I have had the same experience with a gm clack valve casting. Tried to straighten casting by heating first and it broke when I tried to bend it.

Paul.

Heating up gunmetal to bend it is probably the problem. I recall trying to" bump up" a piece of drawn bronze bar, and heated it up first as if I were a blacksmith battering a piece of steel. The hot bronze fell to pieces when I hit it. Reading up about it after the event, it appears that bronze is (or my bit was) "Hot Short" which means it doesn't like being bopped when hot. It's likely that cast gunmetal will behave similarly.

If I was trying to unbend a GM casting, I'd try to bend it cold. If it broke, I'd silver-solder it back together in the correct position. Whether it's worth heating it up a bit after you've successfully bent it to stress-relieve it, I don't know as I'm not a metallurgist. A quick Google search on" stress-relief of cast gunmetal" suggests 300C for a while.

Hope that helps

This is an armchair observation as I've never tried to straighten gunmetal. However I've read softening brass needs exactly the opposite heat treatment to steel. Gunmetal may be similar.

Heating steel to red-heat and allowing it to cool slowly softens it. Heating steel to red-heat and plunging it into cold water makes it hard, brittle and liable to snap.

Conversely it's heating brass to red-heat and allowing it to cool slowly that makes it hard and brittle. It's rapid cooling that softens it and makes it pliable.

Gunmetal isn't exactly a Brass but it might be safer to bend it after a plunge cool. It may be difficult to bend gunmetal because copper alloys tend to work harden in a blink. Brass items made by extrusion have to be heat-treated repeatedly to keep them malleable through manufacture.

Dave

I was going to say bend it cold too.

I didn't know heating gunmetal made it brittle, but I did know it doesn't make it soft.

Neil

Some years ago, I bought a set of gunmetal castings for the "Vulcan" beam engine. They were fine except for the two "A" frames, which both had a pronounced curve from top to bottom. With the ends on a flat surface the centre was about 5/16" above the surface; same amount on each casting. I used a length of studding, two nuts and a couple of face plate clamps, to clamp the castings together face to face, arced sides facing each other. I nipped up the nuts and then increased the tightness a bit at a time over about three days until the castings touched all the way down. I then brought them to red heat, let them "cook" for a minute or so, then left them to cool. When cold the clamps were loose; all nice and straight. They were a funny colour, but entirely usable.

Maurice