Lighter, Stronger, More Efficient: Why S690QL and S960QL Are Replacing Conventional S355 in Modern Mechanical Engineering

Introduction: When S355 Reaches Its Design Limits

S355 has been a proven standard in steel construction and mechanical engineering for decades. It is widely available, offers good weldability and remains economical for many conventional load-bearing structures. However, modern machinery, lifting equipment, vehicle frames, crane structures, special-purpose constructions and mobile systems increasingly demand lower dead weight, higher payloads, improved energy efficiency and more compact designs.

This is where high-strength fine-grain structural steels such as S690QL and S960QL become highly relevant. Their significantly higher yield strength allows design engineers to create slimmer components without sacrificing load-bearing capacity. For buyers and fabrication companies, this becomes especially attractive when material usage, transport costs and welding effort are assessed as a complete system.

Taferner Stahlhandel supports customers in demanding material procurement projects through access to a global supplier network for fast delivery times, semi-finished products, plate cuttings and customised special solutions.

Benefits Check: Higher Yield Strength, Reduced Wall Thickness

The key difference lies in the yield strength:

S355 has a nominal yield strength of approximately 355 MPa.S690QL reaches approximately 690 MPa.S960QL reaches approximately 960 MPa.

This means that S690QL offers almost twice the yield strength of S355, while S960QL is even significantly higher. In practical terms, tensile or bending-loaded components can often be designed with reduced wall thickness, provided that stability, buckling behaviour, notch effects, fatigue, weld design and applicable standards are properly considered.

A simplified example:

A component made from S355 is designed with a 20 mm plate thickness. If the design is recalculated using S690QL, the required thickness under idealised tensile loading could theoretically be reduced as follows:

20 mm × 355 / 690 = approx. 10.3 mm

In real-world design, engineers would not automatically reduce the thickness to 10 mm, because stiffness, buckling behaviour and manufacturing tolerances also play an important role. Nevertheless, the example clearly illustrates the potential: even a reduction from 20 mm to 12 or 15 mm can generate substantial weight savings in large assemblies.

Economic Efficiency: Less Weight Means Lower Follow-Up Costs

High-strength steels are usually more expensive per kilogram than S355. Their economic advantage therefore does not arise from the material price alone, but from the performance of the entire system.

A practical calculation:

A welded assembly requires approximately 2,000 kg of steel when designed in S355. By switching to S690QL, the total weight can conservatively be reduced by 25%.

That results in:

2,000 kg − 25% = 1,500 kg

The weight saving is therefore 500 kg.

This has several direct benefits.

Lower Transport Costs

Reduced weight lowers freight costs, especially for large format plates, heavy welded assemblies or international supply chains. In-house handling, crane time and installation effort can also be reduced.

Reduced Welding Volume

Thinner wall sections usually mean smaller weld cross-sections. This can reduce:

• welding time

• filler metal consumption

• energy input

• distortion

• straightening effort

• rework

  
  

Especially in heavy steel fabrication, welding effort is often a greater cost driver than the raw material itself. This is where S690QL and S960QL can deliver significant economic benefits.

Higher Payload and Improved Efficiency

In mobile machinery, crane components, vehicle frames and lifting equipment, every kilogram of saved dead weight can increase payload capacity or reduce energy consumption. As a result, the switch to high-strength steels can also improve the performance of the final product.

Material Focus: S690QL and S960QL in Modern Mechanical Engineering

S690QL is a quenched and tempered high-strength fine-grain structural steel with an excellent combination of strength and toughness. It is frequently used wherever high load-bearing capacity and reduced weight are required.

Typical applications include:

• crane construction

• lifting equipment

• heavy machinery frames

• vehicle and trailer construction

• steel structures exposed to high loads

• offshore and special-purpose constructions

  
  

S960QL goes one step further. With its even higher yield strength, it is suitable for highly weight-optimised structures where every kilogram matters. At the same time, however, the requirements for design, welding technology and quality assurance increase.

