- Email: [email protected]
Influence of the application of a PN+CrN hybrid layer on improvement of the lifetime of hot forging tools
Available online at www.sciencedirect.com
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 207 (2017) 514–519
International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom
Influence of the application of a PN+CrN hybrid layer on improvement of the lifetime of hot forging tools. a a* a a Zbigniew Gronostajski , Marek Hawryluk , Paweł Widomski , Marcin Kaszuba , Jacek Ziembaa, Jerzy Smolikb a Wroclaw
Wroclaw University of Science and Technology, Lukasiewicza Street 5, Wroclaw 50-371, Poland Institute for Sustainable Technologies – National Research Institute, K. Pulaskiego Street 6/10, Radom 26-600, Poland
bInstitute
Abstract This article presents the results of field tests of forging tools in a selected hot forging process of cover plate. The tools with PN+CrN hybrid layer, applied to improve their operating life, were tested in comparison to gas-nitrided tools. The hybrid layer was produced on a plasma-nitrided substrate, onto which a PVD Cr/CrN coating was deposited. All analyzed tools were tested in industrial conditions by manufacturing specific quantities of forgings. Next, the wear of each tool was analyzed by surface scanning and comparison to the CAD model. All tools were checked for changes in the surface layer under a scanning electron microscope. The tests confirmed the effect of improved forging tool lifetime thanks to the application of the PN+Cr/CrN hybrid layer. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility responsibility of of the thescientific scientific committee the International Conference on theofTechnology Peer-review under committee of theof International Conference on the Technology Plasticity. of Plasticity. Keywords: hot forging, hybrid layer, durability of forging tools
1. Introduction During work, forging tools are exposed to many destructive factors [1]. Three primary factors causing tool destruction can be distinguished in hot forging processes, i.e. intense heat shocks, cyclically variable mechanical loads and intense friction, and in consequence, the dominant wear mechanisms are thermomechanical fatigue and abrasive wear [1-4]. The lifetime of forging tools, understood as their resistance to the aforementioned destructive factors, has been the subject of research in
* Corresponding author. Tel: +48-71-320-21-64; fax: +48-71-320-21-73. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee Plasticity.
of the International Conference on the Technology of
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.814
2
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many scientific and industrial centers. Attempts are being made to apply various methods intended to improve tool life [2, 5]. Nitriding is currently the most popular method of improving the lifetime of forging tools. Nitriding increases tools' resistance to abrasion, fatigue strength, and improves corrosion resistance. Observations of many industrial forging processes in which nitrided tools were applied have proven that this treatment makes it possible to increase tool lifetime several times over. Studies have shown that tools must have a specific, uniform structure for the nitrided layer to improve tool life effectively [6]. Mechanical working methods intended to improve the functional properties of tool surface layers include ball burnishing. It is based on cold dynamic surface working. Surface layers produced by the ball burnishing process are characterized by high absolute values of maximum internal stresses and yield point. Tools made of hot (and cold) work tool steel are increasingly often subjected to cryogenic treatment, or deep freezing. This makes it possible to rather significantly reduce the amount of retained austenite, increase hardness, and in combination with other technologies, to increase tool life [4, 5]. The latest methods undoubtedly include hybrid technologies involving the application of two or more surface engineering techniques [2,6,7]. Hybrid techniques may combine e.g. thermochemical treatment methods and a PVD technique [2,8]. This technology makes it possible to endow the surface with the appropriate functional properties and to create a barrier that will effectively limit the impact of destructive factors [8]. The authors have conducted field tests of punches used in the hot forging process, which were improved according to two different methods for the purpose of comparing lifetimes. These methods were: thermomechanical treatment through nitriding and coating with a hybrid layer consisting of a nitrided layer and PVD coating. 2. Research material Forming tools used in the hot forging process of a cover plate manufactured at Jawor Forge in three operations (upset forging, preliminary die forging, finishing die forging) on a P - 1800 T press were tested. The preform was a cylindrical billet of C45 steel with a diameter of 55 mm, height 95 mm, weight 1.77 kg and temperature 1150 °C. Figure 1 presents a view of tools mounted on the press (a) as well as the scheme of tool positioning in the final forming stage of the second operation. The top punch (b) was singled out for further analysis. a
c
b Operation II
Top die insert Analyzed punch
Operation III
Bottom die insert
Operation I
Fig. 1. (a) Massey 1800T press with forming equipment; (b) analyzed punch; (c) scheme of the blocking operation for the produced element.
