TWI Frequently asked questions
by Ruth Hammond
Any moisture or other hydrogenous material in a welding arc will be broken down into hydrogen and oxygen. Molten metal will absorb this hydrogen from the arc atmosphere. Any hydrogen content of welding consumables or parent metal may also add to this hydrogen absorbed from the arc.
The principal sources of hydrogen in the arc atmosphere, are moisture or hydrogenous compounds in the welding consumables (flux etc.) and oil, dirt, grease and hydrated oxides (e.g. rust), on the surface of the welding wires. Any hydrogen or water in the shield gas will also provide a source of hydrogen, and there can also be a small contribution from moisture in the ambient atmosphere.
Figure 1 shows the relationship between moisture content of the shielding gas and hydrogen content of the weld metal for both argon and carbon dioxide shielding gases.
Fig. 1. Plot of dependence of weld metal hydrogen content on moisture content of shielding gas after Krahl [1]. The upper and lower pairs of lines are for different electrode types: electrode 1 is a carbon steel wire which was copper coated and electrode 2 is an austenitic Cr-Ni steel. It is unlikely that the hydrogen content actually falls to zero when there is no moisture content in the shielding gas as suggested in this figure. Reproduced by permission of DVS.
The relationship between the amount of hydrogen absorbed by the molten weld pool and the percentage hydrogen in the arc atmosphere is illustrated in figure 2. (This should not be confused with the percentage of hydrogen in the shielding gas.)
Fig. 2. Dependence of hydrogen absorbed by weld pool on concentration in atmosphere surrounding arc at 1900°C[2]. Reproduced with permission of Woodhead.
White and Chionis[3] considered the prediction of the hydrogen in the weld metal from knowing the hydrogen content in the shielding gas. Although the prediction consistently underestimated the amount of hydrogen in the weld, the relationship between the percentage of hydrogen in the shielding gas and the hydrogen in the weld metal was determined. Figure 3 shows the relationship between hydrogen content of the shielding gas (Argon-2% oxygen with hydrogen additions) and weld metal hydrogen content.
