Brass - Stress Relieving



[Page created Dec. 25, 2012, last update Jan. 5, 2013]
[Added additional references Feb. 22, 2013]]

Relieving Stress in Brass by Heat Treating - a Small Experiment

While milling some extruded brass for the purpose of making tapered gibs for my 7x12 minilathe, I experienced the well known problem of the brass warping as the milling removed stresses created by the extrusion process. Since I messed up the second tapered gib I was making and had to make yet another one, I decided to take the opportunity to perform a small experiment. I wondered if heat treating the brass before milling might relieve the stresses in the metal before milling. A quick bit of Internet research showed quite a bit of discussion on this topic, and a fair amount of speculation, but nobody actually seems to have tried it (I find this hard to believe, so I assume this has been done before but I just haven't been able to find the links). I am aware, of course, of the recommendation that in milling a long thin piece, it is advised to mill a small amount from one side, then flip the piece over and mill a small amount of the other side, and so on, so as to evenly relieve the stress so that the stresses cancel out and warping is eliminated or minimized. However, in milling a taper, it is necessary to mill one side extensively with the result that there is significant warping.

Since I had to make another gib anyway, I figured I had nothing to lose by treating a small experiment with heat treating. The little bit of information I was able to find on the web suggested that a temperature of 400 - 500 °F for about an hour would do the job. I put a 6 inch length of extruded brass into a countertop "toaster oven," set the temperature to maximum (450 °F) and let it run for 90 minutes, then shut it off and let it cool down for 30 minutes.



The brass is 360 extruded brass square (0.375").

The picture to the left shows the brass before (below) and after (above) heating. It's difficult to tell from the picture, but the brass was slightly darkened by the heating process. The darkening was very minor, and would be machined off in any case.

In preparation for making the tapered gib, I milled .020 from one side of the brass strip, and then checked it against a straight edge for warping.








To the left is the brass piece after the initial milling. Comparison to a straight edge shows no discernible warping. The photo does not show it, but I held the brass and straight edge in front of a light to check for warping or bending, and could not see any.

However, I really did not expect to see any significant warping from this first test. The real test would be when the taper was milled. I proceeded to mill the taper; after the initial full length taper cut, I was still not able to see any warping with comparison to a straight edge. However, during the milling process observation of the cut indicated that there was a very slight curvature at the ends.

As the milling of the taper proceeded, and the piece became thinner, some small but noticeable warping of the piece was observed. My impression was that the warping of the heat treated piece was significantly less than with the untreated piece. Unfortunately, I did not take any measurements of the warping of the untreated piece, and I had already straightened that piece so I could no longer do so.

I have not included a photo of the tapered piece here, because my camera setup was not adequate to capture the slight warping.

I do believe, however, that heat treating the brass was of some benefit, and it may be worthwhile to repeat the experiment under more controlled conditions.




Conclusion

Heat treating brass (stress relief annealing) before machining may be of some benefit in reducing warping. The simple experiment described here appeared to show that warping was significantly reduced, but not eliminated. Further research indicates that a higher annealing temperature (see below) should be used.

More on heating brass

[Added Jan. 5, 2013]

This page has sparked some interest, as well some confusion, on the benefits and methods of heating brass. In particular there is some confusion between the use of the terms annealing and stress-annealing, which are often used interchangeably, but do not mean the same thing. For this reason, I will use the term stress-relieving in place of stress-annealing.

The best information I have obtained to date is the Copper Development Association publication linked to below.

To summarize from this document:

 TemperatureEffect
 Quote from How to make it in Brass - Copper Development Association
 250-300°C

482-572°F



 stress relief
When cold worked brass is progressively heated, the first effect, at about 250°C, is for the internal stresses to be relieved. This prevents stress corrosion cracking subsequently occurring and also minimises the amount of distortion which may occur during machining. This low temperature heat treatment, which should be applied for 1⁄2 to 1 hour, is known as ‘stress-relief annealing’ and  has little, if any, measurable effect on the mechanical properties of the material. The improved strength due to the cold working is  therefore retained.

In order to relieve internal stresses without loss of properties a low-temperature anneal such as 1⁄2 to 1 hour at 250-300°C should be used, dependent on section size.

 400°C

752°F
 annealing, softening
As the temperature is increased further, a rather more fundamental change occurs at about 400°C and above and the material starts to ‘anneal’ or soften with time at temperature. The strengthening effect of the cold working is progressively lost, until at about 500°C the alloy is in the fully annealed condition. Restoration of the cold worked properties can then only be achieved by further cold work.
 500-550°C

932-1022°F
 full annealing
In order to fully soften most brasses, heat to 500-550°C for 1⁄2 to 1 hour at temperature, then either air cool or, especially for alpha alloys, ensure that excessive grain growth is prevented by a quench or rapid furnace cool.
 >550°C

>1022°F
 flash annealing
‘Flash’ annealing can be carried out at higher temperatures for considerably shorter times, but care is
needed to avoid excessive grain growth.
 900-940 °C

1652-1724 °F
 melting point of brass
 source: Wikipedia

To summarize the above:
  • 250-300°C stress relief
  • 400°C annealing/softening
  • 500-550°C full annealing
  • >550°C flash annealing
It should be noted that "brass" is not a single alloy, but a wide variety of alloys with differing properties (see the Wikipedia article referenced in the table above). Because of this, heat treatment of brass may produce different results with different brass compositions. In addition, brass stock may be produced by rolling, extruding, or casting (and possibly other methods), each of which results in different properties and stress in the final product.


Afterword - Related information

Here are some links related to stress-annealing and warping in machined parts:

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