60Si2CrA round steel (GB/T 1222) has a composition of 0.56–0.64% carbon (C), 1.60–2.00% silicon (Si), 0.40–0.70% manganese (Mn), 0.40–0.70% chromium (Cr), ≤0.030% phosphorus (P), ≤0.030% sulfur (S).
Compared to 60Si2Mn (0.56–0.64% C, 1.50–2.00% Si, 0.60–0.90% Mn, no Cr), 60Si2CrA replaces some manganese with chromium (0.40–0.70% Cr vs. 0.60–0.90% Mn)-this change improves fatigue resistance (resistance to failure under repeated loading) via two chemical mechanisms:
Chromium carbide refinement: Chromium forms fine, uniformly distributed chromium carbides (Cr₇C₃) instead of the coarser manganese carbides (Mn₃C) in 60Si2Mn. Fine carbides pin dislocations more effectively and reduce stress concentrations-stress concentrations are the primary cause of fatigue cracks, as they initiate cracks and accelerate their growth.
Reduced impurity segregation: Chromium reduces the segregation of phosphorus and sulfur at grain boundaries. In 60Si2Mn, P and S tend to cluster at boundaries, weakening them and making the steel more prone to fatigue failure. Chromium binds with these impurities, forming stable compounds (e.g., CrP, CrS) that are evenly distributed throughout the matrix, reducing boundary weakness.
Silicon (1.60–2.00%) remains the primary strengthener, forming solid solutions and enhancing the elastic limit-critical for fatigue resistance. Lower sulfur and phosphorus limits (≤0.030% vs. ≤0.035% in 60Si2Mn) further reduce impurity-induced fatigue cracks.