Hydrogen Bake-Out After Welding CrMo Alloy Steel Pipe Fittings: When It Is Required and How to Specify It
Hydrogen-induced cold cracking (HICC) — also called delayed cracking or hydrogen-assisted cracking — is the most common weld failure mode in CrMo alloy steels. Hydrogen enters the weld metal and HAZ during welding from moisture in flux, electrode coatings, and base metal surface contamination. Post-weld hydrogen bake-out is a thermal treatment applied immediately after welding and before post-weld heat treatment (PWHT) to drive diffusible hydrogen out of the weld region before it can cause delayed cracking.
Why Hydrogen Causes Cracking in CrMo Steels
CrMo alloy steels — particularly those with higher chromium and carbon equivalent — transform to martensite in the HAZ during welding cooling. Martensite is a hard, brittle phase with limited ductility and fracture toughness. Atomic hydrogen dissolved in the austenite at high temperature becomes trapped in the martensite after transformation, because hydrogen diffusion in martensite is slow at ambient temperature. The trapped hydrogen accumulates at stress concentrations (weld toe, root, undercut) where residual welding stress is highest. If the local hydrogen concentration exceeds the threshold for the steel's fracture toughness, a crack initiates. The characteristic feature of HICC is that it is delayed — cracks may not appear until 24–72 hours after welding, after cooling is complete and hydrogen has had time to diffuse to the most highly stressed locations. This makes HICC dangerous because visual inspection immediately after welding will not detect it.
The Bake-Out Temperature Window
Hydrogen bake-out is performed at 200–300°C (well below the PWHT temperature of 650–760°C for CrMo steels). At this temperature, diffusible hydrogen has sufficient mobility to escape from the weld metal and HAZ by diffusion to the surface, where it desorbs as H₂ gas. The treatment must be applied immediately after welding is complete and before the joint cools below the interpass temperature minimum (typically 150–200°C for P11/P22, 200°C for P91) — if the joint is allowed to cool to ambient before bake-out, hydrogen-induced cracking may already have initiated. The hold time at bake-out temperature is typically 1–2 hours per 25 mm of wall thickness, with a minimum of 1 hour. For P91 (thick-wall applications), 2–4 hours at 250–300°C is commonly specified. After bake-out, the joint must be immediately transferred to PWHT without being allowed to cool — or be held at the bake-out temperature until PWHT commences.
When Is Bake-Out Required?
Hydrogen bake-out is not universally required for all CrMo welds — it is triggered by specific risk factors: wall thickness greater than 13 mm (beyond this thickness, hydrogen cannot escape rapidly by natural diffusion during cooling); use of cellulosic-coated electrodes (E6010/E7010 — these generate high hydrogen levels, HD > 15 ml/100g deposited metal), which should be avoided in CrMo service if possible; chrome content above 5% (P5, P9, P91, P92) — the higher hardenability means martensite forms even in slow-cooled welds; and repairs to previously PWHT'd joints, where the softened HAZ from prior PWHT is re-hardened by the repair weld thermal cycle. For P11 (1.25Cr-0.5Mo) and P22 (2.25Cr-1Mo) at wall thicknesses below 13 mm using low-hydrogen electrodes (E8018-B2/B3, HD < 5 ml/100g), bake-out may not be required if preheat is adequate and interpass temperature is maintained throughout welding.
How to Specify Bake-Out in a WPS
The Welding Procedure Specification (WPS) for CrMo pipe fitting welds should specify hydrogen bake-out as a separate operation between welding completion and PWHT. The key parameters to specify: temperature range (e.g., 250–300°C for P91); hold time (e.g., minimum 2 hours after joint reaches temperature); maximum temperature at which the joint may cool before bake-out commences (must not fall below minimum interpass temperature, typically 200°C for P91); method of temperature measurement (thermocouple type and attachment method — thermocouples must be attached to the joint, not to the heating blanket); and sequential relationship with PWHT (bake-out must precede PWHT with no intermediate cool-down). Failure to specify these parameters unambiguously in the WPS is a common source of field errors where bake-out is either omitted or performed after the joint has already cooled to ambient.
Delayed NDE After Welding
Because HICC is delayed, final NDE (volumetric examination by RT or UT, and surface examination by MT) must not be performed immediately after welding. The minimum delay between welding completion and final NDE is: 24 hours for P11/P22; 48 hours for P5/P9; and 72 hours for P91/P92. These delays allow any HICC that initiates to propagate to a detectable size before the NDE is performed. Preliminary NDE (visual and MT) can be performed immediately after welding to identify gross defects, but cannot be used as the final acceptance examination. Where hydrogen bake-out has been properly performed, the delayed NDE requirement may sometimes be relaxed by agreement between the parties — but this should be supported by WPS qualification data showing hydrogen levels in the deposited weld metal are below the threshold for the joint geometry.