Yes metal work hardens. But when bending metal something else is going on. Think of a sheet of metal as two parallel lines (the thickness) and a dotted line down the center.
When you bend metal, the radius of the bend runs down that center, it doesn’t really change length (like in geometry this is a line without thickness). The material on the outside of the bend is stretched - it has a longer distance to travel with the same amount of material, the further from the center the further the stretching occurs. Thicker material is more pronounced. The matrix of the grain is rearranged.
On the inner radius, just the opposite, the metal is being compressed, again the metal grain is realigned. Both is where various forms of work hardening occurs- but they react differently.
If when you bend the metal to 30*, you did not bend it enough, you can continue to bend in the SAME direction without worrying about damage, the metal continues to do what it was doing until it reaches its limit. If you tried to undo what was formed, this is referred to as reverse forming and is a big No No. You might get a away with a very minor tweak. But when you reverse form the material that was stretched it doesn’t go back to the way it was: it sort piles up with no clean grain lines, the part that was compressed, stretches out again, but doesn’t go back either, new grain lines formed.
In the initial forming induces stress into the material, when you reverse form even more stress is induced because because the grain lines are not lining up. Stress risers are created and the metals ductility limits can be exceeded and you get cracks or at least weaker points.
This is what happens when you bend a piece of metal back and forth until it breaks. You can see it happening. Bend it one way, look at both sides of the radius. The outside may have a slight “orange peel” appearance - that is the stretched metal. On the inside, slight waviness as the metal builds up. Keep doing this and you actually see it get worse and worse, more distorted until it breaks.
If you don’t want the residual stress induced into the part reduced you have to anneal it, then reheat heat treat it (assuming you don’t want it in a soft condition).
You’ll see temper numbers on Aluminum like T-0, T-3XX, T-4XX, T-6XX etc… These are giving you the hardness numbers and how it got there: i.e. T-4XXX is material that has naturally aged to its hardness (some metals when annealed unless kept extremely cold will go back to their hardness) T-6XXX is artificially aged meaning it was heat treated my various means to get it there. T-3XX is considered non-treatable. After material is formed at the mill it has residual stress which can make it go wonky when trying to form it or just lay straight and flat. T-35XX is physically stretched in one direction about 5%, the metal takes a set and lines the grain up in one direction. If you have material it should have an arrow near the numbers that show the direction was stretched in. It is considered non-heat treatable becuase if you heat treat/anneal it, the induced stress by stretching and grain direction that was desired is relieved and the benefits disappear. So you lose that benefit.
When I was working and building aircraft, the general rule was:
- NO reverse forming, creates a stress riser that will lead the cracking failure
- If under formed, you can continue in the same direction without submitting to engineering or scraping
- If the angle is slightly off, if by just using finger pressure, it can be moved into position for fastening, you were within spring-back limits (meaning you were within the modulus of elasticity so it doesn’t take a set) and not considered pre-loading the the part.
- Radius should be no tighter than 2T (twice the thickness of the material. E.g 18ga steel is .0478" thick, so the absolute minimum bend radius would be .0956". Better if larger, you can bend it further without cracking.
What is critical is what is the part being used for: if decorative or under no load, you can get away with some degree of reverse forming. But if strength is important, then avoid reverse forming, with tight small radii. If the bend is a large number of degrees, best to use annealed material as it is more ductile and then heat treat it.
This is more than a simple YES, but if you understand what is going you understand what is happening and can apply it to other metal forming.
Hope this helped more than confused.