For design engineers, this means that replacing S355 with S690QL or S960QL is not a simple material substitution. It is a design optimisation process. The full benefit is only achieved when component geometry, load case, manufacturing process and welding concept are considered together.

Processing: Weldability Requires Technical Discipline

S690QL and S960QL are generally weldable, but they require significantly greater process control than S355. The most important factor is heat input.

Excessive heat input can impair the mechanical properties in the heat-affected zone. Too little heat input or unsuitable welding parameters, on the other hand, can increase the risk of hardness peaks and hydrogen-induced cracking. A carefully coordinated welding concept is therefore essential.

Practical Tips for Welding Engineers

When processing high-strength fine-grain structural steels, particular attention should be paid to the following points:

1. Select suitable filler metalsThe filler metal must match the required strength, toughness and application. “Higher strength” is not always automatically better. In some cases, a slightly undermatching filler metal may be appropriate if the design allows it.

2. Control heat inputWelding parameters should be selected to preserve both strength and toughness. Especially with S960QL, the permissible process window is narrower.

3. Observe preheating and interpass temperatureDepending on plate thickness, carbon equivalent, hydrogen content of the welding process and component restraint, preheating may be required. Interpass temperature should be monitored, documented and limited.

4. Use low-hydrogen welding proceduresDry filler metals, suitable shielding gases, clean joint faces and controlled ambient conditions reduce the risk of hydrogen-induced cold cracking.

5. Design for weldingHigh-strength steels cannot deliver their full benefits if unnecessarily large welds, sharp notches or unfavourable stiffness transitions are built into the design. Material-appropriate design is essential.

Design Considerations: Strength Is Not the Same as Stiffness

A common mistake when switching from S355 to S690QL or S960QL is focusing only on strength. Higher yield strength allows smaller cross-sections, but the modulus of elasticity remains almost unchanged. In other words, stiffness does not automatically increase with the strength of the steel.

If deflection, vibration behaviour or structural stability is the governing factor, wall thickness cannot be reduced without further analysis. Design engineers should therefore determine whether the load case is governed by strength, stiffness or stability.

This is particularly important for:

• long booms

• crane and lifting arms

• dynamically loaded frames

• thin-walled box girders

• fatigue-loaded components

  
  

Availability: Plate Cuttings and Format Plates via TSH

For buyers, one question is just as important as technical suitability: is the required material available in the right dimensions and quality?

Taferner Stahlhandel can support customers in sourcing high-strength fine-grain structural steels such as S690QL and S960QL — depending on dimensions and market situation — as plate cuttings, format plates or project-specific special procurement through its partner network. TSH’s strength lies in its access to a global supplier network and customised solutions for technical requirements.

For high-strength grades in particular, early enquiries are highly recommended. Steel grade, plate thickness, inspection certificate requirements, applicable standard, delivery date and cutting dimensions should be specified as completely as possible. This enables faster evaluation of the most suitable technical and commercial supply option.

Outlook: High-Strength Steels Are Becoming Standard for Efficient Structures

S355 will remain an important structural steel. But wherever weight, payload, energy efficiency and welding effort are decisive factors, S690QL and S960QL are becoming increasingly important.

For modern mechanical engineering and steel construction projects, the material comparison should not end with the price per kilogram. The decisive factors are total weight, fabrication time, welding volume, transport effort, installation costs and the performance of the finished structure.

Companies that consider these factors early in the design phase can achieve significant technical and economic benefits with high-strength fine-grain structural steels.

Contact

Are you planning a structure using S690QL, S960QL or looking to optimise an existing S355 assembly for weight reduction? Send your enquiry with steel grade, plate thickness, dimensions, quantity, certificate requirements and desired delivery date to Taferner Stahlhandel. TSH supports design engineers, welding engineers and buyers in selecting and sourcing suitable plate cuttings, format plates and special solutions.

Disclaimer / Technical Note

All technical information, material data and application recommendations provided in this article are intended for general guidance only and are provided without warranty. The suitability of a material must always be assessed based on the specific application, applicable standards, operating conditions, medium, temperature and mechanical loads. Final approval must be carried out by the responsible planner, operator or qualified specialist.