High temperature sliding wear behaviour of Inconel 617 and Stellite 6 alloys

High temperature sliding wear behaviour of Inconel 617 and Stellite 6 alloys

Wear 269 (2010) 664–671 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear High temperature sliding wear...

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Wear 269 (2010) 664–671

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

High temperature sliding wear behaviour of Inconel 617 and Stellite 6 alloys Yucel Birol ∗ Materials Institute, Marmara Research Center, 41470 TUBITAK, Kocaeli, Turkey

a r t i c l e

i n f o

Article history: Received 11 March 2010 Received in revised form 7 July 2010 Accepted 7 July 2010 Available online 15 July 2010 Keywords: Sliding wear Steel High temperature Wear testing

a b s t r a c t The high temperature wear performance of Inconel 617 and Stellite 6 alloys was investigated and compared with that of the X32CrMoV33 hot work tool steel. The wear performance of the latter at 750 ◦ C is judged to be very poor due basically to its inferior oxidation resistance. Extensive oxidation co-occurring with wear at 750 ◦ C leads to substantial material loss basically due to the lack of an adhesive oxide scale, sufficiently ductile to sustain the wear action without extensive spalling. The wear resistance of the Inconel 617 and Stellite 6 alloys at 750 ◦ C is relatively superior. The adhesive oxides growing slowly on Inconel 617 and Stellite 6 alloys sustain the wear action without spalling and are claimed to be responsible for the superior wear resistance of these alloys at 750 ◦ C. © 2010 Elsevier B.V. All rights reserved.

1. Introduction High temperature wear is one of the life-limiting factors when metallic surfaces are in repeated contact [1–3]. High forming temperatures impact the wear behaviour of tools through loss of mechanical strength and enhanced oxidation [4]. The significant role of the latter in high temperature sliding wear was first identified by Fink [5]. It is well known that oxidation leads to material degradation and consequently, reduces the material resistance to wear. However, a surface oxide may reduce the oxidation rate and help to decrease the wear loss if it is dense and strong [6,7]. The role of oxide scale in the wear of metals was discussed extensively both for ambient and high temperature wear [8–16] while the mechanisms of oxidation wear were reviewed by Quinn [17,18]. Some new approaches on the interpretation of oxidation wear mechanisms have also been proposed [19]. High temperature wear is identified to be a potential failure mechanism for thixoforging tools [20,21]. While thixoforging is a very attractive processing route for the manufacture of steel parts for drive units and chassis components, it is very demanding on tool materials with high process temperatures involved (>1300 ◦ C) [22]. The conventional hot work tool steels were shown to rapidly deteriorate under such severe conditions [23–27]. With a dispersion of hard carbide particles in a cobalt-rich solid solution matrix, cobalt-base alloys are exceptionally good for applications requiring resistance to oxidation and wear [28–31]. Ni-base alloys are

∗ Tel.: +90 262 6773084; fax: +90 262 6412309. E-mail address: [email protected] 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.07.005

also attractive high temperature materials owing to an excellent oxidation resistance, creep strength and phase stability at high temperatures [32]. Co- and Ni-based high temperature alloys were tested recently for their potential to withstand the steel thixoforming environment [33–36]. Their thermal fatigue performance is encouraging [33–37]. It is thus of great technological interest to explore their wear resistance at high temperatures. While the ambient temperature wear performance has been investigated in detail, published information on the wear performance of these alloys at high temperatures is scarce. The present work was undertaken to investigate the high temperature sliding wear resistance of Stellite 6 and Inconel 617 alloys and rate their performance against that of the conventional hot work tool steel employed in hot forging of steel components. 2. Experimental A CETR Universal Material Tester-2 model ball-on-disc type tribometer (Fig. 1) was used to investigate the high temperature wear properties of Inconel 617 and Stellite 6 alloys and X32CrMoV33 hot work tool steel (Table 1). Wear tests were carried out at 750 ◦ C with a sliding speed of 0.025 m/s, under 5 N load for 60 min. The test temperature was selected with a consideration of the maximum temperature achieved at the surface of the die cavity during steel thixoforming experiments [33]. Since the tool is abraded by very small ␣-Fe particles that make up the solid fraction of the semi-solid feedstock, the ball diameter and the applied load were selected so as to produce a scratching case. A 0.001 m diameter alumina ball ran over disk samples over a circular path having a diameter of 0.03 m. The disc surfaces were ground with a 1000 mesh grit sandpaper

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Table 1 Chemical composition of the X32CrMoV33 hot work tool steel and Ni- and Co-based high temperature alloys used in the present work. Alloy

