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Analysis of the main types of damage on a pair of industrial dies for hot forging car steering mechanisms
Engineering Failure Analysis 18 (2011) 1143–1152
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Review
Analysis of the main types of damage on a pair of industrial dies for hot forging car steering mechanisms Lejla Lavtar a,⇑, Tadej Muhicˇ b, Goran Kugler a, Milan Tercˇelj a a b
Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 1 June 2010 Received in revised form 13 October 2010 Accepted 4 November 2010 Available online 24 November 2010 Keywords: Hot forging Die Gas nitriding Damage
a b s t r a c t The paper presents an analysis of the main types of damage on industrial upper and lower hot-forging dies for hot forging of car steering mechanisms. The dies were previously gas nitrided at various conditions and exhibit various microstructures after nitriding. They failed after a different number of forging strokes. Measurements of microhardness depth profiles and examinations of nitrided layers with an optical microscope were conducted. The design of die shape in sequential forging steps has the predominant role for a prolonged die life in comparison to the quality of nitrided die cavity surface. Improvements of die cavity design as well as improvements of nitriding process are suggested. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction During the process of hot forging the dies are subjected to complex loading involving simultaneous action of thermal, mechanical, chemical and tribological loads. These loads act also on particular parts of hot-forging dies and cause the occurrence of different types of damage such as wear, plastic deformation, thermal and mechanical cracking, gross cracking. and their progress as function of time. In the past, many authors reported that adhesive wear [1], erosion [1–4], abrasive wear [1,5,6], thermal as well as mechanical fatigue [1,7] or plastic deformation [1,8] are some of the main wear mechanisms on hot forging dies. In-service life of the die plays an important role in the economy of hot forgings. Thus the task of every manufacturing engineer is to accurately predict and possibly prolong the die’s in-service life as much as possible. Actually, expenses for dies-make up about 8–15% of the product’s cost while unexpected die failure leads to an increase in production costs by 30%. Thus companies that manufacture forgings are very interested in improving the in-service life of hot forging dies and support research work in this area in order that more useful knowledge for industrial practice is obtained [1]. In general, the occurrence of various types of damage and their progress are additionally influenced by steel material used for the die and applied technological processing parameters, die shape design, i.e. planning of forging sequences, die manufacturing, forging parameters (strain rates, contact times and contact pressures, lubrication, sliding lengths, etc.), applied forging presses, as well as forging stock properties, e.g. formation of oxide scale, local bonding between the die sand the workpiece, etc. Furthermore, the steel for the die should be produced in an appropriate way, i.e. at optimal process parameters, in order to achieve optimal microstructure, optimal grain size and distribution, type, size and shape of carbides. Finally, the manufacturing process of the die (resulting in the die’s shape) should be carried out without negative impacts on die
⇑ Corresponding author. Tel.: +386 41 851 528. E-mail address: [email protected] (L. Lavtar). 1350-6307/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.11.002
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L. Lavtar et al. / Engineering Failure Analysis 18 (2011) 1143–1152 Table 1 Chemical composition of the die steel in wt.%. C
Si
Mn
Cr
Mo
V
0.37
0.26
0.5
5.0
2.36
0.55
surface quality. The die shapes should be optimized through adequate sequential forging steps in order to avoid areas (parts of die cavity) where loads are essentially higher than in other die areas [9,10]. In order to increase in-service life and resistance to wear and fatigue, die surfaces are improved by PVD, PACVD, CVD coatings, diffusion processes like nitriding, etc. [11]. During nitriding die steels, two different types of microstructure are formed on the die surface, i.e. microstructure with a ‘‘compound layer’’ on top and a diffusion layer under it, and in the second case, a diffusion layer alone. In manufacturing hot-forging dies it is advisable to obtain a microstructure after nitriding without a compound layer since it consists of brittle iron nitrides, i.e. e phase (e-Fe23N), c0 phase (c0 -Fe4N) or mixed phases (e + c0 ). When slightly higher contact pressures (about 20 MPa) are applied, the compound layer usually spalls off the die surface at the very beginning of the forging process, and this usually greatly accelerates the process of wear. Microstructure after nitriding as well as die design should be adapted in each application to actual loads that prevail on the die in the chosen forming process [12–24]. In order to learn more-about the relationships between the occurrence of various types of damage and loads, properties of applied steel, die shapes, etc., we need more comprehensive, additional data on failures of industrial dies, especially on data for dies that were previously nitrided, coated or duplex treated. For more accurate prediction of occurrence of damage, and consequently also for better planning of the forging process, more data on analysis of die damage types from industrial practice are needed. Thus this paper presents an analysis of the main types of damage on two failed gas-nitrided industrial pairs of dies for hot forging in the ultimate and penultimate forging sequence. Suggestions to increase their in-service life are given too. 2. Experimental 2.1. Die material, applied methods and microstructures after nitriding A high performance chromium–molybdenum–vanadium alloyed hot-working die steel was used for industrial dies. This die steel is characterized by excellent toughness, ductility in all directions, good tempering resistance and high-temperature strength, excellent hardenability and good dimensional stability during heat treatment as well as during the coating process. Consequently, this die steel offers very high resistance to thermal fatigue (heat checking), gross cracking, hot wear and plastic deformation [25]. It is used for dies in die casting, hot forging, hot extrusion, etc. Table 1 shows its chemical composition. Dies were sectioned and examined using standard metallographic techniques. Optical microscopy using Olympus GX51 microscope and Vickers microhardness measurements using Leitz Miniload 2 apparatus were performed. Samples were taken from the edge of both dies and measurements were carried out in order to reveal surface damage and influence of thermal loads on lowering the hardness of the nitrided layer. After metallographic preparation of the samples (Nital etchant was used), light metallography was used to estimate the quality of the nitrided layers on the cross-sections of the dies. The dies were gas nitrided at various nitriding conditions. This resulted also in different microstructures on cross-sections that were obtained after nitriding, as presented in Fig. 1a and b. The depth of the diffusion layer on the upper die was not homogenous and it varied between 73 and 122 lm. This