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Use of multiple criteria to map the high-temperature scuffing behavior of Co-based superalloys
Wear 267 (2009) 374–379
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Use of multiple criteria to map the high-temperature scuffing behavior of Co-based superalloys P.J. Blau a,∗ , M. Yao b , J. Qu a , J. Wu c a b c
Oak Ridge National Laboratory, P.O. Box 2008 – M/S 6063, Oak Ridge, TN 37831-6063, USA Deloro Stellite Inc., Belleville, Ontario, Canada K8N 5C4 Deloro Stellite, 555 N New Ballas Rd, St. Louis, MO 63141, USA
a r t i c l e
i n f o
Article history: Received 29 August 2008 Received in revised form 13 November 2008 Accepted 14 November 2008 Keywords: Co alloys Stellite 6B Scuffing Tribaloy T400 Pleuco 33 Wear map
a b s t r a c t The goal of this work was to rank, as quantitatively as possible, the high-temperature scuffing characteristics of a series of Co-based materials, including coatings, for possible use in wear-critical components like those in diesel engine exhaust systems and emission-controls. A cylinder pivoting on its side against a flat tile was used as the testing geometry. All tests were performed at 600 ◦ C in air. The time-dependent torque response was recorded for each 60 min test to investigate trends in friction as surfaces roughened and debris deposits built up. Candidate materials included Stellite 6B, Pleuco 33 (a white cast iron), nitrided stainless steel Type 310, IDM 5399 (a high-silicon, molybdenum iron), and coatings of both Tribaloy T-400 and Stellite 3 on stainless steel. Measurements of the damaged areas on the scuffed tiles were used to calculate a specific normal load and plot it versus the roughness of the worn tiles to map experimental results in a way that indicated the severity of scuffing for each material combination. Data on the map were annotated to reflect visual observations of the extent of damage as well. Using this test method and the foregoing criteria, nitrided type 310 stainless steel against T-400 gave the best overall performance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing demand on the designers of car and truck engines to reduce fuel consumption and minimize emissions. In diesel engines, a turbocharger can help to achieve these goals. A turbocharger, which is powered by hot exhaust gas, forces a pressurized fuel–air mixture into the combustion chamber to generate more power and raise fuel-efficiency. The popularity of turbochargers is spreading to gasoline powered engines as well. There are a variety of turbocharger designs. Invariably, they all have vanes to regulate the pressure of exhaust gas. The vanes could be movable or fixed. In either case, metal-to-metal wear occurs. Some of the designs have wear-critical couples. Such wear occurs at very high temperatures. The operating temperature is 830 ◦ C for diesel engines and 1050 ◦ C for gasoline engines [1]. The exact temperature of the wear parts during operation is not accurately known, but it is expected to be not much lower than the operating temperature. Selecting proper materials to mitigate wear is important to ensure long service life. When a turbocharger is not used, exhaust gas recirculation (EGR) is typically used to direct 5–15% of the exhaust gas back to the engine in order to achieve a reduction in NOx emissions. In that
∗ Corresponding author. Tel.: +1 865 574 5377; fax: +1 865 574 4913. E-mail address: [email protected] (P.J. Blau). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.11.004
case, an EGR valve is exposed to the high temperature gas stream. Again, proper material selection is essential to enable the engine to operate reliably and for an extended period of time. There are essentially two basic types of steel used to make valves for turbochargers. One is martensitic steel and the other is austenitic steel. Martensitic steels typically have a high hardness at room temperature (35–55 Rockwell C) after tempering, which improves strength and wear resistance. But as the temperature goes up, martensitic steel loses hardness and strength. Above 538 ◦ C or so, low carbon alloy martensitic steel loses too much hardness and strength to hold up very well. For this reason, low carbon alloy martensitic steel is only used for intake valves, not exhaust valves. Intake valves are cooled by the incoming air/fuel mixture and typically run around 427–538 ◦ C, while exhaust valves are constantly blasted by hot exhaust gases and usually operate at 649–788 ◦ C or higher. The most popular materials for exhaust valves, however, are austenitic stainless steel alloys such as those known as 21-2N and 21-4N. As engines evolved, running hotter and producing more horsepower, new valve materials and facing alloys were developed. Cobalt-based Stellite® alloys1 are known to stand up to wear at elevated temperatures. However, when the temperature
1 Note: The alloy designations used to describe the test specimen materials; namely Stellite® , Tribaloy, T-400, T-400C, and T-800 are registered trademarks of Deloro Stellite Holdings Corporation.
P.J. Blau et al. / Wear 267 (2009) 374–379
375
Nomenclature AN F HRC N Ra RN SN T TN
normalized contact area on the flat specimen (tile) friction force (N) Rockwell hardness number, C-scale normal force (N) arithmetic average surface roughness (m) normalized surface roughness normalized stress frictional torque (N mm) normalized torque angular arc of oscillation of the cylinder upon the flat specimen (rad) kinetic friction coefficient
exceeds 600 ◦ C, their performance starts to deteriorate noticeably. This is mainly because the carbides start to decompose, since these alloys generally contain a substantial amount of carbon. Developed by DuPont in the 1960s, Tribaloy® alloys are known to resist wear at even higher temperatures, up to 1000 ◦ C. The carbon level in these alloys is low and hardening comes from intermetallic phases. These alloys are cobalt-based or nickel-based with alloying elements of Mo, Cr, and Si. They are used as castings, powder metallurgy parts or hardfacing consumables. Upon solidification, an intermetallic Laves phase forms [2], thereby making these alloys hard (HRC 48–58) and improving their properties [3]. Compared to other hard phases like carbides, Laves phase is relatively stable at high temperatures [4], thus making Tribaloy alloys useful for high temperature applications. In the Tribaloy family, there are T-400® , T-800® , T-900, T-401 and T-700 alloys. Among them, T-400 alloy has been used extensively for high temperature applications in the automotive industry. This alloy has the good combination of hardness and toughness for most high temperature applications [5,6]. More recently T-400C was developed and patented to improve the oxidation resistance of T-400 [7,8]. Either Stellite or Tribaloy alloys can be applied as coatings on low alloy or stainless steels using a slurry dipping and sintering process. Obviously, coatings are desirable if their wear resistance is adequate because they are more economical than using solid components of the same composition. However, coatings have different microstructures than their cast counterparts and their wear resistance has not been fully characterized. Complicating this work is the fact that the quantitative measurement of scuffing at high temperatures is difficult. The surface deformation is non-uniform, oxide growth can interfere with mass loss measurements, and softening of alloys at elevated temperatures can make the displacement of material on the surface as problematic as its removal in the form of wear debris.
