15CrMo round steel (GB/T 3077) has a composition of 0.12–0.18% carbon (C), 0.40–0.70% silicon (Si), 0.40–0.70% manganese (Mn), 0.80–1.10% chromium (Cr), 0.40–0.55% molybdenum (Mo), ≤0.035% P, ≤0.035% S.
Compared to 12Cr1MoV (0.08–0.15% C, 0.17–0.37% Si, 0.40–0.70% Mn, 0.90–1.20% Cr, 0.25–0.35% Mo, 0.15–0.30% V), 15CrMo has two key chemical differences that suit moderate-temperature (350–500°C) service:
Higher molybdenum (0.40–0.55% vs. 0.25–0.35%): More Mo enhances creep resistance at moderate temperatures by slowing atomic diffusion-diffusion is the primary cause of creep (slow deformation). Mo also forms stable Mo₂C carbides that resist coarsening at 350–500°C, maintaining their ability to pin dislocations.
No vanadium: 12Cr1MoV's vanadium (0.15–0.30%) forms VC carbides that are highly effective at high temperatures (500–550°C) but unnecessary at moderate temperatures. Without V, 15CrMo has a simpler carbide structure (Cr₂₃C₆, Mo₂C) that is more stable and less prone to dissolution at 350–500°C.
Higher carbon (0.12–0.18% vs. 0.08–0.15%) in 15CrMo provides more C for carbide formation, increasing strength at moderate temperatures without the brittleness risk associated with higher C in high-temperature steels. Silicon (0.40–0.70%) is higher than 12Cr1MoV's, improving oxidation resistance at moderate temperatures by thickening the SiO₂ layer beneath Cr₂O₃.