Hard turning of interrupted surfaces using CBN tools

Transcription

1 journal of materials processing technology 195 (2008) journal homepage: Hard turning of interrupted surfaces using CBN tools Anselmo Eduardo Diniz, Adilson José de Oliveira Department of Manufacturing Engineering, Faculty of Mechanical Engineering, State University of Campinas, CP 6122, Campinas, SP, Brazil article info abstract Article history: Received 31 January 2007 Received in revised form 26 April 2007 Accepted 10 May 2007 Keywords: Hard turning CBN tools Cutting edge Wear Some of the advantages of using turning instead of grinding for the finish machining of hardened steel surfaces are: high flexibility, the ability to cut complex surfaces with a single machine set up, cost of the process, and the possibility of turning without cutting fluid. On the other hand, turning is not very suitable for cutting hardened interrupted surfaces, since most of the tools used in this operation are not very tough. This work aimed to broaden the use of hardened steel turning tools, comparing the use of two CBN tool grades (low CBN content, called 7020, and high CBN content, called 7050) and two cutting edge micro geometries (chamfered and rounded edge) in radial turning of three kinds of surfaces: continuous, semi interrupted and interrupted. The comparison was in terms of tool wear and tool life. The workpiece material was AISI 4340 steel with 56 HR C of hardness. Results showed that the longest tool life was obtained when the 7020 grade was used, regardless of surface type. However, in continuous cutting, tools with chamfered edges proved to have the longest life, while for interrupted cutting the best results were produced by tools with edge rounding Elsevier B.V. All rights reserved. 1. Introduction Turning of hardened steels has been increasingly used to replace grinding operations, due to the development of very hard tool materials (ceramics and CBN) and very rigid machine tools, which can ensure the same accurate geometrical and dimensional tolerances. Within the last years, hard turning operations have become more and more capable with respect to surface roughness and IT standards. Additionally, these processes offer a high flexibility, increased material removal rates and even the possibility of dry machining (Klocke et al., 2005). However, there are some restrictions to its use in the machining of hardened steels with interrupted surfaces, because tools generally used for this purpose are brittle and, have little resistance against the typical shocks of interrupted cutting. On the other hand, interrupted surfaces are typical for turned parts, lending importance to the study of the turning of such surfaces in hardened steel parts. The main goal of this work is to contribute to the studies about this problem. Turning experiments were carried out on different workpiece surfaces and using tools with two different CBN contents and two different cutting edge micro geometries. The final purpose was to find the best tool material and tool cutting edge micro geometry to turn continuous, semi interrupted and interrupted surfaces of hardened steel, in terms of tool wear and tool life Tools materials and geometries for hardened steel turning Wear resistance and chemical stability are the most important properties for a tool material intended for hardened steel turning. The hardened workpiece surface has an abrasive effect on the tool material, and the high temperature on the cutting edge causes diffusion between tool and chip. Moreover, if the surface has any kind of interruption, toughness is Corresponding author. Tel.: ; fax: address: anselmo@fem.unicamp.br (A.E. Diniz) /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.jmatprotec

