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Heat Treatment of Ductile Cast Iron

The Heat Treatment Process

Mechanical properties of alloys and castings are structure-sensitive. Their magnitude largely depends on size, shape and distribution of various micro constituents. These mechanical properties can be changed quite drastically by varying the relative proportions of these micro constituents. This is practically achieved by a process called as heat treatment.

Heat treatment is an ancient skill that man developed in an attempt to improve the performance of materials. Today, cast iron manufacturers use heat treatment to improve the performance of castings. Heat treatment represents a whole class of techniques that involve thermal, mechanical and chemical treatment. Years of study of casting material and their structure-property relationship by various cast iron manufacturers has converted heat treatment from an art to exact science. Heat treatment of cast iron in general and ductile cast iron in particular results in optimum modifications in the composition of the cast material and in achieving apt microstructure that imparts the desired characteristics in the material.

Heat treatment for cast iron includes annealing, normalizing, quenching and tempering, and various surface hardening treatments. Since carbon is present in graphite forms in all of the castings (except while cast iron), it affects the kinetics of austenite significantly.

Heat treatment is a common mechanical procedure; what separates heat treatment of cast iron is the presence of carbon (as mentioned above). For any kind of cast iron, the presence of carbon, its form and percentage defines the type of cast iron. Ductile cast iron, white cast iron and gray cast iron is some of the important classification of cast iron. The important point to note is that the shape of the graphite in cast iron cannot be changed once it is formed. Heat treatment however can alter the matrix structure of the graphite, both in its grain and carbon content.

Heat Treatment of Ductile Cast Iron

Ductile cast iron is a form of cast iron that has carbon in spheroid form. For this reason, it is also called as nodular cast iron.

When ductile iron is cooled, it introduces internal stresses in the cast components due to different cooling rates at different points. In some components, these stresses may cause cracking or warping. Annealing (a kind of heat treatment) is used to relieve this stress. Annealing involves heating the cast material to higher temperatures and then cooling it uniformly. This decreases the flow stress, and subsequent internal stresses. Annealing improves the machinability of cast iron by minimizing or eliminating massive eutectic carbides.
Why does Ductile Iron Require Heat Treatment?

While both steel and ductile iron have almost the same composition, the higher percentage of carbon and silicon levels in ductile iron imparts different mechanical properties to it than steel. The higher carbon level in ductile cast iron makes it susceptible to quench cracking. Additionally, the higher level of silicon also lowers the solubility of carbon in austentite, which precipitates as graphite during slow cooling to form a ferritic matrix. Heat treatment of ductile cast iron is therefore a critical task. In India, very few quality conscious manufacturers of ductile cast iron do it properly.

There are three types of annealing treatment available for ductile cast iron: ferritizing annealing, full annealing and graphitizing annealing.

Ductile cast iron can be hardened by quenching process as well. This involves austenitization at higher temperatures, followed by quenching to form matensite.
One other form of heat treatment for ductile cast iron is surface hardening treatment. This involves induction and flame hardening to increase the surface hardness and wear resistance of cast iron components.

Here are the benefits of heat treatment in brief. Do note that not all of these may be applicable to ductile cast iron.

  1. It relieves internal stresses
  2. Refines the grain size
  3. Increases hardness or tensile strength
  4. Improved machinability of the ductile castings
  5. Modification in electrical conductivity, etc.

The most important parameters in the heat treatment process include the treatment temperature, holding time, and the rate of heating and cooling.