Engine valve and seat insert wear study with a simulator

Transcription

1 Vol. 44 Supp. SCIENCE IN CHINA (Series A) August 2001 Engine valve and seat insert wear study with a simulator Y. S. Wang & S. Narasimhan Eaton Corp., Engine Components Operations B Drive South, Marshall, MI Received July 5, 2001 Abstract The demands on higher performance and the increasing use of alternative fuels challenge engine valves now with greater wear problems than before. A seat wear simulator was built to evaluate the compatibility and wear of valve and seat insert. The rig test results have been successfully correlated with engine test results. In this study, intake valves made from Sil 1 material were treated with salt bath nitride processes and tested against six different insert materials. Wear resistance of these combinations was ranked and compared to the Sil 1 valve without nitriding. The test results demonstrate that nitriding improved valve seat wear resistance. In the total valve seat recession ranking, the combination of nitrided Sil 1 valve against T 400 insert exhibited the least total recession among the nineteen combinations of valve and insert tested. The results indicate that the valve seat wear mechanisms are a complex combination of adhesion and shear strain. The nitrides in the compound layer of nitrided valves gave strong atomic bonding, higher hardness, compressive residual stresses, and possible low friction, thus resulted in the superior wear performance. Keywords: engine valve, nitride, seat insert, valve seat, wear mechanism, wear simulation. Engine valves control air/gas flow and combustion, consequently, any wear on valve and seat insert can affect engine performance and may eventually cause catastrophic valve and engine failure. and insert manufacturers are continuously working with engine manufacturers to improve valvetrain quality and prolong valvetrain life. Many changes in valve and insert materials, processes, and designs in the past have greatly improved engine durability and performance. The valve materials have been consolidated and valve manufacturers use fewer grades of alloys making valves now than before [1]. Typical valve alloys in North America include martensitic steels Sil 1 and SAE 1547 for intake valves, austenitic alloys 21-2N, 21-4N and 23-8N for exhaust, nickel based alloys Inconel 751 for the extreme high temperature exhaust valves, and Stellite 6, T 400 and Eatonite 6 for seat hardfacing. Seat insert materials, however, can vary significantly among insert and engine manufactures and engine applications. Insert materials include cast alloys and powder metallurgical (PM) alloys [2]. The industry is working towards reducing the number of valve and insert materials. To achieve that, a great numbers of tests need to be carried out by valve, insert and engine manufacturers. Due to the cost, time and great number of variables associated with the engine test, the effectiveness of a seat wear simulator becomes critical in screening the right valve and insert combinations for new engine applications as well as for the current engines having seat recession problems. The seat wear simulation can significantly shorten the cycles of valve seat hardfacing, coating, and insert material development. Scores of seat wear simulators are

