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A novel approach to wear testing in hot stamping of high strength boron steel sheets
Wear 302 (2013) 1319–1326
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
A novel approach to wear testing in hot stamping of high strength boron steel sheets A. Ghiotti, F. Sgarabotto, S. Bruschi n DII, University of Padua, Via Venezia 1, 35131 Padua, Italy
a r t i c l e i n f o
abstract
Article history: Received 1 September 2012 Received in revised form 20 December 2012 Accepted 23 December 2012 Available online 9 January 2013
Hot stamping of high strength steel sheets was developed in the automotive industry for the production of components characterized by a high strength-to-weight ratio and an increased resistance to impact. In order to avoid scaling and decarburization, the steel blanks are usually coated with an Al–Si coating that has proved a relevant influence also on their tribological behaviour during the forming stages. However, the knowledge of the influence that this coating may have on the dies wear mechanisms is still inadequate. The paper proposes a novel approach to wear testing, based on a pin-on-disk testing configuration, capable to reproduce in a laboratory environment the conditions arising at the interface between the dies and the blank, by reproducing the sliding velocities at the interface and the cyclic thermal and mechanical stresses on the die material. Investigations were carried out on a hot working tool steel sliding against high strength steel blanks coated with the Al–Si coating under dry reciprocating sliding conditions. Scanning electron microscopy and 3D profilometer analysis were utilized to evaluate the wear mechanisms. The presented results show that the proposed procedure can properly simulate the thermal and mechanical cycles to which the forming dies are subjected during the hot stamping process, allowing to control and vary a number of parameters characterizing the industrial process. The presence of both adhesive and abrasive wear mechanisms is highlighted and a possible explanation of their appearance is given. & 2013 Elsevier B.V. All rights reserved.
Keywords: Wear Friction Hot stamping Boron steel Al–Si coating
1. Introduction In the last few years, due to the increasing demand in reducing CO2 emissions and improving the safety of car passengers, the automotive industry has promoted the development of new categories of sheet forming processes, which exploit sheet heating to increase the characteristics of the formed components in terms of both obtainable geometrical features and mechanical properties. In this framework, the hot stamping of boron steel sheets represents the most promising process, since it allows producing parts of the car body-in-white with a high ratio between the strength and mass and significant crashworthiness resistance [1,2]. During the process, sheets, usually made of the quenchable boron steel 22MnB5 coated with an Al–Si layer to avoid oxidation, are heated above their austenitization temperature and simultaneously formed and quenched into the forming dies: the conduction of the process at elevated temperatures also allows higher formability limits, lower forming loads on the dies and reduced
n
Corresponding author. Tel.: þ39 0498276821. E-mail address: [email protected] (S. Bruschi).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.051
springback compared to room temperature sheet forming operations; at the same time, the die quenching gives the part a fully martensitic structure, which guarantees its outstanding mechanical properties. A literature survey shows that several aspects of the hot stamping process have been extensively investigated, such as steel behaviour and formability at high temperature [3–6], microstructural evolution [7,8], process numerical modelling [9,10], boundary conditions between the blank and the dies in terms of friction [11–14] and heat transfer coefficients [15,16]. On the contrary, the dies behaviour during the process in terms of wear resistance is still almost unexplored, since only a few papers are devoted to the study of wear during hot stamping. The reported studies focus on conventional wear testing configurations, which are suitable to achieve fundamental knowledge on wear evolution and mechanisms, but they fail in replicating the thermal and mechanical conditions to which the forming dies are subjected during the industrial process. In fact, they usually do not apply any thermal cycle to the material acting as the tool, carrying out the tests at constant temperature [17,18]. Otherwise, the wear phenomena are studied through costly and timeconsuming industrial trials [19,20], where the possibility to vary
A. Ghiotti et al. / Wear 302 (2013) 1319–1326
the process parameters and therefore to assess their influence on the die behaviour is limited. On this basis, the paper has a twofold objective: (i) to propose and assess a novel approach to wear testing, based on a conventional pin-on-disk configuration, but capable of reproducing in a laboratory environment the thermal and mechanical cycles to which the forming dies are subjected during hot stamping; and (ii) to investigate the wear evolution during testing and give a first insight of the wear mechanisms arising at increasing number of cycles. The paper is organized into two main parts: the first describes the testing equipment and procedure designed and setup to replicate the hot stamping conditions within the wear test; the second part shows the results obtained in terms of assessment of the thermal and mechanical characteristics reproducible in the wear test as well as the wear mechanisms arising during testing, which were studied by means of scanning electron microscopy and 3D profilometer analysis.
2. Application case The reference industrial process is the hot stamping of high strength boron steel sheets to produce b-pillar automotive components. Data about the duration of the different process phases, which were later on utilized for the wear test design and set-up, were recorded during industrial trials carried out on a hot stamping plant [21]. The process for industrial practice includes first the heating of the metal sheet inside an external furnace above the steel austenitization temperature, and its transfer by means of a robot arm to the forming press in 5–10 s. Then, the dies are closed and the simultaneous forming and quenching phase starts, which takes approximately 20 s. Finally, the dies are opened and the formed component is extracted in approximately 15 s. Therefore, on an average, every 35 s the forming dies are subjected to a new forming cycle, both thermal and mechanical. The forming process is carried out under dry conditions. The dies are usually substituted after 200,000 cycles, but after the production of 2000–3000 parts they are reground [19].