2 276 journal of materials processing technology 195 (2008) an additional necessary property of the tool material, in order to prolong tool life (Wellein and Fabry, 1998). Mixed ceramics (Al 2 O 3 + TiC) and CBNs are the best tool materials for this kind of operation, due to their high hot hardness and wear resistance. Their hardness and chemical stability enable them to withstand the high thermal and mechanical loads of such machining operations. CBN has a higher hardness than ceramic tools in both low and high temperatures. Other CBN properties, such as high thermal conductivity and low thermal expansion coefficient, are also important when using such tools in hardened steel turning. Ceramic also has good properties for use in hardened steel turning, such as hot hardness, wear resistance, and excellent chemical stability higher than CBN. On the other hand, properties such as low thermal conductivity, and particularly low toughness, make it unsuitable as tool materials in hardened steel turning of interrupted surfaces (Luo et al., 1999; König et al., 1990). CBN tools are usually classified in two grades: high CBN content (around 90%), and lower CBN content (around 60%) with a ceramic phase added to the material, usually titanium nitride. The high CBN content tools exhibit higher toughness than tools with an added ceramic phase (low CBN content tool). Therefore, high CBN content tools are usually recommended for the turning operation of hardened steels with interrupted surfaces. Moreover, the high CBN content of these tools makes them harder than those with a lower amount of CBN. The CBN grade in which part of the CBN content is replaced by a ceramic phase loses in hardness and toughness, but gains in chemical stability. This is important for the finish operations of continuous surfaces, where a high temperature is reached, and diffusive wear must be avoided (Sandvik, 1994). Usually these tools have a chamfer on the cutting edge to strengthen it and protect it against chipping and breakage. In general, for low toughness tools (low CBN content), besides the chamfer, an edge rounding is done at the end of the chamfer (Sandvik, 1994). Several studies about ceramic and CBN use in hard turning with continuous cutting have been done (Chou and Evans, 1997; Matsumoto and Diniz, 2000). However, the study of hard turning on interrupted surfaces is limited to a few publications. Chou and Evans (Chou and Evans, 1999) carried out some experiments to identify CBN tool wear characteristics in interrupted cutting, turning a workpiece made of M50 steel with HR C using a low CBN content (CBN-L) tool and a high CBN content (CBN-H) tool in different cutting speeds. For CBN- H tools, tool life decreased as cutting speed increased, but for the CBN-L tools, tool life increased when cutting speed was increased from 2 to 4 m/s, and then decreased as cutting speed increased from 4 to 7.8 m/s. For the experiment with a cutting speed at 2 m/s, CBN-H exhibited the longest tool life, but at the other cutting speeds tested, CBN-L proved to be the best tool material. The typical wear types of these tools were flank and crater wear. No cutting edge chipping and cracking was observed. Thus, tool impact against the interruptions of the turned surface was not an important factor in shortening tool life, because, if it were, it would cause chipping, cracking and breakage of the cutting edge. For the CBN-H tool, the temperature rise caused by the cutting speed increase was predominant in the shortening of tool life. For the CBN-L tool, the tool life increase when cutting speed was changed from 2 to 4 m/s was caused by workpiece softening around the cutting region, which made chip removal easier. When cutting speed increased from 4 to 7.8 m/s, cutting edge softening due to higher temperatures had a greater effect than workpiece softening, and as result tool life decreased. Fig. 1 Continuous and semi interrupted cutting workpieces.

3 journal of materials processing technology 195 (2008) Fig. 2 Interrupted cutting workpiece. 2. Experimental procedures The experiments were carried out in a CNC lathe with 15 kw of power in the spindle motor. The workpiece material was AISI 4340 steel with 56 HR C of hardness. Three kinds of workpieces were used, as shown on Figs. 1 and 2. These workpieces were built in order to obtain continuous, semi interrupted and totally interrupted cutting during their radial turning. Two kinds of CBN grades were used for the tools: 7020 and According to Sandvik (Sandvik, 2006), the CBN 7020 grade is a low CBN content material with a ceramic phase added (TiN), while the CBN 7050 grade is a high CBN content material. Both are recommended for the machining of hardened steel and cast iron in finish operations. Two kinds of tool cutting edge micro geometries were used: chamfered and with edge rounding. The first had a 0.1 mm 20 chamfer on the cutting edge, and the second, besides the same chamfer, had a rounded segment at the end of the chamfer. The ISO code of the inserts and tool holder were SNGA (with TiN coating) and PSBNR2525M12, respectively. The cutting conditions recommended by the tool manufacturer were: cutting speed v c = 150 m/min, feed f = 0.08 mm/rev, and depth of cut a p = 0.15 mm. Along tool life, the flank wear was inspected with an optical microscope. Tool life was considered finished when flank wear reached VB B = 0.20 mm. The experiment was also terminated if after 100 min of cutting time this value of flank wear had not been reached. After the end of tool life (or the end of the experiment), worn inserts were taken to a scanning electronic microscope equipped with and EDS system. One experiment consisted of successive radial turning passes of one of the surfaces shown on Figs. 1 and 2 with the same cutting edge, till the moment when either the tool reached the end of its life, or cutting time reached 100 min. Each experiment was carried out three times. 3. Results and discussions 3.1. Tool life Fig. 3 Figs. 4 and 5 shows the results of all experiments in terms of tool life. According to analysis of variance, using a 90% confidence interval, it can be stated that the workpiece geometry and tool grade significantly influence the tool life. The 7020 grade contains a ceramic phase and, therefore, a lower percentage of CBN than the 7050 grade. So, it has lower thermal conductivity and hardness, and higher chemical stability than 7050 grade (according to data supplied by Sandvik). Fig. 3 Cutting time for all experiments.

