When weld problems arise in S690 high-strength steel structures (e.g. S690QL) already in service in operational buildings or infrastructure (e.g. fatigue cracks, stress corrosion cracking), the repair difficulties far exceed those of factory prefabrication. At this stage, the structure is under load (load-bearing), on-site space is constrained, and large-scale high-temperature integral heat treatment is not feasible. If the repair process is improper, it can easily trigger secondary cold cracking, HAZ embrittlement/softening, and even local structural instability or collapse.

To ensure the repaired weld fully restores the original design load-bearing capacity and complies with EN 1090-2 EXC3/EXC4 operational maintenance specifications, the following dedicated on-site repair procedure — “live-load dynamic monitoring, flame-free/low-heat repair, dual delayed verification” — must be executed:

Structural Load Assessment and Temporary Support (Unloading): The Registered Structural Engineer (RSE) must perform finite element stress analysis on the weld defect. Prior to repair (grinding/gouging), temporary load-bearing supports or hydraulic jacks must be installed for local unloading (stress relief). Grinding directly into core load-bearing welds under full design load is strictly prohibited.

On-Site Strain Dynamic Monitoring (Live Monitoring): Electrical resistance strain gauges or fibre-optic sensors must be installed around the defective weld. Stress-strain changes in the structure must be monitored in real time throughout the entire gouging, preheating, repair welding, and dehydrogenation process. If data exceeds safe thresholds, work must cease immediately.

Environmental Protection Enclosure: A high-standard windproof, rainproof, temperature-controlled enclosure tent must be erected at the repair zone. Welding is absolutely prohibited when on-site relative humidity exceeds 85% or wind speed exceeds 2 m/s, to prevent moisture from entering the weld and causing cold cracks.

Precise defect inspection and crack-arrest drilling: 100% MT + 100% PAUT (phased-array ultrasonic testing) must be used for three-dimensional scanning of the problem weld, locking in the crack’s start/end points and depth. At positions extending at least 50–100 mm into the base metal beyond both ends of the crack, mechanically drill Φ 8–10 mm circular crack-arrest holes to completely sever the stress path that would allow the crack to propagate deeper into the building’s base metal during subsequent heating.

Pure mechanical cold grinding (carbon arc gouging strictly prohibited on-site): On-site use of arc gouging or flame cutting for defect removal is absolutely prohibited. The building is already under residual stress in service; the extreme heat of arc gouging will instantly tear the high-strength steel. Correct method: use high-rigidity pneumatic/electric straight-type grinding heads, tungsten-carbide rotary milling cutters, or precision portable bevelling machines for pure mechanical cold grinding, producing a smooth U-shaped groove (angle ≥ 60°, root radius ≥ 6 mm). After grinding, the groove surface must cool to ambient temperature and 100% MT must confirm the original crack or defect has been 100% completely removed.

Enhanced fully automatic electrical preheating (130 °C – 150 °C): On-site repair strictly prohibits workers using hand-held gas torches for heating. Uneven local heating will cause severe distortion of building components and local annealing/softening of the high-strength steel surface. Correct method: flexible belt-type ceramic heating pads must be strapped to both sides of the groove within a 4× plate-thickness range, controlled by a fully automatic temperature control box, with preheat temperature locked at 130 °C – 150 °C, and temperature curves recorded in real time using thermocouples.

Ultra-low heat input repair welding and temper-bead technique: Welding consumables: moisture-proof packaged H5-grade or lower-hydrogen (diffusible hydrogen ≤ 3 ml/100 g deposited metal) high-strength flux-cored wire (FCAW) or manual low-hydrogen electrodes (carried in portable quivers for immediate use). Heat input must be strictly limited within a narrow window of 1.0 – 1.8 kJ/mm (small current, high voltage, fast travel speed). Employ multi-pass, multi-run, micro-oscillation, ultra-short beads (each weld length ≤ 50 mm) with skip welding to distribute thermal stress. Interpass temperature must be strictly controlled between 125 °C and 180 °C. The micro-heat of subsequent passes self-tempers preceding passes and the HAZ, maintaining the high-strength steel’s ductility.

Immediate post-weld dehydrogenation (enhanced post-heat treatment — DHT): After repair welding is completed and before the weld cools, immediately raise the ceramic heating pads to 250 °C for dehydrogenation. Holding time: 1.5 hours per 25 mm plate thickness (minimum 2 hours). After holding, wrap the entire repair zone airtight with thick insulation blanket for slow natural cooling to ambient temperature. Technical principle: under high on-site constraint stress, diffusible hydrogen in the weld is forced to escape before crystallisation cooling, maximising release of repair residual tensile stress and preventing hydrogen-induced delayed cold cracking.

Precision flush transition grinding: After repair, use an angle grinder to grind the weld surface to a completely smooth transition with the original building structure base metal (transition radius ≥ 12 mm). The surface must be free of any visible undercut, arc strikes, or mechanical marks.

On-site 48-hour mandatory delayed NDT re-inspection (ultimate defence): After the repair weld has finished slow cooling, it must remain undisturbed on-site for a full 48 hours (hard time-lock). 100% MT + 100% PAUT must then be performed, with acceptance criteria strictly following the highest EN 1090-2 Class B quality standard.

On-site corrosion protection restoration (no acid pickling): After NDT passes, use a portable sandblaster for local mechanical blasting to Sa 2.5 grade (chemical acid pickling is absolutely prohibited on-site to prevent acid infiltrating steel gaps and causing hydrogen embrittlement). Immediately apply high-zinc cold-galvanising coating or epoxy zinc-rich primer compliant with EN ISO 12944 to restore the original DFT (dry film thickness).

This on-site repair must be documented as a standalone Maintenance Record, submitted to the owner’s engineer, the Hong Kong Buildings Department (BD), or relevant consultant representatives, including:

• Structural load assessment, safe unloading plan, and temporary support acceptance form signed by the RSE.
• Daily log of on-site live strain monitoring data during the repair period.
• Dedicated on-site rework WPS (Rework WPS) and the welder’s ISO 9606-1 advanced certificate.
• Full-duration automatic temperature-time curve chart (preheat – welding – 250 °C dehydrogenation) exported from the automatic temperature control box.
• New 100% PAUT and 100% MT compliance reports after the 48-hour delay.