Heat Treatment Strengthening Process and Mechanism for 20SA Aluminum-Clad Steel Wire

Aluminum-clad steel wires are widely used in ultra-high-voltage (UHV) transmission lines due to their excellent beneficial properties. To address the inadequate strength–ductility balance of aluminum-clad steel wires for UHV applications, this study investigates the effects of cold drawing and low-temperature annealing (200~330 °C) on the microstructure, aluminum/steel interface, and mechanical properties of 20SA (20.3% IACS) aluminum-clad steel wires. The results indicate that the cold-drawn steel core exhibits high strength due to its high dislocation density and strong <110> fiber texture. Low-temperature annealing promotes overall recovery and partial recrystallization of the microstructure. Specifically, annealing at 240~280 °C significantly enhances ductility while maintaining high strength, achieving optimal strength–ductility synergy; however, higher annealing temperatures result in an unacceptably low ultimate strength, rendering them impractical for application. Interfacial characterization reveals that the as-clad and cold-drawn processes, as well as annealing below 250 °C for 10 min, maintain stable metallurgical bonding at the aluminum/steel interface, whereas annealing at 300 °C for 10 min induces the formation of brittle Fe-Al intermetallic compounds, thereby compromising service reliability. Furthermore, the precipitation of cementite (Fe3C) with increasing temperature partially compensates for the strength loss caused by overall recovery. This study identifies the optimal heat treatment window, providing a theoretical basis for the strength–ductility design and reliable service of UHV transmission lines.