Preheat is the controlled application of heat to the base metal before welding begins. Raising the temperature of the joint area slows the cooling rate after welding, which reduces the risk of hydrogen-induced cracking (cold cracking), minimizes residual stress, and prevents hard, brittle microstructures from forming in the heat-affected zone (HAZ). ASME B31.3 Table 330.1.1 specifies minimum preheat temperatures by P-number and material group. Preheat is mandatory for carbon steel above certain thicknesses, all Cr-Mo alloy steels, and whenever the ambient temperature falls below 0 deg C.
Why Preheat Is Required
Purpose Mechanism Prevent hydrogen cracking Slower cooling allows diffusible hydrogen to escape from the weld zone before the metal becomes brittle Reduce HAZ hardness Slower cooling produces softer microstructures (bainite/pearlite instead of martensite) Reduce residual stress More uniform temperature distribution = lower thermal gradient = lower residual stress Improve weld quality Better fusion, reduced porosity risk from moisture condensation Remove moisture Drives off condensation from pipe surfaces (critical for low-hydrogen welding)
ASME B31.3 Minimum Preheat Requirements
P-Number Material Min Preheat (deg C) Condition P-1 Carbon steel (A106 Gr. B, A234 WPB) 10 deg C (50 deg F) All thicknesses; 79 deg C (175 deg F) when t > 25 mm or CE > 0.65 P-3 Low-alloy (A335 P1, 1/2 Mo) 79 deg C (175 deg F) All thicknesses P-4 1-1/4 Cr-1/2 Mo (A335 P11) 149 deg C (300 deg F) All thicknesses P-5A 2-1/4 Cr-1 Mo (A335 P22) 177 deg C (350 deg F) All thicknesses P-5B 9 Cr-1 Mo-V (A335 P91) 204 deg C (400 deg F) All thicknesses; maintain through all welding passes P-8 Austenitic SS (304, 316) None required Preheat not beneficial; may cause sensitization P-10H Duplex SS (2205, 2507) None required Max interpass temperature is the controlling factor
Carbon Equivalent and Preheat
Carbon equivalent (CE) predicts the hardenability of carbon and low-alloy steels and determines whether preheat is needed beyond code minimums:
IIW formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
CE Range Weldability Preheat Recommendation < 0.35 Good (low risk) Preheat per code minimum only 0.35-0.45 Fair (moderate risk) 50-150 deg C depending on thickness and hydrogen level 0.45-0.65 Poor (high risk) 150-250 deg C; use low-hydrogen process (GTAW, SMAW with H4 electrodes) > 0.65 Very poor 250+ deg C; controlled hydrogen, mandatory PWHT
Preheat Methods
Method Equipment Application Oxy-fuel torch Propane/acetylene rosebud tip Small-diameter pipes, field tack welds Electrical resistance Ceramic pad heaters, heating blankets Controlled preheat on medium/large pipes Induction Induction coils around pipe Fast, uniform heating; shop and field Furnace Gas/electric furnace Entire spool piece or fitting (shop)
Preheat Verification
Requirement Standard Practice Temperature measurement Contact pyrometer, thermocouple, or temperature-indicating crayon (Tempilstik) Measurement location 75 mm (3 in.) from weld edge or 3x thickness, whichever is greater Measurement timing Verify immediately before welding starts Minimum preheat zone 75 mm on each side of the weld joint Documentation Record on daily weld log and fit-up inspection report
Temperature-indicating crayons (Tempilstiks) melt at a calibrated temperature and provide quick verification. Thermocouples connected to digital pyrometers provide continuous monitoring and documentation, required on critical welds and Cr-Mo piping.
Preheat vs Interpass vs PWHT
Parameter Preheat Interpass temperature PWHT When Before welding starts Between weld passes After welding is complete Purpose Slow initial cooling, prevent cracking Prevent overheating, control cooling rate Stress relief, temper HAZ Measured Base metal before root pass Weld surface before next pass Weld + HAZ during soak Control Minimum temperature enforced Maximum temperature enforced Temperature range + hold time
Measuring preheat on the opposite side of the pipe from the heat source. The temperature across the wall thickness can differ by 50-100 deg C. Measure preheat on the same side as the weld joint, at least 75 mm from the bevel edge. For thick-wall Cr-Mo piping, soak the preheat long enough for through-thickness equalization before welding.
Preheat records are documented in the weld inspection report, verified per the inspection and test plan , and archived with mill test certificates and hydrostatic test records.
Read the full guide to non-destructive testing
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