A filler metal classification is a specification compressed into a handful of characters. Every position in the string carries a separate piece of information, and once you know which position means what, a designator you have never seen before still tells you the strength, the positions it can be run in, the current it wants, and roughly how it will behave in the puddle.
That is why classifications come up so often in examination questions and in procedure review. An inspector who can read a designator can check a consumable against a procedure without opening a catalogue, and can tell immediately when a substitution changes something that matters. This guide decodes the common families one character at a time. It describes widely used industry designators and general behaviour; the filler metal specification itself is the authority for any particular classification, and the procedure governing your work decides what is actually permitted.
Covered electrodes: the four-digit backbone
A carbon steel covered electrode is written as E followed by four digits, as in E7018. The leading E simply means electrode. The first two digits give the minimum tensile strength of the deposited weld metal in thousands of pounds per square inch, so 60 means 60 ksi and 70 means 70 ksi. On higher strength electrodes that field runs to three digits, which is why E9018 has one more character than E7018 and means 90 ksi.
The next digit is position. A 1 means the electrode is intended for all positions. A 2 restricts it to the flat position and to horizontal fillets. A 4 indicates suitability for all positions including vertical progression downward, which is a technique choice rather than a bonus capability. That single character is the field most often overlooked during a substitution, and it is one of the most likely to matter.
The final digit does not stand alone. It is read together with the position digit as a pair that describes the covering chemistry and the welding current the electrode is designed for. That pairing is why E7018 and E7028 share a coating family but differ in position, and why the last digit alone tells you very little.
- E - electrode
- First two or three digits - minimum tensile strength in ksi
- Next digit - position: all positions, flat and horizontal fillet, or all positions with downward progression
- Last digit, read with the position digit - covering type and welding current
What the covering pair actually tells you
The covering determines almost everything about how an electrode feels and what it is good for. Cellulosic coverings burn to produce a strongly penetrating, forceful arc with a thin slag, which is why they are associated with open-root pipe work and with running on direct current electrode positive. They are also the family with the highest hydrogen potential.
Rutile coverings give a soft, quiet arc with a heavy easily removed slag and a good-looking bead, which makes them popular for general fabrication and for sheet. They tolerate less than perfect base metal and they run on alternating or direct current depending on the exact classification. What they do not offer is a low-hydrogen deposit, which is why they are not the choice on restrained or higher-strength work.
Low-hydrogen coverings are the ones an inspector cares about most. They are formulated to keep diffusible hydrogen in the deposit low, which is what makes them the default choice on higher strength steels, thicker sections, and restrained joints where hydrogen-assisted cracking is the risk being managed. They pay for that with sensitivity to moisture, which is why storage is a procedural requirement rather than good housekeeping.
Iron powder, deposition, and why some electrodes are flat only
Several coverings include iron powder, which is added to the coating and transfers into the weld as additional deposited metal. The result is a higher deposition rate for the same core wire and a heavier, more fluid slag. That combination is excellent for productivity and poor for gravity, which is why the heaviest iron powder classifications are restricted to the flat position and horizontal fillets.
This is the practical reason a shop that runs a high-deposition electrode on flat seams still keeps an all-position low-hydrogen electrode for everything else. They are not interchangeable, and a substitution that ignores the position digit will show up as a struggling welder before it shows up as a nonconformance. It is also a useful sanity check on paperwork: a procedure qualifying a vertical position while calling out a flat-only classification contradicts itself, and that is exactly what procedure review is for.
The suffixes: chemistry, toughness and hydrogen
Anything after the four digits is a suffix, and suffixes fall into a few recognisable groups. A plain numeric suffix, as in a low-hydrogen electrode with a trailing dash one, generally signals improved impact toughness at lower temperature compared with the base classification. Suffixes are the part of a designator most often dropped in casual conversation and most often load-bearing in a procedure.
Letter-plus-number suffixes indicate an alloyed deposit. An A suffix points at a carbon-molybdenum deposit, B suffixes at chromium-molybdenum deposits of increasing alloy content, and C suffixes at nickel-bearing deposits intended for low-temperature toughness. These matter for creep service, for elevated temperature piping, and for anything where the weld metal has to match a heat-treated alloy base.
