S960QL is a high-strength structural steel with a minimum yield strength of 960 MPa, produced by quenching and tempering (QT). It is manufactured to EN 10025-6 and used in military vehicles, heavy construction equipment, mobile cranes and load-bearing structural fabrications.
Welding this steel is technically very demanding: incorrect parameters can cause both strength loss in the heat-affected zone (HAZ softening) and cold cracking (hydrogen-induced delayed cracking) risks.
Fundamental rule: There is no universal WPS for S960QL. An approved WPS specific to S960QL material under EN ISO 15614-1 is mandatory.
Metallurgical Properties of S960QL
The mechanical properties, surface-to-core hardness distribution and HAZ toughness of S960QL largely originate from the QT heat treatment. Welding heat can adversely affect this structure:
- HAZ Softening: When welding heat input is high, the microstructure in the heat-affected zone (HAZ) normalises and yield strength drops below 960 MPa.
- Cold Cracking: Occurs in the triangle of diffusible hydrogen + high tensile stress + susceptible microstructure; can appear even hours after welding is complete.
Critical Welding Parameters
| Parameter | Value / Rule | Standard Reference |
|---|---|---|
| Heat Input | Maximum 1.5 kJ/mm (0.5–1.5 kJ/mm range per manufacturer recommendation) | EN ISO 1011-2 Method B |
| Preheat | Based on CEV, thickness and ambient temperature; typically ≥75°C – 150°C | EN ISO 1011-2 Method B |
| Interpass Temperature | Maximum 220°C (≤200°C recommended by most manufacturers) | Must be specified in WPS |
| Diffusible Hydrogen | H5 class or lower (≤5 ml/100g weld metal) | EN ISO 3690 |
| PWHT | Generally not recommended — must not exceed tempering temperature | Manufacturer technical document |
CEV Calculation and Preheat
The preheating temperature is based on the Carbon Equivalent Value (CEV) calculated from the chemical composition of the steel. EN ISO 1011-2 Method B formula:
CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
The typical CEV range for S960QL is 0.65–0.82. This value indicates that preheating is mandatory. The actual preheat is calculated by also accounting for the combined parameter Cp, diffusible hydrogen (HD) and wall thickness.
Practical note: When ambient temperature is below +5°C, the calculated preheat value must be automatically increased per EN ISO 1011-2.
Filler Material Selection: H5 Requirement
Keeping diffusible hydrogen levels low is the most critical step in preventing cold cracking when welding S960QL. Therefore:
- All welding consumables (wire, electrode, flux) must be H5 class or lower.
- Consumables must not be used if they have absorbed moisture; flux-cored wires and electrodes must be stored in heated holding ovens.
- Ar-CO₂ shielding gas mixture is preferred for MAG welding (typically 80% Ar + 20% CO₂ or per manufacturer recommendation).
- Tensile strength matching must also be ensured when selecting consumables (avoid over-matching and under-matching).
WPS (Welding Procedure Specification) Requirements
For S960QL welding, the WPS must be supported and approved by a WPQR (Welding Procedure Qualification Record) under EN ISO 15614-1. Destructive tests for WPQR approval:
- Tensile test (EN ISO 4136)
- Bend test (EN ISO 5173)
- Charpy impact test (EN ISO 9016) — from HAZ and weld metal, typically at −40°C
- Hardness test (EN ISO 9015-1) — critical for HAZ softening control
- Macro examination (EN ISO 17639)
Important: The qualification range of the approved WPS for S960QL — thickness, heat input, preheat, filler material group — must exactly match the production part. Using parameters outside the WPS scope invalidates the CE certificate and EN 1090 compliance.
Comparison with S700MC
S700MC, often confused with similar strength grades, is a TMCP (Thermomechanically Controlled Process) steel and shows fundamentally different welding behaviour from the QT steel S960QL:
- S700MC has a low CEV (≤0.39); preheating is generally not required.
- The heat input limit for S700MC is stricter: maximum 1.5 kJ/mm (1.0 kJ/mm for some manufacturers).
- PWHT is not recommended for S960QL, and not recommended for S700MC either — but for different reasons.
Separate WPS documents are required for each material; using the same WPS for both steels is technically incorrect.
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