Hi John,
from what you have described you have;
a 3 phase 100amp rated submain from meter room to a subboard in your unit (protected by a 100a circuit breaker)
On your subboard, you have a 100amp main switch, which isolates the circuit breakers to your subcircuits, (light, power, etc) including a 70amp 3 phase subcircuit for your welder.
One phase from the meter room to your unit keeps burning out.
Two things spring to mind
1/ High resistive joint (loose connection) in submains
2/ Submain circuit breaker needs to be upgraded to a higher kVA rating
(Industrial is normally 6kVA with the inductive load from your welders,
you may need a 9kVA rated breaker)
+ you really shouldnt need to keep tightening the terminals on your circuit breakers, have your sparky check them for you.
Let me know how you go
cheers