50CrNi round steel (GB/T 1222) has a composition of 0.46–0.54% carbon (C), 0.17–0.37% silicon (Si), 0.60–0.90% manganese (Mn), 1.00–1.40% nickel (Ni), ≤0.035% phosphorus (P), ≤0.035% sulfur (S).
Compared to 50CrVA (0.46–0.54% C, 0.17–0.37% Si, 0.60–0.90% Mn, 0.80–1.10% Cr, 0.10–0.20% V, ≤0.035% P/S), 50CrNi replaces chromium and vanadium with nickel (1.00–1.40% Ni), which reduces temper brittleness via two mechanisms:
Nickel's grain boundary segregation inhibition: Temper brittleness occurs when impurities like phosphorus and antimony segregate at grain boundaries during tempering (300–550°C), weakening intergranular bonds. Nickel has a higher affinity for these impurities than iron, binding with them and preventing their migration to grain boundaries. This reduces segregation and strengthens boundaries, making the steel less brittle after tempering.
Microstructural stabilization: Nickel stabilizes the bainitic microstructure formed during tempering, preventing the formation of brittle cementite (Fe₃C) networks along grain boundaries. In 50CrVA, vanadium forms carbides that can promote cementite precipitation, increasing brittleness-nickel avoids this by keeping the microstructure more ductile.
Carbon content is identical in both grades, as it is needed for strength, but nickel ensures this strength does not come with excessive brittleness.