- Email: [email protected]
CrN hybrid layer on durability improvement of die inserts used in hot forging process of wheel forging
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Procedia Manufacturing 00 (2018) 000–000 Available online atatwww.sciencedirect.com Available www.sciencedirect.com Procedia online Manufacturing 00 (2018) 000–000
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Procedia Manufacturing 15 (2018) 396–403 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, 17th International Conference on MetalToyohashi, Forming, Metal Japan Forming 2018, 16-19 September 2018, Toyohashi, Japan
Influence of application of GN+Cr/CrN hybrid layer on durability
Influence Engineering of application GN+Cr/CrN hybrid layer on2017, durability Manufacturing Societyof International Conference 2017, MESIC 28-30 June improvement of die inserts used in hot forging process of wheel forging 2017, Vigo (Pontevedra), Spain improvement of die inserts used in hot forging process of wheel forging Zbigniew Gronostajski, Marek Hawryluk , Paweł in Widomski, Sławomir Polak, * Costing models for capacity optimization Industry 4.0: Trade-off Zbigniew Gronostajski, Marek Hawryluk , Paweł Widomski, Sławomir Polak, Jacek Ziemba, Maciej Zwierzchowski Jacek Ziemba, Maciej Zwierzchowski between used capacity and operational efficiency Wroclaw University of Science and Technology, Department of Metal Forming and Metrology, Lukasiewicza Street 5, Wroclaw 50-371, Poland *
Wroclaw University of Science and Technology, Department of Metal Forming and Metrology, Lukasiewicza Street 5, Wroclaw 50-371, Poland
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
Abstract a University of Minho, 4800-058 Guimarães, Portugal Abstract b Unochapecó, Chapecó,process SC, Brazil This work presents the results of tests of forging tools 89809-000 in a hot forging of wheel forgings. The upper die inserts with This work presents the results of tests of forging tools in a hot forging process of wheel The upper die hybrid inserts layer with GN+Cr/CrN hybrid layer, applied to improve their durability, were tested in comparison the forgings. a gas-nitrided tool. The GN+Cr/CrN hybrid layer, applied to improve their durability, were tested in comparison the a gas-nitrided tool. The hybrid layer was produced on a plasma-nitrided substrate, onto which a PVD Cr/CrN coating was deposited. All tools were checked and was produced on a plasma-nitrided substrate, which aresearch PVD Cr/CrN coating was of deposited. All tools were checked and analyzed for changes in the surface layer underonto a complex including: analysis process conditions, macroscopic Abstract analyzed foranalysis changes(by in scanning), the surfaceaslayer a complex research including: analysis ofthe process macroscopic and geometrical well under SEM research microscopy. The tests confirmed effect conditions, of an improved forging tool geometrical (by scanning), well SEM research microscopy. The confirmed effect of an improved forging tool durability in analysis case of application of theasGN+Cr/CrN hybrid layer in relation to tests the hitherto usedthe -gas nitriding. Under the concept of "Industry 4.0", production processes will be hitherto pushedused to -gas be increasingly interconnected, durability in case of application of the GN+Cr/CrN hybrid layer in relation to the nitriding. information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2018 The Authors. Published by Elsevier B.V. © 2018 The Authors. Published by Elsevier B.V. goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability © 2018 The under Authors. Published by B.V. committee of the 17th International Conference on Metal Forming. and value. Peer-review responsibility of Elsevier the scientific scientific Peer-review under responsibility of the committee of the 17th International Conference on Metal Forming. instead of Indeed, lean management continuous improvement approaches suggest capacity optimization Peer-review under responsibilityand of the scientific committee of the 17th International Conference on Metal Forming.
maximization. The study capacity optimization and costing models Keywords: Tool; Durability; Hotof forging; Nitriding; Hybrid layer; Duplex treatment; Wearis an important research topic that deserves Keywords: Tool; from Durability; Nitriding; Hybrid layer; perspectives. Duplex treatment; Wearpaper presents and discusses a mathematical contributions bothHot theforging; practical and theoretical This model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity The forging die hide inserts work under difficult conditions, because they are constantly loaded by recurrent large optimization might operational inefficiency. The The forging die Published inserts under difficult conditions, they aretoconstantly by recurrent temperature gradients and work high pressures; therefore they because are requested have highloaded strength, hardnesslarge and © 2017 Authors. by Elsevier B.V. temperature gradients and high pressures; therefore they are requested to have high strength, and simultaneously proper toughness [1, 2]. Only the optimal choice of theEngineering process parameters one mayhardness significantly Peer-review under–responsibility of the scientific committee of the Manufacturing Society International Conference simultaneously – proper toughness [1, 2]. Only the optimal choice of the process parameters one may significantly 2017. increase the life of the tools, improve the quality of the forgings and, consequently, increase the productivity of the increase the lifeThe of the tools, improve the quality of the forgings and, causing consequently, increase productivity of the whole process. most common occurring destructive mechanisms decrease in toolthe durability are: plastic whole process. The ABC; most TDABC; common occurring destructive mechanisms causing decrease in tool durability are: plastic Keywords: Cost Models; Capacity Management; Idle Capacity; Operational Efficiency 1. Introduction
* Corresponding author. Tel: +48-71-320-21-64; fax: +48-71-320-21-73. * E-mail Corresponding Tel: +48-71-320-21-64; fax: +48-71-320-21-73. address:author. [email protected] The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018 Authors. Published Elsevier B.V.hours of the Peer-review underThe responsibility of theby scientific committee 17th International on Metal Forming. in several©ways: tons of production, available manufacturing, etc.Conference The management of the idle capacity Peer-review under responsibility thefax: scientific committee * Paulo Afonso. Tel.: +351 253 510of 761; +351 253 604 741 of the 17th International Conference on Metal Forming. E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming. 10.1016/j.promfg.2018.07.235
