Effect of heat treatment on microstructure and mechanical properties of Nb-containing cryogenic steels

Effects of different heat treatment processes on microstructure and properties of Nb-containing cryogenic steels were investigated.The results show that microstructure of the steel as cast is fine lamellar pearlite and blocky ferrite and that after heat treatments is tempered sorbite.The mechanical properties are best after quenching+quenching and tempering,a little lower after normalizing+quenching and tempering,and worst after annealing+quenching and tempering.The Charpy-V impact energies are 45.7 J and 29.9 J respectively tested at-20 ℃ and-40 ℃,and the tensile strength,yield strength and percentage elongation after fracture are 1010 MPa,910 MPa and 19.5% respectively after quenching+quenching and tempering.