Cast iron is the most numerous group of foundry iron alloys (Fe). More specifically, they are iron-carbon alloys with a carbon content above 2%, with a maximum content ranging from 3.8% to 6.7%. Today, in turn, we will discuss a special type of cast iron, namely ductile iron, which happens to be one of the greatest discoveries in the field of foundry materials.
If you want to learn more about cast iron, its properties and applications, check out our article What is the difference between cast iron and cast steel.
Cast iron is a material that is made of iron and carbon in the form of graphite. Ductile iron, on the other hand, is one of the types of gray cast iron. In gray cast iron, the alloy takes the form of the so-called “flakes''. On the other hand, thanks to the addition of magnesium, ductile iron contains spherical graphite nodules. Because of the spherical shape of the graphite, ductile iron is often called spheroidal iron or nodular cast iron.
This type of cast iron has been known since the 1940s. It was then discovered that by introducing a small amount of magnesium into gray cast iron, carbon does not crystallize in the form of flakes, but in the form of graphite spheres, giving ductile cast iron excellent mechanical properties. Although this casting material was used sporadically at first, it eventually revolutionized the production and use of cast iron. Currently, its share in the production of all castings reaches at least 30% in Europe, the USA and Japan.
Due to its properties highly superior to gray cast iron, they must be distinguished - calling spheroidal iron a gray iron is a mistake.
The chemical composition of spheroidal iron differs from the composition of gray cast iron mainly because of the higher content of carbon and silicon, as well as significantly lower permissible sulfur content (<0.02%). This composition is the result of striving to avoid the so-called dark spots, which are the effect of graphite floatation, which is favored by high carbon content. It also prevents excessive shrinkage of castings.
Shrinkage of castings is a change in the dimensions of the material during solidification and cooling of the casting. In combination with uneven cooling, shrinkage is responsible for the formation of, for example, stresses and cracks.
If the phenomenon of shrinkage is a critical value, one should strive to obtain ductile cast iron with a ferritic matrix immediately after casting (without heat treatment).
“Odlewnictwo”, Wyd. Naukowo-Techniczne (own translation)
The amount of manganese and phosphorus in the casting will depend on the type of spheroidal iron, and in particular on the type of metal matrix. Thus, for example, in cast iron with a ferritic matrix, to ensure that the plastic properties would be excellent, their content should be as low as possible. In ductile iron castings, which require high strength, the manganese content should be correspondingly higher.
Depending on the chemical composition, we can obtain different grades of spheroidal iron with different metal matrix microstructures:
Within the spheroidal iron group, there are 13 grades classified on the basis of tensile strength and elongation.
These are, for example:
Want to know why these properties are important? Read the article How to choose the casting material?
More control is required in the manufacture of ductile iron than other types of cast iron. In addition to the appropriate composition, the temperature of the liquid cast iron is equally important and should not be lower than 1400˙C before spheroidization. This is important because in the process of spherodization the heat is absorbed - the introduction of every 1% of magnesium reduces the temperature by 88˙C.
After spheroidization, the liquid alloy is modified to break up the graphite "balls" to ensure their proper density, which is an average of 125 ➗ 175 spheroids (grains) per mm².
Ductile iron, depending on the stress ranges, is rigid and elastic. However, after exceeding the yield point, it becomes flexible. Its hardness and impact strength, in turn, are high in the entire range of occurring stresses. In addition, it has good sliding properties, is resistant to heavy loads, compression and abrasion.
Due to the fact that ductile iron:
Spheroidal iron, thanks to its mechanical properties, copes well with changing conditions, thanks to which castings made of it have a high safety factor.
For this reason, for example, water pipes are made of it. Firstly, the high mechanical strength of the materials and joints ensures that excavation and soil compaction at backfill will require less attention, without compromising the service life of the pipeline. Due to the fact that ductile iron is resistant to pressure, load and destabilization of the substrate, and works well under the influence of static and dynamic deforming forces acting on pipes, it is a material that allows you to avoid costly repairs, failures, cracks and loss of tightness.
What are the pros and cons of this grade of cast iron? We have summarized them in the table below.
| advantages of ductile iron | disadvantages of ductile iron |
| very good strength | low thermal conductivity |
| great plastic properties | relatively high cost of production |
| high impact strength | tendency to create stresses in the casting |
| relatively high fatigue strength | |
| low tendency to stress concentration | |
| abrasion resistance |
What are the applications of spheroidal iron? Due to the wide range of its properties, it is used in various industries.
These are among others:
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