An H followed by a number is a diffusible hydrogen designator: the number is the maximum diffusible hydrogen in millilitres per hundred grams of deposited metal, so a lower number is a tighter limit. A trailing R indicates a covering formulated to resist moisture pickup. On crack-sensitive work these two are often the whole reason a particular product was specified.
- Numeric suffix - improved low-temperature impact properties
- A, B and C letter suffixes - molybdenum, chromium-molybdenum and nickel-bearing deposits
- H with a number - maximum diffusible hydrogen, lower number meaning a tighter limit
- R - covering formulated to resist moisture absorption
Solid wire: the ER family
Solid wires for gas metal arc and gas tungsten arc welding are written as ER followed by a strength number, then S, then a dash and a number or letter. The ER means the product can be used as an electrode or as a filler rod, which is why the same designator covers a spool for wire feeding and a cut length for manual tungsten arc work.
The strength number is the minimum tensile strength in ksi as before, and the S means solid. The character after the dash identifies the chemistry, and the practical difference it makes is deoxidiser content. More silicon and manganese means better tolerance of mill scale and surface contamination, a more fluid puddle, and a flatter bead; less means a wire intended for cleaner material or for a specific mechanical property target.
Alloy and non-ferrous wires follow their own conventions within the same shape. Stainless wires carry the alloy type directly, so the digits identify the grade and an L means a low carbon variant chosen to limit carbide precipitation. Aluminium wires use a four-digit alloy number, where the choice between a silicon-bearing and a magnesium-bearing filler is driven by the base alloy and by whether the joint will be anodised or heat treated.
Flux-cored wire: the T family
A flux-cored designator such as E71T-1 reads with the same logic and one extra idea. The E is electrode, the first digit is tensile strength in tens of ksi rather than the full number, the second digit is position with 1 meaning all positions and 0 meaning flat and horizontal, and the T means tubular.
The number after the dash is a usage designator, and it is doing a lot of work. It bundles together the intended shielding arrangement, the polarity, the transfer behaviour, and whether the wire is single or multiple pass. This is why two wires of identical strength and position can be completely different products: one needing an external shielding gas and one self-shielded, one designed for a smooth spray-like transfer and one for a globular arc that copes with wind.
Further letters can follow to identify the shielding gas the classification was tested with, and a G anywhere in the string means general, meaning the properties were agreed between supplier and purchaser rather than fixed by the classification. A G designator is a flag during procedure review, because it means the designator alone does not tell you what you are getting.
- First digit - tensile strength in tens of ksi
- Second digit - all positions, or flat and horizontal only
- T - tubular, meaning a flux-cored or metal-cored product
- Usage designator after the dash - shielding, polarity, transfer and pass capability
- G anywhere - properties agreed between supplier and purchaser, not fixed by the classification
Reading a designator as an inspector
The review question is never whether a consumable is good. It is whether this consumable is the one the qualified procedure permits, in the condition the procedure requires, used within the parameters the procedure allows. A designator that differs from the procedure by a single character is a different consumable, and the difference may be strength, position capability, hydrogen level or shielding requirement.
Condition is the half that gets skipped. Low-hydrogen coverings absorb moisture from the air, so the procedure will normally set out how they are stored, how long they may be exposed, and whether and how they may be reconditioned. An electrode of exactly the right classification that has spent a shift in a damp bag is not the consumable the procedure qualified.
The same care applies to identification. Consumables need to remain traceable to their classification and batch through storage and issue, and a bin of unmarked electrodes is a finding regardless of what somebody remembers putting in it. Reading designators fluently is only useful if the designator on the packet still means something by the time the metal reaches the arc.
- Read the strength field and compare it against the procedure and the base metal
- Read the position field against the positions the work actually requires
- Read the covering or usage designator for current, shielding and transfer
- Check every suffix, since suffixes carry toughness, alloy and hydrogen requirements
- Confirm the storage and exposure condition matches what the procedure demands
- Confirm the consumable is still traceable to its classification and batch