Frame on an 87 XJ

What's the durabuility of the Uni-Frame in an 87, I heard that it was made from tempered steel, and when heated enough, it would loose its strength. I'm asking cause I'm thinking about turning an 87 Pioneer that I got free for parts into a budget (Cheap as hell) off roader. It runs, but the only thing is, the frame is getting a little ripe in the back. My friend welded the frame on my GOOD 87 Laredo a bit, but it was a Mig he uesd, and he was only welding pices to the top. This Frame probably requires Major patching to the lower part, I was just wondering if this would severly weaken the frame.

Having cut up an 89 with a sawzall, I would venture to say that the steel is not very highly tempered, if at all. It cuts like regular steel. I'd weld it anyway, because if it's rusty enough to need reinforcing, what's to weaken?

When you weld in a patch, it often causes problems, where the patch stops and the regular frame continues. Right next to the weld. A patch some longer than you need and spot welds, would probably work out better.

It is nearly impossilbe to heat-treat a unibody. SO whomever told you "that" is running on odd info. To heat treat it, they would need to anneal the entire unibody after any heating is done above around 500 degrees (read:any forming/cutting/welding), and heat treating only benefits certain alloys( steel or otherwise). XJ's are not heat treated. Welding on them is typical to any welded area. The area welded with a patch will exhibit higher tensile and yield strength than the parent if welded, simply because it is relied upon that the filler metal will alloy with the parent metal. Whenever patching, the rule of thumb I was taught is to go half the distance farther than the patch is required. Example....if the unibody is buckled for a 3 inch stretch, from the centerline go 3 inches to each side of the buckle in order to patch it. That way you rely upon the alloy you are creating to help dissipate the stress. The weakest part of ANY weld (assuming the weld is of high quality) is the union between parent and filler metal (ie-the line drawn between the weld and the parts being welded).

Pick a spot and further your education..there is always more each of us needs to know! I have been welding professionally since 1989 and I admit I know next to nothing compared to some gurus!

http://www.millerwelds.com/education/library.html

Speaking (or writing) from experience, the sheet steel of a uni-frame and/or body parts, can be made some softer than it is (after all it´s sheet Steel). The weld being the hardest part of the patch, from added carbon and the temp. radiating from the weld (uncontrolled temper). Often end up, with very hard, to some hard, to softer, then back to original tension strength. It´s not a very controlled process, sometimes the weld or very near the weld, cracks, from disimiliar hardness, sometimes next/near to the weld area it can crack/tear from softness. Temper is a pretty exacting temp. controlled process, with welding you have many temp.s radiating from the weld out.
But heck the patch is often stronger than the rust, so it´s an improvement anyway. Heat can be controlled somewhat (localized) by spraying the surrounding area with a film of water (from an old window pump sprayer), doesn´t bother a wire welder a bit and makes the demarkation lines of different temper/temperature, much narrower, (the areas that flex, expand and contract at different rates).
Much of the problem isn´t the temper, so much as dissimiliar thicknesses and strengths, next to each other. A larger patch spreads out the stress over a larger area. A 2MM thick sheet steel, is easy to weld and much stronger than the original frame, good material for patches. 1,5 MM is acceptable on most areas, have to study/estimate the stress. I usually, think of the process as controlled crush, I´d rather have it bend than break.
Making the side frame rail patches some longer than the bottom/top frame rail patches, helps distribute the stress and adds a lot of overall strength.
Using a very much thicker metal for the patch than original, doesn´t help much, just, moves the stress points. Continious welds are seldom necessary, spot welds and/or short runs, help with heat control.
Have heard, the newer XJ´s use a water hydrolic press process, that realigns the metal (chrystal) structure and improves tension properties (makes it harder/springier).
I´ve been welding awhile also, like to watch the old guys, always something to pick up.
Thanx for the site, Colorado_XJ. I weld by instinct, don´t think about it much anymore. My sons in school for his certification.

It has been common place to use a mix of high strength (alloy) and mild steel panels in unibody construction for well over a decade and the XJ is no exception. Factory spot welding keeps the heat affected zone localized to maintain the integrity of the HSS panels.

According to the manager at my local dealers body shop, the uni-rails, shock towers and if I remember right, the core support are HSS panels.

The thing that would concern me when welding to the rails is the loss of corrosion protection in the weld area. You can paint and undercoat the outside of the rail, but the inside is a little tougher to get to. This goes for welding on any panels with blind surfaces.

MaXJohnson said:

The thing that would concern me when welding to the rails is the loss of corrosion protection in the weld area.

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Kind of moot if you're welding it because it already rusted through, I think.