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Improving the wear resistance of tools for stamping
Wear 269 (2010) 693–697
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Improving the wear resistance of tools for stamping G. Straffelini a,∗ , G. Bizzotto b , V. Zanon b a b
Department of Materials Engineering and Industrial Design, University of Trento, via Mesiano 77, 38050 Trento, Italy Mevis S.p.A., Vicenza, Italy
a r t i c l e
i n f o
Article history: Received 10 March 2010 Received in revised form 5 June 2010 Accepted 7 July 2010 Available online 15 July 2010 Keywords: Tools wear Stamping Sliding wear PVD coatings Hard metals Cryogenic treatments
a b s t r a c t In the present investigation, the wear resistance of tools for the precision stamping of a mechanical part for the automotive industry is optimised. The investigation is divided into two stages. In the first one, the progression of damage in the punches and dies is assessed. Wear mechanisms are evaluated by means of observations and measurements in a scanning electron microscope (SEM). In the second stage, specific improvements are proposed based on the results of the first stage and are evaluated by specific field tests. The improvements include coating by physical vapour deposition with a AlCrN layer, production of the tools with hard metal and cryogenic treatment of the high-speed steel tools. The results show that all the proposed treatments help improve the performance of the tools, and the best improvements are displayed by the use of hard metals and coated tools. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Punches and dies undergo severe wear during precision stamping of mechanical parts, especially in the case of blanks made by heat-treated steels. Wear is caused by sliding and may be characterised by adhesion, abrasion, transfer phenomena and also by brittle surface microcracking. Such damage impairs the quality of the stamped product. The tools must therefore be periodically replaced, often at a significant economic cost [1–3]. The present investigation makes reference to the production of a mechanical part for the automotive industry. Punches and dies are made with a powder metallurgy high-speed steel (HSS), and the operation takes place under constant lubrication. The investigation is divided into two stages. In the first stage, the progression of tool damage is assessed as a function of the parts produced. In this stage, a simple approach to follow the damage progression of the tools is also established. In the second stage, specific improvements are proposed based on the results of the first part of the investigation. Such improvements include coating by a ceramic thin film, which is produced by physical vapour deposition (PVD), production of the tools by hard metal, and deep cryogenic treatment of the HSS. As is well known, several PVD coatings are now available to increase the tool wear performance in boundary conditions that are usually encountered during precision stamping [3–5]. The basic requirements are good adhesion to the base steel in order to coun-
∗ Corresponding author. Tel.: +39 0461 882458; fax: +39 0461 881977. E-mail address: [email protected] (G. Straffelini). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.07.004
teract the shearing stresses without microcracking or delaminating and high hardness in order to induce low wear rates and a low friction coefficient. Hard metals are produced by powder metallurgy, and they are very hard because of the presence of ceramic carbide particles in their microstructure. They also show a fracture toughness larger than that of ceramics because of the presence of a metal matrix (typically cobalt). Because of this, hard metals display a very high resistance to dry and marginally lubricated sliding [6,7]. Finally, deep cryogenic treatment is reported to increase the sliding resistance of tools steels. It is usually applied in between hardening and tempering treatments, and the improvement in properties is attributed to the complete transformation of austenite into martensite and to the precipitation of ultrafine carbide particles [8–10]. The performance of the proposed treatments is thus evaluated and compared to that of the reference high-speed steel. Indications for the selection and practical application of the proposed treatments are thus obtained from specific field tests. 2. Experimental methods The objective of the investigation was the optimisation of the shaving step in the production of a mechanical part for the automotive industry. In Fig. 1 a, a detail of the part is shown. The blank has a thickness of 2.2 mm and is made of a cold rolled strip submitted to spheroidisation annealing. Its chemical composition is: 0.7% C, 0.7% Mn and 0.2% Si. In Fig. 1b, a sketch of the process is displayed. Punches and dies are made of a powder metallurgy S390 HSS produced by Böhler. Its
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Fig. 2. Detail of the worn region at the end of the punch after 200,000 strokes (SEM).
