MACHINABILITY AND TOOL WEAR DURING THE HIGH SPEED MILLING OF SOME HARDENED TOOL STEELS

MACHINABILITY AND TOOL WEAR DURING THE HIGH SPEED MILLING OF SOME HARDENED TOOL STEELS H.Chandrasekaran and U. Persson Swedish Institute for Metals Research Drottning Kristinas väg 48 114 28 Stockholm Sweden Abstract High speed milling of tool steels in the hardened state is an expanding field. Results from machinability and tool wear studies during the finish milling of some hot and cold working tool steels using both cemented carbide and CBN tool inserts are presented in this paper. Recommended cutting speeds (75 to 600 m/min) and feeds (0,025 to 0,15 mm/tooth) were used to compare tool performance as well as the surface integrity of the milled surface. Microstructural study of the work material and SEM investigation of the worn tools also formed part of this investigation. While cemented carbide tools appear to be capable of milling the different steels in our study, use of proper CBN tools and cutting conditions could improve the milling productivity appreciably in the case of cold working steels. However, in the case of hot working steels (50 HRC) the optimal cutting conditions associated with CBN tools seem to be very sensitive to nominal compositional changes in the work material. Obvious microstructural damage or tensile residual stress was not seen during the present studies. The limited SEM study of the tools indicated that adhesion induced micro chipping could be the probable limiting factor during the high speed milling of hotworking steels. The critical role of alloy content and structure was also seen to play an important role on their machinability during hard milling. 1237

1238 6TH INTERNATIONAL TOOLING CONFERENCE INTRODUCTION The demand on the quality and productivity of dies, moulds and press tools for metal and plastic forming industry is increasing continuously. Such cold and hot working tools are traditionally manufactured through rough machining, finish machining followed by heat treatment. Grinding and EDM (electro discharge machining) are often the final operation carried out. However, cutting tool and machine tool developments give us the possibility to machine materials in the hardened state. This technology of hard machining is capable of enhancing the overall production economy, arising from reduction in lead time and elimination of the grinding operation [1]. Improvement in working environment due to the elimination of a cutting fluid is an additional outcome affecting the overall production economy very favourably. Successful implementation of this technology however, requires the selection of proper tools and cutting conditions [2]. The effective range of cutting conditions for the milling of hardened steels (50 60 HRC) with conventional or super hard tool materials is rather narrow. Despite this, industrial experience shows that similar tool steels with marginal compositional difference do display substantial difference in machinability in the hardened state. Since it is possible to achieve the required hardness both through composition and processing, the resulting effect on machinability is difficult to predict, mainly because available information about the specific effect of microstructural features such as matrix, carbide/other particles on hard machinability is very little. This is one of the prime motives for the present research effort. Reliable information about the role of material microstructure on the milling machinability of tool steels in the hardened state could be very useful in practice and the present work is a step in this direction. The general objective of this study is to expand our knowledge base in the field of high speed milling of typical tool steels in the hardened state. As both technical and practical demands indicated, for our study we have chosen the fine milling of these steels in the hardened state. The work material being the main parameter of interest, standard tools and cutting conditions were used during the controlled milling tests. Systematic mapping of the evolution of tool wear was followed by an examination of the worn tools and the machined surface. Both SEM and optical microscopy was used in these investigations.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...1239 Limited surface integrity investigations were also carried out. These results will be presented now. WORK MATERIALS USED It was proposed to use standard hot and cold working tool steels from the market in the study and included the following materials, namely five hot working tool steels, namely ORVAR Supreme, THG2000 and 3 variants of the grade DIEVAR five cold working tool steels namely VANADIS 4 and CALDUR, P/M HSS ASP 2023 and ASP2023S and bearing steel 100Cr6. All the five hot working grades were heat treated to a hardness of 50 HRC. The hardness of the cold working grades varied from 61 HRC (CAL- DUR), 62HRC (VANADIS 4, ASP 2030 and ASP2030S) and 63 HRC (steel 100Cr6). Chemical analysis of the steels indicated that THG 2000 and ORVAR to differ mainly in Si, and S content, while DIEVAR contains more Mo, but lesser amounts of Si and V. All the other grades contain much more C. The P/M grades VANADIS 4 and ASP2023 have comparatively higher alloy content, especially V. Cold working grades CALDUR and the ball bearing steel 100Cr 6 are low alloyed carbon steels. The microstructural features of the work materials used in our study were obtained from metallographic sections of the work material after nital (4%) etching. All the tool steels investigated have a fine martensitic microstructure in the hardened state. The P/M grade VANADIS 4 contains in addition a fine distribution of primary carbides of CrC and VC, while ASP2023 contains mainly primary carbides of WC and VC. MILLING CUTTERS AND TOOL MATERIALS USED Since both hot and cold working steels were involved, the required tool material and recommended geometry also varied. Since the number of material variants were many, it was proposed to compare their machinability under one cutting condition in terms of speed, feed and depth of cut, suitable for the chosen tool. The tool holders and the tools (material type and geometry) as well as the cutting conditions for the milling tests were selected on the

