Isothermal ductile iron (IDI)

Isothermal ductile iron (IDI) as a solution for very thick parts

Isothermal ductile iron IDI: the Zanardi Fonderie patent for cast parts requiring high static and fatigue strength

Isothermal ductile iron (IDI) is obtained by heat treating ferritic spheroidal graphite iron without the addition of costly alloying elements. In terms of efficiency and cost-savings, IDI is an extremely valid solution for the manufacture of very thick parts requiring high levels of static and fatigue strength, such as pump heads for hydraulic applications, steering knuckles and centre bodies for axles.

IDI is higher performing than spheroidal graphite iron GJS and more affordable than austempered ductile iron ADI.

The structure of the matrix in isothermal ductile iron (IDI) consists mainly of ferrite and pearlite, two elements, which, thanks to a different distribution with respect to traditional spheroidal graphite irons, increase the mechanical performance of finished castings.

Properties of pearlitic isothermal ductile iron IDI

Isothermal ductile iron (IDI) is obtained by heat treating a cast part in ferritic spheroidal graphite iron (spheroidal graphite iron grade GJS400), through isothermal hardening in a salt bath without the addition of alloying elements. One of the most interesting properties of IDI is the consistent behaviour of the material as the thickness varies, even in the presence of low temperatures. This makes the use of isothermal ductile iron (IDI) extremely advantageous when producing very thick parts.

The benefits of IDI in terms of cost and efficiency

Isothermal ductile iron (IDI), in particular when used for very thick castings, represents a valid technical and economic compromise for the production of parts that need to overcome technical and production limits, and the relative cost of alloying elements, pearlitic ductile irons, new high silicon ductile irons and austempered ductile irons.

IDI therefore represents a valid alternative with respect to GJS800-2, ADI800-10, GJS600-10. With machinability comparable to that of pearlitic ductile irons commonly used in mechanical machine shops, IDI is also suited to mechanical processing. It has a mixed ferritic-pearlitic structure, which means it can also be surface hardened only where necessary, thus increasing the wear resistance of parts.

benefits of isothermal ductile iron (IDI), we highlight:


  • Lightness: the specific weight of IDI is 10% lower compared to steel, thus allowing vehicles to either increase their maximum load or reduce fuel consumption;
  • Complex moulds: thanks to the melting technology and excellent castability, the material can be applied only where necessary;
  • Fatigue strength: in terms of performance, it is comparable to that of pearlitic ductile irons;
  • Cost-saving: no alloying elements are required;
  • Resilience: even at low temperatures (up to -40°C), the material guarantees consistent mechanical performance in very thick cast parts or those with variable thickness.

Mechanical properties Isothermal ductile iron (IDI)

Symbolic Designation
IDI 800
Tensile strength Rm (N/mm2 min)
Proof Strenght Rp0,2 (N/mm2 min)
Elongation A5 (% min) LO=5xd
Brinell Hardness (HB)

The values reported into the table are relative to separately cast samples and having relative wall thickness 30 mm.

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