An Introduction to Ductile Iron

Chemically this material is the same as grey iron and is iron-carbon-silicon alloy. It is one of the more recent developments in cast iron technology and has been around since 1948. As the name suggests, it was developed to overcome the brittle nature of grey and white irons. It is also quite ductile in the as-cast form and negates the need for long heat treatments such as those required to produce malleable iron.

Chemistry

Like grey iron, ductile iron is an iron-carbon-silicon alloy. Typical carbon content is in the range 3 to 4%, while silicon content is slightly higher than grey iron at 2 to 3%. It is also common for ductile irons to contain significant additions of nickel.

Ductile iron is often made from scrap, pig iron and recycled ductile iron. Formation of flake graphite (and a grey iron structure) is avoided through the addition of small amount of magnesium to the melt. The interaction is extremely vigorous as the melt temperature of iron is higher then the boiling temperature for magnesium.

Ferrosilicon and other additives may also be used to promote graphitisation, control nodule size and promote formation of spherical graphite particles. Such additions may be added to the ladle rather than the furnace, a process called ladle inoculation.

Structure

The main difference between ductile iron and grey iron is the morphology of the graphite particles which take on a nodular or almost spherical form after suitable treatments are made to the melt. They resemble those found in malleable iron, but are more spherical.

Similar to grey iron the matrix may be ferritic, pearlitic or martensitic depending factors such as chemistry and other process variables.

Designations

The most commonly used system for these materials is outlined in ASTM A536. Designations consist of three numbers and cover 5 grades. The fist number relates to the minimum tensile strength (in kips per square inch), the second to the minimum yield strength (in kips per square inch), while the last refer to elongation during the tensile test (in per cent). The grades of ductile iron are as follows:

• Grade 60-40-18
• Grade 65-45-12
• Grade 80-55-06
• Grade 100-70-03
• Grade 120-90-02

Separate ASTM standards pertain to austenitic ductile irons (ASTM A439 and A571) and special purpose ductile irons (ASTM A476, A716, A395 and A667).

Key Properties

As with most grades of cast iron, ductile irons display:

• Good hardness and good wear resistance
• Good corrosion resistance
• Have tensile and yields strengths that vary widely across the various grades.
• Have compressive strengths that can be utilised more widely (than tensile strengths), with values tending to be about twice the tensile strength.
• Impact strengths are better than grey irons, with lower grades approaching values common for mild steel.
• Fatigue strengths are approximately 40 to 50% of tensile strengths.
• Electrical resistivities are significantly lower compared to grey irons
• Corrosion resistance is similar to grey iron

Machinability is dependent on hardness, with ferritic grades machining better.

Heat Treatments

Ductile irons can be heat treated similarly to grey irons, i.e. they can be:

• Normalised – resulting in increased strength
• Annealed – to relieve internal stresses and aid machining
• Stress relieved – to remove internal stresses from uneven cooling and other effects
• Quench hardened – to produce a stronger, harder, more wear resistant material. Pearlitic matrix ductile irons produce the best results for quench hardening. Prime candidates for this operation include the grades 80-55-06 and 100-70-03.

Heat treatments are often integral in the production process Typically:

• Grade 60-40-18 – is annealed
• Grade 65-45-12 – is used as-cast
• Grade 80-55-06 - is used as-cast
• Grade 100-70-03 – is normalised
• Grade 120-90-02 – is quenched and tempered

This information has been sourced, reviewed and adapted from materials provided by Fonderie Saguenay.

For more information on this source, please visit Fonderie Saguenay.