Effect of Tempering Temperature on Precipitates, Microstructure, and Mechanical Properties of Quenched Cu-Cr-Ni Ultra-High Strength Weathering Steel
The effects of tempering temperature on the microstructure, strength-toughness balance, and precipitates of a quenched Cu-Cr-Ni ultra-high strength weathering steel were systematically investigated. The steel was austenitized at 920°C, quenched, and then tempered at 500°C, 550°C, and 600°C. Microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), while mechanical properties were evaluated via tensile and low-temperature impact tests. Results showed that as the tempering temperature increased from 500°C to 600°C, the microstructure transformed from lath-shaped tempered sorbite to a non-lath morphology. The fraction of rod-like cementite decreased, while spheroidized cementite increased, and the size of MC (M = Ti, Nb, V, Mo) precipitates decreased from an average of 12.2 nm to 9.9 nm. Consequently, yield strength and tensile strength decreased from 935 MPa and 958 MPa to 866 MPa and 888 MPa, respectively, whereas total elongation and impact energy at -40°C increased continuously, reaching maximum values of 5.0% and 280 J at 600°C. When tempered at 550°C, the steel exhibited a yield strength of 895 MPa, tensile strength of 921 MPa, elongation of 4.3%, and impact energy of 271 J at -40°C, demonstrating an optimal combination of strength and toughness. This improvement is primarily attributed to the spheroidization of cementite and the uniform dispersion of fine MC precipitates, which alleviate stress concentration, along with the softening of the acicular ferrite matrix during tempering.