The Effect of Co–N Addition on Mechanical Properties, Microstructure and Erosion of 17Cr Steels

The effect of Co–N addition on mechanical properties, microstructure and water-jet erosion of heat-treated 17Cr steels was examined focusing on hardness, grain size, phase volume fraction and erosion damage. In the heat-treatment temperature range, the hardness and strength of Co–N added 17Cr steels are somewhat higher than those of non-added 17Cr steel. In the 950 to 1100°C quenching temperature range, grain size, hardness and volume fraction of retained γ and δ-ferrite increased with increasing quenching temperature. The grain size in Co–N added 17Cr steels was finer than that in non-added 17Cr steel and the volume fraction of retained γ and δ-ferrite in Co–N added 17Cr steels was smaller than that in non-added 17Cr steel. The Co and N content measured by EDAX and Auger analysis were mostly contained in the matrix; therefore, Co–N added 17Cr steels probably contributed to solid solution strengthening and restraint of δ-ferrite formation in the matrix. Under the tempering treated conditions, hardness patterns are divided into three regions (maximum softening region in 250 to 350°C, secondary hardening and temper brittleness region in 400 to 500°C, maximum softening and precipitation region in 550 to 700°C). All of the Co–N added and non-added 17Cr steels had similar hardness patterns when plotted against tempering temperatures; however erosion damage depth of the Co–N added 17Cr steels in high-pressure water-jet tests was somewhat less than that in Co–N non-added 17Cr steel.