The metal sheets of 1.5 mm thickness are provided with an Al–Si layer of approximately 25 mm of thickness, which prevents the metal oxidation thanks to the formation of an Al–Si–Fe ternary alloy at elevated temperature characterized by a higher melting temperature compared to the one of the Al–Si layer [22]. The pin material hardness was also characterized as a function of the temperature by measuring it with an InstronTM hardness tester equipped with a heating chamber. Fig. 1 shows the evolution of the pin material hardness up to 650 1C; a significant decrease in the hardness is observed from 300 1C. 3.2. Testing equipment A universal UMTTM tribometer by BrukerTM equipped with a pin-on-disk configuration and with a heating furnace was utilized to carry out the wear tests at elevated temperature. The pin is connected to both a vertical and lateral linear motion system. Strain-gauge sensors perform simultaneous measurements of load and torque in two axes (FZ and FX), in order to calculate the friction coefficient during the test. The normal-load sensor provides feedback to the vertical motion controller, actively adjusting the specimen position to ensure a constant load during testing. The pin was machined with an axial hole of 2 mm diameter and 7 mm depth, where a K-type thermocouple (TC) was positioned with the aim of monitoring the pin temperature evolution during testing (see Fig. 2b and c). A locking system ensures the contact between the thermocouple and the pin for the whole duration of the test. An overall image of the pin and pin holder set-up is shown in Fig. 2a.
60 Hardness [HRC]
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55 50 45 40 35 30 0
3. Experimental This section presents the characteristics of the materials involved in the experimentation, the adopted testing equipment, and the proposed testing procedure aimed at reproducing the thermal and mechanical conditions of the forming dies.
200
400 600 Temperature [°C]
800
Fig. 1. Pin material hardness as a function of the temperature.
Ø12 Ø2
9
7
3.1. Materials The pin material is the hardened hot working tool steel AISI H11 and the metal sheet material is the boron steel 22MnB5. Their chemical compositions are reported in Table 1, together with the hardness values at room temperature and initial surface roughness Sa measured by means of the 3D optical profilometer SensoSCAN NeoxTM 3.2.3.
Heat exchanger wings
Thermocouple wire
R0.5
Ø4
TC Table 1 Pin and sheet material chemical compositions (in wt%), hardness at room temperature and initial surface roughness Sa. C
Mn
Cr
Si
B
Mo
V
Hardness (HRC)
Sa (mm)
0.22 0.38
1.40 0.40
0.30 5.00
0.35 1.10
0.005 –
– 1.30
– 0.40
20 71.5 51 71.5
0.5 70.1 1.54 70.1
Pin holder Pin Pin
22MnB5 AISI H11
Fig. 2. Pin and pin holder details: overall set-up (a) and details of pin (b) with the position of the thermocouple (c).
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The actual wear test starts with the pin entrance inside the heating chamber (entry step) and its location at the zero position, which is at a distance of 1.25 70.25 mm from the surface of hot metal sheet sample. Then the pin is brought into contact with the metal sheet, a normal force and a sliding speed are applied for 5 s (contact step). Afterwards, the pin is moved out from the heating chamber (exit step) and cooled down by the air nozzle (cooling step). No lubricant is used in order to replicate the tribological conditions of the industrial process. The above-described cycle is repeated 100 times; after that the pin is made to slide in a virgin portion of the metal sheet. Every 400 cycles the pin is dismounted, cleaned with ether in an ultrasonic cleaner and analyzed with the 3D optical profilometer and the SEM, in order to monitor the wear evolution. The duration of the test was set to 2000 cycles in accordance with the common number of cycles after which the industrial dies are reground. Table 2 reports the adopted testing parameters. It is worth noting that the presented procedure allows controlling and varying the sliding speed and length, the applied normal load, and the metal sheet temperature, thus making it possible to replicate the thermal and mechanical conditions characteristic of the hot stamping process.
4. Results and discussions This section presents the preliminary results in terms of assessment of the thermal and mechanical conditions reproducible in the wear test as well as the methods for investigating the pin wear evolution and mechanisms during testing. 4.1. Assessment of the thermal testing conditions Fig. 3. Detail of the disc (a), the air nozzle (b) and scheme of the pin-on-disk testing equipment (c).
The assessment of the thermal testing conditions was carried out by proving the reproducibility of the thermal cycle imposed on the pin during testing and its closeness to the one the forming
The metal sheet (Fig. 3a), cut in the form of a disc of 45 mm diameter, is fixed to a lower rotary drive equipped with a heating chamber that allows testing temperatures up to 1000 1C. A temperature sensor is located inside the chamber and connected with a computerized controller that uses the output of the temperature sensor to control the chamber temperature. In order to cool down the pin during the test and simulate the thermal cycle to which the forming dies are subjected during the process, an air nozzle (Fig. 3b) is used, which is placed outside the heating chamber and can be positioned, in conjunction with the air flow, to impart the desired cooling rate on the pin sample. A manometer is used to control the pressure of the sprayed air from 0 to 8 bar, in order to obtain different levels of cooling and cooling rates. An overall scheme of the test equipment is shown in Fig. 3c.
Table 2 Testing parameters. Sheet heating rate Sheet temperature Normal load (contact pressure) Sliding speed Sliding distance per cycle Pin cooling time Pin cooling rate Duration
0.43 1C/s 8007 1 1C 507 2 N (7 70.1 MPa) 15 mm/s 75 mm 15 s 3.6 1C/s 2000 cycles
Total cycle time = 36.8 s 12
3.8
Contact
Exit
6
15
6
12
3.3. Testing procedure
160
130 120
Contact
140
Entrance
150
Cooling
Temperature [°C]
170
Entrance
The developed testing procedure can be divided into two main phases: the first one comprises some preliminary steps carried out to set up the testing equipment and assure the repeatability of the testing conditions, while the second phase is the actual wear test. The preliminary steps include the pin cleaning in ether in an ultrasonic cleaner and then its drying in air, its connexion to the pin holder, ensuring the contact between the thermocouple and the bottom of the pin hole, the metal sheet fastening on the rotation table inside the heating chamber, the metal sheet heating to the target temperature, and the air nozzle pressure set up. During the heating phase, the pin is kept outside the heating chamber to avoid any heating.
