EFFECTS OF ALLOYING ELEMENTS ON MECHANICAL-PROPERTIES OF 3CR-MO-W STEELS FOR HEAVY SECTIONAL HIGH-TEMPERATURE AND HIGH-PRESSURE VESSEL

Development of heavy sectional pressure vessel steels which are applicable to higher pressure and higher temperature hydrogen environment, is much in demand in recent years. In this study, creep rupture properties, temper embrittlement susceptibility and resistance to hydrogen attack were investigated in 3Cr-Mo-W steels which was developed by modifing 2 1/4Cr-1Mo steel mainly in order to suppress hydrogen attack. As results, effects of V, W, Ni and Cr on mechanical properties and precipitation behavior of carbides which affect creep rupture properties and hydrogen attack susceptibility were revealed. For instance, V addition increased creep rupture strength through precipitation of fine VC carbides, Cr decreased an amount of VC, replacing a part of Mo with equivalent W stimulated the precipitation of M6C carbides and increased creep rupture strength. Through autoradiography of B10, it was found that 10 ppm boron addition suppressed effectively the transformation of austenite to ferrite and temper embrittlement due to its grain boundary segregation. It was shown that 3Cr-0.7Mo-1.3W-0.3V-0.03Nb-B steel yielded high creep rupture strength together with good resistance to temper embrittlement and hydrogen attack.