Experimental study on hard turning hardened GCr15 steel with PCBN tool

Experimental study on hard turning hardened GCr15 steel with PCBN tool

Journal of Materials Processing Technology 129 (2002) 217±221 Experimental study on hard turning hardened GCr15 steel with PCBN tool X.L. Liua,*, D.H...

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Journal of Materials Processing Technology 129 (2002) 217±221

Experimental study on hard turning hardened GCr15 steel with PCBN tool X.L. Liua,*, D.H. Wenb, Z.J. Lia, L. Xiaoa, F.G. Yana a

Department of Mechanical Engineering, Harbin University of Science and Technology, Harbin 150080, PR China b School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, PR China

Abstract This paper discusses experimental results of turning experiments on GCr15 bearing steel hardened to 60±64 HRC. The objective was to determine the effect of the cutting parameters on cutting force, chip morphology and resultant workpiece surface quality, more speci®cally surface texture, microstructural alterations, changes in microhardness and residual stresses distribution. Experiment results show that tensile stress can be produced under some cutting conditions, the machined super®cial hardened layer depth shows an increasing tendency with the improvement of the workpiece hardness, and that the surface roughness value shows a decreasing tendency when the workpiece hardness is over 50 HRC. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hard turning; Surface integrity; PCBN tool; Hardened bearing steel

1. Introduction Hard turning of machine parts is a new machining method to be used instead of grinding when machining hardened steels [1±4]. Instead of machining a product in the soft state, the part is hardened by heat treatment, then providing the required ®nish and dimensional accuracy by turning, so that the expensive grinding operation is eliminated [5±9]. It has recently been shown that it is feasible to use hard turning under selected conditions to super®nish surfaces, hardened to 64 HRC, to a surface ®nish of 2±8 min., thus making it possible to eliminate the need for separate grinding and abrasive-based super®nish over a broad range of production activities involving hardened workpieces [10,11]. The surface integrity after machining hardened steel is superior and more consistent than that of ground and super®nished surfaces. Surface properties such as roughness are critical to the functionality of machined components. Increased understanding of surface generation mechanisms can be used to optimize machining processes and to improve component functionality. As a result, numerous investigations have been conducted to determine the effect of parameters such as feed rate, tool nose radius, cutting speed and depth of cut on surface roughness in hard turning operations [3±10,13,15,18±20]. * Corresponding author. Tel.: ‡86-451-6673-747. E-mail address: [email protected] (X.L. Liu).

Hard turning serves as an ideal process to examine the effects of workpiece properties and tool edge geometry on the surface roughness and on additional responses such as the cutting forces. Workpiece properties such as hardness are signi®cant in hard turning because this process is de®ned by a characteristic type of chip formation (segmented), resulting typically from the machining of high-hardness materials [5,6]. Additionally, hard turning encompasses a relatively wide range of workpiece hardness values (45±70 HRC). ToÈnshoff et al. [1] considered the effects of tool composition on surface integrity for the hard turning of ASTM 5115 steel. The results of this study showed that a martensitic white layer characterized by tensile residual stress on the surface of the workpiece is prevalent when machining with worn tools. Matsumoto et al. [5] studied chip morphology and cutting forces when machining different hardness hardened steel with ceramic inserts. The cutting forces involved in cutting soft steels were relatively high and decreased as the hardness increased. When the hardness exceeded 50 HRC and a segmental chip appeared, the cutting forces suddenly increased. Wu et al. and Matsumoto et al. studied the effects of workpiece hardness on the machined workpiece surface quality. Wu identi®ed that the stress ®eld generated in the workpiece determined the residual stress. This ®eld is affected by the chip formation process, more speci®cally the orientation of the shear angle, the stress ratio between the tangential and normal stresses acting in the primary shear zone and the work material

0924-0136/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 6 5 7 - X

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properties. This involves the temperature rise in the workpiece and the yield stress/hardness of the material [12]. Liu studied the features of cutting quench bearing steel under different conditions with PCBN tool, the main cutting force features a increasing tendency with the improvement of the workpiece hardness, but the changing extent is difference at the two sides of the workpiece hardness. With the further addition of the workpiece hardness, the cutting temperature shows a decreasing tendency. Both the changing of the chip shape and the machined surface roughness are divided by 50 HRC [13]. Matsumoto et al. [20] studied the effect of the workpiece hardness on the residual stresses produced in the tube facing of AISI 4340 steel. Their study revealed the existence of a white layer in the machining of a highhardness workpiece with a chamfered tool. Chou and Evans [18], AbraÄo and Aspinwall [8] and Davies et al. [15] support the existence of a white layer for the hard turning of AISI 52100 steel and additional steels. Davies et al. and Elbestawi have investigated the effects of cutting conditions on chip morphology in hard turning. It is clear from the literature reviewed above that the effects of the tool cutting edge geometry and the workpiece hardness on surface generation at low feeds characteristic of ®nish machining are not well understood, especially in hard turning. Consequently, the paper presents the results of a detailed investigation of the effects of cutting edge geometry and workpiece hardness on the surface ®nish and cutting forces in the ®nish hard turning of through-hardened VIM/VAR AISI 52100 steel [15,16,21]. Thiele and Melkote investigated the effects of tool cutting edge geometry and workpiece hardness on the surface roughness and cutting forces in the ®nish hard turning of AISI 52100 steel with cubic boron nitride inserts. The study shows that the effect of the two-factor interaction of the edge geometry and workpiece hardness on the surface roughness is also found to be important. It is also shown that the effect of workpiece hardness on the axial and radial components of force is signi®cant, particularly for large edge hones [22]. In Part I the authors discuss the effect of workpiece hardness on the cutting temperature and tool wear, showing that the critical hardness is a important factor in hard turning. The goal of the present study was to identify other cutting mechanisms of PCBN tools when machining GCr15 bearing steel hardened to 60±64 HRC. A series of cutting experiments was designed to investigate the effect of the cutting parameters on cutting force, chip morphology and resultant workpiece surface quality, more speci®cally surface texture, microstructural alterations, changes in microhardness and residual stresses distribution.

