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 heattreated 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 1 100°C quenching temperature range, grain size, hardness and volume fraction of retained g and d-ferrite increased with
... eased 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 g and d-ferrite in Co-N added 17Cr steels was smaller than that in nonadded 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 d-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. Fig. 4. The relationship between hardness and tempering temperatures for various 17Cr steels tempered at various temperatures for 1 h after quenched at 1 000°C for 0.5 h. Fig. 5. Optical micrographs showing microstructures of (a) 0Co-0.015N and (b) 1.0Co-0.04N 17Cr steels tempered at various temperatures for 1 h after quenched at 1 000°C for 0.5 h. © 2008 ISIJ Fig. 8. Erosion damage of various 17Cr steels after 40 impacts at an impact velocity of about 400 m/s in 8 mm distance nozzle to sample.