Crystallographic Determinants of Cu-Induced Sensitization at Austenite Grain Boundaries in Steel

Residual copper in recycled steel scrap causes surface hot shortness during high-temperature reheating: a copper-rich liquid forms at the scale–metal interface and infiltrates prior austenite grain boundaries (PAGBs). The attack is sharply selective: some boundaries fail completely while immediately adjacent ones stay intact. We resolve its crystallographic basis. Commercial secondary steel wire rod (0.21 wt.% Cu) was sensitized at 1200 °C and water-quenched to martensite. Parent grain reconstruction of high-resolution electron backscatter diffraction (EBSD) data recovered the vanished austenite boundary network, and correlative overlay with scanning electron microscopy crack maps paired 199 classified PAGB segments with their misorientation angle and a binary sensitized/unsensitized state. Wetting begins near 20–22°: no boundary below 20° is penetrated, the sensitized fraction reaches 50% at 24° (95% CI 22–26°), and above 35° every non-Σ3 boundary is penetrated. The onset coincides with the misorientation at which the austenite boundary energy crosses the Smith wetting limit of 2γsl ≈ 0.86 J/m2, calculated to fall at 16–20°. Σ3 boundaries (approximately 60°) are entirely immune despite their high misorientation, their energy lying far below that limit. In this steel, boundary energy rather than misorientation angle alone governs copper wetting, identifying grain boundary engineering as a candidate route to scrap-tolerant steels.