MACHINING OF GRAY CAST IRONS AND COMPACTED GRAPHITE IRON

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1 MACHINING OF GRAY CAST IRONS AND COMPACTED GRAPHITE IRON B. Tasdelen 1, M. Escursell 2, G. Grenmyr 3, L. Nyborg 4 1, 3, 4 Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE Gothenburg, Sweden 2 Machining Development Center, SKF, Gothenburg, Sweden bulent@chalmers.se Abstract: The aim of this paper is to compare the machinability of gray cast iron and compacted graphite iron (CGI), in terms of tool life and microstructure. Two gray cast irons with different graphite morphologies and one CGI were tested. After the tool life tests with carbide and Cubic Boron Nitride (CBN) inserts, microstructural changes on the machined surfaces was investigated and SEM analysis of the inserts were performed. The results show that CBN performed much better than Carbide tools when machining gray cast irons but did not work when machining CGI due to high diffusional wear. The shorter and thicker graphite flakes for one of the gray cast iron caused different chip contact and wear mechanism on the inserts. The comparison of machinability and tool wear gave better understanding of the wear mechanism of CGI. Keywords: Compacted graphite iron (CGI), milling, tool wear, surface integrity, microstructure 1. INTRODUCTION New regulations in USA and Europe will decrease the maximum allowed emission level for heavy duty diesel engines. To fulfil the new demands, combustion pressure in the cylinders must be significantly increased and therefore new materials with higher strength are needed. Compacted graphite iron (CGI), which has improved mechanical properties, is therefore a potential candidate to substitute the conventional engine material, gray cast iron (Dawson and Schroeder, 2000; Sahm, et al., 2002). On the other hand, CGI has lower machinability than conventional gray cast irons (Sahm, et al. 1998; Mocelin, et al., 2004). For gray cast iron and ductile iron the chip formation can be described as follows (Mocelin, et al., 2004). First, the tool starts to compress the material beneath the flank and creates fracture that propagates ahead and below the cutting edge. The tool proceeds and a material fragment is completely detached. In some cases, especially for gray cast iron, a part of the work piece material can be torn off in front of the tool. This causes a temporary loss in the contact of the tool and the work piece, until the next chip forms. As a consequence, the surface roughness of the machined work-piece is increased. The vermicular shape of the graphite in CGI creates stronger bonding between the matrix and the graphite than in ordinary gray cast iron and there is hence less crack propagation for CGI compared to gray cast irons (Cohen, et al., 2000; Dawson, et al., 2001). Furthermore, CGI is also more ductile then gray cast iron. This entails considerably larger contact with the cutting insert, resulting in increased temperature and longer time for chemical reactions between the cutting tool and the work-piece material (Reuter, U et al., 1999). 2. EXPERIMENTS AND ANALYSES 2.1 Work piece materials Table 1 presents data for the work piece materials, including nominal tensile strength values, average matrix compositions and graphite characteristics. All Swedish Production Symposium 2007 Tasdelen... 1

materials have predominately pearlitic matrix. However, there is a certain content of free ferrite for CGI. The gray cast irons were sand cast in blocks.

Despite this, both gray cast irons have Type 1 shape A graphite according to ASTM.

A comparison of graphite morphology between the materials is illustrated in Figure 1. The sulphur content is kept between 0.08 wt% and 0.14 wt% for the gray cast irons and it was 0.

