The Behavior of Hydrogen in Arc Welding

Some considerations on the sources of hydrogen in shielded metal-arc welding

Abstract

Studies are made to determine the sources of hydrogen in shielded metal arc weld metal.
First, by assuming that dissolution of hydrogen in molten metal obeys Sieverts'law, an equation is theoretically derived which is able to distinguish between the effects of humidity in the ambient atmosphere and those of moisture from the electrode coating.
Next, the coefficients involved in the equation are determined experimentally.
Finally, a formula which can predict the diffusible hydrogen content in the weld metal of HT50-HT100 steels is proposed.
The results are summarized as follows:
1) Evaporation of water from coating flux of low hydrogen type electrode occurs in two stages; one is the evaporation of moisture up to about 200°C and the other is that of bound water with a peak temperature at 750°C.
2) The effects of slag covering and its basicity in weld puddle on the diffusible hydrogen content are negligible in the case of low hydrogen type electrode.
3) Moisture evaporated from the coating of electrode at elevated temperatures contributes as the cause of the diffusible hydrogen in weld metal much more than that evaporated at relatively low temperatures.
4) The diffusible hydrogen content [H]_D in weld metal in ml/100g is given by
[H]D=(A+12a+0.64b)^1/2-1.8,
where A is a material constant of electrode which depends mainly on the evaporation of moisture at elvated temperatures in ml/100g, a being moisture content in the coating in wt.% and b the partial pressure of vapour in the ambient atmosphere in mmHg.
5) The above equation gives the practical information to control the diffusible hydrogen content in weld metal.