30 Time [s]
40
50
110 100 0
10
20
Fig. 4. Pin temperature evolution during each cycle.
60
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Temperature [°C]
180 150 120 90 60 10 cycles
30
50 cycles
20 cycles
100 cycles
0 0
500
1000
1500
2000 Time [s]
2500
3000
3500
4000
Fig. 5. Pin temperature evolution at increasing number of cycles.
Mean COF
0.8 0.6 0.4 0.2 0 0
500
1000 N°of cycle
1500
2000
0 166
168
170 Time [s]
172
174
0.8 COF
0.6 0.4 0.2
Fig. 6. Mean coefficient of friction (a) at increasing number of cycles and example of acquisition of the coefficient of friction during a cycle (b).
Spectrum
Al dies are subjected to. Fig. 4 shows the pin temperature evolution during each cycle of the wear test as recorded by the embedded K-type thermocouple, with the indication of the testing steps and their duration. The total cycle time is 36.8 s, close to the one the forming dies are subjected to during the hot stamping process. The maximum temperature reached by the pin thermocouple is far below the temperature at which the pin material starts losing its hardness characteristics (see Fig. 1). It is worth noting that the maximum and minimum temperatures of the cycle are not reached when the pin moves out from the heating chamber and at the end of the cooling step, but after a short while: this is due to the pin thermal inertia and to the delay in the response of the thermocouple being placed at a distance of 2 mm from the pin sliding surface. Fig. 5 shows the pin temperature as a function of the number of cycles. It can be observed that 10 cycles are not enough to reach a temperature steady state, whereas, after 20 cycles, the regime condition is reached, assuring a reproducible and stable thermal cycle, with the pin temperature ranging between 120 1C and 170 1C. By changing either the cooling time or the pressure of the air sprayed by the nozzle, it is possible to modify the pin temperature range. 4.2. Assessment of the mechanical testing conditions The assessment of the mechanical testing conditions was carried out by verifying the reproducibility of the friction characteristics between the metal sheet and the pin during testing. Fig. 6a shows the mean friction coefficient at increasing number of cycles; every point of the curve represents the mean friction coefficient evaluated in a single cycle (example in Fig. 6b).
Fe
O
Si
Fe 0.8
Mn 1.5
2.2
2.9
3.6
4.3
5.0
5.7
6.4
7.1
8
Fig. 7. SEM images of the as-received metal sheet after heating up to 800 1C and its spectrum analysis.
The mean coefficient of friction, COF, is equal to 0.51, comparable to others values reported in various literature studies devoted to the evaluation of tribological performance of Al–Si-coated 22MnB5 steel under industrial hot stamping conditions (COF equal to 0.53 in [11], and to 0.55 in [12]). Furthermore, it was important to evaluate if the number of 100 cycles after which the pin was made to slide on a virgin portion of metal sheet (see Section 3.3) provoked any change of the characteristics of the Al–Si–Fe ternary alloy formed on the sheet surface at elevated temperature. To do that, a sheet portion on which the pin slid 100 times was analyzed by using a Quanta FEI 450 SEM equipped with an EDX detector. In Fig. 7 a SEM image of the as-received metal sheet after being heated to the testing temperature (800 1C) and its surface spectrum are shown.
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Spectrum 1
Al
Al O
Fe
0.8
Si
1.5
2.2
3.6
4.3
5.0
5.7
6.4
7.1
8
Spectrum 3
Spectrum 2
Al
2.9
Fe Fe
O Fe
Fe
Si
O
Mn 0.8
1.5
2.2
2.9
3.6
4.3
5.0
5.7
6.4
7.1
8
0.8
1.6
2.4
3.2
4.0
4.8
5.6
6.4
7.2
8
Fig. 8. SEM image of a sheet zone in the wear path and three EDX spectra of different areas of the wear path.
As expected, the EDX sensor detected a high percentage of aluminium and iron, besides the presence of oxygen. Fig. 8 shows a SEM image of a sheet zone belonging to the wear path together with the EDX spectra acquired in three different points of the path. Areas characterized by different percentages of aluminium and silicon can be seen in spectra 1 and 2: the lighter area (spectrum 1) indicates a lower amount of aluminium and a higher percentage of iron compared to the area from spectrum 2. This clearly indicates that in this area the layer of the Al–Si–Fe ternary alloy is thinner, with the steel substrate starting to appear. Therefore, 100 cycles can be considered suitable to still have the coating layer. It is worth noticing that in both spectra 1 and 2 the presence of oxygen was observed, meaning that the surface of the metal sheet was oxidized during the tests. Finally, the third spectrum shows the presence of an iron oxide particle, since neither aluminium nor silicon can be detected. 4.3. Analysis of the wear evolution and mechanisms Both the SEM equipped with the EDX detector and the 3D optical profilometer were used to identify the wear phenomena arising during the pin thermo-mechanical cycling and the wear evolution on the pin contact surface. Fig. 9 shows a back-scattered SEM image of the pin contact surface after 200 cycles: material transfer from the metal sheet to the pin can be clearly observed. The EDX spectra in fact highlight the presence of mainly aluminium coming from the metal sheet surface, with the pin surface darker areas containing a higher percentage. This proves the occurrence of adhesion type wear
since the beginning of the test, with aluminium material transferred from the sheet to the pin sample. The spectrums also show a significant percentage of oxygen, proving that an oxide layer developed during the test on the pin surface. Fig. 10 shows the back-scattered SEM images of the pin contact surface at different cycles: the material adhesion from the metal sheet is evident, resulting in darker areas on the pin surface. At increasing number of cycles, only a slight increase of the aluminium deposition on the pin surface can be detected, proving that the mechanism of adhesion wear is effective since the very beginning of the test. The pin contact surface topography was analyzed by using the optical 3D profilometer and is shown in Fig. 11 at increasing number of cycles. Scratches are evident on the pin surface in all the images; a significant increase of them can be observed, proving the increase of abrasive type wear during cycling due to wear debris formation. The wear debris formation may be attributed to the oxidation of both the pin (see EDX spectra in Fig. 8) and metal sheet (due to the long permanence at high temperature) surfaces, which acts in synergic combination with the sliding wear action. These wear particles may behave in different ways [23,24]: the debris can be completely removed from the contacting surfaces, and therefore play no further role in influencing wear process. Alternatively, the debris may be retained as freelymoving particles between the contacting surfaces, where they may act as three-body abradents, causing abrasion damage to both the surfaces, or may become embedded in one surface, where they can act as two-body abradents, causing damage to the other
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2
2 mm
1
Spectrum1
Al
O
Si 1.0 2.0
Fe
Cr 3.0
4.0
5.0
6.0
7.0
8.0
9.0 10.0 11.0 12.0
Spectrum2
Fe
Al O Cr
Si
1.0 2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0 11.0 12.0
Fig. 9. Back-scattered SEM image and EDX analysis of the pin contact surface after 200 cycles.
