HOW TO DETERMINE THE DIFFUSIBLE HYDROGEN OF MILD STEEL WELD FILLER MATERIAL

To avoid hydrogen-induced cracking, the hydrogen level in the welding filler material must be restricted to a certain maximum level. What exactly is the method for measuring the hydrogen content of a carbon steel weld filler material?

From the outset of my career I was taught the virtues of using an electrode capable of producing low hydrogen deposit. During my training the terms “cold cracking” and “hydrogen-induced cracking” were frequently conveyed as something to be wary of when welding of high-strength, low-alloy steels.

Only recently have I given much thought as to how an electrode is actually determined to provide a low-hydrogen deposit. But in my current role, it’s become critical for me to take a keen interest. So what exactly is the method for measuring the hydrogen content of a carbon steel weld filler material? I’ll share some of the highlights of this process and offer some perspective.

It’s fairly well known that in order to avoid hydrogen-induced cracking, the hydrogen level in the welding filler material must be restricted to a certain maximum level. Hydrogen-induced cracking, which is better known to some as underbead cracking, is characterized by separation of the weld from the base material in the heat affected zone immediately adjacent to the weld metal. This phenomenon occurs within a few hours after welding has been completed and is partially a result of the diffusion of hydrogen from the weld metal into the base material. But migration of hydrogen to the heat-affected zone is not the only factor leading to this type of cracking.

While there does exist a certain threshold value for weld metal hydrogen content leading to underbead cracking, it is very much situation-specific. In addition to diffusible hydrogen content, underbead cracking is dependent upon base metal microstructure susceptibility, weld joint restraint, and welding residual stresses. Microstructure susceptibility to hydrogen-induced cracking most often increases with increasing steel base material strength. This means that for higher strength steels the use of electrodes with lower levels of hydrogen is important. But to apply the qualitative statement “use low hydrogen electrodes” is not sufficient because what is low for some steels may not be low enough for others.

While the effects on the base material due to weld metal diffusible hydrogen in welding operations (such as structural steel fabrication) can be significant, the effect of hydrogen on the weld metal itself is usually far less dramatic. In the course of filler metal qualification testing, the effect of excess levels of hydrogen may be observed during tensile testing. However, those effects are generally limited to a reduction in the ductility of the weld metal but have almost no appreciable effect on its tensile strength, yield strength, or toughness.

Occasionally a tell-tale sign of a high level of diffusible hydrogen content in the weld metal will present itself when a broken tensile specimen exhibits “fisheyes” on the fracture surface, as seen in Figure 1. Otherwise, it may not be readily determined from mechanical or impact testing that a weld metal has relatively high diffusible hydrogen. After all, an E6010 electrode that has been shown to have inherently high hydrogen content regularly provides in weld metal conformance tests CVN impact toughness results of 20 ft-lb or greater at a test temperature of minus 20 deg F.

For these reasons engineers and fabricators (are encouraged to) use specific statements in contract documents such as “only electrodes or electrode-flux combinations capable of depositing weld metal with a maximum diffusible hydrogen content of 4 ml/100 g (H4) are permitted to be used”, rather than general statements such as “use low hydrogen electrodes”.

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