Welding fine grain steel: S460 to S690

High strength steel delivers thinner plate, lighter structures and less weld metal. You only keep that benefit if the welding process leaves the properties of the steel intact. Unlike S235 or S355, S460 and above have both an upper and a lower limit on heat input, and those two limits sit close together.

This article explains where those limits come from, the difference between normalised, thermomechanically rolled and quenched and tempered fine grain steel, how to determine preheat and why low hydrogen practice is a condition rather than a recommendation on these grades.

Three routes to the same strength

Fine grain steel derives its strength from a very fine grain structure, not from high carbon. The route to that structure differs, and that determines how the steel responds to welding heat. Normalised steel to EN 10025-3 (designation N or NL) is the most forgiving: the structure comes from a heat treatment and tolerates a wide heat input range.

Thermomechanically rolled steel to EN 10025-4 (M or ML) gets its grain from precisely controlled low temperature rolling. That structure cannot be restored with heat: excessive heat input grows the grain and strength drops permanently. Quenched and tempered steel to EN 10025-6 (Q or QL, such as S690QL) is hardened and tempered; there too excessive heat breaks down the structure, while too little heat produces martensite and crack sensitivity.

The heat input window

On S355 you can work with a wide margin. On S690QL the window is often no more than 0.8 to 2.0 kJ/mm, and the steel maker issues a table per thickness. Above the upper limit you lose proof strength and notch toughness through grain growth and tempering of the HAZ; below the lower limit the weld cools so fast that hard, crack sensitive martensite forms.

Those mill tables are not a suggestion: on thermomechanical and quenched and tempered steel they are the governing document alongside EN 1011-2. Transfer the limits literally into the WPS and measure them, rather than deriving them from current and voltage alone. The calculation is covered in calculating heat input.

Preheat and the role of carbon equivalent

Preheat temperature is determined with the method in EN 1011-2, annex C. The inputs are the carbon equivalent CET or CEV from the material certificate, the combined thickness, the heat input and the hydrogen content of the consumable. S460M at 20 mm often lands at 50 to 100 degrees, while S690QL at 30 mm quickly requires 125 to 175 degrees.

Preheat does two things at once: it slows cooling so less martensite forms, and it gives hydrogen time to diffuse out of the weld. Measure at the correct location, roughly four times the wall thickness from the joint on the face opposite the heating, and maintain it between passes. See preheating: when it is required.

Low hydrogen practice

The higher the strength, the more sensitive the steel is to hydrogen cracking. On S690 low hydrogen consumables are mandatory: basic coated electrodes with a guaranteed maximum of 5 ml per 100 grams, preferably 3 ml. Those electrodes belong in a drying oven and a quiver, because they absorb moisture from the air.

Hydrogen class also matters for flux cored wire, while solid wire under gas is naturally low. Moisture, rust and paint on the workpiece are just as effective hydrogen sources as a damp electrode. The full method is covered in preventing hydrogen cracking.

Filler metal: matching or undermatching

The default is matching: filler of the same strength class as the parent material. On S690 that means a consumable in the 690 class. There is however a deliberate exception, undermatching, where a lower strength class is chosen. That reduces residual stress and crack risk at the cost of joint strength.

Undermatching is common on fillet welds in S690, where the joint is often not the governing section, and on very heavy sections. The choice belongs in the structural calculation and in the WPS, and must be covered by procedure qualification: a WPQR on matching filler does not automatically cover undermatching.

Qualification and testing

Procedure qualification runs through EN ISO 15614-1, with fine grain steel in material groups 2 and 3 of ISO/TR 15608. Note that qualification in a higher group does not automatically cover a lower group or the other way round; the rules are in the standard and are frequently misapplied. See WPQR validity range.

On these grades the test programme almost always includes low temperature impact testing plus hardness surveys in the HAZ. The hardness limit is usually 380 or 450 HV10, depending on the application and whether post weld heat treatment is applied. What that survey looks like is covered in hardness testing on welded joints.