Why do some standards require that you wait for a few days to inspect welds?

Most of welding standards include clauses such as:

CSA W59: Visual inspection and NDT of welded joints subject to high restraint and/or joints of quenched and tempered steels shall be delayed as long as practicable and preferably not less than 48 hours after completion of the welds.

AWS D1.1: Acceptance for ASTM A514, A517 and A709 Grade HPS 100W [690W] steels shall be based on NDT performed not less than 48 hours after completion of the welds.

This requirement is applicable when material is subject to cold cracking, also called Hydrogen Assisted Cold Cracking (HACC). This phenomenon is likely to happen several hours after welding is completed, when the weld and base metal have cooled to ambient temperature. Three factors will promote cold cracking:

  • Diffused hydrogen in sufficient concentration.
  • High level of residual stress.
  • Brittle microstructure.

Hydrogen introduced in the weld metal during welding will migrate into the steel in its atomic form (H), then combines to form gas (H2) in the microstructure voids. If enough H2 is formed, these microscopic gas pockets create localised spots of extremely high pressure into the steel, causing cracks. This can happen in the weld metal or in the Heat Affected Zone (HAZ), beside or under the weld. High cooling rate will increase the possibility of having cold cracking. It can create a hard and brittle microstructure, a high residual stress and leaves less time to the hydrogen to migrate away from the sensitive area to lower its concentration. Pre-heating and post-heating to reduce cooling rate and using low hydrogen and dry electrodes are the most common ways of preventing cold cracking.

If welds inspection is performed immediately after colling of cold cracking sensitive steels, cold cracks can form in the next 24-48 hours and would not be detected before the piece is released for service.

Richard Sanfacon
Regional Director - Metal and Corrosion Engineering