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The problem of variable weld penetration or 'cast-to-cast' variation in GTA/TIG welding is discussed. It is shown that for normal GTA/TIG welding conditions the Heiple-Roper theory is valid, i.e. that weld penetration is controlled by the fluid flow in the weld pool which, in turn, is controlled by the direction and magnitude of the thermocapillary forces. For most steels the direction and magnitude of these forces are determined by the sulphur content, since the temperature coefficient of surface tension (dγ/dT ) is negative when S < 30 ppm and this leads to a radially outward flow and poor penetration whereas a steel with S > 60 ppm has a positive (dγ/dT ) which produces a radially inward flow giving good weld penetration. Thermocapillary forces were shown to play a part in the problems of 'off-centre welding', 'porosity' and 'arc wander' in GTA/TIG welding and in the surface rippling of welds.

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