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deformation and abrasive wear in warm forging and hot forging, thermal fatigue cracking and thermo-mechanical fatigue [3]. The durability of forging tools, understood as their resistance to destructive factors, has been the subject of research in many scientific as well industrial centers. For example, in [4, 5] presents a comprehensive review of the most frequently used methods and methods allowing to increase the lifetime of forging equipment. Currently still the most popular and most frequently (due to low price) used method of improving the forging tools durability is gas nitriding. This thermo-chemical treatment increases the resistance to abrasion, improves the fatigue strength as well corrosion resistance of forging tools. In many hot industrial forging processes in which the tools were nitrided showed that by this treatment may be repeatedly extended tool life [6]. Increasingly, many die forges, in order to improve durability, use surface engineering techniques, including hybrid technologies. These techniques can combine e.g. thermo-chemical treatment methods and the PVD or CVD technique. Using these two mentioned microstructure components at the same time, i.e. a nitrided layer and a selected coating (such as: CrN, ALCrTiN, ALCrTiSiN, TiCrN) or duplex treatments consisting of plasma nitriding and hard PACVD coating, causes mutual cooperation, which provides very good properties [7-9]. The application and analysis of the impact of hybrid layers on the increase of the durability of forging tools has been the subject of research at many scientific centres, including that which, for many years now, has been performed by the authors of this article [9, 10]. The authors have conducted industrial tests of upper die inserts used in a hot forging process, which were improved according to two different methods, with the purpose to compare their durability. These were: thermochemical treatment by gas nitriding (GN) compared with a hybrid layer consisting of a gas nitrided layer and a PVD coating (GN+Cr/CrN). 2. Research material For the investigations, the hot forging process of a front wheel forging was selected (Fig. 1(c)). The wheel, after toothing, is used in the gear box as the front wheel of the reverse gear in motorcars.
Fig. 1. View of forging process: (a) tools used in front wheel forging installed on press, (b) upper die insert for operation II with marked control points (A and B) and (c) ready front wheel forging (after punching and trimming, before toothing).
The analyzed process is realized on a crank press of the nominal force of 25 MN, in three forging operations (Fig. 1 (a)): operation I - upsetting, operation II – preliminary forging and operation III – finishing forging. The forged material is the QS1816 steel (similar to 16MnCr5), in the shape of a cylinder. The initial temperature of the charge material equals 1150 °C. In the analyzed process, the tools are made of the X37CrMoV5-1 steel. All the tools undergo thermal treatment including hardening and high tempering at 600 ºC. Before the process, they are preliminarily heated by a scrap material to the temperature of 250oC. A detailed analysis was performed on the upper die insert used in the second forging operation, the durability of which is the lowest in the analyzed process, i.e. about 7000 forgings (Fig. 1(b)). 3. Test methodology To improve the upper tool durability, a hybrid layer of the GN/PVD coating type - gas nitriding+Cr/CrN coating was applied in order to make a comparison with the traditionally used nitrided tools. Additionally, with the purpose to compare the nitrided tools with the tools covered with the hybrid layer, a few different die inserts after an increased number of forgings (1000, 8000, 9000, 10000, 11000) were tested. A detailed comparison was made for a nitrided insert,
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which forged 7500 pieces and for an insert with a GN+Cr/CrN layer, which made 8,000 forgings. The tools prepared in this way underwent a performance test under the industrial conditions of Jawor Forge S.A. in Poland. Table 1 presents the properties of the nitrided layer being part of the hybrid layer, as well as the properties of the applied PVD coatings. Table 1. Properties of surface layer of examined die inserts. GN+Cr/CrN Thickness [µm] Coating Hardness
GN layer
GN layer
≈ 8,0 2100±140
Young modulus [GPa]
215±25
Thickness
g800 ≈ 100 µm
Hardness
800HV – effective hardness 1100HV – max. hardness
-
Next, after the operation, the tools were cleaned and tested by way of: a macroscopic analysis combined with scanning, a microstructure analysis and microhardness tests. 4. Macroscopic analysis combined with scanning For a detailed macroscopic analysis, some die inserts which had produced similar numbers of forgings were selected: a nitrided tool (after 7500 pcs) and a tool with a GN+Cr/CrN layer (after 8000 pcs), due to the fact that the average durability of an upper die insert is about 7000 forgings (Fig. 2). Additionally, during the operation, in the case of the tool with the hybrid layer, its progressing wear was observed and the state of its surface was recorded based on the surface state of periodically collected forgings, with the use of the 3D scanning method, elaborated by the authors [11]. (a)
(b)
Fig. 2. View of macro-analyzed die inserts: (a) with nitrided layer after 7500 pieces and (b) with GN+CrN layer 8000 pieces.
The performed macroscopic analysis of the die inserts after their operation showed that, in different areas, for both selected tools, surface changes occur in respect to the state before the forging process. In the front section, cracks caused by thermal and thermo-mechanical fatigue were detected, as well as a clear abrasive wear, intensified by the fatigue. Slightly larger and deeper grooves progressing outside the tool were observed for the nitrided insert (Fig. 2(a)) in respect of the tool with the hybrid layer (Fig. 2(b)). For the die insert with only the nitrided layer, it was also observed that its surface was covered with a network of cracks, yet its abrasive wear proceeded faster and in the whole area. Also, from the inside, a deep local spalling had expanded, which is the most undesirable type of wear due to its unpredictability and size. In turn, on the bridge, for both inserts, strong abrasion of the radius in the whole perimeter can be observed. In this area, as a result of an intensive flow of the forging material, the dominating destruction mechanism is abrasive wear. A slightly smaller wear is visible for the insert with the hybrid layer compared to the nitrided insert. The observations confirm a lower abrasive wear resistance of the insert with only the nitrided layer in respect of the one with the GN+Cr/CrN layer, which is additionally more resistant to wear and thermal fatigue at elevated temperatures.
