Hard Turning of High Speed Steel
Turning instead of grinding
The conventional process chain for the fabrication of components with a high hardness is composed of a sequence of several manufacturing techniques and thus includes high processing times. If products of mould and tool construction are pre-machined, while being in a malleable state, clamping, hardening, unclamping together with final processing, such as grinding and eroding, incur great expenses as a result of logistics, time and accuracy requirements. Concerning the finishing processes of steel materials with hardness value in excess of 60 HRC, grinding or EDM are mostly applied. The fact that turning is also applicable for steels of a high hardness permits a higher productivity since a complete treatment is manageable within one clamping operation. Furthermore, the time-consuming and cost-intensive grinding- and EDM-processes, which are limited by geometrical inflexibilities and low removal rate can be substituted (Figure 1).
User define goals
In order to use entirely and to disseminate new tool materials, not only economic manufacture, but also a process technology, rendering the production of complex geometries - provided that costs, time, and quality are optimised - possible, is strongly demanded. Accordingly, the Institute for Machine Tools and Factory Management (IWF) was aiming to develop a manufacturing strategy including cutting edge material, tool geometry, and kinematic machining parameters in order to facilitate a reliable and economical hard turning process for materials with a hardness value in the range of 65 HRC. As a first step, kinematic cutting parameters had to be defined for every examined cutting edge material group. The process optimisation regarding several goal criteria was accomplished based on very few publications, focussing on turning of high-hardness steels, and on cutting data recommended by tool manufacturers for hard machining up to a hardness value of 60 HRC. Since the industrial partner SWZ Zella-Mehlis GmbH defined the application area being parts of large broaching tools (Figure 2), that prospectively will be manufactured through hard turning instead of grinding, the reliable hard machining of turning elements with diameters up to d = 150 mm and feed lengths up to lf = 350 mm had priority. In order to replace the cost- and time- intensive grinding operation, several tool geometries and cutting parameters were chosen, based on the criterion that high surface qualities comparable to grinding operations will be accomplished. Moreover, the examined tools were also evaluated in respect of their economic efficiency by comparing their maximum material removal V.
Turning of hardened powder metallurgical steel
A vital aim of this study was to ensure that the acquired solutions are not only suitable for niche applications, but also serve as an economic alternative especially to the processing of hard-to-cut hardened steel materials. Those already are, or will be of economic significance due to their range of application. As a result of increasing requirements by the constructers, driven by high innovation pressure, the focus in the area of tool material development of high performance steels is on the constant improvement of alloy components and combinations together with the production methods. The powder metallurgy production is considered to be a key method for the manufacture of homogenous fine disperses structures. The result of this new production technology is the raise of viscosity while still maintaining the typical high hardness and a virtually isotropic material composition. In contrast to conventionally fabricated steel, in the production of powder metallurgical steel, a steel melt is atomized with the aid of an inert gas, filled up in capsules, and hot isostatically compressed [1, 2]. The powder metallurgical High Speed Steel (HSS) examined in this study is HS6-5-3, material number 1.3344 (also referred to as ASP23, PM23) with an application-typical core hardness of 65 ± 3 HRC. It is used as a cutting tool material for cutting tools and increasingly likewise applied for the mould production in the manufacturing of matrices and stamps. The hardness measurements of the external zone of HS6-5-3 ,realised at the IWF, revealed a Vickers hardness between 1000 ... 1100 HV0.1, which is equivalent to a Rockwell-hardness of more than 70 HRC. Scientific paper and cutting data recommendations by tool and tool material manufacturers indicate that the turning treatment of these materials is manageable for hardnesses up to approximately 60 HRC, on condition that the appropriate cutting tool materials are employed. In this connection, it is a fact that the separation process of this hard workpiece material causes high loads in the cutting tool during turning process. Reasons for that are high hydrostatic compressive stresses that are not only caused by the high material hardness, but also by the typical cutting edge geometries, used in the field of hard machining. Because of these loads, the flow of the material and thereby the chip formation is induced. In order to obtain a preferably stable wedge and thus prevent tool failure, indexable inserts with large corner radii, wedge angles, and also chamfers, or roundings are applied. During hard turning processes, the cutting tool is subject to additional thermal and mechanical loads since the depths of cut are in the same range as the protective chamfers. Hence, the typically slightly negatively adjusted chip angle is added to the chamfer angle, resulting in a highly negative effective chip angle [3, 4, 5].
