Heat Treating of Ductile Irons

Ductile cast iron (also known as nodular or spheroidal graphite iron) is heat treated primarily to create matrix microstructures and associated mechanical properties not readily obtained in the as-cast condition. The microstructures achievable can be depicted using a continuous cooling transformation (CCT) diagram and cooling curves for furnace cooling, air cooling, and quenching. Slow furnace cooling results in a ferritic matrix (the desired product of annealing); whereas air cooling, or normalizing, results in a pearlitic matrix; and quenching produces a matrix microstructure consisting mostly of martensite with some retained austenite. Tempering softens the normalized and quenched conditions, resulting in microstructures consisting of the matrix ferrite with small particles of iron carbide (or secondary graphite). Actual annealing cycles usually involve more than just furnace cooling, depending on alloy content and prior structure.

To produce austempered ductile iron (ADI), austenitizing is followed by rapid quenching (usually in molten salt) to an intermediate temperature range for a time that allows the unique metastable carbon-rich (~2%C) austenitic matrix (gamma subscript H) to evolve simultaneously with nucleation and growth of a plate-like ferrite (alpha) or ferrite plus carbide, depending on the austempering temperature and time at temperature. The austempering reaction progresses to a point at which the entire matrix has been transformed to the metastable product (stage I), and that product is "frozen in" by cooling to room temperature before the true banitic ferrite plus carbide phases can appear (stage II). The presence of 2-3%C prevents the rapid formation of iron carbide (Fe₃C), and, thus, the carbon rejected during ferrite formation in stage I enters the matrix austentite, enriching it and thermally stabilizing it to prevent martensite formation upon subsequent cooling.

ADI is a unique cast iron material, having tensile properties attributable to γ_H, but with the fine dispersion of ferrite. Austempering is accomplished by heating the casting to a temperature in the austenite-phase range (usually 815 to 925°C, or 1500 to 1700°F), holding for the time required to saturate the austenite with carbon, cooling to a temperature above the M_s temperature at a rate sufficient to avoid the formation of pearlite or other mixed structures, and them holding at that austenitizing temperature for the time required to produce the optimum structure of acicular ferrite and carbon-enriched austenite. The properties of ADI can be varied by changing the austempering temperature.

The objective of austenitizing is to produce an austenitic matrix having as uniform a carbon content as possible prior to thermal processing.

For a typical hypereutectic ductile cast iron, an upper critical temperature must be exceeded so the austenitizing temperature is in the two-phase (austenite-graphite) field; this temperature varies with alloy content. The "equilibrium" austenite carbon content in equilibrium with graphite increases with increasing austenitizing temperature. The ability to select (within limits) the matrix austenite carbon content makes austenitizing temperature control important in processes that depend on carbon in the matrix to drive a reaction. This is particularly true in structures to be austempered, in which the hardenability (or austemperability) depends to a significant degree on matrix carbon content. Austenitizing temperatures in the range of 900 to 940°C (1650 to 1750°F) typically are used with times ranging from 1 to 3 h.

Ductile iron castings generally are given a full ferritizing anneal when maximum ductility and good machinability are required and high strength is not required. The microstructure is converted to ferrite, and the excess carbon is deposited on the existing nodules. This treatment produces ASTM grade 60-40-18. Amounts of manganese, phosphorus, and alloying elements such as chromium and molybdenum should be as low as possible if superior machinability is desired, because these elements retard the annealing process. Three types of annealing treatment are full anneal for unalloyed 2-3% Si iron having no eutectic carbide, full anneal with carbides present, and subcritical anneal to convert pearlite to ferrite.