2.1. Analysis of process conditions The top punch, which was analyzed in detail, is characterized by low, unstable and variable tool life. To identify the causes of damage, tool working conditions during the forging process were simulated using FEM. The local presence of high pressing forces and intense temperature spikes, the values of which were verified by thermovisual tests, was demonstrated (Fig. 2). a
MPa
b
ºC
c
d
Fig. 2. (a) normal forces [MPa] and (b) punch temperature [°C] on the contact surface of the punch with the forging at the end of forging process, obtained from FEM simulations; (c) thermogram of analyzed tool; (d) surfaces selected for further analysis marked on CAD model.
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Based on simulations, the tool's temperature was determined to change within the range of 200-550ºC and normal forces within the range from 0 to 1283 MPa. This indicates the destructive influence of thermomechanical fatigue, which may be the main destructive mechanism. Temperature tests conducted on tool surfaces using a thermovisual camera confirmed FEM results and the fact that critically high thermal loads were present, which may cause tempering of the material in the surface layer. 2.2. Applied lifetime improvement methods To improve the lifetime of the tested tools, thermochemical treatment technologies were applied, such as: plasma nitriding (PN), gas nitriding (GN) as well as hybrid layers consisting of a plasma nitrided layer coated with a PVD coating, which make it possible to increase wear resistance through the synergic interaction of two surface engineering methods. A nitrided layer with a surface hardness of 1200 HV0.1 and layer of effective hardness 800 HV0.1 on thickness of 70 µm was produced on plasma nitrided tools. The process of plasma/ion nitriding was performed on a multi-functional CDS Standards process station with the use of an optical nitriding intensity control system and the Arc-PVD process. Gas nitrided layers were made in Jawor Forge with a surface hardness of 1200 HV0.1 and layer of effective hardness 800 HV0.1 on thickness of 90 µm. The initial value of hardness of tool steel after heat treatment before nitriding was 550-600 HV. Additionally, the friction coefficient of nitrided layer to steel was measured by ball-on-disc tester and value was: 0,33. The Cr/CrN PVD coating was deposited using single-ingredient cathodes (Cr), which were vaporized by a lowpressure electric arc in a nitrogen atmosphere in an MZ 383 Metaplas Ionon vacuum coater. Cathodes were properly distributed within the chamber, and the substrate was set into rotation in order to obtain the Cr/CrN coating, which is a monolayer coating of chromium nitride deposited on a thin substrate of pure chromium. Table 1 contains data concerning the properties and structure of the obtained coating. Table 1. Properties of deposited PVD coating. Coating Layer thicknesses Overall thickness Hardness Young's Modulus Adhesion to substrate Friction name gw [µm] g[µm] H [GPa] E [GPa] Lc1/2/3 [N] coefficient µ PN+CrN
Cr – 0.1 CrN – 7.2
9
24±1.4
215±25
70/80/140
0.32
The 10 tools were produced using the technologies presented above: 5 gas nitrided tools and 5 tools with the PN+CrN layer, and these tools were used to manufacture a specific quantity of forgings. Because the process was performed under industrial conditions, the quantities of forgings manufactured needed to be adjusted to the production line's schedule, so the number of manufactured forgings oscillates around the expected value. Information concerning the analyzed punches is given below in table 2, according to the type of surface treatment applied and the number of forgings manufactured by each punch. Table 2. List of analyzed tools. No. 1 2 3 4 5
Material/treatment
WCL/ gas nitriding (GN)
Number of forged pcs. 1000 2500 6000 9000 12 830
No.
