# Forced Circulation of Nitrogen Gas for Accelerated and Eco-Friendly Cooling of Metallic Parts
## 1 Introduction  
An innovative nitrogen cooling system has been developed for accelerated and eco-friendly cooling of metallic part. In this cooling process, the dry gaseous nitrogen is rapidly circulated and exchanges heat with the workpiece that it contacts, resulting in an accelerated cool-down. The heated nitrogen gas is forcibly circulated and transported to the chiller in a closed loop, where the heat was removed, and the cooled gas was circulated again. Selection of a cooling medium is mainly based on chemical composition, dimension, and desired surface quality of the parts [1; 2; 3]. The function of the cooling medium is to control the rate of heat transfer from the surface of the quenched parts, and the most commonly used cooling media is water. However, the surface oxidation, environmental contamination, and thermal distortion during water cooling have been problematic in industrial applications [1; 2]. Argon and nitrogen are the gases that are most utilized for eco-friendly cooling during thermal processing in vacuum furnaces. Gases with smaller and lighter molecules exhibit a greater thermal conductivity, because they can move faster and farther without a collision [1; 4; 5]. As nitrogen of atomic weight 14.0067 amu is approximately 2.9 times lighter than argon of atomic weight 39.948 amu, it is approximately four times faster than argon [6; 7]. Consequently, nitrogen exhibits a greater heat transfer coefficient than argon. Furthermore, the heat transfer coefficient of nitrogen can be significantlyincreased by controlling the temperature and pressure [7; 8; 9; 10]. As nitrogen is nonreactive, the inert environment created by nitrogen prevents the occurrence of combustible reactions. As nitrogen leave behind no residues on workpieces or in a chamber space, these properties make it unnecessary to invest in washing facilities and fire monitors and lower the operating cost, since maintenance procedures and disposal of the quenching media are eliminated. Several studies have been performed on the nitrogen cooling process [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. To enhance the cooling rate under operating conditions, liquid nitrogen cooling has been performed. Investigations were carried out in a drilling operation under flood and liquid nitrogen cooling separately. The experimental results indicated a reduction in cutting temperature, increase in thrust force, and surface roughness when liquid nitrogen cooling is applied [11]. The machinability characteristics of turning using dry turning and liquid nitrogen cooling methods has also been investigated. The experimental results indicated improved cutting tool performance under liquid nitrogen cooling by control of wear mechanisms which, in turn, reduced the wear rate [12]. High-pressure gas cooling can be a form of heat treatment process used for hardening of tool and die steels. The use of gas quenching can significantly improve the mechanical and physical properties of a material for obtaining the near-net shape of metal components. The liquid nitrogen die cooling effect has been estimated on the conventional extrusion process [13]. Results showed a significant impact of the design aspects on the thermal efficiency of the cooling and an important heat removal when the liquid nitrogen cooling is used. [PERSON] et al. [14] simulated the high-pressure gas quenching effect on the cooling of a large H13 die by using computational fluid dynamics. Laser surface melting is a high-energy surface treatment that allows modification of the microstructure and surface properties. An attempt of laser surface melting with liquid nitrogen-assisted cooling was carried out to obtain a higher cooling rate and improve the surface properties. The experimental results showed a thinner melted layer, a highly homogeneous, refined melted microstructure [15]. [PERSON] et al. [16] reduced microsegregation by enhancing the weld cooling rate using liquid nitrogen cooling during the gas tungsten arc welding process.