High-Strength Steel Welding Guide
A practical summary of preheat, carbon equivalent, filler metal and heat input control for sound, crack-free welds in S690QL and S700MC plate.
General Principles
S690QL (quenched and tempered) and S700MC (thermomechanically rolled) steels are weldable materials, but they require a more careful approach than standard structural steel (S235/S355). The aim is to preserve the strength and toughness of the base material in the heat-affected zone (HAZ) and to prevent hydrogen-induced cold cracking.
Low Hydrogen
Use low-hydrogen electrodes/wire and dry consumables; moisture is the main cause of cold cracking.
Controlled Heat
Avoid both excessive and insufficient heat input; this preserves HAZ properties.
Preheat
Apply preheat according to thickness and CEV; it slows down cooling.
Carbon Equivalent (CEV / CET)
The carbon equivalent expresses the total effect of alloying elements on hardenability (and therefore cracking tendency) as a single number. Two common formulas are used:
| Index | Formula | Use |
|---|---|---|
| CEV (IIW) | C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 | Common for Q+T steels (S690QL) |
| CET | C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40 | For preheat calculation (EN 1011-2) |
For S690QL the typical CEV value is in the range of approximately 0.65 – 0.82 (depending on thickness). Thanks to its low carbon and total alloy content, S700MC has a markedly lower carbon equivalent and therefore easier weldability.
Preheat Temperatures
Recommended preheat and interpass temperatures by thickness (reference values):
| Plate Thickness | Preheat | Interpass Max. |
|---|---|---|
| ≤ 20 mm | 100 °C | 250 °C |
| 20 – 40 mm | 125 – 150 °C | 250 °C |
| 40 – 80 mm | 150 – 175 °C | 250 °C |
| > 80 mm | 175 – 200 °C | 250 °C |
| Plate Thickness | Preheat | Interpass Max. |
|---|---|---|
| ≤ 10 mm | Usually not required | ≈ 230 °C |
| 10 – 20 mm | 100 – 125 °C | ≈ 230 °C |
| > 20 mm / cold environment | 125 – 150 °C | ≈ 230 °C |
Note: Low ambient temperature (< 5°C), high joint thickness and high restraint increase the preheat requirement. Preheat should be measured over a width of about 75 mm around the joint.
Filler Metal Selection
The filler metal is selected according to the strength and toughness target of the joint. There are two approaches:
| Approach | Rationale | Typical Use |
|---|---|---|
| Undermatch (recommended) | A consumable somewhat lower in strength than the base material; reduces cracking and stress risk | Most structural joints |
| Matching | A consumable with strength equal to the base material | Critical joints requiring full strength |
- MIG/MAG: Low-hydrogen, high-strength class solid/flux-cored wires (e.g. ~70 ksi class and above).
- Stick electrode: Basic, low-hydrogen electrodes; oven-dried and kept warm.
- Submerged arc: A suitable wire-flux combination; flux protected from moisture.
Heat Input & Interpass Temperature
Heat input (kJ/mm) is calculated as welding current × voltage / travel speed and directly affects HAZ properties. In high-strength steels the window is narrow:
- Too low heat input: Fast cooling → hard, brittle HAZ → cracking risk.
- Too high heat input: Slow cooling → HAZ softening, loss of toughness (especially TMCP/S700MC).
- Typical range: 0.5 – 2.5 kJ/mm; selected from manufacturer curves according to thickness and procedure.
- Interpass temperature: Kept at max. ~250°C for S690QL and lower (~230°C) for S700MC to limit softening.
Cold (Hydrogen) Cracking Prevention
Cold cracking occurs when three factors come together. Bringing any one of the three under control greatly reduces the risk:
1. Hydrogen
Low-hydrogen consumables, dry environment, clean surface (no moisture, oil, rust or paint).
2. Hard Microstructure
Reduce the cooling rate with preheat and controlled heat input (limit hardening).
3. Stress
Reduce restraint, use a suitable pass sequence and, when needed, hold heat immediately after the final pass (hydrogen escape).
In thick, highly restrained joints, a short post-weld hydrogen removal bake (e.g. holding at 200–250°C) can be applied. Post-weld heat treatment (PWHT) is generally not required; if needed, it is done below the tempering temperature.
Procedure Summary
| Step | S690QL | S700MC |
|---|---|---|
| Surface preparation | Clean, dry, burr-free | Clean, dry, burr-free |
| Preheat | 100–200°C (by thickness) | Not required in thin sections |
| Consumable | Low H₂, matching/undermatch | Low H₂, matching/undermatch |
| Heat input | 0.5–2.5 kJ/mm | Keep low–medium |
| Interpass max. | ~250°C | ~230°C |
| PWHT | Generally not required | Generally not required |
Frequently Asked Questions
Is S690QL weldable?
Yes. S690QL is welded with low-hydrogen processes (MIG/MAG, TIG, submerged arc). Depending on thickness, 100–200°C preheat is recommended and the interpass temperature is kept below ~250°C. With the correct filler metal and controlled heat input, reliable joints are achieved.
Why is preheat required in welding?
Preheat slows the cooling rate; this gives hydrogen time to escape from the material and reduces the formation of a hard/brittle microstructure in the HAZ. As a result, the risk of cold (hydrogen) cracking drops significantly. As thickness and carbon equivalent increase, the required preheat also increases.
What is the carbon equivalent (CEV)?
The carbon equivalent reduces the total effect of the alloying elements in the steel on hardenability to a single number. A high CEV requires more preheat and tighter hydrogen control. For S690QL the typical CEV is ~0.65–0.82; thanks to its low carbon and alloy content, S700MC has lower values.
Which filler metal is used?
Generally a matching or slightly undermatching (somewhat lower strength) low-hydrogen consumable is used. The undermatch approach reduces the risk of cracking and residual stress in most structural joints. For critical joints requiring full strength, matching consumables are preferred. The choice is made according to the design requirement and the applicable WPS.
The Right Steel + The Right Certificate
Source our S690QL and S700MC plate with a 3.1 inspection certificate; plan your welding procedure with confidence based on the actual chemical analysis.