DOM and HREW are both mild steel (usually 1020) and can easily be welded with MIG (or stick). The chromoly (4130) tubing is an alloy of steel, chromium and molybdenum and is quite a bit stronger and more expensive than mild steel. Most mild steel yiled strength is in the 60,000 psi range where as chromoly is in the 95,000 psi range.
Although chromoly can be welded with MIG, TIG is prefered and the welds must be heat treated after welding to relieve stress.
There is a good article on the FEB 06 4wheel and off road mag
I'm no TIG expert but I'm pretty sure it's the opposite. With Chromo, when you weld it it gets brittle if not pre/post heated. If you cool it down too fast the weld area is actually weaker since the area surrounding the weld is brittle and can snap. What the post heat is doing is stress relieving actually softening the area, I believe.... Or making it more uniform... :
when I redo my cage I am going to use DOM as well as my sliders and exo. My current cage is HREW and it has held up fine so far but with one good roll on it and how easy it bent and dented I am considering DOM now. I will probably use .120" wall for almost everything and then .188 for the sliders.
cromo is in my opinion is then best but not as easy to work with like DOM or HREW and most of us don't need a cage that can handle a 120 mph roll when I flipped my junk it was backwards and the windshield frame and cage too the most of it but the cage got it worse it was wierd
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