Flame straightening is generally believed to be one of the “black arts”. Tain’t so guys 🙂 . It’s relatively straightforward if you have a basic understanding of the principles involved.
You Broke It
You Bought It
When a piece of steel is bent, two things occur.
The inside radius has been compressed, and the outside radius has been stretched. All of the steel is still there, but it’s been altered. Not permanently altered though, it can be brought back to life. We need to shrink the stretched area. The compressed area, unless cracked, is of relatively little consequence when straightening.
Understanding The Mechanics
It all begins with restraint:
We don’t want the bent area to elongate when heated (heat causes expansion). After all, it was elongation/stretching that got us into the trouble we’re in. So……we need to keep it from moving.
Applying force with weight, a jack, clamps, etc. will supply the restraint. ONLY ENOUGH FORCE TO PREVENT MOVEMENT WHEN IT’S HEATED. IT SHOULD NOT BE ENOUGH FORCE TO BEND THE STEEL IF IT WAS COLD (TONS AND TONS OF PRESSURE). I applied force, in this case, with a 4 ton come-along (which is only 4 tons when using the snatch block) in single line pull. It could only generate 2 tons of force max. Clearly not enough to bend the steel on its own.
Exercise restraint when applying restraint:
It can be hard to judge just how pressure to apply. The ideal amount of pressure is just enough to keep expansion in check. Too much pressure will cause the metal to shrink faster, and shrink more than you want it to. Remember that heat reduces the yield point of the steel while it’s at the required temperature.
The ideal solution is to clamp a piece of identically shaped un-damaged piece of steel to the inside of the radius of the damaged piece. The damaged piece will stop all/most movement once it’s formed to the good piece. Kind of like a die in a hydraulic press.
In the real world, it’s very often necessary to do a “free bend”. Force has to be applied somewhere along the bent member. In this case, I used a cable to apply the force at the tip of the spike. A place which would generate the most force along the bent piece. Here’s where a problem can occur.
I need to know just how much force to apply. Think of the force as a ram pushing forward. You want the ram to stop when you’ve reached the desired distance. Steel is springy (the fancy term is ductility…the ability to return to its original shape if the yield point isn’t exceeded). The force you apply has to be little enough to keep from over bending the piece. This is hard to do with mechanical jacks, hoists, etc. So start out with as little force as possible. Just put some strain on it. Don’t put enough force that will attempt to actually move it. Too much force, and the “ram” will want to keep moving past where you want it to stop. Be gentle. THE MAIN PURPOSE OF THE FORCE IS TO PREVENT EXPANSION WHILE HEATING. In this case the cable should go slack when the metal starts to shrink, problem is……the cable itself is under tension, and stores force in that tension, so it’s possible to over apply force. This can help with shrinkage, but has to be judged very carefully. Heat shrinking should be done in small baby steps. Little force, let it cool, little force, let it cool. This is the black art part 😀 . After a few tries, and observations, you’ll catch on just fine. Take some scrap, and play with it.
How hot is too hot:
You can go out and buy an infrared thermometer, or eyeball it.
The ideal temp is around 1200 degrees F.Use a calculator to convert the table to F. SUCH AS
Go any hotter than 1200, and you will damage mild steel. Best to start out with too little heat than too much. You’ll notice that I didn’t have sufficient heat on my first go-around. No biggy, I simply hadn’t heated the metal completely through its required profile (more on that later).
Try to do your heating in the shade, or indoors. You’ll get a true view of the dull red color. Heating outside, in full sun, will drown out the red color. If outside, you can generally stop heating when the steel starts to turn a silver/grey color. Don’t bring it to red in full sun, you’ve gone too far, and are probably already into the orange before you can actually see any appreciable color change. Play it safe.
True color is when you’ve removed the torch from the steel for a few seconds. It should remain glowing dull red for a bit (enough time to cool it with water if you wish). Sometimes the surface of the steel will appear orange while heating a thick piece……this can’t be avoided…..outer surface temps will rise a bit high because you have to keep applying heat to reach the inner part of the thick piece and bring it up to temp. Keep the torch moving while heating a large area.
Cooling the Metal:
Use water if you’re in a hurry, let it air cool if you’re not.
Air cooling will also let you observe the movement at a slower pace, and allow time to back off the force you’ve applied. Sometimes the metal will move too fast when water cooled.
Mild steel responds well to the rapid cooling without damage. We’re not talking dumping it in a pail of water…..just a reasonable application of short water bursts, or a mist.
The heating patterns are pretty much the same for most types of steel profiles, but will differ somewhat. At a later date, when I run across different types of types of structural shapes (channel, tubing, angle, pipe), I’ll cover the process. This should get a person started though. The wedge shaped/pie shaped heating pattern is the most common.