3. Results and discussion 3.1. Analysis of damage progression
Fig. 1. (a) Detail of the produced mechanical part and (b) sketch of the stamping process.
nominal chemical composition is 1.6% C, 4.8% Cr, 2% Mo, 5% V, 10.5% W and 8% Co. The blocks are heat-treated with following cycle: austenitising at 1150 ◦ C for 60 min under vacuum, gas quenching, cold treatment at −70 ◦ C for 20 h, and three tempering treatments at 540 ◦ C for 4 h. The blocks are machined by electric discharge machining (EDM) to the final shape. They are then sharpened to remove the white layer left by the EDM process [11]. The hardness of the heat-treated steel is 67.5 HRC (load: 150 kg). The shaving tools operate at room temperature under constant lubrication at a rate of 100 strokes/min. Around 100,000 parts are produced before tool sharpening is needed because of wear. To investigate the damage progression as a function of strokes (i.e., the number of produced parts), a field test was designed. Specific punches and dies were produced and tested up 40,000, 80,000, 120,000, 160,000 and 200,000 strokes. In each case, a new set of tools was used. The damage evolution has been determined by observations of the worn regions in a scanning electron microscope (SEM) and using an optical microscope (OM). In addition, the burr height in the part produced was measured as a function of the number of parts produced. On the basis of the experimental results, specific improvements were then proposed. The details of the alternative materials and treatments investigated will be provided in the next chapter. In the second stage of the investigation, specific field tests were then carried out to assess the relative performance of each proposed treatment. In each case, the same initial edge geometry was adopted.
We consider first the damage to the punch. Because of the sliding between the punch and the part to be shaved, the punch edges undergo sliding wear. This is shown in Fig. 2, which shows a punch edge after 200,000 strokes. Three regions can be recognised. In region 1 the worn track is flat and adhesive wear prevails. In region 2, some microcracks, normal to the sliding direction, are present. Such microcracks form under the action of the local friction stresses, which means that a boundary lubrication regime was present in the mating surfaces during each operation. In region 3, some transfer debris from the cut blanks can be observed. They remain attached to the surface of the punch because of adhesion [12]. The phenomenon of the transfer of debris to the surface of the punch (region 3 in Fig. 2), was shown to start after 80,000 strokes. The formation of the small cracks (region 2 in Fig. 2) was shown to start after 160,000 strokes. This means that both of these phenomena started after the occurrence of a critical amount of tool wear. The progression of wear with the number of strokes was evaluated by measuring the length of the worn region at a distance of 0.5 mm from the punch edge under the SEM. The results of this examination are shown in Fig. 3. They are the average of three
Fig. 3. Evolution of damage in the punch as a function of the number of parts produced.
G. Straffelini et al. / Wear 269 (2010) 693–697
Fig. 4. Damage appearance at the corner of the matrix after 200,000 strokes (SEM).
measurements taken in different tool positions. As expected, wear increases with the stroke number. The wear rate, however, is shown to increase in passing from 160,000 to 200,000 cycles, and this can be attributed to the formation of small cracks and the interaction of microcracking with adhesive wear. At the cutting edges of the die, the same wear process encountered in the case of the punch was observed. The only difference is that wear is more severe, probably because of worse lubrication. Both transfer and microcracking phenomena were seen to start from the beginning of the tests. As shown in Fig. 4, the contribution of microcracking is particularly severe at the corners. Wear progression was also characterised by measuring the height of the burr protruding below the bottom surface of the produced part. The measurements were carried out on 10 specimens using a 3D touch probe. The average burr height is reported in Fig. 5 as a function of strokes. The experimental standard deviation was found to be low and to increase from 0.02 mm after 40,000 strokes to 0.07 mm after 200,000 strokes. In Fig. 5 the evolution is reported as a function of strokes. It can be noted that the burr height increases with the strokes and that a plateau is reached after about 150,000 strokes. In practice, the production process is stopped when the burr height exceeds the limit of 0.3 mm. The tools are then resharpened and repositioned to continue the production. This occurs after the production of around 100,000 parts.
