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Introduction
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 (K. B. Rundman, 1991).
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
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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 (K. B. Rundman, 1991).
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.

Fig. 1 CCT diagram showing annealing, normalizing, and quenching. Ms, martensite start;
Mf, martensite finish
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Fig. 2 IT diagram of a processing sequence for austempering, with the Ms and Mf decreasing
as the γ is enriched with carbon during stage I
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Fig. 3 Optical micrographs of ductile iron with (a) a ferritic matrix in an annealed casting, (b)
fine pearlitic matrix in a normalized casting, and (c) a martensitic matrix in a quenched
casting. Etched in nital; approximate magnifications shown (K. B. Rundman, 1991).

Figure 2 shows an isothermal transformation (IT) diagram for a ductile cast iron, together
with a processing sequence depicting the production of ADI. In this process, 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 ( H) to evolve
simultaneously with nucleation and growth of a plate-like ferrite (α) or of ferrite plus carbide,
depending on the austempering temperature and time at temperature. This austempering
reaction progresses to a point at which the entire matrix has been transformed to the
metastable product (stage I in Fig. 2), and then that product is "frozen in" by cooling to room
temperature before the true bainitic ferrite plus carbide phases can appear (stage II in Fig. 2).
In ductile cast irons the presence of 2 to 3 wt% Si prevents the rapid formation of iron carbide
(Fe3C). Hence the carbon rejected during ferrite formation in the first stage of the reaction
(stage I in Fig. 2) enters the matrix austenite, enriching it and stabilizing it thermally to
prevent martensite formation upon subsequent cooling. Thus the processing sequence in Fig.
2 shows that the austempering reaction is terminated before stage II begins and illustrates the
decrease in the martensite start (Ms) and martensite finish (Mf) temperatures as γH forms in
stage I. The kinetics of stage I and stage II have been described in detail in the literature (Ref
1, 2, 3, 4). Typical austempering times range from 1 to 4 h, depending on alloy content and
section size. If the part is austempered too long, undesirable bainite will form. Unlike steel,
bainite in cast iron microstructures exhibits lower toughness and ductility.
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Tensile Properties.
Figure 4(a) shows the relationship between minimum specified values for tensile strength and
elongation representing the range of values covered by the ISO, ASTM, and SAE
specifications. Figure 4(b) shows the difference between minimum values for ASTM grades
of austempered and other types of ductile iron. The actual values of properties to be expected
from good-quality ductile irons produced to meet any given specified grade will normally
cover a range, as shown in Fig. 4(c) and 5.

Fig. 4 Tensile strength versus elongation of ductile iron. (a) Minimum values given in various
standards. (b) Minimum values of austempered ductile iron grades specified in ASTM A 897.
(c) Range of tensile strength and elongation values with different heat treatments
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Fig. 5 Strength and ductility versus hardness for ductile iron

Austempered Ductile Iron.

sufficient to avoid the formation of pearlite or other mixed structures, and then holding at that
austempering 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 (see the
section "Austempering Temperature and Time" in this article). A lower transformation
temperature (260 °C, or 500 °F) produces a fine, highstrength, wear-resistant structure (Fig.
6a). A higher transformation temperature (370 °C, or 700 °F) results in a coarser structure
(Fig. 6b) that exhibits high fatigue strength and good ductility. The various grades of ADI
have been quantified in ASTM specifications A 897 and A 897M (Table 1).

Fig. 6 Micrographs of ductile iron treated at different austempering temperatures. (a) Ductile
iron austempered at 260 °C (500 °F) exhibits a fine acicular structure with the following
properties: tensile strength, 1585 Mpa (230 ksi); yield strength, 1380 MPa (200 ksi);
elongation, 3%; unnotched impact, 54 J (40 ft · lbf); hardness, 475 HB. (b) Same iron as in
(a) austempered at 370 °C (700 °F) exhibits a coarse acicular structure with the following
properties: tensile strength, 1035 MPa (150 ksi); yield strength, 825 MPa (120 ksi);
elongation, 11%; unnotched impact, 130 J (95 ft · lbf); hardness, 321 HB. Both etched with
3% nital. 300×. Courtesy of Applied Process, Inc.
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REFERENCE
K. B. Rundman. (1991). ASM Handbook Volume 4 Heat Treatment. In ASM Handbook
Commitee (Ed.), ASM International the Material Information Company (9th Editio, Vol.
4). ASM International Handbook. https://doi.org/10.1016/S0026-0576(03)90166-8