nonhomogenity could be attributed to inappropriate preparation of the die surface (surface activation) before the nitriding process that resulted in different adsorption abilities of nitrogen to the die surface layer [18]. The depth of the diffusion layer of the lower die was regular, it was 125 lm. The thickness of the compound layer on the upper die was in the range of 0.0–3.5 lm (Fig. 1a). Further, the presence of nitrides was detected on grain boundaries. It was also evident that the microstructure of the upper die showed slight overnitriding (nitrides on grain boundaries were in this area perpendicular to the diffusion front, i.e. almost parallel to the die surface) that could result in premature spalling of material from the die surface layer. The nitrided microstructure of the lower die was without the compound layer, and on grain boundaries no nitrides were observed (Fig. 1b). Thus, the general conclusion about the quality of the nitrided microstructures was that the lower die had a more appropriate, better microstructure after nitriding. Initial microhardness profiles of both dies were added to Figs. 6a and 13a which represent damaged surface layers of both dies. Maximum values were above 1200 HV0.1 for both dies, but as could be seen also in Fig. 1a and b, nitriding depths were different. The diffusion depth of the lower die was higher in comparison to that of the upper die. 2.2. Shape of final forged product Initial billet (stock) in forging a car steering mechanism had a diameter of £ 32 mm and a weight of 930 g, it consisted of C35E steel that was heated in the temperature interval of 1230–1280 °C. There were 6 preforms in the forging process design (Fig. 2) to achieve the final shape and dimensions of the forged product (Fig. 3). After hot forging, the punching process was performed.
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Fig. 1. Microstructures on cross-sections from the edges of both dies: (a) nitrided diffusion layer with compound layer on the upper die, (b) nitrided diffusion layer on the lower die.
Fig. 2. Preforms for steering mechanism: (a) 4 preforms made by rolling, (b) the last 3 preforms made by forging.
Fig. 3. Shape of steering mechanism: (a) shape after the last hot forging sequence, (b) final product after punching.
2.3. Description of hot-forging dies The main types of damage on a pair of dies, on the upper (Fig. 4) and lower die (Fig. 5), with two cavities for the penultimate forging sequence and two cavities for the ultimate forging sequence were analyzed after their in-service life. However, the analysis was predominately focused on cavities for the ultimate forging sequence since they give the final shape
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Fig. 4. Damage on the upper die after 45,766 strokes; (a, f) plastic deformation, (b–e and g) mechanical damage, (d) spalling, (d, h) cracks, (i) wear.
Fig. 5. Damage on the lower die after 38,312 strokes; (a) mechanical damage, (b, g) cracks, (c–f) plastic deformation, (a, b, e) spalling.
and dimensions to the product. Both dies had a similar design of cavities. An essential difference between the upper and the lower die was the height of mandrels. Considering Ref. [1], it can be assumed that the temperature reached 600–700 °C and the contact pressure exceeded 1000 MPa on most of the die surfaces. A graphite lubricant suspended in water was used for lubrication. Die life is determined by surface quality of a forged product and by die dimensions. In our case, the customer set special requirements about the quality of the shaped cavity surface and on dimensions at the bottom of mandrels (see Figs. 4 and 5). These areas were indicated as critical areas on hot-forging dies. The upper die had higher mandrels and it failed after 45,766 forging strokes due to distortion of the shape of the forged product in the cavity around high mandrels. The lower die had lower mandrels and it failed after 38,312 strokes, again due to distortion of the shape of the forged product in the cavity around low mandrels.
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3. Results and discussion 3.1. Types of damage on die surfaces The main types of damage on the macro level of the upper and the lower die are shown in Figs. 4 and 5, respectively. Relatively sharp edges on both dies were most often observed on the upper edges of mandrels and around mandrels where wear was usually combined with plastic deformation and spalling (see Figs. 4a, c, e and f and 5c, d and f). On these edges, high contact pressures as well as large sliding lengths between the deformed material and die surface were noted [14,15]. These conditions led to higher heating and consequently to higher tempering of the die surface layer that favoured spalling of fragments off the nitrided layer, and plastic deformation too. Spalling of relative large fragments was observed on the upper edge of the low mandrels (see Fig. 5d and f) and on the upper edge of the high mandrels (Fig. 4c, d and e) in the penultimate forging sequence. Another possible reason for spalling was also the presence of nitrides on grain boundaries of the diffusion layer (see Fig. 1a) that reduced the toughness of die steel. But spalling on the high mandrels took place only on one mandrel (Fig. 4e) while merely emphasized wear was visible on the second mandrel (Fig. 4b). The area of spalling on the high mandrel was relatively small in comparison with the low mandrel (compare Fig. 4e with Fig. 5d and f). In fact, plastic deformation in Fig. 5d and f was visible since previous sharp corners became rounded after spalling due to sliding of deformed material. Further, spalling on top of both mandrels (see Figs. 4i and 5a) was observed but on the lower mandrel this phenomenon was more emphasized despite better microstructure after nitriding (see Fig. 1b) in comparison with the upper die (see Fig. 1a). Spalling can be attributed to thermal fatigue. On the radius of the bottom edge of the high mandrel, presented in Fig. 4g, plastic deformation did not take place, thus only furrows as consequence of abrasion were observed. Contact pressures in this area were relatively high because the radius of the mandrel was smaller. Since the quality of die surface determines die life as well as the quality of forged products, it is worth presenting some evidence of abrasion in Fig. 5e as a consequence of higher sliding lengths. Spalling of the nitrided layer was also observed on the left side in this figure. The bottoms of mandrels indicated above as critical areas of hot-forging dies (see Figs. 4 and 5) were also critical for inservice life of dies and the quality of forged products. In these areas higher stress concentrations occurred since the forged material was pushed into this part of the cavity and it was pressing against both surfaces. This consequently led to higher deflection of the die cavity bottom that resulted in formation of cracks. Thus mechanical fatigue as the main load in combination with thermal fatigue occurred in this area. Cracks were observed at the bottom of high mandrels (see Fig. 4d and h) and in the cavities around low mandrels (Fig. 5b and g). Because of these cracks, forged products did not have adequate geometry and shape. For further explanation of surface damage, microstructure cross-sections of critical areas, i.e. on the top of both mandrels, on the upper and on bottom edge as well as in the central area of mandrels, will be presented in the next Sections 3.2 and 3.3.
