1. Impact on Strength: Mild Solid Solution Strengthening
Yield strength: In standard Cu content (0.20–0.50%), yield strength increases by 15–30 MPa compared to plain carbon steel of the same base composition.
Tensile strength: Tensile strength rises by 20–40 MPa with the same Cu range, a moderate boost that does not over-harden the steel.
Excess Cu (>0.50%): Beyond the standard range, copper segregates at grain boundaries and forms brittle intermetallic phases (e.g., Cu-Fe alloys), which can cause a slight drop in strength and a sharp reduction in toughness (avoided in standard weathering steel formulations).
2. Impact on Ductility & Toughness: Minimal Loss, Synergistic Gain
Ductility (elongation): Copper addition within 0.20–0.50% causes only a 1–2% reduction in elongation (e.g., from ~22% to ~20% for thin plates), a negligible change that preserves the steel's formability for cold bending, rolling, and cutting.
Low-temperature toughness: Copper alone has a mild toughening effect, and when combined with nickel (Ni) (e.g., in ASTM A588) or chromium (Cr) (e.g., in Q355NH), it lowers the ductile-to-brittle transition temperature (DBTT) by 10–20°C. This synergy ensures weathering steel remains ductile at subzero temperatures (e.g., -20°C to -40°C) without brittle fracture.
3. Impact on Formability & Weldability: No Severe Compromise
Formability: The moderate strength increase and minimal ductility loss mean weathering steel with standard Cu content retains good cold formability-it can be bent, curved, and roll-formed into complex shapes (e.g., corrugated plates, pergolas) with standard fabrication techniques (only slightly larger bend radii may be needed for thick plates, vs. plain carbon steel).
Weldability: Copper is a low-carbon equivalent (CE) element, so it does not increase the steel's hardenability or risk of weld cracking. With standard welding electrodes (e.g., E7018-W for weathering steel), weld joints maintain matching strength and corrosion resistance, with no special pre-heating or post-weld heat treatment required for most applications.
4. Key Synergy with Other Alloy Elements
Chromium (Cr): Complements copper's solid solution strengthening and refines the microstructure, further balancing strength and toughness.
Nickel (Ni): Enhances copper's toughening effect and improves weld joint ductility.
Phosphorus (P): Promotes grain refinement, which works with copper to boost strength without excessive ductility loss.
