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The four seized bearings submitted for metallurgical analysis were determined to have suffered deterioration of their grease to the point that it was inhibiting motion rather than enabling. The grease had turned to a thick (viscous) black sludge that was full of particulate. Electrical pitting was found in the seized bearings, especially the inner races. Signs of axial (thrust) loads were noted, mainly in the outer raceways. Larger arc strike regions were found on the balls. Worn and fractured cages were discovered. Metallurgical signs of electrical arcing induced overheating were noted in the balls and in the raceways. It was determined that heating caused by electrical arcing, thrust loading, and the resulting mechanical damage of the bearing components caused the deterioration of the grease.
The hardness of the seized bearing races and balls were measured to be 60 to 61 Rockwell C. The inner and outer races and a ball from a large seized bearing were determined to be made of E52100 bearing steel. The composition and hardness of the components were found to be typical and acceptable for ball bearings.
Six ball bearings were sent for metallurgical analysis - three smaller and three larger. The three smaller bearings were (double) sealed XXX XXX XXX. The inner races of two of these bearings were seized relative to the outer races. The third small bearing was new. The three larger bearings were (double) sealed XXX XXXX XXX. The inner races of two of these bearings were seized relative to the outer races. The third large bearing was new.
The six submitted bearings are shown in Figure 1. There were no signs of mechanical, electrical or corrosion damage on the outer surfaces of the seized bearings. The seals on the four seized bearings were intact and undamaged. There were no signs of grease leakage. There was no heat tint or signs of corrosion observed on any of the bearing outer surfaces.
Diametrically opposite cuts were made in the outer races of the four seized bearings. The outer races were then parted from the rest of the bearings to facilitate visual examination. It was immediately noted that the grease in the four bearings had turned to a thick (viscous) black sludge, full of particulate. The sludge was caked on the seals and between the balls, and generally inhibiting motion rather than enabling it. See Figures 2-6. A sample of the grease from the new bearings was taken for comparison. The new grease was a light aqua color and thought to be XXXXXX XXXX electric motor bearing grease as shown in Figure 7.
The cages showed signs of excessive wear, and in one small and one large bearing the cages were fractured. The fractured cages can be seen in Figures 4, 5, and 8. Most of the deteriorated grease was collected and saved, and then the bearings were cleaned for further examination. Thrust loading was observed in some of the bearings by the presence of wear off to one side of the race. This was especially evident in one of the large bearing outer races shown in Figure 9. Smearing and what looked like electrical pitting (frosted appearance) was observed in some areas of the races. The electrical pitting was most evident on the inner races, as seen in Figures 10 and 11.
Most of the balls in each of the four bearings exhibited at least one, and as many as four small spots of surface spalling apparently due to arc strikes and subsequent enlargement by contact fatigue. Figures 12 through 14 show the spots were typically circular or semi-circular shaped.
Electrical current arcing can cause pitting and in turn roughen the rolling contact surfaces, liberate fractured pieces, and ultimately lead to mechanical damage and frictional heating. It is the heating which deteriorates the grease to the point where it inhibits motion rather than enables it.
Scanning Electron Microscopy
Scanning electron microscopic (SEM) examination of a large and small inner race clearly showed the flattened edges of the (electrical) pitting. Each patch of pitting was made up of numerous small pits that resulted from small electrical arcs. This can be seen in Figures 15 through 18. Areas of wear and plowing from liberated particles were also observed on the races.
SEM examination of balls from a large and a small seized bearing revealed larger localized arc strikes with contact fatigue spalling around the edges in the ball surfaces. Spherical regions with globular features in the center (arc strike) and crack progression marks around the edges (contact fatigue) were observed. See Figures 19 through 22.
Transverse cross-sections were taken through the electrically pitted and damaged areas of the inner and outer races of a small and large seized bearing. These sections were prepared for metallographic examination per ASTM E3-01. Etching in 2% nital was done to ASTM E407-99. The microstructure of all four races was predominantly tempered martensitic with some undissolved carbides, typical of E52100 bearing steel.
White and light etching layers were observed on the surface of the races at the electrical pitting. Fine localized regions of arc-shaped white areas were also noted along the fine pitted regions. This was confirmation of the electrical pitting as these layers represented re-cast and untempered martensite from tiny arc strikes. More of the white and light etching surface layers were present on the inner races. See Figures 23 and 24.
One bearing ball with surface spalling each from a small and large seized bearing was selected for metallographic examination. Both balls were sectioned through the spalling and prepared for metallographic examination. The microstructure of both balls was predominantly tempered martensite with some undissolved carbides, typical of E52100 bearing steel.
The small ball exhibited a complete loss of material at its surface spall. There were remnants of contact fatigue cracking noted at the edges of the spall. This can be seen in Figures 25 and 26. Light etching areas of untempered martensite, indicative of severe overheating were also observed at some locations around the perimeter of the ball. See Figure 27.
The large ball exhibited late stages of contact fatigue cracking and surface material spalling. The cracks started radial and turned to run parallel to the surface, typical of contact fatigue. This is seen in Figures 28 and 29. Nearly the entire circumference of the large ball exhibited light etching untempered martensite, indicative of severe overheating, as shown in Figure 30.
Microhardness testing was performed on a ball from a large seized bearing per ASTM E384-99e1. The results were converted to Rockwell C (HRC) per ASTM E140-05. The converted ball hardness was found to be 60 HRC. The hardness of the inner and outer races from the same seized large bearing was measured to be 61 HRC, respectively. This was typical of through-hardened E52100 bearing steel.
Chemical composition analysis of a large seized bearing inner race, outer race, and ball was performed according to ASTM E419-99a. The results of the analyses are shown in Table 1 along with the requirements of E52100 bearing steel per SAE J404.
The results of the analyses show that the inner and outer races and the ball met the chemical composition requirements of E52100 per SAE J404.
Metallurgical Technologies, Inc. is a Metallurgical Engineering Service Laboratory specializing in Forensic Failure Analysis of Metallurgical Components. They also offer testing, analysis, and consulting services.
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