Fig. 5. Measured burr height as a function of strokes.
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Fig. 6. Measured burr height as a function of wear length at 0.5 mm from the punch edge.
As shown in Fig. 6, there is a clear experimental relationship between the evolution of wear length at 0.5 mm from the punch edge and the evolution of the burr height, which can be explained by considering that as wear is increased the clearance between the punch and the die is increased too, and a large clearance favours the formation of the burr. The comparison of Figs. 3 and 5 highlights that up to the limit of 0.3 mm in burr height, the punch underwent wear by adhesion with some debris transfer but with no microcracking. 3.2. New treatments and evaluation of their performance The tools under investigation underwent sliding wear under boundary lubrication. This is demonstrated by the observation of the wear traces and, in particular, by the occurrence of transfer phenomena and local microcracking. To increase the wear resistance it is therefore necessary to increase the hardness of the mating materials and reduce friction. In addition, it is also necessary to control the fracture toughness of the mating materials in order to reduce the risk of microcracking because of process characteristics, i.e., the occurrence of impact loading at the onset of shaving. These goals may be reached with different approaches that include the deposition of a ceramic thin film by physical vapour deposition (PVD), the production of the tools by hard metal, and the performance of cryogenic treatment of the HSS tools. On the bases of specific literature investigations [13–15], we decided to investigate the performance of a PVD AlCrN coating that is reported to give optimal behaviour for a variety of tools. A commercial AlCrN (Alcrona) coating was thus selected, and it was realised by Oerlikon Balzers Italy in standard Balzers conditions [16]. The coating was deposited on the S390 HSS tools in the mirror polished condition. For the present investigation, specific tools were also produced by EDM of commercial hard metal blocks, namely Ceratizit H40S blocks. This material has a hardness of 1340 HV30, and it is reported to have a fracture toughness of 12 MPa m1/2 [17]. Two cryotreatments of the S390 HSS were investigated. In the first (codenamed: QCT), austenitisation was carried out at 1150 ◦ C for 60 min under vacuum and quenching was done with N2 with a pressure of 5.5 bar. The cryogenic treatment was then carried out at −196 ◦ C (in liquid nitrogen) for 20 h. It was then followed by three tempering treatments at 540 ◦ C for 4 h. In the second cycle (codenamed QTC), the same deep cryogenic treatment was carried out after the three tempering treatments. This last procedure is more
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Fig. 7. Measured burr height as a function of cycles for the different materials investigated.
flexible than the other one, and it has been shown that it may produce an effective increase in the wear resistance of tool steels. No difference was detected between the hardness of the cryotreated steels: a value of 68.5 HRC for both cycles was measured. This value is slightly higher than that of the treated HSS without the cryogenic stage. The performance of the selected treatments was then evaluated in field tests up to 120,000 strokes. Following the indications of the first stage of the present investigation, the wear evolution was followed in each case by simply measuring the burr height of the stamped part at intervals of 30,000 strokes. The results of the investigation are shown in Fig. 7. They are average values of 10 measurements. In all cases, the standard deviations were low and proportional to the average burr height. For example, after 120,000 strokes the standard deviation of S390 was 0.07 mm and that of the AlCrN coating was 0.02 mm. It can be noted that the measured burr height in the tools made by the S390 HSS (the reference condition) is higher than that recorded in the first stage of the investigation and shown in Fig. 5. This result confirms the fact that field tests are characterised by some scatter because of the variability of the operating conditions. The results in Fig. 7 further show that the cryogenic treatments induce an improvement in the wear resistance but the comparison with the data in Fig. 5 shows that such an improvement is limited and it may be within the experimental scatter. No particular difference may be detected between the QTC and QCT treatments. On the other hand, both the use of the hard metal and the AlCrN coating produced a significant increase in the wear resistance of the tools. In both cases, at the end of the test the burr height was well below the limit of 0.3 mm and it seems that the tools may work for a very large number of strokes before the need for sharpening. In particular, the best improvement is displayed by the thin film coating. To gain further information on the damage progression in the proposed treated tools, the worn traces were observed by OM after 120,000 strokes. In Fig. 8 the observations of the worn surfaces in the dies are shown. To allow comparison, the adhesive wear of the reference S390 die is shown (Fig. 8a). As expected, the damage is very similar to that encountered in the first stage of the investigation. Similar behaviour was displayed by the cryotreated tools, and therefore the relevant observations are not reported here.
