas should not be confused with ferrite. Loss of surface carbon is generally to be avoided due to lower hardness and wear resistance. If a part is to have 0.6-0.9% carbon, decarburization is indicated if a surface layer measures less than 0.6% carbon. Minor amounts of surface decarburization are seldom a problem but if the layer is deeper than 0.010 inch (0.25 mm) it may be necessary to prove that function has not been impaired. (See ASTM E1077 for measuring the depth of decarburization.)

Heat-treated ferrous parts generally show a mixture of martensite and fine pearlite. This is particularly true for the PM nickel and carbon steels that are of low hardenability. The maximum tensile strength for these materials has been found to occur in parts with 10% to 35% fine pearlite. The prealloyed steels usually show all martensite because of their greater hardenability. The formation of a carbide network embrittles the martensite in a hardened part and is generally to be avoided. Minor amounts of carbide in the outer 0.005 inch (0.13 mm) of parts usually cause no problem. Minor amounts of retained austenite toughen the martensitic structure and usually cause no problem. Higher percentages are generally avoided because retained austenite can transform to brittle martensite in service.

In preparing PM specimens for microstructural examination, the following etchants and procedures are recommended. Ferrous parts containing carbon are generally etched in 2% nital or a combination of nital/picral. Austenitic stainless steel may be etched in glyceregia (10 mL HNO3,20 mL HCI, 30 mL glycerine) by swabbing for one to two minutes. Discard the solution after 30 minutes. Marble's reagent may also be used (10 grams Cu2S04, 50 mL HCI, 50 mL H20). Swab 5 to 60 seconds. To develop grain boundaries in small grain clusters in bronze, etch by swabbing for 10 to 20 seconds in a mixture of 2 grams of K2Cr2O7, 4 mL of concentrated NaCI solution, 8 mL of H2SO4, 100 mL of H2O. To develop a red color in copper-rich regions in bronze, etch by swabbing 10 to 20 seconds in 4% FeCI3 and H2O. For etching brasses, swab for 20 seconds in a solution of 5 mL of NH4OH, three drops of H2O2, 5 mL of H20. This solution is unstable and should be replaced after 20 minutes of usage. The K2Cr2O7 solution may also be used on nickel silver.

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