4 278 journal of materials processing technology 195 (2008) These properties explain the longer tool life of 7020 grade in almost all cases. Even in interrupted cutting, in which experiments were terminated after 100 min of cutting time, i.e., before the tool reached the end of its life, the 7020 CBN tools exhibited lower wear, as will be shown later. Thus, the supposedly higher toughness of the 7050 grade was not relevant even in interrupted cutting. Therefore, 7020 grade is more suitable than 7050 grade for the turning operation of hardened steels with surfaces similar to those used in these experiments. This does not exclude the possibility that a high content CBN could have a better performance than a low content CBN tool, if the number of interruptions were greater than in this work. Another interesting point apparent in Fig. 3 and confirmed in the analysis of variance is that interrupted cutting resulted in a higher tool life than both semi interrupted and continuous cutting, regardless of edge micro geometry and CBN tool grade. A temperature increase would stimulate wear mechanisms such as abrasion and diffusion (Trent, 1991; Diniz et al., 2000). It reduces tool hardness, facilitating the removal of tool particles by abrasion, and stimulates the exchange of particles between tool and chip (diffusion). Therefore, keeping lower temperatures generally increases tool life. There were three reasons why the tool remained in a lower temperature when interrupted cutting was used: (a) due to the interruptions, the heat propagation inside the workpiece was hindered, and the tool reached a colder part of the workpiece at each 90 of workpiece rotation; (b) with the tool rotation, an air flow is generated through the grooves of the interrupted surfaces, which helps keep workpiece and tool cold; (c) because the tool cuts just a small part of the workpiece between two grooves, there is not enough time to build a seizure zone (Trent, 1991) between chip and tool rake face. When this seizure zone occurs, compressive stresses, strain rate and temperatures are high, and particle exchange between chip and tool is stimulated, causing crater wear on the tool rake face. As will be seen later, crater wear was smaller in interrupted cutting than in continuous and semi interrupted cutting. Remaining in Fig. 3, tool life for semi interrupted and continuous cutting were similar slightly longer for 7050 tools in Fig. 4 View of the worn flank face of chamfered tools.

5 journal of materials processing technology 195 (2008) Fig. 5 View of the worn flank face of edge rounded tools. semi interrupted cutting, and slightly longer for 7020 tools in the continuous cutting. This shows that the four interruptions represented by the holes on the surfaces were not determinant for the tool wear mechanisms. Regarding cutting edge micro geometry, according to analysis of variance, using a 90% confidence interval, it does not significantly influence the tool life. However, the interaction between cutting edge micro geometry and tool grade has a significant influence on tool life. In other words, tools with edge rounding exhibited longer lives when 7050 grade was used, and chamfered tools had longer lives when 7020 grade was used. Edge rounding makes the edge more rigid and resistant to chipping and breakage. On the other hand, it causes a greater chip deformation due to the lower rake angle, especially when a small chip thickness is used. A more detailed explanation for this occurrence is given below during the discussion of wear mechanisms Tool wear Flank and crater tool wear were observed during the experiments. To help understand the wear phenomena occurring on the cutting edges, show pictures of the worn edges taken in a scanning electronic microscope. It is important to note that no chipping or breakage can be seen on the cutting edges, even when interrupted surfaces were turned. This fact proves that all tools were tough enough to withstand the impacts typical of interrupted cutting, no matter what the tool material or cutting edge geometry was. In almost all pictures, abrasion marks are seen in the region of the secondary cutting edge (left side of each picture). This formation is associated to the very small chip thickness in that region. Fig. 6 shows the chip profile; it can be seen that all chip is formed inside the curved region of the cutting edge (nose radius), since the depth of cut is smaller than the nose

