Welding Weldox or RQT or (generic) S690 Steels

Weldox, RQT and S690 are the same steels and are usually known by the trade name Weldox. They are very high strength steels but have lean compositions (low alloy). Their strength is achieved by thermo-mechanical means: rolling and quenching and tempering (RQT).

These days they find use in all sorts of fabrications from battery powered vehicles to skips, in fact anywhere where their increased strength can be used to lighten the fabrication. If the strength is higher you need less thickness to provide the same overall strength. Thinner equals lighter and alighter skip can hold more rubbish but maintain the same overall weight.

These steels provide little problems in welding providing the correct consumables are used in the correct procedure.

As far as consumables are concerned you need an E10018-G or a 11018-G electrode or an ER100S-G or ER110S-G wire.

The procedure is reliant on the correct preheat for the thickness of plate involved. Preheats increase with thickness. Over 30mm you need 75°C and over 70mm thick its 100°C. These thicknesses are combined so if its 20mm to 20mm its 40mm combined. These preheats assume a heat input of 1.7kJ/mm which is common with electrodes but more difficult with MIG (the tendency in MIG is for lower heat input so higher preheats are required).

Heat Input is a number that relates to the energy applied to the weld. It is Volts x Amps x Time / Length.

Given that a process such as MIG will weld faster than MMA, the time taken for a unit length will be shorter so the divider will be greater and thus the heat input will be lower (even though the amps might be higher). Think of it like waving a gas flame across the weld. Do it slowly and it will get the plate hotter than wafting it across very quickly.

The greater the Heat Input the more heat in the weld.