Fig. 1. Schematic diagram of the high-temperature scuffing apparatus. The inset at the upper right shows a tile specimen and the arc-shaped areas of oscillating contact.
Therefore, the purposes of this work were two-fold. The primary goal was to gain a better understanding of the high-temperature scuffing characteristics of a series of Co-based coatings and compare them to other candidate materials for high-temperature, wearcritical, engine applications. The secondary goal of the work was to develop a procedure by which the wear and scuffing resistance of the alloys could be quantified in such a way that the performance of the materials could be effectively ranked or displayed graphically for comparison. 2. Materials and testing procedure Compositions of the materials used in this research and their Rockwell C scale hardness numbers are presented in Table 1. All were commercial alloys and represent bearing steels, bulk Co-based alloys, nitrided stainless steel, and Co-based coatings. High-temperature scuffing tests were performed on eight material couples using a custom-designed apparatus. The contact geometry, shown in Fig. 1, consists of a cylindrical pin (6.35 mm diameter × 25.4 mm long) oscillating on its side through a prescribed angle of arc against a flat tile (31.8 mm × 31.8 mm × 6.35 mm). The tile contained a center hole (6.35 mm diameter) to avoid fretting at the center of pin rotation. A schematic diagram of the apparatus is given in Fig. 1. A load cell attached to the drive mechanism was used to monitor the torque on the oscillating shaft at a data sampling rate of 100 s−1 . The torque was calibrated by pivoting the shaft end on a lubricated ball bearing and subtracting the measured forces arising from the mechanical linkage from those measured during the pinon-tile tests. In reporting the torque, the distance to the frictional contact from the center of rotation was taken as the average radius of contact on the tile specimen, accounting for the center hole.
Table 1 Materials used in scuffing experiments. Alloy
Nominal alloy composition (wt%)
Specimen form
HRCave
440C stainless steel Nitrided 310 stainless steel Stellite 6B Pleuco 33 IDM 5399 T-400C coating on 416 stainless steela T-400C coating on 440C stainless steela Stellite 3 coating on 416 stainless steela Stellite 3 coating on 440C stainless steela
17 Cr, 1 Mn, 1 Si, 0.75 Mo, 1.1 C, 0.04 max P, 0.03 max S, bal. Fe Base alloy: 25 Cr, 21 Ni, 2 Mn, 1.5 Si, 0.25 max C, 0.045 max P, 0.40 max S 30 Cr, 4.5 W, 3 max. Ni, 3 max. Fe, 1.1 C, 1 Mn, 1.5 Mo, 2 Si, balance Co 1–1.4 C, 1.8–2.1 Si, 33–35 Cr, 2–2.5 Mo, 0.02–0.08 Ti, bal. Fe 3–3.4 C, 4.4–4.9 Si, 0.5–0.7 Mo, bal. Fe 29 Mo, 8.5 Cr, 1.5 (max.) Ni, 1.5 (max.) Fe, 0.08 (max.) C, 2.6 Si, balance Co 29 Mo, 8.5 Cr, 1.5 (max.) Ni, 1.5 (max.) Fe, 0.08 (max.) C, 2.6 Si, balance Co. 31 Cr, 12.5 W, 3 Ni, 1 Mn, 1 Si, 3 Fe, 2.4 C, 1 B, 1 other, 44 Co 31 Cr, 12.5 W, 3 Ni, 1 Mn, 1 Si, 3 Fe, 2.4 C, 1 B, 1 other, 44 Co
Tile Tile Pin and tile Pin Pin Pin Tile Pin Tile
54 68 37 29 24 48 51 50 50
a
Coating hardness 15HRN was converted to HRC. The typical coating thickness was 0.3 mm.
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Table 2 Material pairs and tile roughness. Pin material
Tile material
Initial tile, Ra a (m)
Stellite 6B Pleuco 33 IDM5339 IDM5399 T-400C coating T-400C coating Stellite 3 coating Stellite 3 coating
Stellite 6B 440C stainless steel 440C stainless steel Nitrided 310 stainless steel Nitrided 310 stainless steel T-400C coating T-400C coating Stellite 3 coating
1.37, 1.01 2.09, 1.56 0.74, 1.40 1.24, 1.00 0.42, 0.61 0.37, 0.35 0.31, 0.35 0.34, 0.36
The different stress between pin and tile and the varying friction coefficient at different contact points between pin and tile results in different tile roughness. As described in the Results, a normalized post-test tile roughness, using the roughness of a Stellite 6B/Stellite 6B wear pair as a reference, was also used to rank the wear performance. As following results indicated, the smaller the normalized tile roughness the better was the wear performance. 3. Results Table 3 lists the post-test surface roughness of the tile specimen, the average torque at the conclusion of the test, and two derived parameters: normalized roughness (RN ) and normalized torque (TN ). To serve as a basis for comparison, the hardness and roughness of the Stellite 6B pin was selected. Since the indentation hardness number at a given load is inversely related to the contact area, one can define a normalized contact area relative to that for the reference pin material (HRC = 37). Expressed as a percentage:
a Two tests each combination. Ra is the arithmetic average roughness of the two tile specimens.
