machinability and tool wear during ......steels in our study, use of proper cbn tools and cutting...

MACHINABILITY AND TOOL WEAR DURINGTHE HIGH SPEED MILLING OF SOME HARDENEDTOOL STEELS

H.Chandrasekaran and U. PerssonSwedish Institute for Metals Research

Drottning Kristinas väg 48

114 28 Stockholm

Sweden

AbstractHigh speed milling of tool steels in the hardened state is an expanding

field. Results from machinability and tool wear studies during the finishmilling of some hot and cold working tool steels using both cemented carbideand CBN tool inserts are presented in this paper. Recommended cuttingspeeds (75 to 600 m/min) and feeds (0,025 to 0,15 mm/tooth) were usedto compare tool performance as well as the surface integrityof the milledsurface. Microstructural study of the work material and SEMinvestigationof the worn tools also formed part of this investigation.

While cemented carbide tools appear to be capable of millingthe differentsteels in our study, use of proper CBN tools and cutting conditions could im-prove the milling productivity appreciably in the case of cold working steels.However, in the case of hot working steels (50 HRC) the optimal cuttingconditions associated with CBN tools seem to be very sensitive to nominalcompositional changes in the work material. Obvious microstructural dam-age or tensile residual stress was not seen during the present studies. Thelimited SEM study of the tools indicated that adhesion induced micro chip-ping could be the probable limiting factor during the high speed milling ofhotworking steels. The critical role of alloy content and structure was alsoseen to play an important role on their machinability duringhard milling.

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1238 6TH INTERNATIONAL TOOLING CONFERENCE

INTRODUCTION

The demand on the quality and productivity of dies, moulds and press toolsfor metal and plastic forming industry is increasing continuously. Such coldand hot working tools are traditionally manufactured through rough ma-chining, finish machining followed by heat treatment. Grinding and EDM(electro discharge machining) are often the final operationcarried out. How-ever, cutting tool and machine tool developments give us thepossibility tomachine materials in the hardened state. This technology ofhard machiningis capable of enhancing the overall production economy, arising from reduc-tion in lead time and elimination of the grinding operation [1] . Improvementin working environment due to the elimination of a cutting fluid is an addi-tional outcome affecting the overall production economy very favourably.

Successful implementation of this technology however, requires the se-lection of proper tools and cutting conditions [2]. The effective range ofcutting conditions for the milling of hardened steels (50∼ 60 HRC) withconventional or super hard tool materials is rather narrow.Despite this,industrial experience shows that similar tool steels with marginal compo-sitional difference do display substantial difference in machinability in thehardened state. Since it is possible to achieve the requiredhardness boththrough composition and processing, the resulting effect on machinability isdifficult to predict, mainly because available informationabout the specificeffect of microstructural features such as matrix, carbide/other particles onhard machinability is very little. This is one of the prime motives for thepresent research effort.

Reliable information about the role of material microstructure on themilling machinability of tool steels in the hardened state could be very usefulin practice and the present work is a step in this direction. The general objec-tive of this study is to expand our knowledge base in the field of high speedmilling of typical tool steels in the hardened state. As bothtechnical andpractical demands indicated, for our study we have chosen the fine millingof these steels in the hardened state. The work material being the main pa-rameter of interest, standard tools and cutting conditionswere used duringthe controlled milling tests. Systematic mapping of the evolution of toolwear was followed by an examination of the worn tools and the machinedsurface. Both SEM and optical microscopy was used in these investigations.

Machinability and Tool Wear During the HighSpeedMillingofSomeHardened...1239

Limited surface integrity investigations were also carried out. These resultswill be presented now.

WORK MATERIALS USED

It was proposed to use standard hot and cold working tool steels from themarket in the study and included the following materials, namely

five hot working tool steels, namely ORVAR Supreme, THG2000 and3 variants of the grade DIEVAR

five cold working tool steels namely VANADIS 4 and CALDUR, P/MHSS ASP 2023 and ASP2023S and bearing steel 100Cr6.

All the five hot working grades were heat treated to a hardnessof 50HRC. The hardness of the cold working grades varied from 61 HRC (CAL-DUR), 62HRC (VANADIS 4, ASP 2030 and ASP2030S) and 63 HRC (steel100Cr6).

Chemical analysis of the steels indicated that THG 2000 and ORVAR todiffer mainly in Si, and S content, while DIEVAR contains more Mo, butlesser amounts of Si and V.

All the other grades contain much more C. The P/M grades VANADIS 4and ASP2023 have comparatively higher alloy content, especially V. Coldworking grades CALDUR and the ball bearing steel 100Cr 6 are low alloyedcarbon steels.