C

Si

Mn

Cr

X32CrMoV33 Inconel 617 Stellite 6

0.281 0.080 1.089

0.190 0.945 1.099

0.200 0.513 1.154

3.005 21.88 28.272

Mo 2.788 8.177 0.004

Ni

Al

Co

Cu

Nb

Ti

V

W

Fe

0.221 53.861 2.802

0.025 0.167 0.094

<0.010 10.872 58.241

0.1651 0.304 –

0.0015 0.010 0.033

<0.001 0.211 –

0.413 – 0.009

0.020 – 4.512

92.63 2.850 2.660

Fig. 1. Experimental set-up for high temperature ball-on-disc sliding wear test.

and were ultrasonically cleaned in acetone and dried before each test. Disc samples were allowed to warm up to the test temperature before the start of dry sliding. Wear tracks were investigated with a VeecoWyko NT1100 3D model optical profilometer. Wear quantification was done by measuring the volumetric loss of the worn area. The wear behaviour was characterized by stereo, optical and a JEOL 6335F model field emission gun scanning electron microscope (FEG-SEM) fitted with an Oxford Instruments INCA model energy dispersive X-ray analyzer (EDS). XRD analysis with

Fig. 3. Wear volume loss of the X32CrMoV33, Inconel 617 and Stellite 6 disc samples submitted to high temperature ball-on-disc sliding wear test.

Cu K␣ radiation and step size of 0.02◦ was also used to identify the oxides formed on the worn surfaces. The hardness of the samples were measured in Vickers units with a load of 1 kg (HV1) before and after the wear test.

Fig. 2. Two- and three-dimensional profilometer images and two-dimensional surface profiles of the X32CrMoV33, Inconel 617 and Stellite 6 disc samples submitted to high temperature ball-on-disc sliding wear test.

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Fig. 4. Friction coefficient curves of the X32CrMoV33, Inconel 617 and Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

3. Results and discussion Two- and three-dimensional profilometer images and twodimensional surface profiles of the tested surfaces are illustrated in Fig. 2. The widest and the deepest wear track, and thus the high-

Fig. 5. Optical micrographs of (a) X32CrMoV33, (b) Inconel 617 and (c) Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

est volume loss occurred in the hot work tool steel. It is clear from Fig. 2 that the surface of the hot work tool steel disc sample has deteriorated not only inside but also outside the wear track, due to the extensive oxidation suffered by this material at the test temperature. The width and the depth of the wear tracks are relatively smaller in the Inconel 617 alloy and the smallest in the Stellite 6 alloy. These are consistent with the wear volume loss measurements which clearly identify the hot work tool steel to be the least and the Stellite 6 alloy the most resistant to sliding wear at 750 ◦ C (Fig. 3). The friction coefficients measured during the sliding wear tests are shown in Fig. 4. The friction coefficient of the X32CrMoV33 hot

Fig. 6. Optical micrographs showing transverse section of the wear track of (a) X32CrMoV33, (b) Inconel 617 and (c) Stellite 6 disc samples.

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work tool steel is as low as 0.2 at the start of the test and increases with time to approximately 0.4. The thick oxide layer formed on the surface of the tool steel at 750 ◦ C is believed to have served as a lubricant leading to a low friction coefficient initially. Low friction coefficients are linked with poor adherence and thick oxide layers [38,39] which help to enlarge the contact surface thereby decreasing the strain and thus the friction coefficient [40]. Small initial friction coefficient values may also be accounted for by the sudden loss of strength upon thermal exposure. The decohesion of

oxide scales, generation and accumulation of debris in the contact zone are responsible for the relatively larger fluctuations and for an ever-increasing friction coefficient. It is inferred from these features of the friction coefficient curve that the oxide layer on the tool steel disc sample is not stable. The friction coefficient curves of the Inconel 617 and Stellite 6 alloys are markedly different. That of the former is stabilized at

Fig. 7. Scanning electron micrographs of (a) X32CrMoV33, (b) Inconel 617 and (c) Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

Fig. 8. Scanning electron micrographs of (a) X32CrMoV33, (b) Inconel 617 and (c) Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

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Fig. 9. Scanning electron micrograph of the glazed layer in Inconel 617 disc sample submitted to ball-on-disc sliding wear test at 750 ◦ C.

approximately 0.24 and remains more or less constant with sliding time after an initial running-in period of about 500 s. The friction coefficient of Stellite 6 alloy shows a similar trend but runs at a higher value, at approximately 0.48. It is fair to conclude that the friction and wear conditions are quite stable in the Inconel 617 and Stellite 6 alloys owing to a stable oxide layer. The optical micrographs of the wear tracks are shown in Fig. 5. Interestingly, the microstructural features are readily identified on Inconel 617 and Stellite 6 disc samples without the benefit of chemical etching. This is typical of a well known practice in metallography [41] and evidences a thin oxide film which helps to delineate the microstructure under cross polarizer. This effect is not offered by the hot work tool steel disc sample simply due to a much thicker oxide all over. Further evidence for the extent of oxidation in the three alloys is available in the transverse sections of the wear tracks in the respective disc samples (Fig. 6). A very thick oxide scale is evident in the hot work tool steel sample while oxide films on the Inconel 617 and Stellite 6 alloys are apparently