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In order to perform a qualitative analysis of the surface, scanning of the impressions of the analyzed die inserts was conducted (Fig. 3). For the measurements of the changes in the forgings' geometry, the measuring arm ROMER Absolute ARM 7520si was used together with the Polyworks software and the Real Time Quality Meshing technology. The arm enables non-contact measurements with the use of the linear laser scanner RS3 integrated with it, which provides the possibility to collect up to 460 000 points/s for 4600 points on the line with the linear frequency of 100 Hz and the declared accuracy at the level of 2 sigma 30 μm.
Fig. 3. Scan images of worn die inserts after 1000 forgings: (a) with GN+Cr/CrN layer after 1000 forgings, (b) only gas nitrided (GN) after forging 7500 pieces, (c) with GN+Cr/CrN layer after 8000 forgings, (d) with GN+Cr/CrN layer after 9000 forgings, (e) with GN+Cr/CrN layer after 10000 forgings and (f) with GN+Cr/CrN layer after 11 000 forgings.
Analyzing the presented scans after of the tools' operation, we can observe that after 1000 forgings (Fig. 4(a)), no traces of wear are visible. In turn, for the inserts which have produced a larger number of forgings (over 7000 pieces, Fig. 4(b)-(e)), such wear occurs in the front part as well as on the bridge. Comparing directly the nitrided layer insert (Fig. 4(b)) with the insert coated with the Cr/CrN layer (Fig. 4c), after a similar number of produced forgings, we can see that despite the fact that the insert with the GN+Cr/CrN layer has worked through 500 forgings more, the material loss on its working surface is slightly smaller. This proves that the Cr/CrN layer, subtly but still, improves the durability of the tools. Fig. 4(d)-(e) shows tool wear at a higher than assumed average durability (7,500 forgings). However, the results are not very satisfactory, despite the fact that the obtained forgings are within the scope of dimension-shape tolerance, even for the insert after 11000 forgings. Additionally, in order to determine the size of material loss in the normal direction for the two selected tools: a nitrided tool (after 7500 pcs) and a tool with a GN+Cr/CrN layer, the 3D reverse scanning method was applied for the performance tests, to enable a comparison of the progressing tool wear for an increasing number of forgings. This method uses the reflection phenomenon and consists in using a scanner to measure the progressing wear of the selected forging tool (its material loss), on the basis of the changes in the shape of the forgings periodically collected from the process (material growth of the forging). Also, based on a macroscopic analysis, it was established that, for the analyzed tools, two areas with different dominant destruction mechanisms could be distinguished. That is why a detailed analysis of both tools was performed, with a distinction between the front area and the bridge area, as it was determined that using different variants of surface treatment could significantly affect the material loss in those areas, while the total material loss for the whole tool can provide only averaged values. An analysis of this type will enable a better selection of the protective layer depending on the conditions and the area of the tool (Fig. 5).
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Fig. 4. Exemplary die insert: (a) new – before work, together with forging from the beginning of operation (500 items), (b) worn out – after producing 7 500 forgings, together with forging from end of tool’s operation and (c) comparisons of scans.
In turn, Fig. 5(a) shows the results referring to the loss for both selected tools and in their particular areas (Fig. 5(b)).
(b)
(a)
Fig. 5. Compilation of results of volumetric wear calculated based on the forgings for both die inserts: (a) in whole area of cavity and (b) in analyzed area A and B separately.
Based on the presented diagrams-curves (Fig. 5) of the wear for both the whole surface and for the particular areas, a slightly smaller wear in respect of the material loss occurs for the insert with the GN+Cr/CrN layer, which is especially visible in the final period of operation, that is in the case when the average durability of these tools equals about 7500 forgings. 5. Microscopic research In order to evaluate the microstructure in the surface layer after the operation for both selected die inserts, the samples' surface was observed in the cross-section of the two analyzed areas: the central part (A) and in the vicinity of the tool's bridge (B). The microstructural test results for the front area (A) are presented in Fig. 6, whereas for the bridge area (B) – in Fig. 7. (b)
(a)
SEM MAG. 117x Det. BSE, SEM HV: 30kV
500 µm
(c)
(d)
SEM MAG. 146x Det. BSE, SEM HV: 30kV
500 µm
Fig. 6. Results of microstructural tests for area A: (a) image of nitrided die – metallographic microscope, (b) image of nitrided die - SEM, (c) image of the die insert with the hybrid layer GN+Cr/CrN – metallographic microscope and (d) image of the die insert with the hybrid layer GN+Cr/CrN – SEM.
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In Fig. 6(a), one can observe a complete lack of a nitrided layer on the front surface and the presence of fatigue cracks, about 0.4 mm deep. Fig. 6(c) shows a visible hybrid layer GN+Cr/CrN, in which deep cracks have developed towards the inside from the tool's working surface. These cracks also reach 0.3 – 0.4 mm, whereas the hybrid layer has been damaged only locally, in the vicinity of those cracks. Despite a relatively long operation (8000 forging cycles), we can still see the PVD coating on the surface, about 5 µm thick, under which an untouched nitrided layer has remained. The coating is cracked as a result of the operation of changing pressures, but despite the cracks, it remains in cohesion with the substrate. The SEM observations presented in Fig. 6(b) showed deep abrasive grooves, which confirms the occurrence of intensive abrasive wear in this area. The SEM image of the surface of the tool with the hybrid layer showed a characteristic, relatively regular, network of fatigue cracks in the whole tool's working area.
Fig. 7. Results of microstructural tests for area B: (a) image of the nitrided die insert – metallographic microscope, (b) image of the nitrided die insert - SEM, (c) image of the die insert with the hybrid layer GN+Cr/CrN – metallographic microscope and (d) image of the die insert with the hybrid layer GN+Cr/CrN – SEM.
