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

A systematic study was conducted to examine the effectiveness of pre- and post-heating to bring down the diffusible hydrogen (H_D) content in the welds to levels that are unlikely to cause cracking. Specimens for H_D 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 H_D content in the welds. Results also indicate that if the H_D content of welding consumable is known, it is possible to choose the preheat and post-heat temperatures that will bring down the H_D 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.