TechnicAl, I use a 312 on 4130, I know most people say an ER80 or ER90 rod for BS4T45,
are there any reasons why a 312 rod could negatively affect the weld metalurgy of a joint in T45?
No not really (theres a long shot possibility of reduced fatigue life) but its a sledgehammer to crack a nut. Some "experts" will tell you 312 is the way to go (I used to be one of them) until I realised that the material is really thin and the weld is almost always proud. Now I know that A15 / A18 does the job its all I ever recommend. Its a tried and tested method. All the Ford WRX were done this way. I know, I recommended it.