Most recent actual example coincidentally involves a bent fork.
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Recently, a fellow used [abused] his accessory forks on his tracked skid steer and bent one of them down. In a panic and being frustrated with himself, he called me and I gave him by best guess and advice as how to proceed.
He installed a small amount of restraint ( a loop of taught chain, I believe he mentioned that). Then he got his torch ready and his garden sprayer. He ran a narrow line of heat across the inner bend of the fork approximately where it seemed to be bent the most. Cooled it with the garden sprayer right after the heating step was completed. Before starting the next attempt, each time he made for absolute certainty the metal was quite cool (so he could easily lay his hand and feel
no heat).
Deadcold I believe this can be termed.
This behavior was mentally accomplished by tending to his other unrelated chores.
Over the course of the day, he did four "swipes" in the same general area, each time going a wee bit slower with the torch allowing the heat to penetrate a bit more, and as stated above,
always assuring cooling prior to the next time. Each event corrected the bent fork, but the last one did a bit too much, prolly cause he was going too slow and allowing the heated area to be a little bit deeper. This is where experience is invaluable. LOL
For the theorists on the forum....How do I explain how does this work to fix the bent part?
As to the cooling.
As far I as I can discern the forceful cooling is needed to remove the heat from the part asap once heating is completed to maintain the
differential. Creating this
differential is one of the actions that is doing the task at hand. Times I tried this with no garden sprayer did not work well at all or didn't work. I surmise one may use a garden hose, I propose the garden sprayer is less messy (no water puddles or mud to drag one's torch hoses or get one's boots wet or muddy....perhaps the garden sprayer was something they utilized in a shop environment as they didn't want another hose dragged around to be cut or ruined. Also from my experience, the amount of maintenance to keep a garden hose from leaking is another contributor to 'not generally advised or recommended'). Since I do concrete work I always have a garden sprayer or two or three laying around and most time the potable water is already in it.
As to the heating.
The theory of this flame straightening as best I can recite goes like this: the portion of the part, ie the metal being heated 'wants' to expand ... and it does do so permanently. This is due to this: the metal being heated the strength capacity becomes less and less....the behavior under heat is a rather rapid transition which is one reason structural steel must be fire protected. The movement to expand is resisted by the other portion of the metal that is not heated and remaining quite strong. One wins the battle. In effect that permanent change to the
dimension of that portion of the steel is the key to this concept. In the example, the upper portion of the fork metal is being heated and it expands, yes indeed this makes an attempt to make the fork bend more [there are
two topics: Restrained and UnRestrained flame straightening. If there is no Restraint, the item initially upon heating will indeed be looking worse, but once it cools it passes back by the point at which this was started. Been there and seen that; tis kind of wild really.] But the overall fork shape in this example of restrained bending will not be seen bending greater from an overall perspective because of the restraint (the loop of taught chain). But that particular exact section / portion that was heated will indeed yield, ergo permanently change its shape. In this case, once cooled it will be smaller than when it started as to the length and this will ultimately cause the fork to bend in the desired direction, resolving the
overall deformity.
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Why do I fiddle and suggest this could be done with this so called Flame Straightening?
Folklore & sage evaluation by proponents and/or investigators of this method indicates the metallurgy is not changed because of two reasons: one the metal is a type of metal that is able to be welded and withstand processes of fabrication, and two because the time at temperature isn't long enough to rearrange the crystal lattice. Next time I think about it, I will order some tempilsticks and try to add some metrics to what I am observing.
Other reasons is: it is curiously attractive 'cause it takes some experience and some thinking to pull it off, but in many cases it is way easier than rigging or applying big forces and applying loads of heat....that event is what I call "hercules method" being done by me many times (as we all have done) in order to save or correct a piece.
Please Note: there will most likely be an instructive mindful person who feels by either belief or learning or both this heating business is damaging the part
....and if that is in the concern to the extent that prohibition of heating is held to high esteem, then that policy should be held true and complied with. I made this point, because like anything, it depends on the experience and judgement of the person doing the task.
The best example (kind of lame) would be a nice straight piece of heavy wall shrink tube for electrical wire, if only a portion was heated and allowed to cool, the piece would be bent due to the differential in the lengths.
Then there was the broken loader arm near the pivot on a Case 580K that was out in a field that I flame straightened just so perfectly that once the piece was welded up, the pin passed through both the bushings with ease. That job was my first time with this method and it was very sketchy and scary as my mind having only vague ideas and memories of assorted comments....and it was Unrestrained. Another story for another day.
Emoji with burning (torch) shades and happy with the result. I should add, practicing with a scrap piece may be useful prior to making a mess out of something that cant be easily or inexpensively replaced.