Fig. 10. Element distribution profiles (a, b, c) and oxygen distribution profiles (d, e, f) across the wear tracks of (a, d) X32CrMoV33, (b, e) Inconel 617 and (c, f) Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

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too thin to be resolved with an optical microscope. The former has apparently failed to sustain the wear loading and has fractured to produce oxide debris in the wear track. Abrasive wear with grooving in the sliding direction, a very thick oxide layer and an appreciable quantity of debris accumulated at the edges of the track were the basic wear features for the hot work tool steel (Figs. 7 and 8). The oxides along the edges of the wear track were inferred from their colour to be hematite, in contrast to the dark-coloured magnetite covering the disc surface. The oxide debris was apparently carried to the edge of the track by the alumina ball where it has oxidised again. Magnetite reacts with oxygen to produce hematite. Oxidation, fresh surface generation via frac-

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ture and removal of the surface oxides inside the wear track and reoxidation of the fresh surface are claimed to be responsible for the substantial wear loss suffered by the hot work tool steel. The removed oxide itself might have acted as an abrasive agent whilst still within the wear interface producing an abrasive element in the wear of X32CrMoV33. Fig. 7a suggests that this is a likely mechanism when the oxide debris does not readily sinter to form a glaze and act as a third body abrasive [11]. The features of the worn surfaces of the Inconel 617 and Stellite 6 alloys are markedly different (Figs. 7 and 8). The oxides on the Inconel 617 and Stellite 6 samples are very thin. The oxide debris, although much less in quantity, was somehow retained inside the

Fig. 11. XRD spectra obtained from the tested surfaces of (a) X32CrMoV33 and (b) Stellite 6 disc samples submitted to ball-on-disc sliding wear test at 750 ◦ C.

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wear track and was compacted into a glazed surface [2,42,43]. The high hardness of the alumina ball and its capacity to form large groves so as to retain the oxide debris inside the wear track might have been critical in glazed layer formation. While the glazed layer is continuous in the Inconel 617 alloy and marks the boundary of the wear track all around the disc sample, it is revealed as discontinuous patches inside the wear track in the Stellite 6 alloy. The glazed surfaces in the contact zone have been reported to be responsible for the relatively lower friction coefficients at high temperatures as they increase the carrying surface [44]. This could be a plausible account of the low friction coefficient in the Inconel 617 alloy where the glazed layer in the wear track is uninterrupted (Fig. 9). It is inferred from the increased signal intensities across the wear tracks of the Inconel 617 and Stellite 6 disc samples that the oxygen concentration is greater inside the wear tracks than it is outside where oxidation has occurred statically (Fig. 10). Generation of fresh surface and defects due to abrasion via sliding wear and subsequent reoxidation may be responsible for the relatively higher oxygen levels inside the wear tracks. Oxygen signals become even stronger when crossing the glazed layers. Such a signal profile is not evident in the case of the hot work tool steel which is believed to be heavily oxidised both inside and outside the wear track. It is fair to conclude from the foregoing that the hightemperature wear performance of the three alloys tested in the present work is closely linked with their oxidation behaviour at 750 ◦ C. The tribological behaviour is strongly affected by the nature, the thickness, the adherence, and the morphology of the oxide scales [45,46]. The thick surface oxide layer on the tool steel sample is shown by XRD analysis to consist of Fe3 O4 and Fe2 O3 (Fig. 11a). The poor adherence and limited ductility of these oxides promote the failure of the oxide scale impairing the resistance to wear at elevated temperatures [47]. Lack of oxide debris sinterability, which appears to be adequate in the case of Inconel 617 and Stellite 6 alloys, as inferred from Fig. 8c, might have also contributed to the poor wear resistance of the tool steel sample [11]. The adhesive and highly plastic Cr2 O3 film, identified to be the predominant oxide on the surface of both Inconel 617 and Stellite 6 samples (Fig. 11b), on the other hand, has sustained the abrasion and is claimed to be responsible for the improved wear resistance of these alloys at 750 ◦ C as suggested in [47,48]. The reduced oxidation rate in these two alloys suppresses the synergy between oxidation and wear, thus improving the resistance to wear at 750 ◦ C. High-temperature alloys rely on Cr to form protective scales and require a minimum of 20 wt% Cr to develop a continuous Cr2 O3 film to enjoy protection [49]. With a Cr content only as much as 3 wt% and with hardly any Si and Al, the present hot work tool steel evidently lacks a continuous protective oxide and cannot take advantage of such protection. The strain-induced phase transformation from face-centred-cubic to hexagonal-close-packed structure and alignment of the basal plane to the direction of sliding, could also be responsible for the reduced wear of the Stellite 6 alloy [50,51]. Wear resistance of the X32CrMoV33 tool steel is impaired at high temperatures also via loss of mechanical strength. X32CrMoV33 tool steel responded to thermal exposure at 750 ◦ C with a sharp hardness drop (Fig. 12). This is not surprising since most hot work tool steels are known to soften starting around 600 ◦ C [52]. The substantial softening in the X32CrMoV33 hot work tool steel is believed to have been critical in the wear volume loss it has suffered. Inconel 617 and Stellite 6 alloys, on the other hand, retain their hardness at 750 ◦ C and are thus much more wear resistant owing to a higher resistance to abrasion. The wear volume loss and hot hardness are inversely proportional suggesting that the wear resistance of the three alloys tested in the present work is closely linked with their hardness at this temperature.