Table 2 shows the material pairs used in this study and the initial arithmetic average surface roughness of the tile specimen (Talyruf 10TM , 2.0 m tip radius). The following conditions were used in these tests: normal force (FN ) = 14.05 N; pin specimen oscillation rate = 1 Hz; oscillation angle = 30◦ ; test temperature = 600 ◦ C; and atmosphere = air. The test duration was 60 min, selected to provide sufficient time to initiate scuffing, and the torque was sampled by capturing 30 s of data at 5 min intervals. For the reasons mentioned earlier, special attention was paid to the quantification of scuffing damage. Weight loss was unacceptable since there was transfer, lateral material displacement, and oxide growth. Also, due to the presence of compressed debris deposits and transfer patches, neither wear depth nor wear track cross-sectional area could be used. After evaluating several possible measures for scuffing severity, a method that relied on applied load normalized by the measured length of apparent contact damage (in the radial direction on the tile), the frictional torque measurements, and visual assessments of surface damage was employed. It is recognized that the friction coefficient varies at different contact points between pin and tile and as such can vary with oscillation angle and the contact radius. The force that overcomes the friction force together with the oscillation movement results in the externally-measured torque. Considering that the frictional contact length along a given pin specimen is not necessarily equal to the total length of the pin, one can normalize the friction force by dividing the torque by the width of contact damage on the tile specimen, measured in a radial direction (i.e., perpendicular to the arc of oscillation). The normalized torque, relative to the values obtained for a self-mated Stellite 6B wear pair, can then be used to rank the wear performance of the other materials. It was assumed here that the smaller the normalized torque, the better the wear performance.
AN = 100 ×
37 HRCtile
(1)
and the normalized stress SN for each material, expressed as a percentage, is SN =
100 AN
(2)
To account for the initial roughness of each test tile, Ra , and the stress placed on the tile, the normalized roughness RN is RN = 100 ×
Ra SN
(3)
Normalized torque TN was computed from the final value of the torque after 60 min (Tf ) as follows: TN = 100 ×
Tf SN
(4)
Substituting from Eqs. (1) and (2), the normalized torque has the same units of the simple torque (N mm): TN = 370 ×
Tf HRCtile
(5)
Figs. 2 and 3 display the normalized torque from the tests and the normalized average roughness data, respectively. Material combinations tend to cluster into two categories: higher and lower values
Table 3 Roughness and torque data for high-temperature scuffing tests. Pin
Tile
SN
RN
Tf (N mm)
TN (N mm)
Stellite 6B
Stellite 6B
100 100
1.37 1.01
82.5 67.0
82.5 67.0
PL33
440C
43.2 43.2
4.83 3.61
121.6 120.6
281.2 278.9
IDM5399
440C
27.0 27.0
2.81 5.18
81.6 117.9
301.9 436.2
IDM5399
Nitrided 310
27.0 27.0
4.59 3.70
112.3 117.6
415.5 435.1
T-400C coating
Nitrided 310
129.7 129.7
0.32 0.47
49.7 68.1
38.3 52.5
T-400C coating
T-400C coating
129.7 129.7
0.29 0.27
165.1 154.9
127.3 119.4
Stellite 3 coating
T-400C coating
135.1 135.1
0.23 0.27
119.0 86.1
88.1 63.7
Stellite 3 coating
Stellite 3 coating
135.1 135.1
0.25 0.27
106.3 102.2
78.7 75.6
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Fig. 2. Normalized torque data for the test pairs.
Fig. 4. Stellite 3 pin (a) against a T-400C coated 440C stainless steel tile: (a) pin specimen; (b) flat specimen. Fig. 3. Normalized average roughness data for the same test pairs in Fig. 1.
for torque and roughness. With the exception of the Stellite 6B couple, pairs of materials also tended to rank similarly with respect to their magnitudes of torque and roughness. Despite, the slightly larger roughness of the Stellite 6B tile specimen, it was still observed to be among the least damaged of the test couples. 4. Discussion Any laboratory test is its own tribosystem and the relationship between its results and those from the field, another tribosystem, represents a best-effort compromise to simulate the most important variables and to achieve a reasonable ranking of material performance. Scuffing tests are particularly problematical because there might be a long period of mild wear in real vanes, and in turbochargers scuffing can be associated with inadequate or failed lubrication. Scuffing can roughen a surface with no net loss of material (a condition not strictly defined as ‘wear’). The current tests were designed to accelerate the response under scuffing conditions, so the mild wear regime was suppressed in favour of inducing a transition to more severe damage. Several quantities were measured or derived in order to portray the results of these experiments, but the numerical values alone reveal only a part of the picture. Each cylindrical pin and flat tile was
examined in a low-power optical microscope and photographed to document its surface damage. Table 4 summarizes visual observations of the contact surfaces of the scuffing pairs. Based on the scar appearance, they could be grouped into three categories. Category I was the best performance with only minor abrasion and even contact on the specimen surfaces, Category II displayed moderate damage, and Category III displayed the rough grooves and plastic deformation associated with heavy scuffing or incipient galling. Examples of Category I pin and flat specimen surfaces are shown in Figs. 4 and 5, and an example of Category III damage is shown in Fig. 6. In Figs. 4 and 5, note that the wear is relatively uniform across the contact region, but in Fig. 6, there are more localized score marks and within them, the surface is deeply grooved and deformed. The non-uniform contact is also reflected in the uneven scar on the mating surface of the pin in Fig. 6b. Roughness data, torque data, and visual observations were combined to produce what might be called a ‘scuffing severity map’ (see Fig. 7). The specific load is obtained by dividing normal force by the actual contact length, similar to the approach used to compute TN . The lower the specific load, the lower the tendency for scuffing, because a low value for specific load implies a larger area of uniform contact, whereas a high value for the specific load implies localized, non-uniform contact. Couples located to the lower left of the map performed best.
