General feature: Residual stress systems
Stress relief pre-heat treatment vs. straighten/ flatten post-heat treatment M
ost engineering components have a residual stress system. Such systems are inherent in bar stock, plates, and, to some extent, in forgings and castings. While the residual stress system can be modified by removal of material, further forming of the raw material, and heat treatment processes, modification invariably leads to distortion or cracking. “Distortion may be rectified by straightening or flattening operations. However, there is always a risk of breakage or cracking with such operations,” cautions Roger Haw, Managing Director of Flame Hardeners Ltd. Problems associated with changes of residual stress systems are often found in the heat treatment of rollers, shafts,
Problems arising from distortion during
es the risk of cracking. It is necessary to
and pins – together with items such as
treatment can be minimised by stress-
heat the component to a temperature in
gear rings, bearing rings, guide bars,
relieving at intermediate machining stag-
the range of 180-250°C and to keep it at
machine tool beds and press brake
es, and correct jigging during heat treat-
such a temperature for as long as possi-
tools.
ment. This can eliminate the requirement
ble. Larger objects may require several re-
for straightening after heat treatment.
heats during the straightening operation.
Modern machining techniques have
It’s a relatively low-cost heat treatment
led to increased rates of metal removal
operation, undertaken under controlled
“Over many years of processing compo-
and, in many cases, use of bar stock to
parameters and often giving a predicta-
nents that may distort, we have reached
produce complex machined parts from
ble response, whereas straightening and
the conclusion that it is far more econom-
which 50 percent or more of the original
flattening are risky operations due to the
ical to stress-relieve at an intermediate
material volume has been removed.
high risk of cracking or breakage.
machining stage prior to heat treatment
“Cases have been seen where, for the
“Economically, the cost of stress-reliev-
it is to straighten or flatten following heat
benefit of expedient delivery of cylin-
ing can be easily estimated and includ-
treatment. A component that cannot be
drical items, such as rollers and piston
ed in the overall estimate of cost for the
straightened or flattened is just a piece of
rods, available bar stock has been used
component,” continues Haw. “The cost
scrap – and often a very expensive piece
which can be 20 percent greater than the
of straightening is not easily estimated
of scrap,” Haw concludes.
diameter required. Gear shafts, which
as the degree of any distortion to be cor-
may have been made from forgings
rected cannot be easily assessed.”
to minimise or eliminate distortion than
some years ago, are now manufactured
Your specialist heat treatment contractor can often assist you at the design stage
from bar stock and diameters of the
For larger diameters of rollers, piston rods,
by recommending suitable treatment
length of a shaft can vary by as much as
guide bars etc., warm straightening is the
routes.
200 percent,” Haw elaborates.
most desirable process, as this makes actual straightening easier and minimis-
8 dpaonthenet.net
www.flamehardeners.co.uk
