Manufacturing Induced Hydrogen Embrittlement of 52100 Bearing Steel Outer Ring
J Fail. Anal. and Preven.
Manufacturing Induced Hydrogen Embrittlement of 52100 Bearing Steel Outer Ring
P. Roffey 0
0 P. Roffey (&) Forensic Engineering & Materials, ESR Technology , Whittle House, 410 The Quadrant, Birchwood Park, Warrington, Cheshire WA3 6FW , UK
An axle roller bearing failed due to full width axial cracking of the outer ring, an uncommon occurrence in roller bearings. Typically, this mechanism of failure is more consistent with inner rings that have been installed with excessive hoop stress from an incorrect interference fit on a rotating shaft. Fractography of the axial fracture revealed a shallow intergranular crack initiating from the surface of an external corner. The remainder of fracture propagation was high-cycle fatigue. Detailed metallographic analysis revealed hydrogen embrittlement on all four corners of the ring. Final machining processes, such as grinding and polishing performed on the side faces, raceway and outer surface had removed the embrittled material in these areas. This indicated the embrittlement process was not service related, but rather a result of the manufacturing heat treatment, most likely performed following the first rough cut of the ring prior to finishing.
Roller bearing; Hydrogen embrittlement; Intergranular fracture; Fatigue; Heat treatment
The most common material used to manufacture ball and
roller bearing rings is alloy steel AISI 52100, a 1.0%
carbon steel with a chromium content of 1.3–1.6% [
52100 is easily forged, heat treated and machined, with
through hardening producing the fracture toughness
required to avoid, or more specifically reduce, surface and
sub-surface cracking from rolling contact fatigue (RCF).
Despite attempts to increase fracture toughness
properties, RCF remains a common failure mechanism in
bearings and the resistance to RCF can be summarised by
the L10 fatigue life; an engineering statistical approach that
estimates 90% of a particular bearing type will avoid
fatigue damage in specific operating conditions, Fig. 1 [
Although RCF is a common bearing failure mechanism,
it is not the only mechanism observed in bearings, with
others including wear and abrasion, contamination-related
failures (etching, corrosion or fretting corrosion),
inadequate lubrication-related failures (peening, overheating,
smearing) and false brinelling, true brinelling or electrical
All of the mechanisms above are identified by
characteristic features such as pitting, spalling or blueing on the
raceways and rollers. The current study however describes
an investigation of an axle roller bearing which failed due
to a full width axial crack of the outer ring initiating from
the outer surface and not the raceway, an uncommon
failure mode in roller bearings.
The failed roller bearing had been used on a tractor wheel
axle. Each of the components, the outer ring, inner ring,
cage and rolling elements, was examined in detail using
stereomicroscopy. Although all of the components showed
some form of damage, the most distinctive feature was the
full width axial crack observed on the outer ring, Fig. 2.
The crack was oriented perpendicular to the raceway and
showed clear evidence of origination from the machined
corner near to the outer surface at the thicker diameter,
Fracture direction could be identified due to propagation
marks from the origin towards raceway surface. The
surface possessed no coarse, jagged final overload region with
only high-cycle fatigue observed across the whole section.
The fracture surface had suffered limited post fracture
damage which often impedes fractographic analysis. Light
polishing and a narrow band of fatigue damage were
identified on the outer ring raceway although no material
loss had occurred, Fig. 4. The outer surface of the ring,
which mated with the housing, had small patches of light
fretting but no corrosion or mechanical damage near to the
fracture origin, Fig. 5.
Two fatigue spalls were present on the raceway of the
inner ring, along with polishing and secondary mechanical
damage on the guide flange, Fig. 6. The cage was intact but
had undergone some plastic deformation, Fig. 7. Several of
the rollers had catastrophically failed, although the
majority remained in relatively good condition with slight
indentation damage from entrained debris, Fig. 8.
Detailed examination of all the components, including
subsequent metallography and electron microscopy,
confirmed all of the features observed on the cage, rollers and
inner ring were secondary failure related with the axial
crack in the outer ring being the primary root cause of
failure. Thus, the remainder of this study focuses only on
the detailed examination of the outer ring.