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Table 4 Contact surface observations. Category
Cylindrical pin
Flat tile
Observations
I
Stellite 3 coating Stellite 3 coating T-400 coating Stellite 6B T-400 coating
T-400 coating Stellite 3 coating T-400 coating Stellite 6B Nitrided 310 st. steel
Very smooth and uniform wear scars on both pin and tile, low torque and low wear damage Fine and uniform abrasive markings, low wear damage Fine and uniform abrasive markings, low wear damage Wear marks are smooth and uniform on pin and tile Even wear, full contact apparent, low wear on pin and tile
II
IDM 5399
Nitrided 310 st. steel
Wear damage greater than the self-mated Stellite 6B pairs
III
Pleuco 33 IDM 5399
440 C st. steel 440 C st. steel
Very uneven scuffing with deep metallic grooves and severe wear Uneven scuffing with metallic grooves and severe wear
Comparing Fig. 7 with the data in Table 2, the worst results happen to correspond to couples in which there was a large difference in the hardness between the pin and tile specimens. However, no single physical quantity was sufficient to determine the least scuffing prone of the material pairs. Therefore, a combination of metrics was required to rank the materials for scuffing resistance at 600 ◦ C. Scuffing behavior, whether it occurs at room temperature or at elevated temperature, is difficult to measure quantitatively because the surface damage is often non-uniform. Scuffing may initiate at one part of a surface and gradually spread to other parts. In the case
of high-temperature scuffing, the role of oxide scales and the development of third-body layers can complicate the process, leading to an indistinct and difficult-to-replicate time to scuffing initiation. From an engineering and materials selection point of view, the goal of obtaining an accurate and precise measure of the scuffing tendencies of certain materials for use in rotary bearings, face seals, fasteners, or oscillating engine components may not be fully achievable due to the inherent in-homogeneous nature of scuffing phenomena. Therefore, it may be necessary to use a combination of visual and measured parameters, as was done here, to enable
Fig. 5. Appearance of self-mated Stellite 6B test specimens: (a) pin specimen; (b) flat specimen. The asymmetry of wear on the flat specimen illustrates the stochastic nature of scuffing initiation. In this case, the normalized torque was computed based on the average contact width on both sides of the hole.
Fig. 6. Wear on a Pleuco 33 cylindrical pin and a 440C stainless steel flat tile specimen: (a) pin specimen; (b) flat specimen. Like the specimen in Fig. 5b, damage was not uniform and symmetrical with scoring at some locations, and lighter scuffing at others.
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mass loss or wear depth, several normalized quantities were used to present the data and rank material combinations. (b) Observations of damage were combined with measurements of torque and surface roughness to develop a ‘scuffing map’ on which to plot the performance of the test couples. (c) Based on the combination of measured torques and observational criteria, the best performing metallic pair was the Tribaloy T-400C coating mated with nitrided type 310 stainless steel. Pairs of Fe-based alloys displayed the least satisfactory hightemperature scuffing performance. Acknowledgements
Fig. 7. Scuffing severity map.
a merit ranking of candidate material pairs. Depending on the application, contact geometry, and other tribosystem variables, a different combination of parameters than the ones used here may be appropriate to select scuffing-resistant materials. In summary, the development of scuffing tests and the assignment of criteria to rank the merits of various material combinations therefore needs to be done on a case-by-case basis. 5. Conclusions An oscillating cylindrical pin-on-flat tile test method was used to compare and rank the scuffing tendencies of several high-temperature, corrosion-resistant metallic alloys, including sub-millimeter thick Co-based coatings at 600 ◦ C in air. The following summarize the results of this work: (a) Since the scuffing damage on metallic surfaces tends to be nonuniform and does not result in easily-measured changes like
The authors would like to thank reviewers at Oak Ridge National Laboratory and reviewers for the Wear of Materials Conference for their constructive suggestions. A portion of this research was supported by the U.S. Department of Energy, Office of Vehicle Technologies, High Temperature Materials Laboratory User Program. Oak Ridge National Laboratory is managed for the U.S. Department of Energy under contract DE-AC05-00OR22725 with UT-Battelle, LLC. References [1] K. Matsumoto, M. Tojo, Y. Jinnai, N. Hayashi, S. Ibaraki, Development of compact and high-performance turbocharger for 1050 ◦ C exhaust gas, Mitsubishi Heavy Industries, Ltd., Technical Review 45 (3) (2008) 1–5. [2] G.D. Smith, Cobalt alloys, United States Patent 3,180,012, April 27, 1965. [3] R. Liu, M.X. Yao, P.C. Patnaik, X. Wu, Effects of heat treatment on mechanical and tribological properties of cobalt-based Tribaloy alloys, J. Mater. Eng. Perform. 14 (5) (2005) 634–640. [4] A. Halstead, R.D. Rawlings, Structure and hardness of Co–Mo–Cr–Si wear resistant alloys (Tribaloys), Met. Sci. 18 (1984) 491. [5] H. Yamagata, The Science and Technology of Materials in Automotive Engines, CRC Press International LLC, 2005, p. 9, ISBN: 9780849325854. [6] S. Tanaka, M. Yoshida, T. Kokubun, T. Uesugi, T. Kodama, Development of 6M61CNG engine for medium-duty trucks, Mitsubishi Motors, Technical Review (15) (2003) 64–67. [7] J.B.C. Wu, M.X. Yao, Wear-resistant, corrosion-resistant cobalt-based alloys, United States Patent 6,852,176, February 8, 2005. [8] M.X. Yao, J.B.C. Wu, S. Yick, Y. Xie, R. Liu, High temperature wear and corrosion resistance of a Laves phase strengthened Co–Mo–Cr–Si alloy, Mater. Sci. Eng. A 435–436 (2006) 78–83.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Use of multiple criteria to map the high-temperature scuffing behavior of Co-based superalloys P.J. Blau a,∗ , M. Yao b , J. Qu a , J. Wu c a b c
Oak Ridge National Laboratory, P.O. Box 2008 – M/S 6063, Oak Ridge, TN 37831-6063, USA Deloro Stellite Inc., Belleville, Ontario, Canada K8N 5C4 Deloro Stellite, 555 N New Ballas Rd, St. Louis, MO 63141, USA
a r t i c l e
i n f o
Article history: Received 29 August 2008 Received in revised form 13 November 2008 Accepted 14 November 2008 Keywords: Co alloys Stellite 6B Scuffing Tribaloy T400 Pleuco 33 Wear map
