Development and Production of Creep and Hydrogen Resistant Grade 91 (9Cr1MoV) Heavy Plates for New Generation High Efficiency Refining Reactors

Abstract The overall efficiency of refining reactors is strongly linked to process parameters, i.e. service temperatures and pressures. For years, low alloyed ferritic materials - 2,25Cr1Mo and 2,25Cr1MoV steel grades mainly - are used to build heavy processing reactors, thanks to their good mechanical properties at high temperatures and under high hydrogen partial pressures. In particular, their good resistance to High Temperature Hydrogen Attack is of major interest for end-users. Depending on the grades, the ASME(1) Code gives limitations in terms of maximum allowable temperature that can limit the use of these low alloys in the case of advanced processes. Moreover, when these grades are allowed and above a given temperature, maximum allowable stresses are driven by time dependent mechanical properties (i.e. by their creep behaviour), leading to a strong reduction of the considered resistance and then to extra-thickness and weight when considering the vessels. Many developments have been done in the last decades to increase the efficiency of petrochemical/refining processes. In particular, this can lead to increase service temperatures and therefore the actual pressure vessel wall temperatures. Indeed, more and more temperatures around 500-510 °C are likely to be used, leading to a much reduced choice in terms of permitted steel grades. Regarding 2,25Cr low alloy family, the vanadium enhanced grade is not allowed whereas the usual grade has reduced creep allowable stresses. With a view to allowing strong improvements in admissible process parameters, Industeel1 developed a V-modified 9Cr1Mo Creep Strength enhanced material with advanced hydrogen resistance and improved toughness. Very thick plates (up to 200 mm thick) were produced and tested. This contribution reports both mechanical and metallurgical assessments performed on these heavy plates. Evaluation of hydrogen resistance (HTHA) as well as creep resistance under high hydrogen pressure is also reported.