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Failure of a Rear Axle Shaft of an Automobile Due to Improper Heat Treatment
Article in Journal of Failure Analysis and Prevention · June 2013
DOI: 10.1007/s11668-013-9682-5
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J Fail. Anal. and Preven. (2013) 13:353–358
DOI 10.1007/s11668-013-9682-5
TECHNICAL ARTICLE—PEER-REVIEWED
Failure of a Rear Axle Shaft of an Automobile Due to Improper
Heat Treatment
H. M. Tawancy • Luai M. Al-Hadhrami
Submitted: 29 November 2012 / in revised form: 4 March 2013 / Published online: 22 March 2013
Ó ASM International 2013
Abstract A section of fractured rear axle shaft made of
induction-hardened steel and removed from the scene of
overturned automobile was analyzed to determine the most
probable cause of failure. Light optical metallography and
scanning electron microscopy combined with energy dispersive spectroscopy were used to characterize the
microstructure and the mechanical strength was evaluated
by microhardness measurements. Chemical analysis verified that the shaft was made of AISI 4140 steel as per
specifications. However, microstructural characterization
and microhardness measurements revealed that the shaft
was improperly heat treated resulting in a brittle case,
where crack propagation was found to occur by an intergranular mode in contrast with cleavage within the core.
This behavior was related to differences in microstructure,
which was observed to be martensitic-type within the case
with microhardness equivalent to Rc 58, and a mixture of
pearlite and ferrite within the core with Rc 25. Although it
was not possible to reconstruct the exact sequence of
events leading to fracture, it is possible that it was initiated
by large overload within the extremely hard brittle case,
which could lead to overturning of the vehicle and final
fracture could have occurred by the impact of overturning.
However, crack initiation due to hydrogen generated by
rust and water pickup as well as the possibility that overturning of the vehicle was the cause of the fracture could
not be ruled out.
H. M. Tawancy (&) L. M. Al-Hadhrami
Center for Engineering Research, Research Institute, King Fahd
University of Petroleum and Minerals, P.O. Box 1639,
Dhahran 31261, Saudi Arabia
e-mail: tawancy@kfupm.edu.sa
Keywords Brittle fracture Characterization Cleavage Electron fractography Hardness Steel
Introduction
Axles are installed in vehicles to perform two important
functions: (i) they transmit driving torque to the wheels,
and (ii) they maintain the position of the wheels relative to
each other and to the body of the vehicle. In most noncommercial vehicles, the circular motion of the drive
wheels is maintained by means of axle shafts, which are
integral component of the rear axle. The shafts are installed
in the tire’s wheel well near the differentials and stretch
across the bottom of the vehicle. Often during operation,
the shafts are subjected to tremendous torque due to heavy
loads or quick acceleration and therefore, they are manufactured from various grades of hardened steels. However,
for various reasons, the shafts may fail particularly by
fatigue modes, e.g., [1–5]. In extreme cases, cracks in the
shafts during driving can lead to overturning of the vehicle.
A typical symptom of cracks in rear axle shafts is
skidding of the vehicle to one side similar to driving on ice,
which is commonly known as fishtailing. Complete spinning occurs if the driver does not properly respond by
counter steering, i.e., turning the front wheels in the same
direction as the skid. Therefore, cracks in the shaft can lead
to overturning of the vehicle particularly during driving at
high speed such as occurs on highways. However, the shaft
may also fracture upon impact as a result of an accident.
Therefore, the key question to be answered in respective
failure analysis investigations is whether the fractured shaft
is the cause or result of the accident.
In the present case and according to the information
received from the proponent, a vehicle overturned during
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driving on a smooth highway at a speed of about 100 km/h
(62 miles/h), which was within the speed limit. Also, it was
stated that no collision was involved and there was no
indication that the driver was trying to avoid an object.
Since the accident was fatal no information could be
obtained from the driver. As per specifications the fractured
shaft with diameter of about 7 cm was manufactured by
forging of steel grade AISI 4140 given an inductionhardening treatment to produce a case of 3–4 mm in depth.
Although respective specifications regarding microstructure and hardness were not available, it is usually required
that the case consists of tempered martensite with hardness
of HV 500–550, and the core consists of a mixture of ferrite
and pearlite [6, 7]. A section of fractured rear axle shaft
removed from the scene was received to determine the
most probable cause of failure.
