I don't believe the frames are heat treated. They are made of relatively soft metal and are meant to flex instead of being brittle hard. That prevents cracking. Welding heat can change the temper of the metal. I think it makes it brittle.

I have seen many frames boxed for strength however and it seems if done right it's not a problem.

Maybe there's a professional welder over in the garage forum who could clear things up.

 

Original from the 1979 Light Truck Shop Manual


REPAIR OPERATIONS

FRAME REPAIR

DRILLING PRECAUTIONS

Do not drill holes in the frame flanges, since this tends to reduce the frame strength.

WELDING PRECAUTIONS

Disconnect the negative battery ground cable before using electric welding equipment.

all frame welding should be cone with electic welding equipment, and the heatshould be kept in a small areal to prevent change in hardness of the metal. Do not apply heat to or weld 50,000, 80,000 and 110,000 psi frames. When a reinforcement is welded to a side member, run the welds lengthwise along the reinforcement.

FRAME STRAIGHTENING

Frame misalighment can be corrected by straightening the out-of-lineparts or by replacing the crossmembers, braces, or brackets if they are badly damaged.

Straightening should be attempted on frames that fail to meet specifications of the diagnal checking method or where damage is visually apparent.

To prevent internal stresses in the metal, frame straightening should be limited to parts which are not severely bent. If heat is needed to straighten a frame member, keep the temperature below 1200 degrees F (a dull red glow). Excessiveheat may weaken the metal in the frame members and cause permanent damage. Do not apply heat to 50,000, 80,000 and 110,000 psi frames.

FRAME REINFORCING

After a bent frame member has been straightened, inspect the member closely for cracks. If any cracks show, the frame member should be reinforced or replaced. If the frame is a 50,000, 80,000 or 110,000 psi frame, NO REPAIR METHOD IS APPROVED.

Reinforcements can be made from channel, angle or flat stock. The reinforcement stock should be of the same material and the same thickness as the frame member being reinforced and should extend a minimum of six inches to either side of the crack.

To insure sound repair, the crack should be prepared before the reinforcement is welded to a cracked frame member. DO NOT WELD 50,000, 80,000 AND 110,000 PSI FRAMES. THE ONLY FRAME THAT WELDING IS PERMISSIBLE ON IS THE 36, 000 PSI FRAME. THE CRACKED AREA SHOULD BE SIRE BRUSHED TO REMOVE PAINT AND EXPOSE THE CRACK. To prevent the crack from spreading, drill a 1/4 inch hole at the root of the crack. Grind out the crack to form a slot which will allow the weld to penetrate to the surface of the reinforcement.

The proper location of the reinforcement depends upon the location of the crack. If
the crack is at the bottom of the channel ,the reinforcement should be welded to the channel bottom. If the crack extends along the side of the channel, an additional reinforcement should be welded to the side. Drill clearance holes in the reinforcements to clear rivet and bolt heads when necessary.

FRAME MEMBER REPLACEMENT

If a damaged frame member is to be replaced, new bolts, grade 8 fasteners and
rivets required for replacement of parts, should be of the same specifications as the original bolts or rivets. In cases where it is necessary to substitute a bold for a rivet, use the next larger size bolt.

 

Great info Square Wheels!
How do you know if you have a 36, 50, 80, or 100,000 psi frame?
I assume that the gvw of your truck would determine which frame you have?

 

Did not find that info in the ford manual. I think your gvw assumption is probably right. Maybe the 450's & up. Also the old 500's & up. I have 5 Ford trucks, including a 1 ton & a 95 150 that i straightened after it hit a bridge. (salvage rebuilder) You would not it to look at it ,or drive it. I have not come across any (hard) steel in any light truck. I think some of what Ford was describing was CYA for lawyers.

 

Ford pickup frames up to at least 97 were standard mild steel. You can weld on them with properly handled 7018 rods, or 70s solid/71T1 dual shield wire. I wouldn't use 6011 rods on them, as those welds tend to be rather brittle. Any time you weld on the frame, you should make it a full penetration weld. Grind both pieces at a 45* angle, leaving a flat land area of about 1/8". Weld up one side, then grind into the weld from the backside until you are into solid metal, and fill it back in with weld. If you can't get to the backside, grind both sides 45* all the way, leaving no land area. Set it up with about 1/8" of space between the two pieces, and tack a piece of 1/4"x1" flat bar to the backside. Weld up the front side, and it will be a full penetration weld. With either procedure, grind the face of the weld smooth and put a doubler plate over it to prevent excess stress on the weld. Might be a little overkill, but I have never had a weld break on my truck, and using this process, none should ever break.
I always use grade 8 hardware to replace rivets. It's too high stress to chance a junk bolt in, and it's not much more expensive to use 8s.

 

I read that information to indicate the light truck (36,000psi) frames are heat treated.

The higher strength frames were probably used on heavy trucks in specific applications.

 

If anybody wants to pay for the metalurgy lab work, I'll gladly donate steel from a couple of different frames, I have pieces from an 84 F250 frame and pieces from a 70 F-100 frame.

 

I seen some frame rust like powder on the frame and I seen other frames rail peel off or flake off. Just shows how some frames are manufactured.

 

Digging up this fine thread, here's an official ICAR doccument on Ford Truck frame repairs:


http://www.i-car.com.au/pdf/fordfram.pdf