The average grain size of BN300 is about 1 mm. All cutting inserts were fabricated to have a 25  0:1 mm chamfer. The cutting geometry was 308 rake angle, 588 clearance angle, 0.8 mm nose radius, and about 12.5 mm edge radius. The work material was conventional GCr15 hardened bearing steel (equal to AISI 52100 and SUJ2), and the workpieces were 38 mm diameter and about 90 mm long bars. The cutting process was outside diameter turning conducted on a lath CA6140 without coolant. Cutting forces measurement was conducted by a piezoelectric dynamometer. The continuous cutting conditions are the same as in Part I. The in¯uence rules of the machined material hardness on the cutting force and temperature were found out by some experimental studies of the cutting force and the cutting temperature through systematically changing the cutting parameters (cutting speed, feed, cutting depth) and the workpiece hardness under the condition of dry cutting. A turning strain gauge was used for the measurement of the cutting force, and the natural thermocouple was used to measure the cutting temperature. 3. Results 3.1. Cutting forces The changing rules of the main cutting force are shown as in Fig. 1, the changing rules of the main cutting force featuring an increasing tendency with the improvement of the workpiece hardness within the cutting parameter scope, which accords with traditional metal cutting theory. The cutting force involved in cutting soft steels was relatively high and decreased as the hardness increased. When the hardness exceed 50 HRC and a saw-tooth chip appeared, the cutting force suddenly increased. The force was lower when machining at higher cutting speed, when the energy input into the system and strain stress were higher. This can be explained by increased heat generated in the shear zone that was suf®cient to plasticize the workpiece material, and hence reduce its strength. At lower cutting speed, lower temperatures were generated and the cutting force was consequently higher. The mechanical strength of GCr15

2. Experimental details Tool materials was polycrystalline cubic boron nitride from Sumitomo BN500, a low CBN content grade, has 60% volume fraction of CBN grains with titanium nitride binder.

Fig. 1. The effect of workpiece hardness and cutting speed on cutting force.

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Fig. 4. The subsurface residual stress between the two experiments. Fig. 2. The effect of workpiece hardness and cutting speed on shear flow stress.

decreases signi®cantly at approximately 650 8C. The effect of cutting speed and workpiece hardness on the shear ¯ow stress obtained from experiment is shown in Fig. 2. The stress value decrease with increasing cutting speed and workpiece hardness. When cutting at high speed, the strain rate in the shear zone would be expected to be high, thus more heat would be generated, resulting in higher temperatures. The experimental results indicate that 50 HRC is the critical workpiece hardness. When the workpiece hardness is over 50 HRC, because of the cutting heat effect the workpiece material hardness falls sharply, while when the tool hardness decreases a little, the hardness difference between the tool and the workpiece increases, which makes the machining easy. This phenomenon is named the metal soften effect. The 50 HRC is the critical hardness of the metal soften effect when cutting GCr15 as well as the critical hardness of dividing common metal cutting from hard cutting. 3.2. Surface integrity The comparison of the machined surface roughness and hardened layer depth of the machined surface for different hardness is shown in Fig. 3. The machined surface roughness is the worst when the workpiece hardness is around 50 HRC.

When the workpiece hardness is over 50 HRC, the surface roughness value shows a decreasing tendency with the addition hardness. The rule shows that the machined surface integrity is worst around the critical hardness. The machined super®cial harden layer depth shows an increasing tendency with the improvement of the workpiece hardness. When the workpiece hardness is 50 HRC the machined super®cial harden layer depth is optimum. When the workpiece hardness is over 50 HRC the depth changes little with the increase of the workpiece hardness. The residual stress status of the machined surface is shown in Fig. 4 under two kinds of cutting conditions. The residual stress status of the machined surface is compressive stress both in the surface and in the interior for lesser values of the cutting parameters. When the cutting parameters are large the residual stress status of the machined surface is different. The surface stress is tensile stress. From 50 HRC to the interior the stress is compressive stress. From the studies that have been done it was found that most of the stress of the machined surface is the residual compressive stress. This stress is bene®cial for resisting fatigue, which provides a better quality of PCBN cutters, but the test results show that not all the machined surface stress is compressive while cutting with PCBN cutters: unsuitable cutting conditions can create the residual tensile stress also. 4. Discussion 4.1. Analysis of the segmental chip formation process

Fig. 3. The surface finish vs. workpiece hardness.