2 materials have predominately pearlitic matrix. However, there is a certain content of free ferrite for CGI. The gray cast irons were sand cast in blocks. The difference between the gray cast irons was the treatment for gray cast iron 2 to obtain thicker graphite flakes. This also induced certain porosity. Despite this, both gray cast irons have Type 1 shape A graphite according to ASTM. The CGI material was treated specially with Mg in order to get the vermicular shape of the graphite which has also resulted in 4% nodularity. A comparison of graphite morphology between the materials is illustrated in Figure 1. The sulphur content is kept between 0.08 wt% and 0.14 wt% for the gray cast irons and it was 0.02 wt% for the CGI material. Insert Carbide (HM) S-N W CBN S-N F Table 2 Cutting data and insert information v c (m/min) f z (mm/tooth) a p (mm) a e (mm) D (mm) Table 1 Selected data for the tested materials Material Name Gray cast Iron 1 Gray cast Iron 2 CGI Tensile Matrix Graphite Strength Morphology 275 MPa Pearlite Type 1, Size MPa Pearlite Type 1, Size MPa 7 % ferrite Interconnected vermicular 2.3 Analyses of work piece materials After the tool life tests, the microstructural analysis of the milled surfaces was performed with the optical microscope. Cross sectional samples were cut from the milled surfaces with water-jet to minimize the impact of cutting on residual stresses and effects on work piece microstructure within the surface zone of the milled surfaces. 2.4 Tool wear analyses (c) Fig. 1. Graphite morphology: gray cast iron 1, gray cast iron 2 and (c) CGI. 2.2 Tooling The face milling of the blocks were performed with two different inserts and different cutting data in order to investigate the machinability of the materials, see Table 2 for cutting data and insert information. In order to make terminology easy the hardmetal insert, S-N W, will be denoted as HM and the cubic boron nitride insert, S-N F will be denoted as CBN. High temperature and stresses are the main causes of the tool wear. The two basic wear zones on the inserts namely, seizure region (adhesive wear) and sliding region (abrasive wear) can easily be investigated with scanning electron microscopy (SEM). In this study, the rake face of the inserts was investigated and different regions of the chip-insert contact were imaged with secondary electron imaging in a Leo Gemini 1550 SEM instrument. Since sliding of the chip causes work hardening and oxidization of the material with different thickness depending on temperature, electron dispersive spectroscopy (EDS) mapping was also used to find the different elements in the oxidised clad material. The tests were stopped at equivalent time intervals to measure the tool wear. The wear characteristics were pictured with a Nikon optical microscope and the amount of tool wear was measured with a special programme, Mätmodul. The contact of the insert with the work piece is seen in Figure 2 below. hd ap Fig. 2. Insert and work piece contact. Swedish Production Symposium 2007 Tasdelen... 2

$The fracture is a fatigue fracture evidenced by the presence of beach marks on the fracture surface, see arrows in Figure 5 (c).$ HM Inserts; The tool life tests at the lower cutting speed (275 m/min) showed that gray cast iron 1 has the lowest tool wear for both speeds compared to gray cast iron 2.

HM Inserts; The tool life tests at the lower cutting speed (275 m/min) showed that gray cast iron 1 has the lowest tool wear for both speeds compared to gray cast iron 2.

3 3. RESULTS AND DISCUSSION 3.1 Tool life analyses insert that milled CGI. The fracture is a fatigue fracture evidenced by the presence of beach marks on the fracture surface, see arrows in Figure 5 (c). HM Inserts; The tool life tests at the lower cutting speed (275 m/min) showed that gray cast iron 1 has the lowest tool wear for both speeds compared to gray cast iron 2. The highest wear was observed for CGI material as seen in Figure 3. Carbide Insert 0,40 0,30 Grey Iron 1 Grey Iron 2 CGI Vb (mm) 0,20 0,10 0, Ts (min) Fig. 3. Tool life results for HM inserts at 275 m/min. The higher wear on the inserts when milling gray cast iron 2 compared to gray cast iron 1 is a result of the combination of graphite morphology (increased mechanical properties), porosity and a rather higher carbide content that resulted from nitrogen treatment. The higher wear resulted in a crack (see arrow in Figure 4 a) on the end side of depth of cut for gray cast iron 1 and a breakage (see Figure 4 b) for gray cast iron 2 at the same place for the higher speed tests, see Figure 4. Fig. 4. The edge of the inserts after milling gray cast iron 1 and gray cast iron 2. When the inserts were examined on the other side of depth of cut, SEM images indicated crater wear on the inserts that milled gray cast iron 1 and gray cast iron 2. The insert that milled CGI material broke. The crater wear changed the contact area and increased the fatigue loading (interrupted machining). The varying stress level caused a crack on the insert corner when milling gray cast iron 2, see arrow on Figure 5, and a fracture for the (c) Fig. 5. Crater wear and insert breakage, rake face on the HM inserts at 275 m/min, gray cast iron 1 after 45 min, gray cast iron 2 after 45 min and (c) CGI after 49 min. CBN Inserts: The flank wear was low for CBN inserts compared to HM inserts when milling gray cast iron 1 and gray cast iron 2 at 900 m/min, see Figure 6. When machining CGI material even with the lower speed (700 m/min), the CBN inserts experienced severe wear and the tests were stopped after 8 minutes due to high diffusion wear. This high diffusion wear resulted in a deep crater, see Figure 7 for the side views of the inserts. Vb (mm) 0,40 0,30 0,20 0,10 CBN Insert 0, Ts (min) Grey Iron 1 Grey Iron 2 CGI Fig. 6. Tool life results with CBN inserts for gray cast iron 1 (900 m/min), gray cast iron 2 (900 m/min) and CGI (700 m/min). Swedish Production Symposium 2007 Tasdelen... 3

