# Forced Circulation of Nitrogen Gas for Accelerated and Eco-Friendly Cooling of Metallic Parts
## 1 Introduction
Additionally, as the liquid nitrogen cooling increased the fracture degree inside the solids, liquid nitrogen has also been used to bring about thermal damage cooling as a fracturing fluid [17; 18]. As described above, the primary efforts have been mainly focused on discovering appropriate approaches to generate sufficiently high cooling efficiency through the direct contact of cryogenic liquid nitrogen [11; 12; 13; 14; 15; 16; 17; 18] and produce cooling that results in uniform mechanical properties of the products [19; 20]. However, practical applications of dry nitrogen gas in the cooling process of metallic parts have not been sufficiently reported [1; 2; 3; 4; 5; 7; 8; 9; 10]. Gases have a fundamental disadvantage in comparison to fluids, which is that they have poor heat transfer characteristics under normal condition. For their use as cooling media in cooling system, they have to be optimized by proper adjustment of the gas pressure and flow speed. In this study, the nitrogen gas cooling system has been developed to exchange heat with the workpieces, resulting in an accelerated and eco-friendly cool-down. The heated nitrogen gas is forcibly circulated and transported to the chiller in a closed loop, where the heat is removed, and the nitrogen gas is cooled. This cooled nitrogen gas is rapidly circulated again using a circulation pump rotating at a high speed. To estimate the performance of the proposed nitrogen cooling system, sintered tungsten heavy alloy (THA) was cooled using nitrogen gas. It has been observed that the mechanical properties of the sintered THA parts have significantly improved after the heat treatment. Especially, accelerated cooling after heat treatment was advantageous for the improved quality of THA parts [21; 22; 23]. THAs are composites consisting of nearly spherical tungsten particles embedded in a ductile matrix phase with lower melting point elements, such as nickel, iron, copper, and cobalt [24; 25]. As tungsten melts at excessively high temperatures, a mixture of tungsten powders with a minimal amount of a lower-melting-point metal was used to form a liquid phase when heated to a moderate temperature. The lower-temperature melting elements were considered so that the liquid phase could dissolve some tungsten in the solution, thereby inducing wetting of the tungsten grains. The THA is usually processed using powder technology by employing the liquid phase sintering (LPS) process [26; 27]. This process is suitable for alloys that melt over a range of temperatures. It can be conducted under conditions, in which solid grains coexist with a wetting liquid. As the final product is a multi-phase with customized properties, LPS is considered one of the most dominant commercial sintering processes. THAs are widely used in manufacturing radiation shielding and some parts for the military defense, such as missile weapons and armor piercing ammunition, in tubular or hollow shapes. In general, tubular parts are fabricated by conventional metal forming processes. Rotary swaging [28; 29] and roll forming [30] methods are usually applied to manufacture tubular parts. For more complicated tubular products, hydroforming [31; 32; 33] and electromagnetic forming [34] are used. In general, the excessively high consolidation temperature of tungsten does not allow the application of solid-state metal forming method to fabricate tubular parts. Thus, one possible alternative to handle this situation is to combine the segmented THA pieces by solid-state diffusion bonding to build up a tubular component. As the sintering process is required for fabricating the THA tubular part, controlling the sintering temperature is very important. Accurate measurement and control of temperature during high-temperature processes are important because it affects the product quality and operational performance [35; 36; 37; 38; 39; 40; 41; 42]. An extremely dynamic and rapid cooling process does not permit a stable and consistent measurement of cooling rates.