THE INFLUENCE OF STEEL GRADE AND STEEL HARDNESS ON TOOL LIFE WHEN MILLING IN HARDENED TOOL STEEL

Transcription

1 THE INFLUENCE OF STEEL GRADE AND STEEL HARDNESS ON TOOL LIFE WHEN MILLING IN HARDENED TOOL STEEL S. Gunnarsson, B. Högman and L. G. Nordh Uddeholm Tooling AB Research and Development Hagfors Sweden Abstract Keywords: The rapid development of cutting tools and machining processes have made it possible to machine hardened tool steel. Milling operations that would have been impossible to carry out only a few years ago are today used in die and mould production. However, the chemical composition of the tool steel and the steel hardness, has a very big influence on which cutting tool life that can be achieved when machining in hardened tool steel. In this investigation coated solid cemented carbide milling cutters have been tested in different tool steels at hardness levels up to 62 HRC. The cutting speed has been varied in an attempt to find the optimal tool life for best productivity. The investigation shows significant difference in production cost between two steels with the same hardness, but different chemical analysis. Hardened Die and Mould Steel, High Speed Machining INTRODUCTION Machining of moulds and dies direct in hardened state is possible today due to the rapid development of cutting tools and machine tools [1]. A common way to machine a die in the past, was first to rough machine the die in soft condition, followed by hardening and then to spark erode the die to the finished dimension. This procedure will create a hard re-melted layer on the mould surface, which, if it is a die casting mould, must be 1199

2 1200 6TH INTERNATIONAL TOOLING CONFERENCE polished away to give an acceptable mould surface. This polishing is a very time consuming operation, and can sometimes be difficult to carry out on the whole surface if the mould has deep grooves and pockets. A typical example is a die casting mould for escalator steps, which contains a lot of ribs and grooves. At one customer who is using our material DIEVAR hardened to 51 HRC, a comparison was done between to spark erode (EDM) the mould and to machine it, using the high speed milling technique (HSM) [2]. It took 300 hours to EDM the mould, which was followed by 400 hours of polishing to get rid of the re-melted layer. To machine the mould direct in hardened state using the HSM technique took 80 hours. The example shows that huge time saving can be achieved when machining with cutting tool direct in hardened tool steel. However to succeed with this new technique, the following factors are of very big importance: Mould and die design Cutting tool Machine tool Tool passes Work material The hardness of the work material This paper shall present what influence the chemical composition of the steel and the hardness has on the tool life when milling with solid cemented carbide milling cutters TEST PERFORMANCE EXPERIMENTAL The tests have been performed in a vertical Modig MD 7200 machining centre at the R & D at Uddeholm Tooling AB, see Fig. 1. The machine has a maximum spindle speed of rpm and the power 20 kw. It is equipped with a HSK 63 taper and the carbide tools were mounted in a collet chuck. The machining trials were made in hardened steel samples with the dimension mm. A cavity was machined with the size mm. The walls of the cavity were angled 45.

3 The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in Figure 1. Machining centre Modig MD7200. The cutting speed used in the trials is the true cutting speed calculated on the working diameter of the milling cutter. When milling with small depth of cut with a ball nose end milling cutter, the working diameter on the cutter is small. In the finishing milling trial the axial depth of cut was 0.15 mm. This means that the working diameter for a 10 mm cutter with radius 5 mm was only 2.4 mm. This explains, why the maximum cutting speed in the finishing milling trial only was 135 m/ min, as this cutting speed represents rpm in spindle speed, which was the maximum limit in the machining centre. Two different kinds of milling operations were investigated, rough milling and finishing milling. The tool paths used in the investigation were of constant Z-level pocketing strategy beginning from the centre of the work piece and out towards the cavity sides. After finishing one level a helical interpolation was done in the centre of the work piece to the next cutting level, see Fig. 2. Compressed air was used to evacuate the chips. The cutting tools used were two fluted coated 10 mm solid carbide ball nose end mills VC2-SSB manufactured by Kobelco. The flank wear was measured frequently during the tests, and the trial was stopped when a

