Standard specifications for engineering grades of ductile
iron castings classify the grades according to the tensile
strength of a test bar cut from a prescribed test casting.
The International Standards Organization (ISO)
specification ISO 1083:1976 and most national specifications
also specify the ductility in terms of percentage of
elongation and the 0.2% proof strength or offset yield
strength. The impact values of those grades with the highest
ductility are frequently specified in the ISO, UK, and German
specifications, and a guide to microstructure is included
in most specifications. Hardness is usually specified, but is
only mandatory in SAE J434C.
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 that more than
satisfies the requirements of the specification.
Specifications for the highest-strength grades usually
mention the possibility of hardened-and-tempered structures,
but for the most recently reported austempered ductile irons,
which have the highest combinations of tensile strength and
ductility, there are as yet only tentative unofficial
specifications.
Factors That Affect Properties
Graphite Structures. The amount and form of the
graphite in ductile iron are determined during solidification
and cannot be altered by subsequent heat treatment. All of
the mechanical and physical properties of this class of
materials are a result of the graphite being substantially or
wholly in the spheroidal nodular shape, and any departure
from this shape in a proportion of the graphite will cause
some deviation from these properties. It is common to attempt
to produce greater than 90% of the graphite in this form
(>90% nodularity), although structures between 80 and 100%
nodularity are sometimes acceptable.
All properties relating to strength and ductility decrease as
the proportion of non-nodular graphite increases, and those
relating to failure, such as tensile strength and fatigue
strength, are more affected by small amounts of such graphite
than properties not involving failure, such as proof
strength.
The form of non-nodular graphite is important because thin
flakes of graphite with sharp edges have a more adverse
effect on strength properties than compacted forms of
graphite with rounded ends. For this reason, visual estimates
of percentage of nodularity are only a rough guide to
properties. Graphite form also affects modulus of elasticity,
which can be measured by resonant frequency and ultrasonic
velocity measurements, and such measurements are therefore
often a better guide to nodularity and its effects on other
properties. A low percentage of nodularity also lowers impact
energy in the ductile condition, reduces fatigue strength,
increases damping capacity, increases thermal conductivity,
and reduces electrical resistivity.
Graphite Amount. As the amount of graphite increases,
there is a relatively small decrease in strength and
elongation, in modulus of elasticity, and in density. In
general, these effects are small compared with the effects of
other variables because the carbon equivalent content of
spheroidal graphite iron is not a major variable and is
generally maintained close to the eutectic value.
Matrix Structure. The principal factor in determining
the different grades of ductile iron in the specifications is
the matrix structure. In the as-cast condition, the matrix
will consist of varying proportions of pearlite and ferrite,
and as the amount of pearlite increases, the strength and
hardness of the iron also increase. Ductility and impact
properties are principally determined by the proportions of
ferrite and pearlite in the matrix.
The matrix structure can be changed by heat treatments, and
those most often carried out are annealing to produce a fully
ferritic matrix and normalizing to produce a substantially
pearlitic matrix. In general, annealing produces a more
ductile matrix with a lower impact transition temperature
than is obtained in as-cast ferritic irons. Normalizing
produces a higher tensile strength with a higher amount of
elongation than is obtained in fully pearlitic as-cast
irons.
Section Size. As section size decreases, the
solidification and cooling rates in the mold increase. This
results in a fine-grain structure that can be annealed more
rapidly. In thinner sections, however, carbides may be
present, which will increase hardness, decrease machinability,
and lead to brittleness. To achieve soft ductile structures
in thin sections, heavy inoculation, probably at a late stage,
is desirable to promote graphite formation through a high
nodule number.
As the section size increases, the nodule number decreases,
and micro segregation becomes more pronounced. This results
in a large nodule size, a reduction in the proportion of
as-cast ferrite, and increasing resistance to the formation
of a fully ferritic structure upon annealing. In heavier
sections, minor elements, especially carbide formers such as
chromium, titanium, and vanadium, segregate to produce a
segregation pattern that reduces ductility, toughness, and
strength. The effect on proof strength is much less
pronounced. It is important for heavy sections to be well
inoculated and to be made from a composition low in trace
elements.
Composition. In addition to the effects of elements in
stabilizing pearlite or retarding transformation (which
facilitates heat treatment to change matrix structure and
properties), certain aspects of composition have an important
influence on some properties. Silicon hardens and strengthens
ferrite and raises its impact transition temperature;
therefore, silicon content should be kept as low as practical,
even below 2%, to achieve maximum ductility and toughness.
Nickel also strengthens ferrite, but has much less effect
than silicon in reducing ductility. When producing as-cast
grades of iron requiring fairly high ductility and strength
such as ISO Grade 500-7, it is necessary to keep silicon low
to obtain high ductility, but it may also be necessary to add
some nickel to strengthen the iron sufficiently to obtain the
required tensile strength.
Almost all elements present in trace amounts combine to
reduce ferrite formation, and high-purity charges must be
used for irons to be produced in the ferritic as-cast
condition. Similarly, all carbide-forming elements and
manganese must be kept low to achieve maximum ductility and
low hardness. Silicon is added to avoid carbides and to
promote ferrite as-cast in thin sections.
The electrical, magnetic, and thermal properties of ductile
irons are influenced by the composition of the matrix. In
general, as the amount of alloying elements increases,
resistivity and the magnetic hardness of the material
increase and thermal conductivity decreases.
Heat Treatment of Ductile Iron
The first stage of most heat treatments designed to change
the structure and properties of ductile iron consists of
heating to, and holding at, a temperature between 850 and
950
oC for about 1h plus 1h for each 25 mm of
section thickness to homogenize the iron. When carbides are
present in the structure, the temperature should be
approximately 900 to 950
oC, which decomposes the
carbides prior to subsequent stages of heat treatment. The
time may have to be extended to 6 or 8h if carbide-stabilizing
elements are present. In castings of complex shape in which
stresses could be produced by nonuniform heating, the initial
heating to 600
oC should be slow, preferably 50 to
100
oC per hour.
To prevent scaling and surface decarburization during this
stage of treatment, it is recommended that a nonoxidizing
furnace temperature be maintained using a sealed furnace; a
controlled atmosphere may be necessary. Care must also be
taken to support castings susceptible to distortion and to
avoid packing so that castings are not distorted by the
weight of other castings placed above them.
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 give
improved strength and higher proof stress ratio
- Austempering - to yield bainitic structures of high
strength, with some ductility and good wear resistance
- Surface hardening - by induction, flame, or laser to
produce a local wear-resistant hard surface