3.2. Presentation of damage on cross-section of high mandrel Micrographs of cross-sections of the high mandrel for the ultimate forging sequence revealed a compound layer on top of the mandrel (Fig. 6c) and in the central area (Fig. 8b). On the other hand, the compound layer was removed on the upper edge (Fig. 7b) and on the bottom edge (Fig. 9c) during the forging process, and formation of cracks in the diffusion layer was observed. Removal of the compound layer could be attributed to higher contact pressures and longer sliding lengths as
Fig. 6. Top area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the penultimate and (c) ultimate forging sequence.
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Fig. 7. Upper edge area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the ultimate forging sequence.
Fig. 8. Center area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the ultimate forging sequence.
Fig. 9. Bottom edge area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the penultimate and (c) ultimate forging sequence.
mentioned in Section 3.1. They prevailed in these areas and this led firstly to cracking of the compound layer and then to its spalling; the compound layer is namely very brittle in comparison to the diffusion layer. A crack with a length of about 1.5 mm was detected at the bottom of the mandrel for the ultimate forging sequence (Fig. 9c, compare it with Fig. 4h) while just one crack with a length of 103 lm was observed on the mandrel for the
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penultimate forging sequence (Fig. 9b). Cracks were also present in the diffusion layer on top of the mandrel, being about 69 lm long (Fig. 6c) and a 96 lm long crack was found on top of the mandrel for the penultimate forging sequence (Fig. 6b). The compound layer on top of the mandrel also exhibited cracking as a consequence of thermal fatigue (compare Fig. 6c with Fig. 4i). On the upper edge of the mandrel for the ultimate forging sequence, a crack of a length about 96 lm (Fig. 7b) was detected. Relative sliding between the die surface and deformed material was in these areas reduced, thus also abrasion was not a dominant wear mechanism. The upper edge and the central area showed a similar formation of cracks on mandrels for the ultimate and penultimate forging sequence. Effective nitriding depth on all the cross-sections of the mandrel for the ultimate forging sequence was about 150– 160 lm, while nitriding depth on the mandrel for the penultimate forging sequence was about 40–140 lm. Microhardness measurements showed the highest drop of hardness on the upper edge of the high mandrel for the ultimate forging sequence, the drop was from 1292 HV0.1 after nitriding to 993 HV0.1 after die failure (Fig. 7a), while for the penultimate forging sequence the highest drop of hardness was in the central area, the drop was to 709 HV0.1 (Fig. 8a). Increased microhardness values were detected on the mandrel for the ultimate forging sequence at the depth of about 50 lm and higher. They are represented in Figs. 6a–9a. This could be attributed to diffusion of nitrogen from the die surface layer into bulk material [14] or to possible increase of carbon content since a graphite-based lubricant was used for lubrication and cooling of die cavities. 3.3. Presentation of damage on cross-sections of low mandrel The cross-sections of the low mandrel for the ultimate and penultimate forging sequence did not reveal any compound layers (Figs. 10b, 13b and 11c), and there was no presence of nitrides in the diffusion layer as observed in Section 3.1 on the die edge (compare them with Fig. 1b).
Fig. 10. Top area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Fig. 11. Upper edge area of low mandrel: (a) hardness profile, (b) microstructure cross-section for penultimate and (c) ultimate forging sequence.
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Fig. 12. Central area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Fig. 13. Bottom edge area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Longitude cracks in the diffusion layer with a length of about 46 lm were observed on top of the mandrel for the ultimate forging sequence (Fig. 10b, compare it with Fig. 5a). The same was observed also on the mandrel for the penultimate forging sequence. Cracks depth on the upper edge of the mandrel for the ultimate forging sequence was approximately the same, i.e.
Fig. 14. Cavity-critical area around low mandrel: (a) microstructure cross-section and cracks on critical area for ultimate forging sequence, (b) spalling of die material on critical area for penultimate forging sequence.
