Hydrogen Cracking Prevention and Management

Hydrogen cracking has and continues to be observed in both heat affected zones (HAZ) and weld metals. High carbon equivalent weldment HAZ combined with rapid cooling produce susceptible microstructures. Weld metal cracking is observed in both high- and low-strength weldments and is a particular concern for root passes due to parent metal dilution, applied loading, and weld fault stress riser effects which promote cracking. The risk of HAZ and weld cracking are increased for repair and in-service welds and/or welds deposited on older generation materials (e.g., pipe or fittings) and this risk can, in turn, pose a significant risk to the integrity of welded connections. The delay time to cracking is likewise affected by the welding procedure and environmental conditions. Thus the time for inspection to preclude hydrogen crack from entering service needs to consider these factors.

A number of factors have been shown to influence the risk of hydrogen cracking in multi-pass welds (welding process, restraint, material composition, preheat temperature, interpass time and temperature, etc.)., However, these factors influence cracking and cracking delay time through their effect on the three necessary conditions to produce hydrogen cracks, namely:

• the presence or local hydrogen concentration,
• the stress or strain level at the crack initiation point, and
• the susceptibility of the weld metal or HAZ microstructure.

The elimination of any one of these factor would preclude hydrogen cracking; however, practical experience has shown that in field welding one cannot eliminate any of these factors. This paper will describe research inspired techniques and limits that have been developed by BMT Fleet Technology (BMT) to consider hydrogen cracking susceptibility and delay time for multi-pass welds. These approaches are being developed to provide greater assurance in the design and inspection of welds in preventing hydrogen cracks from entering service.