Material/treatment
6 7 8 9 10
WCL/PN+CrN
Number of forged pcs. 4000 5300 6000 10 000 13 000
3. Test results 3.1. Tests of gas nitrided tools Tools were made from X37CrMoV5-1 steel according to technology covering preforming, heat treatment, finishing work and gas nitriding, which was performed in an ammonia atmosphere at a temperature of 550ºC for a time of approx. 40h. As a result of such treatment, a thin layer of ε nitrides is formed on the tool's surface, and a diffusive layer of α+γ’ nitrides is formed at a depth of approx. 0.2 mm. Fig. 3 presents the microstructure of the obtained layer's cross-section.
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a
517
b
Fig. 3. a) microstructure of gas nitrided layer, b) detailed view of nitrides in the surface layer
3.1.1. Macroscopic analysis As a result of exploitation, the tested tools underwent visible wear, which was observed to initially assess its size and set out the direction of further studies. Fig. 4 shows the surface of a tool after exploitation. a
b
c
d
e
Fig. 4 Macroscopic view of the surface of nitrided punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
Furrows and deep chipping of the surface layer appear on the frontal surface of punches after just 2500 pieces are manufactured (fig. 5b). Next, the volume of lost material is enlarged until it covers the entire frontal surface. After that, wear progresses deeper through abrasive wear, leaving behind characteristic furrows in the direction of the material's flow. A tool can be used even with such deep wear until cracks and damage form on the edge between the frontal surface and side surface. 3.1.2. Geometrical analysis Tool surfaces were scanned and compared to the geometrical CAD model for the purpose of measuring wear depth. The results of this comparison are presented in fig. 5. a
b
c
d
e
Fig. 5. Comparison of the geometry of used punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
Conducted analysis confirmed the presence of deep (1-2 mm) material losses on the frontal surfaces of tested punches, which appear after forging of 2500 pcs. and develop until wear covers the entire analysed surface. 3.1.3. SEM analysis Observations made under a Tescan Vega electron microscope are presented in Fig. 6. a
b
c
d
e
Fig. 6. SEM image of selected in Fig. 2 area for nitrided punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
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SEM analysis revealed the presence of a mesh of fatigue cracks on tool surfaces, appearing after forging of 1000 pcs. (Fig. 6a). Areas with cracks are chipped away (Fig. 6b and 6c), causing a deep loss to develop from the center of the tool. Abrasive wear occurs in worn areas, manifesting as furrows (Fig. 6d) and plastic deformations, and a secondary mesh of thermomechanical cracks is formed (Fig. 6e). 3.1.4. Description of wear on nitrided tools The analyzed nitrided tools exhibit low wear resistance. They are mainly damaged by thermomechanical fatigue, which forms a crack mesh on their surface over forging of just several hundred forgings. Next, tempering of the material under the nitrided layer takes place at a depth of 0.2 mm, which deepens the structural indentation and leads to gradual chipping and removal of the surface layer up to a depth of 2 mm after approx. 2000 forgings. In the final stage of exploitation (above 9000 forgings), the tool works without the nitrided layer, undergoing abrasive wear and small plastic deformations. 3.2. Tools with the PN+CrN hybrid layer Tools were thermochemically treated by ion (plasma) nitriding and coated with a monolayer CrN coating. Tests showed the following initial structure of ready-for-work tools, which is presented in Fig. 7. b
a
Fig. 7. Polished and etched cross-section of tool with PN+CrN hybrid layer, magnification: (a) 500x; (b) 1000x.
The coating that can be seen in the figure is uniform and does not contain many droplet phase precipitates. Its thickness is approx. 9µm and remains constant throughout the analysed surface area. 3.2.1. Geometrical analysis of wear To conduct quantitative wear assessment, tool surfaces were scanned after exploitation and compared to the CAD model. The results are presented in Fig. 8. a
b
c
d
e
Fig. 8. Wear distribution in punches with the PN+CrN layer after forging of: (a) 4000; (b) 5300 pcs.; (c) 6000 pcs.; (d) 10000 pcs.; (e) 13000 pcs..
