PART 1: Understanding and overcoming machining challenges presented
by compacted graphite iron (CGI).

(See recent test data in PART 2 of this article.) 

The use of compacted graphite iron (CGI) is on the rise. The use of CGI is increasing, particularly in the manufacturing of truck and automotive diesel engine blocks and cylinder heads. CGI provides a number of advantages that engine designers can exploit to improve fuel efficiency and engine performance. It provides higher strength-to-weight ratio—as compared to conventional grey or high-molybdenum iron—as well as effective wear resistance, thermal conductivity and vibration damping properties—all attributes necessary to use in engine designs.

However, manufacturers find that CGI is more difficult to machine than conventional cast irons. When they use the same metalworking fluid in machining CGI as they use when machining typical cast irons, tools can wear as much as 90% faster—resulting in more costs and downtime. In addition, because cutting speeds and feed rates may need to be reduced, it can take longer to machine CGI as compared to gray cast iron. The use of specialized cutting fluids can mitigate the machining challenges presented by CGI.

Understanding the properties that make CGI difficult to machine. CGI is more difficult to machine than conventional iron or ductile cast irons. Tool wear rates under comparable machining conditions can be up to 30 times greater than when machining conventional cast gray iron. This is especially true when cutting at elevated speeds (250-700 m/min), where the wear rate differences can be most pronounced. These higher speeds are commonly used in operations such as engine cylinder boring.

In order to design new metalworking fluids to improve tool wear, it is important to first understand the underlying properties responsible for the low machinability of CGI.

The major differences between gray cast iron and CGI are:
– Differences in graphite structure.
    • Gray cast iron contains graphite with a flake-like structure that facilitates chip formation and enhances machinability
    • CGI contains graphite with a coral-like or vermicular shape, resulting in higher strength properties and lower machinability.
    (See microphotograph below).
– Differences in the presence of manganese sulfide inclusions. 
   In gray cast iron the presence of manganese sulfide (MnS) inclusions provide lubrication during cutting, particularly at elevated 
   cutting speeds where the benefit of MnS is more pronounced. However, MnS inclusions are not present in CGI and therefore a
   comparable benefit in lubrication and protection of the cutting tool is not obtained. (See microphotograph below).

(MnS inclusions observable in microstructure of gray cast iron shown above)

New metalworking fluids can help. With an understanding of the underlying factors that contribute to lower machinability of CGI, new metalworking fluid technology has been developed that yields improved tool life and surface quality in many CGI machining operations. These operations include high-speed continuous cutting processes such as those typically used in engine cylinder boring operations. While newly developed metalworking fluid technology has significantly bridged the machinability gap between conventional gray cast irons and CGI, it is important to note that to further improve the machinability of this metal and the related costs to machine it, continued research and development will be necessary.

If you are currently or planning to machine CGI, it is beneficial to work with your metalworking and tooling supplier to obtain useful advice regarding the best fluids and tooling to use.

PART 2: Recent test data show dramatic improvement in tool wear.

In 2010, Quaker Chemical Corporation introduced a new metalworking fluid technology that was developed specifically for enhanced machining of compacted graphite iron. It yields significantly reduced tool wear rates when machining CGI as compared to those typically obtained using conventional cast iron machining fluids.

Testing tool wear in high-speed, continuous cutting. To demonstrate the utility of QUAKERCOOL 7020® CG, machining tests were conducted using a turning operation (continuous cutting) on Grade 450 CGI, using tungsten carbide cutting inserts at a cutting speed of 250 m/min. In addition, a conventional cast iron metalworking fluid was tested to provide a baseline for this study. Machining using dry (un-lubricated) conditions was also performed to assess the impact of wet versus dry machining in this operation.

The insert wear that occurs on the tool under these machining conditions can be seen in the figure above.
As seen, flank wear occurs rapidly and eventually reaches a point where the integrity of the cutting insert is lost.
 

QUAKERCOOL^® 7020 CG offers enhanced performance. The enhanced performance offered by QUAKERCOOL 7020 CG over the conventional ferrous machining fluid and certainly over dry machining under such cutting conditions is clearly seen, (plot of insert wear vs. distance of cutting—shown below). Using QUAKERCOOL 7020 CG a 32.6% increase in tool life is obtained over that for the conventional ferrous machining fluid. A 124% insert life was seen over that obtained for dry machining.