a b s t r a c t The goal of this work was to rank, as quantitatively as possible, the high-temperature scuffing characteristics of a series of Co-based materials, including coatings, for possible use in wear-critical components like those in diesel engine exhaust systems and emission-controls. A cylinder pivoting on its side against a flat tile was used as the testing geometry. All tests were performed at 600 ◦ C in air. The time-dependent torque response was recorded for each 60 min test to investigate trends in friction as surfaces roughened and debris deposits built up. Candidate materials included Stellite 6B, Pleuco 33 (a white cast iron), nitrided stainless steel Type 310, IDM 5399 (a high-silicon, molybdenum iron), and coatings of both Tribaloy T-400 and Stellite 3 on stainless steel. Measurements of the damaged areas on the scuffed tiles were used to calculate a specific normal load and plot it versus the roughness of the worn tiles to map experimental results in a way that indicated the severity of scuffing for each material combination. Data on the map were annotated to reflect visual observations of the extent of damage as well. Using this test method and the foregoing criteria, nitrided type 310 stainless steel against T-400 gave the best overall performance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing demand on the designers of car and truck engines to reduce fuel consumption and minimize emissions. In diesel engines, a turbocharger can help to achieve these goals. A turbocharger, which is powered by hot exhaust gas, forces a pressurized fuel–air mixture into the combustion chamber to generate more power and raise fuel-efficiency. The popularity of turbochargers is spreading to gasoline powered engines as well. There are a variety of turbocharger designs. Invariably, they all have vanes to regulate the pressure of exhaust gas. The vanes could be movable or fixed. In either case, metal-to-metal wear occurs. Some of the designs have wear-critical couples. Such wear occurs at very high temperatures. The operating temperature is 830 ◦ C for diesel engines and 1050 ◦ C for gasoline engines [1]. The exact temperature of the wear parts during operation is not accurately known, but it is expected to be not much lower than the operating temperature. Selecting proper materials to mitigate wear is important to ensure long service life. When a turbocharger is not used, exhaust gas recirculation (EGR) is typically used to direct 5–15% of the exhaust gas back to the engine in order to achieve a reduction in NOx emissions. In that
∗ Corresponding author. Tel.: +1 865 574 5377; fax: +1 865 574 4913. E-mail address: [email protected] (P.J. Blau). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.11.004
case, an EGR valve is exposed to the high temperature gas stream. Again, proper material selection is essential to enable the engine to operate reliably and for an extended period of time. There are essentially two basic types of steel used to make valves for turbochargers. One is martensitic steel and the other is austenitic steel. Martensitic steels typically have a high hardness at room temperature (35–55 Rockwell C) after tempering, which improves strength and wear resistance. But as the temperature goes up, martensitic steel loses hardness and strength. Above 538 ◦ C or so, low carbon alloy martensitic steel loses too much hardness and strength to hold up very well. For this reason, low carbon alloy martensitic steel is only used for intake valves, not exhaust valves. Intake valves are cooled by the incoming air/fuel mixture and typically run around 427–538 ◦ C, while exhaust valves are constantly blasted by hot exhaust gases and usually operate at 649–788 ◦ C or higher. The most popular materials for exhaust valves, however, are austenitic stainless steel alloys such as those known as 21-2N and 21-4N. As engines evolved, running hotter and producing more horsepower, new valve materials and facing alloys were developed. Cobalt-based Stellite® alloys1 are known to stand up to wear at elevated temperatures. However, when the temperature
1 Note: The alloy designations used to describe the test specimen materials; namely Stellite® , Tribaloy, T-400, T-400C, and T-800 are registered trademarks of Deloro Stellite Holdings Corporation.
P.J. Blau et al. / Wear 267 (2009) 374–379
375
Nomenclature AN F HRC N Ra RN SN T TN
normalized contact area on the flat specimen (tile) friction force (N) Rockwell hardness number, C-scale normal force (N) arithmetic average surface roughness (m) normalized surface roughness normalized stress frictional torque (N mm) normalized torque angular arc of oscillation of the cylinder upon the flat specimen (rad) kinetic friction coefficient
exceeds 600 ◦ C, their performance starts to deteriorate noticeably. This is mainly because the carbides start to decompose, since these alloys generally contain a substantial amount of carbon. Developed by DuPont in the 1960s, Tribaloy® alloys are known to resist wear at even higher temperatures, up to 1000 ◦ C. The carbon level in these alloys is low and hardening comes from intermetallic phases. These alloys are cobalt-based or nickel-based with alloying elements of Mo, Cr, and Si. They are used as castings, powder metallurgy parts or hardfacing consumables. Upon solidification, an intermetallic Laves phase forms [2], thereby making these alloys hard (HRC 48–58) and improving their properties [3]. Compared to other hard phases like carbides, Laves phase is relatively stable at high temperatures [4], thus making Tribaloy alloys useful for high temperature applications. In the Tribaloy family, there are T-400® , T-800® , T-900, T-401 and T-700 alloys. Among them, T-400 alloy has been used extensively for high temperature applications in the automotive industry. This alloy has the good combination of hardness and toughness for most high temperature applications [5,6]. More recently T-400C was developed and patented to improve the oxidation resistance of T-400 [7,8]. Either Stellite or Tribaloy alloys can be applied as coatings on low alloy or stainless steels using a slurry dipping and sintering process. Obviously, coatings are desirable if their wear resistance is adequate because they are more economical than using solid components of the same composition. However, coatings have different microstructures than their cast counterparts and their wear resistance has not been fully characterized. Complicating this work is the fact that the quantitative measurement of scuffing at high temperatures is difficult. The surface deformation is non-uniform, oxide growth can interfere with mass loss measurements, and softening of alloys at elevated temperatures can make the displacement of material on the surface as problematic as its removal in the form of wear debris.
