shot peening residual stress - Metal Improvement Company

C H A P T E R T H R E E
MANUFA CTURING PROCESSES
18
G R I N D I N G
Typically, grinding induces residual tensile stress
as a result of localized heat generated during
the process. The metal in contact with the
abrasive medium heats locally and attempts to
expand. The heated material is weaker than the
surrounding metal and yields in compression.
Upon cooling the yielded metal attempts to
contract. This contraction is resisted by the
surrounding metal resulting in residual tensile
stress. Residual tensile stress of any magnitude
will have a negative effect on fatigue life and
resistance to stress corrosion cracking.
Figure 3-2 graphically depicts residual tensile
stress generated from various grinding processes [Ref 3.9]. A 1020, 150-180 BHN carbon steel (with and
without weld) was ground abusively and conventionally. Figure 3-2 shows that the grinding processes
resulted in high surface tension with the abusive grind having a deeper (detrimental) layer of residual tension.
Shot peening after grinding will reverse the residual stress state from tensile to compressive. The beneficial
stress reversal is similar to that from shot peening welded regions in a state of tension.
Figure 3-3 Plating Micro-Cracks
P L A T I N G
Many parts are shot peened prior to chrome and electroless
nickel plating to counteract the potential harmful effects on
fatigue life. Fatigue deficits from plating may occur from the
micro-cracking in the brittle surface, hydrogen embrittlement
or residual tensile stresses.
Figure 3-3 is a 1200x SEM photograph showing a network of
very fine cracks that is typical of hard chrome plating [Ref 3.10].
Under fatigue loading, the micro-cracks can propagate into the
base metal and lead to fatigue failure.
When the base metal is shot peened, the potential for fatigue
crack propagation into the base metal from the plating is
dramatically reduced. Figure 3-4 illustrates this concept and
Figure 3-4 Compressive Stress Resists assumes dynamic loading on a component.
Micro-Crack Growth
The graphic on the left shows the micro-cracking propagating
into the base material. When shot peened, the graphic on the right shows the compressive layer preventing
the micro-cracking from propagating into the base material.
Shot peening prior to plating is recommended on cyclically loaded parts to enhance fatigue properties. For
parts that require unlimited life under dynamic loads, federal specifications QQ-C-320 and MIL-C-26074 call for
shot peening of steel parts prior to chrome or electroless nickel-plating. Other hard plating processes such as
electrolytic nickel may also lower fatigue strength.
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Figure 3-2 Residual Stresses from Grinding