(c) Fig. 7. Crater wear on CBN inserts, side view for gray cast iron 1 (900 m/min) after 45 min, gray cast iron 2 (900 m/min) after 45 min and CGI (700 m/min) after 8 min.

The seizure region had a clad material in the shape of a band for gray cast iron 1 but not for gray cast iron 2, see arrow in Figure 8.

The EDS mapping revealed that the banded structure is the clad material that was oxidized.

4 (c) Fig. 7. Crater wear on CBN inserts, side view for gray cast iron 1 (900 m/min) after 45 min, gray cast iron 2 (900 m/min) after 45 min and CGI (700 m/min) after 8 min. When examining the rake face of the CBN inserts with SEM, it was observed that a partial layer formation was seen only for gray cast iron 1 on the seizure region of the chip contact. The seizure region had a clad material in the shape of a band for gray cast iron 1 but not for gray cast iron 2, see arrow in Figure 8. Fig. 8. Rake face of the CBN inserts showing the different chip contact and clad material after 45 minutes of cut at 900 m/min for gray cast iron 1 and gray cast iron 2. The EDS mapping revealed that the banded structure is the clad material that was oxidized. Besides O, the clad material in this seizure region consisted of mainly Fe, Mn, S and Si, see Figure 9 for the example of EDS mapping of the region. The same elements were found in the rest of contact area for both gray cast irons. When it comes to CGI, the clad material in the contact area consisted mainly of Fe, O, Si and Mg. Fig. 9. Mapping of CBN insert for gray cast iron 1. Another observation is that a partial layer formation occurred very close to cutting edge for gray cast iron 1, but not for gray cast iron 2 and CGI. This banded layer in the contact is connected to chip formation, heat distribution in the cutting zones and chemical interaction between the insert and work piece material. It was already expected that a layer formation would be missing when cutting CGI. However, the lack of this layer when cutting the gray cast iron 2 means that either a small variation in the sulphur content or the graphite morphology difference is the cause for different contact topography for the two gray cast irons. It has also been pointed in literature (Mocelin, et al., 2004) that the easy cracking leads to short chip formation in gray cast irons. This is also observed in our case and supposed to be a result of the decreasing interatomic locking between insert and chip in the seizure region (Trent and Wright, 2000). This hypothesis is supported by following arguments. There is sliding of the chip in the seizure region for gray cast iron 1 as evidenced by the partial layer formation. However, there is no such layer in the seizure region on the inserts that milled the gray cast iron 2. The plasticity of manganese sulphide inclusions must be adequate to act favourably in the secondary shear zone, meaning that the characteristics of the sulphide should not be too fluid (Araki and Yamamato, 1975). There is an appropriate temperature range for an effective lubricating action of manganese sulphide inclusions, not only in the secondary shear zone but also in the primary shear zone (Araki and Yamamato, 1975). Consequently, if there is another temperature isotherm distribution on the rake face when cutting the gray cast iron 2, there could also be a different banded layer formation. Another reason may also be the different graphite morphology of gray cast iron 2 compared to that of gray iron 1, which in turn should affect the contact area. The combination of these two effects is of course also possible. It has been commonly anticipated that MnS layer formation in the gray cast iron acts as a barrier and Swedish Production Symposium 2007 Tasdelen... 4

decreases the chemical affinity between the insert and the chip ( Cohen, et al., 2000; Dawson, et al., 2001; Gastel, et al., 2000; Abele, et al., 2002).