In Fig. 7(a), one can see a nearly complete lack of a nitrided layer on the whole observed rounding. The SEM image of the surface of the same tool (Fig. 7(b)) shows deep abrasive grooves on the whole rounding's surface. In comparison, the tool with the hybrid layer underwent abrasive wear only partially, which led to a local removal of fragments of the coating and the nitrided layer (Fig. 7(c)). The SEM observations and microscopic tests confirmed an improved durability through the use of a hybrid layer both in the central part, where the hybrid layer demonstrated a significantly higher abrasion wear resistance, intensified by the thermo-mechanical fatigue, and in the vicinity of the bridge, where the observed tool wear is much smaller on the tool with the hybrid layer. 6. Microhardness measurements The microhardness measurements by means of the Vickers method were performed in the direction from the working surface on the section of 1.5 mm towards the inside of the tool, with the load of 100 g. The test was conducted in 2 selected points for both tools, marked in Fig. 1b as no. A (front area) and B (vicinity of the die bridge area). Fig. 8 shows the measurement results for the analyzed die inserts.
Fig. 8. Results of microhardness measurements in selected control points (A and B) for the nitrided insert and for the die insert with GN+Cr/CrN layer.
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On the front surface, which is a flat working area localized in the central part of the tool (area A), the hardness of the tool material significantly decreased. The nitrided layer underwent tempering and removal by way of abrasive wear, and so, the measured hardness refers to the substrate material, that is tool steel X37CrMoV5-1. Based on the obtained hardness values, it can be concluded that the hybrid layer did not undergo abrasive wear, as the coating remained on the tool's surface, protecting it through the whole operation period, which is also confirmed by the microstructural test results (Fig. 6(c)). However, the hardness in the nitrided layer and directly beneath it became lower, being reduced even to 400 HV. In turn, in the vicinity of the bridge, where intensive abrasive wear was observed, a local removal of both layers took place, while the wear of the hybrid layer was smaller, the latter having partially remained on the analyzed surface. There is a visible effect of a hardness loss as a result of the material's tempering even down to the depth of 0,4mm for the hybrid layer, which proves that the nitrided layer is not thick enough, being an insufficiently deep substrate for the PVD coating. Due to this fact, material tempering below the nitrides' diffusion layer is observed. 7. Conclusion The article presents an analysis of the effect of the use of a hybrid layer GN+Cr/CrN, compared to the standard nitrided layer, on the durability of the selected tool (upper die insert in operation II – preliminary forging) in a hot forging process. The performed investigations have demonstrated that the application of gas nitriding in combination with a PVD coating reduces the wear of the tool and improves its durability by over a 8% to 3000 forging and even 25% at the end of work – for 7000 pieces. This effect especially refers to the final operation period (about 7000 pieces), in which the Cr/CrN coating (despite partial cracking and damage) remains on the tool's surface, protecting it from abrasion and acting as a barrier against thermal shocks. Stopping the progress of wear in this way makes the actual tool wear begin later, thus prolonging the durability of the tool. The use of the reverse scanning method for the analysis of the tool's state enables a quantitative and qualitative evaluation of the material loss, and selecting two characteristic areas on the tool, with different dominant degradation mechanisms, makes it possible to perform a more thorough analysis of the effect of the use of different surface engineering techniques. The advantageous properties of hybrid layers compared to the traditional techniques of thermo-chemical treatment (nitriding) observed and proved in the article justify further research on the improvement of tool durability, while, due to additional costs resulting from the application of the coatings, these layers should be especially used for tools assigned for precisions forging, where high quality and dimension-shape precision of the forged products are expected. Also, the investigations performed by the authors on other forging processes have shown that, for each process or even each tool, the coatings should be selected individually, as the existing operation conditions, despite being similar, are not identical, which makes them reliable in one case and unreliable in another. Acknowledgement This study was founded by National Centre for Research and Development, Poland (grant no. TECHMATSTRATEG1/348491/10/NCBR/2017). References [1] Ch. Choi, A. Groseclose, T. Altan, Estimation of plastic deformation and abrasive wear in warm forging dies, Journal of Materials Processing Technology, 212-8 (2012) 1742–1752. [2] L. Lavtar, T. Muhic, G. Kugler, J. Tercelj, Analysis of the main types of damage on a pair of industrial dies for hot forging car steering mechanisms, Engineering Failure Analysis, 18-10 (2011) 1143–1152. [3] Z. Gronostajski, M. Kaszuba, M. Hawryluk, M. Zwierzchowski, A review of the degradation mechanisms of the hot forging tools, Archives of Civil and Mechanical Engineering, 14-4 (2014) 528–539. [4] K. Lange, L. Cser, L.M. Geiger, J.A.G. Kals, Tool life and tool quality in bulk metal forming, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 207 (1993) 223–239. [5] M. Hawryluk, Review of selected methods of increasing the life of forging tools in hot die forging processes, Archives of Civil and Mechanical Engineering, 16-4 (2016) 845–866. [6] H. Paschke, M. Weber, G. Braeuer, T. Yilkiran, B.A. Behrens, H. Brand, Optimized plasma nitriding processes for efficient wear reduction of forging dies, Archives of Civil and Mechanical Engineering, 12-4 (2012) 407–412.
