This is an excerpt (with DIS comments) from work done by University of Alabama, Birmingham, for the American Foundry Society (AFS) and several foundry companies with support from the US Department of Energy. The original report was presented by Dr. Charles Bates at the AFS Convention, 2002 and is copyrighted by AFS. This is paper # 02 - 160 for those who would like to review the entire paper, which is available from AFS.

The two-year research program was conducted to develop an understanding of the factors that effect Ductile Iron machinability. Samples of continuous cast round bars of various chemistries and microstructures were turned on a lathe and tool wear rates were measured. The CNC lathe used was programmed to keep a constant surface speed. Un-coated fine-grained carbides inserts were the tools and flank wear curves were generated for each case of chemistry and microstructure as well as at different surface speeds.

Test data - final chemistry ranges: 

Physical properties (on tested samples):

Statistical analysis was used to evaluate data and provide a high confidence level to determine the significance of an input variable.

Many graphs of the data were prepared and shown in the original report; they include turning speed vs. tool wear, tool wear vs. combined carbon, silicon, copper, nodule count, pearlite volume, pearlite colony thickness, and BHN hardness. Other variables evaluated in the project include graphite volume percentage, nodule diameter, graphite spacing, graphite shape as measured by the roundness formula, and spacing between pearlite colonies.

Conclusions;

1. With the exception of one soft iron tested (179 BHN) the wear slope in the graphs was higher at 600 sfm (surface feet per minute) than at 450 sfm. They surmised that this was due to increased tool heating at the faster speed, which can accelerate tool wear.
2. Higher tool wear rates were seen in irons with a higher BHN hardness.
3. Lower wear rates were seen in the irons with higher silicon levels, but the data was limited and the silicon content was on the low side at 2.4% max. Note that at higher silicon levels of > 2.6% machinability is generally reduced. 
4. Higher wear rates were seen in irons with a higher combined carbon contents.
5. Higher wear rates were seen in irons with higher copper concentrations. Note, it appeared that in irons of the same hardness that had both copper and tin, strengthening of the iron by using copper gave higher tool wear rates than strengthening with tin. 
6. Higher tool wear was seen in irons with less rounded nodules, but these irons also had higher hardness, so it is difficult to give a direct comparison.
7. Irons with the highest volume of pearlite, lowest pearlite colony spacing and highest pearlite colony thickness had the highest tool wear rates. The pearlite colony thickness seemed to be the most significant on predicting wear rate.
8. Higher nodule counts can make irons with lower pearlite colony thickness.
9. Overall this was not a truly comprehensive study of a typical 80-55-06 grade of material since continuous cast bars will have some large changes in nodule count and hence microstructure based upon the severity of cooling. They tried to compensate for this in the project by pre-machining the bars removing the surface layer. This allowed them to study some of the other casting parameters more closely. However, sand foundries must keep in mind that the surface condition (sand, slag, etc as well as the amount of shot peening the part receives) will have a significant effect on the first cut of machining.