2 236 SCIENCE IN CHINA (Series A) Vol. 44 now being used in the industry for the above mentioned purposes and claimed different degrees of success [3 ü6]. Historically, when valve alloys lack adequate wear and/or corrosion resistance to withstand the operating environment at the seat face, a hardfacing alloy may be welded to the seat face. The combination of T 400 facing against T 400 insert has demonstrated superior wear resistance in harsh heavy duty intake applications. More recently, in searching for the replacement of high cost cobalt based hardfacing alloys, salt bath nitriding (SBN) has shown to be an excellent alternative surface treatment to improve wear properties, fatigue strength, fretting resistance, and corrosion resistance [7 ü10]. In addition, the SBN process also provides low distortion and high tempering resistance associated with the high hardness property at service temperatures below the nitriding temperature. The important combination of increased wear resistance and fatigue strength results in increased engine valve applications in the automotive industry. 1 Experimental procedures materials used in this study were Sil 1 and SBN Sil 1 (using Sursulf and Melonite processes respectively). Insert materials were Sil XB, PL 7, Stellite 3, PL 33, Eatonite, and T 400. Chemical compositions, microstructure, and hardness of valves and inserts are listed in Table 1. Table 1 Chemical compositions, trademark, microstructure and hardness of the tested engine intake valve and insert materials Chemical composition (weight %) Material Hardness Microstructure C Mn Si Cr Ni Fe Other Sil Bal. 40B5HRc Martensite. Sil XB Bal. 42B2HRc carbide + martensite. PL Bal. Mo: B2HRc graphite+martensite+carbide Stellite W:12.5, Co: bal. 54B2HRc carbide + solid solution PL Bal. Mo: B3HRc carbide + martensite Eatonite W:15, Co:10 42B2HRc carbide + solid solution T Mo:29, Co: bal. 53B2HRc Laves + solid solution Trademark: PL = Plueco, Stellite = Delco Stellite, Eatonite = Eaton. Sursulf process was carried out in a molten salt bath which consisted of alkaline cyanates and carbonates and was stabilized by the addition of lithium compounds. The bath was therefore free from cyanides when initially used (<0.8%), and also specifically contained very small additions of sulfur compounds (normally on the order of a few ppm by weight). The cyanide content of the salt was as low as 0.2% CN. A treatment time of 90 minutes in a Sursulf bath at 565k produced at the surface of treated parts a compound zone rich in nitrogen and containing sulfur. Melonite process did not contain sulfur. The uniqueness of the Melonite is that it may be used in combination with an oxidizing quenching salt. In the absence of sulphur the cyanide content cannot be kept below than 2.5%. The Melonite process started by preheating the valves to 343k, then melonized at 580k for 20 min, followed by quench in the KQ 500 bath at 400k, a water quench and a water rinse. The end result was similar to that of Sursulf process, a compound zone rich in nitrogen plus a deep nitrogen diffusion zone.

3 Supp. ENGINE VALVE & SEAT INSERT WEAR STUDY WITH A SIMULATOR 237 The seat wear simulator consisted of the hydraulic system, the electronic control system and the mechanical equipment, is schematically shown fig. 1(a). A natural gas flame was used to simulate combustion heating. The hydraulic actuator was used to apply the load to the valve head to simulate engine combustion pressure. The loading frequency was 10 Hz, and the valve displacement was set at 1.27 mm. The valve and insert geometries and dimensions are shown in fig. 1. The simulator was designed to have a lateral offset of 0.76 mm, which produced a crescent wear scar on the valve and insert seat. seat wear tests were conducted at a temperature of 510B30k, and load of 17640B900 N for 24 h (864 cycles). A profilometer was used to measure valve seat wear at the maximum wear position due to the maximum contact stress introduced by the offset LVDT 45º φ 9.75 Frame Guide Exhaust Channel Insert Retainer Plate Heat Chamber & Burner Assembly Return Spring Cooling Channels (water) Insert Push Rod Load Cell Hydraulic Actuator (piston) φ 53 47º 0 φ 54.2 φ Fig. 1. (a) (a) Schematic of the simulator, valve and insert dimensions. 2 Results and discussion Sil 1 and nitrided Sil 1 valves were tested against six different insert materials. T 400 hardfaced valve against T 400 insert was also tested for comparison with a total of mm wear including mm valve wear. Typical worn surface morphologies of valve seats at the maximum wear region are shown in three dimension in fig. 2. Fig. 2(a) shows valve seat wear profile of the Sil 1 valve against T 400 insert, Fig. 2 against PL 7 insert respectively. Fig. 3 shows the wear resistance ranking in terms of the recession (i.e., valve plus insert wear). Fig. 3(a) shows the recession ranking order of the Sil 1 valve against the Sil XB, Stellite 3, PL 33, T 400, PL 7 and Eatonite inserts. Fig. 3 shows the valve seat recession ranking of Sursulf Sil 1 valve in the order of against the T 400, PL 33, Sil XB, Eatonite, Stellite 3 and PL 7 inserts. The Melonite Sil 1 valve had the similar valve seat recession ranking, fig. 3(c). The lowest

