Evaluation of hydrogen-assisted cracking susceptibility in modified 9cr-1mo steel welds

The hydrogen-assisted cracking susceptibility of modified 9Cr-1Mo steel weld is evaluated by Y-groove and gapped bead-on-plate test. It is found that a combination of pre + post heat is able to prevent cracking. However, the pre + post heat temperature combination determined from G-BOP test to prevent cracking (200 °C) is found to be higher than that in Y-groove test (100 °C). This is attributed to the higher weld restraint experienced in G-BOP than Y-groove test. For both the tests, the weld metal is found to be more susceptible to cracking than the heat affected zone. In addition to the diffusible hydrogen content of the welding consumables, diffusible hydrogen present in the G-BOP specimens prepared with different preheating and combinations of pre + post heat is also estimated by maintaining identical cooling conditions. The diffusible hydrogen in the weld for which no cracking is reported for G-BOP test (200 °C pre + post heat) is less than 1 ml/100 g. Considering the high restraint conditions present in the G-BOP test, it can be concluded that to prevent HAC in P91 steel, the choice of welding parameters (hydrogen content in welding consumable, heat input, pre/post-heating) should be such that diffusible hydrogen in the weld is brought down to below 1 ml/100 g.