FS960 ultra-high strength steel welding refers to top-precision welding technology applied to quenched and tempered ultra-high strength structural steel with a minimum yield strength of 960 MPa (compliant with European standard EN 10025-6, grades S960Q / S960QL / S960QL1).

For international high-end steel structure and heavy machinery projects implemented in accordance with EN 1090-2 EXC3/EXC4 or AWS D1.1 standards, S960 represents the extreme limit of material performance. With a carbon equivalent (CEV) generally ranging from 0.55 to 0.68, S960 has an extremely high hardening tendency and high constraint stress after welding, resulting in a much narrower welding parameter window than S690. The welding process must adopt strictest control measures against three fatal defects: cold cracks (hydrogen-induced delayed cracking, HIDC), excessive softening of the heat-affected zone (HAZ) leading to sharp strength loss, and joint embrittlement.

A complete and rigorous set of welding technical documents and personnel credentials must be established prior to formal production:

Closed-loop Welding Procedure Specification (WPS) An International Welding Engineer (IWE) compiles a Preliminary Welding Procedure Specification (pWPS). Welding Procedure Qualification Record (WPQR) tests shall be carried out in accordance with the highest Class One requirements of EN ISO 15614-1.
Mandatory hardness and toughness criteria: The maximum hardness of the HAZ on qualified test plates shall be strictly limited to ≤ 450 HV10. Meanwhile, both weld metal and HAZ must pass Charpy V-notch low-temperature impact tests at -40°C or -60°C.

Advanced ISO 9606-1 Welder Qualification All welders shall hold ISO 9606-1 certificates specially covering ultra-high strength steel (Group 9 steel and above). QC inspectors shall verify that the qualification scope (welding processes, plate thicknesses, all welding positions including 3G/4G) fully matches actual production conditions.

Dual Crack-Resistant Groove Preparation (3mm mechanical machining highest standard) Step 1: Thermal cutting – High-precision laser or fine plasma cutting for initial blanking and groove forming.
Step 2: Mechanical milling – Core crack prevention technology. S960 steel is extremely sensitive to the hardened layer on thermally cut edges; the heat-affected deteriorated layer (microscopic martensitic microstructure) is the main cause of root cracks. This process mandates mechanical edge milling to remove a minimum 3mm layer from all edges, restoring the groove surface to pure base metal crystalline structure completely.

Fit-up Specification (Strictly implemented per AWS Welding Handbook 9.4) Root gap, groove angle and misalignment are controlled strictly in accordance with AWS 9.4 standards. Forced assembly is prohibited: tensioning by dead load or any non-standard external force is forbidden. Components must be positioned naturally via special fixtures to minimize internal additional constraint stress of the structure and prevent lamellar tearing of thick plates during welding.

Accurate Electric Preheating Before Welding (100°C – 150°C, in accordance with EN 1011 standard) Local heating by conventional gas torches is completely prohibited to avoid overheating annealing and softening of S960 surfaces, which would cause permanent sharp decline in material strength. Uniform automatic electric heating shall be applied using flexible heating blankets or crawler-type ceramic heating pads. Preheating range: no less than 4 times the plate thickness on both sides of the weld (minimum 75mm), monitored at multiple points in real time by infrared thermometers or thermocouples.

Welding Consumable Selection and Ultra-Low Hydrogen Control (H5 Grade) H5 grade or lower hydrogen welding consumables (diffusible hydrogen content ≤ 5ml per 100g deposited metal) are mandatory, including ultra-high strength flux-cored welding wire (FCAW, e.g., AWS A5.28 E110C-G/E111T1 or equivalent European grades) or submerged arc welding (SAW) materials.

Ultra-Low Heat Input Control and Multi-Layer Multi-Pass Welding (Key to prevent HAZ softening) Heat input locking: Different from S690, the biggest challenge of S960 welding is preventing HAZ softening. Welding heat input shall be strictly limited to a narrower range of 0.8 – 1.8 kJ/mm as specified in the formal WPS; high heat input and high-current operation are strictly forbidden.
Interpass temperature control: Interpass temperature is maintained between 100°C and 150°C. Welding must be suspended immediately if interpass temperature exceeds 150°C and resumed only after cooling down to the specified range.
Temper bead technique: Low heat input, multi-layer multi-pass, slight oscillation and narrow weld pass techniques are mandatory. The minor heat from subsequent weld beads provides self-tempering to prior weld layers, refining HAZ grains while minimizing microstructure softening.

Immediate Post-Weld Hydrogen Removal (Direct Heat Treatment for Stress Relief, DHTO) Timely hydrogen removal at 200°C – 250°C: Before the weld cools to ambient temperature after welding, ceramic heating pads shall be reactivated immediately to reheat the weld and HAZ to 200°C – 250°C.
Slow cooling with thermal insulation blankets: Once the target temperature is reached, the welded area shall be fully wrapped with thick asbestos insulation blankets for natural slow cooling. This accelerates the escape of diffusible hydrogen in the weld and greatly releases residual tensile stress, eliminating the microscopic driving force for delayed cracking.

Precision Weld Finishing Standard All welds shall be finely ground with angle grinders after welding to form a smooth arc transition between weld metal and base metal (transition radius ≥ 10mm). All spatter, slag, undercut and arc strikes shall be completely removed to avoid stress concentration caused by abrupt geometric changes.

Specification for 100% Delayed Nondestructive Testing (Delayed NDT) Mandatory delay period: S960 has an extremely high tendency of delayed cracking, as hydrogen requires time to diffuse inside steel. After hydrogen removal and cooling to ambient temperature, components must stand for a full 48 hours before NDT inspection.
Inspection coverage: 100% Ultrasonic Testing (PAUT phased array ultrasonic testing is highly recommended) plus 100% Magnetic Particle Testing (MT). Penetrant Testing (PT) is strictly prohibited as a replacement for MT.

Sa2.5 Shot Blasting Shot blasting is applied to the entire component for rust removal, complying with ISO 8501-1 Sa2.5 grade standard to achieve ideal surface roughness.

Complete elimination of pickling Due to the ultra-high strength of S960 steel, hydrogen atoms will be massively absorbed at grain boundaries instantly when immersed in conventional pickling tanks, resulting in catastrophic hydrogen embrittlement fracture. Therefore, this process mandates 100% replacement of conventional pickling for steel structures with automatic shot blasting before hot-dip galvanizing and paint coating.

To meet pre-commencement ITP review by clients and consulting engineers, every critical weld must be accompanied by two quality record forms:

1. S960 Preheat & Post-Heat Temperature Record: Documents ceramic pad preheating temperature (100–150°C), interpass temperature, post-weld hydrogen removal temperature (200–250°C) and insulation duration.
2. S960 Welding Construction Record: Documents welder’s ISO 9606-1 certificate number, WPS reference number, actual voltage and current, calculated heat input (0.8–1.8 kJ/mm), confirmation of 3mm mechanical milling inspection, and final closed-loop linkage to 100% UT/MT report numbers obtained after 48 hours delayed inspection.