Metalurgical Technologies, Inc Call an Engineer: 704-663-5108
Quick Quote Contact an Engineer
Failure Analysis, Corrosion Investigation, Metallurgical Testing and Engineering • Located in Mooresville, NC

FAILURE ANAYSIS OF BALL BEARINGS

Report for: Report by: 
**************
**************
**************
**************
**************
METALLURGICAL TECHNOLOGIES, INC., P.A.
160 Bevan Drive
Mooresville, NC  28115
704-663-5108
Failure Analysis of ball bearings

Sections:

 

 

SUMMARY:

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.

 

ANALYSIS:

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. 

 

Visual Examination

 

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.

 

Metallography

 

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.

  

Hardness Test

 

Microhardness testing was performed on a ball from a large seized bearing per ASTM E384-99ε1.  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 ANALYSIS

Table 1

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.

 

TABLE 1

Chemical Composition Analysis Results

(weight %)

Element

Inner Race

Outer Race

Ball

SAE E52100

Carbon

0.98

0.99

0.99

0.98 - 1.10

Manganese

0.41

0.35

0.33

0.25 -0 .45

Phosphorus

  0.012

<0.001

<0.001

0.25 max

Sulfur

<0.001

  0.011

<0.001

0.25 max

Silicon

0.18

0.25

0.26

0.15 - 0.35

Nickel

0.13

0.23

0.02

-

Chromium

1.43

1.51

1.49

1.30 - 1.60

Molybdenum

0.04

0.08

<0.01

-

Vanadium

  0.01

  0.01

  0.01

-

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.

CONCLUSIONS:

  • The grease in all four seized bearings had turned to a thick (viscous) black sludge full of particulate.  The deteriorated grease was caked on the seals and on the cage between balls.  The deteriorated grease was inhibiting motion rather than enabling it.
  • Worn and fractured cages were found in the disassembled bearings.  The balls exhibited evidence of arc strikes and contact fatigue surface spalls.
  • Evidence of electrical pitting was noted in the raceways.  This was confirmed by metallographic examination.
  • Evidence of axial (thrust) loading was observed mainly in the outer raceways of the seized bearings.

IMAGES:

Failure Analysis of ball bearings
Figure 1: Photograph of the six submitted bearings showing the new bearings in the left column, two small seized bearings in the center column, and two large seized bearings in the right column.  (Photo XXXXX-N1)
seized bearing disassembled after cutting the outer race
Figure 2: Photograph of a small seized bearing disassembled after cutting the outer race showing the grease had turned to a black thick sludge.  (Photo XXXX-N2)
seized bearing disassembled after cutting the outer race
Figure 3: Photograph of the cage, balls and inner race in the seized small bearing showing the thick, black sludge and particulate coating the inside.  (Photo XXXX-N6)
seized bearing disassembled after cutting the outer race
Figure 4: Photograph of a large seized bearing disassembled after cutting the outer race showing the grease had turned to a black thick sludge.  Note the fractured cage pieces (arrows) and the liberated balls.  (Photo XXXXX-N7)
Photograph of the cage, balls and inner race from a large seized bearing showing the thick, black sludge and particulate coating the inside. Note the fractured cage.
Figure 5: Photograph of the cage, balls and inner race from a large seized bearing showing the thick, black sludge and particulate coating the inside.  Note the fractured cage. (Photo XXXX-N10)
typical grease particles in the seized bearings showing it had deteriorated to be a thick, black sludge full of particulate
Figure 6: Photomacrograph of typical grease particles in the seized bearings showing it had deteriorated to be a thick, black sludge full of particulate.  (Photo XXXX-U17)
Photomacrograph of a sample of new grease on the left in comparison with the deteriorated grease on the right
Figure 7: Photomacrograph of a sample of new grease on the left in comparison with the deteriorated grease on the right.  (Photo XXXX-U18)
Photograph of a seized small bearing showing the heavily worn and cracked (arrows) cage. Note the caked black grease chunks on the sides.
Figure 8: Photograph of a seized small bearing showing the heavily worn and cracked (arrows) cage.  Note the caked black grease chunks on the sides.  (Photo XXXX-U24, Mag. 5X)
Photograph of a large bearing outer race ID ball path showing wear on one side, indicative of axial (thrust) loading.
Figure 9: Photograph of a large bearing outer race ID ball path showing wear on one side, indicative of axial (thrust) loading.  (Photo XXXX-U15, Mag. 8X)
Photograph of the inner race on a small seized bearing after cleaning showing electrical pitting (frosted area enclosed in dashed outline). 
Figure 10: Photograph of the inner race on a small seized bearing after cleaning showing electrical pitting (frosted area enclosed in dashed outline).  (Photo XXXX-U9, Mag. 16X)
Photograph of the inner race on a large seized bearing showing corrugated patches of electrical pitting – frosted regions (arrows).
Figure 11: Photograph of the inner race on a large seized bearing showing corrugated patches of electrical pitting – frosted regions (arrows).  (Photo XXXX-U12, Mag. 10X)
Photograph of a typical ball showing a semi-circular patch of contact fatigue surface spalling.
Figure 12: Photograph of a typical ball showing a semi-circular patch of contact fatigue surface spalling.  (Photo XXXX-U1, Mag. 16X)
hotograph of a typical ball showing a circular patch of contact fatigue surface spalling. 
Figure 13: Photograph of a typical ball showing a circular patch of contact fatigue surface spalling.  (Photo XXXX-U4, Mag. 16X)
Photograph of a typical ball showing a circular patch of contact fatigue surface spalling. 
Figure 14: Photograph of a typical ball showing a circular patch of contact fatigue surface spalling.  (Photo XXXX-U5, Mag. 16X)
SEM image of the inner race on a small seized bearing showing a localized patch of electrical pitting made of numerous individual pits.
Figure 15: SEM image of the inner race on a small seized bearing showing a localized patch of electrical pitting made of numerous individual pits.  (SEM Photo XXXX-SB2, Mag. 100X)
Increased magnification SEM image of the inner race on a small seized bearing showing the electrical pitting damage. 
Figure 16: Increased magnification SEM image of the inner race on a small seized bearing showing the electrical pitting damage.  (SEM Photo XXXX-SB7, Mag. 1,000X)
SEM image of the inner race on a large seized bearing showing a localized patch of electrical pitting made up of numerous individual pits. 
Figure 17: SEM image of the inner race on a large seized bearing showing a localized patch of electrical pitting made up of numerous individual pits. 

