Research on the Temperature Control and Protection Effects of Low-Temperature Nitrogen on the Water Jet Cutting Process of Alloy Steel

During abrasive waterjet cutting of alloy steel in complex field environments, local heat sources can induce temperature rise and oxidation risk. To address this issue, this study proposes a low-temperature nitrogen jet (LTNJ) method for simultaneous local cooling and air displacement near the cutting zone. A three-dimensional fluid-solid coupled heat-transfer and species-transport numerical model was established to investigate the coupled cooling and gas-coverage behavior. The study was further supported by infrared thermal-imaging measurements, in which the measured maximum apparent surface temperature remained within 32.1–33.6 °C during cutting under LTNJ assistance. The simulation results show that a nitrogen-enriched low-oxygen coverage region can be formed above the local heat source; for example, when the local nitrogen volume fraction reaches 64.74%, the estimated oxygen volume fraction decreases to approximately 7.39%. The gas temperature above the heat source decreases by more than 240 °C under the simulated conditions, and the relative position between the nozzle and heat source is the dominant factor affecting local cooling and gas coverage. Considering cooling effect, nitrogen utilization, and field implementation feasibility, the recommended parameter combination within the simulated range is a 10 mm horizontal distance between nozzle axis and heat-source center, a 30 mm vertical distance from nozzle outlet to workpiece surface, an initial nitrogen temperature of −50 °C, and a nitrogen flow rate of 5 m3/h. These results provide a numerical and preliminary experimental basis for low-temperature nitrogen-assisted temperature control and low-oxygen protection during abrasive waterjet cutting in complex field environments.