shot peening residual stress - Metal Improvement Company

C H A P T E R T W O
RESPONSE OF METALS
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Shot peening has proven to be effective in restoring most of the fatigue strength lost due to decarburization
[Ref 2.7]. Because the decarburized layer is not easily detectable on quantities of parts, peening can
insure the integrity of the parts if decarburization is suspected. If a gear that is intended to have a high
surface hardness (58+ HRC) exhibits unusually heavy dimpling after peening, decarburization should
be suspected.
Decarburization is often accompanied with the undesirable metallurgical condition of retained austenite.
By cold working the surface, shot peening reduces the percentage of retained austenite.
A p p l i c a t i o n C a s e S t u d y
REDUCTION OF RETAINED AUSTENITE - 5120 CARBURIZED STEEL,
SHOT PEENED AT 0.014" (0.36 mm) A INTENSITY
Retained Austenite
(Volume %)
Depth (inches) Depth (mm) Unpeened Peened
0.0000 0.00 5 3
0.0004 0.01 7 4
0.0008 0.02 14 5
0.0012 0.03 13 6
0.0016 0.04 14 7
0.0020 0.05 14 7
0.0024 0.06 15 8
0.0028 0.07 15 9
0.0039 0.10 15 10
0.0055 0.14 12 10
[Ref 2.8]
A U S T E M P E R E D D U C T I L E I R O N
Improvements in austempered ductile iron (ADI) have allowed it to replace steel forgings, castings, and
weldments in some engineering applications. ADI has a high strength-to-weight ratio and the benefit of
excellent wear capabilities. ADI has also replaced aluminum in certain high strength applications as it is at
least 3 times stronger and only 2.5 times more dense. With the addition of shot peening, the allowable
bending fatigue strength of ADI can be increased up to 75%. This makes certain grades of ADI with shot
peening comparable to case-carburized steels for gearing applications [Ref 2.9].
C A S T I R O N
There has been an increased demand in recent years for nodular cast iron components that are capable of
withstanding relatively high fatigue loading. Cast iron components are often used without machining in
applications where the cast surface is subject to load stresses. The presence of imperfections at casting
surfaces in the form of pinholes, dross or flake graphite can considerably reduce the fatigue properties of
unmachined pearlitic nodular irons. The unnotched fatigue limit may be reduced by as much as 40%,
depending on the severity of the imperfections at the casting surface.
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