Why NDT may be delayed after welding and when post-heating can help
The delay before MT or UT is linked to delayed hydrogen cracking. A new equivalent-holding-time method can support shorter delays, but it is not a universal authorization.
A crack may not form immediately
Hydrogen-assisted cold cracking requires a critical combination of diffusible hydrogen, a susceptible microstructure and tensile stress. Hydrogen continues to redistribute through weld metal and the HAZ after cooling. A critical local concentration may therefore develop hours later, when an immediate inspection would not yet reveal a crack.
This is why some product standards and specifications require delayed MT or UT. The delay is not intended to let a weld “settle”; it increases the likelihood that a delayed crack has formed before inspection.
Equivalent holding time
Researchers described a temperature-dependent equivalent holding time. Post-heating at elevated temperature is selected to provide a degree of hydrogen removal comparable with a chosen reference time at ambient temperature. Increased temperature accelerates diffusion, so the required period may be shorter.
The approach has been incorporated into NORSOK M-101. That does not automatically make it applicable to every contract governed by EN 1090, EN ISO or another product standard.
Three different thermal operations
- preheating influences cooling rate and formation of hard microstructures,
- hydrogen-removal post-heating promotes hydrogen diffusion after welding,
- PWHT has broader metallurgical and stress-related purposes and is not synonymous with short post-heating.
Any reduction in inspection delay must be permitted by the applicable specification, supported by documented time and temperature records and, where required, approved by the client.
Source
- Jemblie, L. et al.: Equivalent holding time to reduce NDT delay and prevent hydrogen-induced cold cracking, *Welding in the World*, 28 September 2026.
This article is an original explanation of the principle. Specific times and temperatures must only be taken from the applicable, legally obtained specification.