Hey All,
After reading the exchanges on the question of heating parts I thought I
would provide a little more information to help each of you decide for
yourselves whether or not you want to use this technique. Knowledge is
power as they say.
The question of whether heating a part can be unsafe depends entirely on
the material being heated and the way it is done. Steel alloys all undergo
heat treatment (some by default) in the manufacturing process in order to
obtain the correct properties for the application. These properties include
ultimate strength, yield strength, hardness, ductility etc. all of which
affect the ability of the part to handle the stresses to which it is
subjected. Those properties are obtained by heat treatment processes such
as quenching, tempering, annealing, and combinations and variations of
these. In addition, some metal parts will have other treatments such as
cold working, case hardening or metallurgical surface treatments. The two
primary factors which will affect the material properties when heating are
the maximum temperature reached and the cooling rate. Keep in mind though
that special surface treatments for hardness or corrosion protection can
also be affected by oxidation or other chemical interactions while hot.
The material phase of iron based alloys begin to change at what is called
the critical temperature which is a little over 1300F. The material
properties, however, for fully tempered alloys can be affected by
temperatures as low as 400F. The only time that you could heat a part to
high temperatures without changing its designed properties are in cases
where the original heat treatment is emulated by what you are doing. For
example if that tie rod end you are heating was manufactured by hot forging
followed by air cooling and without any further treatment and Joe Mechanic
heats it and allows it to air cool he probably won't significantly affect
its material properties. Likewise for a part that was machined, annealed at
high temperature and then allowed to air cool.
However for parts that have undergone specific treatments for material
properties, e.g. heat treatments, cold working, metallurgical surface
treatments etc. heating to a high temperature will affect material
properties and can potentially result in a reduction in service life or
outright component failure.
The good news is that many automotive components fall into the first
category and are not adversely affected by heating.
The bad news is that its not obvious which components they are.
The mixed news is that design factors of safety for automobiles are generous
and thats probably why you don't see more failures.
For myself, if I think heat is needed I will use it. But I only heat the
part (using propane torch) to the lowest possible temperature necessary for
removal. If I suspect that the part is critical, i.e. failure could cause
significant damage or injury, is a heat treated component and a temperature
was reached greater than about 500F I will replace the part with a new one.
Everyone needs, and will, decide for themselves what the prudent approach
is. I hope this will give you a little better understanding of what the
real issues are.
By the way - there have been some excellent and very helpful postings lately
to which I say thank you to those who have contributed them. I keep a file
of those tips so your contributions are both meaningful and lasting.
Andrew
#614 and #2755
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