25Cr2Mo1V round steel (GB/T 3077) is designed for creep resistance (resistance to slow deformation under heat/pressure) with a composition: 0.22–0.29% carbon (C), 0.17–0.37% silicon (Si), 0.40–0.70% manganese (Mn), 2.10–2.50% chromium (Cr), 0.90–1.10% molybdenum (Mo), 0.25–0.35% vanadium (V), ≤0.035% P, ≤0.035% S.
Three elements work synergistically to enhance creep resistance:
Molybdenum (0.90–1.10%): Mo atoms slow the diffusion of iron and carbon atoms in the steel matrix-diffusion is the primary driver of creep. Mo also forms stable molybdenum carbides (Mo₂C) that resist coarsening at high temperatures.
Vanadium (0.25–0.35%): V forms ultra-fine vanadium carbides (VC) that pin grain boundaries and dislocations. These precipitates block the movement of dislocations (the atomic defects that cause deformation), reducing creep rate.
Chromium (2.10–2.50%): Cr forms chromium carbides (Cr₂₃C₆) that strengthen the matrix and improve oxidation resistance-oxidation weakens the steel and accelerates creep. Cr also enhances the stability of Mo and V carbides, preventing their dissolution at high temperatures.
Carbon (0.22–0.29%) provides enough C to form these carbides without excessive brittleness, while Si/Mn act as deoxidizers to minimize inclusions that could initiate creep cracks.