Cast iron is used widely in the automotive and heavy equipment industries for structural components such as engine, brake, suspension, and steering parts. Cast iron is formed by adding carbon (graphite) to steel. The carbon can precipitate in different forms, for example, in nodules or spherulites (in ductile iron), in a vermicular or flake form (gray iron) or in compacted aggregates (malleable iron). The proportion of nodules, their size and their distribution is a key factor in determining the properties of ductile iron, the greater the proportion of nodules, the greater the strength and ductability. Other properties such as ductility and impact properties are also determined by the structure of the matrix. The matrix, for example, consists of varying proportions of ferrite and pearlite. The greater the amount of pearlite present, the greater the strength and hardness. The characteristics of cast iron are formed during solidification and once solid cannot be changed through further heating.
To ensure that cast iron meets specifications required by the customer, at the end of each heating process a small amount of iron is poured into a disc-shaped mold. The disc is examined to ensure that the carbon has precipitated to agreed manufacturing standards and specifications before further processing can continue. Optical microscopy is a key tool in this examination process. The test disc is usually cut and polished an examined under the microscope using brightfield, darkfield, DIC and / or polarizing microscopy.
Structures visible under the microscope may be compared to a standard by the operator or analysis may be carried out automatically using appropriate image analysis software (such as Nikon’s NIS-Elements Metallo module). Automated image analysis techniques eliminate subjective errors, are calibrated to work to industry standards (such as JIS), and, in the case, of NIS-Elements can additionally carry out other analyses such as the proportions of ferrite and pearlite in the matrix.