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about 48 lm (Fig. 11c), while the depth of cracks on the mandrel for the penultimate forging sequence varied from 64 to 70 lm (Fig. 11b). Wear of the diffusion layer was detected in the central area of the low mandrel for the ultimate forging sequence (Fig. 12b). Relatively high wear in this area could be attributed to greater sliding action between the forged material and the die surface. Consequently, the highest drop of hardness occurred; from 1044 HV0.1 after nitriding to 841 HV0.1 after die failure, while the hardness drop on the mandrel for the penultimate forging sequence was to 493 HV0.1 (Fig. 12a). On the bottom edge of the mandrel for the ultimate forging sequence were observed a few single cracks with lengths of about 14 lm (Fig. 13b). The same was observed also on the mandrel for the penultimate forging sequence. Effective nitriding depth on all the cross-sections on the low mandrel for the ultimate forging sequence was about 110– 160 lm and on the mandrel for the penultimate forging sequence about 60–130 lm. As mentioned above, the lower die failed after 38,312 strokes because of crack formation in the cavity around low mandrels in the critical areas. Cracks with lengths of about 1.18 mm were observed in this area on the mandrel for the ultimate forging sequence (Fig. 14a, compare it with Fig. 5b), while spalling after cracking occurred in the cavity around the mandrel for the penultimate forging sequence, as can be seen in Fig. 14b (compare it with Fig. 5g). Since the cracks in the die cavity for the penultimate forging sequence were lower than in the cavity for the ultimate forging sequence, this means that a prolongation of die life could be achieved with improved shape of die cavities, i.e. by changing deformation sequences in such a way that deflection of the die cavity in the ultimate forging sequence would be smaller.
4. Conclusions Occurrence of damage on a pair of hot-forging dies made of hot-working die steel has been analyzed. Dies, gas nitrided at various conditions, were sectioned and examined after their in-service life using optical microscopy and Vickers microhardness measurements. The most important conclusions were: 1. Three mechanisms of wear were detected on die surfaces, i.e. thermal fatigue, mechanical fatigue and abrasion. 2. More appropriate microstructure after nitriding, without a compound layer and without nitrides in the homogenous diffusion layer was found in the lower hot-forging die, but nevertheless it failed at a lower number of forging strokes than the upper die. 3. The upper hot-forging die had a less appropriate microstructure with a compound layer and nitrides present on grain boundaries. Additionally, a non-homogenous depth of the diffusion layer was observed. This could be a consequence of improper surface preparation prior to nitriding, but despite this fact the upper die failed after a higher number of forging strokes than the lower die. The research team is of opinion that a better selection of proper nitriding parameters as well as proper die surface preparation prior to nitriding in order to improve surface activation are needed. 4. Metallographical analysis revealed that mechanical fatigue cracking was a dominant wear mechanism of die failure. Cracks were found in critical areas of the mandrels for the ultimate forging sequence; on the bottom edge of the high mandrel a crack with length of about 1.5 mm was observed and in the cavity around the low mandrel cracks with a length of about 1.18 mm were detected. Several shorter cracks of about 103 lm were observed at the bottom of the cavity around the high mandrel for the penultimate forging sequence, and cracks with a length of about 21 lm next to spalling of die material were observed at the bottom of the cavity around the low mandrel for the penultimate forging sequence. 5. Effective nitriding depths in all the cross-sections of the high mandrel for the ultimate forging sequence were in the range of about 150–160 lm and for the penultimate forging sequence in the range of about 40–140 lm. 6. Effective nitriding depths in all the cross-sections of the low mandrel for the ultimate forging sequence were in the range of about 110–160 lm and for the penultimate forging sequences in the range of about 60–130 lm. 7. Design of die shape (cavity) in sequential forging steps had a predominant role for prolonged die life in comparison to the quality of nitrided die cavity surface. Spalling with subsequent plastic deformation (rounding of sharp edges) was found to be the main mechanism of damage on low mandrels. 8. In order to improve the life time of hot-forging dies, it is first of all necessary to improve the design (shape) of dies in preceding forging sequences with the goal to reduce deflection of die cavities in the ultimate forging sequence. The sequence of deformation steps has a dominant influence on die life and not the quality of nitriding.
References [1] Hansen PH. Analysis of wear distribution in forging dies. PhD. Technical University of Denmark; 1990. [2] Rooks BW, The effect of die temperature on metal flow and die wear during high speed hot forging. In: Proceedings of the 15th international MTDR conference. London, Birmingham: Macmillan; 1974. p. 487–94. [3] Singh AK. Factors affecting die wear. Wear 1973;25:271–9. [4] Silva TM, Dean TA. Wear in drop forging dies. In: Proceedings of the 15th international MTDR conference. London, Birmingham (UK): Macmillan; 1974. p. 479–86. [5] Sharma R, Arrowsmith DJ. The wear of forging dies in the first five forging blows. Wear 1981;74:1–10. [6] Siedel R, Luig H. Friction and wear processes in hot die forging. New materials processes experiences for dieing. In: Proceedings of the international european conference on dieing materials. Switzerland; 1992. p. 467–80.
