Ductile iron is not a single material, but a family of materials offering a wide range of properties obtained through micro-structure control. The common feature that all ductile irons share is the roughly spherical shape of the graphite nodules. These nodules act as “crack-arresters and make ductile iron “ductile”.
Ductile iron casting offers designers versatility and higher performance at lower cost. This versatility is especially evident in the area of mechanical properties where ductile iron offers the designer the option of choosing high ductility, with grades guaranteeing more than 18% elongation, or high strength, with tensile strengths exceeding 120 ksi (825 MPa). Austempered Ductile Iron (ADI) offers even greater mechanical properties and wear resistance, providing tensile strengths exceeding 230 ksi (1600 MPa).
Ductile iron offers cost advantages, when compared to steel and malleable iron castings, through reduced shrinkage. The formation of graphite during solidification causes an internal expansion of ductile iron as it solidifies and as a result, it may be cast free of significant shrinkage defects. Ductile iron can be cast with feeders that are much smaller than those used for malleable iron and steel. Large ductile iron castings produced in rigid molds do not require feeders. This reduced requirement for feed metal increases the productivity of ductile iron and reduces its material and energy requirements, resulting in substantial cost savings. The use of the most common grades of ductile iron “as-cast” eliminates heat treatment costs, offering a further advantage.
Depending upon the micro-structure control ductile iron offers a wide range of properties. The ductile iron family includes these grades:
- Ferritic Ductile Iron – Graphite spheroids in a matrix of ferrite provides an iron with good ductility and impact resistance and with a tensile and yield strength equivalent to a low carbon steel. Ferritic ductile iron can be produced “as-cast” but may be given an annealing heat treatment to assure maximum ductility and low temperature toughness.
- Ferritic Pearlitic Ductile Iron – This is the most common grade of ductile iron and is normally produced in the “as cast” condition. The graphite spheroids are in a matrix containing both ferrite and pearlite. Properties are intermediate between ferritic and pearlitic grades, with good machinability and low production costs.
- Pearlitic Ductile Iron – Graphite spheroids in a matrix of pearlite result in an iron with high strength, good wear resistance, and moderate ductility and impact resistance. Machinability is also superior to steels of comparable physical properties.
The preceding three types of ductile iron are the most common and are usually used in the as-cast condition, but ductile iron can be also be alloyed and/or heat treated to provide the following grades for a wide variety of additional applications.
- Martensitic Ductile Iron – Using sufficient alloy additions to prevent pearlite formation, and a quench-and-temper heat treatment produces this type of Ductile iron. The resultant tempered martensite matrix develops very high strength and wear resistance but with lower levels of ductility and toughness.
- Bainitic Ductile Iron - This grade can be obtained through alloying and/or by heat treatment to produce a hard, wear resistant material.
- Austenitic Ductile Iron - Alloyed to produce an austenitic matrix, this ductile iron offers good corrosion and oxidation resistance, good magnetic properties, and good strength and dimensional stability at elevated temperatures.
- Austempered Ductile iIron (ADI) – ADI, the most recent addition to the ductile iron family, is a sub-group of ductile irons produced by giving conventional ductile iron a special austempering heat treatment. Nearly twice as strong as pearlitic ductile iron, ADI still retains high elongation and toughness. This combination provides a material with superior wear resistance and fatigue strength.