softening copper

[brightspark]

The only difference that I've noticed is that most of the scale tends to fall off when quenched.

yep thats all; it does. both ways soften it after heating to cherry red

 

[Maker]

It is not something I have ever considered, but can you work harden mild (non-carbon) steel?

Yes, and if it helps convince yourself, bend a piece of 1/8" round in your hands, then bend it straight again, the first bend hardens the steel slightly so the second bend forms either side of it, so rather than getting a nice straight rod back, you get a wavy piece of scrap.

When I was at school, in metalwork we made some copper bowls, we quenched them in sulphuric acid to anneal & clean the copper, then buffed them on an industrial buffer. This was in 1962ish, can you imagine that being allowed for 12 year olds today! Still got the bowl.

I quench in acid too, though it tends to be citric because I'm not too keen on sulphur dioxide, chemical burns or going blind.

 

[R Kraft]

A brass (copper alloy) cannon will work harden from use, becoming brittle, and will eventually burst.
An iron or steel cannon will last indefinitely if not abused.

 

[outofthefire]

This is quite clear from the two usual types of mild steel bar/sheet - hot-rolled and cold-rolled.

Cold-rolled is merely hot rolled steel that has been further rolled (worked) while cold. Both strength and hardness of the cold-rolled product are improved over the hot-rolled. But in terms of is it worth it? Probably not really, unless one is not going to machine it further.

Mild steel isn't hardenable there isn't enough carbon in it. Work hardening mild steel and copper just makes it less ductile, resulting in tears and tears when shaping sheet steel or copper if you don't anneal regularly.

 

[merryman]

Nobody has mentioned age hardening. I'm fairly sure copper does. certainly a newly quenched copper washer is soft enough to squidge when you tighten it down, whereas the ones you get in the boxes of washers seem quite a lot harder.

 

[Seadog]

Does that not just mean that the manufacturers have not annealed them fully, or that they're a copper alloy rather than pure copper?

 

[Misterg]

The hardness and ductility of a pure metal depends on the size of the "grains" in the metal (areas of semi ordered crystal structure). Heating causes the grains to merge together and grow, making the metal more ductile and softer. Working the metal causes the grains to divide and you get a finer grain structure which makes the metal harder and less ductile - work hardening.

Copper follows those rules - it matters not how quickly it is cooled, it's the time at high temperature that grows the grains and makes it "softer" (quenching it does help get rid of the oxide layer though).

Pure (1XXX series) aluminium also follows those rules. Aluminum alloys (e.g. 6XXX series) commonly contain ingredients that are dissolved by the aluminium at high temperatures (like sugar in tea) but which will crystalise out again at lower temperatures - even at room temperature in some cases. These tend to form at grain boundaries and 'pin' them in place, making the metal less ductile and harder. The slower that this happens, the finer and more numerous are the 'pins' - this is age hardening - the crystals develop slowly over time.

There is no age hardnening mechanism for pure aluminium (or copper). Chances are that the copper used for the washers in the box will have been rolled and punched, work hardening it somewhat in the process. (There are copper *ALLOYS* that can be age hardened.)

Steel is iron+carbon. The carbon can be dissolved in the iron, or it can form different structures with the iron, all depending on temperature. Some of these structures form the bulk material and can be glass hard and brittle, others aren't; some deposit at the grain boundaries, 'pinning' them in place. Quenching steel is used to "freeze" the structure in its high temperature state; Tempering is to allow this 'frozen' structure to revert towards some intermediate state.The strength and ductility of steel therefore also depends (strongly) on it's thermal history and the amount of carbon presesent. If you want to get it back to it's room temperature state, you need to let it cool *slowly* so that the different structures can work themselves out.

This is overlaid on the basic work hardening mechanism - steel / iron will also work harden regardless of the carbon content.

The video is reasonable (if you can stand the voice):

 

[arther dailey]

Does that not just mean that the manufacturers have not annealed them fully, or that they're a copper alloy rather than pure copper?

I wondered the same thing as the new pack of washers I got were distincly hard , so aneal as I use.

 

[Newlands]

The hardness and ductility of a pure metal depends on the size of the "grains" in the metal (areas of semi ordered crystal structure). Heating causes the grains to merge together and grow, making the metal more ductile and softer. Working the metal causes the grains to divide and you get a finer grain structure which makes the metal harder and less ductile - work hardening.

We had a brilliant metallurgy lecturer when I was at college who explained crystal structure, grain boundaries and dislocations perfectly for all us oiks. I remember he had a big board with loads of small ball bearings he used to demonstrate how the lattice structures would align in patches, meeting up with another patch that was aligned in a different direction. He had loads of anecdotes to show how all this applied to real life.