This part of ISO 945 specifies a method of classifying the microstructure of graphite in cast irons by comparative visual analysis. The purpose of this part of ISO 945 is to provide information about the method of graphite classification. It is not intended to give information on the suitability of cast-iron types and grades for any particular application. The particular material grade is specified by results from tensile tests or hardness testing and, in the case of austenitic cast irons, by their chemical composition. The interpretation of graphite form and size does not allow a statistically valid statement on the fulfilment of the requirements specified in the relevant material standard. The structure of the metallic matrix (e.g. ferrite, pearlite) has a significant effect on the material properties. Such an interpretation is not the purpose of this part of ISO 945.