Effect of Heat Treatment on Structure and Mechanical Properties of 12Cr-3Co Steel with Low N and High B Contents

The effect of heat treatment on the microstructure and mechanical properties of Fe-bal-0.1 wt.%C-12Cr-3Co-2.5W-1Cu-VNbTaBN steel was investigated. After normalization in the range of 1050-1150 degrees C, the martensitic structure was dominant; delta-ferrite content was about 10%. When the temperature of normalizing increased from 1050 degrees C to 1150 degrees C, the average size of prior austenite grains increased from 44 to 68 mu m. After tempering at 750, 770 and 800 degrees C, tempered martensitic lath structure with a high dislocation density within martensitic laths contained nanosized M23C6 carbides along the boundaries of prior austenite grains and laths and (Ta,Nb)X carbonitrides randomly distributed in the ferritic matrix. The average sizes of M23C6 carbides and (Ta,Nb)X carbonitrides were about 50 nm and 40 nm, respectively, regardless of tempering temperature. The M6C carbide particles were also observed along the boundaries of prior austenite grains and pockets as well as along the boundaries between d-ferrite and martensite; their amount was negligible. When tempering temperature increased from 750 to 800 degrees C, the particle density on the boundaries between delta-ferrite/martensite decreased from 3.8 to 0.2 mu m(-1). A relationship between the lath size (h) and the density of free dislocations (rho) can be described as h = (5.41 root rho) - 0.07. Increasing the tempering temperature to 800 degrees C led to a decrease in the hardness up to similar to 220 HB, yield stress and ultimate tensile stress up to 520 and 700 MPa, respectively.