Steady state micro welding was an advanced engineering technique pioneered by the Lamadines which the Third Doctor used to try and repair his dematerialisation circuit during his exile to Earth.
The Third Doctor attempting to use steady state micro welding to repair his dematerialisation circuit. (TV: Terror of the Autons)
When smoke started to come from the circuit, Jo Grant thought it would start a fire and sprayed it with a fire extinguisher. However, the Doctor said that steady state micro welding "always produces more smoke then fire". (TV: Terror of the Autons)