Compacted Graphite Iron: Part Two

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Understanding Compacted Graphite Iron Properties

Compacted graphite iron represents a unique class of cast iron materials that bridges the gap between traditional grey cast iron and ductile iron. One of the most critical properties required for engine blocks and cylinder heads is excellent thermal conductivity and the ability to dampen mechanical vibrations effectively. The thermal conductivity of CGI demonstrates intermediate characteristics when compared to other cast iron types.

The thermal conductivity of CGI (approximately 36 W/m.K) is around 25% higher than that of GJL-250 (approximately 46 W/m.K) but only 10% lower compared to GJL-300 (approximately 39 W/m.K). As the tensile strength of compacted graphite iron increases, the associated carbon content decreases, consequently reducing thermal conductivity. The amount of spheroidal graphite in cast iron strongly influences thermal conductivity, which decreases as spheroidal graphite content rises.

CGI provides lower mechanical vibration absorption capability compared to lamellar graphite cast iron but demonstrates superior performance when compared to spheroidal graphite cast iron. The damping capacity of CGI increases as the amount of spheroidal graphite decreases, making this relationship crucial for engineering applications.

Table 1. The properties of grey iron, CGI and ductile iron

Property	Grey Iron	CGI	Ductile Iron
Tensile strength (MPa)	250	450	750
Elastic Modulus (GPa)	105	150	160
Elongation (%)	0	1.5	2.5
Endurance Ratios: - Rotating Bending - Three Point Bending - Tension Compression	0.35-0.50 0.45-0.50 0.45-0.50	0.55-0.65 0.60-0.70 0.65-0.75	0.20-0.30 0.25-0.35 0.65-0.75
Thermal Conductivity (W/m-K)	48	38	32

Graphite Morphology and Material Differentiation

Grey cast iron, compacted graphite iron, and ductile iron are differentiated by the distinctive shape of their graphite particles. Grey iron is characterized by randomly oriented graphite flakes, while graphite particles in ductile iron appear as individual spheres. In contrast, graphite particles in CGI appear as individual 'worm-shaped' or vermicular particles. These particles are elongated and randomly oriented similar to grey iron; however, they are shorter, thicker, and feature rounded edges.

Figure 1: Grey iron, compacted graphite iron and ductile iron are differentiated by the shape of the graphite particles

Compositional Differences and Machinability Challenges

Beyond graphite structure differences, significant compositional variations exist between gray cast iron and CGI, which are largely responsible for the differences in machinability of cast iron. The presence of sulfur in gray cast iron is considered a critical factor associated with the high machinability of this metal. During machining operations of gray cast iron, the sulfur alloyed in the metal combines with manganese to form manganese sulfide (MnS) inclusions.

During cutting operations, MnS inclusions assist in the chip breaking process and adhere to the cutting tool surface, forming a lubricating layer that reduces friction, protects against oxidation and diffusion, and subsequently minimizes tool wear machining (especially at high cutting speeds). In machining of compacted graphite iron, such layer formation does not occur since the normal amount of sulfur content cast iron added to CGI is around 0.01%, approximately ten times lower than that added to gray iron.

Additionally, residual sulfur in compacted graphite iron tends to combine with magnesium (an element added to enhance graphite nodulization), leaving little sulfur available to combine with manganese and form the protective MnS layer. The lack of sulfur in compacted graphite iron is believed to be a primary reason for poorer machinability and higher tool wear associated with machining this metal.

Machining Performance and Tool Life Studies

Due to graphite morphology and sulfur content factors, the machinability of CGI is considerably lower, and tool wear is considerably higher than that experienced in gray cast iron machining. Previously reported studies demonstrate that tool life for milling and drilling operations of CGI can be reduced to one half, while tool life in CGI boring operations has been observed to be just one-tenth of that obtained in comparable machining operations with gray cast iron.

Understanding the lubrication and fluid requirements needed for improving the machinability of CGI will greatly benefit its current and future applications. The research conducted by R.Evans, F.Hoogendoorn, and E.Platt investigated the machining properties of CGI and the metalworking fluid properties and composition that impact and potentially extend tool life in CGI machining.

Experimental Methodology and Results

To study the differences in machinability between gray cast iron and CGI, machining tests were conducted on a Bridgeport V2XT machine using a standard water-based metalworking fluid. The fluid used was an oil-in-water macroemulsion known to provide high levels of lubrication in ferrous machining operations. Testing involved drilling and subsequent reaming of Grade 450 compacted graphite iron as well as Class 40 gray cast iron.

Assessment of the metals' machinability was made by measuring cutting forces and tool wear occurring during operations. The machining conditions used in this study are detailed in the experimental setup.

Table 2. Machining conditions

	CGI Drilling	CGI Reaming
Workpiece	Grade 450CGI	Grade 450CGI
Tool	Gehring 5514 0.25" dia. Firex coated solid carbide	0.266" dia. Six straight fluted solid carbide
Speed	3000 rpm (196 sfm)	900 rpm (62.6 sfm)
Feed	10.4 IPM (.00346ipr)	5.1 IPM (.00556ipr)
Depth	1.25" Through hole	1.25" Through hole
Fluid	8% in 130 ppm water	8% in 130 ppm water
Measured Parameters	Cutting Forces Tool Wear	Hole Finish

Research Findings and Conclusions

The machining test results clearly demonstrate the higher level of difficulty encountered in machining compacted graphite iron compared to standard gray cast irons. This was evident in both cutting forces and tool wear measurements. While differences in graphite morphology are largely responsible for the machinability differences of CGI relative to gray cast iron, SEM/EDX analysis supports the belief that the lack of sulfur in CGI and the inability to form lubricating manganese sulfide inclusions during cutting also contribute to the poor machinability of cast iron.

While it is necessary to cast CGI with minimal sulfur content, the research showed that lubricating sulfur-based additives can be utilized in machining fluid to compensate for the lack of sulfur in the metal and provide enhanced lubrication necessary for reducing cutting forces and tool wear machining. The role of sulfur-based additives in metalworking fluid in forming a protective lubricating layer on the workpiece and tool surface during machining was supported by SEM/EDX analysis of the used tool.

The analysis revealed high sulfur content on the tool surface following machining, with sulfur levels comparable in concentration to those found on tools used for gray cast iron machining. The formation of a sulfur-based tribological film requires a certain level of heat, as supported by EDX analysis results showing high sulfur levels on the cutting edge surface where high friction, heat generation, and wear occurred, contrasting with only trace sulfur levels at locations farther up on the tool rake face where minimal metal-to-metal contact occurs.