Machinability of ADI and IDI ductile irons

Machinability of cast irons

The careful design of the mechanical machining of ADI and IDI ductile irons, and likewise of the casting process, plays an extremely important role in achieving profitable results.

By analysing the formation of swarf and the surface quality obtained during mechanical machining, behaviour can be appreciated similar to that of machining untreated spheroidal graphite iron. As with all spheroidal graphite irons, the mechanical machining of ADI and IDI ductile irons similarly generates lower average shear values than steels of the same hardness. In spite of this, one must consider the greater dynamic components of the forces in play, which increase the tendency, in this case of ADI ductile irons, to vibrate.

To avoid reductions in tool life, workpiece flasks should be used that guarantee maximum stability and, therefore, a lower tendency to vibrate. The same precautions must be applied to the choice of tools and their respective holders. Adapters and/or reducers are not recommended.

Fig 1 Turning tool
Fig 2 small-pitch finishing cutter for cast iron

Process parameters

Taking pearlitic spheroidal graphite iron as a reference (JS/700-2 ISO 1083) and comparing its data with ADI 800 (JS800-10 ISO 17804), tool wear being equal, it is possible to establish an average reduction of about 15% in the cutting parameters (cutting speed [Vc] for turning and feed to tooth [fz] for milling) necessary to work in optimum conditions; the behaviour and cutting parameters of Isothermal Ductile Iron are comparable to JS/700-2 ISO 1083.

The tool coating, for all types of machining (turning, milling and drilling) must be able to guarantee a good seal, considering the high temperatures generated in the machining of ADI ductile irons; the use of a titanium nitride and aluminium nitride coating is the best choice. Depending on the type of machining, even ceramic tools may be a valid choice, especially considering their high wear resistance.

For the cutting edge, it is important to consider the self-hardening of the material due to the SITRAM (Stress Induced Trasformation of Retained Austenite into Martensite) effect. Therefore, the right compromise between sharpness and the geometry capable of supporting the load on the cutting edge, must be found. This will allow extremely competitive results in roughing.

The geometry of the insert with zero tilt angle is most often the best choice. The radius of the cutting edge, like machining on other materials, must be optimised based on the accuracy of the process; during roughing stages it is recommended not to exceed a radius of 1.6 mm to avoid high loads that may trigger self-hardening.

The table below provides suggested values for various types of operations.

Coolant is necessary in most situations due to the high machining temperatures. Proper management of the coolant flow, in addition to the general lowering of the temperature at the point of contact, also allows the correct evacuation of swarf near the cutting edge.

Another advantage of correctly using a coolant is a reduction in the material work-hardening effect, thus making it easier to manage the dimensional characteristics with tight tolerances.

In the case of non-continuous machining (milling), dry machining is possible in order to avoid thermal shock phenomena. Where this is not possible, an 8% emulsion is recommended, making sure the coolant is properly directed to the point of contact.

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Infosheet 4.3-1

Machinability

Infosheet 4.3-2

ADI Machinability – Process parameters and type of tool

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