How to Pick Filler Metal: Strength Match, Toughness, Hydrogen and Machinability

Published October 14, 2026. How we research this.

Filler metal is the part of a weldment that nobody sees after the weld cools, and it carries as much load as the steel on each side. A weld can only be as strong, as tough and as machinable as the filler allows. A drawing that leaves the choice open gets whatever each shop has on the shelf.

This guide explains the four questions that govern filler choice and what a buyer should state so that every shop quotes the same weld.

Question 1: how strong must the weld metal be

For a carbon or low alloy structural steel, the filler is usually chosen to match the parent steel’s strength. A filler for S355 or A572 Grade 50 is in the 70 ksi tensile class, which in the AWS system means a minimum tensile strength of around 490 MPa and in the European system a code such as G 42 for a common solid wire.

The strength levels are described in three ways.

Term	Meaning	Typical use
Matching	Weld strength about equal to the parent steel	Most load carrying joints
Overmatching	Weld stronger than the parent steel	Joints where the weld is the weak point by geometry, such as small fillets
Undermatching	Weld weaker than the parent steel	Very hard or high strength plate, to reduce cracking

Overmatching is not always better. A weld much stronger than the parent can be harder and less ductile, and it can concentrate strain in the heat affected zone. The design engineer decides how much to match. Buyers rarely do, and should not guess.

Question 2: what classification to write down

Fillers carry a classification that fixes their properties. The classification is printed on the electrode or the wire spool.

Process	Example classification	What it tells you
Stick (SMAW)	AWS A5.1 E7018	Tensile class, position, low hydrogen coating
Solid wire (GMAW)	AWS A5.18 ER70S-6	Solid wire for gas shielded welding on carbon steel
Flux cored (FCAW)	AWS A5.20 E71T-1	Flux cored, gas shielded, all positions
Submerged arc (SAW)	AWS A5.17 F7A2-EM12K	Flux and wire combination with tested impact value
TIG rod (GTAW)	AWS A5.18 ER70S-2	Solid rod for gas tungsten arc welding

European standards use a different code. For example, EN ISO 14341 describes solid wire for gas shielded welding with a code that shows yield level, impact temperature and shielding gas. The designation system differs, but the idea is the same.

The right classification depends on the parent steel, the process and the toughness the joint needs. A drawing or a procedure that names the classification takes the guesswork out. If you are unsure, state the performance you need, such as the minimum tensile strength and the impact energy at the service temperature, and let the shop propose the classification for approval.

Question 3: how much hydrogen can the joint tolerate

Hydrogen cracking is the main risk on thicker and stronger steels. The filler contributes hydrogen through moisture in the flux or coating and through contamination. Fillers are sold with a diffusible hydrogen designation, such as H4, H8 or H16, which states the maximum millilitres of hydrogen per 100 g of deposited weld metal.

A low number is better. A low hydrogen filler is a basic coated electrode such as E7018, or a clean solid or metal cored wire, handled and stored dry. Cellulosic stick electrodes and wet flux add hydrogen and are not suited to thick or high strength joints.

Practical consequences:

• Low hydrogen electrodes are stored in a heated oven and used within a set time after they come out
• Electrodes exposed beyond the time limit are rebaked according to the maker’s instructions or scrapped
• Flux for submerged arc welding is stored dry and rebaked if it picks up moisture
• Joints are clean and dry, and plate that has been in the cold is brought up to temperature before welding

Hydrogen control works together with preheat, which is explained in thick plate bevel prep and preheat. A shop with a clear drying and storage routine can describe it in a few sentences.

Question 4: how does the weld machine

This question matters for a machined weldment more than for a plain frame. The weld metal and the parent steel have to cut together.

Hardness. A weld metal harder than the parent steel resists the cutter, and the tool may skip or dig in at the transition. Matching strength fillers on structural steel generally machine well. Very high strength fillers and some low alloy types make a harder zone.

Work hardening. Austenitic stainless and nickel based fillers harden as they are cut. Machining them needs sharp tools and positive feed, as discussed in stainless steel weldments.

Uniformity. A weld with uneven hardness along its length gives a changing cut. A procedure with controlled heat input and consistent passes gives a more uniform bead.

Colour and appearance. If the weld will be visible after machining, the filler should match the parent steel so that the finish is uniform. This is mainly a cosmetic point.

What stress relief does to weld metal

If the weldment is stress relieved at 580 to 650 degrees C, the heat treatment changes the weld metal too. Some fillers gain or lose strength and toughness during the hold. A filler that passes impact tests as welded can fall below the requirement after a long stress relief.

Fillers are therefore tested in two conditions, as welded and after post weld heat treatment, and the maker states which are available. If your part will be stress relieved, tell the shop at quote stage and make sure the filler has been qualified in the heat treated condition. See stress relief before machining for the reasons and the usual cycle.

Filler for different steels

The table below shows typical filler families for common base metals. It is a map, not a recommendation, and the welding procedure decides.

Base metal	Typical filler family	Note
Structural carbon steel such as S355 or A572	Carbon steel solid wire or low hydrogen electrode, 70 ksi class	Most common case
Low temperature steels	Nickel containing low alloy filler with impact testing	Charpy requirement, see low temperature steels
304L and 316L stainless	308L and 316L filler	Matching low carbon grades
Carbon steel to stainless	309L filler	Higher alloy to dilute the mixing
Duplex 2205	Over-alloyed duplex filler	See duplex stainless welding
Quenched and tempered wear plate	Low strength or austenitic filler	See wear plate in weldments
Aluminium 5xxx and 6xxx	5356 or 4043 type	Choice affects strength and colour after anodising

Dissimilar metal joints need particular care. Mixing carbon steel with stainless needs a filler that can absorb the dilution without forming brittle structures, which is why the over-alloyed 309L is common.

Records the buyer can ask for

A shop that controls filler can show records.

• The filler classification and the manufacturer’s certificate for the batch
• The consumable storage and drying routine
• The welding procedure that names the filler, with qualification results
• For stainless and special alloys, material identification of the filler before use

If the part has a code requirement, the filler lot may be recorded against the weld. That is traceability, and it adds time. Ask for it only where the part needs it, and say so at the quote stage. See WPS, PQR and welder qualification for the procedure side.

What to write on the drawing

Keep it short and specific.

1. The parent steel grade
2. The welding process or processes allowed
3. The minimum weld metal strength and the impact requirement at temperature
4. The maximum diffusible hydrogen level where cracking is a risk
5. The post weld heat treatment, if any
6. The qualified procedure or the standard it must follow

Leave the exact wire or electrode brand to the shop. State the performance, and the shop picks a filler that meets it and shows the classification for approval.

Closing checklist

Before you send the drawing, check these points.

• The filler requirement is written as strength, impact and hydrogen, not as “suitable wire”
• The post weld heat treatment is named if it applies
• Dissimilar joints are marked and the filler for them is agreed
• The shop describes how it stores and dries low hydrogen consumables
• The filler is covered by the welding procedure qualification
• You have asked for the classification of the filler in the quote

Send your weldment RFQ with the filler performance stated and each shop can quote a weld that meets it.

Frequently asked questions

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