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A question regarding tube bending 1
- Thread starter zebmahar
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EdStainless
Materials
Hot bending can induce changes in microstructure.
Cold bending also can offer tighter control of the dimensions.
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P.E. Metallurgy, Plymouth Tube
Cold bending also can offer tighter control of the dimensions.
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P.E. Metallurgy, Plymouth Tube
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1
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racookpe1978
Nuclear
A hot bend is easier to bend = true. BUT, if you are doing hundreds of bends an hour, or even only two or three a day, both a hot bend and a cold bend require a form, a die and a process or machine to pull the tube through the bend. You are actually rolling the tube past a fixed die with a mandrel and lever arm against the fixed die.
A hot bend MUST BE exactly the same temperature across the entire bend length (and be very, very carefully kept exactly at the same temperature EVERY time the same part is made.) Otherwise, the part is ruined - it bends too far at the hot spots and too little (is still straight) at any or all of the cold spots. So, as you do more and more bends, the mandrel and die and machine heat up - you get different results. Then you go to lunch, or lose power, or do something else for a few minutes, and the parts change again., Start up again the next day, you make all of the same differences in each part.
You need more energy for a cold bend, but that bend is simple and repeatable. Build the machine for a big tube, buy a new mandrel and die and you can use it for any smaller tube diameter, wall thickness, and bend diameter. A cold bend is the same bend across the whole tube every time because the metal yields predictably every time. (Unless wall thickness changes.)
A hot bend MUST BE exactly the same temperature across the entire bend length (and be very, very carefully kept exactly at the same temperature EVERY time the same part is made.) Otherwise, the part is ruined - it bends too far at the hot spots and too little (is still straight) at any or all of the cold spots. So, as you do more and more bends, the mandrel and die and machine heat up - you get different results. Then you go to lunch, or lose power, or do something else for a few minutes, and the parts change again., Start up again the next day, you make all of the same differences in each part.
You need more energy for a cold bend, but that bend is simple and repeatable. Build the machine for a big tube, buy a new mandrel and die and you can use it for any smaller tube diameter, wall thickness, and bend diameter. A cold bend is the same bend across the whole tube every time because the metal yields predictably every time. (Unless wall thickness changes.)
moltenmetal
Chemical
The easiest, fastest and cheapest bend for stainless steel tubing is a 3D externally guided cold bend. From 1/8" through 1/2", you can do this with hand-operated lever tube benders- and at 3/4" and 1" you can manage with hand-operated crank benders.
Need to bend tighter than 3D? Now you need the bother of either internal guiding or heat. The bother associated with heat has been very articulately explained by racookpe1978.
Need to bend tighter than 3D? Now you need the bother of either internal guiding or heat. The bother associated with heat has been very articulately explained by racookpe1978.
EdStainless
Materials
We cold bend SS and Ni alloy tubes, a lot.
We actually use mandrels up to about r=4d, that way we can control the balance between wall thinning on the extrados and ovality in the bend.
And in most cases we post bend anneal the ubends. Clamp the legs just past tangent point and pass DC current through them using optical pyrometers to control the temp. The real critical part is the cooling. You need fast enough to preserve the structure, but slow enough to not introduce severe residual stresses.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
We actually use mandrels up to about r=4d, that way we can control the balance between wall thinning on the extrados and ovality in the bend.
And in most cases we post bend anneal the ubends. Clamp the legs just past tangent point and pass DC current through them using optical pyrometers to control the temp. The real critical part is the cooling. You need fast enough to preserve the structure, but slow enough to not introduce severe residual stresses.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
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