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[7] Doege E, Groche P, Bobke T. Application of adhesion theory to friction and wear processes in hot die forging. In: Proceedings of the 3rd international conference on technology on plasticity; 1990. p. 27–32. [8] Baar C. Schmelzmetallurgische massnahmen zur standmengenerhöhung von schmiedewerkzeugen. In: Proceedings of the seminar on werkzeug – und formenbau. Hannover: Univerität Hannover; 1993. p. 2/1–2/13. [9] Behrens BA, Schaefer F. Prediction of wear in hot forging tools by means of finite-element-analysis. J Mater Process Technol 2005;167:309–15. [10] Ebara R, Kubota K. Failure analysis of hot forging dies for automotive components. Eng Fail Anal 2008;15:881–93. [11] Pye D. Practical nitriding and ferritic nitrocarburizing. Materials Park: ASM International; 2003. [12] Tercelj M, Smolej A, Fajfar P, Turk R. Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium. Tribol Int 2007;40:374–84. [13] Castro G, Fernández-Vicente A, Cid J. Influence of the nitriding time in the wear behaviour of an AISI H13 steel during a crankshaft forging process. Wear 2007;263:1375–85. [14] Tercelj M, Panjan P, Urankar I, Fajfar P, Turk R. A newly designed laboratory hot forging test for evaluation of coated tool wear resistance. Surf Coat Technol 2006;200:3594–604. [15] Turk R, Perus I, Tercelj M. New starting points for the prediction of tool wear in hot forging. Int J Mach Tools Manuf 2004;44:1319–31. [16] Karamis MB, Gerçekcioglu E. Wear behaviour of plasma nitrided steels at ambient and elevated temperatures. Wear 2000;243:76–84. [17] Miola EJ, De Souza SD, Olzon-Dionysio M. Systematic study on influence of the nitriding parameters on pure iron superficial layer properties. Surf Coat Technol 2003;167:33–40. [18] Baranowska J, Szczecinski K, Wysiecki M. Increasing of gas nitriding kinetics via surface pre-treatment. Surf Coat Technol 2002;151–152:534–9. [19] Alsaran A, Çelik A, Çelik C. Determination of the optimum conditions for ion nitriding of AISI 5140 steel. Surf Coat Technol 2002;160:219–26. [20] Pellizzari M, Zadra M, Molinari A. Tribological properties of surface engineered hot work tool steel for aluminium extrusion dies. Surf Eng 2007;23:165–8. [21] Matteis P, Scavino G, Quadrini E, Perucci P, Firrao D. Damage of repeatedly nitrocarburised steel dies for aluminium extrusion. Surf Eng 2009;25:507–16. [22] Wen DC. Microstructure, corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding. Appl Surf Sci 2009;256:797–804. [23] Chang S-H, Tang T-P, Chen Y-C, Chen J-K. Enhancement of erosion resistance on AISI H13 tool steel by oxynitriding treatment. ISIJ Int 2009;49:421–4. [24] Akhtar SS, Arif AFM, Yilbas BS. Evaluation of gas nitriding process with in-process variation of nitriding potential for AISI H13 tool steel. Int J Adv Manuf Technol 2010;47:687–98. [25] http://www.ramada.pt/acos/pdf/pdf/Dievar.pdf.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Review
Analysis of the main types of damage on a pair of industrial dies for hot forging car steering mechanisms Lejla Lavtar a,⇑, Tadej Muhicˇ b, Goran Kugler a, Milan Tercˇelj a a b
Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia Faculty of Mechanical Engineering, University of Ljubljana, Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 1 June 2010 Received in revised form 13 October 2010 Accepted 4 November 2010 Available online 24 November 2010 Keywords: Hot forging Die Gas nitriding Damage
a b s t r a c t The paper presents an analysis of the main types of damage on industrial upper and lower hot-forging dies for hot forging of car steering mechanisms. The dies were previously gas nitrided at various conditions and exhibit various microstructures after nitriding. They failed after a different number of forging strokes. Measurements of microhardness depth profiles and examinations of nitrided layers with an optical microscope were conducted. The design of die shape in sequential forging steps has the predominant role for a prolonged die life in comparison to the quality of nitrided die cavity surface. Improvements of die cavity design as well as improvements of nitriding process are suggested. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction During the process of hot forging the dies are subjected to complex loading involving simultaneous action of thermal, mechanical, chemical and tribological loads. These loads act also on particular parts of hot-forging dies and cause the occurrence of different types of damage such as wear, plastic deformation, thermal and mechanical cracking, gross cracking. and their progress as function of time. In the past, many authors reported that adhesive wear [1], erosion [1–4], abrasive wear [1,5,6], thermal as well as mechanical fatigue [1,7] or plastic deformation [1,8] are some of the main wear mechanisms on hot forging dies. In-service life of the die plays an important role in the economy of hot forgings. Thus the task of every manufacturing engineer is to accurately predict and possibly prolong the die’s in-service life as much as possible. Actually, expenses for dies-make up about 8–15% of the product’s cost while unexpected die failure leads to an increase in production costs by 30%. Thus companies that manufacture forgings are very interested in improving the in-service life of hot forging dies and support research work in this area in order that more useful knowledge for industrial practice is obtained [1]. In general, the occurrence of various types of damage and their progress are additionally influenced by steel material used for the die and applied technological processing parameters, die shape design, i.e. planning of forging sequences, die manufacturing, forging parameters (strain rates, contact times and contact pressures, lubrication, sliding lengths, etc.), applied forging presses, as well as forging stock properties, e.g. formation of oxide scale, local bonding between the die sand the workpiece, etc. Furthermore, the steel for the die should be produced in an appropriate way, i.e. at optimal process parameters, in order to achieve optimal microstructure, optimal grain size and distribution, type, size and shape of carbides. Finally, the manufacturing process of the die (resulting in the die’s shape) should be carried out without negative impacts on die
⇑ Corresponding author. Tel.: +386 41 851 528. E-mail address: [email protected] (L. Lavtar). 1350-6307/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2010.11.002
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L. Lavtar et al. / Engineering Failure Analysis 18 (2011) 1143–1152 Table 1 Chemical composition of the die steel in wt.%. C
Si
Mn
Cr
Mo
V
0.37
0.26
0.5
5.0
2.36
0.55