Therefore, the affinity of the CBN insert to the work piece material is another important concern.

The CBN itself is also not stable and decomposes forming volatile boron and nitrous oxides (Gastel, et al., 2000).

5 decreases the chemical affinity between the insert and the chip ( Cohen, et al., 2000; Dawson, et al., 2001; Gastel, et al., 2000; Abele, et al., 2002). However, although we observed no layer formation in the seizure contact zone for either gray iron 2 or CGI when cutting with CBN, the high diffusion wear was only observed for CGI as a deep crater. Therefore, the affinity of the CBN insert to the work piece material is another important concern. At temperatures above 700 C (at high cutting speeds for cutting CGI and also gray cast irons), the binder phase in the CBN tools are not stable. The CBN itself is also not stable and decomposes forming volatile boron and nitrous oxides (Gastel, et al., 2000). The diffusion of Co, C and tungsten to the ferrous work piece materials have been mentioned in the literature (Pereira, et al., 2006). However, Gimenez (Gimenez, et al., 2006) has found that WC-Co substrate dissolves even under 700 C when in contact with iron, whereas PCBN composites are chemically compatible with iron even up to 1000 C for 1 hour. Dawson, et al., 2001 has mentioned that small amount of free ferrite in gray cast iron can reduce PCBN tool life. Nevertheless, they concluded in their work that small amounts of free ferrite is not responsible for high wear rates in CGI machining. The real mechanisms behind the high diffusion effect in CGI machining with CBN has not been clarified yet. The chips from the CGI tests are longer and have different colour than the chips from the two gray cast irons. This means that the chips are less easily broken due to the vermicular graphite shape of CGI. As mentioned before, the flake graphites in conventional gray cast iron contributes to the easy cracking path and thereby shorter chips. It is supposed that the low content of S in CGI resulting in no MnS layer formation is not the only reason for high diffusional interaction and associated tool wear. The observations in terms of cladding on the tool were similar for the gray cast iron 2 and the CGI tested, although the former showed much less wear. Thus, the short chip formation due to the graphite flakes geometry and distribution is therefore expected to be an important reason for chip-insert contact time. Longer contact time in this context means higher temperature in contact zone and then larger degradation of the insert. microstructure development within the surface zone of the work piece material during machining. Milling with HM resulted in a plasticized layer below the cut surface. This heat and deformation affected layer can be observed the orientation of the matrix and the graphites along the cutting direction, see Figure 10. The effect is almost similar for gray cast iron 1 and gray cast iron 2. However, the layer is thinner for CGI which means that the flow zone is thinner, see Figure 10. On top of the surface, the white layer formed for gray cast iron 1 and gray cast iron 2 is clearly observable and thicker than that observed for CGI. Microhardness measurements, Hv 0.05 showed that this layer had a hardness of about (kg/mm 2 ). Two alternative explanations exist. Either the layer is martensite or it is composed of a zone experiencing mechanical grain refinement. In both cases, significant hardness increase would result. Further studies including detailed X-ray diffraction studies and transmissions electron microscopic studies would be need to clarify the issue. However, the change in microstructure in the surface results from the combination of high deformation and heat. The thinner white layer for CGI is then an evidence of a different chip formation mechanism that is connected to different heat distribution between the chip, insert and work piece material. Fig. 10. Plasticized zone and microstructural change at the surfaces of milled at 275 m/min for gray cast iron 1(after 30 min.) CGI (after min). The same plasticized region and white band were not observed on the surfaces that were cut by CBN inserts. This resulted in a better and smoother surface finish for CBN inserts, see Figure Surface Integrity of milled work piece materials Milling with either HM or CBN inserts leads to different conditions in the cutting zone. For the same depth of cut, the latter kind of inserts can be run at much higher cutting speed closer to adiabatic conditions in the surface. This is also reflected in the Fig. 11. Machined surfaces with CBN inserts. 900 m/min, 30 minutes of cut for gray cast iron 1 and 700 m/min, 8.08 minutes of cut for CGI. Swedish Production Symposium 2007 Tasdelen... 5