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[7] K. Klimek, H. Ahn, I. Seebach, M. Wang, K.T. Rie, Duplex process applied for die casting and forging tools, Surface and Coatings Technology, 174–175 (2013) 677–680. [8] A. Mazurkiewicz, J. Smolik, The innovative directions In development and implementations of hybryd technologies in surface engineering, Archives of Metallurgy and Materials, 60-3 (2015) 2161–2172. [9] M. Prabakaran, S.S. Kumar, K.R. Ramyesh, R.V. Srinivasan, Characterization and optimization of CrN coatings on tool steels (6959), International Journal of Mechanical and Industrial Technology, 2-1 (2014) 108–112. [10] M. Hawryluk, Z. Gronostajski, M. Kaszuba, S. Polak, P. Widomski, J. Ziemba, Application of selected surface engineering methods to improve the durability of tools used in precision forging, International Journal of Advance Manufacturing Technology, 5-8 (2017) 2183– 2200. [11] M. Hawryluk, J. Ziemba, Ł. Dworzak, Development of a method for tool wear analysis using 3D scanning, Metrology and Measurement Systems, 24-4 (2017) 739–757.
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17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, 17th International Conference on MetalToyohashi, Forming, Metal Japan Forming 2018, 16-19 September 2018, Toyohashi, Japan
Influence of application of GN+Cr/CrN hybrid layer on durability
Influence Engineering of application GN+Cr/CrN hybrid layer on2017, durability Manufacturing Societyof International Conference 2017, MESIC 28-30 June improvement of die inserts used in hot forging process of wheel forging 2017, Vigo (Pontevedra), Spain improvement of die inserts used in hot forging process of wheel forging Zbigniew Gronostajski, Marek Hawryluk , Paweł in Widomski, Sławomir Polak, * Costing models for capacity optimization Industry 4.0: Trade-off Zbigniew Gronostajski, Marek Hawryluk , Paweł Widomski, Sławomir Polak, Jacek Ziemba, Maciej Zwierzchowski Jacek Ziemba, Maciej Zwierzchowski between used capacity and operational efficiency Wroclaw University of Science and Technology, Department of Metal Forming and Metrology, Lukasiewicza Street 5, Wroclaw 50-371, Poland *
Wroclaw University of Science and Technology, Department of Metal Forming and Metrology, Lukasiewicza Street 5, Wroclaw 50-371, Poland
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
Abstract a University of Minho, 4800-058 Guimarães, Portugal Abstract b Unochapecó, Chapecó,process SC, Brazil This work presents the results of tests of forging tools 89809-000 in a hot forging of wheel forgings. The upper die inserts with This work presents the results of tests of forging tools in a hot forging process of wheel The upper die hybrid inserts layer with GN+Cr/CrN hybrid layer, applied to improve their durability, were tested in comparison the forgings. a gas-nitrided tool. The GN+Cr/CrN hybrid layer, applied to improve their durability, were tested in comparison the a gas-nitrided tool. The hybrid layer was produced on a plasma-nitrided substrate, onto which a PVD Cr/CrN coating was deposited. All tools were checked and was produced on a plasma-nitrided substrate, which aresearch PVD Cr/CrN coating was of deposited. All tools were checked and analyzed for changes in the surface layer underonto a complex including: analysis process conditions, macroscopic Abstract analyzed foranalysis changes(by in scanning), the surfaceaslayer a complex research including: analysis ofthe process macroscopic and geometrical well under SEM research microscopy. The tests confirmed effect conditions, of an improved forging tool geometrical (by scanning), well SEM research microscopy. The confirmed effect of an improved forging tool durability in analysis case of application of theasGN+Cr/CrN hybrid layer in relation to tests the hitherto usedthe -gas nitriding. Under the concept of "Industry 4.0", production processes will be hitherto pushedused to -gas be increasingly interconnected, durability in case of application of the GN+Cr/CrN hybrid layer in relation to the nitriding. information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2018 The Authors. Published by Elsevier B.V. © 2018 The Authors. Published by Elsevier B.V. goes beyond the traditional aim of capacity maximization, contributing also for organization’s profitability © 2018 The under Authors. Published by B.V. committee of the 17th International Conference on Metal Forming. and value. Peer-review responsibility of Elsevier the scientific scientific Peer-review under responsibility of the committee of the 17th International Conference on Metal Forming. instead of Indeed, lean management continuous improvement approaches suggest capacity optimization Peer-review under responsibilityand of the scientific committee of the 17th International Conference on Metal Forming.
