Hello and Happy New Year Miguel;

I suspect the issue is the amount of aluminum whose primary function is as a deoxidizer and grain refiner used with the self-shielded T-11 electrodes. While aluminum does a great job of both deoxidizing and grain refinement, it isn't favorable when notch toughness is an issue. Notch toughness isn't usually an issue on thin carbon steels or high strength low alloys steels, it becomes more of a concern when the thickness of the base metal increases. The thickness of the joint influences the degree of restraint. As rule of thumb, thicker base metals inherently increase the degree of restraint in proportion to the square of the thickness. 

The other issue is that single pass welds typically require a higher level of deoxidation if welding over mill scale, lightly oxidized surfaces (rust), etc. However, in the case of multiple pass welds, the same level of deoxidation is not required. The deoxidizers that are not utilized for that purpose become alloy additions in the deposited weld metal. Increases in the alloy content can increase the strength and hardness at the expense of ductility. So, thick joints that require multiple weld beads increase the degree of restraint. The weld requires good ductility to accommodate the forces that develop as the weld cools and contracts. Yielding in the weld deposit accommodates the residual forces that are on par with the yield strength of the weld and base metal. Something has to "give" to accommodate the residual stresses as the weld and adjacent base metal cools. High levels of deoxidation that isn't needed results in an increase in the alloying of the weld deposit resulting in increases in strength at the expense of ductility. This fact works counter to what is needed to accommodate the high restraint and residual stresses. Add to the mix is the high percentage of aluminum in the weld deposit. As it turns out, too much of a good thing is detrimental to the properties of the weld. Excess aluminum reduces the toughness of the weld deposit. There is a loose relationship between good toughness and good ductility. With the reduction in ductility, there is typically a reduction in toughness.

Limiting the thickness of the weld and base metal welded with the EXXT-11 electrodes mitigates the issue of increased alloying in the weld deposit, minimizes the amount of aluminum in the weld deposit, and minimizes the degree of restraint in the welded joint.

Several years ago, I had a project where the contractor utilized an electrode intended for single pass welds deposited at high speed (read that as a robotic processes) on thick moment connections (20 mm up to 40 mm). He chose the electrode because the deposition rates were high compared to other electrode types. The field welding was performed with semi-automatic equipment with travel speeds that were a fraction of what a robot would have used. The end result was that every groove weld cracked down the center-line. The contractor made a comment that it wasn't a problem because the cracks were superficial. A minimal amount of grinding would eliminate the cracks. On that point, he was correct. A light grinding operation that extended down to the backing bar was all that was needed to eliminate the cracks (sarcasm). It took weeks to excavate every weld and reweld the connections.

The axiom "The less they know, the more we make" held true.

Best regards - Al