High-magnification imaging was performed on the outer
ring using scanning electron microscopy (SEM). The
fracture origin at machined radius possessed a 300 lm
intergranular (IG) crack, Figs. 9 and 10. IG cracking in
bearing rings is typically associated with environmentally
assisted cracking (EAC), i.e. corrosion related.
Examination of the external surface close to the origin however
showed no indication of corrosive attack, either general
corrosion or pitting corrosion, Fig. 11. A small impact
mark was identified adjacent to the origin; this however
was not directly at the point of fracture and was more likely
a result of the bearing removal process following failure.
The remainder of the fracture surface showed a high-cyclic
fatigue propagating towards the raceway with no final
overload region present, Fig. 12.
Optical Microscopy Microsections were prepared through the fracture surface to observe the crack origin, with transverse sections taken
close to the fracture point to examine remote areas. The
metallurgically prepared sections were etched using
Picral solution to reveal the microstructure and carbide
The IG propagation when viewed in cross-section
revealed a coarse morphology with no branching; this was
followed by the relatively smooth fatigue propagation,
Fig. 13(a). The microstructure was mottled bainitic with
prior austenite boundaries visible; the mottling is the result
of localised variations in composition. High-magnification
examination of the machined radius surface revealed
evidence of IG embrittlement along the entire length with a
high distribution of grain boundary carbides, Fig. 13(b).
White non-etching material was also present along the
radii surfaces, a result of material smearing from the coarse
machining process. Ideally, non-etching material should be
completely removed during the final grinding and polishing
operation. Further examination of all the microsections
showed similar features on all four corners of the ring.
Compositional checks were carried out using spark optical
emission spectroscopy (OES), Table 1. Alloy 52100
typically possesses a carbon content of 1% with a chromium
level of 1.3–1.6%; the inner ring was shown to be within
specification. The phosphorous content was also below the
allowable maximum level.
Hardness testing was performed on polished
microsections using a Vickers indenter (HV) and a load of 100 g
(HV0.1). A profile obtained from the origin to a depth of
2 mm is plotted in Fig. 14. The average hardness was
measured at 736 HV with no larges increases or decreases
within the embrittled region.
Generally, steels with a hardness value above 320 HV
are more susceptible to hydrogen diffusion and
embrittlement; bearing steels are well above this HE threshold.
Axial cracking is failure mode often associated with
bearing inner rings having been mounted on to a shaft
through an interference fit. The rings are heated to provide
Fig. 13 (a) Cross-section through fracture surface with coarse IG
crack followed by ‘smooth’ fatigue propagation (b) detail of
embrittled surface with high carbide distribution at grain boundaries
a small amount of expansion allowing the ring to slide over
the shaft. Cooling then shrinks the ring on to the shaft
surface, resulting in the interference fit. This process
inherently generates a tensile stress in the ring. Excessive
hoop stress may then result in the perpendicular axial
cracking. This mode of cracking through an outer ring,
however, which typically sits in a housing with a small
amount of clearance, is rarely observed. The clearances
between the ring and housing are associated with strict
tolerances, typically 150 lm. If this is exceeded, failure
tends to occur due to RCF, resulting in a large amount
secondary damage as the rolling elements and ring have the
freedom to move and contact each other.
The examination of the outer ring showed little evidence
of secondary damage with limited rubbing damage,
indicating that the clearance was likely to have been correct,
not allowing the two fractures to contact each other
following failure. This was further supported due to
uninterrupted high-cycle fatigue propagation through the
full cross-section with no final overload region.
The lack of substantial raceway damage eliminated the
possibility of lubrication issues or RCF as contributions to
failure. Furthermore, no evidence of corrosion was seen
within the contact zone or on the external surfaces which
eliminated the ingress of water during service.