maximization. The study capacity optimization and costing models Keywords: Tool; Durability; Hotof forging; Nitriding; Hybrid layer; Duplex treatment; Wearis an important research topic that deserves Keywords: Tool; from Durability; Nitriding; Hybrid layer; perspectives. Duplex treatment; Wearpaper presents and discusses a mathematical contributions bothHot theforging; practical and theoretical This model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity The forging die hide inserts work under difficult conditions, because they are constantly loaded by recurrent large optimization might operational inefficiency. The The forging die Published inserts under difficult conditions, they aretoconstantly by recurrent temperature gradients and work high pressures; therefore they because are requested have highloaded strength, hardnesslarge and © 2017 Authors. by Elsevier B.V. temperature gradients and high pressures; therefore they are requested to have high strength, and simultaneously proper toughness [1, 2]. Only the optimal choice of theEngineering process parameters one mayhardness significantly Peer-review under–responsibility of the scientific committee of the Manufacturing Society International Conference simultaneously – proper toughness [1, 2]. Only the optimal choice of the process parameters one may significantly 2017. increase the life of the tools, improve the quality of the forgings and, consequently, increase the productivity of the increase the lifeThe of the tools, improve the quality of the forgings and, causing consequently, increase productivity of the whole process. most common occurring destructive mechanisms decrease in toolthe durability are: plastic whole process. The ABC; most TDABC; common occurring destructive mechanisms causing decrease in tool durability are: plastic Keywords: Cost Models; Capacity Management; Idle Capacity; Operational Efficiency 1. Introduction
* Corresponding author. Tel: +48-71-320-21-64; fax: +48-71-320-21-73. * E-mail Corresponding Tel: +48-71-320-21-64; fax: +48-71-320-21-73. address:author. [email protected] The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018 Authors. Published Elsevier B.V.hours of the Peer-review underThe responsibility of theby scientific committee 17th International on Metal Forming. in several©ways: tons of production, available manufacturing, etc.Conference The management of the idle capacity Peer-review under responsibility thefax: scientific committee * Paulo Afonso. Tel.: +351 253 510of 761; +351 253 604 741 of the 17th International Conference on Metal Forming. E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming. 10.1016/j.promfg.2018.07.235
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deformation and abrasive wear in warm forging and hot forging, thermal fatigue cracking and thermo-mechanical fatigue [3]. The durability of forging tools, understood as their resistance to destructive factors, has been the subject of research in many scientific as well industrial centers. For example, in [4, 5] presents a comprehensive review of the most frequently used methods and methods allowing to increase the lifetime of forging equipment. Currently still the most popular and most frequently (due to low price) used method of improving the forging tools durability is gas nitriding. This thermo-chemical treatment increases the resistance to abrasion, improves the fatigue strength as well corrosion resistance of forging tools. In many hot industrial forging processes in which the tools were nitrided showed that by this treatment may be repeatedly extended tool life [6]. Increasingly, many die forges, in order to improve durability, use surface engineering techniques, including hybrid technologies. These techniques can combine e.g. thermo-chemical treatment methods and the PVD or CVD technique. Using these two mentioned microstructure components at the same time, i.e. a nitrided layer and a selected coating (such as: CrN, ALCrTiN, ALCrTiSiN, TiCrN) or duplex treatments consisting of plasma nitriding and hard PACVD coating, causes mutual cooperation, which provides very good properties [7-9]. The application and analysis of the impact of hybrid layers on the increase of the durability of forging tools has been the subject of research at many scientific centres, including that which, for many years now, has been performed by the authors of this article [9, 10]. The authors have conducted industrial tests of upper die inserts used in a hot forging process, which were improved according to two different methods, with the purpose to compare their durability. These were: thermochemical treatment by gas nitriding (GN) compared with a hybrid layer consisting of a gas nitrided layer and a PVD coating (GN+Cr/CrN). 2. Research material For the investigations, the hot forging process of a front wheel forging was selected (Fig. 1(c)). The wheel, after toothing, is used in the gear box as the front wheel of the reverse gear in motorcars.
Fig. 1. View of forging process: (a) tools used in front wheel forging installed on press, (b) upper die insert for operation II with marked control points (A and B) and (c) ready front wheel forging (after punching and trimming, before toothing).
The analyzed process is realized on a crank press of the nominal force of 25 MN, in three forging operations (Fig. 1 (a)): operation I - upsetting, operation II – preliminary forging and operation III – finishing forging. The forged material is the QS1816 steel (similar to 16MnCr5), in the shape of a cylinder. The initial temperature of the charge material equals 1150 °C. In the analyzed process, the tools are made of the X37CrMoV5-1 steel. All the tools undergo thermal treatment including hardening and high tempering at 600 ºC. Before the process, they are preliminarily heated by a scrap material to the temperature of 250oC. A detailed analysis was performed on the upper die insert used in the second forging operation, the durability of which is the lowest in the analyzed process, i.e. about 7000 forgings (Fig. 1(b)). 3. Test methodology To improve the upper tool durability, a hybrid layer of the GN/PVD coating type - gas nitriding+Cr/CrN coating was applied in order to make a comparison with the traditionally used nitrided tools. Additionally, with the purpose to compare the nitrided tools with the tools covered with the hybrid layer, a few different die inserts after an increased number of forgings (1000, 8000, 9000, 10000, 11000) were tested. A detailed comparison was made for a nitrided insert,
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which forged 7500 pieces and for an insert with a GN+Cr/CrN layer, which made 8,000 forgings. The tools prepared in this way underwent a performance test under the industrial conditions of Jawor Forge S.A. in Poland. Table 1 presents the properties of the nitrided layer being part of the hybrid layer, as well as the properties of the applied PVD coatings. Table 1. Properties of surface layer of examined die inserts. GN+Cr/CrN Thickness [µm] Coating Hardness
GN layer
GN layer
≈ 8,0 2100±140
Young modulus [GPa]
215±25
Thickness
g800 ≈ 100 µm
Hardness
800HV – effective hardness 1100HV – max. hardness
-
Next, after the operation, the tools were cleaned and tested by way of: a macroscopic analysis combined with scanning, a microstructure analysis and microhardness tests. 4. Macroscopic analysis combined with scanning For a detailed macroscopic analysis, some die inserts which had produced similar numbers of forgings were selected: a nitrided tool (after 7500 pcs) and a tool with a GN+Cr/CrN layer (after 8000 pcs), due to the fact that the average durability of an upper die insert is about 7000 forgings (Fig. 2). Additionally, during the operation, in the case of the tool with the hybrid layer, its progressing wear was observed and the state of its surface was recorded based on the surface state of periodically collected forgings, with the use of the 3D scanning method, elaborated by the authors [11]. (a)
(b)
Fig. 2. View of macro-analyzed die inserts: (a) with nitrided layer after 7500 pieces and (b) with GN+CrN layer 8000 pieces.
The performed macroscopic analysis of the die inserts after their operation showed that, in different areas, for both selected tools, surface changes occur in respect to the state before the forging process. In the front section, cracks caused by thermal and thermo-mechanical fatigue were detected, as well as a clear abrasive wear, intensified by the fatigue. Slightly larger and deeper grooves progressing outside the tool were observed for the nitrided insert (Fig. 2(a)) in respect of the tool with the hybrid layer (Fig. 2(b)). For the die insert with only the nitrided layer, it was also observed that its surface was covered with a network of cracks, yet its abrasive wear proceeded faster and in the whole area. Also, from the inside, a deep local spalling had expanded, which is the most undesirable type of wear due to its unpredictability and size. In turn, on the bridge, for both inserts, strong abrasion of the radius in the whole perimeter can be observed. In this area, as a result of an intensive flow of the forging material, the dominating destruction mechanism is abrasive wear. A slightly smaller wear is visible for the insert with the hybrid layer compared to the nitrided insert. The observations confirm a lower abrasive wear resistance of the insert with only the nitrided layer in respect of the one with the GN+Cr/CrN layer, which is additionally more resistant to wear and thermal fatigue at elevated temperatures.