Fig. 8. Sliding wear in the matrix after 120,000 strokes. (a) S390 reference steels; (b) AlCrN coated steel; (c) hard metal.
In Fig. 8b the worn surface of the coated die is shown. It can be noted that in some regions the film is removed. As demonstrated by the experimental results and already reported in other investigations [18], however, the coating is still able to protect the tool and to guarantee a good quality of the stamped part, even if partially destroyed. In Fig. 8c the worn surface of the hard metal is shown. The wear extent is low but the presence of small cracks may be detected. The presence of such cracks may be better appreciated by a bottom view of the die, as shown in Fig. 9. Such cracking is due to the relatively low fracture toughness of the hard metals coupled with the presence of impact loading during the stamping process.
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produced by physical vapour deposition (PVD); production of the tools by hard metal; and two deep cryogenic treatments of the HSS. The comparative behaviour of the treated tools was investigated in the second stage of the investigation by means of specific field tests. The results show that the cryogenic treatments induce an improvement in the wear resistance. The comparison with the data obtained in the first stage, however, shows that such an improvement is limited and it may be within the experimental scatter. On the contrary, both the use of the hard metal and the AlCrN coating gave rise to a significant increase in the wear resistance. In both cases, at the end of the test the burr height was limited and it seems that the tools may work for a very large number of strokes before the need for sharpening. References
Fig. 9. Wear damage in the matrix by hard metal, after 120,000 cycles.
4. Conclusions In the first stage of the present investigation, the progression of tool wear during a precision stamping operation was investigated. Punches and dies wear made by a heat-treated HSS and each operation took place in a boundary lubrication condition. Observed wear was due to adhesion (with some transfer) and after 160,000 strokes microcracking damage was also shown to start in the punch. Such microcracking is present at the die edges from the beginning of the test. The wear progression was also monitored by recoding the burr height of the stamped part and an experimental relationship between the punch wear and burr height progression was established. To improve the wear behaviour of the tools, three treatments were investigated: coating by a ceramic thin film, namely AlCrN
[1] S. Kalpakjian, Manufacturing Processes for Engineering Materials, AddisonWesley, 1984. [2] J.A. Shey, Introduction to Manufacturing Processes, 2nd ed., McGraw-Hill Book Company, 1987. [3] X.T. Zeng, S. Zhang, T. Muramatsu, Surface and Coating Technology 127 (2000) 38–42. [4] J. Vetter, R. Knaup, H. Dwuletzki, E. Schneider, S. Vogler, Surface and Coating Technology 86–87 (1996) 739–747. [5] H. Holleck, V. Schier, Surface and Coating Technology 76–77 (1995) 328–337. [6] W.A. Glaeser, Materials for Tribology, Elsevier, 1992. [7] L.J. Yang, Wear 250 (2001) 366–375. [8] A. Molinari, M. Pellizzari, S. Gialanella, G. Straffelini, K.H. Stiasny, Journal of Materials Processing Technology 118 (2001) 350–355. [9] D. Mohan Lal, S. Renganarayanan, A. Kalanidhi, Cryogenics 41 (2001) 149–155. [10] D. Das, K.K. Ray, A.K. Dutta, Wear 267 (2009) 1361–1370. [11] G. Cusanelli, A. Hessler-Wyser, F. Bobard, R. Demellayer, R. Perez, R. Flukiger, Journal of Materials Processing Technology 149 (2004) 289–295. [12] G. Straffelini, Wear 249 (2001) 79–85. [13] J. Vetter, E. Lugsheider, S.S. Guerreiro, Surface and Coating Technology 98 (1998) 1233–1239. [14] J.L. Mo, M.H. Zhu, Wear 267 (2009) 874–881. [15] J.L. Mo, M.H. Zhu, B. Lei, Y.X. Leng, N. Huang, Wear 263 (2007) 1423–1429. [16] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino, International Journal of Refractory Metals & Hard Materials 24 (2006) 399–404. [17] Hard Metal for the Tool and Die Industry, Ceratizit Product Catalogue, 2009. [18] U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (1999) 262–269.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Improving the wear resistance of tools for stamping G. Straffelini a,∗ , G. Bizzotto b , V. Zanon b a b
Department of Materials Engineering and Industrial Design, University of Trento, via Mesiano 77, 38050 Trento, Italy Mevis S.p.A., Vicenza, Italy
a r t i c l e
i n f o
