Nanofluids and heat treatment of materials
Many industrial processes involve the heat treatment of materials, such as metals, plastics and composites. Commonly used heat transfer fluids such as water, ethylene glycol, and oils have relatively low thermal conductivity compared to solids.
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By using nanofluids, which are colloidal suspensions of solid nanoparticles in a base fluid, researchers and engineers can increase the thermal conductivity of the heat transfer fluid, thereby saving energy and reducing the processing time required.
Heat treatment is a process in which materials are heated or cooled in a specific sequence to aid material processing, prevent materials from suffering irreversible thermal damage, or improve their mechanical properties.
Advantages of Heat Treatment of Materials
The heat treatment of metals and metal alloys is a particularly important process used in steelmaking. By choosing the appropriate heat treatment method, manufacturers can improve the hardness, strength and wear resistance of the metal or improve its ductility. However, not all metal properties can be changed in one operation. For example, quench hardening of a metal, a process commonly used in the steel industry to increase the reliability of the end product, can cause brittleness.
Here, once the material reaches a specific temperature after heating, it is held and then quenched in a cooling medium. The quenching process is performed to trigger physical processes such as heat transfer, which triggers other processes, phase transformation, and strain or strain evolution.
The result of the quenching process mainly depends on the heat transfer characteristics of the quenching medium. In the context of industrial heat treatment of metal, commonly used quenching fluids, such as water, brine solution and oils, the possibilities for improving heat transfer are very limited. Thus, materials scientists from industry and academia are focusing on designing quench media with improved heat transfer characteristics.
Developments in nanotechnology have facilitated the emergence of nanofluids
Modern nanotechnology allows the large-scale manufacture of nanomaterials with an average particle size of less than 50 nanometers. The unique properties of these nanomaterials include size-dependent physical properties, extremely large specific surface area, and tunable surface properties. Colloidal suspensions of nanoparticles in carrier fluids are called nanofluids. Nanomaterials commonly used in nanofluids are oxide ceramics (Al2O3, CuO), carbides (SiC), nitrides (AlN, SiN), metals (Al, Cu) and non-metals (graphite, carbon nanotubes and others). Base fluids include water, ethylene or triethylene glycol, oils, and polymer solutions.
Thermal and physical properties of nanofluids
Nanofluids are mainly produced using a two-step method, where the nanoparticles are first synthesized and then dispersed in the base fluid. Extensive nanofluid research over the past two decades has demonstrated that adding nanoparticles to conventional fluids causes dramatic changes in the thermophysical properties of the fluid. The high specific surface of the nanoparticles improves the heat transfer between the nanoparticles and the fluid.
The boiling behavior of the fluid at the surfaces is also affected, as the nanoparticles fill in the discontinuities at the solid-fluid interface, thus affecting the critical heat flux. The stability of the nanofluid dispersion, mainly governed by the Brownian motion of the nanoparticles, ensures a stable and isotropic heat exchange. Experimental evidence shows that nanofluids allow a reduction in pumping power, compared to pure liquid, to achieve an equivalent heat transfer rate. Additionally, nanofluids offer tunable properties, such as wetting kinetics and heat removal characteristics, by varying the concentration and composition of the particles to suit different applications.
It is important to note that a large improvement in thermal conductivity can be obtained at very low concentrations of nanoparticles, which completely preserves the Newtonian behavior of the fluid without noticeable effects on the viscosity of the fluid and the pressure drop required during the fluid pumping. All these unique characteristics of nanofluids can be exploited in the industrial heat treatment of materials.
Biobased nanofluids for steel quenching
Researchers from SSN College of Engineering in Chennai, India have demonstrated the benefits of using a low-cost, sustainable biochar-based nanofluid (biochar is carbon residue left over after the pyrolysis of biomass) as quenching medium in low carbon processing.
Adding only 0.25 wt% thermally annealed biochar to distilled water yielded a nanofluid that achieved a much higher cooling rate and resulted in smaller surface roughness, higher hardness and more refined microstructure of treated low carbon steel compared to conventionally hardened specimens.
Surfactants improve the performance of organic nanofluids
Similarly, a research group from the Department of Metallurgical and Materials Engineering of Universitas Indonesia studied how the use of a nanofluid prepared by adding carbon nanoparticles to oil as a quenching medium affects the properties of JIS S45C medium carbon steel.
The microstructure and hardness of the steel sample showed a significant dependence on the volume fraction of nanoparticles in the nanofluid. Compared to oil quenching, quenching in an oil-based nanofluid with a nanoparticle content of 0.2% by volume resulted in higher sample hardness. Higher concentrations of nanoparticles resulted in lower steel hardness, possibly due to particle agglomeration.
Adding anionic surfactants, such as sodium dodecylbenzene sulfonate, to the nanofluid allowed the researchers to further improve the quenching process and increase the hardness of the steel.
Ceramic nanoparticles help reduce steel shape distortion
Improving the hardness and microstructure of low carbon steel when using ceramic-based nanofluid (Al2O3 nanoparticles suspended in a water-ammonia solution) as an extinguishing medium has also been reported by scientists from the Poznań University of Technology in Poland. Their results established that quenching the nanofluid resulted in less shrinkage of the steel specimen while improving its hardness and reducing processing time.
The addition of nanoparticles to conventional quenching fluids improves their heat transfer and wetting properties, thereby minimizing shape distortion of treated components and improving the reproducibility of quenching treatment. Additionally, the use of bioderived nanomaterials to prepare nanoquenchants can improve the cost effectiveness and sustainability of industrial heat treatment processes.
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References and further reading
Hassoni, SM, et al. (2020) The role of ZnO nanofluids on heat treatments of medium carbon steel. IOP 2020 Lecture Series: Materials Science and Engineering. 881, p. 012095. doi.org/10.1088/1757-899X/881/1/012095
Arularasan, R & Babu, K (2021) Thermally annealed biochar-assisted nanofluid as quench on mechanical and microstructural properties of AISI-1020 heat-treated steel – a cleaner production approach. Biomass conversion and biorefinery. doi.org/10.1007/s13399-021-01737-x
Riza, RI, et al. (2020) Observation of the cooling rate in the quenching process using carbon nanofluids for S45C carbon steel. Key Engineering Materials, 833, p.13–17. Trans Tech Publications, Ltd. doi.org/10.4028/www.scientific.net/kem.833.13
Oktavio, L. et al. (2019). Effect of sodium dodecylbenzene sulfonate as an anionic surfactant on the performance of water-based carbon nanofluids as a quenching medium in heat treatment. IOP Lecture Series. Materials Science and Engineering, 622(1). doi.org/10.1088/1757-899X/622/1/012009
Mahiswara, EP, et al. (2018) Characterization of an oil-based nanofluid for quenching medium. IOP Lecture Series: Materials Science and Engineering, 299, p. 012068. doi.org/10.1088/1757-899X/299/1/012068
Łyduch, K., Gęstwa, W. (2018) The influence of chemical composition on the change in dimensions of quenched steels in nanofluids. Archives of Mechanical and Materials Technology, 38p. 29-34. oi.org/10.2478/amtm-2018-0005