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In order to perform a qualitative analysis of the surface, scanning of the impressions of the analyzed die inserts was conducted (Fig. 3). For the measurements of the changes in the forgings' geometry, the measuring arm ROMER Absolute ARM 7520si was used together with the Polyworks software and the Real Time Quality Meshing technology. The arm enables non-contact measurements with the use of the linear laser scanner RS3 integrated with it, which provides the possibility to collect up to 460 000 points/s for 4600 points on the line with the linear frequency of 100 Hz and the declared accuracy at the level of 2 sigma 30 μm.
Fig. 3. Scan images of worn die inserts after 1000 forgings: (a) with GN+Cr/CrN layer after 1000 forgings, (b) only gas nitrided (GN) after forging 7500 pieces, (c) with GN+Cr/CrN layer after 8000 forgings, (d) with GN+Cr/CrN layer after 9000 forgings, (e) with GN+Cr/CrN layer after 10000 forgings and (f) with GN+Cr/CrN layer after 11 000 forgings.
Analyzing the presented scans after of the tools' operation, we can observe that after 1000 forgings (Fig. 4(a)), no traces of wear are visible. In turn, for the inserts which have produced a larger number of forgings (over 7000 pieces, Fig. 4(b)-(e)), such wear occurs in the front part as well as on the bridge. Comparing directly the nitrided layer insert (Fig. 4(b)) with the insert coated with the Cr/CrN layer (Fig. 4c), after a similar number of produced forgings, we can see that despite the fact that the insert with the GN+Cr/CrN layer has worked through 500 forgings more, the material loss on its working surface is slightly smaller. This proves that the Cr/CrN layer, subtly but still, improves the durability of the tools. Fig. 4(d)-(e) shows tool wear at a higher than assumed average durability (7,500 forgings). However, the results are not very satisfactory, despite the fact that the obtained forgings are within the scope of dimension-shape tolerance, even for the insert after 11000 forgings. Additionally, in order to determine the size of material loss in the normal direction for the two selected tools: a nitrided tool (after 7500 pcs) and a tool with a GN+Cr/CrN layer, the 3D reverse scanning method was applied for the performance tests, to enable a comparison of the progressing tool wear for an increasing number of forgings. This method uses the reflection phenomenon and consists in using a scanner to measure the progressing wear of the selected forging tool (its material loss), on the basis of the changes in the shape of the forgings periodically collected from the process (material growth of the forging). Also, based on a macroscopic analysis, it was established that, for the analyzed tools, two areas with different dominant destruction mechanisms could be distinguished. That is why a detailed analysis of both tools was performed, with a distinction between the front area and the bridge area, as it was determined that using different variants of surface treatment could significantly affect the material loss in those areas, while the total material loss for the whole tool can provide only averaged values. An analysis of this type will enable a better selection of the protective layer depending on the conditions and the area of the tool (Fig. 5).
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Fig. 4. Exemplary die insert: (a) new – before work, together with forging from the beginning of operation (500 items), (b) worn out – after producing 7 500 forgings, together with forging from end of tool’s operation and (c) comparisons of scans.
In turn, Fig. 5(a) shows the results referring to the loss for both selected tools and in their particular areas (Fig. 5(b)).
(b)
(a)
Fig. 5. Compilation of results of volumetric wear calculated based on the forgings for both die inserts: (a) in whole area of cavity and (b) in analyzed area A and B separately.
Based on the presented diagrams-curves (Fig. 5) of the wear for both the whole surface and for the particular areas, a slightly smaller wear in respect of the material loss occurs for the insert with the GN+Cr/CrN layer, which is especially visible in the final period of operation, that is in the case when the average durability of these tools equals about 7500 forgings. 5. Microscopic research In order to evaluate the microstructure in the surface layer after the operation for both selected die inserts, the samples' surface was observed in the cross-section of the two analyzed areas: the central part (A) and in the vicinity of the tool's bridge (B). The microstructural test results for the front area (A) are presented in Fig. 6, whereas for the bridge area (B) – in Fig. 7. (b)
(a)
SEM MAG. 117x Det. BSE, SEM HV: 30kV
500 µm
(c)
(d)
SEM MAG. 146x Det. BSE, SEM HV: 30kV
500 µm
Fig. 6. Results of microstructural tests for area A: (a) image of nitrided die – metallographic microscope, (b) image of nitrided die - SEM, (c) image of the die insert with the hybrid layer GN+Cr/CrN – metallographic microscope and (d) image of the die insert with the hybrid layer GN+Cr/CrN – SEM.
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In Fig. 6(a), one can observe a complete lack of a nitrided layer on the front surface and the presence of fatigue cracks, about 0.4 mm deep. Fig. 6(c) shows a visible hybrid layer GN+Cr/CrN, in which deep cracks have developed towards the inside from the tool's working surface. These cracks also reach 0.3 – 0.4 mm, whereas the hybrid layer has been damaged only locally, in the vicinity of those cracks. Despite a relatively long operation (8000 forging cycles), we can still see the PVD coating on the surface, about 5 µm thick, under which an untouched nitrided layer has remained. The coating is cracked as a result of the operation of changing pressures, but despite the cracks, it remains in cohesion with the substrate. The SEM observations presented in Fig. 6(b) showed deep abrasive grooves, which confirms the occurrence of intensive abrasive wear in this area. The SEM image of the surface of the tool with the hybrid layer showed a characteristic, relatively regular, network of fatigue cracks in the whole tool's working area.