surface. This phenomenon is shown well in Fig. 8, where oxide particles are detected embedded on the surface of the metal sheet. Finally, the debris may be retained as non-moving particles on one or both the surfaces, leading to the development of particle layers that can provide protection against further wear damage [25]. This last phenomenon was observed on the pin surface (see Figs. 9 and 10), where the Al–Si coating adhered on the contact surface.
From the SEM and 3D profilometer analysis, it is clear that both adhesive and abrasive wear take place during testing: the adhesion can be attributed to aluminium transfer from the metal sheet coating to the pin, as revealed by the SEM/EDX analysis; on the contrary, the abrasive wear of the pin, observed thanks to the profilometer analysis, can be probably ascribed to the formation of oxide particles acting as wear debris between the two contacting surfaces.
A. Ghiotti et al. / Wear 302 (2013) 1319–1326
400 cycles
1200 cycles
1325
2000 cycles
Fig. 10. Back-scattered SEM images of the pin contact surface at increasing number of cycles.
400 cycles
1200 cycles
2000 cycles
65 µm
-100 µm Fig. 11. Topography of the pin contact surface at increasing number of cycles.
However, further investigations are needed to understand these wear mechanisms better, in particular as regards the possible detachment of portions of the Al–Si coating that adhered to the surface of pin due to severe adhesive wear.
5. Conclusions A novel experimental approach to study wear mechanisms under hot stamping conditions was presented. The proposed test, based on a conventional pin-on-disk configuration, allows replicating the thermal and mechanical cycles to which the forming dies are subjected during the hot stamping. The assessment of the test was carried out by proving the reproducibility of the thermal cycle imposed to the pin and its closeness to the industrial one as well as by proving the reproducibility of the friction characteristics during testing. The worn pin surface analysis by SEM/EDX and 3D profilometer showed that both adhesive and abrasive wear mechanisms take place; the adhesion can be attributed to the aluminium transfer from the sheet protective AlSi coating, whereas the formation of oxide particles acting as wear debris can provoke the occurrence of abrasive wear. However, further investigations are needed to understand these wear phenomena better. Nonetheless, it is worth underlining that the presented experimental procedure allows controlling and varying all the process parameters that can affect the wear behaviour of hot stamping dies.
Acknowledgements The authors gratefully acknowledge the project GPHS ‘‘Green Press Hardening Steel Grades’’, founded by the EU Research Fund for Steel and Coal.
References [1] R. Neugebauer, T. Altan, M. Geiger, M. Kleiner, A. Sterzing, Sheet metal forming at elevated temperatures, CIRP Annals—Manufacturing Technology 55 (2) (2006) 799–816. [2] H. Karbasian, A.E. Tekkaya, Review in hot stamping, Journal of Materials Processing Technology 210 (2010) 2103–2118. [3] P.F. Bariani, S. Bruschi, A. Ghiotti, A. Turetta, Testing formability in the hot stamping of HSS, CIRP Annals—Manufacturing Technology 57 (1) (2008) 265–268. [4] Y. Chastel, Y. Dahan, E. Massoni, P. Duroux, J. Wilsius, P. Hein, Formability of quenchable steels in hot stamping, in: Proceedings of the 9th ICTP, 2008. [5] M. Merklein, J. Lechler, Investigation of the thermo-mechanical properties of hot stamping steels, Journal of Materials Processing Technology 177 (2006) 452–455. [6] M. Naderi, L. Durrenberger, A. Molinari, W. Bleck, Constituitve relationships for 22MnB5 boron steel deformed isothermally at high temperatures, Materials Science and Engineering A 478 (1–2) (2008) 130–139. [7] S. Serajzadeh, Modelling of temperature history and phase transformations during cooling of steel, Journal of Materials Science and Engineering 146 (2004) 311–317. [8] N. Xiao, M. Tong, Y. Lan, D. Li, Y. Li, Coupled simulation of the influence of austenite deformation on the subsequent isothermal austenite–ferrite transformation, Acta Materialia 54 (2006) 1265–1278. [9] A.E. Tekkaya, H. Karbasian, W. Homberg, M. Kleiner, Thermo-mechanical coupled simulation of hot stamping components for process design, Production Engineering 1 (2007) 85–89. [10] Z.W. Xing, J. Bao, Y.Y. Yang, Numerical simulation of hot stamping of quenchable boron steel, Materials Science and Engineering A 499 (2009) 28–31. [11] M. Geiger, M. Merklein, J. Lechler, Determination of tribological conditions within hot stamping, Production Engineering—Research and Development 2 (2008) 269–276. [12] A. Azushima, K. Uda, A. Yanagida, Friction behavior of aluminum-coated 22MnB5 in hot stamping under dry and lubricated conditions, Journal of Materials Processing Technology 212 (2012) 1014–1021. [13] A. Yanagida, T. Kurihara, A. Azushima, Development of tribo-simulator for hot stamping, Journal of Materials Processing Technology 210 (2010) 456–460. [14] X. Tian, Y. Zhang, J. Li, Investigation on Tribological behavior of advanced high strength steels: influence of hot stamping process parameters, Tribology Letters 45 (2012) 489–495. [15] M. Wieland, M. Merklein, Characterization of heat transfer coefficients of tool materials and tool coatings for hot stamping of boron–manganese steels, Key Engineering Materials 438 (2010).
