The development of very high strength levels in many alloy steels is achieved by a secondary hardening reaction. In high Co-Ni steels containing the strong carbide-forming elements, Mo and Cr, secondary hardening is accomplished by the precipitation of fine scale $M_2C$ alloy carbides. Thermodynamic stability and coarsening resistance of $M_2C$ precipitates depends on the alloy content of these elements.
The effect of tempering on the growth behavior of $M_2C$ precipitates was examined. Applied tempering temperatures are 468, 482, 510℃ and tempering hours are varied from 1 to 400 hours. The value of peak hardness was HRc 55 in all temperatures and the shift of the apex of peak hardness to the left along tempering time was appeared with increasing temperature. When the average length and diameter of $M_2C$ carbides are 4∼8 nm and 1.5∼2.5 nm, respectively, the peak hardness was accomplished. The more the calculated volume percent of $M_2C$ phase, the higher value of hardness maintained after 200 hours tempering. Necessary information pertaining to the equilibrium in this steel was obtained using Thermo-Calc software and database developed at the Royal Institute of Technology, Stockholm, Sweden.
The size of $M_2C$ carbides was increased with time and those of 510℃ were largest than those of 482 and 468℃ in all tempering time. The aspect ratios of $M_2C$ carbides were almost 3 in all tempering conditions. The amount of precipitated austenite was increased with tempering time and temperatures.
Experimental studies of growth behavior of the carbides in this steel have been made by using TEM, SEM and X-ray Diffraction. Coarsening kinetics of $M_2C$ carbides in this steel is found to be controlled by combined effect of volume diffusion of alloying element Cr and Mo.
