ASME B31.12 (Hydrogen Piping and Pipelines) governs piping in hydrogen service across hydrogen economy + green steel + Power-to-X applications — covering Part GR (general), Part IP (industrial piping), Part PL (pipeline). Hydrogen embrittlement screening drives material from carbon steel (HV ≤235 HAZ) → austenitic stainless → SA-372 Class IV/V → Inconel. 5 welding requirements (WPS Sec IX+B31.12 Appendix / low-H consumable ≤4 mL/100g / hardness HV ≤235 / PWHT mandatory CS+low-alloy / 100% NDE + helium leak high-pressure). 5 project scenarios + B31.12 vs B31.3 vs API 1104 comparison.