Spalling of a Ball-Peen Hammer

Carmine D'Antonio, Department of Metallurgy and Materials Science, Polytechnic University


From: Handbook of Case Histories in Failure Analysis, Vol 2, K.A. Esakul, Ed., ASM International, 1992

Abstract: 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.

Keywords: Automotive Automotive; Drive shaft

Material: Fe-0.85C-0.71Mn (Nonresulfurized carbon steel)

Failure types: Spalling wear; Brittle fracture


Background

A carbon steel ball-peen hammer ejected a chip that struck the user's eye.

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 hammer.

Pertinent specifications

The hammer was manufactured to conform to ANSI B173.2-1978, “Safety Requirements for Ball Peen Hammers.”

Specimen selection

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

Figure 1 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.
Figure 1

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

Metallography

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. Figures 2 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.
Figure 2

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×.

Figure 3 shows a cross section of the chipped area. It is evident that the fracture occurred through a white band.
Figure 3

Fig. 3  Micrograph taken at the chipped area. Fracture occurred through a white band. 315×.

Chemical analysis/identification

The results of chemical analysis are presented in Table 1. The hammer material conformed to the chemical requirements for ball-peen hammers (ANSI B173.2-1978).

Table 1   Results of chemical analysis

Element
Composition, %
Chemical requirement
Hammer
(ANSI 173.2–1978)
Carbon
0.85
0.45–0.88
Manganese
0.71
0.30–0.90
Phosphorus
0.020
0.04 (max)
Sulfur
0.0.26
0.05 (max)
Silicon
0.24
0.10–0.30

Mechanical properties

Hardness. 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 46 HRC.

Discussion

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 zones.

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.

Remedial action

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.

Reference

  1. Zener and Holloman, J. Appl. Phys., Vol 15, 1944, p22.

Related Information

G.F. Vander Voort, Failures of Tools and Dies, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 1986, p 563–585
W.T. Becker and D. McGarry, Mechanisms and Appearances of Ductile and Brittle Fracture in Metals, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 587–626
D.L. McGarry, G.W. Powell and C. Lonsdale, Spalling from Impact Events, Failure Analysis and Prevention, Vol 11, ASM Handbook, ASM International, 2002, p 975–988