The use of 4130 over mild steel is for wt.# saving's.1.75X.095 4130 weighs the same per foot as 1.75x.095 mild steel tubing.4130 is about 60% stronger than mild steel so you can use a thinner wall thickness than mild steel to abtain the same amount of strenght in your roll cage or chassis.Most of the drag race chassis builder's I've seen use a gas lense as Greg say's and never post heat the tube joint anymore.They just hold the torch over the weld puddle after finishing the weld and let the post flow cool it down.If you keep the ambiant tempure down you should not have any weld joint failure.You will know when your weld temp is right your weld on 4130 will a rainbow of gold to blue.To hot and the weld will turn out all grey & will see small stress cracks if you use a mag. glass.
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