Mechanical Properties and the Effect of Hydrogen on Base Metal and Welds of 9Cr-1Mo-V Steel

Abstract

9Cr-1Mo-V steel or Grade 91 has been typically used up to now in thermal power plants operating at high temperatures, e.g. 600°C. In another potential application, such as refinery equipment like hydrocracker and hydrotreater reactors, the choice is still the 2¼Cr-1Mo-¼V. The main reason is that the average design temperature of hydrocracker and hydrotreater reactors, approximately 450°C, has not been increased in the last 20 years making the use of 2¼Cr-1Mo-¼V steel more suitable and cheaper in comparison to Grade 91. However, some recent interest has been observed for its application in the petrochemical and chemical sector, including for new process technologies related to green economy market sectors. In these applications the influence of hydrogen could be detrimental for the steel and in the past, some investigations concluded positively on the use of Grade 91. In recent years, an improved control of the chemistry intended to improve creep properties has led to the industrial production of the SA-387 Grade 91 Type 2. Therefore, due to a renewed interest and an improved chemistry, tests were planned and carried out with the aim of assessing the hydrogen effect on the Grade 91 Type 2. This was done using welded test samples covering both full plate butt joints and weld overlays using two stainless steels, 308L and 347. In the butt joints, the impact transition curves of the weld metal, heat affected zone and base materials were compared to those after a heat treatment under high hydrogen pressure. The resistance to hydrogen induced disbonding was evaluated on the weld overlays. Finally, the influence of step cooling treatment on impact toughness was also investigated for both base and weld metal.