Microstructure and Mechanical Properties in Isothermal Tempering of High Co-Ni Secondary Hardening Ultrahigh Strength Steel

The effect of isothermal tempering on the microstructure and mechanical properties in new-type high Co-Ni secondary hardening ultrahigh strength steel was investigated. The tempering temperature was 480°C and the tempering time spanned in the range from 10 minutes to 200 hours. The results showed that the hardness and tensile strength at the secondary hardening peak was about HRc 56.5 and 2222 MPa, respectively. With prolonged tempering time, the decrease of hardness and strength in this steel was attributed to the coarsening of the M2C carbides and decreasing of its coherency with matrix. The impact toughness of the steel decreased when it was tempered for a longer time, which was caused by the segregation of phosphorus and sulfur at grain boundaries.