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 phenomenon might affect the steel's long-term creep rupture properties, although a steel with the original chemical composition has demonstrated high creep strength at temperatures up to 600°C for more than 110,000 h.

The growth behavior of oxide scale in a laboratory steam environment has been conducted for the shot-peened 18Cr-8Ni stainless steels differing in grain size. Both steels (fine grained and coarse grained) have demonstrated almost the same steam oxidation behavior reacted at 700°C for up to 2000h, which had excellent oxidation resistance due to formation of a protective Cr 2 O 3 scale. After the exposure of 4000h, however, nodule-like oxide occurred on the coarse grained steel, while the fine grained steel still remained the uniform Cr 2 O 3 scale. These behaviors well explained in terms of changes of the outward Cr flux due to recovery and recrystallization of the deformed structure. This result has proven that the shot-peened tube composed of fine grain structure is capable of combat against the steam oxidation at high temperatures.