Plain Carbon Structural Steels — Materials science

Unalloyed structural steels are the workhorses of steel construction. You'll find them in every hall, every crane, and every steel frame: , , – the 'S' stands for Structural Steel, and the number behind it tells you the guaranteed in megapascals. An thus withstands at least 355 MPa before it permanently deforms. 'Unalloyed' means: apart from (below 0.25%) and manganese (up to about 1.6%), there's deliberately hardly anything else in it. It's exactly this simplicity that makes structural steels cheap, readily available, and above all, easy to weld.

The 0.22% rule: One of the most important rules of thumb in welding technology is: steels with more than about 0.22% are considered difficult to weld, if not impossible. The reason: is the number one hardener – the more C in the steel, the harder and more brittle the structure that forms when the weld cools quickly. From about 0.22% C onwards, the becomes so hard that it cracks under the shrinkage stresses of the weld if you don't counteract. Difficult to weld doesn't mean impossible – it means you absolutely need , low-hydrogen fillers, and controlled heat input. Below 0.22% C, the normal measures suffice.

Your key parameter is the (CEV). It converts all hardening-effective elements into a single numerical value. Rule of thumb: If the CEV is below about 0.35, the steel is generally weldable without much fuss. If it rises above 0.45, you need to be careful – then hard structures and cracks threaten in the ().

Why? A look at the structure shows it. During , the heats the material next to the weld to over 900 °C and then cools it rapidly. In the , this can form – a glass-hard, brittle structure – especially with thick plates. Combined with from the weld shrinking and from damp electrodes or , that's the classic recipe for .

You have three levers to counter this. First, the right : slows down the cooling and lets the hydrogen escape before it can cause damage. Second, dry, low-hydrogen . Third, joint preparation that keeps under control.

For the , the rule is 'same or slightly better'. For and , use an unalloyed like G3Si1. For , reach for a higher-strength wire so the weld achieves the of the . Welding is almost always with a like argon + 18% CO₂ – that gives a stable and clean penetration.

Two weld types dominate: the for corner and lap joints, and the for forces transverse to the weld. Important: The fillet weld transmits its force over the weld throat – the a-dimension on the drawing is binding.

Modern structural steels are during melting, i.e., freed from dissolved oxygen ('killed steel'). Fine-grain structural steels like or S460M additionally get such as niobium, , or titanium. These make the grain fine and tough – and that's exactly why an S460M, despite its significantly higher strength, is often still cleanly weldable.