In-Service Welding: Burn-Through Is Not the Only Risk — Hydrogen Cracking Can Control Integrity

Why Hydrogen Cracking Is a Specific Risk in In-Service Welding

In conventional shop welding, preheat is used to slow cooling, reduce HAZ hardness, and allow hydrogen to diffuse out of the weld zone before cracking can initiate. In in-service welding, the internal product flow provides significant cooling that may remove heat rapidly from the weld area, increasing the cooling rate despite normal ambient preheat application. This can create hard HAZ microstructures and high hydrogen saturation conditions more rapidly than in standard shop or field welding.

Key Drivers of Hydrogen Cracking Risk in In-Service Welding

• Carbon equivalent (CE) of the base material — higher CE increases hardenability and hydrogen cracking susceptibility
• HAZ cooling rate — accelerated by internal flow, affecting HAZ microstructure and hardness
• Hydrogen content from welding consumables and environment
• Residual stress and mechanical restraint at the weld joint
• Wall thickness — affects heat input, cooling rate, and constraint
• Operating temperature and product chemistry — may affect hydrogen availability and diffusion
• Weld joint geometry — affects restraint and stress concentration

Controls for Hydrogen Cracking in In-Service Welding

A defensible in-service welding program should include:

• Carbon equivalent calculation for the base material and assessment against relevant susceptibility models
• Use of low-hydrogen welding consumables — hydrogen-controlled electrodes, stored and handled per manufacturer requirements
• Preheat where feasible and technically effective given the cooling conditions
• Heat input control to maintain HAZ properties within acceptable limits
• Qualification of the WPS for the specific in-service conditions including cooling effects
• Post-weld hardness verification at defined timing to check HAZ hardness
• NDE timing — not only immediate visual or MT/PT, but also delayed inspection after a defined hold period when hydrogen cracking risk is elevated

Delayed Inspection and Hold Period Requirements

One of the most important — and most frequently overlooked — aspects of hydrogen cracking management in in-service welding is defining an appropriate inspection timing protocol. Hydrogen cracking may initiate and propagate hours to days after welding, particularly in highly restrained joints, high-CE materials, or situations where post-weld cooling conditions are severe. Immediate NDE after welding may not detect delayed cracking that initiates after the inspection.

The in-service welding procedure should define: whether a delayed inspection hold period is required based on material and joint assessment; the timing of delayed NDE; the NDE method to be used for detecting HAZ cracking; and the acceptance criteria and disposition path if cracking is detected.

Integration with FFS and ECA

If hydrogen cracking is detected after in-service welding, the findings should be evaluated through FFS or ECA to determine whether the crack is acceptable for continued service, whether monitoring is sufficient, or whether repair is required. Attempting to weld over a detected HAZ crack without proper engineering assessment can create additional risk.