The current study was conducted to assess the performance of Quakercool 7020-CG with regard to its ability to reduce tool wear in the continuous cutting of CGI at high speeds using both carbide and polycrystalline boron nitride tool materials. This product was developed for CGI machining and has to date provided effective performance in both controlled testing as well as in production use.
The machinability of compacted graphite iron (CGI), and the performance of selected metalworking fluids, were assessed in a turning operation utilized to simulate the continuous cutting conditions which occur during engine cylinder boring. A turning operation was performed on the outside surface of both CGI and gray cast iron test cylinders. Testing involved the use of tungsten carbide cutting inserts at cutting speeds of 250 m/min, and polycrystalline boron nitride (PCBN) cutting inserts at cutting speeds of 700 m/min.
CGI and Gray Cast Iron Cylinders
Two metalworking fluids were tested, which included one considered to be standard for conventional cast iron machining, (Fluid 37), and one (Quakercool 7020-CG) engineered specifically for use in machining CGI. In addition to fluid lubricated machining, machining was also done under dry conditions.
Recent studies in CGI machining have focused on continuous cutting conditions which occur during high speed cylinder boring. Boring of engine cylinders is one of the more critical operations in engine production, requiring high quality surfaces to be produced at relatively high cutting speeds. It is at such elevated cutting speeds (250-700 m/min), where the machinability differences between CGI and conventional gray cast irons are most pronounced. Previous studies have reported insert wear rates to be 20-30 times greater in the continuous cutting of CGI relative to that obtained in the machining of gray cast iron under equivalent conditions.
The differences in machinability between gray cast iron and CGI have been attributed to differences in graphite structure and also to the presence of manganese sulfide inclusions in gray cast iron. Manganese sulfide inclusions, (not present in CGI), form a protective and lubricating film on the cutting tool surface during machining of gray cast iron, resulting in protection of the tool surface and decreased rates of tool wear. It has been shown that the formation and activity of these lubricating films is highly dependant on cutting speed (likely cutting temperatures) with gray iron machinability and tool life improving with increasing cutting speeds.
Based on these factors and the current level of understanding of the microstructural and compositional features of CGI, recent work at Quaker has led to the development of new metalworking fluid technology (Quakercool 7020-CG) useful for increasing tool life in CGI machining. To further assess the utility of this new technology and especially its impact during CGI machining at high speeds under continuous cutting conditions, tests were conducted to assess the performance of this fluid in a turning operation using tungsten carbide cutting inserts at cutting speeds of 250 m/min, and also PCBN cutting inserts at cutting speeds of 700 m/min. Fluid 37, a fluid commonly used for the machining of conventional gray cast iron was included in this testing and served to provide a baseline for this study. In addition to assessing fluid performance in the machining of both gray cast iron and CGI, dry machining of CGI was performed using both tool materials and conditions. This was done to assess the impact of wet versus dry machining in this operation.
Machining was performed on a Victor Fortune TNS-2 Turning Center where multiple turning passes were made on test cylinders of gray cast iron and Grade 450 CGI, both supplied by SinterCast.
Victor/Fortune TNS-2 Turning Center
Two separate cutting insert materials were used. Coated carbide inserts were used at cutting speeds of 250 m/min and PCBN inserts were used at 700 m/min.
Cylinders were pre-machined to ensure roundness and remove outer skin.
Carbide Cutting Inserts @ 250 m/min
Carbide Insert Wear
In turning of the CGI cylinders using the carbide inserts at 250 m/min cutting speed, abrasive wear on the flank face of the cutting insert was seen to develop quickly and progressed rapidly with continued machining. In the current study, machining was continued until 0.3 mm flank wear length was reached. At 0.3 mm flank wear, considered as the failure point for the tool, noticeable loss of the rake face geometry was also observed.
Fluid Performance & Wet Versus Dry Machining
The insert wear measured for the two metalworking fluids as well as for dry machining, using the carbide inserts are shown in the chart. As seen, wet machining offers significant benefit with regard to tool wear over that obtained under dry machining conditions.
Tool life improvements of between 79 to 124% were obtained using fluid lubrication. In assessing the relative performance of the two fluids tested, the enhanced performance offered by Quakercool 7020-CG over the conventional ferrous machining fluid (Fluid 37), and certainly over dry machining, is clearly seen. Quakercool 7020-CG resulted in a 32.6% increase in tool life over that for Fluid 37. A 124% increase in insert life was seen over that obtained for dry machining.
