Microstructure and mechanical properties of low-carbon steels quenched and tempered after gas carburizing, carbonitriding, and nitriding

Kawata Kazuki - Kawata PE Office (Japan)

Low-carbon steel specimens were subjected to either gas carburizing, carbonitriding, or nitriding at 800°C. The carburizing atmosphere included a mixture of nitrogen and methanol as the carrier gas and propane gas as the enriched gas with the carbon potential controlled by an oxygen sensor. The gas carbonitriding was performed using a carrier gas with a mixture of nitrogen and methanol and an enriched gas with propane and ammonia gas wherein the carbon potential and the amount of residual ammonia in the furnace was controlled by an oxygen sensor and an infrared ammonia analyzer, respectively. The gas nitriding was performed using a mixture of nitrogen, ammonia, and ammonia decomposition gas with the nitriding potential controlled by a thermal-conductivity-type hydrogen sensor. Subsequent to gas carburizing, carbonitriding, or nitriding, the steel specimens were oil quenched and tempered at 300°C. The microstructure for each of the specimens used was determined subsequent to oil quenching. The carburized specimen comprised martensite, fine pearlite (unhardened structure), and a small amount of retained austenite. The carbonitrided specimens comprised martensite and retained austenite. The nitrided specimen comprised martensite, retained austenite, and voids. The amount of retained austenite in the specimens subsequent to oil quenching was observed to be higher in specimens with a larger amount of nitrogen. X-ray diffraction analysis revealed that the carbonitrided and nitrided specimens subsequent to tempering had higher amounts of Fe4N in the specimens with higher nitrogen content. The surface hardness subsequent to tempering increased as the amount of nitrogen increased, except for the nitrided specimen, wherein voids were formed. Using a ball-on-disk type friction and wear test, the carbonitrided specimen with a high nitrogen content showed the best wear resistance subsequent to tempering.