4 238 SCIENCE IN CHINA (Series A) Vol. 44 seat recession for the Melonite Sil 1 valve was against the T 400, followed by the Sil XB, PL 33, Stellite 3 and Eatonite inserts. It had the worst recession when against the PL 7 inserts. #80, Sil 1/T 400 (a) (d) #121, M-Sil 1/PL 7 Fig. 2. Two representative 3-D valve seat wear scar profiles of Sil 1 valve at the maximum wear scar area against insert: (a) T 400, PL Insert Insert (a) Insert (c) Wear, mm Sil XB Stellite 3 PL 33 T 400 PL 7 Eatonite Insert Materials Wear, mm T 400 Sil XB PL 33 Stellite 3Eatonite PL 7 Insert Materials Wear, mm T 400 PL 33 Sil XB EatoniteStellite 3 PL 7 Insert Materials Fig. 3. Overall seat wear (valve and insert) resistance ranking of (a) Sil 1, Melonite Sil 1 and (c) Sursulf Sil 1 against different inserts. The best overall seat wear resistance combination among all combinations tested was the Sursulf Sil 1 against the T 400 insert, followed by Melonite Sil 1 valve against T 400 insert. Both are better than the combination of T 400 hardfaced valve against T 400 insert (0.320 mm) used in the premium heavy duty diesel engine applications. The valve seat wear simulation was an accelerated wear test and tries to cover the progression of series of wear mechanisms encountered in an actual engine from break-in through long range operation. The actual field engines may be subjected to severe cycles, changes in operating conditions and environments that may affect valve seat/insert wear. Therefore in certain engines, the wear results may vary. Nevertheless, the simulator appears to be able to provide a list of materials to choose from for further actual engine testing and to give the satisfactory direction in selecting the compatible valve and insert materials. Shear strain type wear occurs when strain at asperity contacts exceeds the critical limits of the ductile materials by the repeated tangential stress and the surface layer breaks away as wear debris. Fig. 2(a) shows that valve seat materials were pushed-up towards the major dimension of at OD sides of wear scars. These push-ups at OD side of wear scar indicate shear strain or radial flow played a significant role of valve seat wear due to high shear stress at the seat surfaces. The increased

5 Supp. ENGINE VALVE & SEAT INSERT WEAR STUDY WITH A SIMULATOR 239 hardness in the compound layer and the sulfides which had lubricating function formed in the compound layer reduced friction coefficient, thus lessened shear stresses on the interfaces of valve seat and insert seat, eventually diminished the shear strain type wear [21ü23]. The general approach in reducing shear strain type wear would be to minimize the contact stress or strain and to use the high yield strength materials. Any paring materials with lower coefficient of friction could decrease contact stress/strain, and thus reduce the shear strain related wear. Oxide films, acting as lubricants or parting agents, can be beneficial in reducing shear strain type wear. Adhesive wear occurs due to the repeated microwelding and subsequent breakage of the microwelding joints. Some material combinations under high stress and/or poor lubrication are prone to microwelding and breakage thus can lead to severe adhesive wear. Fig. 2 exhibit material accumulation at the valve seat surface inside the wear scar instead of wearing, which indicates that adhesion or material transferring from the seat insert to the valve seat occurred. Wear primarily occurred on the insert seats when the Sil 1, Melonite and Sursulf Sil 1 valves were run against the PL 7 inserts. The combination of Sil 1 and T 400 is an example. The affinity and the activation force between two atoms are believed to have the controlling effect on the adhesive wear. Fig. 4 show typical microstructure, SEM micrographs and EDX spectra of nitrided Sil 1 valve T 400 inserts. (a) 30 µm 30 µm (c) (d) CrCo O Si Mo Cr Fe Co Fig. 4. (a) & Sursulf Sil 1 and T 400 microstructure sectioned at seat wear area. (c) & (d) SEM and EDX spectra of Sursulf Sil 1 valve and T 400 insert at wear scar area.