Analysis of the tool scans shown in Fig. 8 clearly indicates a two-stage wear process. The first stage covers production of approx. 5-6 thousands forgings and is characterized by low, nearly zero, tool wear, since the tool is protected by the applied PVD coating. In the second stage, this coating is removed, and abrasive wear progresses gradually, forming uniformly distributed furrows, extending radially and reaching a depth up to approx. 2 mm. 3.2.2. SEM observations Changes occurring on the surfaces of the tested tools were observed using a Tescan Vega microscope. Selected photographs are presented in fig. 9.
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c
d
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e
Fig. 9. SEM of the surface from analyzed area for PN+CrN punches after forging of: : (a) 4000; (b) 5300 pcs.; (c) 6000 pcs.; (d) 10000 pcs.; (e) 13000 pcs..
A coherent mesh of fatigue cracks can be seen on the tool's surface in fig. 9a and 9b, with an average crack length of approx. 500 µm, and these cracks do not exhibit a tendency to expand, thanks to which the coating continues to adhere to the surface despite numerous cracks. Fig. 11c shows the first chipped fragments of the CrN coating, which corresponds to the time of coating destruction, after forging of approx. 6000 forgings. The next photograph (fig. 9d) presents the surface after removal of the coating and deepened abrasive wear, with a secondary fatigue crack mesh, and fig. 11e also shows furrows indicating deep abrasive wear. 3.2.3. Description of the wear mechanism of tools with the PN+CrN hybrid layer The application of a PVD coating, besides just nitriding, which forms a hybrid layer together with the nitrided layer, clearly results in improvement of lifetime. Wear occurs in two stages, the first being based on the formation of a fatigue crack mesh without abrasion-related loss of material up to approx. 6000 forgings, as long as the PVD coating protecting the nitrided layer against abrasive wear and partially against heat is preserved. Next, after the coating is damaged and removed, a secondary crack mesh is formed and gradual abrasive wear occur, and the surface layer is tempered as a result of contact with hot material. 4. Conclusion This article presents an analysis of the influence of the application of a hybrid layer on tool lifetime in the hot forging process. It has been demonstrated that the application of ion nitriding in combination with a PVD coating has a significant impact on tool wear and lifetime. This impact particularly pertains to the early stage of exploitation (up to 6000 pcs.), while the coating remains on the tool, protecting it against abrasive wear and serving as a barrier against thermal shocks. Arresting the progression of wear in this way causes real tool wear to begin later and thus extends tool life, particularly in the case of a CrN coating. Research [2] has shown that the CrN coating is characterized by high resistance to damage and heat. This is due to the combination of several beneficial properties, including good adhesion to the substrate, good tribological properties, and the characteristic ability to dissipate and carry variable stresses occurring during forging. The beneficial properties of hybrid layers will continue to be researched for the purpose of improving tool lifetimes and can successfully be applied to forging tools, particularly tools for precision forging, where high dimensional accuracy of forgings is expected. Acknowledgements This study was found by National Centre for Research and Development, Poland (NCBiR) (grant no. PBS2/A5/37/2013). References
[1] B.A Behrens, A. Bouguecha, C. Buse, K. Wölki, A. Santangelo, Potentials of in situ monitoring of aluminum alloy forging by acoustic emission, Arch of Civ. and Mech. Eng. 16 (2016) 724-733. [2] J. Smolik, Role of hybrid nitrided/PVD layers in the process of improving the lifetime of forging dies, WITE, Radom, 2007. [3] Z. Gronostajski, M. Kaszuba, M. Hawryluk, M. Zwierzchowski. A review of the degradation mechanisms of the hot forging tools. Arch. of Civ. and Mech. Eng. 14 vol.4 (2014) 528-539. [4] L. Rauch, A. Chmura, Z. Gronostajski, S. Polak, M. Pietrzyk, Cellular automata model for prediction of crack initiation and propagation in hot forging tools Original Research, Arch. of Civ. and Mech. Eng. 16 (2016) 437-447. [5] M. Hawryluk, Review of selected methods of increasing the life of forging tools in hot die forging processes. Arch. of Civ. and Mech. Eng. 16 vol. 4 (2016) 845-866. [6] J. Wendland, L. Małdziński, J. Borowski, K. Ostrowska, T. Darul: Trwałość matryc azotowanych metodą ZeroFlow [Lifetime of dies nitrided using the ZeroFlow method]. Obróbka Plastyczna Metali [Plastic Working of Metals] vol. XXV no. 3 (2014), pp. 185–200. [7] A. Sawka, A. Kwatera, A. Woźnicki, J. Zasadziński, Cemented carbide cutting tools life with nanocrystalline Al2O3 layer deposited by MOCVD, Arch. of Civ. and Mech. Eng. 16 vol.4 (2016) 845-866. [8] H. Paschke, M. Steuber, C. Ziebert, M. Bistron, Composition, microstructure and mechanical properties of boron containing multilayer coatings for hot forming tools, Surf. and Coat. Techn. 205 vol. 2 (2011) 24-28.
