20CrMo round steel (GB/T 3077) has a composition of 0.17–0.24% carbon (C), 0.17–0.37% silicon (Si), 0.40–0.70% manganese (Mn), 0.80–1.10% chromium (Cr), 0.15–0.25% molybdenum (Mo), ≤0.035% phosphorus (P), ≤0.035% sulfur (S).
Chromium (Cr) and molybdenum (Mo) work synergistically to boost high-temperature stability via complementary mechanisms:
Chromium's oxide protection: Cr reacts with oxygen to form a dense, adherent Cr₂O₃ passive layer on the steel surface. This layer blocks oxygen from penetrating the matrix, resisting oxidation at temperatures up to 500°C. Unlike iron oxides (which are porous and flake off), Cr₂O₃ remains intact under thermal cycling, maintaining long-term protection.
Molybdenum's creep inhibition: Mo slows the diffusion of iron and carbon atoms in the steel matrix-diffusion is the primary driver of creep (slow deformation under heat and pressure). Mo also forms stable molybdenum carbides (Mo₂C) that resist coarsening at high temperatures; coarsened carbides lose their ability to pin dislocations, which accelerates creep.
Together, Cr prevents surface degradation, while Mo preserves internal structural integrity-this combination ensures the steel retains its chemical structure and strength at temperatures where low-alloy steels (e.g., 20#) would degrade rapidly.