Cutting tool materials for hard cutting processes
Hardened steel workpieces are mainly turned by super hard cutting tool materials, such as PcBN and mixed ceramics. Reasons for that are the extremely high demands on the hot hardness and the diffusion resistance of the cutting material, that are a consequence of the high temperatures and cutting forces within the cutting zone. Even though Polycrystalline diamond is ranked among the hardest cutting tool materials, it is unsuitable for the hard machining because of its affinity to iron. Whereas cemented carbides were primarily used for drilling and thread cutting in the past, nowadays they are increasingly deployed for hard turning and hard milling processes. This is possible due to improvements of their thermal and mechanical features as well as the availability of new high-performance coatings. The major disadvantages compared to high-hardness cutting tools are low practicable cutting speeds and low tool life [5, 6, 7]. As a part of the cutting tests, the wear behaviour of commercially available tools was examined during hard turning of powder metallurgical HSS. For this purpose, cutting tool materials and tool geometries, which are explicitly recommended for the turning of hardened materials by the suppliers, were chosen. In this connection, mixed ceramics, PcBN, and coated cemented carbide tools were applied. Furthermore, nanocomposite-coated PcBN tools were employed and analyzed since they are currently intensively examined within the scope of an IWF project. On that account, several PVD-coatings were deposited on different PcBN tools in order to analyse performance during hard machining . The cutting tool materials for the machining of HS6-5-3, which have been examined at the IWF, are listed in table 1.
The applied machining process was cylindrical turning, where a maximum width of flank wear land was set to VBmax = 0.3 mm. The initial diameter of the cylinder amounted to 150 mm. In the context of the investigations, the flank wear VB, the mechanisms of wear and the wear form, such as complete breakaways and cutting edge chipping, were analysed. In addition the achieved surface roughnesses on the workpiece were determined. These criteria are essential for the evaluation of the performance of the cutting edge regarding the material as well as the geometry. The cutting tests were carried out under dry cutting conditions on the universal lathe by DMG Mori, Bielefeld, Germany, of the type CTX gamma 1250TC. Every utilised tool had a tool orthogonal clearance of αo = 6° and a tool orthogonal rake angle of γo = -6°. A summary of the cutting tests with different cutting tool materials with regard to the tool life volume V as well as the material removal rate Qw is shown in Figure 3.
All PcBN-tools, analyzed in the experiments, had a chamfered cutting edge (chamfer width bγ = 100 µm). The PcBN-tip of the type BL - 49 (Kieninger Technologie GmbH) and nACo are soldered onto cemented carbide. As opposed to this the tool Type CB7025 show a complete corner consisting of PcBN of the cutting edge with a wiper geometry (Sandvik Coromant). The PcBN-tools were tested with a cutting speed of vc = 150 m/min, a feed of f = 0.15 mm, and a depth of cut of ap = 0.25 mm. The results show that the PcBN-cutting edge type BL 49 acquired an average tool life volume of V = 17 cm³. A rapidly increasing, constantly growing wear progress was noticed, leading to cutting edge chipping. Due to the high thermal and mechanical loads in dry machining of the hardened steel, the uncoated PCBN grade showed significant signs of tribo-oxidation and abrasive wear. In particular, the crater wear lead to abrupt tool failure of the uncoated cutting tool. On the contrary, the nACo-coated PcBN-cutting edge has shown a constantly growing wear progress, without any breakaways. With the aid of the nACo-coating, a tool life volume of V = 27 cm³ could be achieved, representing an increase of about 60 % compared to the uncoated tool. Since high process temperatures occur in the chip formation zone during turning of hardened HSS, the heat resistance of the cutting tool material and the coating are of high importance. Due to the nanocomposite coating the heat and wear resistance of the PcBN-substrate could be increased and preserved against tribo-chemical wear, which leads to the improvement of tool life. The highest tool life volume of V = 36 cm³ was accomplished by using the tool type CB 7025 (wiper geometry), and increased tool life by 110 % compared to uncoated PcBN. Concerning the wear forms, no significant differences to the other PcBN-tools have been determined. A constantly growing crater and flank wear progress as well as coating delamination on the rake face were observed. The