6 280 journal of materials processing technology 195 (2008) Fig. 6 Scheme of cutting geometry. radius. The chip thickness in the average point of contact toolworkpiece is 0.02 mm and the length of the chip-tool contact is 0.62 mm. However, in the end of the contact region between the workpiece and the secondary cutting edge, chip thickness is very small, causing chip side flow and further increasing the specific cutting force in that region. Figs. 4 and 5 also reveal that abrasion occurred not just in the secondary cutting edge, but it was also the most important wear phenomenon for flank wear formation when CBN 7020 tools were used, regardless of edge micro geometry or workpiece shape. Moreover, crater wear in these tools was always smaller than crater wear in 7050 tools. For the 7050 tools, abrasion was only important in the interrupted cutting experiments; with continuous and semi interrupted cutting, the small grooves typical of abrasion wear were not so clear, and crater wear occurred intensively. As previously mentioned, a 7020 tool is made of CBN plus a ceramic phase. The added ceramic decreases its thermal conductivity, and increases its chemical stability with some metallic elements of the chip. Therefore, this tool is very resistant to diffusion, which is the main mechanism for the crater wear formation. Thus, even in continuous cutting where high temperatures occurred, crater wear was always small. For the 7050 tools in continuous and semi interrupted cuttings, the grooves on the flank face typical of abrasion wear occurred only in the region of the secondary cutting edge. In the other parts of the flank face in contact with the workpiece, the smooth appearance of the flank wear land indi- cates that diffusion was the mechanism responsible for flank wear. Because this type of tool has no ceramic phase, it has a lower chemical stability and a higher thermal conductivity than the 7020 grade. Consequently, tool temperature is higher, and resistance against diffusion is lower. Therefore, diffusion was the wear mechanism for both flank and crater wear. However, for interrupted cutting, abrasion marks can be seen on the whole wear land of the 7050 tools. As previously said, tool temperature is lower in interrupted cutting. Therefore, diffusion cannot occur and abrasion takes place instead. The fact that, for 7020 tools, chamfered edges provided the longest tool life (with exception of interrupted cutting, in which tools with both edges presented the same performance), whereas for the 7050 tools, the longest tool life occurred with edge rounding, is associated with the kind of tool wear and the geometric contribution of the cutting edge. Fig. 7 illustrates this point. With both kinds of edges the rake angle is very negative ( 26 ). But it can be seen in Fig. 7 that, with edge rounding, the chip-tool and workpiecetool contact areas are larger, and chip formation is hindered, mainly for very thin chips (secondary cutting edge). Moreover, for small chip thickness, when edge rounded tools are used, the rake angle is even more negative than for the chamfered edge. Another point to note is that edges are supposedly rounded to increase tool resistance against chipping and breakage. However, neither was observed, even for the 7020 tool with chamfered edges in interrupted cutting the situation in which edge chipping and breakage are most likely to occur. The main wear mechanism for the 7020 tool was abrasion, which caused flank wear. This increase of chip deformation and, consequently, chip and tool temperatures when edge rounding is used, reduced tool hardness and made it more susceptible to abrasive wear. Nevertheless, the temperature was not high enough to cause crater wear, due to the high chemical stability of this tool material. Edge rounding, and the resulting larger contact region between chip and tool and between workpiece and tool, are important for resistance against diffusion. In 7050 tools (which do not have a high chemical stability), despite the fact that heat generation is higher, the larger contact area made heat dissipation easier. Therefore, diffusion was not as intense as when chamfered tools were used, and tool life was prolonged for the edge rounded tool. 4. Conclusions Based on the results above, it can be concluded that for the radial turning of AISI 4340 steel with 56 HR C, with CBN tools and in conditions similar to those used in this work: Fig. 7 Scheme of cutting edge area for both tools. in continuous cutting and in semi interrupted cutting, the best performance in terms of tool life was achieved with the 7020 tool with chamfered cutting edge; in interrupted cutting, again the 7020 tool was the most suitable, and any kind of cutting edge geometry can be used;

7 journal of materials processing technology 195 (2008) tool life was longer when cutting interrupted surfaces than either semi interrupted or continuous surfaces, because tool temperature was lower in interrupted cuttings; no chipping or breakage was observed on the cutting edges, even when interrupted surfaces were turned. This fact proves that all tools displayed enough toughness to withstand the impacts typical of interrupted cutting, no matter what the tool material or the cutting edge geometry was. Acknowledgements The authors wish to thank Sandvik Coromant for the tools supplied for the experiments and also for support during execution of the tests and analysis of the results. references Chou, Y.K., Evans, C.J., Tool wear mechanism in continuous cutting of hardened tool steel. Wear 212, Chou, Y.K., Evans, C.J., Cubic boron nitride tool wear in interrupted hard cutting. Wear , Diniz, A.E., Marcondes, F.C., Coppini, N.L., Technology of Machining of Materials, 2 ed., São Paulo: Artiliber Editora, In Portuguese. Klocke, F., Brinksmeier, E., Weinert, K., Capability profile of hard cutting and grinding processes. Ann. CIRP 54, König, W., Klinger, M., Link, R., Machining hard materials with geometrically defined cutting edges field of applications and limitations. Ann. CIRP 39, Luo, S.Y., Liao, Y.S., Tsai, Y.Y., Wear characteristics in turning high hardness alloy steel by ceramic an CBN tools. J. Mater. Process. Technol. 88, Matsumoto, H., Diniz, A.E., Tool life in turning hardened steel. Máquinas e Metais 411, , In Portuguese. Sandvik Coromant, Modern Metal Cutting. AB Sandvik Coromant, Sandviken, Sweden, Sandvik Coromant. Main Catalogue Available in: Acess in: 10 april Trent, E.M., Metal Cutting, 3 ed. Butteworths-Heinemann, Oxford. Wellein, G., Fabry, J., Hard Turning is More Than the Application on CBN. Cutting Technology, Kennametal Hertel.