Lawrence is right in the fact that 5P and 5P+ is a 60KSI series rod and is not low hydrogen.
This is why it is not allowed on higher strength structural steel.
The AWS D1.1 Structural Welding Code, in Table 3.1, lists the electrode classifications that are allowed to be used on particular grades of steel. It divides many steel grades into 4 groups I, II, III and IV. Group I steels, such as the common A36 can be welded with 60KSI series rods, which is what I always considered to be "mild steel". Group II steels, such as A572 grade 50, require 70KSI series electrodes. Group II requires 80KSI, and group IV requires 90 ksi. This table also lists the preheat requirements for the different groups as well.
Steel grade A 106 is what most pipe is made of and is considered by AWS to be a group I steel, and is weldable with 60KSI series electrodes, such as 5P.
The main reason structural steel designers like to use the higher strength steels, like A572 grade 50, is because they can use a thinner or lower weight piece of steel to hold up the same amount of weight as A36. When you're paying for steel by the pound, it makes a big difference, considering that, for the most part, grade 50 costs the same as A36.
Does this answer your question?
Tim
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