increased tool life volume compared to other examined PcBN-cutting inserts is attributed to the specific micro geometry (wiper). Furthermore, it is assumed that the achieved improvement is due to the fixation system of the PcBN-cutting edge. Every applied ceramic tool was chamfered with bγ = 0.1 mm and an angle of chamfer γf = 20°. Through preliminary tests for the process configuration, a cutting speed of vc = 60 m/min, a feed of f = 0.1 mm, and a cutting depth of ap = 0.1 mm was acquired for these tools. The whisker-reinforced aluminum oxide ceramic type KY4300 (Kennametal) accomplished the highest tool life volume of V = 55 cm³ of all examined tools. Furthermore, high resistance to crater wear and also high cutting edge stability were noticed. Typical wear characteristics contained a continuously growing flank wear and a large-area notch wear along the minor cutting edge because of the high mechanical and thermal loads. This type of wear occurs in consequence of high passive forces generated in hard machining . The aluminum-oxide-based mixed ceramic CC650 by Sandvik Coromant merely acquired a tool life volume of V = 24 cm, representing approximately 40 % of the tool life volume of the KY4300 cutting edge. In the wake of breakaways of the rake face, due to brittle cutting tool material properties, the tool failed abruptly. Moreover, notch wear developed on the minor cutting edge. The coated cemented carbide cutting edge produced by Sandvik Coromant (wiper geometry, CVD-TiCN-Al2O3 coating) with chip breaker accomplishes a maximum tool life volume of V = 14 cm³ due to its adapted cutting speed of vc = 25 m/min, feed of f = 0.25 mm, and cutting depth of ap = 0.2 mm. These cutting parameters were also predominantly determined in respect of their reliability. Nevertheless, in the field of hard machining, the chip breaker has to be rated critically since it weakens the wedge angle. Additionally, it often leads to breakaways at the corner in connection with high cutting forces. The low heat resistance of the cemented carbide substrate and the observed coating delaminations resulted in a change of micro geometry of the cutting edge and massive abrasive wear, reflected in flank and crater wear. Finally, this led to the end of tool life. The material removal rate Qw, expressing a characteristic variable for the efficiency of the cutting process regardless of the accomplished tool life time or tool expenses, is calculated with the help of kinematic cutting parameters. The comparison of material removal rates of different tools, provided that all cutting parameters are optimized, revealed that it is highly productive to cut HSS with PcBN-materials (fig. 3B). The application of ceramic and cemented carbide indexable inserts is due to their low heat resistance only reliable if the cutting speeds and feed rates are distinctly reduced, causing in turn low material removal rates. Studies, in which parameters were applied that are usually adapted for PcBN-tools, exposed that the ceramic cutting material was overstressed and thus concluded with early non-reproducible tool failure. Figure 4 illustrates the generated surface qualities after the cutting process, using different cutting tool materials. In the upper chart in figure 4A, the arithmetical mean deviation of the assessed profile Ra after the first 30 seconds and at the end of tool life is plotted. Accordingly, in the lower chart 4B, the results concerning the maximum height of profile Rz can be found. It is noticeable that both ceramic tools exhibit the lowest surface qualities at the end of tool life. This circumstance is linked to the type of wear. The changes in the accurately defined shape of the cutting edge, due to the partially large-area breakaways, have an impact on the surface quality. Not only the coated, but also the uncoated PcBN-cutting edges generated conspicuously low surface qualities. However, they slightly improved while approaching the end of tool life, which is explained by the changing tool geometry in consequence of the crater wear. The nACo-coated indexable insert shows a slightly better performance compared to the uncoated PcBN-cutting edge (BL 49). An explanation for that are the advantageous friction properties of the nanocomposite coating at high temperatures . High surface qualities were acquired through the application of the coated cemented carbide cutting edge and the PcBN-cutting edge, type CB7025, both corners featuring wiper geometry. Further, the constant data for the CB7025-cutting edge from the lead till the wear criterion was reached is remarkable. With a roughness values Ra of around 0.25 µm, a turning process could be implemented, accomplishing surface qualities similar to a grinding treatment.