Article history: Received 10 March 2010 Received in revised form 5 June 2010 Accepted 7 July 2010 Available online 15 July 2010 Keywords: Tools wear Stamping Sliding wear PVD coatings Hard metals Cryogenic treatments
a b s t r a c t In the present investigation, the wear resistance of tools for the precision stamping of a mechanical part for the automotive industry is optimised. The investigation is divided into two stages. In the first one, the progression of damage in the punches and dies is assessed. Wear mechanisms are evaluated by means of observations and measurements in a scanning electron microscope (SEM). In the second stage, specific improvements are proposed based on the results of the first stage and are evaluated by specific field tests. The improvements include coating by physical vapour deposition with a AlCrN layer, production of the tools with hard metal and cryogenic treatment of the high-speed steel tools. The results show that all the proposed treatments help improve the performance of the tools, and the best improvements are displayed by the use of hard metals and coated tools. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Punches and dies undergo severe wear during precision stamping of mechanical parts, especially in the case of blanks made by heat-treated steels. Wear is caused by sliding and may be characterised by adhesion, abrasion, transfer phenomena and also by brittle surface microcracking. Such damage impairs the quality of the stamped product. The tools must therefore be periodically replaced, often at a significant economic cost [1–3]. The present investigation makes reference to the production of a mechanical part for the automotive industry. Punches and dies are made with a powder metallurgy high-speed steel (HSS), and the operation takes place under constant lubrication. The investigation is divided into two stages. In the first stage, the progression of tool damage is assessed as a function of the parts produced. In this stage, a simple approach to follow the damage progression of the tools is also established. In the second stage, specific improvements are proposed based on the results of the first part of the investigation. Such improvements include coating by a ceramic thin film, which is produced by physical vapour deposition (PVD), production of the tools by hard metal, and deep cryogenic treatment of the HSS. As is well known, several PVD coatings are now available to increase the tool wear performance in boundary conditions that are usually encountered during precision stamping [3–5]. The basic requirements are good adhesion to the base steel in order to coun-
∗ Corresponding author. Tel.: +39 0461 882458; fax: +39 0461 881977. E-mail address: [email protected] (G. Straffelini). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.07.004
teract the shearing stresses without microcracking or delaminating and high hardness in order to induce low wear rates and a low friction coefficient. Hard metals are produced by powder metallurgy, and they are very hard because of the presence of ceramic carbide particles in their microstructure. They also show a fracture toughness larger than that of ceramics because of the presence of a metal matrix (typically cobalt). Because of this, hard metals display a very high resistance to dry and marginally lubricated sliding [6,7]. Finally, deep cryogenic treatment is reported to increase the sliding resistance of tools steels. It is usually applied in between hardening and tempering treatments, and the improvement in properties is attributed to the complete transformation of austenite into martensite and to the precipitation of ultrafine carbide particles [8–10]. The performance of the proposed treatments is thus evaluated and compared to that of the reference high-speed steel. Indications for the selection and practical application of the proposed treatments are thus obtained from specific field tests. 2. Experimental methods The objective of the investigation was the optimisation of the shaving step in the production of a mechanical part for the automotive industry. In Fig. 1 a, a detail of the part is shown. The blank has a thickness of 2.2 mm and is made of a cold rolled strip submitted to spheroidisation annealing. Its chemical composition is: 0.7% C, 0.7% Mn and 0.2% Si. In Fig. 1b, a sketch of the process is displayed. Punches and dies are made of a powder metallurgy S390 HSS produced by Böhler. Its
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G. Straffelini et al. / Wear 269 (2010) 693–697
Fig. 2. Detail of the worn region at the end of the punch after 200,000 strokes (SEM).