surface quality. The die shapes should be optimized through adequate sequential forging steps in order to avoid areas (parts of die cavity) where loads are essentially higher than in other die areas [9,10]. In order to increase in-service life and resistance to wear and fatigue, die surfaces are improved by PVD, PACVD, CVD coatings, diffusion processes like nitriding, etc. [11]. During nitriding die steels, two different types of microstructure are formed on the die surface, i.e. microstructure with a ‘‘compound layer’’ on top and a diffusion layer under it, and in the second case, a diffusion layer alone. In manufacturing hot-forging dies it is advisable to obtain a microstructure after nitriding without a compound layer since it consists of brittle iron nitrides, i.e. e phase (e-Fe23N), c0 phase (c0 -Fe4N) or mixed phases (e + c0 ). When slightly higher contact pressures (about 20 MPa) are applied, the compound layer usually spalls off the die surface at the very beginning of the forging process, and this usually greatly accelerates the process of wear. Microstructure after nitriding as well as die design should be adapted in each application to actual loads that prevail on the die in the chosen forming process [12–24]. In order to learn more-about the relationships between the occurrence of various types of damage and loads, properties of applied steel, die shapes, etc., we need more comprehensive, additional data on failures of industrial dies, especially on data for dies that were previously nitrided, coated or duplex treated. For more accurate prediction of occurrence of damage, and consequently also for better planning of the forging process, more data on analysis of die damage types from industrial practice are needed. Thus this paper presents an analysis of the main types of damage on two failed gas-nitrided industrial pairs of dies for hot forging in the ultimate and penultimate forging sequence. Suggestions to increase their in-service life are given too. 2. Experimental 2.1. Die material, applied methods and microstructures after nitriding A high performance chromium–molybdenum–vanadium alloyed hot-working die steel was used for industrial dies. This die steel is characterized by excellent toughness, ductility in all directions, good tempering resistance and high-temperature strength, excellent hardenability and good dimensional stability during heat treatment as well as during the coating process. Consequently, this die steel offers very high resistance to thermal fatigue (heat checking), gross cracking, hot wear and plastic deformation [25]. It is used for dies in die casting, hot forging, hot extrusion, etc. Table 1 shows its chemical composition. Dies were sectioned and examined using standard metallographic techniques. Optical microscopy using Olympus GX51 microscope and Vickers microhardness measurements using Leitz Miniload 2 apparatus were performed. Samples were taken from the edge of both dies and measurements were carried out in order to reveal surface damage and influence of thermal loads on lowering the hardness of the nitrided layer. After metallographic preparation of the samples (Nital etchant was used), light metallography was used to estimate the quality of the nitrided layers on the cross-sections of the dies. The dies were gas nitrided at various nitriding conditions. This resulted also in different microstructures on cross-sections that were obtained after nitriding, as presented in Fig. 1a and b. The depth of the diffusion layer on the upper die was not homogenous and it varied between 73 and 122 lm. This nonhomogenity could be attributed to inappropriate preparation of the die surface (surface activation) before the nitriding process that resulted in different adsorption abilities of nitrogen to the die surface layer [18]. The depth of the diffusion layer of the lower die was regular, it was 125 lm. The thickness of the compound layer on the upper die was in the range of 0.0–3.5 lm (Fig. 1a). Further, the presence of nitrides was detected on grain boundaries. It was also evident that the microstructure of the upper die showed slight overnitriding (nitrides on grain boundaries were in this area perpendicular to the diffusion front, i.e. almost parallel to the die surface) that could result in premature spalling of material from the die surface layer. The nitrided microstructure of the lower die was without the compound layer, and on grain boundaries no nitrides were observed (Fig. 1b). Thus, the general conclusion about the quality of the nitrided microstructures was that the lower die had a more appropriate, better microstructure after nitriding. Initial microhardness profiles of both dies were added to Figs. 6a and 13a which represent damaged surface layers of both dies. Maximum values were above 1200 HV0.1 for both dies, but as could be seen also in Fig. 1a and b, nitriding depths were different. The diffusion depth of the lower die was higher in comparison to that of the upper die. 2.2. Shape of final forged product Initial billet (stock) in forging a car steering mechanism had a diameter of £ 32 mm and a weight of 930 g, it consisted of C35E steel that was heated in the temperature interval of 1230–1280 °C. There were 6 preforms in the forging process design (Fig. 2) to achieve the final shape and dimensions of the forged product (Fig. 3). After hot forging, the punching process was performed.
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Fig. 1. Microstructures on cross-sections from the edges of both dies: (a) nitrided diffusion layer with compound layer on the upper die, (b) nitrided diffusion layer on the lower die.
Fig. 2. Preforms for steering mechanism: (a) 4 preforms made by rolling, (b) the last 3 preforms made by forging.
Fig. 3. Shape of steering mechanism: (a) shape after the last hot forging sequence, (b) final product after punching.
2.3. Description of hot-forging dies The main types of damage on a pair of dies, on the upper (Fig. 4) and lower die (Fig. 5), with two cavities for the penultimate forging sequence and two cavities for the ultimate forging sequence were analyzed after their in-service life. However, the analysis was predominately focused on cavities for the ultimate forging sequence since they give the final shape
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Fig. 4. Damage on the upper die after 45,766 strokes; (a, f) plastic deformation, (b–e and g) mechanical damage, (d) spalling, (d, h) cracks, (i) wear.
Fig. 5. Damage on the lower die after 38,312 strokes; (a) mechanical damage, (b, g) cracks, (c–f) plastic deformation, (a, b, e) spalling.
and dimensions to the product. Both dies had a similar design of cavities. An essential difference between the upper and the lower die was the height of mandrels. Considering Ref. [1], it can be assumed that the temperature reached 600–700 °C and the contact pressure exceeded 1000 MPa on most of the die surfaces. A graphite lubricant suspended in water was used for lubrication. Die life is determined by surface quality of a forged product and by die dimensions. In our case, the customer set special requirements about the quality of the shaped cavity surface and on dimensions at the bottom of mandrels (see Figs. 4 and 5). These areas were indicated as critical areas on hot-forging dies. The upper die had higher mandrels and it failed after 45,766 forging strokes due to distortion of the shape of the forged product in the cavity around high mandrels. The lower die had lower mandrels and it failed after 38,312 strokes, again due to distortion of the shape of the forged product in the cavity around low mandrels.