J Fail. Anal. and Preven. (2013) 13:353–358
fracture surface, (ii) specimens in the as-polished condition
to measure the microhardness across the case and into the
core, and (iii) specimens etched in 3% nital to reveal the
grain structure within the case and core. A Vicker’s hardness tester was used to measure the microhardness using
10 g load.
Experimental Results and Discussion
Figure 1a is a schematic illustration of the rear axle shaft
showing the approximate location of the fracture near the
wheel mounting flange. A photograph of the section
received for analysis is shown in Fig. 1b. In general, there
Experimental Procedure
Representative metallographic specimens were removed
from the as-received section of the shaft for metallurgical
evaluation using light optical microscopy and scanning
electron microscopy combined with microchemical analysis employing a windowless x-ray detector. Inductively
coupled plasma-atomic energy spectroscopy (ICP-AES)
was used to measure the chemical composition of the steel
used in the application with the exception of the carbon
content, which was determined by combustion calorimetry
(CC). Three types of specimens were included in the study:
(i) specimens in the as-received condition for visual
inspection and characterization of the morphology of
Fig. 1 (a) A schematic
illustration of the rear axle shaft
showing the approximate
location of fracture. (b) A
photograph of the section of
received for analysis
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Fig. 2 A light optical macrograph showing two distinct regions A
(case) and B (core) of the fracture surface (as-received condition)
J Fail. Anal. and Preven. (2013) 13:353–358
355
Fig. 3 Light optical
macrograph showing the
characteristic features of regions
A and B in Fig. 2 as viewed at
higher magnification (asreceived condition). (a)
Chevron marks in region A
pointing to the fracture origin at
the surface. (b) Shiny faceted
fracture in region B
Table 1 Chemical composition of AISI 4140 steel (wt.%)
Element
Nominal
Fe
Balance
Balance
Cr
0.80–1.00
0.89
Mn
0.75–1.00
0.96
C
0.38–0.43
–
0.41
Si
0.15–0.35
0.28
Mo
0.15–0.25
0.17
S
0.040a
0.03
P
0.035a
0.01
a
Measured (ICP-AES)
Measured (CC)
Maximum
was no evidence for macroscopic deformation in the vicinity
of the fracture, however, there was some rust at the surface.
As can be seen, the fracture runs normal to the axis of the
shaft. It is well known that fracture propagation is generally
determined by local stress condition [6, 8]. Although rear
axle shafts are often subjected to heavy torques as pointed out
earlier, torsion or shear fractures run at 45° to the direction of
stress in contrast with tensile fractures, which run normal to
the stress [5, 6]. Therefore, the observation of Fig. 1b suggests that the fracture is of the tensile type, which could have
resulted from large overload.
A light optical macrograph showing the fracture surface
in the as-received condition is shown in Fig. 2. It is
observed that the fracture surface consists of two distinct
regions: (i) a relatively smoother perimeter region marked
A where the fracture was initiated with depth corresponding to that of the case, and (ii) a rougher core marked B
where final fracture occurred. Light optical macrographs
illustrating details of the two regions as observed at higher
magnifications are shown in Fig. 3. It is observed from
Fig. 3a that region A contains chevron marks converging at
the surface region where the fracture has been initiated. As
Fig. 4 Characteristic microstructural features of the case (etched
specimen). (a) Backscattered electron image showing martensitictype microstructure within the case. (b) Corresponding energy
dispersive x-ray spectrum showing the elemental composition of the
steel
shown later, fracture in this region has propagated by an
intergranular mechanism and MnS inclusions could have
enhanced the notch sensitivity of the shaft. Shiny faceted
fracture in the core (region B) such as that observed in
Fig. 3b typifies a cleavage mechanism. Further confirmation of these crack propagation modes is provided by the
SEM observations shown later. Also, it is shown that the
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Fig. 5 Analysis of MnS inclusions observed in the case (etched
specimens). (a) Backscattered electron images showing the inclusions
as viewed at different magnifications (regions of dark contrast in the
vicinity of the particles could be due to preferential etching and/or
microcracks). (b) Energy dispersive x-ray spectrum derived from
MnS particle as indicated by the arrow
Fig. 6 Secondary electron
images showing the
morphology of fracture surface
corresponding to the case (asreceived condition). (a) Gross
morphology of the fracture
surface. (b) The fracture surface
as viewed at high magnification
showing crack propagation by
an intergranular mechanism
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J Fail. Anal. and Preven. (2013) 13:353–358
observed difference in fracture behavior is caused by variation in microstructure of the case and core resulting from
the hardening treatment.
