Choosing Alloys for High-Temperature Service

## Why Temperature Changes Everything At elevated temperatures, metals suffer creep (time-dependent deformation under sustained stress), accelerated oxidation, thermal fatigue, and microstructural instability. ## Temperature-Based Alloy Selection ### Up to 550 degrees C: Low-Alloy Steels | Alloy | Max Temp | Key Application | |-------|---------|-----------------| | 2.25Cr-1Mo (SA-387 Gr 22) | 595 degrees C | Refinery reactors | | 9Cr-1Mo-V (P91) | 600 degrees C | Supercritical boiler piping | P91 provides double the allowable stress of P22 at 600 degrees C, enabling thinner walls. ### 550-900 degrees C: Stainless Steels **304H and 316H**: H grades (0.04-0.10% C) have better creep strength than L grades. **Alloy 800H/HT (UNS N08810)**: Standard for ethylene cracker furnace tubes up to 900 degrees C. ### 900-1100 degrees C: Nickel Superalloys **Inconel 617**: Exceptional combination of strength and oxidation resistance to 1000 degrees C. **Haynes 230**: Excellent thermal stability to 1150 degrees C. ## Design with Creep Data High-temperature design uses allowable stress values from ASME Section II, Part D that decrease with temperature and design life through the Larson-Miller parameter. ## Common Mistakes - Using 304 where 304H is required (lower creep strength) - Designing for short-term tensile strength rather than creep-rupture strength - Ignoring thermal cycling effects during startups and shutdowns