304H stainless round steel (ASTM A276; GB 07Cr19Ni10H) has a composition of 18.0–20.0% chromium (Cr), 8.0–10.5% nickel (Ni), 0.04–0.10% carbon (C), ≤1.00% silicon (Si), ≤2.00% manganese (Mn), ≤0.045% P, ≤0.030% S.
Compared to 304 (≤0.08% C), 304H's key difference is a controlled higher carbon content (0.04–0.10%), which serves a critical chemical role in high-temperature service:
Carbide strengthening: At temperatures above 500°C, carbon reacts with chromium to form fine chromium carbides (Cr₂₃C₆) along grain boundaries. These carbides "pin" grain boundaries, preventing grain growth (a major cause of strength loss at high temperatures). 304H's higher C content forms more Cr₂₃C₆ than 304, maintaining strength at 600–800°C-where 304's lower C leads to insufficient carbide formation and rapid strength degradation.
Creep resistance enhancement: The Cr₂₃C₆ carbides reduce creep (slow deformation under sustained load) by blocking the movement of dislocations and inhibiting grain boundary sliding. 304H's creep rate at 700°C is ~50% lower than 304's, thanks to its higher carbide density.
Chromium and nickel function the same as in 304-forming the passive layer and stabilizing austenite-but 304H's controlled C ensures these benefits are retained at elevated temperatures.