15 ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 Fig. 1 — (a) Schematic shows the stacking of pre-etched plates that form a homogeneous diffusion bonded block after applying high temperature and pressure. (b) Optical image shows the cross section of CHX with 1.6 channel diameter[2]. (c) Actual stainless steel 316L diffusion bonded CHXs, and (d) Schematic shows the size difference between CHX and a stack of three shell and tube exchangers with the same thermal duty, at the same pressure drop[3]. An ongoing effort to advance various energy systems (nuclear, solar, geothermal, and fossil) is underway around the world to meet the increasing demand for clean, affordable, and resilient energy, while enhancing the safety and efficiency resource use. These systems could benefit from advanced compact heat exchangers (CHXs) with unique designs and configurations that optimize heat transfer at reduced cost. Advancements in manufacturing processes and new innovative developments and designs have resulted in significant improvements in heat exchanger technology. CHXs are characterized by low space and weight requirements, high thermal effectiveness, low-pressure drop, moderate to high design pressure capability, and high effectiveness approaching 95%, as shown in Fig. 1. This combi- nation of attributes is desirable to improve cost and efficiency of advanced nuclear reactor technologies now under development. Solid state diffusion bonding has been used to create CHXs for various applications. It is a solid-state welding process in which two contacting surfaces bond under high temperature and pressure in a vacuum environment. The bonding process is performed at elevated temperature, around 80% of the melting point of the bonded materials. The high temperature allows atomic diffusion across the bonding line, making a permanent bond between similar or dissimilar materials with no residual strain or deformation in the bonded parts. During the manufacturing of diffusion bonded CHXs, small channels (on the order of a mm in size) are chemically etched or machined into thin metallic sheets and these sheets are stacked and diffusion bonded into a solid block resulting in a high surface area to volume ratio in comparison to other traditional heat exchangers technologies. In general, diffusion bonding has been a successful manufacturing process for low to intermediate temperature CHXs made of various stainless-steel alloys such as 316L and 304, as well as other copper and titanium alloys[1]. However, there is limited information about the diffusion bonding (and manufacturing) of CHXs in high temperature applications and associated selection of bonded materials, bonding conditions, mechanical perfor- mance, and thermo-fluid characteristics. This article aims to shed light on the available knowledge and the ongoing research being conducted to address gaps in information and application. AVAILABLE KNOWLEDGE Substantive knowledge exists in the literature for the successful diffusion bonding of stainless steel 316 and 316L, achieving properties equivalent to that of the wrought metal at both room and elevated temperatures[1,4]. However, those alloys are limited for high-temperature applications. Of particular interest for the nuclear industry, there are a total of six alloys qualified in Section III, Division V of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) for elevated-temperature nuclear construction: Nickel- based Alloy 617, 316H stainless steel (SS316H), 304H (SS304H), and Alloy 800H; and ferritic/martensite steels grade 22 (2.25Cr-1Mo) and 91 (9Cr-1Mo-V). There have been multiple endeavors to diffusion bond Alloy 617 and only a subset of these efforts characterized the elevated- temperature mechanical properties. Precipitate formation at the interfaces is frequently cited as impeding grain growth, therefore preventing successful diffusion bonding. In general, the elevated-temperature time dependent and cyclic properties, particularly creep- fatigue of the diffusion bonded Alloy 617, were significantly reduced compared to the as-received base metal, with failure in the diffusion-bonded specimens occurring at the weakest interface[5-8]. Recent studies focused on various methods to enhance the joints mechanical properties using post-bonding heat treatment and pre-bonding oxidation followed by removal of surface oxides[5-7]. Diffusion bonding of Alloy 800H suffered the same issues as Alloy 617, but the use of an Ni interlayer was found to enhance the bonding process and achieve reasonable strength and ductility[9-11]. On the other hand, diffusion bonding of 316H stainless steel did not lead to significant reduction in strength and/or ductility. The diffusion bonding lines were found to contain carbide particles which compromise the mechanical properties, but post-bonding heat treatment was found to completely dissolve them[4]. Although the aforementioned studies showed the limitations of the current technology and potential for improved high-temperature per- (a) (b) (c) (d)
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