# Forced Circulation of Nitrogen Gas for Accelerated and Eco-Friendly Cooling of Metallic Parts
## 2 Materials and Methods
### Methods
#### 2.2.2 Application of the Nitrogen Cooling System to Cool the Sintered THA  
The characteristics of nitrogen cooling were compared with those of water cooling. The sintered THA was heat-treated at 1150 \({}^{\circ}\)C for 8 h and then cooled using water as well as nitrogen cooling systems. Figure 6 shows the heat treatment cycle performed in this study. Conventional water cooling system mainly includes a heating zone and a water-quenching zone. In the heating zone, the workpieces are heated under controlled atmosphere. In the water-quenching zone, the heat-treated products are rapidly cooled in the water pool. Figure 7 shows the furnace with the water cooling system used in this study. Figure 7a shows the workpiece before heat treatment. For performing the heat treatment, the workpieces are loaded in the heating zone, and the loading gate is closed. The workpiece positioned in the heating zone is heated to 1150 \({}^{\circ}\)C under controlled atmosphere, as shown in Figure 7b. Then, the heat-treated workpieces are moved to the water-quenching zone and cooled by water, as shown in Figure 7c. Cooling in the water pool is continued, until the temperature of the product attains room temperature. Then the product is extracted from the water pool and air-dried. Figure 5: Schematic drawing of the door system designed for the cooling process. Figure 6: Heat treatment cycle. An extremely dynamic and rapid cooling process does not permit a stable and consistent measurement of cooling rates. Furthermore, as each configuration suffers from non-uniform cooling rates in the longitudinal and circumferential directions, and an experimental measurements of reliable cooling rate is difficult to accomplish. In the case of nitrogen cooling, the cooling of the workpiece progressed in the cooling chamber. Therefore, the cooling of the workpiece was monitored by the temperature of nitrogen in the cooling system. Figure 8 shows the on-line measured temperature profiles of nitrogen gas before and after the heat exchange in the chiller. As shown in this figure, the hot nitrogen gas was rapidly cooled by the heat exchanger and, eventually, the temperatures of the nitrogen gas before and after the heat exchange turned similar after about 2000 s. This similarity in temperature before and after the heat exchange implies that the actual cooling of the workpiece was completed. Therefore, the nitrogen cooling of the workpiece (until it attained the room temperature) was completed after 2000 s. To estimate the overall heat transfer coefficient of the nitrogen cooling system, the transient heat analysis was conducted using the commercial finite element analysis code ABAQUS(tm) (Abaqus 6.13, Dassault Systems, Providence, RI, USA) [46]. To analyze the cooling process that employed water and nitrogen gas, the tubular workpiece was modeled with an element dimension of 0.02 mm \(\times\) 0.02 mm \(\times\) 0.02 mm, as shown in Figure 9. The \(D_{i}\), \(D_{o}\), and \(L\) of the tubular model are 0.4 m, 0.6 m, and 0.5 m, respectively. Figure 8: Measured nitrogen temperatures before and after passing the heat exchanger. Figure 7: Schematic drawing of the furnace with the water cooling system: (**a**); Before workpiece loading (**b**) workpiece loading and heating; (**c**) water cooling. The element type was an eight-node heat transfer brick element (DC3D8). As thermal properties of THA, \(\rho\) = 18,500 kg m\({}^{-3}\), \(k\) = 125 W m\({}^{-1}\)\({}^{\circ}\)C\({}^{-1}\), and \(C_{p}\) = 133 J kg\({}^{-1}\)\({}^{\circ}\)C\({}^{-1}\) were used in this study. During the cooling process, the heat was removed using three main heat transfer mechanisms, i.e., convection, conduction, and radiation. Varied film coefficients were applied at the surface of specimen for cooling with water and nitrogen gas.