Originally Posted by matts65ss
Do not use chromoly I dont know the exact specifics but i think if welded incorrectly it can become brittle and crack. Especially for a glorified harness bar haha. Ive probably built around 15 cages mostly complete offroad chassis' but a few road course cars and never needed to use chromoly.If your dead set on this then use .120 wall DOM if i was doing a cage in my wrx i would probably go with 1.5''
but dont get me wrong I have had to talk myself out of putting a custom cage in my car numerous times
there is some truth to this.
Chromoly is a great material to use due to its strength, but you are correct - it can be severely weakened through the welding process which is caused by letting it cool too quickly. The trick to 4130 is temp is heat treating. When done correctly, 4130 can achieve a yield of 150,000psi vs the normalized 90,000psi.
You can achieve close to the same yield strength as normalized 4130 with most mild steels.