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3. Results and discussion 3.1. Types of damage on die surfaces The main types of damage on the macro level of the upper and the lower die are shown in Figs. 4 and 5, respectively. Relatively sharp edges on both dies were most often observed on the upper edges of mandrels and around mandrels where wear was usually combined with plastic deformation and spalling (see Figs. 4a, c, e and f and 5c, d and f). On these edges, high contact pressures as well as large sliding lengths between the deformed material and die surface were noted [14,15]. These conditions led to higher heating and consequently to higher tempering of the die surface layer that favoured spalling of fragments off the nitrided layer, and plastic deformation too. Spalling of relative large fragments was observed on the upper edge of the low mandrels (see Fig. 5d and f) and on the upper edge of the high mandrels (Fig. 4c, d and e) in the penultimate forging sequence. Another possible reason for spalling was also the presence of nitrides on grain boundaries of the diffusion layer (see Fig. 1a) that reduced the toughness of die steel. But spalling on the high mandrels took place only on one mandrel (Fig. 4e) while merely emphasized wear was visible on the second mandrel (Fig. 4b). The area of spalling on the high mandrel was relatively small in comparison with the low mandrel (compare Fig. 4e with Fig. 5d and f). In fact, plastic deformation in Fig. 5d and f was visible since previous sharp corners became rounded after spalling due to sliding of deformed material. Further, spalling on top of both mandrels (see Figs. 4i and 5a) was observed but on the lower mandrel this phenomenon was more emphasized despite better microstructure after nitriding (see Fig. 1b) in comparison with the upper die (see Fig. 1a). Spalling can be attributed to thermal fatigue. On the radius of the bottom edge of the high mandrel, presented in Fig. 4g, plastic deformation did not take place, thus only furrows as consequence of abrasion were observed. Contact pressures in this area were relatively high because the radius of the mandrel was smaller. Since the quality of die surface determines die life as well as the quality of forged products, it is worth presenting some evidence of abrasion in Fig. 5e as a consequence of higher sliding lengths. Spalling of the nitrided layer was also observed on the left side in this figure. The bottoms of mandrels indicated above as critical areas of hot-forging dies (see Figs. 4 and 5) were also critical for inservice life of dies and the quality of forged products. In these areas higher stress concentrations occurred since the forged material was pushed into this part of the cavity and it was pressing against both surfaces. This consequently led to higher deflection of the die cavity bottom that resulted in formation of cracks. Thus mechanical fatigue as the main load in combination with thermal fatigue occurred in this area. Cracks were observed at the bottom of high mandrels (see Fig. 4d and h) and in the cavities around low mandrels (Fig. 5b and g). Because of these cracks, forged products did not have adequate geometry and shape. For further explanation of surface damage, microstructure cross-sections of critical areas, i.e. on the top of both mandrels, on the upper and on bottom edge as well as in the central area of mandrels, will be presented in the next Sections 3.2 and 3.3.
3.2. Presentation of damage on cross-section of high mandrel Micrographs of cross-sections of the high mandrel for the ultimate forging sequence revealed a compound layer on top of the mandrel (Fig. 6c) and in the central area (Fig. 8b). On the other hand, the compound layer was removed on the upper edge (Fig. 7b) and on the bottom edge (Fig. 9c) during the forging process, and formation of cracks in the diffusion layer was observed. Removal of the compound layer could be attributed to higher contact pressures and longer sliding lengths as
Fig. 6. Top area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the penultimate and (c) ultimate forging sequence.
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Fig. 7. Upper edge area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the ultimate forging sequence.
Fig. 8. Center area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the ultimate forging sequence.
Fig. 9. Bottom edge area of high mandrel: (a) hardness profile, (b) microstructure cross-section for the penultimate and (c) ultimate forging sequence.
mentioned in Section 3.1. They prevailed in these areas and this led firstly to cracking of the compound layer and then to its spalling; the compound layer is namely very brittle in comparison to the diffusion layer. A crack with a length of about 1.5 mm was detected at the bottom of the mandrel for the ultimate forging sequence (Fig. 9c, compare it with Fig. 4h) while just one crack with a length of 103 lm was observed on the mandrel for the
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penultimate forging sequence (Fig. 9b). Cracks were also present in the diffusion layer on top of the mandrel, being about 69 lm long (Fig. 6c) and a 96 lm long crack was found on top of the mandrel for the penultimate forging sequence (Fig. 6b). The compound layer on top of the mandrel also exhibited cracking as a consequence of thermal fatigue (compare Fig. 6c with Fig. 4i). On the upper edge of the mandrel for the ultimate forging sequence, a crack of a length about 96 lm (Fig. 7b) was detected. Relative sliding between the die surface and deformed material was in these areas reduced, thus also abrasion was not a dominant wear mechanism. The upper edge and the central area showed a similar formation of cracks on mandrels for the ultimate and penultimate forging sequence. Effective nitriding depth on all the cross-sections of the mandrel for the ultimate forging sequence was about 150– 160 lm, while nitriding depth on the mandrel for the penultimate forging sequence was about 40–140 lm. Microhardness measurements showed the highest drop of hardness on the upper edge of the high mandrel for the ultimate forging sequence, the drop was from 1292 HV0.1 after nitriding to 993 HV0.1 after die failure (Fig. 7a), while for the penultimate forging sequence the highest drop of hardness was in the central area, the drop was to 709 HV0.1 (Fig. 8a). Increased microhardness values were detected on the mandrel for the ultimate forging sequence at the depth of about 50 lm and higher. They are represented in Figs. 6a–9a. This could be attributed to diffusion of nitrogen from the die surface layer into bulk material [14] or to possible increase of carbon content since a graphite-based lubricant was used for lubrication and cooling of die cavities. 3.3. Presentation of damage on cross-sections of low mandrel The cross-sections of the low mandrel for the ultimate and penultimate forging sequence did not reveal any compound layers (Figs. 10b, 13b and 11c), and there was no presence of nitrides in the diffusion layer as observed in Section 3.1 on the die edge (compare them with Fig. 1b).
