In the ASME Boiler and Pressure Vessel Code, Sec. VIII, Div. 1, design allowable stress-intensity values are influenced by the creep rupture strength at temperatures higher than 450°C. Pressure vessels for such high-temperature service inevitably tend to be thickwalled unless a steel with high creep strength is used. In the case of a coal liquefaction reactor, for instance, a possible wall thickness would be approximately 300 mm or even greater with the use of conventional 2¼Cr-1Mo steel (SA336 F22).

The objective of the present work was to develop a heavy-section 2¼Cr-1Mo steel with better creep strength by conducting a set of experiments with regard to the effect of microalloying elements. The best composition to obtain improved creep-rupture properties was found to be low Si-0.10C-2¼Cr-1Mo-¼V-Ti-B. With this composition, remarkable reduction in temper embrittlement susceptibility was also achieved. Other properties, such as weldability, low temperature toughness, and hydrogen embrittlement resistance, were similar to those of conventional 2¼Cr-1Mo steel.