Metallographic techniques for cast irons are similar to those for steels; with the exception that graphite retention is a more challenging task. Recommended procedures to prepare cast irons are given. Colloidal silica is an excellent final polishing abrasive for many metals and alloys. However, for pearlitic cast iron grades, colloidal silica often produces small etch spots on the specimen surface. In this case, OP-AN alumina suspension yields excellent results, much better than standard alumina abrasive powders made by the calcination process. Examples of cast iron structures revealed using a variety of etchants is presented.
New concepts and new preparation materials have been introduced that enable metallographers to shorten the process while producing better, more consistent results. But first, the specimens must be sectioned. Many metallographers do not use a blade designed for metallography work, and the depth of damage will be much greater when production-type abrasive saws are used. So, as a first rule, produce a cut with the least possible amount of damage. If an automated device is used that holds a number of specimens rigidly (central force), then the first step must remove the sectioning damage on each specimen and bring all of the specimens in the holder to a common plane. This first step is often called “planar grinding.” SiC paper can be used for this step, although more than one sheet may be needed. Alternatively, the metallographer could use MD-Piano 120 or 220 (for specimens with hardness >150 HV) for the initial grind, followed (if desired) by MD-Piano 600 for a second grinding step. If the cast iron has a low hardness (<250 HV), one can planar grind with MD Primo 220. Alternatively, MD-Allegro could also be used to planar grind for specimens >150 HV hardness. If the hardness is <150 HV, MD-Largo can be used.