Fig. 10. Top area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Fig. 11. Upper edge area of low mandrel: (a) hardness profile, (b) microstructure cross-section for penultimate and (c) ultimate forging sequence.
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Fig. 12. Central area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Fig. 13. Bottom edge area of low mandrel: (a) hardness profile, (b) microstructure cross-section for ultimate forging sequence.
Longitude cracks in the diffusion layer with a length of about 46 lm were observed on top of the mandrel for the ultimate forging sequence (Fig. 10b, compare it with Fig. 5a). The same was observed also on the mandrel for the penultimate forging sequence. Cracks depth on the upper edge of the mandrel for the ultimate forging sequence was approximately the same, i.e.
Fig. 14. Cavity-critical area around low mandrel: (a) microstructure cross-section and cracks on critical area for ultimate forging sequence, (b) spalling of die material on critical area for penultimate forging sequence.
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about 48 lm (Fig. 11c), while the depth of cracks on the mandrel for the penultimate forging sequence varied from 64 to 70 lm (Fig. 11b). Wear of the diffusion layer was detected in the central area of the low mandrel for the ultimate forging sequence (Fig. 12b). Relatively high wear in this area could be attributed to greater sliding action between the forged material and the die surface. Consequently, the highest drop of hardness occurred; from 1044 HV0.1 after nitriding to 841 HV0.1 after die failure, while the hardness drop on the mandrel for the penultimate forging sequence was to 493 HV0.1 (Fig. 12a). On the bottom edge of the mandrel for the ultimate forging sequence were observed a few single cracks with lengths of about 14 lm (Fig. 13b). The same was observed also on the mandrel for the penultimate forging sequence. Effective nitriding depth on all the cross-sections on the low mandrel for the ultimate forging sequence was about 110– 160 lm and on the mandrel for the penultimate forging sequence about 60–130 lm. As mentioned above, the lower die failed after 38,312 strokes because of crack formation in the cavity around low mandrels in the critical areas. Cracks with lengths of about 1.18 mm were observed in this area on the mandrel for the ultimate forging sequence (Fig. 14a, compare it with Fig. 5b), while spalling after cracking occurred in the cavity around the mandrel for the penultimate forging sequence, as can be seen in Fig. 14b (compare it with Fig. 5g). Since the cracks in the die cavity for the penultimate forging sequence were lower than in the cavity for the ultimate forging sequence, this means that a prolongation of die life could be achieved with improved shape of die cavities, i.e. by changing deformation sequences in such a way that deflection of the die cavity in the ultimate forging sequence would be smaller.
4. Conclusions Occurrence of damage on a pair of hot-forging dies made of hot-working die steel has been analyzed. Dies, gas nitrided at various conditions, were sectioned and examined after their in-service life using optical microscopy and Vickers microhardness measurements. The most important conclusions were: 1. Three mechanisms of wear were detected on die surfaces, i.e. thermal fatigue, mechanical fatigue and abrasion. 2. More appropriate microstructure after nitriding, without a compound layer and without nitrides in the homogenous diffusion layer was found in the lower hot-forging die, but nevertheless it failed at a lower number of forging strokes than the upper die. 3. The upper hot-forging die had a less appropriate microstructure with a compound layer and nitrides present on grain boundaries. Additionally, a non-homogenous depth of the diffusion layer was observed. This could be a consequence of improper surface preparation prior to nitriding, but despite this fact the upper die failed after a higher number of forging strokes than the lower die. The research team is of opinion that a better selection of proper nitriding parameters as well as proper die surface preparation prior to nitriding in order to improve surface activation are needed. 4. Metallographical analysis revealed that mechanical fatigue cracking was a dominant wear mechanism of die failure. Cracks were found in critical areas of the mandrels for the ultimate forging sequence; on the bottom edge of the high mandrel a crack with length of about 1.5 mm was observed and in the cavity around the low mandrel cracks with a length of about 1.18 mm were detected. Several shorter cracks of about 103 lm were observed at the bottom of the cavity around the high mandrel for the penultimate forging sequence, and cracks with a length of about 21 lm next to spalling of die material were observed at the bottom of the cavity around the low mandrel for the penultimate forging sequence. 5. Effective nitriding depths in all the cross-sections of the high mandrel for the ultimate forging sequence were in the range of about 150–160 lm and for the penultimate forging sequence in the range of about 40–140 lm. 6. Effective nitriding depths in all the cross-sections of the low mandrel for the ultimate forging sequence were in the range of about 110–160 lm and for the penultimate forging sequences in the range of about 60–130 lm. 7. Design of die shape (cavity) in sequential forging steps had a predominant role for prolonged die life in comparison to the quality of nitrided die cavity surface. Spalling with subsequent plastic deformation (rounding of sharp edges) was found to be the main mechanism of damage on low mandrels. 8. In order to improve the life time of hot-forging dies, it is first of all necessary to improve the design (shape) of dies in preceding forging sequences with the goal to reduce deflection of die cavities in the ultimate forging sequence. The sequence of deformation steps has a dominant influence on die life and not the quality of nitriding.
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