The two main factors that determine the machinability of ductile irons are the microstructure and hardness of ductile iron castings.
Machinability is measured by determining the relationship between cutting speed and tool life. These two factors will influence the machine tool productivity and machining costs.
The complex microstructure of Irons (Cast, Malleable, and Ductile) is based around the presence of graphite in iron. This presence is one of the biggest factors involved in the superior machinability of irons versus steels. Graphite influences cutting force and surface finish. Hardness of the casting is dependent on the microstructure of the alloy. The microstructure matrix is made up of a few components and will be the primary determinant of the cutter tool life.
What makes up the Complex Microstructure of Ductile Irons?
Listed below are the components of the microstructure matrix and the effects each component has on machinability:
Ferrite- is the softest matrix constituent in ductile iron and exhibits the best machinability. This is due the effect of Silicon (Si) combined with ferrite which will decrease the toughness of ferrite. Machinability will increase when up to 3% Si is added, but if more content it will consequently decrease the characteristic.
Pearlite- consists of soft ferrite and hard lamellar iron carbide. The volume fraction of pearlite and the fineness of lamellae determine the hardness and machinability of ductile iron. This is matrix is common in intermediate strength grades of ductile iron.
Carbide- hardest component and consequently has the poorest machinability. Carbide can however be sheared and machinable when it is present in thin lamella in pearlite. However, there are processes are available to offset the effects of carbide.
Martensitic- extremely hard matrix phase that is produced by quenching ductile iron. When Martensitic is quenched it is not machinable due to the hardness and brittle characteristics of Martensitic. Is there a solution? If Martensitic is tempered, then it is more machinable than pearlite of similar hardness.
Which Microstructure Matrix is used in which Ductile Iron Alloy?
Ferritic- Pearlitic Matrix - are used in ductile iron 60-45-12 and ductile iron 80-55-06 and can be machined as-cast.
Pearlitic- Martensitic Matrix- used in ductile iron 100-70-03 and will need to Normalized or Tempered for machining.
Martensitic Alone- used in ductile iron 120-90-02 and will need to be Quenched and Tempered before machining.