Cutting edge geometry is essential
The presented results point out that an adapted tool geometry is of a particular importance in the field of hard machining due to the high mechanical loads. From protective chamfers through to wiper geometries and complex rake face geometries are the state of the art in the scope of coated cemented carbide tools. Concerning PcBN and ceramic complex shaped cutting edge and corner geometries can rarely be found thus far by reason of high manufacturing costs and particularly missing treatment technologies [5, 10]. After the evaluation of various cutting tool materials in cutting applications of PM-HSS-steel, several cutting edge geometries were examined regarding their properties in process within the studies at the IWF. Since the high potential of PcBN has been recognized in former studies, tools made of this cutting material were used and analysed in view of diverse corner radii and cutting edge geometries (fig. 5).
In initial series of tests, only PcBN-cutting edges, having a cutting edge rounding, were used. The applied cutting edge type for cutting tests with hardened PM-steel was BL 49 with a corner radii between rε = 0.4 mm and rε =1.2 mm. At this, it could be demonstrated that the usage of large corner radii (rε = 1.2 mm) increases the tool life volume by up to 80 % compared to smaller ones (fig. 5A). Conforming to the general theory in hard machining, the studies could show that the bigger the corner radius is, the more ruggedly the tool cutting edge works when cutting hardened materials. However, the application range is limited due to the bigger corner radii up to rounded indexable inserts. Components to be manufactured with small radii, or finishing operations with depths of cut of ap ≤ rε are unrealisable, or because of unfavourable chip forms and high radial cutting forces not reliable. As a next step, tools with different cutting edge corner shapes, but a constant corner radius of rε = 0.8 mm were evaluated in terms of tool life, cutting forces, and created surface qualities of the component. Chamfers with bf = 100 µm and edge roundings (rβ ≈ 100 µm) have shown comparable dimensions. In figure 5B, the development of the width of flank wear land up to the test criterion of VBmax = 0.3 mm is visible. It explains that here, too an increase in the stability of the tool through cutting edge chamfers, or roundings leads to an increase of the tool life volume in hard turning applications. Considerable differences of existing forces, or generated surface roughnesses have not indeed been determined. For this reason, a summarized statement concerning the effectiveness of chamfers compared to roundings cannot be made. In order to be able to make a qualitative declaration concerning an experimentally-verified, significant influence of the shape of the cutting edge in hard turning processes, the IWF is currently working on the simulative dimensioning, the manufacturing, and the analysis of the performance of specifically edge-modified PcBN-turning tools.
The results indicate that a reliable hard turning process of High Speed Steel produced by powder metallurgy with about 65 HRC is also practicable. In the series of tests, both, statements about materials and geometries to be applied could be made, and application standard values could be developed. Whereas the hard turning process with PcBN-cutting tool materials appears to be effective with high material removal rates and surface qualities, ceramic tools have to be applied at average cutting parameters to avoid cutting edge chippings and tool failure. Furthermore, the application of coated PcBN-cutting tool materials exposes the great potentials of these recent developments and hence, possibilities for prospective research and development in the field of hard machining. However, in all conducted tests, the significant effect of cutting edge and cutting edge corner geometry on the tool performance and on the generated surface quality of the component has been revealed. Therefrom, the focus of the research work at the IWF is on the specific dimensioning, manufacturing, and evaluation of PcBN and ceramic tools having cutting edge geometries that are appropriate for hard machining processes.
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Institut für Werkzeugmaschinen und Fabrikbetrieb (IWF) der TU Berlin
Telefon: +49 (0) 30 / 314-24962
Telefax: +49 (0) 30 / 314-25895
Dipl.-Ing. Javier A. Oyanedel Fuentes
Institut für Werkzeugmaschinen und Fabrikbetrieb (IWF) der TU Berlin
Telefon: +49 (0) 30 / 314-22424
Telefax: +49 (0) 30 / 314-25895