The microstructural features of the work materials used in our study wereobtained from metallographic sections of the work materialafter nital (4%)etching. All the tool steels investigated have a fine martensitic microstructurein the hardened state. The P/M grade VANADIS 4 contains in addition a finedistribution of primary carbides of CrC and VC, while ASP2023 containsmainly primary carbides of WC and VC.

MILLING CUTTERS AND TOOL MATERIALS USED

Since both hot and cold working steels were involved, the required toolmaterial and recommended geometry also varied. Since the number of ma-terial variants were many, it was proposed to compare their machinabilityunder one cutting condition in terms of speed, feed and depthof cut, suitablefor the chosen tool. The tool holders and the tools (materialtype and geome-try) as well as the cutting conditions for the milling tests were selected on the


recommendations of the tool supplier. Bulk of the milling was carried outusing different grades of super hard tool material, namely cubic boron nitride(CBN) from more than one supplier. For one material (ASP2023) other ce-mented carbide and cermet grades were also used. Finish facemilling usingend mills/face mills was the main test mode.

MILLING TESTS

Finish milling in the hardened state is characterised by small feeds anddepths of cut using small diameter cutters rotating at high speeds. In viewof the large number of material and tool combinations involved in our studyhigh speed milling tests were carried out of different laboratories, namelyMachining laboratory of Uddeholm Tooling AB at Hagfors, Secotools AB,Fagersta and Sandvik Coromant AB, Stockholm. All tests werecarriedout 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 casesof uneven wearthe 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 conservethe resources. Tool wear was monitored at regular intervalsusing low powertoolmakers microscope.

The consolidated test conditions used are shown in Table 1. As a stepin mapping the observed tool wear and machinability behaviour of the toolthe worn tools were examined in SEM. Further an effort was also made tomap the surface integrity parameters of the milled surfaces. This includedmeasurement of surface quality (Profilometer), state of microstructure (met-allographic section), subsurface deformation (microhardness) and residualstresses (X-ray diffraction). These investigations were carried out only forchosen test conditions only. We will now present the tool wear results andthe results obtained from SEM only.

RESULTS FROM TOOL WEAR STUDIES

Evolution of flank and notch wear during the milling of three DIEVARvariants using CBN inserts is shown in Fig. 1. If both notch and flank wearwere present these were monitored. It can be readily seen that in the initialstages both wear modes follow similar path. However, the resulting toolwear for the three DIEVAR variants clearly shows that with marginal mate-rial variation even at comparable hardness the tool wear could be affected.


Table 1. The cutting conditions used for tool wear studies in milling

Tool steels tested Tool grade z V fz ap ae

used mm/tooth m/min mm mm

ORWAR supreme BN300 1 600 0,08 0,5 7THG 2000DIEVAR (1,2,3)ASP 2023 CBN 20 2 400 0,15 0,3 4ASP 2023S CBN 20 2 400 0,15 4 0,3VANADIS 4 CBN 20 2 400 0,15 4 0,3ASP 2023S F30M 2 75 0,15 4 0,6ASP 2023S 390 CB50 2 400 0,15 1,5 0,3ASP 2023S 390 CT530 2 75 0,15 1,5 0,3ASP 2023S 390 1025 E 2 75 0,15 1,5 0,3ASP 2023S 390 1025 M 2 75 0,15 1,5 0,3CALDUR CBN20 2 600 0,025 3 0,3100Cr6 CBN20 2 600 0,025 3 0,3CALDUR CBN100 1 400 0,05 0,15 47100Cr6 CBN100 1 400 0,05 0,15 47CALDUR CBN300 1 400 0,05 0,15 47100Cr6 CBN300 1 400 0,05 0,15 47

Similar wear development was also seen in the case of other hotworkinggrades. From these curves the tool life was computed using a flank wearcriterion of VB = 0,2 mm.

The consolidated tool life results for the five hotworking grades is shownin Fig. 2. The machinability difference between the nominally similar (hard-ness) grades in hard milling at identical cutting conditions could still differby almost two orders of magnitude as in the case of ORVAR and one of theDIEVAR variants.

In the case of the cold working grades the four P/M processed variants, asthe workmaterial was in the form of circular bars, circular interpolation modewas used to face mill using a milling cutter with two cutting edges. BothCBN and a cemented carbide grade were used. The consolidatedresults areshown in Fig. 3. The critical role of tool grade selection wasalso evident inthe milling of the cold working grades. An almost 100% increase in tool lifewas possible through the use of correct grade of CBN as shown in Fig. 4.