Intergranular cracking of bearing rings is a failure mode
often associated with EAC mechanisms such as stress
corrosion cracking (SCC) from corrosive electrolytes such
as moisture, water or lubricants. SCC is an IG mechanism
but typically initiates from a corrosion pit and propagates
through branched IG cracks; no branching or corrosion was
observed. Other forms of hydrogen-induced cracking
include cracking due to the presence of volatile substances
such as hydrogen sulphide in sour gas; this particular
bearing however would not have been exposed to such
environments. The absence of corrosion however
eliminated in-service embrittlement with the likely source being
Optical microscopy of microsections showed embrittled
material present on all four corners of the ring along with a
high distribution of grain boundary carbides. The surface of
the corners also possessed white non-etching material,
remnants of the coarse machining process. The side face,
raceway and outer surface however showed none of these
The initial shape of a forged bearing ring means that the
rounded edges on the side faces require a greater amount of
material removal from these locations to achieve tolerance
and surface finish. Likewise, the outer surface and raceway
will undergo substantial material removal, with the
raceway being finished to a polished surface. The corners of the
ring, or radii, however require less strict tolerance and can
be left with a relatively coarse machined finish, Fig. 15.
Following this first rough cut a heat treatment is then
performed, before final grinding and polishing processes.
The presence of the defective material on the corners
would indicate the embrittlement occurred during the first
heat treatment process, an excluded possibilities of
embrittlement during the final stages of finishing.
The presence of embrittled material will increase the
risk of crack initiation under shock loading conditions, and
the service duty of a tractor axle will undoubtedly lead to
shock loading. Grain boundary embrittlement will result in
IG cracking through the embrittled zone, this will then act
as a stress concentration or notch for fatigue propagation
through the remainder of the section under
high-cycle/lowload cyclic conditions.
A common form of material embrittlement during the
manufacturing stage is hydrogen diffusion into the structure,
i.e. hydrogen embrittlement (HE), particularly for materials
above the 320 HV threshold. Thus, heat treatments should be
performed correctly to avoid hydrogen diffusion. The
diffusion of the hydrogen follows the path of least resistance
along ferrite grain boundaries, through the ferrite phases and
along the interface between the ferrite and pearlite phases
resulting in the loss of ductility and tensile strength. Grain
boundary separation and separation through the ferrite phase
were observed on the fracture surface, Fig. 16.
Bearing steels possess a network of dispersed carbides
though the structure, or more precisely along the grain
boundaries. It is the presence of carbides and unwanted
defects such as non-metallic inclusions (both sub-surface
and surface breaking) that often result in RCF.
Furthermore, the carbides may also act as traps under hydrogen
diffusion conditions, further increasing susceptibility
where carbide distribution is high [
Heat treatments aim to refine the microstructure by
increasing homogeneity. This is achieved by dissolving the
carbides (or reducing the size) and transforming the
austenite phase to bainite. A post-heat treatment tempering is
then performed to achieve desired toughness [
heat treatments are performed incorrectly, i.e. incorrect
temperature or insufficient time, where the carbide network
will remain unrefined or become clustered as observed in
this study. This increases the risk and likelihood for RCF
and grain boundary embrittlement.
High levels of phosphorous have also been shown to
increase the propensity of hydrogen diffusion, particularly
at crack tips, which drives cracking forward, although the
levels in the outer ring measured by Spark OES were
It should be noted that localised carbide formation may
also occur during machining processes; however, the
temperature required to form the carbides would result in
clear temperature damage, and the bearing would have
been rejected at the quality control stage, or would have
been clearly visible at installation/failure investigation.
To allow the steel to effuse the hydrogen and avoid
embrittlement, baking processes can be carried out to
components. This process however may be only applicable
to certain material and may affect the materials properties,
thus it should be performed to the correct parameters.
The primary root cause of failure of the outer ring was due
to intergranular embrittlement, or HE, from incorrect heat
treatment performed prior to machining.
Material removal from the side faces, outer surface and
the running track removed the embrittled layer although
only coarse machining of the corners left the embrittled
material in place.
A relatively shallow IG crack on the radii of the outer
surface of the outer ring, most likely the result of a shock
load, created a notch for fatigue initiation.
High-cycle/low-load operation of the bearing
propagated a fatigue crack through the bearing ring. Fracture of
the outer ring increased the clearance, which lead to the
secondary damage to the rollers, cage inner and outer ring
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