Spalling of a Ball-Peen Hammer
Handbook of Case Histories in Failure Analysis,
Vol 2, K.A. Esakul, Ed., ASM International, 1992
A carbon steel ball-peen hammer ejected a chip that struck the user's eye. Failure occurred when two hammers were struck together during an attempt to free a universal joint from an automotive drive shaft. Two samples were cut from the face of the hammer one through the chipped area on the chamfer and the other from the undamaged area on the chamfer. The shape and texture of the fracture surfaces were typical of spalling. The fracture was conchoidal and exhibited a complete lack of plastic deformation. White etching bands that intersected the face and chamfer were revealed during metallographic examination. Fracture occurred through a white band. Failure was attributed to formation of envelopes of untempered martensite under the chamfer that ruptured explosively during service.
Automotive Automotive; Drive shaft
(Nonresulfurized carbon steel)
Spalling wear; Brittle fracture
A carbon steel ball-peen hammer ejected a chip that struck the user's
Circumstances leading to failure
The user was attempting to free a universal joint from an automotive
drive shaft. The shaft was secured in a vise and two ball-peen hammers were
being used, one face up and the other face down. The hammers were struck together,
causing sparking of the hammer faces and ejection of a chip from the striking
The hammer was manufactured to conform to ANSI B173.2-1978, “Safety
Requirements for Ball Peen Hammers.”
Two samples were cut from the face of the failed hammer, one through
the chipped area on the chamfer and the other from an unchipped area on the
chamfer. Both were mounted in cold-setting resin to show the chamfer (and
chipped area), face, and bell in cross section. A third piece was cut from
the cheek for chemical analysis.
Visual Examination of General Physical Features
shows the chipped
area at the chamfer of the hammer and the ejected chip. The shape and texture
of fracture surfaces were typical of those on spalled hammers. The fracture
was conchoidal and exhibited a complete lack of plastic deformation. These
features were caused by the underlying microstructure.
Fig. 1 The face and bell of the ball-peen hammer, showing the area where
the chip was ejected. The chip in shown in the insert at the right. The fracture
was conchoidal and brittle in appearance.
Testing Procedure and Results
The general microstructure of the hammer material under the face was
tempered martensite. Of greater significance, however, was the presence of
white etching bands that intersected the face and chamfer.
show a network of these bands found in the
specimen taken from the undamaged area. Note that a portion of one of the
bands is cracked.
Fig. 2 White etching bands of untempered martensite that intersected the
face and chamfer of the hammer in a zone away from the chipped area. A crack
is present in one of the bands. 34.65×.
shows a cross
section of the chipped area. It is evident that the fracture occurred through
a white band.
Fig. 3 Micrograph taken at the chipped area. Fracture occurred through a
white band. 315×.
The results of chemical analysis are presented in
. The hammer material conformed to the chemical
requirements for ball-peen hammers (ANSI B173.2-1978).
Table 1 Results of chemical analysis
Microhardness tests (Knoop, 1 kg load) conducted
on the metallographic samples showed that the hardness of the material under
the face (converted to the Rockwell C scale) was 51 to 52 HRC, well within
the allowable range of 45 to 60 HRC. Microhardness tests were also performed
on the white bands and the material immediately adjacent to them. These tests
were conducted using lighter loads of 200 and 500 g. The hardness of the bands
was 64 to 65 HRC, and the adjacent matrix material had a hardness of 42 to
The mechanism of chipping is generic to striking (or struck) tools.
When a hardened steel tool is struck against a material as hard or harder
than itself, conchoidal envelopes (appearing as white bands when viewed in
cross section) of impact-induced untempered martensite can form at the striking
surface, particularly under the chamfer. This envelope of brittle material
can fracture explosively, expelling a chip either as the result of a tensile
wave reflected back from the free surface of the hammer head or by a shock
wave caused by a subsequent impact.
The mechanism of formation of white bands has been described by investigators
as zones of intense adiabatic shear. Zener and Holloman (Ref 1
) proposed that the bands resulted from high strain rates
that caused shear instability in an otherwise uniformly straining solid. These
high strain rates can cause deformation to change from isothermal to adiabatic,
resulting in an instability where deformation cannot be homogeneous. They
concluded that the white bands were martensite formed by the high temperatures
resulting from the concentration of shear strain in the band regions, which
changed the structure to austenite. The austenite was then rapidly quenched
by the surrounding material.
This phenomenon was probably responsible for the hammer failure. The
high hardness of the white bands, the evidence of incipient cracking in the
bands, and the fracture path in the chipped area through a white band all
support this conclusion. Additional proof that formation of the white martensite
was caused by localized heating is provided by the lower hardness values of
the material adjacent to the bands, which indicate further tempering of these
Conclusion and Recommendations
Most probable cause
When the two hammers were struck together, spalling occurred. Because
the hammers possessed the same hardness, envelopes of untempered martensite
were formed under the face and chamfer. One of these envelopes fractured,
ejecting a chip.
The applicable standard sets forth comprehensive safety requirements
and limitations of use, one of which states that a ball-peen hammer should
never be struck against hard or hardened objects. All hammers must be marked
with a warning to that effect as well as a caution to wear safety goggles.
The warnings notwithstanding, in most cases it is difficult to determine the
hardness of an object without hardness data. In this instance, however, the
user should have known that both hammers were of similar hardness and should
have worn safety glasses. It should also be stressed that in this instance
the hammer had been repeatedly used to strike hard objects. This is indicated
by the damage present on the hammer face. Such conditions of use greatly increase
the likelihood of this type of failure.
Zener and Holloman,
J. Appl. Phys.,
Vol 15, 1944, p22.
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