1240 6TH INTERNATIONAL TOOLING CONFERENCE recommendations of the tool supplier. Bulk of the milling was carried out using different grades of super hard tool material, namely cubic boron nitride (CBN) from more than one supplier. For one material (ASP2023) other cemented carbide and cermet grades were also used. Finish face milling using end mills/face mills was the main test mode. MILLING TESTS Finish milling in the hardened state is characterised by small feeds and depths of cut using small diameter cutters rotating at high speeds. In view of the large number of material and tool combinations involved in our study high speed milling tests were carried out of different laboratories, namely Machining laboratory of Uddeholm Tooling AB at Hagfors, Secotools AB, Fagersta and Sandvik Coromant AB, Stockholm. All tests were carried out in the dry condition. A flank/notch wear of 0,2 mm (VB = 0,2 mm) was the evaluation criterion used in these studies. In cases of uneven wear the tests were stopped when the maximum wear level reached 0,2 mm. Similarly, when the tool wear was too little the test was stopped to conserve the resources. Tool wear was monitored at regular intervals using low power toolmakers microscope. The consolidated test conditions used are shown in Table 1. As a step in mapping the observed tool wear and machinability behaviour of the tool the worn tools were examined in SEM. Further an effort was also made to map the surface integrity parameters of the milled surfaces. This included measurement of surface quality (Profilometer), state of microstructure (metallographic section), subsurface deformation (microhardness) and residual stresses (X-ray diffraction). These investigations were carried out only for chosen test conditions only. We will now present the tool wear results and the results obtained from SEM only. RESULTS FROM TOOL WEAR STUDIES Evolution of flank and notch wear during the milling of three DIEVAR variants using CBN inserts is shown in Fig. 1. If both notch and flank wear were present these were monitored. It can be readily seen that in the initial stages both wear modes follow similar path. However, the resulting tool wear for the three DIEVAR variants clearly shows that with marginal material variation even at comparable hardness the tool wear could be affected.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...1241 Table 1. The cutting conditions used for tool wear studies in milling Tool steels tested Tool grade z V f z a p a e used mm/tooth m/min mm mm ORWAR supreme BN300 1 600 0,08 0,5 7 THG 2000 DIEVAR (1,2,3) ASP 2023 CBN 20 2 400 0,15 0,3 4 ASP 2023S CBN 20 2 400 0,15 4 0,3 VANADIS 4 CBN 20 2 400 0,15 4 0,3 ASP 2023S F30M 2 75 0,15 4 0,6 ASP 2023S 390 CB50 2 400 0,15 1,5 0,3 ASP 2023S 390 CT530 2 75 0,15 1,5 0,3 ASP 2023S 390 1025 E 2 75 0,15 1,5 0,3 ASP 2023S 390 1025 M 2 75 0,15 1,5 0,3 CALDUR CBN20 2 600 0,025 3 0,3 100Cr6 CBN20 2 600 0,025 3 0,3 CALDUR CBN100 1 400 0,05 0,15 47 100Cr6 CBN100 1 400 0,05 0,15 47 CALDUR CBN300 1 400 0,05 0,15 47 100Cr6 CBN300 1 400 0,05 0,15 47 Similar wear development was also seen in the case of other hotworking grades. From these curves the tool life was computed using a flank wear criterion of VB = 0,2 mm. The consolidated tool life results for the five hotworking grades is shown in Fig. 2. The machinability difference between the nominally similar (hardness) grades in hard milling at identical cutting conditions could still differ by almost two orders of magnitude as in the case of ORVAR and one of the DIEVAR variants. In the case of the cold working grades the four P/M processed variants, as the workmaterial was in the form of circular bars, circular interpolation mode was used to face mill using a milling cutter with two cutting edges. Both CBN and a cemented carbide grade were used. The consolidated results are shown in Fig. 3. The critical role of tool grade selection was also evident in the milling of the cold working grades. An almost 100% increase in tool life was possible through the use of correct grade of CBN as shown in Fig. 4.

1242 6TH INTERNATIONAL TOOLING CONFERENCE Figure 1. Evolution of tool wear (flank and notch) for three variants of DIEVAR (E10615, E10685 and E10283) using CBN (BN300) inserts; Vc=600 m/min, fz=0,08 mm/tooth, a p =7 mm and a e=0,5 mm. This was also evident in the case of cemented carbides shown in the case of milling ASP2023S using CBN and a set of three cemented carbides (Fig. 5). Evidently the cermet grade (CT530) is unsuitable for this workmaterial. DISCUSSION OF RESULTS In order to understand and interpret the observed machinability behaviour in our studies additional investigation of the tools in SEM was carried out. In the absence of systematic observation of the tools as a function of time the observed difference in the appearance of the tools at the end of tool life was used to draw qualitative information. This indicated that in most cases associated with long tool life a stable evolution of wear along both rake and flank surface was evident. This was typical in the cases of ORVAR-CBN (BN300), CALDUR/100Cr6 CBN (CBN300).

