Effect of preheat and post-heating on diffusible hydrogen content of welds

Abstract

A systematic study was conducted to examine the effectiveness of pre- and post-heating to bring down the diffusible hydrogen (HD) content in the welds to levels that are unlikely to cause cracking. Specimens for HD measurement were prepared as per ISO standard 3690, and the measurement was carried out using standard mercury method and the newly developed hot extraction technique, which uses a hydrogen sensor. Results show that pre- and post-heating of the weld at lower temperatures are more effective than preheating alone at higher temperatures to bring down the HD content in the welds. Results also indicate that if the HD content of welding consumable is known, it is possible to choose the preheat and post-heat temperatures that will bring down the HD content remaining in the weld to sufficiently low levels so that danger of delayed cracking is minimum. Such information can help to optimise the pre- and post-heating requirements during weld fabrication, resulting in substantial energy savings.