Specifications for Ductile Iron

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: