Cutting of Hardened Steel

Cutting of Hardened Steel

Keynote Papers Keynote Papers Presented at the ScientificTechnical Committees Paper Discussion Sessions Cutting of Hardened Steel H. K.Tonshoff ( l )...

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Keynote Papers

Keynote Papers Presented at the ScientificTechnical Committees Paper Discussion Sessions Cutting of Hardened Steel H. K.Tonshoff ( l ) ,C. Arendt, R. Ben Amor Institute for Production Engineering and MachineTools, University of Hannover, Germany Abstract Cutting of hardened steel is a topic of high interest for toda 'sindustrial production and scientific research. Machine parts consisting of hardened steel are high peiormance components which are often loaded near their physical limits. The functional behavior of machined parts is decisively influenced by the fine finishing process which represents the last step in the process chain and can as well be undertaken by cutting as grinding. An overview of the mechanisms of chip removal in hard cutting and the thermomechanical influence of the work area is presented. Furthermore, several models of chip removal in hard turning are introduced and discussed summarizing the metallurgical fundamentals and giving an overview on stress and temperature distributions in the work area. Boundary conditions for hard cutting as e.g. machine tools, cutting materials and others are subject to discussion to determine the achievable workpiece quality and economic efficiency of hard cutting processes in comparison with grinding. Keywords: Hard cutting, chip removal, machine, cutting tool, quality of work

1 INTRODUCTION Cutting of hardened steel is a topic of high interest for today's industrial production and scientific research. Tests and first introductory steps have been made in automotive-, gear-, bearing-, tool and die making industry. In research institutes and universities, basic investigations experimentally as well as theoretically In the have been made since more than 25 years [l]. ClRP community, the subject has been worked out by several researchers. For this review many colleagues contributed b discussion and publications. The authors thank the f&owing contributors for their valuable assistance:

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-

M. Balazinski, Montrkal, Kanada E. Brinksmeier, Bremen, Germany G. Byrne, Dublin, Ireland T. H. C. Childs, Leeds, UK M. A. Davies, Gaithersburg, USA M. A. Elbestawi, Hamilton, Canada C. J. Evans, Gaithersburg, USA F. Klocke, Aachen, Germany W. Koenig, Aachen, Germany R. Komanduri, Stillwater, USA C. R. Liu, W. Lafayette, USA A. Moisan, Aix-en-Provence, France C. A. van Luttervelt, Delft, Holland K. Nakayama, Tokyo, Japan t M. C. Shaw, Tempe, USA H. Schulz, Darmstadt, Germany K. Weinert, Dortmund, Germany R. Wertheim, Tefen, Israel

Machine parts consisting of hardened steel are high performance components which are often loaded near their physical limits. The functional behavior of machined parts is decisively influenced by the fine finishing process which represents the last step in the process chain and can as well be undertaken by cutting as grinding. For this reason, fine finishing is defined as an important process and its results have to satisfy high quality requirements. The product specific issues and demands also meet general trends in cutting such as flexibility, ecology, cost effectiveness, time to market and process agility.

Annals of the ClRP Vol. 49/2/2000

Developments in machine tools as well as in process technology focus on cutting hardened steel and rapidly lead to a highly raised industrial relevance of hard cutting. In fact, hard cutting can seriously be regarded as an alternative for grinding operations under certain circumstances. A comparison between hard cutting and grinding is, due to variability of workpiece geometry and required quality, difficult. Koch specified economy, flexibility, ecology and quality as important criteria for process evaluation [2]. High flexibility and the ability to manufacture complex workpiece geometry in one set-up represent the main advantages of hard cutting in comparison to grinding. Furthermore, the substitution of grinding processes with cutting processes enables to avoid coolants and therefore can actually be regarded as interestin alternative even from the ecological point of view. In gure 1.1, a comparison of the processes hard cutting and grinding is given.

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Figure 1.1: Comparison of hard cutting with grinding In the following, several aspects of the cutting of hardened steel are examined and discussed. Metallurgical fundamentals of hardened ferrous materials serve to characterize the workpiece material concerning required process quality as well as quality of work. Furthermore, an overview of the mechanisms of chip removal in hard cutting and the thermo-mechanical 547

influence of the work area is presented. Moreover, several models of chip removal in hard turning are introduced and discussed summarizing the metallurgical fundamentals and giving an overview on stress and temperature distributions in the work area, too. Boundary conditions for hard cutting as e.g. machine tools, cutting materials and others are subject to discussion to determine the achievable workpiece quality and economic efficiency of hard cutting processes in comparison with grinding. 2 METALLURGlCAL FUNDAMENTALS Mechanical properties of hardened or case hardened steel can be adjusted in a wide range and consequently influence the cutting process. Depending on alloys and the heat treatment, the hardness of hardened ferrous materials can be varied between 50 and 70HRC (figure 2.1). The relatively high hardness of ferrous work materials can be reached by martensitic transformation and/or carbide precipitation.

processes is low applying an increased cooling rate and leads to a reduced displacement of carbon atoms. If the critical cooling rate is exceeded, a tetragonal distorted martensite will be produced at the martensitestarting-temperature Ms. During this extremely fast transformation the carbon remains in a constrained soluble state in the a-lattice. A comparison between tetragonal and cubic structured lattices shows that the carbon atoms are arranged on interstitial in the lattice as far as the production of martensite is concerned (figure 2.3). All three edges of the elementary cell of the body centered cubic a-iron are of the same length. By elongation of one and compression of the other two axes, a tetragonal lattice is formed. In the tetragonal lattice C-atoms are not obligatory arranged in every interstitial. The dissolved C-atom makes slip dislocations almost impossible and effectively hinders plastic deformation causing high hardness of martensite [3]. The specific volume of martensite is higher than the specific volume of austenite [4] [5]. The dilatometric curve of a steel with 0,9% carbon content (figure2.3) shows the change of length and therefore of volume during the transformation process. A characteristic sign is the increase in volume at the beginning of the generation of martensite at temperatures of Ms = 210°C. The dilatometric curve does not develop linearly in case of rising the temperature up to starting temperature. The increase of martensite volume results in compressive stress in the austenite and obstructs further martensitic transformation. Thermal stress basically caused by temperature differences between workpiece surface and core is superposed to these transformation stresses [6] [A [8]. Both types of stresses suppress further progress in the martensite transformation [5] [9]. work(pIece material: steel with 0,9%C

Figure 2.1 : Hardness and microstructure of hardened ferrous work materials

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Physico-chemlcal fundamentals In contrast to annealing in which an equilibrium state of texture is adjusted, hardening serves to initiate an imbalance status. Depending on cooling rate and consequently on carbon diffusion deceleration, pearlite, bainite and/or martensite is formed. The timetemperature-transformation curve (TITcurve) indicates the composition of the texture. Figure 2.2 shows an example of the carbonized case hardening steel 16MnCr5 (comparable to AlSl5115) which has been achieved by continuous cooling. 2.1

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Figure 2.2: Time-temperature-transformation curve of case hardening steel 16MnCr5 (AISI 5115) The composition of the produced workpiece material is a result of a controlled cooling speed. Nucleation and c stal growth occur in the pearlitic structured area by dzusion of carbon and iron. Ferrite and cementite lamellas appear in the material structure in case of transformation of austenite. The time for diffusion

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Figure 2.3: Change of structure during martensite formation The carbon content influences martensitic hardening and should not be less than approximately 0,25%. Otherwise, the cooling rate necessary for martensitic transformation is difficult to reach. Case hardening is adapted to improve mechanical properties of workpiece materials with low carbon content. Carbonizing consists of enriching the subsurface with carbon in order to enable the subsequent hardening process. Other chemical elements such as boron or nitrogen are also used in the heat treatment of ferrous materials. Boronizing is usually applied to increase the hardness and improve the resistance against corrosion. The precipitation hardness of the material of the subsurface improves the stability of components exposed to static and especially to oscillating loads. Hereby, the disperse nitride precipitation ameliorates the local fatigue strength and obstructs the movement of dislocations. The attainable fatigue strength depends on the amount of absorbed nitride, the depth of nitride precipitation and the strength inside the workpiece.

Keynote Papers

2.2 Properties and demands In order to design mechanical products and machines with optimal economical and technical properties, possible origins of failures have to be identified. In machines with moving parts surfaces as well as volumes are loaded. Volume loads such as mechanical and thermal loads lead to deformation and fracture while surface loads cause corrosion and wear. Often both types of loads are superimposing (figure 2.4).

Machine components being in contact with moving parts are simultaneously subject to wear and corrosion. This effect is called wear corrosion or frictional corrosion. It presents the main origin of friction fatigue fracture and has to be taken into account in machine part design [lo].

mation. Therefore the stress, strain and temperature distribution the cutting zone is of interest. The proper understanding of the material removal mechanisms taking place during hard cutting is essential for process evaluation. The analysis of the work area is necessary to describe the chip generation in hardened materials. Depending on cutting parameters and workpiece material properties, cutting may either lead to continuous or discontinuous chip formation. Komanduri and Brown have compiled a detailed classification of chips produced by non homogeneous cutting, which are called wavy segmented and discontinuous chips 1121(figure 3.1).

endangering functionality of machine parts

is. volume loads:

surface loads:

-mechanical

Figure 2.4: Main origins for failure of machine parts Steel components as e.g. camshafts, gearwheels, bearings and cutting tools being exposed to high mechanical loads are often heat treated. Hardening and tempering or case hardening of ferrous materials lead to an improvement of mechanical properties of workpieces. Besides the improvement of strength and hardness, the I]. Figure 2.5 fatigue strength is considerably increased [I shows the influence of the workpiece hardness on rolling contact fatigue having tested gearwheels of different alloyed steels. It can be indicated that the rolling contact fatigue is increased with increasing hardness. The working life is also remarkably improved with increased workpiece hardness and so reaches about 10 time the initial state.

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3.1 Plastomechanics The stress-strain curve of hardened steel, gained in a tension test, is almost linear till fracture. There is practically no plastic deformation. Therefore, a main question is how and why smooth surfaces can be achieved with obviously required large plastic deformation. There are two theories to explain plastic deformation which are 0

the thermodynamic theory and

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the hydrostatic theory.

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The thermod namic theory explain the formability of the work materia by heating up the chip formation zone. The applied mechanical energy is almost exclusively transformed to thermal energy which heats up the material in front of the cutting edge. Under elevated temperature steel softens and get a high formability [I6 [I7 [I81 [I91 [20] [21] [22]. The effect is called "seI/ induced hot machining". According to this theory there is a decisive influence of the heat conductivity of the tool material. It should not be too high to keep the heat in the chip formation zone and to generate a sufficient level of temperature. To examine this theory, Brand made cutting experiments under constant conditions varying only the heat conductivi of the tool material [3]. He could not find a significant in uence of the heat conductivity on the cutting force (figure 3.2). This indicates that the thermodynamic theory can not fully explain the hard cutting phenomenon. This is underlined by force measurement if varying the cutting speed. Here, if self induced heating of the chip formation zone would be the dominant effect, the cutting force should decrease with increasing the cutting speed. Basically, the heat conduction is a time dependent effect. If there is less

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Figure 3.1: Classification of chip The wavy chip is generated by a cyclic variation of the chip thickness due to an oscillation of the shear angle. The discontinuous chip tends to form in case brittle materials are cut at low speeds. The segments of the discontinuous chip are completely separated by fractured surfaces. Meanwhile, a distinction between the segmented chip and the shear chip is difficult to draw and normal1 they are both classified as segmented chip U21 [I 31 [I 4f The behavior of materials causing elastic deformation, plastic deformation, cracks or localized shearing, also called adiabatic shearing, is decisively influenced by temperature and stress distribution in interaction with strain and strain rate [15].

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Figure 2.5: Influence of heat treatment on rolling contact fatigue Heat treatments which aim to increase the hardness of components or of work surfaces lead to the improvement of wear resistance. The subsurface hardness adjusted after machining or after being loaded in the required function is of high importance. A higher workpiece subsurface hardness generally increases the resistance against wear. Further factors like structure constituents additionally influence the resistance against wear.

3 MECHANISMS OF CHIP REMOVAL DURING HARD

CUTTING Applying hard cutting as a finishing process requires the generation of machined surface by pure plastic defor-

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549

time the thermal energy is more concentrated near the heat source. Thus, the temperature in the plastically deformed zone is higher and consequently the specific cutting ene y is lower. Investigations show that there is only a s m 5 influence of the cutting speed on force components [3]. Therefore, it is reasoned that the thermodynamic theory can not explain the plastic deformation of hardened material alone.

material is not plastically deformed in this region and instead of fbw chips, segmented chips are formed. The transition from continuous chip to segmented chiplshear localized chip also takes place with increasing cutting speed [27l. The material behavior in the work zone and so the thermo-mechanical mechanisms strongly depend on cutting parameters and especially on chip thickness h and cuttins speed vc Figure 3.4 shows the influence of the chip thickness on the chip formation process [26]. In 1964, Recht developed a classical model for describing catastrophic shear instability in machining [28]. According to Recht, catastrophic shear will occur in plastically deformed regions in a material if the slope of the true stress true strain curve becomes zero. Semiatin and Rao developed another model of shear localization which incorporates a heat transfer ana sis and material properties, such as the strain har ening rate, the temperature dependence of the flow stress and the strain rate sensitivity of the flow stress, to establish the tendencv towards localized flow 1291.

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Figure 3.2: Influence of heat conductivity on the cutting force The hydrostatic theory is based on the fact that the formability of a material is strongly dependent on of the stress state. This was first shown by v. Karmann who undertook his famous experiments on brittle marble. Under high hydrostatic pressure marble is plastically deformable. The model based on this effect is given in (231. Both low chip thickness and a strongly negatrve chi angle induce high compressive stresses and hig hydrostatic pressure.

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Figure 3.4: Influence of chip thickness on chip formation

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In 1985, Recht introduced the adiabatic shear theory to characterize the chi segmentation process during hard cutting operations. lierefore, themeplastic instability is where a decrease in flow stress due to thermal softening associated with an increase in strain more than offsets the associated strain hardening. Proponents of this theory refer to this to explain the chip segmentation. R is assumed that for some reasons a themeplastic instability occurs along a line, extending from the tool tip and curving upwards to the free surface of the workpiice [30].

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Figure 3.3: Chip formation and Mohr's circle in hard cutting Appl ing Mohr's circle different, stress and strain limits can explained [24] (figure 3.3). Under biaxial tension (cirde 1) the material is fractured when the normal stress reaches the yield strength. The shear fracture limit is relevant under biaxial and compressive stress (circle 2). Final the material even brittle is deformed lastically if the s ear flow limit is achieved (arde3). f h e circle3 with considerable high hydrostatic pressure (center of the circle) stands for stress state in front of the cutting edge with a little undeformed chip thickness and a highly negative rake angle. The chip thickness in this region in which the workpiece surface is generated is extremely small in comparison with the cutting edge radius and measure some micrometers. This geometrical condition A high leads to an effective rake angle of -60' to a. pressure is generated to remove the material in this part of the cutting edge.

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Flow chi s and segmented chips can be observed in cutting Lrdened steel. Generally, flow chips are generated in case the chip thickness is smaller than hc20pm. If the chip thickness exceeds 20pm a segmented chip will be formed [2] [25] [XI. For a chip thickness above 20pm the mentioned condition fails near the free workpiece subsurface. Hardened workpiece 550

Figure 3.5: Crack initiationin the formation of saw toothed chip

In addition to themplastic instability which leads to shear localization other mechanisms can reduce the shear stre th within the shear band without a thermal softening e k . According to Nakayama, the segmented chip will generate, if the shear strain on the free surface of the workpiece attains an ultimate value . In a simplified model the crack initiates at a point Q o k e free workpiece surface. The free surface at Q must be a principle stress direction. Hence the shear plane includes an angle of d 4 with the free surface. If W, is the inclination angle and @ the shear angle, it is @ = d 4 W,.

-

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From figure 3.5 it can be seen that this relation is quite different from the theory based on the model of continuous chip formation. In cutting experiments of a bearing steel of 760 HV (- 62 HRC), the rake angle is varied between y = -10" and y = -50". In spite of this wide variation of the rake angle, crack inclination 'Y, is almost constant and amounts to 'Y, = 30 [31]. Elbestawi investigated the chip formation during machining of hardened steel working out criteria for crack initiation and propagation [32]. Surfaces which have to be machined are not perfectly smooth but rough and composed of microscopic ridges, cracks, voids etc. Machining case hardened materials, high compressive stress creates subsurface material flow but particularly leads to the formation of crack in the free surface depending on the brittleness of the workpiece material. The irregularities in the free surface play an important role in crack initiation. In 1974, Sih tried to incorporate the surface effects into a continuum by considering a layer thickness d to separate surface and interior regions. Sih suggested a surface layer energy failure criterion. According to this criterion, the crack will initiate at a location along the boundary layer of the workpiece where the local energy caused by loading exceeds some experimentally determined material constant. This layer is assumed to be a continuum such that its gross properties may be treated as an isotropic and homogeneous medium [33].

plane. A shear crack should be expected to initiate near a point of maximum shear where the compressive stress is a minimum. Finite element analysis of stress along the shear plane shows a substantial increase in normal stress in processing along the shear plane from the free surface to the tool tip [35]. A higher normal stress will be encountered if a shear crack progresses from the free surface to the tool tip. A continuous gross crack may gradually be converted into a discontinuous microcracked region [34]. Figure 3.7 summarize the different models for chip formation in hard cutting. ___4 ;4 .;i

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Cutting experiments show that the chip formation in hard turning starts with crack initiation near the free surface. Cracks propagate and end up in a plastically deformed region close to the tip of the cutting edge (figure 3.6).

3.2 Forces and stress distribution

Figure 3.6: Microstructure and chip formation According to the hypothesis mentioned above, the crack initiates from the free surface to a point where transition from brittleness to ductility takes place due to higher hydrostatic pressure near the tool tip area. On the basis of this hypothesis Elbestawi concluded that the crack in the free surface will initiate at the critical angle cpcr against the cutting direction when the surface layer energy reaches its maximum value at the minimal applied cutting pressure. This angle of crack initiation will remain constant and will not be affected by changing cutting conditions provided that the cutting material is very hard, brittle and homogeneous. Elbestawi analyzed the crack angle introducing an energetic theory. In accordance to Nakayama's theory he showed that the crack initiates at the free surface of the workpiece with an angle of cpcr = 33" [32]. In 1999, Shaw compared the theory of segmented chip to the adiabatic shear theory for large undeformed chip. He suggests that a thermally initiated process should be expected to have its origin where the temperature has its maximum which is at the tool tip. This corresponds to Recht's model but does not fit with observations made by [31] [34] who showed that the crack starts from the free surface to the tool tip along a relatively straight shear

In cutting hardened steel, resultant forces and stress distributions in the contact area are substantially influenced by the hardness of the workpiece material as observed by Matsumoto. Figure 3.8 shows the relationship between components of machining force and the workpiece material hardness. It has to be mentioned that in this case an extremely high feed was applied. The forces occurring in cutting soft steels are relatively high and decrease if hardness increases. With the hardness exceeding 50 HRC the cutting force increases suddenly. The passive force is known to alter the maximum strain value in the chip and the chip type [36]. cumng speed vC = 90 mlmin deplh ofcul ap = 0 15 mm feed f =09mm

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Dynamic forces that fluctuate at a frequency over 10 kHz are difficult to measure. A conventional piezoelectric dynamometer limits the frequency response to about 1.5kHz. However, an estimation of the relative change in the force components and the frequency of force fluctuation may be obtained by using wire resistance strain gages [34]or accelerometers [43].The force to time relationship can also be related to the cutting process through high speed motion pictures [13]. 400 workpece hardness *A HV760

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Figure 3.10: Effect of negative rake angle on cutting forces The behavior of the cutting force to time dependence is shown in figure 3.11. The fluctuation in percent can be given applying the strain gage and measure 6 % of the cutting force. Lemaitre qualitatively analyses the behavior of the cutting force depending on the chip formation. The results are presented in figure 3.11. Just prior to the maximum force, the chip begins to slide up the rake face, and deformation in the chip is confined to a narrow zone extending from the tip of the cutting tool to the free surface of the workpiece. 800 Ibr

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Figure 3.9:Cutting geometry and forces in hard turning Chip formation takes place mainly in the region of the corner radius and the chamfer of the cutting tool. Due to small values of depth of cut, the effective tool cutting edge angle is determined by the tool holder [41].The passive force F is the largest cutting force component. With an increasfng cutting length leading to an increasing width of flank wear, a significant rise of the passive force F can be observed. Consequently, in contrast to c8nventional cutting the force ratio FJF, is reversed [42]. Larger negative rake angle increases the cutting force F, only to a minor extent while increasing the passive force F remarkably. Nakayama explained these phenomena d t h the formation of saw-toothed chip. In machining of annealed steels, both force components increase with the increase of the negative rake angle [31](figure 3.10). As shown above, cutting of hardened steel often leads to the formation of segmented chip. This phenomenon is directly related to a fluctuation of cutting force and stress distribution in the work area and influences the temperature distribution and so the process result. 552

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As stated above the rake angle in hard cutting is strongly negative because of a more or less large cutting edge roundness rB. a small depth of cut ap and a large corner radius r,. The rake angle is of significant influence on the stress distribution in the chip forming zone. Schmidt made interesting experiments with varying edge radii r or effective rake angle yefi respectively (figure 3.12)[44f In a depth of cut range of 2,5 yrn to 50 pm and an edge radius range of 35 pm to 115 pm the effective rake angle varied between -30"and -75".With the depth of cut ,a, and the material removal rate Q, the cutting and passive force increase. Larger edge radii result in larger forces. Comparable results show the investigation of Nakayama [31] (see also figure 3.10). Schmidt demonstrate the strong influence of the effective rake angle on the chip formation. Smaller rake angle between -62" and -64" leads to chip segmentation. The steep increase of the passive force with smaller rake angle is due to two causes: On one hand there is a pure geometric effect. With decrease of the rake angle a larger position of the thrust force (vector addition of feed and passive force) is transmitted in the passive direction. On the other hand the higher cutting force, a conse-

Keynote Papers

quence of higher shearing, increase the passive force by friction on the rake face. 120 I

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Figure 3.12: Influence of cutting edge radius on cutting and passive force

Figure 3.14: Influence of cutting speed and workpiece material on the temperature

Concerning tool load and chip formation, the stress distribution in the work region is of decisive importance.

The investigation of the influence of the workpiece material hardness on cutting temperatures confirm the results of the examination of the machining force (figure 3.8). In a higher range of hardness leads an increase of the material hardness to an increase of the cutting force. With increasing the cutting force cutting energy is becoming higher and results in elevated temperatures.

Fallbohmer simulated both, temperature and stress in tool and workpiece (figure 3.13). He was interested in the free surface of the chip-work interface. The simulation is carried out for orthogonal cutting process of 35CrMo4 (AIS14130) with PCBN cutting tools varying the depth of cut between 10 and 60 pm. As soon as the depth of cut exceeds 18 pm, c7 drops quickly. From a = 10 pm to a = 30 pm the daximum shear stress Jecreases to 88 % of its original value whereas the principle stress in the y-direction decreases to 71 %. Additionally, local temperatures decrease because the distance to the tool tip increases with a depth of cut preventing from material softening which is necessary for plastic deformation. It can be concluded that the highest shear stress can be found at the free end of the chip workpiece interface. The above-mentioned shear stresses in combination with decreasing principle stress in the y-direction as well as decreasing local temperature lead to crack initiation with increasing depth of cut. Thus, a segmented chip is generated [25].

With respect to the surface integrity the temperature distribution in the workpiece subsurface is the focus of the investigations carried out by Schmidt. During the machining operation with a prepared tool holder, the temperature of the workpiece surface is directly measured below the zone of material separation with an infra-red camera. The measurements show that two areas with different temperatures are formed below the cutting edge. The maximum temperature on the component surface develops at the beginning of the contact area near the material separation. A second maximum is produced if the minimum cutting depth is exceeded and only plastic and elastic deformation as well as friction take place. The temperature of the workpiece surface is about 350°C. These measured temperatures enable to determine thermal load in the workpiece subsurface [44].

Figure 3.13: Simulated stresses and temperatures 3.3 Energy and temperature During machining processes, the cutting energy is almost completely converted into heat [45]. The mechanisms of material deformation, friction and material removal lead to the initiation of heat sources in the work area. Regarding the surface integrity of machined components, the friction between rake face and the new generated workpiece surface is the most important mechanism.

In 1999, Ueda et al. investigated the influence of cutting parameters and workpiece material on the temperature of the cutting edge Tte (figure 3.14). A two-color pyrometer was used to measure the temperature over the thermal radiation of the tool conducted through a hole of an internal tumed tube. Different kinds of steels in different hardness level have been investigated. It can be

Figure 3.15: Contactless temperature measurement The thermographical measurement of the workpiece surface temperature as shown in figure 3.15 does not allow the measurement of temperature in the contact area tooVworkpiece. Schmidt adapted a simulation program to determine this maximum value of temperature in the new generated surface [44]. Furthermore, he determined thermal power and thermal energy load of the workpiece subsurface in the contact area. Thermal power, thermal energy and maximum temperature for varied cutting parameters such as depth of cut and feed rate are determined with temperature simulation. The thermal power per unit length Pa' is defined as a physical parameter which appoints the maximum temperature in the workpiece subsurface and predicts the appearance of white layers.

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The prediction of temperature maximum has also been tried by Wobker introducing the contact surface related power Pa". pa

=P,p . W lk VB,

. v ,

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With increasing cutting time, the contact surface related power Pa" decreases. This is contradictory to measurements showing higher temperature with increasing flank wear. Subsequently, the correlation between contact surface related power Pa" and the subsurface temperaure is not possible (figure 3.16). wear

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I = 0 1 rnrn dry cuning 312/1957OC 0 IN(

Figure 3.16: Relationship between force contact area and thermal power The relationship between thermal power per length unit and maximum temperature in the subsurface of workpiece is verified taking further results of Brandt and Winands into consideration. Higher temperature generally results in higher residual stress and in the appearance of white layers. Figure 3.17 shows that residual stress measured using x-ray diffraction increases with growth of thermal power per length unit. If the thermal load per length unit exceeds Pa' = 150 W/m white layers appear in the subsurface of the workpieces.

deformations of the tool tip in the contact area of tool and workpiece. Therefore, a high resistance of the contact area against strong impact and stress is necessary. A high hardness to Young's modulus ratio is required in order to minimize the quantity of local plastic deformation after the cutting tool has passed over. Hence follows that the accuracy deviation becomes essential constituent for the finishing of hard materials [31]. Due to the heat generated in the cutting process, the thermal conductivity of the cutting tool material influences the expansion of tool and workpiece. Materials with high thermal conductivity reduce the probability that deviations in the geometric accuracy of the workpiece occur [3] [47]. The high specific forces cause stress on the contact area between tool and workpiece. For this reason, cutting tool materials must have high resistance against mechanical stress combined with a high wear resistance. High stability against abrasive particles in the microstructure of the workpiece material is required to prevent that grooves in the cutting edge develop as well as to promote that even in case of progressing wear the tool tip keeps its original geometrical shape [23]. High thermal stability of the cutting tool material has to be guaranteed reliably because the energy resulting from high specific cutting forces is almost completely transformed into heat so that extremely high process temperatures are produced in the area of the contact zone. 4.2 Applicable cutting materials and their properties As to hard cutting processes, the cutting tool materials require especially high resistant properties against specific cutting forces in connection with process temperatures which are caused by the high deformation resistance of the workpiece material [26] [48]. In figure 4.1, mechanical and thermal properties of different cutting tool materials are given as an overview. Cemented carbides are available in variations with different characteristic properties. The most interesting properties are the high tensile strength and the high fracture toughness. Due to their high hardness, only fine and ultra-fine grained cemented carbides are of interest for hard cutting operations. These cutting tool materials are applied in kinematicly more difficult processes demanding complex tool geometry. These processes are hard drilling and milling operations especially for dies and mold manufacturing [26] [49] [50] [51]. Apart from this, the hardness of cemented carbides is insufficient for hard turning or face milling. The application of cemented carbide tools is also limited by their comparatively low temperature stability.

Figure 3.17: Subsurface quality and friction power per unit length 4 CUTTING MATERIALS AND TOOLS 4.1 Demands and process interference Reflecting upon the surface roughness, surface integrity and workpiece accuracy, demands on hard cutting operations are usually comparable to those on grinding processes. Therefore, certain boundary characteristics and requirements of applicable cutting tools can be defined with regard to the accuracy of the machined surface. High indentation hardness of the cutting tool is required. Usually, it has to be higher than three times the workpiece hardness [31]. In hard cutting operations, this is important in order to prevent

554

The most often applied cutting tool materials for hard turning and face milling operations are AIzOdliCceramics and PCBN [3] [26]. Their high hardness combined with a high temperature stability enables these materials to resist the thermal and mechanical loads in the hard cutting process. A most important difference between ceramics and PCBN is the value of fracture toughness. Compared to ceramics, PCBN-tools are favorable in interrupted cutting operations. Considering the effects on dimensional and form accuracy of the workpiece, a high thermal conductivity and a low thermal expansion coefficient is of importance. Both characteristics favor PCBN as the more adapted tool material for hard cutting processes [20] [52] [53]. The hardness of PCBN is surpassed by PCD. Even at low temperatures, the diffusion ability of carbon in ferrous materials is too high so that it cannot be applied in hard cutting of steel [23].

Keynote Papers

I

cementedcarbide K 10

- density [glcrnl] u s ? __ 30 r"C Cg hardnessHV__ [GPa] z p p Youngs modulus -___ y)

!kn fracture lauohnerr lMPal

140150

1500 1700

I

Ceram,C

I

3850

I

PCD

-

35-42

__

30004500

4000 5000

580480

680-810

3 7-63

6 8-88

1800 2500

_ ____300400

. .

1108

2 0-30

=54

75-80

i

i

3649

I

__

__

590-630

source Sum!lomo. De Been Herlel Sandvik Kwpp Widia

PCBN

3443

I

4249

I

317110775cO IFW 9609

Figure 4.1 : Mechanical and thermal properties of different cutting tool materials [23] 4.3 Wear mechanisms

The cutting processes of hardened steels are characterized by high mechanical and thermal loads that have to be endured by the cutting tool material. As to hard turning processes, hard and ultra-hard cutting materials are therefore required. Usually, pure ceramic cutting tool materials applied in hard cutting do not reveal special advantages [ I ] [I61 [54]. AlzO3-ceramic possesses high chemical stability and hardness but the resistance to fracture is insufficient. Therefore, the main criterion for the end of tool life are micro-chippings on the cutting edge [55]. The higher toughness of SisN4-ceramics can not be utilized in hard turning. Definitely, the hardness is lower compared to aluminum oxide ceramics but the high chemical reactivity of the binder phases lead to dissolution of tool material. For this reason, usually mixed ceramic tools are applied as cutting tool materials for turning hardened steels.

PCBN is the cutting tool material with the longest possible tool life. But the composition of the tool material exerts an influence on the wear mechanisms. The material properties of PCBN can be influenced by the PCBN content, the grain size and distribution as well as the composition of the binder phase which can be ceramic or metallic. For hard turning operations usually ceramic binders are preferred. The range of PCBNcontent in the different tool materials in figure 4.2 is varied in a percentage of 50% to 90%. Even if a high content of PCBN increases the hardness of the cutting material, the tool wear by using these materials is comparable to the A120dTiC-ceramics. The mechanical and thermal loads in the cutting process require additional properties of the tool material, especially at low values on uncut chip thickness. Here, the tool material with a lower content of PCBN shows advantages due to lower thermal conductivity and higher toughness. Furthermore, additional properties of the cutting tool material are of importance, This is based upon possible deviations of tool life in relation to different cutting tool manufacturers. Finally, the highest potential for turning hardened steel is shown by PCBN-tools with a low content of PCBN of small grain size [20] [53] [56] [57]. Figure4.3 exemplifies wear pattern of a PCBN tool characterized by fine scourings. Chips formation exclusively takes place on the chamfer and in the cutting edge radius. The rake face as well as the flank wear land does not show any deep grooves from the cutting action. Especially the cutting edge itself does not show excavations. Such wear pattern indicates that it is possible to achieve high surface quality and size accuracy of the workpiece.

Figure4.2 shows tool life in turning of hardened steel, using different ceramic and PCBN inserts. 100

min

c

Figure 4.3: Wear pattern of a CBN-tool after hard turning

h ( ~ i r c e o 0ranat IFW

Figure

3171iV112cQIFW B E 4

4.2: Efficiency of different cutting tool materials

Sic-whisker reinforced ceramics show potential for the cutting of hardened steel, but compared to Tic-whisker reinforced ceramics, the achieved tool life is low. Here, the micro-resistance of the matrix shows the same wear behavior as pure AlzO3-ceramics. Although the toughness of Sic-whisker reinforced ceramics is higher, this advantage cannot be used in hard machining. The explanation is given facing the chemical resistance of Sic which shows solubility against the machined steel material, so that the effects of the reinforcement are poor. As to the tool wear behavior of SisN4-ceramics, this is similar. Concerning the application of ceramic based tool material, the highest potential for the cutting of hardened steel is shown by Tic-reinforced Alz03-ceramics. Here, a comparatively low rate of flank wear is combined with a high resistance against cutting edge chippings. The comparison between cutting tool materials of different producers reveals just small deviations [3] [26].

Tool geometry determines the contact conditions between tool and workpiece in the shear zone. This exerts an influence on the wear mechanisms because different tool geometry causes specific mechanical and also thermal loads in the contact area. The interaction of the tool wear progression and the cutting forces using tools with different cutting edge radii is shown in figure 4.4 [44] [58]. Generally, the tool wear progression divides into two areas of different gradients. The gradient of the width of flank wear decreases in dependence from increasing cutting time, so that a linear relation does not exist. Tools with lower cutting edge radius show higher wear resistance. These tools allow a 1.4 times higher cutting time until reaching the width of flank wear of VBc = 100 pm, compared to the tool with the higher cutting edge radius. Independent from the tool wear, the cutting forces are on a higher level if using a tool with higher cutting edge radius. Taking the cutting time into account, the tool with the lower cutting edge radius shows smaller flank wear. This advantage is limited by the possibility that cutting edge chippings may occur. A higher cutting edge radius stabilizes the cutting wedge.

555

I

wear

cornponenls of resultant force

-5

150

;up 3

100

8 ; (o

The presence of a flow area of workpiece material also influences the cutting tool material. PCBN supports that boron can be solubilized to form out a boron-oxide film on the tool surface at high cutting temperatures, which can act as a layer decreasing friction. Such boron-oxide films were detected in former investigations [25] [60].

.

-

LL

75

E 8

?i

50

g

25

g U

‘0

25

50

mm

30pm

100

tWpm

30pm

100pm

cutting matenal

flank face wear VB, cutting parameters vc = 150 mlmm f = 0 05 mm ap = 0 05 mm

PCBN CNMA i 2 M o a

dry Culling

cutting time workpiece matenal 16MnCr5lAIS15115 60 62 HRC source J Schmidt IFW

boundary layer between tool and workpiece in the contact area.

A detailed representation of wear patterns on cutting edges recovers the contact conditions at the active surface of cutting tool with highly negative rake angles in hard turning, see figure 4.6.

312125Mx1c Q I F n

Figure 4.4: Influence of tool wear on components of resultant force The SEM pictures in figure 4.5 show the wear pattern of two tools with the cutting edge radii rp = 35 pm and r - 130 pm at the flank wear of VBc = 100 pm reached at Jierent cutting times.

cutting paramelen vC=150m/mln 1 = 0 0 5 m m a p = 0 0 5 m m t = t 2 m m d,yccul 3 1 ~ ~ 7 8m 4 0IFM ~

Figure 4.6: Typical wear pattern of a PCBN-tool and chip in longitudinal microsection

outer diameter turning workpiece 16MnCr5. AIS15115 60-62 HRC cutting t c d PCBN BN 4 dry cutting s o ~ r u tJ Schmidt IFW

loo1 geometry CNMA 120408

1-

cunlng parameters vc = 150 mlmin f =O05mm ap = 0 0 5 mm 31Z2560tc 0 IFW

Figure 4.5: Views of cutting edges with same wear progression In both cases, the wear is very regular and characterized by abrasive and fine scouring areas on the flank. This indicates, that the mechanical and thermal loads are in a suitable range for reliable process conditions. Both tools show crater wear. Concerning the tool with the higher cutting edge radius, the crater wear is localized completely in the area of the cutting edge radius. The tool with the smaller cutting edge radius predominately shows the crater wear in the rake face. This weakens the cutting edge, because the possibility of chippings increases in further use of this tool. The left-sided SEM picture shows a detail of the worn cutting edge with build up layers. These build up layers are confirmed by EDX-investigations and indicate the presence of a flow layer of workpiece material in the area of the cutting edge during the cutting process. The flow layer is generated by the high pressure due to the high mechanical and thermal load in the small contact area between the tool and the workpiece [44]. A model developed for grinding operations confirms that a highly viscous boundary layer between the strongly negative rake angle of the grinding grains is formed out which influences the chip formation [59]. It can be assumed that no material removal does occur if the uncut chip thickness is below a critical value. Until the critical chip thickness is reached, both pressure and temperature in the contact zone increase to rather high values until the temperature of chip formation is reached. There is a “chip formation equilibration temperature” which depends on the main tensile strain of the workpiece material. Such behavior can also be observed in hard cutting processes. Different thermal conductivities of tool materials do not cause different levels of cutting forces so that the conclusion can be drawn that there is a constant temperature of the chip formation [3]. Because of this, it has to be assumed that there is a highly viscous

556

A tool with a high negative cutting edge radius (rll,= 130”) has been selected. There, the characteristic conditions at the cutting edge can be observed in a comparatively wide area. The contact area of tool/workpiece/chip can be divided into five areas [44]. The zone 1 and 5 do not show tribological contact between the workpiece material and the flank of the tool. Also in zone 3 wear cannot be found. It seems that there is no relative velocity between tool and workpiece which indicates that a stable wear preventing flow layer has been formed. Here, the wear protection of possible boron-oxide films is possible. The flank in zone 4 is characterized by scouring areas due to abrasive wear. Elastic and plastic deformed workpiece material slides across the tool area. This zone is beyond the flow layer in the contact zone so wear occurs at the cutting tool. Zone 2 is also characterized by wear due to the material deformation and continuous contact to the chips. The contact zone along the cutting edge radius in the transition region between major and minor flank can be divided into four areas. In zone B, there is intense contact between tool and workpiece. In the further area of the contact arc, the minimum chip thickness is increased and no chip formation occurs. Zone C is characterized by point contact between workpiece-tool material. Because of the elastic deformation of the workpiece, the zone C has a relatively wide tangential extension along the cutting edge. The boundaries to the zones A and D are reached if no contact between tool and workpiece material can be observed. The bottom side of the chip in figure 4.6 shows a strong white layer which is formed almost independent from tool wear. This indicates the strong plastic deformation of the tool material during the chip formation [3] [61]. It c an be estimated that this area contains constituents of the flow layer from the contact zone which is essential for the chip formation in hard cutting processes with negative rake angles. A model is given in figure 4.7. In this model, the uncut chip thickness exceeds the cutting edge radius. In this case, the minimum chip thickness hmnis decreased below the point “I”. Here, chip formation does not occur but elastic and plastic deformation of the workpiece material. A more detailed representation of this behavior is given in the detail “Y’. The point “I” is localized at the cutting edge and defined by the stagnation angle 0 = 25“ = 90”-lyeffl. After the

Keynote Papers

workpiece material has passed the po,int “II”, the elastic proportion Ahel springs back success~velymoving with the tool while behind the point “111” the plastic deformed proportion Ahp[ leads to a lasting deformation. The velocity profile in the flow layer (detail “ X ) is a result of the local temperatures, viscosity and shear stresses in this area [62].

Precision turning lathes with components designed for increased accuracy and high-precision turning lathes correspond with the demands of grinding machines. Typical characteristics of precision and high-precision machines are shown in figure 5.2. The precision lathes are equipped with roller bearing spindles, slide ways and NC-controls with a resolution of 1 pm. Here, many applications reach the accuracy of grinding processes. These values can be quantified in ISO-qualities IT6-7. Surface roughness can be realized in the area between R, = 2-6 pm. Higher demands for workpiece accuracy exceed the range of application of these machines conceptually based on conventional machine components. For high grade applications as e.g. for gear components or roller bearings with dimensional and form accuracy in the range of IT3-5 and surface roughness Rz c 1.5 pm, the precision of hard turning operations was usually insufficient, so the finishing operation must be grinding or honing.

Figure 4.7: Material flow at the cutting edge As shown, a favorable wear behavior of the cutting tool in cutting hardened steel depends on the theoretical base but also on low continuous friction behavior in the contact area between tool and workpiece. Depending on the cutting tool, even cutting edges, homogenous fine grained microstructure of the tool and possibly low cutting edge radius lead to an increase tool wear. Additionally, a small cutting edge radius decreases the extension of the zones 3 and C and supports the chip formation on a wider area on the contact arc. 5 QUALITY OF WORK

5.1 Machine tool requirements Up to now, conventional lathes are usually used for hard turning. This kind of machine tools is often modified in some machine parts to be suited for industrial scale manufacturing. The modification of vital machine components like headstock, tailstock, foundation and slides show higher accuracy, and linear path-measuring systems lead to higher dimension and position accuracy. The application of such types of machine tools enables to reach workpiece qualities in the range of IT6-7 in hard cutting under appropriate condition reliability [63].

Figure 5.1 : Characteristicsof high-precision lathes suited for hard turning Recent efforts in machine tool development lead to new machine concepts of high-precision lathes to improve the quality of hard turned workpieces significantly (see figure 5.1). The accuracy demands on these machine tools are comparable with grinding machine tools concerning static, dynamic and thermal stiffness as well as accuracy of the spindle system and slides [32] [64]. Dimensional shape and accuracy in the range of IT 4-6 becomes possible by the use of high-precision lathes [2].

The substitution of these processes by hard turning requires an adapted machine conception (see figure 5.2). Special attention must be paid to the geometrical and kinematical accuracy as well as on the stiffness of the machine tool. Additionally, the stress relief of the workpiece due to the possibility of sparking-out processes in grinding must be realized by higher precision of hard turning machines. Each deviation of the relative position between cutting tool and workpiece causes inaccuracies of the workpiece [42] [65] [66]. precision lathes

1 high precision lathes

machine type

slant bed 30’ 45’ 60’ vertically i ptck-up

slant bea 4~

machine bed

rneehanttecasl/ stlicateConcrete mineralile ConcrBIe filled gray cast iron

natural granile

spindle drive

wnv bell dnve dired dnve

direct dnve

spindle bearing

preoslon roller bearing (steel ceramic). hydrostatical beanng

hydrosbtial beartng aetostalicalbearing

3500. 10000 t/mw

honzantaie

max revlmin

4000.6300 llmin

power

16/22 - 4 3 5 7 kW (100%/50% WT)

15 75kW

transverse slide

slide way linear roller Slide

hydroslallc siMe

longitudinal slidc

slide way linear roller slide hydrostatic slide

hydroslalic slide

cooling system

spindle cooling circuit (waterlair). 011 coaling urwbt (spmdle slmcl~recolumn)

011 cooling

IT-quality

5 7 mainly 6 (measunng sensor shafl encoder longitudinal. linear scale for plannmg

5 5 (measuring by linear scale) 31324648 0 IFW

Figure 5.2: Characteristics of hard turning lathes Recent developments of high-precision turning lathes adapted to the special demands in hard turning lead to an increase in workpiece quality [42]. These highprecision turning lathes are characterized by hydrostatic guideways as well as spindles and supports which are arranged on both sides of the machine bed of natural granite. Hydrostatic bearing systems embody the possibility to increase the deviation of radial run out of the spindle center axis on values distinctly lower as 1 pm. Furthermore, a sensitive adjustment of position and motion with a resolution in the area of 0.1 pm is enabled preventing stick-slip. To increase the thermal stability of the machine tool, heat producing components like electric and hydraulic devices are located separately from the machine. High-precision turning lathes equipped with components comparable to the technology of grinding machines enable to gain workpiece accuracy of grinding processes by hard turning. In this case it must be mentioned that the reproducibility of higher workpiece accuracy also depends on the tool wear so the constraints of process monitoring is more extensive than in comparable grinding processes. 5.2 Geometric accuracy A main criterion of the workpiece quality especially for finishing processes is the surface roughness. In many applications, conventional lathes deliver sufficient surface qualities in the range of R, = 2-4 prn.

557

the theoretical path has to be implemented in the NCcontrol. With linear interpolation of these bases the required profile can be achieved. wnvex microprofile numenc wntrol

2

0 mm

01

02

6

&

8

12

02

mm

feed f

feed f

6 0 4 2 ~ ~~ j 7 . ~ / ; . I r l s a n j rG 6' j -26O 14"I 5 5 ' / 6' 2 0 mm

cumng 1001 rnatenal PCBN unworn 1001

01

005

0

1

a,=olmm 3131201878

max crowoing 1 6 pm

0

IFW

requ'redraceway profile

workpiece matenal

Figure 5.3: Surface quality on high-precision lathes Higher requirements on surface quality can be fulfilled by the use of high-precision lathes. Figure 5.3 shows that a surface roughness in the range of Rz = 0.5-1 pm can be achieved. Fine grinding or honing qualities are defined up to a surface roughness of R, < 1 pm. Feed rates of f = 0.1 mm deliver surface qualities in hard turning comparable to fine grinding or honing [42] [67]. Besides the surface roughness, for some precision components like bearings or gear parts the surface microprofile is of high importance for the functional behavior. Some of these precision parts are characterized by a very fine and smooth contour profile. Fine grinding machine tools can realize the production of fine contour requirements by highest NC-control resolution and an appropriate dressing of the grinding tool. Due to the fact that hard turning processes only offer limited possibilities in case of selecting the tool geometry, the precision of the surface profile completely depends on the NC-control resolution [42] [68]. To realize sophisticated requirements on microprofiles, the application of high-precision lathes is necessary, as figure 5.4 shows. By the use of conventional lathes, steps in the microprofile in the range of the NC-control resolution occur. In the case shown in figure 5.4, the NC-control resolution is about 1 pm which is a typical value for conventional turning lathes. Using high-precision lathes with an NC-control resolution of about 0.05 pm, single steps cannot be recognized. conventional lathe

high-precisionlathe

NC-control resol~lion1 pm

NCconIrol resoIuI10n 0 05 um

I

1

I

Jf"s.cllr-

I

-

mm

4

005

0

cylindnca roller bearing NU208 nner nng

1 W C 6 AlSl 52100 60 62 HRC wning malerial PCBN tool geometry DNMA 150620

r'i

313i20192e 0 FW

Figure 5.5: Microprofile of a cylindrical roller bearing raceway

One of the main benefits of hard turning is the possibility to avoid the use of cooling lubricants. On the other hand, in the case of dry machining, form errors gain more importance. Regarding hard turning as a finishing process, low tolerances have to be fulfilled. One important source of error that has not been focussed so far is the effect of process heat in hard cutting. The form errors of the workpiece occur due to the thermal expansion of the workpiece and cutting material and lead to a reduction of workpiece diameter in feed direction [69]. Additionally, the thermal effects lead to deviation of parallelism on the surface lines. Since being able to move in steps in the range of 0.05 pm, newly developed high-precision lathes offer the possibility to compensate form errors due to thermal effects. This is a requirement to implement a correction in the NC-control and to receive a smooth profile without recognizable NC-control steps after dry turning. Figure 5.6 gives an example of the compensation of form errors caused by thermal expansion in machining the microprofile of the cylindrical roller bearing mentioned above in figure 5.5.

?+t'::,!:: pfirequ,red racway pmfl1e

canectfon referewe prollle

tml entry d,

-

0495

wne

e

d, = d,

cuning material PCD geometry SPUN 120304

+

10 pm

d,

c u m 9 Parameter vC = 150 d m i n a, = 0 05 mm f =O05mm

workpie~emalerial

CuZn35N12 yellow brass 270

laa1

313122309e 0 IFW

Figure 5.4: Surface microprofile in cone turning Because of the hydrostatic bearing, the ability of the slides to move in steps in the range of 0.05 pm avoiding stick-slip behavior is the pre-condition to achieve a convex microprofile. Due to this property it is possible to generate microprofiles with shape accuracy required e.g. by roller bearings to avoid unfavorable edge pressures. An example for the application of high-precision lathes is the production of an inner ring of a roller bearing with very close tolerances for the profile contour. As figure 5.5 shows, a smooth profile without recognizable NC-controlsteps can be realized after machining. Hence, cylindrical roller bearings permit the use of hard turning as a finishing process. In order to machine this microprofile, 558

wohpnu,

L C d

c y l h d m l mller beannp NU208 inner r q

cutllng mslenal PCBN

geanetry

DNMA 150616 S

VB, = I00 prn

d,

= d,

f

K

-.

arlbnp paramelw vC i t50mlmin ap = 0 05 mrn

WkplBCemaIalal tCCC6 AlSl 52100 6042 HRC

I =005mm

313122310e 0 IFW

Figure 5.6: Compensation of form errors due to thermal expansion For the compensation, a correction reference profile with different gradients described by the correction factor K is used. Lower or higher K-factors lead to unallowable deviations of parallelism. Thus, by using high-precision lathes it is possible to compensate form errors, above all in case of dry machining, to fulfill the high requirements of workpiece quality. A more common way to increase workpiece quality preventing form errors due to thermal effects is the use of cooling lubricants. In figure 5.7, the differences in dimensional accuracy between dry and coolant supported cutting processes are shown [69]. The thermal expansion of tool and workpiece material causes a diameter deviation in a range of 12 pm

Keynote Papers

respecting a feed length of 40 mm. Respecting the thermal expansion of the tool and workpiece material, the application of cooling lubricants exerts a positive effect, so that no deviation of diameter does occur during the cutting. This means that the dimensional accuracy is not infected by the process heat, and a compensation of the NC-control is not necessary. Hence, the use of cooling lubricants is recommended if the control resolution is not sufficient to compensate form errors.

vc = 180 mlmin f =008mm

ap=02rnm 1wI matenal

A a g n e u c chuck

lml geometry CNMA 120412 /_I VB, = 0.09 rnm

workpiece: 1W)Cffi. AlSl 52100 62 HRC workpiece geameQy

11

1

Do=lOOmm D,= 80 mm

b sourca J Liarmann

10

20

.

, . mm

I

40

feed length

friction. This is indicated by higher cutting forces which usually increase linear to the tool wear. Here, the back force shows the steepest gradient. The development of structural changes in connection with increasing tool wear is compiled in figure 5.9 [70] [71]. In relation to the tool wear, a white layer connected with a tempering zone is formed out. In this rehardened zone, an increased hardness of approximately 1000 HV0.025 can be found in contrast to the hardness of 800 HV00.25 in the primary structure [70]. Depending on increasing flank wear land VBc, the back force and the structural changes increase. Furthermore, in all cases tensile residual stresses are found directly on the surface. The amount of tool wear leads to an increase of tensile residual stresses, here up to 600 MPa for VB, = 200 pm. Not only the surface value of residual stresses is influenced, but also depth profiles are measured in order to investigate residual stresses in deeper heat affected material regions, see figure 5.10. Depending on the tool wear, the residual stresses beneath the machined surface are measured from the surface to a depth of 90 pm. The cutting tool material has no significant influence on the depth profiles, the crucial criterion is the tool wear.

Figure 5.7: Form errors in hard turning due to thermal effects 5.3 Physical properties The functional characteristics of a machined part essentially consist of volume effects, workpiece geometry and surface roughness as well as surface integrity. Especially geometry and surface attributes are strongly influenced by the cutting process. Regarding hard cutting is a finishing process, the influence on the machined part is very important for the functional behavior of the workpiece. To achieve high applicability of machined parts, the final cutting operation must be carried out with small cutting depth. So, the influence of mechanical and thermal loads on the functional characteristics of the workpiece is decreased. Therefore, deviations in dimensions should be reduced due to low cutting forces, simultaneously low values of surface roughness can be achieved and influences on the surface integrity are minimized.

Main criteria of the functional workpiece quality are the geometrical surface integrity (surface roughness, dimensional and form accuracy) and the physical surface integrity (microstructure, hardness, residual stresses), see figure 5.8.

Figure 5.9: Surface structure as a function of tool wear With increasing flank wear, residual stresses on the surface shift to tensile range. Close beneath the surface, compressive residual stresses are detected. The maximum level occurs in small depth. Increasing flank wear shifts the maximum to deeper material regions. Tensile stresses at the surface are caused by thermal influence due to friction and plastic deformation. On the other hand, mechanical loads lead to compressive residual stresses which admittedly influence deeper material regions [3] [36] [68] [72] [73] [74] [75]. 800 MPa Mx)

400

200

0 200 -400

'

0

1 10

20

30

40

50

I 60

. 70

l pm

90

depth yxim 0 Brandl IFW

Figure 5.8: Stress distribution and surface integrity in hard cutting Due to the combination of extremely high temperatures and mechanical loads in the contact zone, the austenite temperature of the workpiece material is reached in contact times of approximately 0.1 ms. The workpiece material is self-quenched, structural changes will have to be expected and residual stresses influence the surface integrity. In this case, the relation to the tool wear must be taken into account. Increased tool wear leads to higher thermal process energy related to increased

317111038~0IFW 9113

Figure 5.10: Residual stress as a function of tool wear Besides the influence on surface integrity in case of residual stresses, the formation of white layers characterizes the surface of hard cutting processes. In order to discuss the mechanisms of transformation in the material structure, it is necessary to analyse the material composition and structure. The distribution of the chemical elements near the surface is an important criterion for the examination of the effects during the turning process. In case of hard

559

turning, it is of interest whether the development of white layers changes the distribution of chemical elements in the transition region to the annealing zone compared to the crude material structure. Figure 5.1 1 shows the results of measurements with optical glow-discharge spectroscopy on a workpiece. workpiece material 15 MnCr S5 (ASTM 5115) 50 62HRC

9.

s,

1

= 145 rn rnin =01mm

a,=02mm

measurement technique

glow discharge aplical emlssion spdrometty

317112985 0 IFLV

Figure 5.1 1: Element distribution in hard turned surface The white layers with a thickness of 10 pm are produced by a worn tool of mixed ceramic (VBc= 280 pm). All elements which have been examined (Mn, Cr, C, Si, S) are equally distributed related to the removed workpiece material, and neither in the zone near the white layer nor in the other material structure changes in the distribution can be found. References for diffusion of elements or chemical reactions can not be derived. Figure 5.12 compares the result of EDX-examinations of workpieces which have been machined with differently worn PCBN-tools. Additionally, the microstructure of subsurface area and the remaining austenite is determined and compared to the crude workpiece material. If the tool wear is low, the microstructure can be compared to the unmachined part. The amount of remaining austenite does not essentially differ from this part. But as soon as the white layers are formed, a considerable increase of the amount of austenite can be observed in the subsurface. On the confirmed supposition that the measured values have mostly been taken within the area of the white layer, the percentage by mass of austenite has been 73%. White layers which have been gained by means of hard turning consist of about 213 of austenite notwithstanding the applied cutting tool material.

surface of hard-turned workpieces with white layers is, as examinations confirm, not reduced. In order to receive white layers by the machining of hard turned materials, two conditions must be given: The contact temperature must exceed austenizing level of the workpiece material. The cooling of the surface area must take place in very short time. Therefore, a description of the mechanism about the formation of white layers must be able to explain the extremely fast heating phase which is connected with austenization. In hard cutting, the contact time between tool and workpiece is often less than 0.1 ms even in case of increasing tool wear. For this reason, the conditions leading to changes in the material structure fundamentally differ from the conditions in the grinding operations. In hard turning, the contact time is very short as well as the time for heat conduction. Besides, just a single contact takes place between tool and regarded element on the workpiece surface. Based on these conditions, the reason for the high temperatures of the machining process and the short time of heating must be a combination between friction and plastic deformation. Plastic deformations can produce a high amount of heat in a short time which also heats up the workpiece. Moreover, the time which is necessary to austenize is considerably short. The following thesis resulting from these reflections will be discussed now: "In hard turning without plastic deformations no white layers are produced." Comparing the processes on the rake face and flank face in case of different tool wear, the absolute and specific effective forces are on a similar level. Merely considering chip formation and chip deflection in the contact area, the wear of the flank exerts just small influence. This can be regarded in the shape of the chip bottom sides in the chip roots of figure 5.13 where significant white areas are formed out. Here, the high process temperature in combination with the high grade of plastic deformation is significant [3] [76].

Figure 5.13: Chip roots in hard turning depending on tool wear Figure 5.12: Relation between subsurface microstucture and austenite content White layers consist of over 60% austenite which is a non-etching white component in contradiction to the dark martensite scorching. The martensite components also show a mostly tetragonal twisted lattice, and can hardly be etched. The fact must be emphasized that no amorphous layer exists. Apart from the EDXexamination, additional links come from metallographical analyses. Another argument confirming that white layers are crystal-structured is the following: Amorphous material is not conductive, but the conductivity of the

560

Regarding the subsurface of the workpiece material after turning, the formation of white layers significantly depends on the tool wear. The plastic deformation caused by unworn tools appears as material separation directly in the chip formation zone. Increased tool wear changes the deformation conditions in the contact area as well as at the region of the tool flank. The effects of wear cause increased upsetting deformation and chip deflection as well as shear by friction. Thus, the increase of the wear of the tool flank influences the formation of the workpiece subsurface zone with respect to two main aspects:

Keynote Papers

Increased frictional heat is caused by extended contact time due to geometrical change of the flank of the tool. Plastic deformations lead to higher temperatures and support the lattice shearing so the austenization of the workpiece material is supported in the machined subsurface area.

these conditions a white layer of small thickness is generated. Interesting conditions of obseyation are the beginning of rolling contact load 9 N = 10 overruns and at the end of the test at N = 5 x 10 overruns.

The combination of high plastic deformations and extreme short heat up times cause the lattice shearing with the formation of a very fine austenitic grain structure. Grain growth is almost impossible due to the fast chilling after tool contact. The combination of martensite and extremely fine grained austenite structure constituents causes the high hardness in the area of the white layer. Due to the microstructural changes during hard cutting, the surface integrity is affected. The residual stresses of the austenite material components are clearly shifted towards compressive strain. In the white layer, measurements of martensite show more than 600 MPa, the dominating austenite is only about 150 MPa (figure 5.14).

Figure 5.15: Microstructure after hard turning and roller contact fatigue test

Figure 5.14: Structure and residual stress of white layers

The rolling contact load does not show an influence on the microstructurewithin the first N = lo4 ovejruns. At the end of the rolling contact test (N = 5 x 10 overruns), petty damages in the guide ways can be observed. The micrographic analysis of the stressed area reveals the damages as planishing and removal of material. In macroscopic view, the damages are localized as lines of pittings following the peeks of the feed marks which occur in turning. Additional tests using tools with increased wear rate or higher feed rates for preparation of the test samples showed that the origin of pittingappearance is connected with the geometry of feed mark and the highest surface roughness peaks. This indicates that by the mere presence of white layers, the surface integrity is of minor relevance for roller bearing fatigue life of machined parts. Of essential effect is the surface geometry the latter mainly influenced by the feed and wear rate in the production process.

The martensite tensile residual stresses are caused by lower specific volume and consequently higher density of the austenite. The surrounding martensite is obligatory set under tensile stress while the austenite is forming. Because white layers only occur directly on the surface, the tensile residual stresses are restricted to the boundary layer of the workpiece, too. Under the surface and as a consequence of high mechanical stress, compressive residual stress occurs. Its maximum is localized under the surface. From these overlaying influences, the curve of the residual stress which is typical for hard turned work pieces with increasing tool wear is derived.

Higher demands on surfaces in rolling contact loads are given if slip contact is possible. In figure 5.16, the effects of slip-load in rolling contact is described. The slip free contact allows the achievement of the defined roller contact fatigue life even if the test sample is turned with the relatively high feed rate o f f = 0.15 mm and a tool with increased wear (VB, = 200 pm).

5.4 Functional behavior The reliable behavior of machined components, especially the functional surfaces, is an important criterion in many technical applications. Hard cutting processes have decisive influence on the surface integrity. This affects the fatigue behavior and is a crucial aspect for the application. Important applications are rolling contact and durable alteration of loads [74].

In motive application, the rolling load of the surfaces of components is common. According to this, the evaluation of rolling fatigue life concerning hard turned surfaces is a decisive prerequisite for applicability. An adapted testing method represents the long time investigation in roller type test stands [77] [78]. Test conditions for rolling fatigue life are performed at a Hertzian stress of po = 3600 MPa. The limit yf fatigue strength is achieved at a number of N = 5 x 10 overruns in case the area of pittings is smaller than 1% of the contact area. Figure 5.15 shows the influence on the subsurface microstructure in rolling contact loads. The original functional surfaces represented by the sample with N = 0 overruns have been turned with a wear affected cutting tool (VB, = 100 vm) and a feed rate o f f = 0.1 mm. Due to

Figure 5.16: Surface topography after rolling contact testings concerning slip-load When slip load occurs, the samples parts fail, even if surface roughness of the sample is decreased by the reduced feed rate to f = 0.1 mm. Tolerable surface damage in the rolling contact fatigue life tests under slip load is achieved by turning the test sample with low wear and feed rates. Here, the surface roughness of the roller exercises the main influence on the rolling contact fatigue life. Besides microstructural influences from hard turning, residual stresses in the subsurface area of functional surface are of high interest concerning the fatigue life of components. Figure 5.17 describes the development of 561

On sample pieces turned with worn tools, significant differences in the fatigue life could not be found, see figure 5.19.

residual stresses in dependence on the load time in the roller time test stand. The samples were produced concerning low feed (f = 0.05 mm) and tool wear (VBc= 0.1 mm), slip and slip-free contact conditions can be observed.

8W

The slip-contact conditions show that surface tensile stresses parallel to the cutting direction 011 decrease strongly after the first overruns so the area of compressive stresses is reached very soon. In contrast to this, the reduction of tensile residual stress 011 in slipfree load is small. The residual stresses perpendicular to the cutting direction were originally in the compressive area and change just slightly in compressive direction. In case of roller contact loads, residual stresses in hard turned surfaces change to values in direction of compressive stresses which is known for its positive influence in case of the fatigue live of components.

d

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U 0

n

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load time t

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Figure 5.19: Oscillation fatigue life of hard turned samples

sample matenat lOOC6 AlSl52100 6 M 2 HRC

SllP

1o5

Figure 5.1 7: Surface residual stresses depending on load time in roller contact load Another important component characteristic is the fatigue strength against bending or oscillating loads. Here, sensitive influences from the area of the subsurface, surface topography, microstructure and residual stresses are known. As an example, case hardened components are common in automotive applications and the reliability is very important. Bending fatigue strength is a main criterion for components because the maximum of the load directly occurs in the surface, the machined area. Here, the changes of the surface integrity are also lying on a maximum. In this case, the subsurface residual stresses caused by hard machining are of special meaning.

In contrast to this, sample pieces produced with unworn tools which cause more compressive residual stresses in the subsurface show much higher values of oscillation fatigue strength. Here, the striking decrease of fatigue strength that characterizes the samples which have been turned with worn tool cannot be found. The examination of a relation to the surface roughness of the samples did not show significant influence on the fatigue strength of oscillating loads, in contrast to the rolling contact fatigue life.

As shown in figure 5.20, on characteristic fracture surfaces wedge shaped fissures occur on the sample pieces which show tensile residual stresses. The fracture shape of sample pieces characterized by compressive residual subsurface stresses are very homogeneous so that consequently premature failures can be avoided. For this reason, the requirements for hard cutting processes to achieve components with high oscillating fatigue strength have to be chosen in the way that compressive subsurface residual stresses are generated.

For investigations on the influence of hard turning to bending fatigue life, sample pieces with different residual stresses by the turning processes were selected. Stress cycle diagrams of a Wohler fatigue test reveal the influence of hard cutting conditions to the oscillating fatigue strength, see figure 5.18.

Figure 5.20: Surfaces of fracture of hard turned Wohlerfatigue tested sample pieces 6 POTENTIAL OF HARD CUTTING

0 source

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TU ciausma1

10’

10’ oscillation number

106

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10’ 31326M3 0 IFW

Figure 5.18: Residual stress in relation to the oscillation number The results of the investigations show that the residual stresses are almost not influenced by the number of oscillating loads. A reduction of residual stresses as shown in rolling contact loads does not occur.

562

6.1 Economics Apart from further influencing criteria, the material removal rate is a most important economical aspect to evaluate the productivity of a cutting process. For finishing operations, additionally the generated surfaces are of high importance. Figure 6.1 shows the relations between machining conditions and material removal rate. In grinding, the defining quantity is the width related volume rase. For finishing, it might be less than Q ’, = 1 mm @ms, in roughing high values of more than Q’, = 20 mm lmms can be reached. Feasible process removal rates are able to achieve values exceeding 200 mm3/mms. From these values, the derived material removal rates Q, for grinding are at least as high as in hard cutting. Besides, the volume related material

Keynote Papers

removal rate A,. is an important criterion concerning productivity. It may be implied from the calculation that in grinding the surface rate is superior to hard cutting. This points out that the grinding time is often shorter than the cutting time in turning [42]. I

I

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I

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characteristic parameter

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3

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2 - 12 mm lqms

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6.2 Environmental aspects The following example shows the ecological benefits of cutting compared to grinding, (figure 6.3). Assumed that 5000 parts of a gear component have to be produced per year, approximately 50 kg chips will result from the overmeasure machining. This is independent from the chosen manufacturing process.

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1 2 5 - M O m m lo

An additional bonus of hard turning is avoiding cutting fluids. The possibility of dry machining means saving considerable costs otherwise caused by buying, monitoring, treatment and disposal of cutting fluids.

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8

However, in grinding a consumption of coolant has to be considered additionally which reaches up to 8 tons per year. Furthermore, degendent on grinding wheel composition, about 20 cm of abrasive and bond particles are generated during machining and dressing. These particles are mixed with coolant, chips and filter material. In industrial application, it is nearly impossible to separate these materials. Thus, the waste consists of a variety of different, usually detrimental to health and environmentally harmful waste materials, the whole amount has to be disposed under special security conditions. \

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Figure 6.2: Turning versus grinding - hydraulic component Compared to the former production by grinding operations, the production sequence is reduced noticeably. Only one machine tool and one single set up is necessary. Workpiece quality achieved in hard turning is at least at the level of the grinding operation. Significant improvements can be seen concerning production times and costs. For instance machining time for batch size is about 60% shorter compared to grinding. In comparison to hard turning, grinding is a more complex process. The repeated conditioning of the grinding wheel occupies the machine itself while tool preparation in hard turning can be done off-line.

Cutting leads to more favorable conditions. Due to the possibility of dry machining, there are only chips consisting of not contaminated workpiece material that can easily be recycled. Very small amounts of tool material are to be neglected because they are dissolving easily in the steel matrix. Tool can be disposed (ceramics) or reused after sharpening, but there is no mixture of different materials. Thus, cutting of hard materials can be considered as a very efficient possibility for protection of environment. 7 SUMMARY First of all, the availability of hard and super hard cutting tool materials enabled to machine hardened steels. Investigations in the cutting tool sector accompanied with developments and design of machine tools supports the successful story of this relatively new production technology.

Independent from cutting parameters high compressive stresses due to relatively high rake angles are adjusted in the tool tip region. High hydrostatic pressure is defined to be the most important physical quantity of plastic deformation of hardened ferrous materials which is essential for manufacturing technical surfaces [23]. By increasing the distance to the tool tip compressive stress is increased [25] and plastic deformation cannot take place. Shear localization occurs and results in segmented chip formation. Local thermal softness, crack initiation in the free workpiece surface and microcracks in the shear zone represent the most denunciated phenomena initiating shear localization [12] [26] [30] [31] [32] [34]. In literature, different theories exist concerning the crack growth. Crack initiation and crack growth are subject to temperature distribution, stress distribution, strain and strain rate in the work area, and on time and location. Depending on resulting time and location 563

dependent thermo-mechanical loads, crack initiation and crack growth is decisively influenced. If the workpiece hardness is increased in a range up 50 HRC, the machining force and the energy consumption are increased [36][39].The increase of converted energy in the work area results in higher temperatures and influences the thermal load of the tool and of the workpiece subsurface [44][46].The passive force F is the largest cutting force component. With an increashg cutting length leading to an increasing width of flank wear, a significant rise of the passive force F can be observed. Consequently, the force ratio FJP, is reversed in contrast to conventional cutting [42]. Newly developed high precision lathes offer the possibility to improve the quality of hard turned workpiece significantly. Turning tests on roller bearings with high demands on surface roughness and shape accuracy show the potential of precision hard turning. Due to the ability of high-precision lathes to move in steps less than 1 pm especially fine and smooth microprofiles can be realized which are necessary for some precision parts. Furthermore, compensation of form errors as a result of thermal expansion of the workpiece becomes possible. Changes of boundary conditions require the consideration of ecological aspects in manufacturing. In order to promote the development of new manufacturing concepts, detailed analyses of single processes as well as whole production chains are necessary although in future, the acceptance of ecological manufacturing technologies will depend on their economical benefits. Using dry machining, the environmental pollution as well as the production costs can be decreased. For this, necessarily a consequent adaptation of tools and machines is required. Every application has to be checked for economical suitability. A comparison between hard cutting and grinding shows that the first offers several important advantages. Different surfaces and shapes can be formed applying only a single tool, and the surface quality is as well comparable, as might be shown in finishing of a hydraulic component. Therefore, hard turning turns out to have high potential to replace grinding operations. 8 REFERENCES

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Elbestawi, M. A., Srivastava, A. K., El-Wardany, T. I.: A Model For Chip Formation During Machining of Hardened Steel, Annals of the ClRP 45 (1996)1, pp. 71-76

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564

Liedke, D., Jonsson, R.: Warmebehandlung Grundlagen und Anwendungen fur Eisenwerkstoffe. Expert Verlag, Ehningen, 1991

Keynote Papers

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[42] Toenshoff, H. K.: Environmentaland Ecological Aspects of Cutting -State, Developments, Potential, I.C.E.M. Meeting, Ecology and Economy in Metal Forming and Cutting, s'-Hetogenbosch, Netherlands 1997

[62] Schaller, E.: Beitrag zur Untersuchung von Spannungen und dynamischen Vorgangen in der Grenzschicht zwischen Werkzeug und Span bei der Zerspanung mit Hartmetallwerkzeugen.Dr.-lng. Diss. RTWH Aachen 1964

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[63] Toenshoff, H.K., Karpuschewski, B., Borbe, C.: Prazisions-Hgrtdrehenvon Walzlagerringen. 12.-14. Mai 1997, 14 International Plansee Seminar '97, Reutte, Tirol

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[46] Ueda, T., Al Huda, M., Yamada, K., Nakayama, K.: Temperature Measurement of CBN Tool in Turning of Hardened Steel: Annals of the CIRP, 48 (1999) 1, pp. 63-66

[66] Martens, M., van Luttervelt, C. A.: Precisjeharddraaien in de kleinseriefabricage. MB Jg. 64, No. 617 1998

[47] Winands, N.: Hartdrehen aus der Umformwarme geharteter Walzlagerringe. Dr.-lng. Dissertation RWTH Aachen, 1996

[67] Davies, M. A., Burns, T.J., Evans C. J.: On the Chip Morphology, Tool Wear and Cutting Mechanics in Finish Hard Turning. Annals of the ClRP Vol. 451111996 pp. 77-82

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