1、13FTM14 AGMA Technical Paper Metallurgical Investigation of “Tiger Stripes” on a Carburized High Speed Pinion By M. Li, Lufkin Industries, P. Terry, P. Terry and Associates and R. Eckert, Northwest Laboratories, Inc. 2 13FTM14 Metallurgical Investigation of “Tiger Stripes” on a Carburized High Speed
2、 Pinion March Li, Lufkin Industries, Phil Terry, P. Terry and Associates and R. Eckert, Northwest Laboratories, Inc. The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstr
3、act “Tiger stripes” on a high speed pinion made of a carburized SAE 9310 steel were investigated. The stripes were on lines of action on the load side of the teeth coinciding with different angular positions of the gear mesh. Scanning Electron Microscopy (SEM) of the affected areas showed fused meta
4、l particles, with a diameter of 1-3 microns, and gas pockets. The morphology of the damage was typical of electric discharge damage shown in ANSI/AGMA 1010-E95. This indicates that the stripes were in fact electric discharge damage. Microhardness surveys on a metallurgical transverse section of a to
5、oth showed a hardness loss due to the discharge, with load side surface hardness even lower than 58 HRC. The cause of the “tiger stripes” and potential damage to the gear tooth were analyzed. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virgini
6、a 22314 September 2013 ISBN: 978-1-61481-071-1 3 13FTM14 Metallurgical Investigation of “Tiger Stripes” on a Carburized High Speed Pinion March Li, Lufkin Industries, Phil Terry, P. Terry and Associates and R. Eckert, Northwest Laboratories, Inc. Introduction There are lots of gear failure modes dep
7、ending on the material and related strengthening/hardening processing, working condition (power, speed and load) and environment (temperature, lubrication, corrosion, etc.). It is important to identify the failure mode in order to take necessary measures to mitigate it or to prevent it from occurrin
8、g. Among the various failure modes, electric discharge is a common one. It is caused by electric arc discharge across the oil film between mating gear teeth. This discharge may produce temperature high enough to locally melt the gear tooth surface 1. Electric discharge also causes bearing failures.
9、It was reported 2 that electric discharge pits initiated spalling on a bearing, which created vibration and overheating, leading to fatigue failure of the bearing. The electric current typically originates from electric motors especially variable frequency drives (VFD), sources of rapidly switching
10、electric currents such as electric clutches, or accumulation of static charge and subsequent discharge. Accordingly, it can be prevented by providing adequate electrical insulation or grounding. To the unaided eye, a surface damaged by electric discharge appears as an arc burn similar to a spot weld
11、. The density of the spots on the affected surface increases with the increase of the electric current intensity. On a microscopic level, small hemispherical craters can be observed. The edges of the crater are smooth and they may be surrounded by burned or fused metal in the form of rounded particl
12、es that were once molten. ANSI/AGMA 1010-E95 includes some macro- and micrographs showing the morphology of this failure mode. However, it does not mention any other surface appearance or property change. This paper introduces a specific appearance of electric discharge “Tiger Stripes”, on a high sp
13、eed pinion made of carburized SAE 9310 steel. Morphology characterization was performed by means of scanning electron microscopy (SEM). The hardness profile across the carburized case depth was measured with a microhardness tester to reveal the damage due to electric discharge. Application, chemistr
14、y, and material tensile properties The application is a speed increaser gearbox with a 4.735:1 ratio driven by an 1800 rpm VFD electric motor and driving a centrifugal compressor. The high speed pinion has 34 teeth and a normal diametral pitch of 4. A sample of material was cut from the pinion shaft
15、 and analyzed using a mass spectrometer. The results of the analysis are shown in Table 1 along with ranges specified for SAE 9310 steel. It shows that the carbon content is a little high, but all other elements are within the specification. Two tensile test specimens were prepared from the pinion s
16、haft and the measured mechanical properties are shown in Table 2. The manufacturing records show that the surface hardness of the pinion teeth is 59-60 HRC with 10% retained austenite and dispersed carbides. Table 1. Chemistry of the sample, wt.% Chemistry C Si Mn Cr Mo Ni S P Sample 0.14 0.26 0.66
17、1.32 0.12 3.28 0.011 0.007 SAE 9310 0.07 0.13 0.15 0.35 0.40 0.70 1.00 1.45 0.08 0.15 2.95 3.55 0.040 max 0.030 max 4 13FTM14 Table 2. Mechanical properties of the samples Ultimate tensile strength, psi Yield strength, psi Elongation, % Area reduction, % Sample 1 148,300 107,500 17.7 58.1 Sample 2 1