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[16] H. Hoffmann, H. So, H. Steinbeiss, Design of hot stamping tools with cooling system, CIRP Annals—Manufacturing Technology 56 (1) (2007) 269–272. [17] J. Hardell, B. Prakash, High-temperature friction and wear behavior of different tool steels during against Al–Si coated high strenghth steel, Tribology International 41 (2008) 663–671. [18] J. Hardell, E. Kassfeldt, B. Prakash, Friction and wear behavior of high strength boron steel at elevated temperatures of up to 800 1C, Wear 264 (2008) 788–799. [19] L. Pelcastre, J. Hardell, N. Herrera, B. Prakash, Investigation into the occurrence of galling during hot forming of Al–Si-coated high-strength steel, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 225 (2010).
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Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
A novel approach to wear testing in hot stamping of high strength boron steel sheets A. Ghiotti, F. Sgarabotto, S. Bruschi n DII, University of Padua, Via Venezia 1, 35131 Padua, Italy
a r t i c l e i n f o
abstract
Article history: Received 1 September 2012 Received in revised form 20 December 2012 Accepted 23 December 2012 Available online 9 January 2013
Hot stamping of high strength steel sheets was developed in the automotive industry for the production of components characterized by a high strength-to-weight ratio and an increased resistance to impact. In order to avoid scaling and decarburization, the steel blanks are usually coated with an Al–Si coating that has proved a relevant influence also on their tribological behaviour during the forming stages. However, the knowledge of the influence that this coating may have on the dies wear mechanisms is still inadequate. The paper proposes a novel approach to wear testing, based on a pin-on-disk testing configuration, capable to reproduce in a laboratory environment the conditions arising at the interface between the dies and the blank, by reproducing the sliding velocities at the interface and the cyclic thermal and mechanical stresses on the die material. Investigations were carried out on a hot working tool steel sliding against high strength steel blanks coated with the Al–Si coating under dry reciprocating sliding conditions. Scanning electron microscopy and 3D profilometer analysis were utilized to evaluate the wear mechanisms. The presented results show that the proposed procedure can properly simulate the thermal and mechanical cycles to which the forming dies are subjected during the hot stamping process, allowing to control and vary a number of parameters characterizing the industrial process. The presence of both adhesive and abrasive wear mechanisms is highlighted and a possible explanation of their appearance is given. & 2013 Elsevier B.V. All rights reserved.
Keywords: Wear Friction Hot stamping Boron steel Al–Si coating
1. Introduction In the last few years, due to the increasing demand in reducing CO2 emissions and improving the safety of car passengers, the automotive industry has promoted the development of new categories of sheet forming processes, which exploit sheet heating to increase the characteristics of the formed components in terms of both obtainable geometrical features and mechanical properties. In this framework, the hot stamping of boron steel sheets represents the most promising process, since it allows producing parts of the car body-in-white with a high ratio between the strength and mass and significant crashworthiness resistance [1,2]. During the process, sheets, usually made of the quenchable boron steel 22MnB5 coated with an Al–Si layer to avoid oxidation, are heated above their austenitization temperature and simultaneously formed and quenched into the forming dies: the conduction of the process at elevated temperatures also allows higher formability limits, lower forming loads on the dies and reduced
n
Corresponding author. Tel.: þ39 0498276821. E-mail address: [email protected] (S. Bruschi).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.051
springback compared to room temperature sheet forming operations; at the same time, the die quenching gives the part a fully martensitic structure, which guarantees its outstanding mechanical properties. A literature survey shows that several aspects of the hot stamping process have been extensively investigated, such as steel behaviour and formability at high temperature [3–6], microstructural evolution [7,8], process numerical modelling [9,10], boundary conditions between the blank and the dies in terms of friction [11–14] and heat transfer coefficients [15,16]. On the contrary, the dies behaviour during the process in terms of wear resistance is still almost unexplored, since only a few papers are devoted to the study of wear during hot stamping. The reported studies focus on conventional wear testing configurations, which are suitable to achieve fundamental knowledge on wear evolution and mechanisms, but they fail in replicating the thermal and mechanical conditions to which the forming dies are subjected during the industrial process. In fact, they usually do not apply any thermal cycle to the material acting as the tool, carrying out the tests at constant temperature [17,18]. Otherwise, the wear phenomena are studied through costly and timeconsuming industrial trials [19,20], where the possibility to vary
A. Ghiotti et al. / Wear 302 (2013) 1319–1326
the process parameters and therefore to assess their influence on the die behaviour is limited. On this basis, the paper has a twofold objective: (i) to propose and assess a novel approach to wear testing, based on a conventional pin-on-disk configuration, but capable of reproducing in a laboratory environment the thermal and mechanical cycles to which the forming dies are subjected during hot stamping; and (ii) to investigate the wear evolution during testing and give a first insight of the wear mechanisms arising at increasing number of cycles. The paper is organized into two main parts: the first describes the testing equipment and procedure designed and setup to replicate the hot stamping conditions within the wear test; the second part shows the results obtained in terms of assessment of the thermal and mechanical characteristics reproducible in the wear test as well as the wear mechanisms arising during testing, which were studied by means of scanning electron microscopy and 3D profilometer analysis.