4 1202 6TH INTERNATIONAL TOOLING CONFERENCE flank- or notch wear of 0,2 mm on the milling cutter was reached For rough milling the measuring unit was the machined volume and for the finishing step the machined surface area. The cutting speed was varied, the remaining parameters were kept constant as follows: Rough milling Axial depth of cut, ap: Radial depth of cut, ae: Tooth feed, fz: Finishing milling Axial depth of cut, ap: Radial depth of cut, ae: Tooth feed, fz: 1.0 mm 2.5 mm 0.06 mm/tooth 0.15 mm 0.15 mm 0.15 mm/tooth WORK MATERIAL For the trials, tool steels with different properties and application areas were chosen [1]. The analyses of the materials are given in Table 1. DIEVAR and HOTVAR are steels used for hot work applications such as die casting, extrusion and warm forging of metals. They are conventionally made by ingot casting but also electro slag remelted (ESR), thus they have Figure 2. Milling strategy used in the investigation.

5 The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in Table 1. Analysis and hardness of the investigated tool steels Steel grade C Si Mn Cr Mo V W HRC DIEVAR 0,4 0,3 0,5 5,0 2,3 0,6 51 HOTVAR 0,5 1,0 0,8 2,6 2,2 0,8 56 STAVAX ESR 0,4 0,9 0,5 13,6 0,3 50 CALMAX 0,6 0,3 0,8 4,5 0,5 0,2 54 CALDUR 0,7 1,0 0,8 2,5 2,1 0,5 60 ELMAX 1,7 0,8 0,3 18,0 1,0 3,0 56 VANADIS 6 2,1 1,0 0,5 6,8 1,5 5,4 60 VANADIS 10 2,9 1,0 0,5 8,0 1,5 9,8 62 VANADIS 23 1,3 0,5 0,3 4,2 5,0 3,1 6,4 61 a very low amount of inclusions in the steel structure. Working hardness is in the range HRC for DIEVAR and HRC for HOTVAR. STAVAX ESR is a conventionally made martensitic stainless steel, also ESR-treated, and the steel is used for plastic injection moulds. The working hardness is in the range HRC. CALMAX and CALDUR are conventionally made cold work tool steels, which can be used for punching and pressing of plate. The working hardness is in the range HRC. The chosen materials are of the type with no primary carbides in the microstructure. ELMAX, VANADIS 6, VANADIS 10 and VANADIS 23 are powder metallurgical made tool steels, and they all contain a lot of hard primary carbides in the steel structure. ELMAX is due to its high chromium content, corrosion resistant and is used for plastic moulds where there is a need for good wear resistance. VANADIS 6 and VANADIS 10 are cold work tool steels mainly used for punching of plates and VANADIS 23 is a high speed steel, which can be used for cutting tools. The working hardness for the PM steels is in the range HRC depending on type of steel grade and application. All the tested steels were hardened and tempered to the hardness given in Table 1.

6 1204 6TH INTERNATIONAL TOOLING CONFERENCE RESULTS FINISHING MILLING When finishing milling, the machined surface area in cm 2 as a function of cutting speed was chosen as the unit of comparison. The results can be seen in Fig. 3. In the figure results of two different cutting speeds are given. The values without brackets are the true cutting speed calculated on the working diameter, the values in brackets are the cutting speed calculated on the maximum diameter on the cutter (10 mm). For the PM steels different cutting speeds have a small influence on the tool life when looking at machined surface area. The machined surface area was more or less the same with different cutting speeds. The machined surface area was also quite small, in the range cm 3. Figure 3. Machined surface as function of cutting speed when finishing milling (wear criteria 0,2 mm flank wear). The reason for the short tool life when milling in PM tool steels is firstly, that the steels contain a lot of hard and abrasive carbides that produce a high wear on the cutting tool, secondly, the steels are hardened to a high hardness. The cold work materials containing no primary carbides have a quite high hardness but due to the absence of primary carbides the machined surface area is higher than for the PM steels. The biggest area machined is obtained with the cutting speed 100 m/min. The tool life is longer for CALMAX than for CALDUR probably due to the hardness difference.

