API 1104 / AWS D1.1 / EN 1011-2
Typical: 25-35 SMAW, 30-45 GMAW, 40-60 SAW
Understand carbon equivalents, hydrogen cracking mechanisms, preheat methods, and PWHT requirements for pipeline welding
Welding preheat is the controlled heating of base metal before welding to reduce the cooling rate in the weld and heat-affected zone (HAZ). Preheat prevents hydrogen-induced cracking (cold cracking) by allowing hydrogen to diffuse out of the weld, reduces residual stress, and prevents the formation of hard, brittle martensite in the HAZ. For pipeline steels with carbon equivalent above 0.35-0.40, preheat is typically required per API 1104 and AWS D1.1.
Two primary carbon equivalent formulas are used: the IIW formula CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, which is best for steels with C > 0.12%; and the CEN/Pcm formula Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B, which is better for modern TMCP low-carbon pipeline steels. Both predict weldability and minimum preheat temperature requirements.
For API 5L X52, preheat depends on the specific heat chemistry and wall thickness. Typical X52 has CE around 0.35-0.43. For CE below 0.35, no preheat is required (above 50 deg F ambient). For CE 0.35-0.45 with wall thickness under 3/4 inch, 50-150 deg F preheat is typical. Thicker walls and higher CE values require 150-300 deg F. Always calculate CE from the actual mill test report (MTR) chemistry.
PWHT is typically required when carbon equivalent exceeds 0.45-0.50 (IIW CE), wall thickness exceeds 1.25 inches per ASME B31.3, or when welding in sour service (NACE MR0175). API 1104 does not mandate PWHT for pipeline girth welds but recommends it for high CE materials. PWHT at 1100-1150 deg F for 1 hour per inch of thickness tempers hard HAZ microstructures and relieves residual stress.