I did not ask this question from the business model point of view. i.e. Can you (should you) make money doing it? No. I was curious how people repair it or get around the problem.
I know people do it for various reasons. Not always to make money. This can be a long and detailed topic so I will stop here.
I know straitening can work in some instances. By the same token it can be catastrophic and you need to use sound judgement. The financial services industry has hijacked our society so everyone is to think litigiously. People are always worried about some kind of legal liability. You better run out and buy more insurance. The lawyers are all over tv now fishing for business. At least in the USA. Who wins? The Financial Services industry and a bunch of lawyers. But this is an other story for another board.
Bert, thanks for your technical expertise and experience. However I will agree to disagree on one point. A cast crank will work harden (strain harden may be the term you are more familiar with). The nodular cast iron used on cranks is designed to be ductile and bend rather than break like most cast irons. Also it is cheaper that forged, so I will stick my neck out and guess all walk behind lawn mowers have cast crankshafts. Someone will correct me if I am wrong. Please don't chop off my head. Work hardening is not something you would intentionally do on this material, but it work hardens when strained. Just science.
Walt, I would love to see a picture of your fixture.
Nodular iron, malleable iron ductile SG iron basically the same stuff different names most of which is misused .
The difference is the shape of the graphite & it's distribution
like all iron carbon alloys it comes in a lot of different chemistries for different applications
Black heart & white heart are the most common terms used.
Work hardening is another term that has been so misused that it has become meaning less
When you cold from a metal the energy you put into the metal is stored within it as strain energy between the layers of atoms in each crystal
The more energy you put in the more strain gets stored in the lattice so the harder it becomes to move a type of irregularity called line disslocations .
Malleable irons are specifically designed not to work harden because the ferrite can shear at the grain boundries to relieve the stresses in the lattice so the linedisslocations are fairly free to travel through the crystals
If you do this very quickly a phenomenon called "high energy rate deformation" happens where the stressed crystals shear to become new smaller crystals
So now in your crankshaft you have chill crystals around the edges then directional growth crystals in the middle and a chunk of recryatalized crystals with a specifi orientation and specific slip planes all lined up in roughly the same plane which is the crank is a toroidal plain
Then you bend it back very slowly so some crystals deform a lot and other deform a little
Thus there is now a complex mix of planes of weakness through the section which previously was uniformly random .
Thus a much lower force than would normally be needed to cause the crankshaft to fail applied to the specific direction of the weak plain will cause a fracture .
So while the actual eleastic / plastic deformation transition point gets shifted higher the ultimate strength lowers and the slope of the stress strain curve above the transition point becomes virtually flat so you now end up with an instantaneous catastrophic failure .
The grains you stressed bending it back, are not the same ones that bent in the first place and these become stiffer due to the strain you introduced in the straitening.
The stiffer lattice resists the movement of line disslocations so lattice strain relief comes in the form of hydrogen atom diffusion.
And yes there are hundreds of millions of hydrogen atoms dissolved in nearly every metal that has not been degassed and cast in a vacuum.
The hydrogen atoms are very small so move easily and very quickly through the lattice till they bump into each other which surprise surprise will be along a slip plane and in particular a slip plane under stress.
The 2 hydrogen atoms then for a hydrogen molecule which further stresses the lattice which encourages more hydrogen atoms to migrate to the same place
The hydrogen atoms no longer are bonded to the lattice because the H-H bond is much stronger than the H-Fe or H-C bond so now you have an ever increasing void in the crankshaft commonly called an internal stress crack and this will continue to grow till the lack of crossectional area becomes so great the crank either bends again or breaks .
In the boxes of stuff you never throw out is a 10 page explanation of this with diagrams regularly presented as evidence to judges & juries and an even longer one with calculations that got used when the correct verdict got appealed against on the grounds of conflicting technical evidence .
