Keys are to be carbon steel or alloy heat treated steel as specified. carbon steel keys are to be 0.30 carbon minimum, with hardness of rockwell B 90 minimum. Alloy steel keys are to be SAE 2330 or 8630 steel, heat treated to a hardness of 40-50 RC; or other alloy steels having equal physical properties at the same hardness. Alloy heat treated keys are to be marked with depressions on the top to distinguish them from carbon steel keys.