When machining a workpiece with different hardness the created chip shapes are different also. When the workpiece hardness is under 50 HRC the chip shape is in strip form, but when the workpiece reaches 50 HRC, the chip shape changes into hackle form (shown in Fig. 5). Numerous studies have been conducted about the forming condition of the hackle chip and its mechanism, Shaw considered that the hackle chip is created when the cutting properties are bad or the cutting heat is less, Davies stated the hackle chip is caused by the local stress when the cutting speed reaches a certain critical value. The present authors think when the workpiece material has a certain hardness,

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Fig. 6. The deformation coefficient and hardness under different hardness. Fig. 5. Saw-tooth chip photography (120).

the cutting edge deformation by extrusion is lesser, the deformation region is smaller also, so that the cutting heat created by plastic deformation is comparative lower and the cutting temperature is lower also. The formation process can be divided into four phases. The ®rst phase is present after the formation of the former hackle chip, strain created by the extrusion of the cutting layer metal on the cutter edge occurring in a local region near to the cutter edge, because the workpiece hardness is too high to transfer the change to the machining surface. With the progress of cutting, the formation process of the hackle chip come into the second phase, in which the moving forwards of the cutter edge leads to the deformation region enlarging with the increasing of the extrusion of the cutter edge. The ¯exibility deformation of the former region of the cutter edge spreads towards to the workpiece body. When the deformation region extends to the machined surface the process come into the third phase. The deformation, which is the same that in the cutting of common plastic material, in this area, generates the shear deformation which follows the shear plane, and much heat is generated because of the increasing of the plastic area. However, the former hackle shear plane is impaired by the heat generated from the deformation, to increase the temperature of the shear area sharply, which makes the material soft. When the temperature of the shear area increases to a certain degree the chip slides along the shear plane and then come into the fourth phase of the hackle chip formation. A new hackle slide plane is formed and the material with plasticity and high temperature features changes into a chip as well as taking away a large amount of transforming heat. The cutting temperature decreases and the temperature below the slide plane falls sharply, the plastic region reduces quickly, the formation process of a hackle is ended and another chip begins to form. 4.2. Influence of chip deformation on cutting temperature The deformation coef®cient for cutting different hardness GCr15 is shown as Fig. 6. With the increase of the machined

material hardness the changing coef®cient is decreased. When the workpiece hardness is over 50 HRC, the changing coef®cient is less than 1. The cutting heat is small if the chip deformation is small, which is one of the reasons why the cutting temperature falls when the workpiece hardness is high. 4.3. Changing of the chip hardness When the hackle chip comes into being, because most of the plastic deformation region was divided by the shear cross-section of the division saws on the ef¯uent chip, most of the heat was taken away by the chip. Thus the cutting temperature features a decreasing tendency when producing the hackle chip (the workpiece hardness is over 50 HRC). The changing of chip hardness can re¯ect this point. The workpiece hardness is the chip hardness for cutting various hardness GCr15, shown in Fig. 6, the chip hardness increasing with the workpiece hardness when it is lower than 50 HRC, when the chip can be quenched by the cutting heat; workpiece material with a hardness over 50 HRC has a lower chip hardness. The chip can be tempered by the cutting heat, which can make the chip hardness fall. If it is serious the cutting heat appears in a melted status of the chip. 5. Conclusions The following conclusions were gained when conducting the experimental cutting study on quench bearing steel under different conditions with PCBN cutters: (1) Under different cutting parameters, the rule of cutting force change with workpiece hardness change accords with traditional metal cutting theory. The main cutting force features an increasing tendency with the increase of the workpiece hardness, but the changing extent is different at the two sides of the workpiece hardness. (2) The deformation created by the chip formation reduces when the workpiece hardness is increased. The chip

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formation mechanism and the chip shape change also. When the workpiece hardness is over 50 HRC, that the hackle chip can bring more heat away is the main reason for the decrease of the cutting temperature. (3) Unsuitable cutting conditions can also create residual tensile stress. For further reading see [14,17]. Acknowledgements This project is supported by the National Natural Science Foundation of China (No. 59975026). The authors would like to thank Harbin University of Science and Technology for supporting this work. References [1] H.K. ToÈnshoffns, C. Arendt, R. Ben Amor, Ann. CIRP 49 (2000) 547±566. [2] W. KoÈnig, Th. Wand, Ind. Diamond Rev. 47 (1987) 117±120. [3] W. KoÈnigni, M. Klinger, R. Link, Ann. CIRP 39 (1990) 61±64.

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