Metallography and Microstructures of Cast Iron

Metallography and Microstructures of Cast Iron / 567can be reduced to the desired size for metallographicspecimens by using a laboratory abrasivecutoff saw or a band saw. If the casting wasFig. 7 Same as in Fig. 6 but after final polishing withthe 1 lm diamond paste applied on a naplesscloth. Graphite is free of any visible pullouts. As-polished.400sectioned by flame cutting, the specimen mustbe removed well away from the heat-affectedzone. The pieces cut out for metallographic examinationmay be ground prior to mounting (thismay be done to round off sharp cut edges or toreduce the roughness of band-saw-cut surfaces)and subsequent preparation. Overheating isavoided by proper selection of the speed of cutoffsaws, the use of the correct wheel, and adequatewater cooling. Overheating during grindingis avoided by using fresh abrasive paper andproper cooling. When metallographic specimensare cut out from the standard cast bars, they aresometimes prepared using standard machineshop equipment, such as turning in a lathe ormilling. These devices can deform the testpiecesurfaces to a considerable depth, so care must beexercised to remove any damage from theseoperations before starting specimen preparation.Mounting. Specimens can be mounted in apolymeric material using either cold or hotmounting procedures. The mounting resin ischosen depending on the cast iron hardness (softor hard) and the need to enhance edge retention.Use of an incorrect resin, or ignoring the mountingprocess, can make it very difficult to obtainproperly polished graphite in the area close tothe specimen edge. Figures 1 and 2 show themicrostructure of spheroidal graphite in ductileiron close to the edge of the specimens, whichwere cut off from a 30 mm (1.2 in.) diameter barand polished with and without embedding in apolymer resin, respectively. In the specimen preparedwithout embedding in a resin, the graphitewas pulled out, while in the specimen that wasembedded in a resin and prepared, the graphitenodules were perfectly retained. Figures 3 and 4show that the uniform grinding of nonmountedspecimens is more difficult, and the flake graphitein gray iron close to the edge of such a specimenis not polished perfectly, in comparison towell-polished graphite in the mounted specimen.Grinding and Polishing. To ensure propergraphite retention, the use of an automatedgrinding-polishing machine is recommendedover manual preparation. The automated equipmentmakes it possible, in comparison to manualspecimen preparation, to properly control theorientation of the specimen surface relative tothe grinding or polishing surface, to maintainconstantly the desired load on the specimens, touniformly rotate the specimens relative to thework surface, and to control the time for eachpreparation step. Proper control of these factorsinfluences graphite retention, although other factorsare also important.A good, general principle is to minimize thenumber of grinding and polishing stages. Also,the load on each specimen, or on all specimensin the holder, must be chosen to obtain a correctlypolished surface in the shortest possibletime. This precludes the risk of pulling out thegraphite phase and ensures that the graphite precipitateswill be perfectly flat with sharp boundaries.The recommended procedure for automatedpreparation of the specimens of nonalloyed andlow-alloyed cast iron with graphite specimens isto grind with a high-quality, waterproof 220- or240-grit (or equivalent) SiC paper until plane,with a load of 100 N for six specimens mountedin the sample holder, with central loading. Pol-Fig. 8 Same as in Fig. 7 but after final polishing withthe 1 lm diamond suspension applied on anapped cloth. The arrows show the pulled-out graphite.Aspolished.400Fig. 9 White high-chromium iron (Fe-3.2%C-4.65%Cr-2.9%Mn-0.51%Si-0.050%P-0.024%S). Eutecticand secondary carbides in the matrix. Specimen was preparedcorrectly. The casting was austenitized at 1000 C(1830 F), held 1 h, furnace cooled to 400 C (750 F) for2 h, taken to salt bath at 400 C (750 F), held for 4 h, andair cooled. Etched with glyceregia. 500Fig. 10 White high-chromium iron (Fe-3.16%C-8.86%Cr-0.50%Si-3.04%Mn-0.051%P-0.018%S). Eutectic and secondary carbides in the matrix.Specimen was prepared incorrectly. The casting was austenitizedat 1000 C (1830 F), held 1 h, furnace cooled to700 C (1290 F) for 2 h, taken to salt bath at 700 C (1290F), held 4 h, and air cooled. Etched with glyceregia. 500