Fig. 1. Schematic diagram of the high-temperature scuffing apparatus. The inset at the upper right shows a tile specimen and the arc-shaped areas of oscillating contact.
Therefore, the purposes of this work were two-fold. The primary goal was to gain a better understanding of the high-temperature scuffing characteristics of a series of Co-based coatings and compare them to other candidate materials for high-temperature, wearcritical, engine applications. The secondary goal of the work was to develop a procedure by which the wear and scuffing resistance of the alloys could be quantified in such a way that the performance of the materials could be effectively ranked or displayed graphically for comparison. 2. Materials and testing procedure Compositions of the materials used in this research and their Rockwell C scale hardness numbers are presented in Table 1. All were commercial alloys and represent bearing steels, bulk Co-based alloys, nitrided stainless steel, and Co-based coatings. High-temperature scuffing tests were performed on eight material couples using a custom-designed apparatus. The contact geometry, shown in Fig. 1, consists of a cylindrical pin (6.35 mm diameter × 25.4 mm long) oscillating on its side through a prescribed angle of arc against a flat tile (31.8 mm × 31.8 mm × 6.35 mm). The tile contained a center hole (6.35 mm diameter) to avoid fretting at the center of pin rotation. A schematic diagram of the apparatus is given in Fig. 1. A load cell attached to the drive mechanism was used to monitor the torque on the oscillating shaft at a data sampling rate of 100 s−1 . The torque was calibrated by pivoting the shaft end on a lubricated ball bearing and subtracting the measured forces arising from the mechanical linkage from those measured during the pinon-tile tests. In reporting the torque, the distance to the frictional contact from the center of rotation was taken as the average radius of contact on the tile specimen, accounting for the center hole.
Table 1 Materials used in scuffing experiments. Alloy
Nominal alloy composition (wt%)
Specimen form
HRCave
440C stainless steel Nitrided 310 stainless steel Stellite 6B Pleuco 33 IDM 5399 T-400C coating on 416 stainless steela T-400C coating on 440C stainless steela Stellite 3 coating on 416 stainless steela Stellite 3 coating on 440C stainless steela
17 Cr, 1 Mn, 1 Si, 0.75 Mo, 1.1 C, 0.04 max P, 0.03 max S, bal. Fe Base alloy: 25 Cr, 21 Ni, 2 Mn, 1.5 Si, 0.25 max C, 0.045 max P, 0.40 max S 30 Cr, 4.5 W, 3 max. Ni, 3 max. Fe, 1.1 C, 1 Mn, 1.5 Mo, 2 Si, balance Co 1–1.4 C, 1.8–2.1 Si, 33–35 Cr, 2–2.5 Mo, 0.02–0.08 Ti, bal. Fe 3–3.4 C, 4.4–4.9 Si, 0.5–0.7 Mo, bal. Fe 29 Mo, 8.5 Cr, 1.5 (max.) Ni, 1.5 (max.) Fe, 0.08 (max.) C, 2.6 Si, balance Co 29 Mo, 8.5 Cr, 1.5 (max.) Ni, 1.5 (max.) Fe, 0.08 (max.) C, 2.6 Si, balance Co. 31 Cr, 12.5 W, 3 Ni, 1 Mn, 1 Si, 3 Fe, 2.4 C, 1 B, 1 other, 44 Co 31 Cr, 12.5 W, 3 Ni, 1 Mn, 1 Si, 3 Fe, 2.4 C, 1 B, 1 other, 44 Co
Tile Tile Pin and tile Pin Pin Pin Tile Pin Tile
54 68 37 29 24 48 51 50 50
a
Coating hardness 15HRN was converted to HRC. The typical coating thickness was 0.3 mm.
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Table 2 Material pairs and tile roughness. Pin material
Tile material
Initial tile, Ra a (m)
Stellite 6B Pleuco 33 IDM5339 IDM5399 T-400C coating T-400C coating Stellite 3 coating Stellite 3 coating
Stellite 6B 440C stainless steel 440C stainless steel Nitrided 310 stainless steel Nitrided 310 stainless steel T-400C coating T-400C coating Stellite 3 coating
1.37, 1.01 2.09, 1.56 0.74, 1.40 1.24, 1.00 0.42, 0.61 0.37, 0.35 0.31, 0.35 0.34, 0.36
The different stress between pin and tile and the varying friction coefficient at different contact points between pin and tile results in different tile roughness. As described in the Results, a normalized post-test tile roughness, using the roughness of a Stellite 6B/Stellite 6B wear pair as a reference, was also used to rank the wear performance. As following results indicated, the smaller the normalized tile roughness the better was the wear performance. 3. Results Table 3 lists the post-test surface roughness of the tile specimen, the average torque at the conclusion of the test, and two derived parameters: normalized roughness (RN ) and normalized torque (TN ). To serve as a basis for comparison, the hardness and roughness of the Stellite 6B pin was selected. Since the indentation hardness number at a given load is inversely related to the contact area, one can define a normalized contact area relative to that for the reference pin material (HRC = 37). Expressed as a percentage:
a Two tests each combination. Ra is the arithmetic average roughness of the two tile specimens.
