Abstract
Nowadays, oil refining processes such as heavy oil up-grading and hydrodesulphurization require higher temperature and pressures in order for hydrotreating reactors to operate efficiently.
To meet these demands, three types of high strength pressure vessel steel have been developed and certified in ASME Code. The newly developed materials of enhanced 21/4Cr-1Mo, V-modified 21/4Cr-1Mo and V-modified 3Cr-1Mo steel have higher design stress intensity values at elevated temperature applications, which results in reduction of wall thickness and total weight of reactor vessels. In the world wide R & D Projects for the advanced materials, extensive studies on material degradation problems during long term hydrogenation services have been carried out to evaluate the feasibility for higher temperature applications. The applicability of the advanced pressure vessel steels is reviewed in terms of toughness degradation by temper embrittlement, hydrogen attack, hydrogen embrittlement and creep embrittlement.
Improved resistance to temper embrittlement was confirmed in the enhanced 21/4Cr-1Mo steel with low J-factor value. And also, two types of V-modified Cr-Mo steel exhibited sufficiently lower susceptibility to toughness degradation due to temper embrittlement. The stable carbide formation of VC produced the excellent resistance to hydrogen attack damage in the V-modified steel. At the same time, fine precipitation of VC carbides contributed significant improvements in the hydrogen assisted crack growth and the hydrogen induced disbonding at the base steel/stainless steel weld overlay interfaces. These beneficial material properties were confirmed to be derived from hydrogen trapping effect of VC carbides in the V-modified steel. It is concluded that two types of V-modified steel are highly applicable to severe service conditions of coal liquefaction and heavy oil up-grading processes. Large size hydrocracking reactors with a weight of 1000 ton were completed using the advanced 3Cr-1Mo-1/4V-Ti-B steel in 1992.