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 207 (2017) 514–519
International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom
Influence of the application of a PN+CrN hybrid layer on improvement of the lifetime of hot forging tools. a a* a a Zbigniew Gronostajski , Marek Hawryluk , Paweł Widomski , Marcin Kaszuba , Jacek Ziembaa, Jerzy Smolikb a Wroclaw
Wroclaw University of Science and Technology, Lukasiewicza Street 5, Wroclaw 50-371, Poland Institute for Sustainable Technologies – National Research Institute, K. Pulaskiego Street 6/10, Radom 26-600, Poland
bInstitute
Abstract This article presents the results of field tests of forging tools in a selected hot forging process of cover plate. The tools with PN+CrN hybrid layer, applied to improve their operating life, were tested in comparison to gas-nitrided tools. The hybrid layer was produced on a plasma-nitrided substrate, onto which a PVD Cr/CrN coating was deposited. All analyzed tools were tested in industrial conditions by manufacturing specific quantities of forgings. Next, the wear of each tool was analyzed by surface scanning and comparison to the CAD model. All tools were checked for changes in the surface layer under a scanning electron microscope. The tests confirmed the effect of improved forging tool lifetime thanks to the application of the PN+Cr/CrN hybrid layer. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility responsibility of of the thescientific scientific committee the International Conference on theofTechnology Peer-review under committee of theof International Conference on the Technology Plasticity. of Plasticity. Keywords: hot forging, hybrid layer, durability of forging tools
1. Introduction During work, forging tools are exposed to many destructive factors [1]. Three primary factors causing tool destruction can be distinguished in hot forging processes, i.e. intense heat shocks, cyclically variable mechanical loads and intense friction, and in consequence, the dominant wear mechanisms are thermomechanical fatigue and abrasive wear [1-4]. The lifetime of forging tools, understood as their resistance to the aforementioned destructive factors, has been the subject of research in
* Corresponding author. Tel: +48-71-320-21-64; fax: +48-71-320-21-73. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee Plasticity.
of the International Conference on the Technology of
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.814
2
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515
many scientific and industrial centers. Attempts are being made to apply various methods intended to improve tool life [2, 5]. Nitriding is currently the most popular method of improving the lifetime of forging tools. Nitriding increases tools' resistance to abrasion, fatigue strength, and improves corrosion resistance. Observations of many industrial forging processes in which nitrided tools were applied have proven that this treatment makes it possible to increase tool lifetime several times over. Studies have shown that tools must have a specific, uniform structure for the nitrided layer to improve tool life effectively [6]. Mechanical working methods intended to improve the functional properties of tool surface layers include ball burnishing. It is based on cold dynamic surface working. Surface layers produced by the ball burnishing process are characterized by high absolute values of maximum internal stresses and yield point. Tools made of hot (and cold) work tool steel are increasingly often subjected to cryogenic treatment, or deep freezing. This makes it possible to rather significantly reduce the amount of retained austenite, increase hardness, and in combination with other technologies, to increase tool life [4, 5]. The latest methods undoubtedly include hybrid technologies involving the application of two or more surface engineering techniques [2,6,7]. Hybrid techniques may combine e.g. thermochemical treatment methods and a PVD technique [2,8]. This technology makes it possible to endow the surface with the appropriate functional properties and to create a barrier that will effectively limit the impact of destructive factors [8]. The authors have conducted field tests of punches used in the hot forging process, which were improved according to two different methods for the purpose of comparing lifetimes. These methods were: thermomechanical treatment through nitriding and coating with a hybrid layer consisting of a nitrided layer and PVD coating. 2. Research material Forming tools used in the hot forging process of a cover plate manufactured at Jawor Forge in three operations (upset forging, preliminary die forging, finishing die forging) on a P - 1800 T press were tested. The preform was a cylindrical billet of C45 steel with a diameter of 55 mm, height 95 mm, weight 1.77 kg and temperature 1150 °C. Figure 1 presents a view of tools mounted on the press (a) as well as the scheme of tool positioning in the final forming stage of the second operation. The top punch (b) was singled out for further analysis. a
c
b Operation II
Top die insert Analyzed punch
Operation III
Bottom die insert
Operation I
Fig. 1. (a) Massey 1800T press with forming equipment; (b) analyzed punch; (c) scheme of the blocking operation for the produced element.
