No abstract

High-nitrogen ductile iron (DI) was prepared by a new method of injecting nitrogen gas into molten iron and nodularizing treatment. The microstructure and mechanical properties of the as-prepared DI for different nitrogen gas injection periods were characterized. The graphite morphology gradually deteriorated with the increase in the nitrogen gas injection time. The maximum nitrogen and pearlite contents were obtained after 20 min of nitrogen gas injection, and the corresponding tensile strength and elongation of the DI were calculated as 492 MPa and 9.5%, respectively, which were 9.3% and 22% higher than those of the DI prepared without the nitrogen gas injection treatment, respectively.

A ductile iron wheel used for a rail-transit vehicle was treated with a recommended heat-treatment process. The ductile iron wheel after heat treatment was composed of graphite nodules and tempered sorbite with an area fraction of 98%. A friction test of the ductile iron and carbon steel wheel materials was systematically performed under different normal loads and sliding velocities. The results indicated that the wear mechanism of the ductile iron wheel changed from adhesion to abrasion with an increase in the normal load level. Adhesion was the main wear mechanism at different sliding velocities and normal load level. The impact of the normal load on the wear mechanism was greater than that of the sliding velocity. Since the ductile iron wheel material had excellent thermal property and higher carbon content, it exhibited a lower wear rate, a smaller difference value of the friction coefficient, and plastic deformation on the worn surface than those of the carbon steel wheel material. This indicates that ductile iron wheels may have a longer wear life, greater traction, and higher stability during operation than carbon steel wheels. The iron wheels have the potential for being applied in rail-transit vehicles.

In order to prepare heavy-section ductile iron with high strength and excellent elongation, a series of quenching- tempering experiments was conducted. A relationship between quenching-tempering time and temperature and the contents of martensite and pearlite was established by adjusting different quenching mediums and process parameters, and different microstructures in the iron matrix led to different mechanical properties. The content of martensite in the iron matrix reached over 94% after quenching at 880°C or a higher temperature. Further, the pearlite content could reach over 91% after tempering at 570°C or a higher temperature, thus resulting in improved mechanical properties. The investigated ductile iron yielded mechanical properties of a tensile strength of 970 MPa and an elongation of 6% after quenching in water at 880°C and tempering at 570°C. This will provide more possibilities for the application of heavy-section ductile iron parts.

In order to prepare heavy-section ductile iron with high strength and excellent elongation, a series of quenching- tempering experiments was conducted. A relationship between quenching-tempering time and temperature and the contents of martensite and pearlite was established by adjusting different quenching mediums and process parameters, and different microstructures in the iron matrix led to different mechanical properties. The content of martensite in the iron matrix reached over 94% after quenching at 880°C or a higher temperature. Further, the pearlite content could reach over 91% after tempering at 570°C or a higher temperature, thus resulting in improved mechanical properties. The investigated ductile iron yielded mechanical properties of a tensile strength of 970 MPa and an elongation of 6% after quenching in water at 880°C and tempering at 570°C. This will provide more possibilities for the application of heavy-section ductile iron parts.