shot peening residual stress - Metal Improvement Company

C H A P T E R N I N E
THERMAL F A T IGUE & HEA T EFF ECTS
32
E F F E C T S O F H E A T
Caution should be exercised when parts are
heated after shot peening. The amount of
compressive stress that is relieved is a
function of temperature, time and material.
Figure 9-1 demonstrates the increasing stress
relief effect of increasing temperature on shot
peened Inconel 718 [Ref 9.1]. Inconel 718 is
commonly used in high temperature jet engine
applications.
Stress relief temperature is a physical property
of the material. Figure 9-2 depicts several
materials and the temperatures at which
residual stresses will begin to relax. Many
shot peened fatigue applications operate
above these lower temperature limits as
fatigue benefits are still realized providing the
operating temperature does not approach the
stress relief temperature of the material.
The following are examples where shot peening
followed by heat treatment is commonly
incorporated into manufacturing.
• Springs – It is common to
perform a post-bake
operation for improvements
in spring performance.
Please see Chapter 5 –
Torsional Fatigue
• Plated Parts – It is
common for shot peening
prior to plating. Peening
is called out for fatigue
benefits and resistance to
hydrogen embrittlement.
Please see Chapter 3 –
Manufacturing Processes.
Plating commonly involves
a hydrogen bake operation at
350-400 °F (175-205 °C) for
several hours.
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Figure 9-1 Residual Stress Patterns in Shot Peened
Inconel 718 Alloy after 100-Hour Exposure
to Elevated Temperatures
Figure 9-2 Approximate Temperature at Which
Compressive Stresses Begin to Dissipate