Table 1 shows that the measured chemical composition
of the shaft material is consistent with the nominal composition of AISI 4140 as-specified. The microstructure of
the case was observed to be typical of martensite as shown
in the example of Fig. 4a indicating that the shaft was
quenched and not tempered while the core was allowed to
cool slowly. A further confirmation was provided by the
observed fracture mode and microhardness measurement as
shown later. Figure 4b shows a corresponding energy dispersive spectrum illustrating the elemental composition of
the steel consistent with the data of Table 1 noting that the
small concentration of Mo (0.2 wt.%) is below the detection limit. Another microstructural feature was the presence
of elongated particles of MnS as demonstrated in the
example of Fig. 5. The regions of dark contrast surrounding the particles could be due to preferential etching as a
result of high local strain and/or microcracks. Such inclusions can act as centers for attracting dislocations leading
to formation of microcracks [9, 10]. Also, it was shown that
elongated particles of MnS promote hydrogen cracking of
steels particularly if the hardness exceeds Rc 25 [11]. As
shown later the hardness within the case was about Rc 58.
Secondary electron images showing typical morphology of
the fracture surface corresponding to the case are shown in
Fig. 6. It is evident that crack propagation within the case
occurred by an intergranular mechanism, which could be
related to a combination of large overload and brittle case
as further shown below.
Figure 7 illustrates a typical microstructure of the core
showing a mixture of pearlite and ferrite. Energy dispersive
spectra derived from the core were the same as that shown
in Fig. 4b. Corresponding morphological features of the
J Fail. Anal. and Preven. (2013) 13:353–358
357
Fig. 7 Backscattered electron image showing typical microstructural
features within the core (a mixture of pearlite and ferrite; etched
specimen)
fracture surface are shown in Fig. 8 consistent with crack
propagation by a cleavage mechanism as reflected by the
cleavage steps indicated by the arrows. This could have
occurred due to the high strain rate experienced by the
impact of overturning the vehicle.
Figure 9 summarizes the results of measuring the
microhardness (HV10) across the case and into the core,
and the equivalent Rc values. As can be seen, the hardness
near the surface exceeded 650 HV10 approaching Rc 58,
which is considerably higher than the expected hardness of
500–550 HV. This appeared to be consistent with the
characteristic microstructure within the case (Fig. 4a) as
well as the fracture behavior (Fig. 6).
The above results suggested that the most probable
cause of failure was improper heat treatment of the shaft
resulting in a case microstructure with poor ductility susceptible to brittle fracture. It is apparent that the shaft was
not subjected to a proper tempering treatment. Although it
was rather difficult to reconstruct the exact sequence of
events leading to fracture of the shaft, it is possible that it
was initiated in the brittle case by large overload leading to
overturning of the vehicle. However, another possibility is
hydrogen-induced cracking due to water pickup and some
rust at the surface, but this could not be confirmed.
Although the final facture could have occurred by the
impact of overturning and the associated high strain rate,
the possibility that overturning of the vehicle was the sole
cause of the fracture could not be ruled out.
Conclusion
It is concluded that the root cause of the problem was
embrittling the case by improper heat treatment. Although
Fig. 8 Secondary electron images showing the morphology of
fracture surface corresponding to the core (as-received condition).
(a) Lower magnification image showing typical cleavage fracture. (b)
Higher magnification image illustrating cleavage steps as indicated by
the arrows
Fig. 9 Vicker’s microhardness profile (HV10) across the case and
into the core and the equivalent Rc values (polished specimen)
it was difficult to determine the fracture initiation mechanism, it could have resulted by large overload, but the
possibility of cracking due to hydrogen generated by rust
and water pickup could not be ruled out. However, final
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fracture could have resulted from the impact of overturning
the vehicle. Another possibility is that overturning of the
vehicle was fully responsible for the fracture.
Acknowledgments It is a pleasure to acknowledge the continued
support of King Fahd University of Petroleum and Minerals.
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