| Secondary graphitization in quenched and tempered ductile cast iron refers specifically to the decomposition of a martensitic structure at high tempering temperatures resulting in the formation of small graphite precipitates throughout a ferrite matrix. This graphitization causes a degredation in the tensile and impact properties, loss of hardness control upon tempering, and difficulties when re-heat treating.;Only heat treatment variables that affect the morphology of the martensite formed upon quenching affect the morphology and distribution of the secondary graphite formed upon tempering. Decomposition of tempered martensite to form secondary graphite commonly occurs when tempering above 1000 F, but can occur at temperatures as low as 800 F. Secondary graphitization cannot be prevented by the addition of alloying elements within normal ranges; low silicon levels delay the graphitization kinetics to a greater extent than the addition of carbide forming elements.;Secondary graphitization is preceeded by carbide formation and carbide decomposition that take place during the early stages of tempering, as well as by some direct nucleation of secondary graphite from the quenched structure. Further tempering results in the growth of graphite on the primary graphite as well as on the easily nucleated secondary graphite, first in the eutectic cell regions and then in the intercellular regions of the matrix.;Secondary graphite significantly reduces the upper shelf impact energy but greatly decreases the impact transition temperature and increases the lower shelf energy compared to ferritic ductile iron. Impact properties are similar to those of ferritic malleable iron.;The following characteristics of secondary graphite formation were investigated experimentally; (a) the effects of heat treatment and chemical composition variables on the formation and distribution of secondary graphite, (b) the specific mechanism of secondary graphitization, (c) the relationship between secondary graphitization and temper embrittlement in quenched and tempered ductile iron, and (d) the effects of secondary graphite on impact toughness. Experiments were performed on commercial ductile cast iron, alloyed ductile cast iron produced in the laboratory, and on various other cast irons and steels from comparison. Optical metallography, scanning and transmission electron microscopy, dilatometry, and impact testing were used as analysis tools. |