(SEM Photo XXXX-SA12, Mag. 100X)

Increased magnification SEM image of the inner race on a large seized bearing showing the electrical pitting damage. 
Figure 18: Increased magnification SEM image of the inner race on a large seized bearing showing the electrical pitting damage.  (SEM Photo XXXX-SA13, Mag. 400X)
SEM image of a large arc strike on the surface of a ball from a small seized bearing. The outer edges exhibit crack progression marks indicative of continued damage by contact fatigue spalling.
Figure 19: SEM image of a large arc strike on the surface of a ball from a small seized bearing.  The outer edges exhibit crack progression marks indicative of continued damage by contact fatigue spalling.  (SEM Photo XXXX-SA31, Mag. 50X)
SEM image of the edge of one of the small seized bearing ball spalls showing crack progression marks indicative of contact fatigue. 
Figure 20: SEM image of the edge of one of the small seized bearing ball spalls showing crack progression marks indicative of contact fatigue.  (SEM Photo XXXX-SA32, Mag. 200X)
SEM image of a localized large arc strike on a ball from a large seized bearing. The outer edges exhibit contact fatigue progression. 
Figure 21: SEM image of a localized large arc strike on a ball from a large seized bearing.  The outer edges exhibit contact fatigue progression.  (SEM Photo XXXX-SA2, Mag. 25X)
Increased magnification SEM image of the contact fatigue area at the outer edges of the arc strike
Figure 22: Increased magnification SEM image of the contact fatigue area at the outer edges of the arc strike shown in Figure 21.  (SEM Photo XXXX-SA3, Mag. 100X)
(Photo XXXX-MB4, Mag. 500X, nital etch)" rel="lightbox(bb)">Photomicrograph of a small seized bearing inner race transverse cross-section showing white and light etching (arrows) areas indicative of electrical pitting along the contact surface. The core microstructure was tempered martensite with fine carbides. 
Figure 23: Photomicrograph of a small seized bearing inner race transverse cross-section showing white and light etching (arrows) areas indicative of electrical pitting along the contact surface.  The core microstructure was tempered martensite with fine carbides.  (Photo XXXX-MB4, Mag. 500X, nital etch)

hotomicrograph of a large seized bearing inner race transverse cross-section showing white and light etching (arrows) areas indicative of electrical pitting. The core microstructure was tempered martensite with fine carbides. 

Figure 24: Photomicrograph of a large seized bearing inner race transverse cross-section showing white and light etching (arrows) areas indicative of electrical pitting.  The core microstructure was tempered martensite with fine carbides.  (Photo XXXX-MB19, Mag. 500X, nital etch)

Photomicrograph of the cross-section through a ball from a small seized bearing showing the complete loss of material at the spalled surface

Figure 25: Photomicrograph of the cross-section through a ball from a small seized bearing showing the complete loss of material at the spalled surface. 

(Photo XXXX-MB10, Mag. 50X, nital etch)

Photomicrograph of the spalled surface of a ball from a small seized bearing showing the remnant contact fatigue cracks (arrows) at the edge of the spall,

Figure 26: Photomicrograph of the spalled surface of a ball from a small seized bearing showing the remnant contact fatigue cracks (arrows) at the edge of the spall, as shown in Figure 25.  (Photo XXXX-MB15, Mag. 500X, nital etch)

Photomicrograph of a ball from a small seized bearing showing light etching untempered martensite on the outer surface, indicative of severe overheating. 

Figure 27: Photomicrograph of a ball from a small seized bearing showing light etching untempered martensite on the outer surface, indicative of severe overheating.  (Photo XXXX-MB12, Mag. 500X, nital etch)

Photomicrograph of the cross-section through the damaged surface in a ball from the large seized bearing showing overheat and later stages of contact fatigue and surface spalling. Note the cracks propagating parallel to the surface. 

Figure 28: Photomicrograph of the cross-section through the damaged surface in a ball from the large seized bearing showing overheat and later stages of contact fatigue and surface spalling.  Note the cracks propagating parallel to the surface. 

(Photo XXXX-MB26, Mag. 100X, nital etch)

Photomicrograph of the damaged surface in a ball from the large seized bearing showing contact fatigue cracks. Note the cracks (arrows) propagating parallel to the surface. 

Figure 29: Photomicrograph of the damaged surface in a ball from the large seized bearing showing contact fatigue cracks.  Note the cracks (arrows) propagating parallel to the surface.  (Photo XXXX-MB23, Mag. 500X, nital etch)

Photomicrograph of a ball from a large seized bearing showing light etching untempered martensite observed around most of the outer surface, indicative of severe overheating.

Figure 30: Photomicrograph of a ball from a large seized bearing showing light etching untempered martensite observed around most of the outer surface, indicative of severe overheating.  (Photo XXXX-MB24, Mag. 500X, nital etch)