3. Results and discussion 3.1. Analysis of damage progression
Fig. 1. (a) Detail of the produced mechanical part and (b) sketch of the stamping process.
nominal chemical composition is 1.6% C, 4.8% Cr, 2% Mo, 5% V, 10.5% W and 8% Co. The blocks are heat-treated with following cycle: austenitising at 1150 ◦ C for 60 min under vacuum, gas quenching, cold treatment at −70 ◦ C for 20 h, and three tempering treatments at 540 ◦ C for 4 h. The blocks are machined by electric discharge machining (EDM) to the final shape. They are then sharpened to remove the white layer left by the EDM process [11]. The hardness of the heat-treated steel is 67.5 HRC (load: 150 kg). The shaving tools operate at room temperature under constant lubrication at a rate of 100 strokes/min. Around 100,000 parts are produced before tool sharpening is needed because of wear. To investigate the damage progression as a function of strokes (i.e., the number of produced parts), a field test was designed. Specific punches and dies were produced and tested up 40,000, 80,000, 120,000, 160,000 and 200,000 strokes. In each case, a new set of tools was used. The damage evolution has been determined by observations of the worn regions in a scanning electron microscope (SEM) and using an optical microscope (OM). In addition, the burr height in the part produced was measured as a function of the number of parts produced. On the basis of the experimental results, specific improvements were then proposed. The details of the alternative materials and treatments investigated will be provided in the next chapter. In the second stage of the investigation, specific field tests were then carried out to assess the relative performance of each proposed treatment. In each case, the same initial edge geometry was adopted.
We consider first the damage to the punch. Because of the sliding between the punch and the part to be shaved, the punch edges undergo sliding wear. This is shown in Fig. 2, which shows a punch edge after 200,000 strokes. Three regions can be recognised. In region 1 the worn track is flat and adhesive wear prevails. In region 2, some microcracks, normal to the sliding direction, are present. Such microcracks form under the action of the local friction stresses, which means that a boundary lubrication regime was present in the mating surfaces during each operation. In region 3, some transfer debris from the cut blanks can be observed. They remain attached to the surface of the punch because of adhesion [12]. The phenomenon of the transfer of debris to the surface of the punch (region 3 in Fig. 2), was shown to start after 80,000 strokes. The formation of the small cracks (region 2 in Fig. 2) was shown to start after 160,000 strokes. This means that both of these phenomena started after the occurrence of a critical amount of tool wear. The progression of wear with the number of strokes was evaluated by measuring the length of the worn region at a distance of 0.5 mm from the punch edge under the SEM. The results of this examination are shown in Fig. 3. They are the average of three
Fig. 3. Evolution of damage in the punch as a function of the number of parts produced.