6 4. CONCLUSIONS The machinability of CGI was evaluated in comparison to conventional gray cast irons with graphite morphology ranging between CGI and conventional gray cast iron structure. The summary of this study can be outlined as follows: Graphite morphology plays a significant role in the chip formation mechanism. This is evidenced from the banded seizure region (different chip contact) of two gray cast irons with different graphite characteristics. It is supposed that the formation of MnS layer may not be the only mechanism behind the difference in machinability between the materials tested. The tests showed only possible presence of MnS clad on tool inserts for one of the gray cast irons, while the other gray cast iron as well as the CGI produced no such clad and still showed significantly different machinability. The thinner flow zone was observed for CGI cutting compared to the gray cast irons pointing out different heat distribution between the insert, chip and work piece material. The CBN inserts are robust and gave a better surface finish and lower tool wear for gray cast irons but did not work for CGI. The affinity between CGI and CBN inserts resulted in high diffusion wear. However, the reason for the high affinity between CBN and CGI has not yet been explained. Besides such possible incompatibility, higher wear was generally experienced also for the carbide inserts when machining CGI. The vermicular graphite morphology was here identified as an important mechanism, leading to longer chips and different chip formation mechanism with potentially different heat impact on inserts. Dawson S., Hollinger I., Robbins M., Daeth J., Reuter U., Schulz H. (2001), The effect of metallurgical variables on the machinability of compacted graphite Iron, SAE 2001 World Congress Detroit, Michigan March 5-8, Gastel M., Konetschuny C., Reuter U., Fasel C., Schulz H., Riedel R., Ortner H. M. (2000), Investigation of the wear mechanism of cubic boron nitride tools used for the machining of compacted graphite iron and gray cast iron, International Journal of Refractory Metals and Hard Materials 18, pp Gimenez S., Vleugels J., Van der Biest O. (2006), Chemical Compatibility of PCD and PCBN Superhard Tools with Iron, Euro PM2006, Volume 3, pp Mocelin F., Melleras E and Guesser W.L. (2004), Study of the machinability of CGI for drilling process, Tupy Fundicoes Ltda. Brazilian Manufacturing Congress, Vol XXVI, pp Pereira A. A., Boehs L., Guesser W. L. (2006), The influence of sulfur on the machinability of gray cast iron FC25, Journal of Materials Processing Technology, 179, pp Reuter U., Schulz, H., McDonald, M. (1999) Compact and Bijou - The Problems Associated with CGI Can be Overcome, Engine Technology International 12/1999, pp 58-60, Dorking, GB Sahm D., Troschel W., Schulz H., Kalhofer E. and Reuter U. (1998). Machining of CGI. Werkstatt und Betrieb (Germany), 131(3), Sahm A., E. Abele, H. Schulz (2002), Machining of Compacted Graphite Iron (CGI), Mat. wiss. U. Werkstofftech. 33, pp , Trent E. M., Wright P. K. (2000), Metal Cutting, Fourth Edition, Ch 9, Machinability, pp REFERENCES Abele E., A. Sahm, H. Schulz (1) (2002), Wear Mechanism When Machining Compacted Graphite Iron, Annals of the CIRP Vol. 51/1/2002, pp Araki T., Yamamoto S. (1975), Influence of Metallurgy on Machinability, Proceedings of the International Symposium on Influence of Metallurgy on Machinability, vol 7, pp, Cohen, P. H. ; Voigt, R. C. and Marvanga, R. O. (2000), Influence of graphite morphology and matrix structure on chip formation during machining of ductile irons. AFS Casting Congress, American Foundrymen`s Society, Pittsburg. pp Dawson, S., and Schroeder, T. (2000). Compacted graphite iron offers a viable design alternative. Engineered Casting Solutions (USA), 2(2), pp Swedish Production Symposium 2007 Tasdelen... 6

LUBRICATION & MACHINING OF COMPACTED GRAPHITE IRON

LUBRICATION & MACHINING OF COMPACTED By R. Evans, F. Hoogendoorn, & E. Platt, Quaker Chemical Corporation, Metalworking Division Laboratory SCOPE Compacted graphite iron (CGI) continues to gain use within