18、48,700 110,700 16.8 58.3 Visual examination Figure 1 shows the “tiger stripes” on the pinion. These stripes appeared to be along lines of contact of one (load) side of the pinion teeth and distributed at different rotational positions of the pinion. Also, it was noted that the “tiger stripes” occurr
19、ed on the pinion only - the mating gear didnt show any stripes. Figure 1. “Tiger stripes” on the pinion. 5 13FTM14 Microstructure investigation One pinion tooth was cut for morphological characterization by scanning electron microscopy (SEM). The observation is illustrated in Figure 2. The SEM showe
20、d clearly that each stripe was composed of a high density of craters (Figure 2c). Under higher magnification, it revealed these craters were caused by electric discharge, indicated by the typical fused metal particles and gas pockets (Figures 2d and 2e). The particles were about 1-3 micron in diamet
21、er. For comparison, Figure 3f is cited from ANSI/AGMA 1010-E95 as an example of electric discharge. Figures 2b and 2c show the original machining marks outside of the tiger stripes. Figure 2. Morphology of “tiger stripes” revealing electric discharge. a - e were taken from the sample, f is cited fro
22、m ANSI/AGMA 1010-E95. Note they are under different magnifications indicated by scale bars. 6 13FTM14 Hardness profile In order to evaluate the extent of damage caused by electric discharge, one pinion tooth was sectioned for microhardness profile checks across the gear flanks (load and no-load side
23、s) and the top land from the surface to the hardened case depth. The results are shown in Figure 3. The load flank hardness profile revealed that although the depth between surface and the location where the hardness number is 50 HRC reached 0.060“, and is deeper than the specified minimum effective
24、 case depth (in this case, 0.036“), both the load flank and top land lost some hardness below the surface due to the “tiger stripes”, especially on the load flank of the teeth. For example, the hardness was only 57.6 HRC on the load flank surface, which is lower than the required minimum 58 HRC. Thi
25、s made the pinion soft and lowered its contact fatigue resistance. As a matter of fact, except at 0.015, its hardness is lower than 58 HRC at any other depth. Even the top land showed some degree of hardness drop. The “tiger stripes” had the least negative influence on the no-load flank of the pinio
26、n teeth. Since the original manufacturing records show that the surface hardness on any flank of the pinion was 59 - 60 HRC, the hardness loss of the inspected pinion was due to the “tiger stripes”, i.e., electric discharge. Discussion The chemical analysis confirmed the material is SAE 9310 carburi
27、zing steel. Mechanical properties were in the normal range. The hardness profiles exhibited hardness loss due to the “tiger stripes”. The load flank surface hardness was even lower than 58 HRC, minimum hardness required by the AGMA standard. SEM analysis revealed that these stripes were electric dis
28、charge defects. The electric discharge generated very high local temperature and local melting of the pinion teeth. As a result, some areas of the pinion were retempered at high temperature. This gave rise to lowered hardness and jeopardized the contact fatigue resistance. Furthermore, as the surfac
29、e hardness is lower than 58 HRC, the original gear rating is not valid. It is recommended that a new pinion should be manufactured to replace this damaged one if the user wants to keep the initial rating. It should be noted that this “tiger stripe” pattern is different from normal electric discharge
30、. While the latter shows random spots on gear tooth, “tiger stripe” takes on regular patterns. Obviously, these stripes came from periodic discharge between the mating flanks. Although some composite bearings have been introducing to minimize this type of damage, proper operating grounding such as b
31、rushes is still considered to be the best solution to prevent this from happening 3. Figure 3. Case hardness profiles of the pinion tooth. 7 13FTM14 Conclusions The steel for this pinion was confirmed to be SAE 9310. Tensile test showed its mechanical properties met the requirements. SEM analysis co
32、nfirmed the “tiger stripes” are electric discharge damage. It generated high temperature and locally melted the pinion surface, giving rise to low surface hardness. Acknowledgement The authors are thankful to Bob Errichello for his discussion and review of this paper. References 1. ANSI/AGMA 1010-E9
33、5, Appearance of Gear Teeth - Terminology of Wear and Failure, pp. 11-12. 2. Lyons, L., Electrical Discharge Pitting PT6A Accessary Drive Gear, Flight Safety Australia, Nov.-Dec. 2001, p. 36-39. 3. Sohre, J.S., Shaft Riding Brushes to Control Electric Stray Currents, 20th Turbomachinery Symposium, Sept. 17-19, 1991, Dallas, Texas, p. 63-75.
