Abstract
The current preference of regulators in the United States and Canada is to require a delay time prior to nondestructive examination (NDE) of welds for cracking; for both welds made manually during construction of new pipelines and during maintenance activities for existing pipelines. A better approach than imposing a delay time is to prevent cracking, by lowering the bulk hydrogen below a threshold value necessary for cracking to occur. The proactive approach of using post-heating immediately after welding to prevent the weld from cracking in lieu of an inspection delay was explored.
An experimental program was carried out to determine the hydrogen diffusion rate in pipe materials, as well as the typical weld hydrogen levels in both a baseline (room temperature) and time and temperature conditions. Experiments were also conducted to measure hydrogen levels in simulated welds in the as-welded condition, after 24 hours at ambient temperature, and after post-heating. The experimental program showed that post-heating newly constructed girth welds made using cellulosic-coated electrodes for approximately 20 minutes at 200 °C (400 °F) and in-service welds made using low-hydrogen electrodes for approximately 20 minutes at 120 °C (250 °F) may be used to mitigate the risk of hydrogen-assisted cold cracking (HACC), thereby justifying immediate inspection.
Various heating methods were evaluated to determine how effectively the post-heating time and temperature targets could be achieved for new construction welds and for in-service welds.
To aid in the successful implementation of post-heating, guidance material suitable for use by field personnel in the application of post-heating, as well as recommendations for industry standards improvement were developed.
