55# round steel (GB/T 699) is a medium-high carbon steel with mechanical properties optimized for spring and washer applications:
Tensile strength: 645–835 MPa after quenching (820–840°C) and tempering (480–520°C), providing the rigidity needed for load-bearing springs.
Yield strength: ≥380 MPa, ensuring springs retain their shape after repeated compression or extension.
Hardness: 38–43 HRC post-heat treatment, balancing wear resistance (for washers) and elasticity (for springs).
Elongation: 13–16%, sufficient to form into coiled springs without cracking.
Its chemistry (0.52–0.60% C, 0.17–0.37% Si, 0.50–0.80% Mn) promotes the formation of a fine pearlitic microstructure, which enhances spring properties. 55# is equivalent to AISI 1055 (USA) and DIN C55 (Germany), commonly used in automotive suspension washers and industrial compression springs. Made-in-China listings often highlight it as "55# Carbon Steel Round Bar for Springs & Washers."