Machinability: CGI Versus Gray Cast Iron
In looking at the insert wear measured in the machining of gray cast iron using the conventional cast iron machining fluid, the lower machinability of CGI relative to gray cast iron is clearly seen. In the turning of CGI, tool failure was reached after 10 Km of metal distance cut, whereas in the machining of the gray cast iron, minimal wear was observed even after 20 Km of cutting distance. The dramatic difference in wear between the two metals can also be seen in the microphotographs of the flank surfaces of the cutting inserts following 10 Km cutting distance.
Rapid abrasive wear and crater wear occurs very quickly with the machining of CGI whereas only slight abrasive wear occurs on the flank face of the tool during gray cast iron machining. This difference in machinability can also be seen in the condition of the rake face surface of the inserts following 10 Km cutting distance on both metals.
While abrasive wear can be seen on the insert rake face surface used with gray cast iron, the cutting edge is still retained. In contrast, the rake surface of the insert used with CGI shows significant abrasive wear with deformation and loss of the cutting edge.
Polycrystalline Cubic Boron Nitride (PCBN) Cutting Inserts @ 700 m/min
PCBN Insert Wear
Polycrystalline cubic boron nitride inserts are typically used at considerably higher cutting speeds, than many other tool materials, and thus can present even greater challenges for effective machining of compacted graphite iron. In turning of the CGI cylinders using the PCBN inserts at 700 m/min cutting speed, abrasive wear developed quickly and at a much greater rate relative to that which occurred with the carbide inserts used at lower cutting speeds. Failure of the PCBN inserts (0.3 mm flank wear length) occurred between 1.1 to 1.8 Km of cutting distance, a very short amount of machining. The progression and type of wear observed can be seen below. As seen, abrasive wear occurs quickly and continues in severity as machining continues. Likely due to the high hardness and wear resistance of this tool material. Along with abrasive wear, a noticeable amount of plastic deformation of the tool material is also seen along the cutting edge.
Fluid Performance & Wet Versus Dry Machining
While wear occurs very rapidly on an insert, benefit of wet versus dry machining can still be seen with a 55% improvement in insert life being obtained with the use of Quakercool 7020-CG relative to that obtained during dry machining conditions. This difference can also be clearly seen in the condition of the cutting inserts used for dry machining and with wet machining using Quakercool 7020-CG. Also, measurable differences in performance between the fluids is also seen, with Quakercool 7020-CG yielding an 18% increase in insert life over that obtained with use of the conventional ferrous machining fluid, Fluid 37.
Machinability: CGI Versus Gray Cast
In looking at the insert wear measured in the machining of gray cast iron relative to CGI, the significant differences and challenges which exist with high speed boring of CGI using PCBN inserts is very apparent. This difference is also seen in the microphotographs of the flank surfaces of the cutting inserts following 2.85 Km cutting distance as well as at 25.7 Km cutting distance for gray iron. Rapid abrasive wear occurs very quickly with the machining of CGI whereas only slight abrasive wear occurs on the flank face of the tool during gray iron machining. This difference in machinability was also be seen in the condition of the rake face surface of the inserts where significant abrasive wear and loss of the cutting edge occurred after approximately 2.85 KM cutting distance in CGI machining, while in the machining of gray iron retention of the cutting edge was maintained through the entire 25.7 Km distance.
Summary & Conclusions
Two important elements in engine cylinder boring are the productivity or cycle times achieved in the roughing semi-finish and finish processes, as well as the tool life obtained. It has been shown in previous studies that the mechanical properties and presence of manganese sulfide inclusions in gray cast iron give rise to higher machinability and greater tool life relative to compacted graphite iron. It has also been shown that the effects of MnS lubrication in gray iron is increased at elevated cutting speeds, which likely is due to a thermal activation process at the higher speeds. Thus it can be understood how the differences in tool life experienced between CGI and gray iron would be large and magnified at the high cutting speeds used in cylinder boring processes.
The continuous cutting of CGI and gray cast iron using carbide inserts at 250 m/min cutting speed, showed the following results:
The continuous cutting of CGI and gray cast iron using PCBN inserts at 700 m/min cutting speed, showed the following results:
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