G. Straffelini et al. / Wear 269 (2010) 693–697
Fig. 4. Damage appearance at the corner of the matrix after 200,000 strokes (SEM).
measurements taken in different tool positions. As expected, wear increases with the stroke number. The wear rate, however, is shown to increase in passing from 160,000 to 200,000 cycles, and this can be attributed to the formation of small cracks and the interaction of microcracking with adhesive wear. At the cutting edges of the die, the same wear process encountered in the case of the punch was observed. The only difference is that wear is more severe, probably because of worse lubrication. Both transfer and microcracking phenomena were seen to start from the beginning of the tests. As shown in Fig. 4, the contribution of microcracking is particularly severe at the corners. Wear progression was also characterised by measuring the height of the burr protruding below the bottom surface of the produced part. The measurements were carried out on 10 specimens using a 3D touch probe. The average burr height is reported in Fig. 5 as a function of strokes. The experimental standard deviation was found to be low and to increase from 0.02 mm after 40,000 strokes to 0.07 mm after 200,000 strokes. In Fig. 5 the evolution is reported as a function of strokes. It can be noted that the burr height increases with the strokes and that a plateau is reached after about 150,000 strokes. In practice, the production process is stopped when the burr height exceeds the limit of 0.3 mm. The tools are then resharpened and repositioned to continue the production. This occurs after the production of around 100,000 parts.
Fig. 5. Measured burr height as a function of strokes.
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Fig. 6. Measured burr height as a function of wear length at 0.5 mm from the punch edge.
As shown in Fig. 6, there is a clear experimental relationship between the evolution of wear length at 0.5 mm from the punch edge and the evolution of the burr height, which can be explained by considering that as wear is increased the clearance between the punch and the die is increased too, and a large clearance favours the formation of the burr. The comparison of Figs. 3 and 5 highlights that up to the limit of 0.3 mm in burr height, the punch underwent wear by adhesion with some debris transfer but with no microcracking. 3.2. New treatments and evaluation of their performance The tools under investigation underwent sliding wear under boundary lubrication. This is demonstrated by the observation of the wear traces and, in particular, by the occurrence of transfer phenomena and local microcracking. To increase the wear resistance it is therefore necessary to increase the hardness of the mating materials and reduce friction. In addition, it is also necessary to control the fracture toughness of the mating materials in order to reduce the risk of microcracking because of process characteristics, i.e., the occurrence of impact loading at the onset of shaving. These goals may be reached with different approaches that include the deposition of a ceramic thin film by physical vapour deposition (PVD), the production of the tools by hard metal, and the performance of cryogenic treatment of the HSS tools. On the bases of specific literature investigations [13–15], we decided to investigate the performance of a PVD AlCrN coating that is reported to give optimal behaviour for a variety of tools. A commercial AlCrN (Alcrona) coating was thus selected, and it was realised by Oerlikon Balzers Italy in standard Balzers conditions [16]. The coating was deposited on the S390 HSS tools in the mirror polished condition. For the present investigation, specific tools were also produced by EDM of commercial hard metal blocks, namely Ceratizit H40S blocks. This material has a hardness of 1340 HV30, and it is reported to have a fracture toughness of 12 MPa m1/2 [17]. Two cryotreatments of the S390 HSS were investigated. In the first (codenamed: QCT), austenitisation was carried out at 1150 ◦ C for 60 min under vacuum and quenching was done with N2 with a pressure of 5.5 bar. The cryogenic treatment was then carried out at −196 ◦ C (in liquid nitrogen) for 20 h. It was then followed by three tempering treatments at 540 ◦ C for 4 h. In the second cycle (codenamed QTC), the same deep cryogenic treatment was carried out after the three tempering treatments. This last procedure is more
696
G. Straffelini et al. / Wear 269 (2010) 693–697
Fig. 7. Measured burr height as a function of cycles for the different materials investigated.