2.1. Analysis of process conditions The top punch, which was analyzed in detail, is characterized by low, unstable and variable tool life. To identify the causes of damage, tool working conditions during the forging process were simulated using FEM. The local presence of high pressing forces and intense temperature spikes, the values of which were verified by thermovisual tests, was demonstrated (Fig. 2). a
MPa
b
ºC
c
d
Fig. 2. (a) normal forces [MPa] and (b) punch temperature [°C] on the contact surface of the punch with the forging at the end of forging process, obtained from FEM simulations; (c) thermogram of analyzed tool; (d) surfaces selected for further analysis marked on CAD model.
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3
Based on simulations, the tool's temperature was determined to change within the range of 200-550ºC and normal forces within the range from 0 to 1283 MPa. This indicates the destructive influence of thermomechanical fatigue, which may be the main destructive mechanism. Temperature tests conducted on tool surfaces using a thermovisual camera confirmed FEM results and the fact that critically high thermal loads were present, which may cause tempering of the material in the surface layer. 2.2. Applied lifetime improvement methods To improve the lifetime of the tested tools, thermochemical treatment technologies were applied, such as: plasma nitriding (PN), gas nitriding (GN) as well as hybrid layers consisting of a plasma nitrided layer coated with a PVD coating, which make it possible to increase wear resistance through the synergic interaction of two surface engineering methods. A nitrided layer with a surface hardness of 1200 HV0.1 and layer of effective hardness 800 HV0.1 on thickness of 70 µm was produced on plasma nitrided tools. The process of plasma/ion nitriding was performed on a multi-functional CDS Standards process station with the use of an optical nitriding intensity control system and the Arc-PVD process. Gas nitrided layers were made in Jawor Forge with a surface hardness of 1200 HV0.1 and layer of effective hardness 800 HV0.1 on thickness of 90 µm. The initial value of hardness of tool steel after heat treatment before nitriding was 550-600 HV. Additionally, the friction coefficient of nitrided layer to steel was measured by ball-on-disc tester and value was: 0,33. The Cr/CrN PVD coating was deposited using single-ingredient cathodes (Cr), which were vaporized by a lowpressure electric arc in a nitrogen atmosphere in an MZ 383 Metaplas Ionon vacuum coater. Cathodes were properly distributed within the chamber, and the substrate was set into rotation in order to obtain the Cr/CrN coating, which is a monolayer coating of chromium nitride deposited on a thin substrate of pure chromium. Table 1 contains data concerning the properties and structure of the obtained coating. Table 1. Properties of deposited PVD coating. Coating Layer thicknesses Overall thickness Hardness Young's Modulus Adhesion to substrate Friction name gw [µm] g[µm] H [GPa] E [GPa] Lc1/2/3 [N] coefficient µ PN+CrN
Cr – 0.1 CrN – 7.2
9
24±1.4
215±25
70/80/140
0.32
The 10 tools were produced using the technologies presented above: 5 gas nitrided tools and 5 tools with the PN+CrN layer, and these tools were used to manufacture a specific quantity of forgings. Because the process was performed under industrial conditions, the quantities of forgings manufactured needed to be adjusted to the production line's schedule, so the number of manufactured forgings oscillates around the expected value. Information concerning the analyzed punches is given below in table 2, according to the type of surface treatment applied and the number of forgings manufactured by each punch. Table 2. List of analyzed tools. No. 1 2 3 4 5
Material/treatment
WCL/ gas nitriding (GN)
Number of forged pcs. 1000 2500 6000 9000 12 830
No.
