Simulation, optimization and development of thermo-chemical diffusion processes

Wei, Yingying

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Simulation, optimization and development of thermo-chemical diffusion processes

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Thermo-chemical diffusion processes play an important part in modern manufacturing technologies. They exist in many varieties depending on the type of diffusing elements used and the respective process objectives and procedures. To improve wear and/or corrosion performance of precisely machined steel components, gas nitriding is selected as the most preferred thermo-chemical surface treatment. Conventional gas nitriding of steels is a multi-hour, sometimes multi-day hardening process carried out at ferritic temperatures and including a complete heat treatment cycle: normalizing, austenitizing, martensitic quenching and tempering. An alternative, subcritical-temperature austenitic nitriding process is evaluated with the purpose of accelerating the treatment and optimizing the hardness and toughness of nitrided layers while minimizing the distortion of steel parts treated. The alternative process involves liquefied nitrogen cryogenic quenching as well as aging. This study presents results of experimental work on AISI 4140 steel, examining the interplay between the nitriding and tempering conditions and phase transformations in both ferritic (525oC) and subcritical, nitrogen-austenitic (610oC) processes. Thermodynamic models, used to design processing conditions, are applied also in the microstructural interpretation of nitrided layers. Results are verified using the SEM, EPMA and EDS techniques. Kinetics of interstitial diffusion, isothermal martensite transformation, as well as dimensional control of nitrided parts is also presented. Carburizing is, by far, the most widely adopted method in surface hardening. Problems with intergranular oxidation (IGO), energy efficiency and carbon footprint of conventional endothermic atmosphere (CO-H2-N2) carburizing is forcing heat treating and manufacturing companies to move toward increasingly capital- and operating-cost expensive, low-pressure (vacuum furnace) carburizing methods. In response, a new activated and alternate carburizing method (A2A carburizing) has recently been developed, bridging the endothermic atmosphere and vacuum processes, where a plasma-activated, oxygen-free, non-equilibrium nitrogen-hydrocarbon gas blend is utilized. The optimization of industrial A2A carburizing processes involves improvement of case uniformity of parts at different locations in the charge as well as between different sides on the parts. Connected to the optimization, a computational fluid dynamics (CFD) study is conducted for examination of gas flow field inside the furnace and trays holding steel parts treated. To mitigate soot in the atmosphere and minimize the poorly carburized contact area between parts, effects of different combinations of nitrogen-hydrocarbons mixture on soot formation in atmosphere, deposition on metal surface and graphite growth at carburizing temperature are investigated. N2- 0.4%C3H8-1%CH4 mixture is proven to be able to provide proper carburizing hardened case with less soot in atmosphere, less coke deposition on metal surface, as well as minimized marginally carburized contact zone. A soot formation mechanism for non-equilibrium atmosphere in A2A carburizing is discussed. The carburizing processes have been investigated for decades, yet it still faces challenges concerning performance, reliability and process control. Since carburized parts must meet tolerances and specifications of particular applications, it is necessary to accurately predict carbon concentration profiles as a function of processing conditions. Proper carbon distribution is critical for satisfactory and reliable service life of carburized parts. Based on experimental work and theoretical developments, a software CarbTool© has been created for atmosphere and low pressure carburizing methods which consider the thermodynamics, mass transfer kinetics and carbon diffusion aspects of the carburizing process and the gas-steel interface condition. The models are capable now to accurately predict the surface carbon concentration and the carbon concentration profile in the steel, i.e. the most important outcomes of the process.

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