The determining role of aluminum on copper precipitation and mechanical properties in Cu-Ni-bearing low alloy steel

We elucidate here the determining role of aluminum on the precipitation behavior and mechanical properties in Cu-Ni-bearing low alloy steel. The microstructure and precipitation behavior were studied by electron microscopy in conjunction with the thermodynamic analysis. Aluminum had significant influence on the precipitation behavior of niobium such that the precipitation temperature of Nb(C, N) was decreased, and prior austenite grain size was governed by Al and Nb precipitation. The nano-size B2 structure of particles was confirmed to be Ni(Al, Mn), and Mn atoms preferred to substitute for Al in the sublattice site in the B2 structure. Furthermore, aluminum effectively increased precipitate density by increasing the nucleation rate of ordered-B2 Ni(Al, Mn) and copper precipitation, thereby increasing the hardness of Cu-Ni-bearing low alloy steel during tempering. Excellent tensile properties were obtained in low carbon steel after quenching and tempering, with tensile strength of 1150 MPa and yield strength of 1020 MPa. The high strength is attributed to the martensite matrix and precipitation of copper.