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What is the difference between tempering and stress relief?Helpful Member!(3) 

mechanicalboy (Mechanical) (OP)
28 Jun 05 14:03
I was doing some research on materials and got real confused. The terms tempering and stress relief seem to be used to describe the same process. I am wondering what is the difference between the two and maybe get pointed in the right direction as far as finding advantages and disadvantages of each.

Thanks
James
Helpful Member!  metengr (Materials)
28 Jun 05 14:16
Here is a reference to an excellent glossary for definitions of engineering terms that might be of help to you;

http://nsm.eng.ohio-state.edu/Stamping_Glossary/html/t.html
EdStainless (Materials)
28 Jun 05 17:28
There is an ASM glossary also.  That I can't find the link to right now.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

Helpful Member!(2)  AlanD (Materials)
28 Jun 05 17:29
From an old copy of "Definitions of Metallurgical Terms"
by the The Australian Institute of Metals - 1971.

Stress Relieving: Heating a metal to, and if necessary holding at, some temperature generally below the recrystallisation range, followed by uniform cooling, for the sole purpose of removing internal stresses.
Other treatments, eg.annealing, tempering, blueing etc., whilst applied primarily to bring about changes in structure or propertie, may also remove internal stresses.

Tempering: Re-heating a quenched steel to some temperature below the transformation range and, if necessary holding at that temperature followed by cooling at a suitable rate.
THis process is usually applied for the purpose of producing desired combinations of hardness, ductility and tooughness, and while correctly applied to the re-heating of normalised steels, it should not be used to described the heating of mechanically worked metals for the sole purpose of removing stresses.
The use of the term "drawing" to describe tempering is not favoured.  
TVP (Materials)
28 Jun 05 22:15
James,

I just checked the link provided by metengr, and I have to say that I am quite disappointed in the definitions that were listed.  These two terms are often misused, especially by non-technical people, and the ERC/NSM site did not use the proper definitions.  The description provided by AlanD is the correct usage.  Also keep in mind that translation from other languages can also be a problem when trying to define these words.
metengr (Materials)
28 Jun 05 22:58
TVP;
I had received this link from an engineering associate some time ago that attended the Ohio State University (which has a reputable engineering program). I glanced over the references when I received the link, which seemed to be rather comprehensive. I will inform him of your comments.
metengr (Materials)
28 Jun 05 23:22
Per the ASM Handbook, Desk Edition, Glossary of Terms and Engineering Data;

tempering; In heat treatment, reheating hardened steel to some temperature below the eutectoid temperature to decrease hardness and/or increase toughness.


stress relief heat treatment; uniform heating of a structure or a portion thereof to a sufficient temperature to relieve the major portion of the residual stresses, followed by uniform cooling.

stress relieving; heating to a suitable temperature, holding long enough to reduce residual stresses, and then cooling slowly enough to minimize the development of new residual stresses.
TVP (Materials)
29 Jun 05 9:03
metengr,

Thanks for the posting the ASM definitions.  I have a great deal of respect for Prof. Altan and the program at the ERC/NSM, which is why I was disappointed to see the poor definitions.  I think the most comprehensive definition for tempering would also include a description of the microstructure evolution: changing from lath martensite to tempered martensite (essentially ferrite + carbides that are too small to be resolved by optical microscopy).
BTIGUY (Industrial)
5 Jul 05 12:16
James,
I am not a metallugist however I have worked in the stress relief industry for decades with the sub-harmonic vibration stress relief system.  It is confusing because many people use stress relief interchangeably with other very specific processes.  Simply put, I see the difference between stress relief and tempering as this - the purpose of each are different:
Stress relief is really for the purpose of one of 3 things:
1. control distortion immediately following machining,
2. control delayed distortion, and/or
3. reduce premature cracking.
Every "good" stress relief process should be able to "consistently" obtain.
IF "heat" is used for the SR process you also have may have some metallugical changes in hardness, ductility, toughness, and strength.  These may be desireable or undesireable.
Tempering in my mind is for the purpose to soften up the real hard, brittle areas of a weldment without causing much softening or reduction of strength to the rest of the part.  Heat is required, which is considerably lower than that of a stress relief.  Tempering can effect a partial stress relief.
Let me know if you need "stress relief" benefits.
BTIGuy

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