What NACE-compliant welding actually means.
In sour-service (H₂S) wells, a weld has to do more than hold. The finished joint — filler metal and heat-affected zone included — must keep the same properties as the original NACE-qualified base metal, so it can operate safely for the design life of the well.
Welding changes the metal
Fusing two sections at welding temperature alters a thin heat-affected zone either side of the weld. Rapid heating and cooling, dissolved hydrogen and residual stress combine to raise hardness — and hard, brittle metal in an H₂S environment is what leads to sulphide stress cracking and sudden failure.
Preheat + PWHT restore it
Controlled preheating slows cooling and lets hydrogen diffuse out; post-weld heat treatment then soaks the joint at high temperature to relieve residual stress and bring hardness back below the NACE limit — restoring base-metal properties across the whole joint.
The failure modes we engineer against
Sulphide stress cracking
The combined effect of H₂S, high hardness and residual stress. Controlling hardness and relieving stress removes the conditions that cause it.
Hydrogen-induced cold cracking
Driven by absorbed hydrogen, stress and a hard microstructure. Low-hydrogen electrodes, preheat and PWHT each remove one of the legs.
Brittle failure
High hardness under impact or sudden temperature change. Restoring ductility through heat treatment keeps the joint tough.
How we apply the heat: controlled, not chemical.
Heat treatment is where wellhead welding is won or lost. The field splits into two approaches — and they are not equivalent.
- Chemical heat source raises temperature quickly in a single burn
- Limited control of heating and cooling rate
- Leaves soot and residue requiring clean-up
- Hard to hold a constant soak or prove what happened
- One-shot — little room to adjust mid-process
- Electrical pads with multi-zone, thermocouple-based control
- Heating, soak and cooling held to a specified, constant rate
- No soot — clean, localized heating
- Calibrated chart recorder logs every weld: verifiable & auditable
- Repeatable to the qualified PQR, weld after weld
Both can reach temperature. Only one gives the operator a calibrated, traceable record that the heat-affected zone was treated to specification — which is what NACE compliance in sour service ultimately rests on.