Material/treatment
6 7 8 9 10
WCL/PN+CrN
Number of forged pcs. 4000 5300 6000 10 000 13 000
3. Test results 3.1. Tests of gas nitrided tools Tools were made from X37CrMoV5-1 steel according to technology covering preforming, heat treatment, finishing work and gas nitriding, which was performed in an ammonia atmosphere at a temperature of 550ºC for a time of approx. 40h. As a result of such treatment, a thin layer of ε nitrides is formed on the tool's surface, and a diffusive layer of α+γ’ nitrides is formed at a depth of approx. 0.2 mm. Fig. 3 presents the microstructure of the obtained layer's cross-section.
4
Zbigniew Gronostajski et al. / Procedia Engineering 207 (2017) 514–519 Zbigniew Gronostajski et al. / Procedia Engineering 00 (2017) 000–000
a
517
b
Fig. 3. a) microstructure of gas nitrided layer, b) detailed view of nitrides in the surface layer
3.1.1. Macroscopic analysis As a result of exploitation, the tested tools underwent visible wear, which was observed to initially assess its size and set out the direction of further studies. Fig. 4 shows the surface of a tool after exploitation. a
b
c
d
e
Fig. 4 Macroscopic view of the surface of nitrided punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
Furrows and deep chipping of the surface layer appear on the frontal surface of punches after just 2500 pieces are manufactured (fig. 5b). Next, the volume of lost material is enlarged until it covers the entire frontal surface. After that, wear progresses deeper through abrasive wear, leaving behind characteristic furrows in the direction of the material's flow. A tool can be used even with such deep wear until cracks and damage form on the edge between the frontal surface and side surface. 3.1.2. Geometrical analysis Tool surfaces were scanned and compared to the geometrical CAD model for the purpose of measuring wear depth. The results of this comparison are presented in fig. 5. a
b
c
d
e
Fig. 5. Comparison of the geometry of used punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
Conducted analysis confirmed the presence of deep (1-2 mm) material losses on the frontal surfaces of tested punches, which appear after forging of 2500 pcs. and develop until wear covers the entire analysed surface. 3.1.3. SEM analysis Observations made under a Tescan Vega electron microscope are presented in Fig. 6. a
b
c
d
e
Fig. 6. SEM image of selected in Fig. 2 area for nitrided punches after forging of: (a) 1000; (b) 2500 pcs.; (c) 6000 pcs.; (d) 9000 pcs.; (e) 12,830 pcs..
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SEM analysis revealed the presence of a mesh of fatigue cracks on tool surfaces, appearing after forging of 1000 pcs. (Fig. 6a). Areas with cracks are chipped away (Fig. 6b and 6c), causing a deep loss to develop from the center of the tool. Abrasive wear occurs in worn areas, manifesting as furrows (Fig. 6d) and plastic deformations, and a secondary mesh of thermomechanical cracks is formed (Fig. 6e). 3.1.4. Description of wear on nitrided tools The analyzed nitrided tools exhibit low wear resistance. They are mainly damaged by thermomechanical fatigue, which forms a crack mesh on their surface over forging of just several hundred forgings. Next, tempering of the material under the nitrided layer takes place at a depth of 0.2 mm, which deepens the structural indentation and leads to gradual chipping and removal of the surface layer up to a depth of 2 mm after approx. 2000 forgings. In the final stage of exploitation (above 9000 forgings), the tool works without the nitrided layer, undergoing abrasive wear and small plastic deformations. 3.2. Tools with the PN+CrN hybrid layer Tools were thermochemically treated by ion (plasma) nitriding and coated with a monolayer CrN coating. Tests showed the following initial structure of ready-for-work tools, which is presented in Fig. 7. b
a
Fig. 7. Polished and etched cross-section of tool with PN+CrN hybrid layer, magnification: (a) 500x; (b) 1000x.
