Solid Height - Spring Manufacturers Institute

Figure 4: Hypothetical relationship depicting the amount of
stress relief vs. time for steel springs.
Stress relaxation can be critical for some hightemperature
applications. Stress relieving cycles can
be optimized to minimize relaxation. An old rule of
thumb is that the stress relaxation temperature
should be at least 100°F higher than the maximum
service temperature.
Practical Application
Springs are typically stress relieved using batch
ovens or in-line conveyer furnaces. The cycle time for
in-line or high-heat-transfer furnaces can be shorter
than for the batch furnaces. This is accomplished by
increasing the heat transfer in the furnace and by
taking advantage of the time/temperature thermal
relationship evident in particular grades of steel.
Larson and Miller created a mathematical model
for this effect in order to produce valid test results
that substituted long test times for elevated test
temperatures. Their model proposed that many different
time/temperature combinations existed that
could produce a similar thermal effect. For stress
relieving steel springs this relationship may not be
perfect, but it can be used as a good starting point
to help predict the proper stress relief cycle. Most
stresses are relieved in the first minutes after the
steel is at temperature. For one particular temperature,
a theoretical relationship is shown in Figure
4, above.
One of the basic assumptions of the Larson-
Miller parameter is that no structural changes occur
in the metal over the range of temperatures being
examined. Certain temperatures on some alloy systems
may produce a significant metallurgical change
that may affect the slope of this relationship. When
this situation occurs, experimental data should be
used [3]. For alloys like 17-7PH that can be age hardened
at a very specific temperature, the Larson Miller
equation would not apply for temperature ranges
around 900°F.
In general, it’s a good idea to stress relieve all
springs in a timely manner, and many springmakers
do this. However, to properly stress relieve springs
fabricated from different materials or containing
severe bends, it is important to realize that one size
doesn’t fit all. Be on the lookout for that occasional
spring that encounters “a few” failures every year.
Does this spring have a low index? Is the material
chrome silicon? For torsion springs, does it tend to
crack at the bends in the arms before plating? If you
can answer “yes” to any of these questions, consider
reviewing the stress-relief process to ensure that it
fits the application.
The author thanks Loren Godfrey, president of
Colonial Spring in Bristol, CT, for consulting on this
article.
For further information on the stress relief of
springs, refer to Mark Hayes’ “Cautionary Tales”
Parts VIII and IX, which appeared in the December
2001 and February 2002 issues, respectively.
References
1. Parrington, Ron, Hydrogen Damage and
Embrittlement, ASM Handbook, Volume 11 Failure
Analysis and Prevention, Editors: W. Becker and R.
Shipley, ASM International 2002, pp. 809-811.
2. Dieter, G.E., Mechanical Metallurgy, Sanjeev
Rao, McGraw-Hill, 1986, pp. 201-203.
3. Dieter, G.E., Mechanical Metallurgy, Sanjeev
Rao, McGraw-Hill, 1986, pp. 461-462. v
Don’t Let OSHA Play
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Hidden Hazards can be Dangerous and
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SPRINGS July 2006 43