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Someone is trying to anneal a brass casting to minimize the segregation of Zinc. He likes the results of annealing at 400 Celsius for 3 hours, but the high temperature produces problems with residual stresses. Instead, he wants to anneal at 250 Celsius. How long will it take to achieve the same results when annealing at 250 Celsius?

I have no idea where to begin with this problem. I couldn't find any examples of a similar problem in the textbook or online, and we didn't do one in class. Can anyone help?

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    $\begingroup$ Reading around, 250C might be below the phase transition temperature of brass which means it won't anneal at all. $\endgroup$
    – DKNguyen
    Oct 8, 2021 at 21:40

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The alloy makes a difference. If it is essentially copper and zinc like 70:30 (cartridge brass, or admiralty if you put in one pre-cent tin) or lower amounts of zinc: Nothing much happens with the phases. Hard to guess why he thinks there is segration or residual stresses after a slow cool from 400 C. Actually a water quench would give a solution anneal, good for improved ductility. I think you had trouble finding references because there is no problem with residual stress in brass unless it is cold rolled. Brasses are going to have annealing twins (metallographically ), maybe that is causing confusion.

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  • $\begingroup$ I think my professor wrote the residual stress part as a random comment for why he wants to redo it at 250C. More of the focus I think is on what the results of annealing would be after 400C for 3 hours and how you could replicate that at 250C. $\endgroup$
    – jorkz
    Oct 9, 2021 at 18:40
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I used a different search engine and may have found something useful.

"When cold worked brass is progressively heated, the first effect, at about 250ºC, 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. As the temperature is increased further, a rather more fundamental change occurs at about 400ºC and above and the material starts to ‘anneal’ or soften with time at temperature."

So with this information, I believe that one could not achieve the same effects at 250C as at 400C no matter how long they held it at that temperature or how long they cooled it. However, they could reduce the residual stresses of the original 400C annealed brass by heating it at 250C for 1/2 to 1 hour.

http://www.nationalbronze.com/pub-117---the-brasses_whole_web-pdf.pdf

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  • $\begingroup$ THis makes sense. $\endgroup$
    – r13
    Oct 9, 2021 at 21:57
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A suggestion here:

"Annealing is often confused with Tempering. Both can be considered forms of Heat Treatment, but, with the risk of oversimplifying matters – Tempering takes the metal to a lower temperature and tends to harden metals, Annealing is a shorter, hotter process and softens them.1

We don’t want to harden the metal any more than it already is. So, we are talking about high temperatures for brief periods of time. Under-anneal and no real benefit will occur, over-anneal, and you risk over softening your brass. Something you don’t really want when it’s meant to be directing and controlling and explosion.

Brass Annealing Temperature Online, the suggested temperature your brass needs to get varies a bit, ranging from 600 to 800 degrees Fahrenheit (that’s 315 to 420 Celsius for us in metric land). The average recommendation seems to sit in the 700 F range though (370 C)."

https://thebloke.co.nz/understanding-brass-annealing/

One one of the online forums, the consensus seems to suggest heating to 650 degrees Fahrenheit for 8s to 15 minutes. I believe the wide range of heating time is due to the various size of the samples.

ADD:

The stress relieving temperature is normally between 550 and 650°C for steel parts. Soaking time is about one to two hours. After the soaking time the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this is especially important when stress relieving larger components.

The temperature for stress relieving copper parts is, depending on the alloy, 150-275°C and for brass components 250-500°C.

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  • $\begingroup$ The "bloke" reference shows a basic misunderstanding of what is happening. They are stress relieving , not annealing, not tempering. I suspect annealed cartridge brass would not work in an automatic gun, too soft. $\endgroup$
    – blacksmith37
    Oct 9, 2021 at 14:42
  • $\begingroup$ @blacksmith37 The article made no mistake, it talks nothing about stress-relieving. I cited it to point out that annealing at 250 degrees C would likely harden the metal instead of softening. (See the last sentence in the first paragraph of the question) $\endgroup$
    – r13
    Oct 9, 2021 at 16:13
  • $\begingroup$ @blacksmith37 Find a web page full of links to articles that talk about annealing gun parts for your info use, though I am not positive any of them cover brass cartridges. bing.com/… $\endgroup$
    – r13
    Oct 9, 2021 at 16:47
  • $\begingroup$ Yeah, the focus of the question is not on residual stresses, but rather on annealing at a lower temperature and producing the same results. The residual stress part was likely a throwaway comment. $\endgroup$
    – jorkz
    Oct 9, 2021 at 18:42
  • $\begingroup$ @blacksmith37 Annealing, stress relieving, and normalizing are all technically distinct terms? $\endgroup$
    – DKNguyen
    Oct 9, 2021 at 18:45

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