Long-Term Creep Properties of 2.25Cr-.1.6W-VNbB Steel (T23/1323) for Fossil Fired and Heat Recovery Boilers

The creep enhanced low alloy steel with 2.25Cr-1.6W-V-Nb (HCM2S; Gr.23, ASME CC2199) has been originally developed by Mitsubishi Heavy Industries, Ltd. and Sumitomo Metal Industries, Ltd. The steel tubes and pipe (T23/P23) are now widely used for fossil fired power plants all over the world. Recently, the chemical composition requirements for ASME Code of the steel have been changed and a new Code Case 2199-4 has been issued with the additional restriction regarding Ti, B, N and Ni, and the Ti/N ratio incorporated. In this study, the effects of additional elements of Ti, N and B on the mechanical properties and microstructure of T23/P23 steels have been evaluated. It is found that N decreases the hardenability of the steel by forming BN type nitride and thus consuming the effective B, which is a key element for hardening of the steel. The addition of Ti, on the other hand, enhances the hardenability of the steel by precipitating TiN and thus increasing the effective B. It is also found that too much addition of Ti degrades the Charpy impact property and creep ductility of the steel to a great extent. This might affect on the long term creep rupture properties of the steel, although the steel with the original chemical compositions has already exhibited high creep strength up to 600 degrees C over 110000h.