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CASE HISTORY—PEER-REVIEWED
Spring Fatigue Fractures Due to Microstructural Changesin Service
J. Maciejewski • B. Akyuz
Submitted: 22 November 2013 / Published online: 31 January 2014
� ASM International 2014
Abstract Multiple in-service fractures of torsion springs
were experienced in the same system, which was the sup-
port assembly to the electrical pickup for an electric-
powered vehicle, similar to a subway rail car or electric
trolley car. Scanning electron microscopy and metallo-
graphic examinations determined that the fractures initiated
due to electric arc damage. Intergranular quench cracks at
the spring surface through the transformed untempered
martensite provided crack initiations for fatigue that
propagated during operation.
Keywords Spring fracture � Fatigue � Stray current �Arc damage � Untempered martensite � Quench cracking �Torsion spring
Introduction
Multiple in-service fractures of torsion springs were
experienced in the same system, which was the support
assembly to the electrical pickup for an electric-powered
vehicle, similar to a subway rail car or electric trolley car.
The remainder of the system details is withheld to protect
client confidentiality. The springs were round-wire helical
construction, made from patented music wire (carbon steel)
that was subsequently electro-galvanized.
Analysis
The fracture surfaces consistently exhibited a flat fracture
transverse to the wire at the spring outer diameter (OD) that
extended across half the wire diameter, with the remainder of
the fracture approximately parallel to the wire longitudinal
direction (Fig. 1). This two zone fracture morphology is
typical of fatigue of a spring, wherein the transverse zone is
the fatigue zone and the longitudinal fracture is ductile
overload of the remaining section [1]. However, the origins
at the OD of the springs were unusual, since it is well known
that the highest stress location on a cycling spring is the inner
diameter (ID) [2, 3]. This is the reason fractures of springs at
the end of their fatigue life normally initiate on the ID of the
coil. Therefore, a surface defect or other feature was sus-
pected that would shift the fatigue initiation site to the OD.
A scanning electron microscope (SEM) was used to
examine the samples in more detail at up to 5,0009
magnification. The flat fracture zones at the OD of the
springs exhibited thumbnail-shaped origins with radial
marks extending into the wire, indicating the OD was
indeed the fracture origin (Fig. 2). Detailed examination of
the origins revealed intergranular fracture between equi-
axed, presumably prior austenite, grains that exhibited
decreasing grain size inward from the surface (Fig. 3).
At first, these features would suggest some form of
embrittlement (i.e., possibly hydrogen embrittlement due to
the electroplating operation); however, embrittlement of
prior austenite grain boundaries should not be possible with
cold-drawn music wire. In music wire the ferrite grains and
pearlite colonies are severely deformed, and any prior
austenite grain boundaries are destroyed during the draw-
ing operation. Indeed, longitudinal metallographic sections
of the wires revealed highly deformed ferrite and pearlite,
the normal and expected microstructure for cold-drawn
J. Maciejewski (&)
Materials Testing, Applied Technical Services, Inc., Marietta,
GA, USA
e-mail: [email protected]
B. Akyuz
Failure Analysis and Metallurgy, Applied Technical Services,
Inc., Marietta, GA, USA
123
J Fail. Anal. and Preven. (2014) 14:148–151
DOI 10.1007/s11668-014-9783-9
wire (Fig. 4). However, the longitudinal sections through
the fatigue cracks exhibited a white-etching microstructural
phase at the origins (Fig. 5).
This microstructural feature was untempered martensite,
which could only result from a highly localized, high tem-
perature event, and self-quenching. This is consistent with an
electric arc from stray current in the system. Examination of
the wire surfaces at the OD of the springs did show areas of
remelted material at the fatigue zone origins (Figs. 6, 7).
Stray current electrical discharge damage can occur due to
the component constituting a transmission pathway to
ground for welding currents, improperly grounded machin-
ery electrical energy, or even lightning strikes.
Untempered martensite is a hard, brittle phase with
significant residual stresses that can often lead to inter-
granular cracking immediately after transformation (i.e.,
Fig. 1 Typical spring fracture, showing flat transverse fracture at the
OD (arrow) and adjacent longitudinal fracture
Fig. 2 SEM image showing the thumbnail zone at the crack initiation
site on the OD
Fig. 3 Detail of a fracture origin, showing intergranular fracture and
decreasing grain size inward from the surface
Fig. 4 Longitudinal microstructure of a typical spring wire, exhib-
iting highly deformed ferrite (white) and pearlite (dark), Nital etch
Fig. 5 Longitudinal section through a fatigue crack origin, showing
white-etching untempered martensite at the initiation, as well as
highly deformed ferrite and pearlite. The longitudinal direction is
indicated
J Fail. Anal. and Preven. (2014) 14:148–151 149
123
quench cracking) and cracking from loads in service [4].
The reaustenitized volume caused by stray current arc
damage also explains the varying prior austenite grain size
observed in Fig. 3. During the event there would be a
significant temperature gradient in the arced zone resulting
in the most grain growth at the hottest point (the exterior
surface). The temperature gradient is estimated by: at least
1500 �C (melting) at the surface to \500 �C (no recrys-
tallization) at a point 0.081 mm deep beneath the surface
(from Fig. 5), which is [12,300 �C/mm.
The remainder of the transverse fracture zone area
exhibited a feathery or mottled morphology (Fig. 8),
commonly observed in fatigue of springs. This morphology
is a result of somewhat microstructure control of the
fracture [5], the transverse microstructure of the cold-
drawn wire being reproduced on the fracture surface
(compare Figs. 8 and 9).
Fig. 6 SEM image of a remelted, arc-damage area at one fracture
origin (arrow)
Fig. 7 Detail of one remelt area at a fatigue origin
Fig. 8 Detail of the flat fracture surface outside of the origin
thumbnail, exhibiting a mottled or feathery morphology
Fig. 9 Typical transverse microstructure of a patented music wire,
exhibiting folded and deformed ferrite grains (white) and pearlite
colonies (dark), Nital etch
Fig. 10 SEM image of typical longitudinal final overload region,
showing a ‘‘woody’’ dimpled morphology
150 J Fail. Anal. and Preven. (2014) 14:148–151
123
The longitudinal fracture zones on the springs exhibited
ductile microvoid dimples with a ‘‘woody’’ appearance
(Fig. 10), typical of ductile overload transverse to the
rolling direction of elongated structures [6]. This is also a
microstructure-controlled fracture morphology.
Conclusion
The fractures of the springs initiated in a brittle mode by
intergranular cracking through untempered martensite
volumes at the surfaces caused by in-service stray current
arcing. The fractures propagated by fatigue transverse to
the spring wire axes, due to the normal cyclic loading in
service. Finally, the remaining cross-sectional area failed in
overload, generating the longitudinal fracture planes.
It is known that a significant portion of the high cycle
fatigue life is spent in initiation [7, 8]. Therefore, the
springs experienced significantly reduced service lives due
to the instantaneous crack initiations by electric arc dam-
age-induced intergranular cracking.
Prevention of further failures and elimination of stray
currents required a structural and electrical assessment of
the design and as-assembled components.
References
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