Figure 1. Evolution of tool wear (flank and notch) for three variants ofDIEVAR (E10615,E10685 and E10283) using CBN (BN300) inserts; Vc=600 m/min,fz=0,08 mm/tooth,ap

=7 mm andae=0,5 mm.

This was also evident in the case of cemented carbides shown in the case ofmilling 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 behaviourin 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 timethe observed difference in the appearance of the tools at theend of tool lifewas used to draw qualitative information.

This indicated that in most cases associated with long tool life a stableevolution of wear along both rake and flank surface was evident. This wastypical in the cases of ORVAR-CBN (BN300), CALDUR/100Cr6 – CBN(CBN300).


In the absence of this the progress of wear was often through one or acombination of following modes, namely abrasive wear alongwith progres-sive micro-chipping and final destruction of edge, Fig. 6, severe notching(many of the DIEVAR variants) and localised chipping. In many instance ofpoor machinability tendency for notch formation and adhesion of the workmaterial to the tool was evident. In other words due to the interaction ofthe microgeometry with the workmaterial at the prevalent cutting conditions(cutting speed, feed and depth of cut), thermally induced tool-work interac-tion enhancing the tendency for workmaterial adhesion takes place. If thisis postponed for a long time as in the case of 100Cr6 –CBN300 shown inFig. 7, the tool life is also prolonged. Indirect evidence for the above hy-pothesis also came from the appearance of the chips. With progressive toolwear the chip morphology and colour changed (thinner and wider) indicatingincreased temperature. Under these conditions the notching at the depth ofcut line is also facilitated due to increased interaction with air (oxygen).

The improvement in the machinability of S alloyed ASP2023S grade isanother strong evidence of the traditional role of machinability improvementadditives. Stable presence of MnS layer could protect the tool from the largeprimary carbides in the workmaterial [3]. The influence of other elementslikes Si and the intermittent nature of the contact phenomenon are additionalfeatures affecting tool wear. This is also the possible reason for the observedvariations in the machinability of the hot working grades inour case.

CONCLUSIONS

Based on our experimental results and discussion, the following conclu-sions are in order.

The finish milling of hardened tool-steel poses no technicalproblemsas such. But to achieve practical tool life optimal tools (material andmicro- geometry) and cutting conditions are critical.

Successful milling of most of the hardened steels with cemented car-bide tools appears feasible. However, correct selection ofcuttingconditions and type of CBN could result in high productivityfor someof the steels.

For a given hot working grade (50 HRC) apparently nominal com-positional difference could affect machinability significantly. Milling


the grade ORVAR SUPREME with the same CBN grade at the samecutting conditions resulted in the tool life variations of 2orders ofmagnitude.

Even at comparable bulk hardness level, increase in carbidecontentcontributes to the deterioration in the machinability of the cold work-ing tool steels, as evidenced by 100Cr6 and VANADIS 4.

There is some evidence indicating that the tool-work interaction andthe formation of protective inclusion layers or reduction in the adhe-sion of work material is closely related to the alloy content.

ACKNOWLEDGMENTS

We are thankful to VINNOVA, Uddeholm AB, Secotools AB, SandvikCoromant AB, Ovako AB and Erasteel Kloster AB for jointly financing thisproject. Thanks are also due to Uddeholm AB (Staffan Gunnarsson), Sec-otools AB (Bengt Högrelius) Sandvik Coromant AB (Rikard Sundström),Erasteel Kloster (Jan Tiberg) and Ovako AB (Tomas Andersson) for thesupply of materials and milling tests.

REFERENCES

[1] K. KÖNIG, A. BERTHOLD and K-F. KOCH, (1993), Turning versus grinding – Acomparison 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 mi-crostructural parameters– A literature survey, Report from Swedish Institute for MetalsResearch, IM-3664.


Figure 2. Tool life in milling of five hot working steels using CBN(BN300) inserts usinga criterion of VB =0,2 mm; Vc=600 m/min, fz=0,08 mm/tooth,ap =7 mm andae =0,5 mm.


Figure 3. Tool life in milling of four P/M cold working steels with CBN (BN20) and ce-mented carbide inserts using a criterion of VB =0,2 mm; Vc=400 m/min, fz =0,15 mm/tooth,ap =1,5 mm andae =0,3 mm. Notice the changed cutting conditions for the carbide insertF30M; Vc = 75 m/min andae =0,6 mm.


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.


(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 therake (b).

(a) (b)

Figure 7. SEM view of the cutting edge after milling 100Cr6 for 420 min.usingCBN(CBN300) showing some material adhesion (a) and the sameregion at a higher magni-fication (b).

machinability and tool wear during ......steels in our study, use of proper cbn tools and cutting...

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