Table 2 shows the material pairs used in this study and the initial arithmetic average surface roughness of the tile specimen (Talyruf 10TM , 2.0 m tip radius). The following conditions were used in these tests: normal force (FN ) = 14.05 N; pin specimen oscillation rate = 1 Hz; oscillation angle = 30◦ ; test temperature = 600 ◦ C; and atmosphere = air. The test duration was 60 min, selected to provide sufficient time to initiate scuffing, and the torque was sampled by capturing 30 s of data at 5 min intervals. For the reasons mentioned earlier, special attention was paid to the quantification of scuffing damage. Weight loss was unacceptable since there was transfer, lateral material displacement, and oxide growth. Also, due to the presence of compressed debris deposits and transfer patches, neither wear depth nor wear track cross-sectional area could be used. After evaluating several possible measures for scuffing severity, a method that relied on applied load normalized by the measured length of apparent contact damage (in the radial direction on the tile), the frictional torque measurements, and visual assessments of surface damage was employed. It is recognized that the friction coefficient varies at different contact points between pin and tile and as such can vary with oscillation angle and the contact radius. The force that overcomes the friction force together with the oscillation movement results in the externally-measured torque. Considering that the frictional contact length along a given pin specimen is not necessarily equal to the total length of the pin, one can normalize the friction force by dividing the torque by the width of contact damage on the tile specimen, measured in a radial direction (i.e., perpendicular to the arc of oscillation). The normalized torque, relative to the values obtained for a self-mated Stellite 6B wear pair, can then be used to rank the wear performance of the other materials. It was assumed here that the smaller the normalized torque, the better the wear performance.
AN = 100 ×
37 HRCtile
(1)
and the normalized stress SN for each material, expressed as a percentage, is SN =
100 AN
(2)
To account for the initial roughness of each test tile, Ra , and the stress placed on the tile, the normalized roughness RN is RN = 100 ×
Ra SN
(3)
Normalized torque TN was computed from the final value of the torque after 60 min (Tf ) as follows: TN = 100 ×
Tf SN
(4)
Substituting from Eqs. (1) and (2), the normalized torque has the same units of the simple torque (N mm): TN = 370 ×
Tf HRCtile
(5)
Figs. 2 and 3 display the normalized torque from the tests and the normalized average roughness data, respectively. Material combinations tend to cluster into two categories: higher and lower values
Table 3 Roughness and torque data for high-temperature scuffing tests. Pin
Tile
SN
RN
Tf (N mm)
TN (N mm)
Stellite 6B
Stellite 6B
100 100
1.37 1.01
82.5 67.0
82.5 67.0
PL33
440C
43.2 43.2
4.83 3.61
121.6 120.6
281.2 278.9
IDM5399
440C
27.0 27.0
2.81 5.18
81.6 117.9
301.9 436.2
IDM5399
Nitrided 310
27.0 27.0
4.59 3.70
112.3 117.6
415.5 435.1
T-400C coating
Nitrided 310
129.7 129.7
0.32 0.47
49.7 68.1
38.3 52.5
T-400C coating
T-400C coating
129.7 129.7
0.29 0.27
165.1 154.9
127.3 119.4
Stellite 3 coating
T-400C coating
135.1 135.1
0.23 0.27
119.0 86.1
88.1 63.7
Stellite 3 coating
Stellite 3 coating
135.1 135.1
0.25 0.27
106.3 102.2
78.7 75.6
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Fig. 2. Normalized torque data for the test pairs.
Fig. 4. Stellite 3 pin (a) against a T-400C coated 440C stainless steel tile: (a) pin specimen; (b) flat specimen. Fig. 3. Normalized average roughness data for the same test pairs in Fig. 1.
for torque and roughness. With the exception of the Stellite 6B couple, pairs of materials also tended to rank similarly with respect to their magnitudes of torque and roughness. Despite, the slightly larger roughness of the Stellite 6B tile specimen, it was still observed to be among the least damaged of the test couples. 4. Discussion Any laboratory test is its own tribosystem and the relationship between its results and those from the field, another tribosystem, represents a best-effort compromise to simulate the most important variables and to achieve a reasonable ranking of material performance. Scuffing tests are particularly problematical because there might be a long period of mild wear in real vanes, and in turbochargers scuffing can be associated with inadequate or failed lubrication. Scuffing can roughen a surface with no net loss of material (a condition not strictly defined as ‘wear’). The current tests were designed to accelerate the response under scuffing conditions, so the mild wear regime was suppressed in favour of inducing a transition to more severe damage. Several quantities were measured or derived in order to portray the results of these experiments, but the numerical values alone reveal only a part of the picture. Each cylindrical pin and flat tile was
examined in a low-power optical microscope and photographed to document its surface damage. Table 4 summarizes visual observations of the contact surfaces of the scuffing pairs. Based on the scar appearance, they could be grouped into three categories. Category I was the best performance with only minor abrasion and even contact on the specimen surfaces, Category II displayed moderate damage, and Category III displayed the rough grooves and plastic deformation associated with heavy scuffing or incipient galling. Examples of Category I pin and flat specimen surfaces are shown in Figs. 4 and 5, and an example of Category III damage is shown in Fig. 6. In Figs. 4 and 5, note that the wear is relatively uniform across the contact region, but in Fig. 6, there are more localized score marks and within them, the surface is deeply grooved and deformed. The non-uniform contact is also reflected in the uneven scar on the mating surface of the pin in Fig. 6b. Roughness data, torque data, and visual observations were combined to produce what might be called a ‘scuffing severity map’ (see Fig. 7). The specific load is obtained by dividing normal force by the actual contact length, similar to the approach used to compute TN . The lower the specific load, the lower the tendency for scuffing, because a low value for specific load implies a larger area of uniform contact, whereas a high value for the specific load implies localized, non-uniform contact. Couples located to the lower left of the map performed best.
