Supercritical boilers operate at pressures above 22.1 MPa and temperatures exceeding 374°C, conditions where water becomes a supercritical fluid. Material selection for boiler tubes in such environments is critical because traditional mild steels rapidly lose strength and oxidation resistance. The primary requirements for supercritical boiler tube alloys include high creep rupture strength, excellent steam-side oxidation resistance, adequate fire-side corrosion resistance, good weldability, and long-term microstructural stability.

For lower temperature zones (up to 450°C), low-alloy ferritic steels such as T22 (2.25Cr-1Mo) and T23 (2.25Cr-1.6W-V) remain viable. However, for superheater and reheater tubes exposed to 580–620°C, advanced ferritic-martensitic steels are the standard. Grade T91 (9Cr-1Mo-V-Nb) offers a good balance of creep strength and oxidation resistance up to 620°C. Its higher chromium content forms a protective chromium oxide scale, reducing steam-side oxidation. For slightly higher temperatures (630°C), T92 (9Cr-0.5Mo-1.8W-V-Nb) provides improved creep properties through tungsten strengthening.

When steam temperatures exceed 650°C, austenitic stainless steels become necessary. TP347H (18Cr-10Ni-Nb) and TP310HCbN (25Cr-20Ni-Nb-N) are commonly used. Their higher chromium and nickel content impart exceptional oxidation and corrosion resistance. However, austenitic steels suffer from higher thermal expansion coefficients, greater susceptibility to stress corrosion cracking in chloride-containing environments, and increased cost. Additionally, dissimilar metal welds between ferritic and austenitic tubes require careful management to avoid premature failure.

Nickel-based superalloys such as Inconel 740H and Haynes 282 are emerging for ultra-supercritical applications (700–760°C). These offer outstanding creep strength and oxidation resistance but are expensive and difficult to fabricate. Economic considerations often drive a graded approach: ferritic steels for lower-temperature sections, austenitic for intermediate zones, and nickel alloys only for the hottest final superheater. Key selection criteria include the maximum metal temperature (accounting for heat flux irregularities), allowable stress values from codes like ASME Section I, and long-term aging data. Ultimately, material selection for supercritical boiler tubes represents a trade-off between performance, fabricability, and lifecycle cost—with creep strength and oxidation resistance being non-negotiable at high temperatures.