Effect of Annealing Time on Microstructure Evolution and Mechanical Properties of Hot-Rolled Cu–Zn–Sn Alloy

The effect of post-rolling annealing time on the microstructural evolution and room-temperature mechanical properties of a hot-rolled Cu–Zn–Sn alloy was systematically investigated. The alloy was isothermally annealed at 500 °C for 4, 5, and 6 h. Results reveal that annealing time non-monotonically governs the recrystallization of the α and β phases and the final content of the Sn-rich intergranular phase, coupled with the α/β phase transformation and Sn partitioning. The as-hot-rolled microstructure comprises a dual α + β structure without the Sn-rich intergranular phase. After 4 h, the α phase recrystallizes completely, and the β phase fraction decreases, leading to Sn rejection and Sn-rich intergranular phase formation after furnace cooling; hardness and strength are minimum, while ductility is high. At 5 h, the β phase fraction recovers and Sn partitions into the β phase, decreasing the final Sn-rich intergranular phase content. The β phase transforms into fine short-rod and lath-like morphologies, refining grains and yielding optimal strength–ductility synergy, with an ultimate tensile strength (UTS) of 451.6 MPa and an elongation of 41.7%. At 6 h, the β phase fraction decreases and grains coarsen, leading to re-formation of the Sn-rich intergranular phase and degraded strength. Quantitative analysis reveals that solid-solution strengthening provides a stable baseline, while variations in yield strength are dominated by dislocation and grain-boundary strengthening. The β phase provides load-transfer strengthening, and the Sn-rich intergranular phase acts primarily as a damage source. Fracture changes from ductile dimples to mixed ductile–brittle with local flat features, depending on the Sn-rich intergranular phase distribution. This study links processing, microstructure, and properties for optimizing tin brass heat treatment.