S690QL and S960QL: High-Strength Fine-Grain Structural Steels in Practice

High-strength fine-grain structural steels such as S690QL and S960QL have become indispensable in modern crane, vehicle and plant construction. They deliver strength values unattainable with classic structural steels – opening up entirely new scope for designers in terms of weight and component design. At the same time, they place higher demands on processing. Those who know and observe these demands get from the material exactly what it was developed for.

What the designation to EN 10025-6 tells you

The short designation follows a clear system. The leading "S" stands for structural steel, the number that follows gives the guaranteed minimum yield strength in MPa for the smallest product thickness: 690 MPa for S690QL, 960 MPa for S960QL. The appended letters specify the delivery condition and the quality class:

• Q – quenched and tempered, i.e. water-hardened and subsequently tempered. This heat treatment produces the fine, strength-optimized microstructure.
• L – guaranteed notch impact toughness at low temperature, typically a minimum impact energy at −40 °C. This is decisive for components that must remain safe in winter or under impact loading.

The practical benefit: strength becomes weight

The high minimum yield strength is not an end in itself. It makes it possible to reduce load-bearing cross-sections and wall thicknesses considerably without diminishing the load capacity. Less material means less weight – and that is precisely what counts wherever moving mass translates directly into payload, range or dynamics.

• Mobile cranes with greater reach at the same dead weight
• Vehicles and bodywork with higher payload and lower fuel consumption
• Transport and lifting equipment as well as heavily loaded defense applications
• Highly stressed components in which every kilogram works structurally

Processing: more demanding than classic structural steel

The fine quenched-and-tempered microstructure that delivers the strength reacts more sensitively to uncontrolled heat input than an ordinary S235 or S355. When welding, consistent heat management is therefore mandatory: controlled preheating, a limited heat input and adherence to the interpass temperature. Excessive heat input softens the heat-affected zone, while too-rapid cooling embrittles it. Both are to be avoided.

A central issue is the risk of hydrogen-induced cold cracks. These arise from the interaction of hydrogen, a hardening microstructure and residual stresses. The effective countermeasures are well established:

• suitable, low-hydrogen filler materials, matched to the base material
• cleanliness and dryness of the joint faces – free of moisture, oil, rust and scale
• dry, properly stored welding consumables
• controlled preheating and maintaining the preheat and interpass temperature

Different rules also apply to cold forming: the high strength calls for larger minimum bending radii than with lower-strength steels, in order to avoid incipient cracks and local hardening. The bending direction relative to the rolling direction and the product thickness also play a part here.

The data sheet and WPS are binding

The specific values – preheat temperature, permissible heat input, interpass temperature, minimum bending radii – depend on thickness, process, weld geometry and product. Do not rely here on rules of thumb, but on the relevant manufacturer's data sheet and a qualified welding procedure specification (WPS). These documents form the basis for a reproducibly safe result. An inspection certificate 3.1 to EN 10204 is, of course, available from us on request.

In short: S690QL and S960QL deliver real weight savings and design freedom through their high yield strength – provided that heat management, cold-crack prevention, filler materials and bending radii are implemented consistently according to the manufacturer's data sheet and WPS.