relaxing metal with Heat?

[McClary]

i was on the West Coast Choppers blogs, and was reading about a frame being built...
it quoted ..

Time to check back in and see how Junior is doing on our DOMINATOR frame. After coping all the tubes to fit, the frame gets welded. Once he is done laying down some sick TIG welds, the frame gets relaxed with the torch before being removed from the jig for the last bit of welding..more to come so stay tuned!

(note: pictures from www.westcoastchoppers.com)

ok , im little confused on this... ive always understood that HEAT will cause metal to expand , but as it cools, it will shrink the metal...
so what is this guy doing? they say he's relaxing the metal to be removed from the jig.... but once it cools , wont it just slightly shrink in the heated are , causing the frame to not match the jig specs ?
or am i missing something?

thanks
mat

[Marty Comstock]

When you heat metal in a localized and it cools, it wants to shrink. This is the basic definition of the effect you see when you weld. What happens is a tad more complicated, but I wont get into that here. What you are seeing is annealing, or what is called process annealing. Taking a part in its whole, or a large enough area to 550 degrees or so and allowing it to cool on its own. I do this often, even on sheetmetal parts that are heavily shaped in a very smal area, helps with arrangment issues. It helps relieve the stresses put into framework after much welding has been done to the joints.

Marty

[Joe Hartson]

Marty is correct in what he says. This was and probably still is standard practice on air craft frames. After the cluster, group of tubes, are welded and still in the jig the entire cluster is heated to about 500 degrees F and allowed to cool at room temperature. The heating and cooling process will remove the stresses built up in the cluster from welding.

[kjc]

A "sick" Tig weld?

LOL

[pro70z28]

I've got the license plate affect in that "camaro" panel I stamped for the firewall. Do you suppose if I clamped it to the firewall where it will eventually reside & then go over it with the torch, then let it cool, I might get rid of that problem?

[McClary]

thanks for the replies!

i understand about heat after cooling shrinking metal.... thats where i was a bit confused how they said "relax" the metal with heat. in my mind i could only see the frame shrinking more when heat is put on it...



is there any links for more info about this process you guys mention , like with the aircraft frames and etc?

thanks

[John Buchtenkirch]

It’s pretty common to stress relieve welded joints on high end chrome molly fabrication by heating the joints and letting them cool slowly, i’m sure it’s helpful with mild steel at times but probably not as critical. When John Force’s funny car frame broke 2 years ago NHRA had a very lengthy write up on the subject, some frame builders were dead set against the idea while some agreed it should be done.


The fellow in the photo isn’t even heating a joint so I’m thinking he could just be doing it for the photographer. A good friend owns a salvage yard and he straightens out angle irons and even I-beams by heating one flange or the other, no hydraulics or jacking at all and he manages to nail a lot of them on the first attempt so shrinking stuff into place works real well for him. ~ John Buchtenkirch

[tkeen]

I have straightened a lot of large, long shafts with heat. You heat small spots and the metal expands with the heat and then when it cools it shrinks more than where it started. You heat the high side and that makes it higher, then when it cools it shrinks and goes lower than where it started.

Tom

[TheRodDoc]

you don't have to anneal mild steel.

he is heating the pipe where another pipe is welded to only one side.
heating on opposite side will take the bends out of the backbone in that picture. (the smaller pipe is welded under it)

welding on only one side of a pipe will draw the pipe down on each side to wards the welded on pipe.

If something was welded on the other side directly across from the other pipe then heating wouldn't be necessary.

This is all the heating is for.


over emphasized drawing of how the larger pipe will bend when the smaller one is welded on. heating to about the same temp as was used for the weld on opposite side will straighten it.

Heatpipe.jpg

[steve.murphy]

Does anyone have a copy of the NHRA report? would be good to have in the library.
Thanks, Steve