DOM steel tubing is manufactured the same way as mild, including the welding. The alloy is typically 1018 up to 1026, the higher the number, the higher the carbon content. DOM means Drawn Over Mandrel, which “trues” the tube and hides the weld, giving it more accurate dimensions. DOM is about twice the cost of mild, and almost as much as 4130.
4130 chromoly steel tubing is a true seamless tube, with chromium and molybdenum added for strength and light weight. 4130 is very expensive and is used most often in big budget builds. It requires heat treating after welding to achieve maximum strength. Some say that if a 4130 chassis is not heat treated, it is no stronger than the other steels. 4130 suspension components should definitely be heat treated.
All that was taken from here -
http://www.pirate4x4.com/forum/showthread.php?t=386480