2. Application case The reference industrial process is the hot stamping of high strength boron steel sheets to produce b-pillar automotive components. Data about the duration of the different process phases, which were later on utilized for the wear test design and set-up, were recorded during industrial trials carried out on a hot stamping plant [21]. The process for industrial practice includes first the heating of the metal sheet inside an external furnace above the steel austenitization temperature, and its transfer by means of a robot arm to the forming press in 5–10 s. Then, the dies are closed and the simultaneous forming and quenching phase starts, which takes approximately 20 s. Finally, the dies are opened and the formed component is extracted in approximately 15 s. Therefore, on an average, every 35 s the forming dies are subjected to a new forming cycle, both thermal and mechanical. The forming process is carried out under dry conditions. The dies are usually substituted after 200,000 cycles, but after the production of 2000–3000 parts they are reground [19].
The metal sheets of 1.5 mm thickness are provided with an Al–Si layer of approximately 25 mm of thickness, which prevents the metal oxidation thanks to the formation of an Al–Si–Fe ternary alloy at elevated temperature characterized by a higher melting temperature compared to the one of the Al–Si layer [22]. The pin material hardness was also characterized as a function of the temperature by measuring it with an InstronTM hardness tester equipped with a heating chamber. Fig. 1 shows the evolution of the pin material hardness up to 650 1C; a significant decrease in the hardness is observed from 300 1C. 3.2. Testing equipment A universal UMTTM tribometer by BrukerTM equipped with a pin-on-disk configuration and with a heating furnace was utilized to carry out the wear tests at elevated temperature. The pin is connected to both a vertical and lateral linear motion system. Strain-gauge sensors perform simultaneous measurements of load and torque in two axes (FZ and FX), in order to calculate the friction coefficient during the test. The normal-load sensor provides feedback to the vertical motion controller, actively adjusting the specimen position to ensure a constant load during testing. The pin was machined with an axial hole of 2 mm diameter and 7 mm depth, where a K-type thermocouple (TC) was positioned with the aim of monitoring the pin temperature evolution during testing (see Fig. 2b and c). A locking system ensures the contact between the thermocouple and the pin for the whole duration of the test. An overall image of the pin and pin holder set-up is shown in Fig. 2a.
60 Hardness [HRC]
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55 50 45 40 35 30 0
3. Experimental This section presents the characteristics of the materials involved in the experimentation, the adopted testing equipment, and the proposed testing procedure aimed at reproducing the thermal and mechanical conditions of the forming dies.
200
400 600 Temperature [°C]
800
Fig. 1. Pin material hardness as a function of the temperature.
Ø12 Ø2
9
7
3.1. Materials The pin material is the hardened hot working tool steel AISI H11 and the metal sheet material is the boron steel 22MnB5. Their chemical compositions are reported in Table 1, together with the hardness values at room temperature and initial surface roughness Sa measured by means of the 3D optical profilometer SensoSCAN NeoxTM 3.2.3.
Heat exchanger wings
Thermocouple wire
R0.5
Ø4
TC Table 1 Pin and sheet material chemical compositions (in wt%), hardness at room temperature and initial surface roughness Sa. C
Mn
Cr
Si
B
Mo
V
Hardness (HRC)
Sa (mm)
0.22 0.38
1.40 0.40
0.30 5.00
0.35 1.10
0.005 –
– 1.30
– 0.40
20 71.5 51 71.5
0.5 70.1 1.54 70.1
Pin holder Pin Pin
22MnB5 AISI H11
Fig. 2. Pin and pin holder details: overall set-up (a) and details of pin (b) with the position of the thermocouple (c).
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The actual wear test starts with the pin entrance inside the heating chamber (entry step) and its location at the zero position, which is at a distance of 1.25 70.25 mm from the surface of hot metal sheet sample. Then the pin is brought into contact with the metal sheet, a normal force and a sliding speed are applied for 5 s (contact step). Afterwards, the pin is moved out from the heating chamber (exit step) and cooled down by the air nozzle (cooling step). No lubricant is used in order to replicate the tribological conditions of the industrial process. The above-described cycle is repeated 100 times; after that the pin is made to slide in a virgin portion of the metal sheet. Every 400 cycles the pin is dismounted, cleaned with ether in an ultrasonic cleaner and analyzed with the 3D optical profilometer and the SEM, in order to monitor the wear evolution. The duration of the test was set to 2000 cycles in accordance with the common number of cycles after which the industrial dies are reground. Table 2 reports the adopted testing parameters. It is worth noting that the presented procedure allows controlling and varying the sliding speed and length, the applied normal load, and the metal sheet temperature, thus making it possible to replicate the thermal and mechanical conditions characteristic of the hot stamping process.
4. Results and discussions This section presents the preliminary results in terms of assessment of the thermal and mechanical conditions reproducible in the wear test as well as the methods for investigating the pin wear evolution and mechanisms during testing. 4.1. Assessment of the thermal testing conditions Fig. 3. Detail of the disc (a), the air nozzle (b) and scheme of the pin-on-disk testing equipment (c).
