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Bending cast gunmetal

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AJW08/02/2019 21:26:45
270 forum posts
117 photos
I have a gunmetal casting which has been cast from a deformed pattern so is slightly out of position on one of its lugs.(rocker arm on Centaur cylinder head)
I had one of these before and attempted to heat it with a view to straighten it out when it was hot - it broke!
Would it be possible to recover this otherwise scrap casting? It's not got to move much, about 2/2.5 mm

Paul Lousick08/02/2019 21:43:11
1095 forum posts
483 photos

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.


AJW10/02/2019 13:49:27
270 forum posts
117 photos
Just dragging this question back up!
Am I to understand that correction of the lug by bending is not going to happen?

Nigel Bennett10/02/2019 14:25:20
289 forum posts
6 photos

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

Dave Wootton10/02/2019 14:30:14
13 forum posts
1 photos

Hi Alan

I have tried in the past heating Gunmetal castings to try and get them to the required shape, every time has ended in disaster!, the only way I have found is to cut the casing with a fine saw, let a piece of thin material in, silver solder, and try and file to the correct shape.

My current project a 5" Aspinall 0-6-0 suffers from some awful castings, so there has been a lot of letting in and silver soldering going on, I know I should return them, but I have given up on the supplier.


SillyOldDuffer10/02/2019 14:48:09
4396 forum posts
956 photos

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 Wootton10/02/2019 15:23:30
13 forum posts
1 photos

Hi Again Alan

I hadn't read Nigel's post above when I made mine, must have been typing at the same time. I hadn't thought of trying to cold bend cast gunmetal, so I just popped out to the workshop and tried a quick experiment.

I had a scrap gunmetal horn stay with a section of 3/16" x 5/8" stuck it in the vice and bent it ( with a hammer), it went to around 20- 25 deg with no apparent ill effects, at about 30 deg minute cracks appeared on the outside of the bend and it fell apart at about 45 deg., Not scientific at all, but I think it shows that minor corrections could be made without too much trouble without any heating.

I'm interested because I have some casings for my loco that need "adjustment" and was going to cut and silver solder, but it would be difficult to hold the parts together, so I will give the cold method a try. if they fall apart I won't be any worse off!


Neil Wyatt10/02/2019 16:01:06
16076 forum posts
675 photos
73 articles

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.


Michael Gilligan10/02/2019 16:51:03
13235 forum posts
578 photos

Just a guess, but enthusiastic local heating probably boils-off some of the lead content ...



Edited By Michael Gilligan on 10/02/2019 16:51:31

Maurice10/02/2019 17:03:41
435 forum posts
50 photos

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.


MichaelR10/02/2019 17:20:07
328 forum posts
55 photos

I had the same problem with my Centaur cylinder head, I straightened the lug cold with no problem.


AJW10/02/2019 17:52:55
270 forum posts
117 photos
Thanks, very informative inputs! Sounds like bending cold might be the way to go.
When I broke my original casting I had got it pretty darned hot, not dull red but HOT.
Upon bending it straight there was no give at all, it just broke off!
I have since silver soldered it all back together and the head is now fully machined with no bits falling off!
I asked the question because I have another head casting which is showing exactly the same out of line lug. This one is in dispute with the supplier who have told me the pattern is in error - and has been since 2001!
I am in correspondence with them to try and encourage them to put the pattern right, instead of keep producing incorrect castings that might or might not be recoverable!

SillyOldDuffer10/02/2019 18:10:58
4396 forum posts
956 photos

Newnes Engineer's Reference Book is more helpful than Machinery's. Gunmetal is a Bronze, of rather wide composition: up to 89% copper, 5-20% Tin, 0-20% Lead and 2-6% Zinc.

Some quotes:

'Bronzes are slightly improved by annealing but the treatment has limitations.'

'Gunmetal should be annealed at about 700C for half an hour.'

'Not all Bronzes are capable of being hot and cold worked. Hot work is, actually, seldom carried out on them.'

'Bronzes with 6% Tin or over should not be cold worked.'

Discouraging! All those who reported negative experiences straightening gunmetal knew what they were talking about!


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