Fig. 7. Results of microstructural tests for area B: (a) image of the nitrided die insert – metallographic microscope, (b) image of the nitrided die insert - SEM, (c) image of the die insert with the hybrid layer GN+Cr/CrN – metallographic microscope and (d) image of the die insert with the hybrid layer GN+Cr/CrN – SEM.
In Fig. 7(a), one can see a nearly complete lack of a nitrided layer on the whole observed rounding. The SEM image of the surface of the same tool (Fig. 7(b)) shows deep abrasive grooves on the whole rounding's surface. In comparison, the tool with the hybrid layer underwent abrasive wear only partially, which led to a local removal of fragments of the coating and the nitrided layer (Fig. 7(c)). The SEM observations and microscopic tests confirmed an improved durability through the use of a hybrid layer both in the central part, where the hybrid layer demonstrated a significantly higher abrasion wear resistance, intensified by the thermo-mechanical fatigue, and in the vicinity of the bridge, where the observed tool wear is much smaller on the tool with the hybrid layer. 6. Microhardness measurements The microhardness measurements by means of the Vickers method were performed in the direction from the working surface on the section of 1.5 mm towards the inside of the tool, with the load of 100 g. The test was conducted in 2 selected points for both tools, marked in Fig. 1b as no. A (front area) and B (vicinity of the die bridge area). Fig. 8 shows the measurement results for the analyzed die inserts.
Fig. 8. Results of microhardness measurements in selected control points (A and B) for the nitrided insert and for the die insert with GN+Cr/CrN layer.
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On the front surface, which is a flat working area localized in the central part of the tool (area A), the hardness of the tool material significantly decreased. The nitrided layer underwent tempering and removal by way of abrasive wear, and so, the measured hardness refers to the substrate material, that is tool steel X37CrMoV5-1. Based on the obtained hardness values, it can be concluded that the hybrid layer did not undergo abrasive wear, as the coating remained on the tool's surface, protecting it through the whole operation period, which is also confirmed by the microstructural test results (Fig. 6(c)). However, the hardness in the nitrided layer and directly beneath it became lower, being reduced even to 400 HV. In turn, in the vicinity of the bridge, where intensive abrasive wear was observed, a local removal of both layers took place, while the wear of the hybrid layer was smaller, the latter having partially remained on the analyzed surface. There is a visible effect of a hardness loss as a result of the material's tempering even down to the depth of 0,4mm for the hybrid layer, which proves that the nitrided layer is not thick enough, being an insufficiently deep substrate for the PVD coating. Due to this fact, material tempering below the nitrides' diffusion layer is observed. 7. Conclusion The article presents an analysis of the effect of the use of a hybrid layer GN+Cr/CrN, compared to the standard nitrided layer, on the durability of the selected tool (upper die insert in operation II – preliminary forging) in a hot forging process. The performed investigations have demonstrated that the application of gas nitriding in combination with a PVD coating reduces the wear of the tool and improves its durability by over a 8% to 3000 forging and even 25% at the end of work – for 7000 pieces. This effect especially refers to the final operation period (about 7000 pieces), in which the Cr/CrN coating (despite partial cracking and damage) remains on the tool's surface, protecting it from abrasion and acting as a barrier against thermal shocks. Stopping the progress of wear in this way makes the actual tool wear begin later, thus prolonging the durability of the tool. The use of the reverse scanning method for the analysis of the tool's state enables a quantitative and qualitative evaluation of the material loss, and selecting two characteristic areas on the tool, with different dominant degradation mechanisms, makes it possible to perform a more thorough analysis of the effect of the use of different surface engineering techniques. The advantageous properties of hybrid layers compared to the traditional techniques of thermo-chemical treatment (nitriding) observed and proved in the article justify further research on the improvement of tool durability, while, due to additional costs resulting from the application of the coatings, these layers should be especially used for tools assigned for precisions forging, where high quality and dimension-shape precision of the forged products are expected. Also, the investigations performed by the authors on other forging processes have shown that, for each process or even each tool, the coatings should be selected individually, as the existing operation conditions, despite being similar, are not identical, which makes them reliable in one case and unreliable in another. Acknowledgement This study was founded by National Centre for Research and Development, Poland (grant no. TECHMATSTRATEG1/348491/10/NCBR/2017). References [1] Ch. Choi, A. Groseclose, T. Altan, Estimation of plastic deformation and abrasive wear in warm forging dies, Journal of Materials Processing Technology, 212-8 (2012) 1742–1752. [2] L. Lavtar, T. Muhic, G. Kugler, J. Tercelj, Analysis of the main types of damage on a pair of industrial dies for hot forging car steering mechanisms, Engineering Failure Analysis, 18-10 (2011) 1143–1152. [3] Z. Gronostajski, M. Kaszuba, M. Hawryluk, M. Zwierzchowski, A review of the degradation mechanisms of the hot forging tools, Archives of Civil and Mechanical Engineering, 14-4 (2014) 528–539. [4] K. Lange, L. Cser, L.M. Geiger, J.A.G. Kals, Tool life and tool quality in bulk metal forming, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 207 (1993) 223–239. [5] M. Hawryluk, Review of selected methods of increasing the life of forging tools in hot die forging processes, Archives of Civil and Mechanical Engineering, 16-4 (2016) 845–866. [6] H. Paschke, M. Weber, G. Braeuer, T. Yilkiran, B.A. Behrens, H. Brand, Optimized plasma nitriding processes for efficient wear reduction of forging dies, Archives of Civil and Mechanical Engineering, 12-4 (2012) 407–412.
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