20CrNiMo round steel (GB/T 3077) has a composition of 0.17–0.23% carbon (C), 0.17–0.37% silicon (Si), 0.60–0.90% manganese (Mn), 0.40–0.70% chromium (Cr), 1.25–1.65% nickel (Ni), 0.20–0.30% molybdenum (Mo), ≤0.035% P, ≤0.035% S.
Nickel and molybdenum collaborate to drastically improve chemical hardenability (the ability to form hard martensite during quenching):
Nickel's role: Nickel (1.25–1.65%) is an austenite stabilizer-it lowers the martensite start temperature (Ms) and slows the transformation of austenite to soft pearlite/bainite during cooling. This gives the steel more time to cool to martensite-forming temperatures before softer phases develop.
Molybdenum's role: Molybdenum (0.20–0.30%) is a strong carbide former and diffusion inhibitor. It slows the movement of carbon and iron atoms, further delaying pearlite formation and extending the "critical cooling rate" (the minimum cooling rate needed to form martensite). Molybdenum also enhances the hardening effect of nickel by increasing the solubility of carbon in austenite.
Together, these elements allow 20CrNiMo to form martensite even with relatively slow cooling (e.g., air cooling for thin sections), whereas lower-alloy steels would require rapid water quenching. Chromium (0.40–0.70%) complements this by forming fine carbides that enhance hardness, while low carbon (0.17–0.23%) ensures the martensite remains ductile.