Fig. 12. Hardness measurements of X32CrMoV33, Inconel 617 and Stellite 6 disc samples before and after ball-on-disc sliding wear test at 750 ◦ C.

4. Conclusions The sliding wear performance of the X32CrMoV33 hot work tool steel is degraded at 750 ◦ C due basically to its inferior oxidation resistance. Extensive oxidation co-occurring with abrasion at 750 ◦ C leads to substantial material loss basically due to the lack of an adhesive oxide scale, sufficiently ductile to sustain the abrasive action without extensive cracking or spalling. Fe3 O4 fails to survive the abrasion conditions and is readily detached from the surface. The wear resistance of the Inconel 617 and Stellite 6 alloys, on the other hand, is much better at 750 ◦ C. The adhesive and the relatively more plastic Cr2 O3 on Inconel 617 and Stellite 6 alloys sustains the sliding wear action without spalling and is claimed to be responsible for the improved wear resistance of these alloys at 750 ◦ C. Acknowledgements D. I˙ sler performed the ball-on-disc wear tests. Prof. M. Urgen is thanked for the provision of the wear test facilities. F. Alageyik and O. Cakır are thanked for their help in the experiments and C. Berk for his help in SEM-EDS investigations. This work was funded by TUBITAK. References [1] I.A. Inman, S. Datta, H.L. Du, J.S. Burnell-Gray, Q. Luo, Microscopy of glazed layers formed during high temperature sliding wear at 750 ◦ C, Wear 254 (2003) 461–467. [2] A. Pauschitz, M. Roy, F. Franek, Mechanisms of sliding wear of metals and alloys at elevated temperatures, Tribol. Int. 41 (2008) 584–602. [3] X. Jiang, W. Liu, S.Y. Dong, B.S. Xu, Surf. Coat. Technol. 194 (2005) 10. [4] Y. Birol, D. I˙ sler, Response to thermal cycling of CAPVD (Al,Cr)N-coated hot work tool steel, Surf. Coat. Technol. doi:10.1016/j.surfcoat.2010.06.038. [5] M. Fink, Wear oxidation—a new component of wear, Trans. Am. Soc. Steel 18 (1930) 1026–1034. [6] J.-N. Aoh, J.-C. Chen, On the wear characteristics of cobalt-based hardfacing layer after thermal fatigue and oxidation, Wear 250 (2001) 611–620. [7] F.H. Stott, The role of oxidation in the wear of metals, Tribol. Int. 31 (1998) 61–71. [8] T.F.J. Quinn, J.L. Sullivan, D.M. Rowson, Origins and development of oxidational wear at low ambient temperatures, Wear 94 (1984) 175–191. [9] J.P. Tu, X.H. Jie, Z.Y. Mao, M. Matsumara, The effect of temperature on the unlubricated sliding wear of 5 CrNiMo steel against 40 MnB steel in the range 400–600 ◦ C, Tribol. Int. 31 (1998) 347–353. [10] I. Radu, D.Y. Li, Investigation of the role of oxide scale on Stellite 21 modified with yttrium in resisting wear at elevated temperatures, Wear 259 (2005) 453–458. [11] I.A. Inman, S.R. Rose, P.K. Datta, Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6, Tribol. Int. 39 (2006) 1361–1375. [12] M. Roy, A. Pauschitz, J. Wernisch, F. Franek, Effect of mating surface on the high temperature wear of 253 MA alloy, Mater. Corros. 55 (2004) 259–273.

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