The assessment of the thermal testing conditions was carried out by proving the reproducibility of the thermal cycle imposed on the pin during testing and its closeness to the one the forming
The metal sheet (Fig. 3a), cut in the form of a disc of 45 mm diameter, is fixed to a lower rotary drive equipped with a heating chamber that allows testing temperatures up to 1000 1C. A temperature sensor is located inside the chamber and connected with a computerized controller that uses the output of the temperature sensor to control the chamber temperature. In order to cool down the pin during the test and simulate the thermal cycle to which the forming dies are subjected during the process, an air nozzle (Fig. 3b) is used, which is placed outside the heating chamber and can be positioned, in conjunction with the air flow, to impart the desired cooling rate on the pin sample. A manometer is used to control the pressure of the sprayed air from 0 to 8 bar, in order to obtain different levels of cooling and cooling rates. An overall scheme of the test equipment is shown in Fig. 3c.
Table 2 Testing parameters. Sheet heating rate Sheet temperature Normal load (contact pressure) Sliding speed Sliding distance per cycle Pin cooling time Pin cooling rate Duration
0.43 1C/s 8007 1 1C 507 2 N (7 70.1 MPa) 15 mm/s 75 mm 15 s 3.6 1C/s 2000 cycles
Total cycle time = 36.8 s 12
3.8
Contact
Exit
6
15
6
12
3.3. Testing procedure
160
130 120
Contact
140
Entrance
150
Cooling
Temperature [°C]
170
Entrance
The developed testing procedure can be divided into two main phases: the first one comprises some preliminary steps carried out to set up the testing equipment and assure the repeatability of the testing conditions, while the second phase is the actual wear test. The preliminary steps include the pin cleaning in ether in an ultrasonic cleaner and then its drying in air, its connexion to the pin holder, ensuring the contact between the thermocouple and the bottom of the pin hole, the metal sheet fastening on the rotation table inside the heating chamber, the metal sheet heating to the target temperature, and the air nozzle pressure set up. During the heating phase, the pin is kept outside the heating chamber to avoid any heating.
30 Time [s]
40
50
110 100 0
10
20
Fig. 4. Pin temperature evolution during each cycle.
60
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Temperature [°C]
180 150 120 90 60 10 cycles
30
50 cycles
20 cycles
100 cycles
0 0
500
1000
1500
2000 Time [s]
2500
3000
3500
4000
Fig. 5. Pin temperature evolution at increasing number of cycles.
Mean COF
0.8 0.6 0.4 0.2 0 0
500
1000 N°of cycle
1500
2000
0 166
168
170 Time [s]
172
174
0.8 COF
0.6 0.4 0.2
Fig. 6. Mean coefficient of friction (a) at increasing number of cycles and example of acquisition of the coefficient of friction during a cycle (b).
Spectrum
Al dies are subjected to. Fig. 4 shows the pin temperature evolution during each cycle of the wear test as recorded by the embedded K-type thermocouple, with the indication of the testing steps and their duration. The total cycle time is 36.8 s, close to the one the forming dies are subjected to during the hot stamping process. The maximum temperature reached by the pin thermocouple is far below the temperature at which the pin material starts losing its hardness characteristics (see Fig. 1). It is worth noting that the maximum and minimum temperatures of the cycle are not reached when the pin moves out from the heating chamber and at the end of the cooling step, but after a short while: this is due to the pin thermal inertia and to the delay in the response of the thermocouple being placed at a distance of 2 mm from the pin sliding surface. Fig. 5 shows the pin temperature as a function of the number of cycles. It can be observed that 10 cycles are not enough to reach a temperature steady state, whereas, after 20 cycles, the regime condition is reached, assuring a reproducible and stable thermal cycle, with the pin temperature ranging between 120 1C and 170 1C. By changing either the cooling time or the pressure of the air sprayed by the nozzle, it is possible to modify the pin temperature range. 4.2. Assessment of the mechanical testing conditions The assessment of the mechanical testing conditions was carried out by verifying the reproducibility of the friction characteristics between the metal sheet and the pin during testing. Fig. 6a shows the mean friction coefficient at increasing number of cycles; every point of the curve represents the mean friction coefficient evaluated in a single cycle (example in Fig. 6b).
Fe
O
Si
Fe 0.8
Mn 1.5
2.2
2.9
3.6
4.3
5.0
5.7
6.4
7.1
8
Fig. 7. SEM images of the as-received metal sheet after heating up to 800 1C and its spectrum analysis.
The mean coefficient of friction, COF, is equal to 0.51, comparable to others values reported in various literature studies devoted to the evaluation of tribological performance of Al–Si-coated 22MnB5 steel under industrial hot stamping conditions (COF equal to 0.53 in [11], and to 0.55 in [12]). Furthermore, it was important to evaluate if the number of 100 cycles after which the pin was made to slide on a virgin portion of metal sheet (see Section 3.3) provoked any change of the characteristics of the Al–Si–Fe ternary alloy formed on the sheet surface at elevated temperature. To do that, a sheet portion on which the pin slid 100 times was analyzed by using a Quanta FEI 450 SEM equipped with an EDX detector. In Fig. 7 a SEM image of the as-received metal sheet after being heated to the testing temperature (800 1C) and its surface spectrum are shown.
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Spectrum 1
Al
Al O
Fe
0.8
Si
1.5
2.2
3.6
4.3
5.0
5.7
6.4
7.1
8
Spectrum 3
Spectrum 2
Al
2.9
Fe Fe
O Fe
Fe
Si
O
Mn 0.8
1.5
2.2
2.9
3.6
4.3
5.0
5.7
6.4
7.1
8
0.8
1.6
2.4
3.2
4.0
4.8
5.6
6.4
7.2
8
Fig. 8. SEM image of a sheet zone in the wear path and three EDX spectra of different areas of the wear path.