Machinability and Tool Wear During the High Speed Milling of Some Hardened...1243 In the absence of this the progress of wear was often through one or a combination of following modes, namely abrasive wear along with progressive micro-chipping and final destruction of edge, Fig. 6, severe notching (many of the DIEVAR variants) and localised chipping. In many instance of poor machinability tendency for notch formation and adhesion of the work material to the tool was evident. In other words due to the interaction of the microgeometry with the workmaterial at the prevalent cutting conditions (cutting speed, feed and depth of cut), thermally induced tool-work interaction enhancing the tendency for workmaterial adhesion takes place. If this is postponed for a long time as in the case of 100Cr6 CBN300 shown in Fig. 7, the tool life is also prolonged. Indirect evidence for the above hypothesis also came from the appearance of the chips. With progressive tool wear the chip morphology and colour changed (thinner and wider) indicating increased temperature. Under these conditions the notching at the depth of cut line is also facilitated due to increased interaction with air (oxygen). The improvement in the machinability of S alloyed ASP2023S grade is another strong evidence of the traditional role of machinability improvement additives. Stable presence of MnS layer could protect the tool from the large primary carbides in the workmaterial [3]. The influence of other elements likes Si and the intermittent nature of the contact phenomenon are additional features affecting tool wear. This is also the possible reason for the observed variations in the machinability of the hot working grades in our case. CONCLUSIONS Based on our experimental results and discussion, the following conclusions are in order. The finish milling of hardened tool-steel poses no technical problems as such. But to achieve practical tool life optimal tools (material and micro- geometry) and cutting conditions are critical. Successful milling of most of the hardened steels with cemented carbide tools appears feasible. However, correct selection of cutting conditions and type of CBN could result in high productivity for some of the steels. For a given hot working grade (50 HRC) apparently nominal compositional difference could affect machinability significantly. Milling

1244 6TH INTERNATIONAL TOOLING CONFERENCE the grade ORVAR SUPREME with the same CBN grade at the same cutting conditions resulted in the tool life variations of 2 orders of magnitude. Even at comparable bulk hardness level, increase in carbide content contributes to the deterioration in the machinability of the cold working tool steels, as evidenced by 100Cr6 and VANADIS 4. There is some evidence indicating that the tool-work interaction and the formation of protective inclusion layers or reduction in the adhesion of work material is closely related to the alloy content. ACKNOWLEDGMENTS We are thankful to VINNOVA, Uddeholm AB, Secotools AB, Sandvik Coromant AB, Ovako AB and Erasteel Kloster AB for jointly financing this project. Thanks are also due to Uddeholm AB (Staffan Gunnarsson), Secotools AB (Bengt Högrelius) Sandvik Coromant AB (Rikard Sundström), Erasteel Kloster (Jan Tiberg) and Ovako AB (Tomas Andersson) for the supply of materials and milling tests. REFERENCES [1] K. KÖNIG, A. BERTHOLD and K-F. KOCH, (1993), Turning versus grinding A comparison of surface integrity aspects and attainable accuracies, Ann. of CIRP 42/1, pp. 39 43. [2] S. GUNNARSSON, (1996) Machining of hardened steels, "Progress in Tool Steels Proc. of 4th Int. Conf. On Tooling, Ruhr University, Bochum, Sept. 11 13, 1996, pp. 457 467. [3] H. CHANDRASEKARAN, (1998) Machinability of ferrous alloys and the role of microstructural parameters A literature survey, Report from Swedish Institute for Metals Research, IM-3664.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...1245 Figure 2. Tool life in milling of five hot working steels using CBN(BN300) inserts using a criterion of VB =0,2 mm; Vc=600 m/min, fz=0,08 mm/tooth, a p =7 mm and a e =0,5 mm.

1246 6TH INTERNATIONAL TOOLING CONFERENCE Figure 3. Tool life in milling of four P/M cold working steels with CBN (BN20) and cemented carbide inserts using a criterion of VB =0,2 mm; Vc=400 m/min, fz =0,15 mm/tooth, a p =1,5 mm and a e =0,3 mm. Notice the changed cutting conditions for the carbide insert F30M; Vc = 75 m/min and a e =0,6 mm.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...1247 Figure 4. Tool life results for two cold working grades using different type of CBN inserts; cutting conditions as in Table 1. Figure 5. Tool life as a function of tool material type in hard milling (circular interpolation) of ASP2023S; cutting conditions as in Table 1.

1248 6TH INTERNATIONAL TOOLING CONFERENCE (a) (b) Figure 6. SEM view of CBN (BN300) tool edge after 496 min of milling in ORVAR; observe the even flank wear (a), notch wear and chipping on the rake (b). (a) (b) Figure 7. SEM view of the cutting edge after milling 100Cr6 for 420 min. using CBN(CBN300) showing some material adhesion (a) and the same region at a higher magnification (b).