flexible than the other one, and it has been shown that it may produce an effective increase in the wear resistance of tool steels. No difference was detected between the hardness of the cryotreated steels: a value of 68.5 HRC for both cycles was measured. This value is slightly higher than that of the treated HSS without the cryogenic stage. The performance of the selected treatments was then evaluated in field tests up to 120,000 strokes. Following the indications of the first stage of the present investigation, the wear evolution was followed in each case by simply measuring the burr height of the stamped part at intervals of 30,000 strokes. The results of the investigation are shown in Fig. 7. They are average values of 10 measurements. In all cases, the standard deviations were low and proportional to the average burr height. For example, after 120,000 strokes the standard deviation of S390 was 0.07 mm and that of the AlCrN coating was 0.02 mm. It can be noted that the measured burr height in the tools made by the S390 HSS (the reference condition) is higher than that recorded in the first stage of the investigation and shown in Fig. 5. This result confirms the fact that field tests are characterised by some scatter because of the variability of the operating conditions. The results in Fig. 7 further show that the cryogenic treatments induce an improvement in the wear resistance but the comparison with the data in Fig. 5 shows that such an improvement is limited and it may be within the experimental scatter. No particular difference may be detected between the QTC and QCT treatments. On the other hand, both the use of the hard metal and the AlCrN coating produced a significant increase in the wear resistance of the tools. In both cases, at the end of the test the burr height was well below the limit of 0.3 mm and it seems that the tools may work for a very large number of strokes before the need for sharpening. In particular, the best improvement is displayed by the thin film coating. To gain further information on the damage progression in the proposed treated tools, the worn traces were observed by OM after 120,000 strokes. In Fig. 8 the observations of the worn surfaces in the dies are shown. To allow comparison, the adhesive wear of the reference S390 die is shown (Fig. 8a). As expected, the damage is very similar to that encountered in the first stage of the investigation. Similar behaviour was displayed by the cryotreated tools, and therefore the relevant observations are not reported here.
Fig. 8. Sliding wear in the matrix after 120,000 strokes. (a) S390 reference steels; (b) AlCrN coated steel; (c) hard metal.
In Fig. 8b the worn surface of the coated die is shown. It can be noted that in some regions the film is removed. As demonstrated by the experimental results and already reported in other investigations [18], however, the coating is still able to protect the tool and to guarantee a good quality of the stamped part, even if partially destroyed. In Fig. 8c the worn surface of the hard metal is shown. The wear extent is low but the presence of small cracks may be detected. The presence of such cracks may be better appreciated by a bottom view of the die, as shown in Fig. 9. Such cracking is due to the relatively low fracture toughness of the hard metals coupled with the presence of impact loading during the stamping process.
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produced by physical vapour deposition (PVD); production of the tools by hard metal; and two deep cryogenic treatments of the HSS. The comparative behaviour of the treated tools was investigated in the second stage of the investigation by means of specific field tests. The results show that the cryogenic treatments induce an improvement in the wear resistance. The comparison with the data obtained in the first stage, however, shows that such an improvement is limited and it may be within the experimental scatter. On the contrary, both the use of the hard metal and the AlCrN coating gave rise to a significant increase in the wear resistance. In both cases, at the end of the test the burr height was limited and it seems that the tools may work for a very large number of strokes before the need for sharpening. References
Fig. 9. Wear damage in the matrix by hard metal, after 120,000 cycles.
4. Conclusions In the first stage of the present investigation, the progression of tool wear during a precision stamping operation was investigated. Punches and dies wear made by a heat-treated HSS and each operation took place in a boundary lubrication condition. Observed wear was due to adhesion (with some transfer) and after 160,000 strokes microcracking damage was also shown to start in the punch. Such microcracking is present at the die edges from the beginning of the test. The wear progression was also monitored by recoding the burr height of the stamped part and an experimental relationship between the punch wear and burr height progression was established. To improve the wear behaviour of the tools, three treatments were investigated: coating by a ceramic thin film, namely AlCrN
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