Effect of specimen preparation on evaluation of cast iron microstructures

Abstract

Preparation of cast iron specimens is difficult due to the need to properly retain the graphite phase. Three procedures using automated grinding/polishing processes were used to study the effect on graphite retention of such factors as, the quality of SiC grit paper, the kind of polishing cloth and diamond paste, as well as the number of preparation steps. The best preparation method for different types of graphite consisted of four steps, with the polishing process carried out on a napless cloth with diamond in paste form. A procedure for specimen preparation of white, high-chromium cast iron is also given. Examples of the use of different etchants for revealing unalloyed cast iron microstructures, such as: pearlite, ferrite and cementite, are presented. The phosphorous eutectic (Steadite) in gray iron can be observed after etching with 4% nital, but selective color etching helps to differentiate the revealing constituents. This method is also helpful in phase differentiation of the ADI matrix as well as revealing silicon segregation in ductile iron and microsegregation in the austenitic matrix of mottled iron. Color etching of chromium carbides in alloyed white irons facilitates the use of the color image analyzer for measurements of the amount and morphology of this phase.