Why has my mild steel bent

It also happens with brass.

Fortunately I was able to batter it straight again and use it, but it does illustrate the locked-in stresses at the surface of drawn material.

It wasn’t along its length it was is the side to side that dished ,as I say it’s not a problem with this part , I was just wondering for future reference when something more critical in size is needed, I have since measured and the slot I cut in first 8.6mm wide now measured 8.67, although it is less the further in towards the closed end but that would make sense as more material there. so not a great amount but if it was some critical part 0.07mm could make a difference between fit and not .

Dell

being a new metalworker someone will correct me if my thinking is wrong so here goes how low in the vice is it being held. would holding it just by the lower section where no material is being machined away stop this happening, that is to say no pressure on the top closing it up. I know that vices for milling machines are meant to hold square.

Hope that makes sense

Dalboy, - unfortunately no, the vise will not prevent the stresses releasing though it will do so until the vise is undone.

Dell's deep cut of the tee slot is what was the main cause and is something I would have expected to happen if done without allowing the stress to ease by lighter machining and releasing from the vise for those stresses to ease gradually.

As Nigel says brass is particularly bad for this but can sometimes be reshaped it the part will allow it.

A similar situation to this was experienced at work. Thin, slightly distorted heat treated parts could not be held down to the magnetic chuck by magnetism to be ground initially - the magnetism would actually straighten the part which would then resume its distortion once released. Parts, sometimes many of them, would be stuck down to the chuck using double sided sticky tape and the initial cut gingerly taken to take the bow out. The sticky job of removing the tape had to be done before holding them in the correct fashion to bring them flat. A one off might similarly be held in a vise to create a flat surface before using the mag chuck.

Not quite the same as distortion caused by stress release on CRS but similar in the way the material is dealt with to create a flat surface

Please note - I am not advocating holding parts down by the sticky tape for milling !!

Tug

Note that normalizing and annealing are different processes. With normalizing, the work is allowed to cool more rapidly than in annealing, usually in free air, not by insulating it or furnace cooling it.

Annealed steel has a coarse grain structure and machines like chewing gum. Normalized steel has a finer grain structure and is more pleasent to machine, while still being stress free.

Hi, cold rolling produces stress though-out the the metal that is rolled, not just at the surface of each side, although the stresses are greater on the surfaces of each side. The only true way to overcome the situation is by heat treatment.

It can be seen on the graph above, that normalising is slightly hotter than annealing, and normalising will be a little stronger than annealing, but the temperature has a narrow window, so a controlled temperature environment would be needed.

Regards Nick.

In the winter I heat any mild steel likely to do that to red-heat in the (log) sitting-room fire and leave it to cool down in the ashes overnight before machining. That seems to get rid of any locked-in stresses. Not done it in the summer, but I'd probably heat it up to red with a propane torch and steal some ashes from the sacks my missus saves for the garden for the cool-down. I've never had it happen with brass though.

On top, or aside, of this - cold rolled steel is significantly better (mechanically) than hot rolled if strength is important.

.

Something conceptually in common with pre-stressed concrete, I suspect.

MichaelG.

Yes, hot rolled is about 20% to 30% weaker than cold rolled. Some example numbers:

Tensile Strength. Hot Rolled 67000psi, Cold Rolled 85000 psi
Yield Strength. Hot Rolled 45000psi, Cold Rolled 70000psi

So,probably better to machine hot-rolled for non-structural parts because the stock won't warp, and to machine cold-rolled whenever strength matters.

I mostly work with cold-rolled steel because starting with clean straight surfaces often saves a lot of time - I rarely have to machine an entire block. Warping is definitely a real problem, but I've only been caught once in 12 years. Took about 5 years before a length warped on me during a cut and it hasn't happened since. Could be the devil looks after his own!

Dave