When sourcing heat resistant steel pipes, you must prioritize grades that maintain mechanical properties at elevated temperatures. Common grades include ASTM A335 (P5, P9, P11, P22, P91) for high-pressure service and AISI 310S or 309S for high-temperature oxidation resistance. Ensure the pipes meet ASTM, ASME, or EN standards. Key specifications include creep strength, oxidation resistance up to 1200°C, and thermal fatigue resistance. Always verify the chemical composition (Cr, Ni, Mo content) via a Mill Test Certificate (MTC) to ensure the alloy meets your specific environment's needs.