Heat Treating of Ductile Irons,Heat Treating of Ductile Irons,Heat Treating of Ductile Irons
DUCTILE CAST IRONS (also known as nodular or spheroidal graphite iron), are primarily heat treated to create matrix
microstructures and associated mechanical properties not readily obtained in the as-cast condition. As-cast matrix
microstructures usually consist of ferrite or pearlite or combinations of both, depending on cast section size and/or alloy
composition. These and other factors that affect the casting of ductile irons are discussed in the article "Classification and Basic Metallurgy of Cast Iron" in Volume 1 of ASM Handbook, formerly 10th Edition Metals Handbook. The purpose of
this article is to discuss the heat treatment of ductile irons.
The most important heat treatments and their purposes are:
· Stress relieving, a low-temperature treatment, to reduce or relieve internal stresses remaining after
casting
· Annealing, to improve ductility and toughness, to reduce hardness, and to remove carbides
· Normalizing, to improve strength with some ductility
· Hardening and tempering, to increase hardness or to improve strength and raise proof stress ratio
· Austempering, to yield a microstructure of high strength, with some ductility and good wear resistance
· Surface hardening, by induction, flame, or laser, to produce a locally selected wear-resistant hard
surface
The normalizing, hardening, and austempering heat treatment, which involve austenitization followed by controlled
cooling or isothermal reaction, or a combination of the two, can produce a variety of microstructures and greatly extend
the limits on the mechanical properties of ductile cast iron. These microstructures can be separated into two broad classes:
· Those in which the major iron-bearing matrix phase is the thermodynamically stable body-centered
cubic (ferrite) structure
· Those with a matrix phase that is a metastable face-centered cubic (austenite) structure
The former are usually generated by the annealing, normalizing, normalizing and tempering, or quenching and tempering
processes. The latter are generated by austempering, an isothermal reaction process resulting in a product called
austempered ductile iron (ADI).
Other heat treatments in common industrial use include stress-relief annealing and selective surface heat treatment.
Stress-relief annealing does not involve major microstructural transformations, whereas selective surface treatment (such
as flame and induction surface hardening) does involve microstructural transformations, but only in selectively controlled
parts of the casting.

General Characteristics
The basic structural differences between the ferritic and austenitic classes are explained in Fig. 1 and 2. Figure 1 shows a
continuous cooling transformation (CCT) diagram and cooling curves for furnace cooling, air cooling, and quenching. It
can be seen from Fig. 1 that slow furnace cooling results in a ferritic matrix (the desired product of annealing), whereas
the cooling curve for 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). Examples of furnace-cooled, air-cooled, and water-quenched microstructures are shown in Fig. 3.
Actual annealing cycles usually involve more than just furnace cooling, depending on alloy content and prior structure.
These processes will be detailed in the next section.

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