6 240 SCIENCE IN CHINA (Series A) Vol. 44 Salt bath nitriding (SBN) has significantly improved Sil 1 valve seat wear resistance, and overall seat recession when run against T 400 insert. SBN is a thermo-chemical diffusion process producing a compound layer of nitrides including zeta iron nitride (Fe 2 N), epsilon iron nitride (Fe 3 N) and gamma prime iron nitrides (Fe 4 N). The nitrides introduce compressive residual stress on the surface and cause an increased surface hardness by the diffusion of atomic nitrogen into the surfaces. Adjacent to the compound zone, a much lower concentration of diffused nitrogen is present in solid solution with iron. This region is termed the diffusion zone. The percentage and distribution of nitrides are depending on the process conditions and matrix material compositions. Fig. 5 shows the residual stress distribution from surface to subsurface and the major phases in the nitrided layer by using X-ray diffraction analysis of the nitride layer of Sursulf Sil 1 valve. The results indicate that substantial compressive residual stresses existed on the nitrided surface, up to 1570 MPa. Some major and minor phases produced from Sursulf on the Sil 1 surface are Magnetite (Fe 3 O 4 ), Zeta iron nitride (Fe 2 N)Epsilonironnitride(Fe 3 N), Gamma-Prime iron nitride (Fe 4 N) and Chrome nitride (CrN). Fig. 6 shows the hardness distribution of Sursulf Sil 1 from surface to subsurface. The surface hardness reached as high as 1250 HK 100. This hardness distribution correlates with the residual stress distribution in fig. 5. In addition to higher hardness, the nitrides, oxides and sulphides in the compound layers provide the inherent lubricious surface which reduces the coefficient of friction under either lubricated or non-lubricated conditions, and result in the superior wear performance Residual Stress (MPa) Fe 2-3 N&CrN Fe 4 N&CrN Hardness, (HK 100 ) Fe 3 O 4 &Fe 2 N Distance from Surface (µm) Distance from Surface (µm) Fig. 5. seat. X-ray diffraction analysis on nitride Sil 1 valve Fig. 6. Hardness distribution of nitride Sil 1 valve. 3 Conclusions Engine valve and seat insert wear simulation results show that the combination of salt bath nitrided Sil 1 valve against T 400 insert had the least total seat recession among all combinations

7 Supp. ENGINE VALVE & SEAT INSERT WEAR STUDY WITH A SIMULATOR 241 tested. Sursulf process appeared slightly better than Melonite process. The test results also indicate that valve seat and insert wear mechanisms were a complex combination of adhesion and shear strain. Adhesive wear was confirmed with the material transfer, while shear strain wear was verified with shear strain or radial flow and abrasion. The superior wear performance of Sursulf treated valves was attributed to the oxides and nitrides which gave the compressive residual stresses, hard and homogeneous compound layer on the surface and prevented the surfaces from the direct metal to metal contact. In addition to the nitrides and oxides, the lubricious sulfides in the compound layer could have reduced the coefficient of friction on the seat surface, thus reduced adhesive and shear strain type wear. Acknowledgements The authors would like to thank J. M. Larson and S. Schaefer at Eaton Corp., Engine Components Operations for their comments and discussions. References 1. Engine Poppet Information Report, SAE Surface Vehicle Information Report, SAE J775, (1993). 2. Seat Insert Information Report, SAE Surface Vehicle Information Report, SAE J1692, (1993). 3. Dooley, D., Trudeau, T., Bancroft, D., Materials and design aspects of modern valve seat inserts, Proceedings of the International Symposium on train System Design and Materials, ed. H. A. Bolton and J. M. Larson., 1997, Malatesta, M. J., Barber, G. C., Larson, J. M. et al., Development of a laboratory bench test to simulate seat wear of engine poppet valves, Tribology Transactions, 1993, 36(4): M. Onoda, N. Kuroishi and N. Motooka, Sintered Seat Insert for High Performance Engine, SAE Ootani, T., Yahata, N., Fujiki, A. et al., Impact wear characteristics of engine valve and valve seat insert materials at high temperature, Wear, 188 (1995) p Lewis, R., Dwyer-Joyce, R., Josey, G., Design and development of a bench test rig for investigating diesel engine inlet valve and insert wear, The Institution of Engineers Australia Austrib 98-Tribology at Work (1998) J. R. Easterday, Basic of Salt Bath Nitriding, Proceedings of Salt Bath Nitriding Seminar, KOLENE, 1985, p3. 9. Astley, P., Liquid nitriding: development and present applications, Proceedings of the International Conference on Heat Treatment, 1973, Mueller, J., Review of salt bath nitriding, Proceedings of SBN Seminar, Kolene, 1985, Radford, B. L., Nitriding in a cyanate based salt bath to improve resistance to scuffing wear and fatigue, Industrial Heating, June 1979, 46(6): 18.