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Table 4 Contact surface observations. Category
Cylindrical pin
Flat tile
Observations
I
Stellite 3 coating Stellite 3 coating T-400 coating Stellite 6B T-400 coating
T-400 coating Stellite 3 coating T-400 coating Stellite 6B Nitrided 310 st. steel
Very smooth and uniform wear scars on both pin and tile, low torque and low wear damage Fine and uniform abrasive markings, low wear damage Fine and uniform abrasive markings, low wear damage Wear marks are smooth and uniform on pin and tile Even wear, full contact apparent, low wear on pin and tile
II
IDM 5399
Nitrided 310 st. steel
Wear damage greater than the self-mated Stellite 6B pairs
III
Pleuco 33 IDM 5399
440 C st. steel 440 C st. steel
Very uneven scuffing with deep metallic grooves and severe wear Uneven scuffing with metallic grooves and severe wear
Comparing Fig. 7 with the data in Table 2, the worst results happen to correspond to couples in which there was a large difference in the hardness between the pin and tile specimens. However, no single physical quantity was sufficient to determine the least scuffing prone of the material pairs. Therefore, a combination of metrics was required to rank the materials for scuffing resistance at 600 ◦ C. Scuffing behavior, whether it occurs at room temperature or at elevated temperature, is difficult to measure quantitatively because the surface damage is often non-uniform. Scuffing may initiate at one part of a surface and gradually spread to other parts. In the case
of high-temperature scuffing, the role of oxide scales and the development of third-body layers can complicate the process, leading to an indistinct and difficult-to-replicate time to scuffing initiation. From an engineering and materials selection point of view, the goal of obtaining an accurate and precise measure of the scuffing tendencies of certain materials for use in rotary bearings, face seals, fasteners, or oscillating engine components may not be fully achievable due to the inherent in-homogeneous nature of scuffing phenomena. Therefore, it may be necessary to use a combination of visual and measured parameters, as was done here, to enable
Fig. 5. Appearance of self-mated Stellite 6B test specimens: (a) pin specimen; (b) flat specimen. The asymmetry of wear on the flat specimen illustrates the stochastic nature of scuffing initiation. In this case, the normalized torque was computed based on the average contact width on both sides of the hole.
Fig. 6. Wear on a Pleuco 33 cylindrical pin and a 440C stainless steel flat tile specimen: (a) pin specimen; (b) flat specimen. Like the specimen in Fig. 5b, damage was not uniform and symmetrical with scoring at some locations, and lighter scuffing at others.
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mass loss or wear depth, several normalized quantities were used to present the data and rank material combinations. (b) Observations of damage were combined with measurements of torque and surface roughness to develop a ‘scuffing map’ on which to plot the performance of the test couples. (c) Based on the combination of measured torques and observational criteria, the best performing metallic pair was the Tribaloy T-400C coating mated with nitrided type 310 stainless steel. Pairs of Fe-based alloys displayed the least satisfactory hightemperature scuffing performance. Acknowledgements
Fig. 7. Scuffing severity map.
a merit ranking of candidate material pairs. Depending on the application, contact geometry, and other tribosystem variables, a different combination of parameters than the ones used here may be appropriate to select scuffing-resistant materials. In summary, the development of scuffing tests and the assignment of criteria to rank the merits of various material combinations therefore needs to be done on a case-by-case basis. 5. Conclusions An oscillating cylindrical pin-on-flat tile test method was used to compare and rank the scuffing tendencies of several high-temperature, corrosion-resistant metallic alloys, including sub-millimeter thick Co-based coatings at 600 ◦ C in air. The following summarize the results of this work: (a) Since the scuffing damage on metallic surfaces tends to be nonuniform and does not result in easily-measured changes like
The authors would like to thank reviewers at Oak Ridge National Laboratory and reviewers for the Wear of Materials Conference for their constructive suggestions. A portion of this research was supported by the U.S. Department of Energy, Office of Vehicle Technologies, High Temperature Materials Laboratory User Program. Oak Ridge National Laboratory is managed for the U.S. Department of Energy under contract DE-AC05-00OR22725 with UT-Battelle, LLC. References [1] K. Matsumoto, M. Tojo, Y. Jinnai, N. Hayashi, S. Ibaraki, Development of compact and high-performance turbocharger for 1050 ◦ C exhaust gas, Mitsubishi Heavy Industries, Ltd., Technical Review 45 (3) (2008) 1–5. [2] G.D. Smith, Cobalt alloys, United States Patent 3,180,012, April 27, 1965. [3] R. Liu, M.X. Yao, P.C. Patnaik, X. Wu, Effects of heat treatment on mechanical and tribological properties of cobalt-based Tribaloy alloys, J. Mater. Eng. Perform. 14 (5) (2005) 634–640. [4] A. Halstead, R.D. Rawlings, Structure and hardness of Co–Mo–Cr–Si wear resistant alloys (Tribaloys), Met. Sci. 18 (1984) 491. [5] H. Yamagata, The Science and Technology of Materials in Automotive Engines, CRC Press International LLC, 2005, p. 9, ISBN: 9780849325854. [6] S. Tanaka, M. Yoshida, T. Kokubun, T. Uesugi, T. Kodama, Development of 6M61CNG engine for medium-duty trucks, Mitsubishi Motors, Technical Review (15) (2003) 64–67. [7] J.B.C. Wu, M.X. Yao, Wear-resistant, corrosion-resistant cobalt-based alloys, United States Patent 6,852,176, February 8, 2005. [8] M.X. Yao, J.B.C. Wu, S. Yick, Y. Xie, R. Liu, High temperature wear and corrosion resistance of a Laves phase strengthened Co–Mo–Cr–Si alloy, Mater. Sci. Eng. A 435–436 (2006) 78–83.