Originally Posted by
WLIU
I have not seen "mild" steel used in aircraft construction or repair. All Aircraft Spruce sells is 4130. I will suggest that for the last 50 years steel tube fuselages have been built with 4130 and you will not find any issues that involve "brittleness". Having seen aircraft built with 4130 tubing hit the ground and other objects, 4130 looks pretty tough to me. I think that the FAA expects repairs to steel tube fuselages to use 4130 replacement or reinforcement following the instructions in AC43.13-1b Chapter 4.
Order up the stock you need an start welding without worry.
Best of luck,
Wes
N78PS