This article introduced structure principle and usage of semi-automatic plasma run-on and run-off tab device which possess some characteristics,such as simple operation,structure novelty and economical and practical under without changing current procedure and no need to add auxiliary equipment.

As for to cracking generating in the course of sub-merge arc weld for longitudinal weld of pressure vessel body by 16MnR steel,This article analyses the causes of the crack,measures such as improving blowout plate,etc.