The coating that can be seen in the figure is uniform and does not contain many droplet phase precipitates. Its thickness is approx. 9µm and remains constant throughout the analysed surface area. 3.2.1. Geometrical analysis of wear To conduct quantitative wear assessment, tool surfaces were scanned after exploitation and compared to the CAD model. The results are presented in Fig. 8. a
b
c
d
e
Fig. 8. Wear distribution in punches with the PN+CrN layer after forging of: (a) 4000; (b) 5300 pcs.; (c) 6000 pcs.; (d) 10000 pcs.; (e) 13000 pcs..
Analysis of the tool scans shown in Fig. 8 clearly indicates a two-stage wear process. The first stage covers production of approx. 5-6 thousands forgings and is characterized by low, nearly zero, tool wear, since the tool is protected by the applied PVD coating. In the second stage, this coating is removed, and abrasive wear progresses gradually, forming uniformly distributed furrows, extending radially and reaching a depth up to approx. 2 mm. 3.2.2. SEM observations Changes occurring on the surfaces of the tested tools were observed using a Tescan Vega microscope. Selected photographs are presented in fig. 9.
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Zbigniew Gronostajski et al. / Procedia Engineering 207 (2017) 514–519 Zbigniew Gronostajski et al. / Procedia Engineering 00 (2017) 000–000
b
c
d
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e
Fig. 9. SEM of the surface from analyzed area for PN+CrN punches after forging of: : (a) 4000; (b) 5300 pcs.; (c) 6000 pcs.; (d) 10000 pcs.; (e) 13000 pcs..
A coherent mesh of fatigue cracks can be seen on the tool's surface in fig. 9a and 9b, with an average crack length of approx. 500 µm, and these cracks do not exhibit a tendency to expand, thanks to which the coating continues to adhere to the surface despite numerous cracks. Fig. 11c shows the first chipped fragments of the CrN coating, which corresponds to the time of coating destruction, after forging of approx. 6000 forgings. The next photograph (fig. 9d) presents the surface after removal of the coating and deepened abrasive wear, with a secondary fatigue crack mesh, and fig. 11e also shows furrows indicating deep abrasive wear. 3.2.3. Description of the wear mechanism of tools with the PN+CrN hybrid layer The application of a PVD coating, besides just nitriding, which forms a hybrid layer together with the nitrided layer, clearly results in improvement of lifetime. Wear occurs in two stages, the first being based on the formation of a fatigue crack mesh without abrasion-related loss of material up to approx. 6000 forgings, as long as the PVD coating protecting the nitrided layer against abrasive wear and partially against heat is preserved. Next, after the coating is damaged and removed, a secondary crack mesh is formed and gradual abrasive wear occur, and the surface layer is tempered as a result of contact with hot material. 4. Conclusion This article presents an analysis of the influence of the application of a hybrid layer on tool lifetime in the hot forging process. It has been demonstrated that the application of ion nitriding in combination with a PVD coating has a significant impact on tool wear and lifetime. This impact particularly pertains to the early stage of exploitation (up to 6000 pcs.), while the coating remains on the tool, protecting it against abrasive wear and serving as a barrier against thermal shocks. Arresting the progression of wear in this way causes real tool wear to begin later and thus extends tool life, particularly in the case of a CrN coating. Research [2] has shown that the CrN coating is characterized by high resistance to damage and heat. This is due to the combination of several beneficial properties, including good adhesion to the substrate, good tribological properties, and the characteristic ability to dissipate and carry variable stresses occurring during forging. The beneficial properties of hybrid layers will continue to be researched for the purpose of improving tool lifetimes and can successfully be applied to forging tools, particularly tools for precision forging, where high dimensional accuracy of forgings is expected. Acknowledgements This study was found by National Centre for Research and Development, Poland (NCBiR) (grant no. PBS2/A5/37/2013). References
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