The effects of carburizing, plasma nitriding and carbonitriding on 25CrMo4 steel were investigated. The aim was to compare their effects on hardness, the depth of harde-ned layers, the depth of surface-modified regions, the distribution of carbon and ni-trogen, and the resulting microstructure. Six specimens were prepared, with two used for each treatment. After thermochemical treatment, Vickers hardness mea-surements at a load of 0.5 kgf, metallographic examinations, scanning electron mic-roscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were car-ried out. The depths of hardened layers were determined from hardness profiles, while the depths of surface-modified regions were estimated metallographically on cross-sections. The maximum measured hardness was 692 HV for the carburized specimen, 730 HV for the plasma-nitrided specimen and 754 HV for the carbonitrided specimen. The hardened layer depth was approximately 0.57 mm for the carburized specimen and 0.42 mm for the carbonitrided specimen, while the nitriding hardness depth was approximately 0.42 mm. The metallographically estimated depths of the surface-modified regions were approximately 0.31 mm after carburizing, 0.23 mm after plasma nitriding and 0.44 mm after carbonitriding. Energy-dispersive X-ray spectroscopy analyses showed enrichment of the near-surface region with carbon after carburizing, with nitrogen after nitriding, and with both carbon and nitrogen after carbonitriding. Microstructural examinations of the carburized and carbonitrided specimens showed a predominantly low-temperature tempered martensitic
microstructure at the surface and a mixed microstructure in the core, whereas the nitrided specimen retained a ferritic-pearlitic microstructure in the core. All three processes increased surface-region hardness. Under the applied processing conditions, carbonitriding produced the highest measured hardness, whereas plasma nitriding caused the least change in the properties of the core.
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