As expected, the EDX sensor detected a high percentage of aluminium and iron, besides the presence of oxygen. Fig. 8 shows a SEM image of a sheet zone belonging to the wear path together with the EDX spectra acquired in three different points of the path. Areas characterized by different percentages of aluminium and silicon can be seen in spectra 1 and 2: the lighter area (spectrum 1) indicates a lower amount of aluminium and a higher percentage of iron compared to the area from spectrum 2. This clearly indicates that in this area the layer of the Al–Si–Fe ternary alloy is thinner, with the steel substrate starting to appear. Therefore, 100 cycles can be considered suitable to still have the coating layer. It is worth noticing that in both spectra 1 and 2 the presence of oxygen was observed, meaning that the surface of the metal sheet was oxidized during the tests. Finally, the third spectrum shows the presence of an iron oxide particle, since neither aluminium nor silicon can be detected. 4.3. Analysis of the wear evolution and mechanisms Both the SEM equipped with the EDX detector and the 3D optical profilometer were used to identify the wear phenomena arising during the pin thermo-mechanical cycling and the wear evolution on the pin contact surface. Fig. 9 shows a back-scattered SEM image of the pin contact surface after 200 cycles: material transfer from the metal sheet to the pin can be clearly observed. The EDX spectra in fact highlight the presence of mainly aluminium coming from the metal sheet surface, with the pin surface darker areas containing a higher percentage. This proves the occurrence of adhesion type wear
since the beginning of the test, with aluminium material transferred from the sheet to the pin sample. The spectrums also show a significant percentage of oxygen, proving that an oxide layer developed during the test on the pin surface. Fig. 10 shows the back-scattered SEM images of the pin contact surface at different cycles: the material adhesion from the metal sheet is evident, resulting in darker areas on the pin surface. At increasing number of cycles, only a slight increase of the aluminium deposition on the pin surface can be detected, proving that the mechanism of adhesion wear is effective since the very beginning of the test. The pin contact surface topography was analyzed by using the optical 3D profilometer and is shown in Fig. 11 at increasing number of cycles. Scratches are evident on the pin surface in all the images; a significant increase of them can be observed, proving the increase of abrasive type wear during cycling due to wear debris formation. The wear debris formation may be attributed to the oxidation of both the pin (see EDX spectra in Fig. 8) and metal sheet (due to the long permanence at high temperature) surfaces, which acts in synergic combination with the sliding wear action. These wear particles may behave in different ways [23,24]: the debris can be completely removed from the contacting surfaces, and therefore play no further role in influencing wear process. Alternatively, the debris may be retained as freelymoving particles between the contacting surfaces, where they may act as three-body abradents, causing abrasion damage to both the surfaces, or may become embedded in one surface, where they can act as two-body abradents, causing damage to the other
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2
2 mm
1
Spectrum1
Al
O
Si 1.0 2.0
Fe
Cr 3.0
4.0
5.0
6.0
7.0
8.0
9.0 10.0 11.0 12.0
Spectrum2
Fe
Al O Cr
Si
1.0 2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0 11.0 12.0
Fig. 9. Back-scattered SEM image and EDX analysis of the pin contact surface after 200 cycles.
surface. This phenomenon is shown well in Fig. 8, where oxide particles are detected embedded on the surface of the metal sheet. Finally, the debris may be retained as non-moving particles on one or both the surfaces, leading to the development of particle layers that can provide protection against further wear damage [25]. This last phenomenon was observed on the pin surface (see Figs. 9 and 10), where the Al–Si coating adhered on the contact surface.
From the SEM and 3D profilometer analysis, it is clear that both adhesive and abrasive wear take place during testing: the adhesion can be attributed to aluminium transfer from the metal sheet coating to the pin, as revealed by the SEM/EDX analysis; on the contrary, the abrasive wear of the pin, observed thanks to the profilometer analysis, can be probably ascribed to the formation of oxide particles acting as wear debris between the two contacting surfaces.
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400 cycles
1200 cycles
1325
2000 cycles
Fig. 10. Back-scattered SEM images of the pin contact surface at increasing number of cycles.
400 cycles
1200 cycles
2000 cycles
65 µm
-100 µm Fig. 11. Topography of the pin contact surface at increasing number of cycles.
However, further investigations are needed to understand these wear mechanisms better, in particular as regards the possible detachment of portions of the Al–Si coating that adhered to the surface of pin due to severe adhesive wear.
5. Conclusions A novel experimental approach to study wear mechanisms under hot stamping conditions was presented. The proposed test, based on a conventional pin-on-disk configuration, allows replicating the thermal and mechanical cycles to which the forming dies are subjected during the hot stamping. The assessment of the test was carried out by proving the reproducibility of the thermal cycle imposed to the pin and its closeness to the industrial one as well as by proving the reproducibility of the friction characteristics during testing. The worn pin surface analysis by SEM/EDX and 3D profilometer showed that both adhesive and abrasive wear mechanisms take place; the adhesion can be attributed to the aluminium transfer from the sheet protective AlSi coating, whereas the formation of oxide particles acting as wear debris can provoke the occurrence of abrasive wear. However, further investigations are needed to understand these wear phenomena better. Nonetheless, it is worth underlining that the presented experimental procedure allows controlling and varying all the process parameters that can affect the wear behaviour of hot stamping dies.
Acknowledgements The authors gratefully acknowledge the project GPHS ‘‘Green Press Hardening Steel Grades’’, founded by the EU Research Fund for Steel and Coal.
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