Welding consumables - Part 3

Job Knowledge 84

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Part 2
Part 4
Part 5

In order to be able to specify the type of flux coating, welding characteristics and chemical composition of an electrode for a particular application, there needs to be some standardised method of unique identification that is universally recognised.

This requirement has led to the writing of a series of consumable specifications that enable an electrode to be easily and uniquely identified by assigning a consumable a 'classification'. The two MMA electrode classification schemes that will be dealt with in this month's article are the EN (Euronorm) and the AWS (American Welding Society) specifications. There is insufficient space to cover in detail the whole range of compositions for MMA electrodes so the emphasis here will be on the carbon steel filler metals.

The European specification for non-alloy and fine grained steel MMA electrodes is EN ISO 2560. This contains two different systems for classifying electrodes: System A uses yield strength and 47J impact energy, and System B uses tensile strength and 27J impact energy. The standard further divides each classification into two parts. Part 1 is a compulsory section that requires symbols for the process, strength and elongation, impact strength, the chemical composition type of flux coating and any post-weld heat treatment (System B only). The second part is optional and includes symbols for the type of current and metal recovery, the welding position(s) that the electrode can be used in and for the maximum hydrogen content of the deposited weld metal (NOT the electrode).

The designation of a covered electrode begins with the letter 'E'. This tells us that this is a covered electrode intended for MMA welding. The next two numbers give the minimum yield strength that may be expected as shown in Table 1a for System A and Table 1b for System B.

Table 1: Strength and elongation symbols

The next symbol under System A indicates the temperature at which an average impact value of 47J can be achieved, as shown in Table 2 (see the standard for the details of System B).

The third mandatory symbol is for the composition. Although the specification title (non-alloy and fine grained steels) suggests that the electrodes have no alloying elements present, up to 3% Ni and NiMo electrodes are included, see Table 3 for the symbols under System A (see the standard for the details of System B). (This symbol is only applied where the electrode contains ≥0.3Mo or ≥0.6Ni).

The fourth symbol indicates the type of flux coating - basic, rutile etc as shown in Table 4 for the symbols under System A (see the standard for the symbols under System B, as this symbol also contains information on welding positions and suitable current for System B).

System B also contains a symbol which System A does not, which can describe whether electrodes properties are for the as-welded or post-weld heat treated condition.

The next three symbols under System A are not compulsory and give additional information on the percentage weld metal recovery and the type of welding current on which the electrode can be operated (Table 5); the welding position (Table 6) and the maximum hydrogen content of the deposited weld metal if the electrodes are dried or baked as recommended by the manufacturer (Table 7). System B only makes use of the diffusible hydrogen symbol.

A full designation may therefore read E42 2 B32H5. This describes a basic carbon manganese steel electrode; weld metal yield strength of 420N/mm2, better than 47J at -20°C, a weld metal recovery of over 105%, capable of being used on AC or DC current in all positions except vertical down and providing less than 5mls hydrogen in the weld metal.

The AWS specification equivalent to EN ISO 2560 is AWS A5.1 - Specification for Carbon Steel Electrodes for Shielded Metal Arc Welding. The classification comprises five characters but in there are two separate schemes. A5.1, based on the US units of tensile strength in pounds per square inch, Charpy -V values in foot-pounds and A5.1M, based on the SI system, with strength in MPa, Charpy-V values in Joules.

It is thus possible to have virtually identical electrodes with different classifications, one using US units, the other SI units. There is insufficient space within this brief article to describe fully all of the types covered by the specification except perhaps for the most commonly used electrodes. For full details of the AWS scheme it is necessary to consult the specification.

To illustrate briefly how the electrodes are classified, the following gives a summary of the key features.

The first character 'E' is common to both classifications and indicates that the electrode is a flux coated manual metal arc electrode. The next two digits indicate the tensile strength. In the A5.1 designation this is either '60',indicating a UTS of 60ksi and a yield strength of 48ksi, or '70', indicating a UTS of 70ksi and a yield strength of 58ksi. In the A5.1M designation these are 43 indicating a UTS of 430MPa, yield strength of 330MPa or 49, indicating a UTS of 490MPa yield strength of 400MPa.

The last two digits give information on flux coating type, welding position, current type and polarity and Charpy-V impact strength, if required. Those electrodes suffixed XX10 or XX11 have cellulosic coatings; those suffixed XX12,XX13, XX14, XX19 or XX24 have rutile coatings and those suffixed XX15, XX16, XX18, XX28 and XX48 are basic low hydrogen. XX18, XX28 and XX48 all have iron powder additions and are therefore high recovery electrodes.

This article was written by Gene Mathers.