7 The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in The plastic mould steel STAVAX ESR gave, in spite of the low hardness, relatively low tool life. The hot work tool steel DIEVAR gave the highest tool life of all the investigated materials. At the cutting speed 100 m/ min, the tool lasted for 20 hours before it was worn out. One of the reasons for the long tool life is probably that this steel had a low hardness. The second hot work tool steel tested, HOTVAR, gave a considerable shorter tool life, which is due to the higher hardness in this material. ROUGH MILLING Some of the materials, which were tested at the finishing milling, have also been tested in rough milling. The results showed big differences between different tool steels even tough the hardness was the same. The results from the rough milling as the removed volume of work material as function of cutting speed can be seen in Fig. 4. The cold work material Figure 4. Machined volume as function of cutting speed when rough milling (wear criteria 0,2 mm flank wear). CALDUR and the PM steel VANADIS 6 have a rather low machinability and the tool life is decreasing when the cutting speed is increased already from m/ min. Even tough the hardness is about the same for the PM steel and CALDUR, the slope is at a lower level for the PM steel. This is mainly due to the high primary carbide content in this material. For the two steels with the lower hardness, DIEVAR and CALMAX, the tool life is longer in CALMAX, which is opposite to the result obtained in

8 1206 6TH INTERNATIONAL TOOLING CONFERENCE the finishing milling trials. The reason for this can be that rough milling gives more heat into the milling cutter. Earlier investigation [3] has shown, that the tool life obtained when milling in DIEVAR is very dependent on the cutting edge temperature. A high cutting edge temperature will give a low tool performance. Nevertheless, for all the materials tested in the rough milling stage, the cutting speed has a rather strong influence on the tool life. If a high cutting speed is chosen it will shorten the tool life considerable. DISCUSSIONS AND ECONOMICAL ASPECTS The investigation has shown that it is a big difference in machinability between different tool steels when machining them in hardened condition. Two materials with the same hardness can give quite different cutting tool life. This means also that the cost to produce a die from PM tool steel will be higher compared to a die produced from low alloyed tool steel. A customer that today is using hot work tool steel for a die, for instance a die for a case for a mobile phone, can have a tool life up to twenty hours on the milling cutter when finishing milling. If he requires a more wear resistant material in the die, for instance the PM steel ELMAX, the tool life will decrease to around 40 minutes. The cost to finish mill a specific surface area in relationship to the cutting speed for the different materials is shown in Fig. 5. In the example it has been calculated with machine costs of 110 Euro/hour and a cost for the milling cutters of 110 Euro/cutter. The time for tool change is assumed to be 10 minutes. For three materials (VANADIS 10, VANADIS 6 and VANADIS 23) 100 m/ min seems to be the cutting speed that gives the lowest production cost for respectively material. For the other materials the lowest cost is at a cutting speed of 135 m/ min or higher. The example shows also the huge difference in cost to produce a specific surface area for different materials. For the steels with the best machinability the cost to mill one square centimetre is about 0,3 Euro, while to machine the same surface in the material VANADIS 10 costs about 1,4 Euro

9 The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in Figure 5. Total cost / cm 2 as function of cutting speed when finishing milling in the different tool steels. CONCLUSION The new technique to machine a mould cavity direct in hardened steel has much to offer regarding shortening of the lead times from design until the mould run in production. This investigation has shown that the production costs to machine a mould or die direct from hardened steel, is very dependent on the work material. Not only the steel hardness has an influence, but also the steel analysis has an important influence on the total cost to produce the mould. In most cases the higher production cost for a mould made from a high alloyed tool steel be can neglected, compared to the better performance and longer life of the mould in production of parts. REFERENCES [1] SANDVIK Coromant; Application Guide, Die & Mould Making. [2] ASM INTERNATIONAL; Metals Handbook, Ninth edition, Volume 16, Machining. [3] Internal report; Forskningsmeddelande, FM

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