Welding for sour service demands strict hardness control to prevent sulfide stress cracking (SSC). NACE MR0175/ISO 15156 sets maximum hardness limits for carbon and low-alloy steels in H2S environments: typically 22 HRC or 250 HV for base metal and weld metal, with lower limits for certain conditions.
The key is controlling HAZ hardness through proper welding parameters. Preheat and interpass temperature, heat input, and cooling rate all influence the final hardness. A slow cooling rate allows hydrogen to diffuse out and reduces martensite formation, which is the primary cause of high hardness.
PWHT is often required to temper the weld and HAZ to below the specified hardness. The temperature and holding time must be carefully selected to avoid sensitization or over-tempering. Verification is done by hardness traverses across the weld cross-section, typically using Vickers method.
Common mistakes include relying solely on preheat without controlling interpass temperature, and assuming all low-hydrogen processes automatically meet hardness limits. Even GTAW can produce hard spots if travel speed is too high.
What hardness testing method do you use to qualify your sour service welds?