Parameter Optimization of Nitriding Process Using Chemical Kinetics
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, FIRAT AKAR, and NILS LIPPMANN
Using the dynamics of chemical kinetics, an investigation to search for an optimum condition for a gas nitriding process is performed over the solution space spanned by the initial temperature and gas composition of the furnace. For a two-component furnace atmosphere, the results are presented in temporal variations of gas concentrations and the nitrogen coverage on the surface. It seems that the exploitation of the nitriding kinetics can provide important feedback for setting the model-based control algorithms. The present work shows that when the nitrogen gas concentration is not allowed to exceed 6 pct, the Nad coverage can attain maximum values as high as 0.97. The time evolution of the Nad coverage also reveals that, as long as the temperature is above the value where nitrogen poisoning of the surface due to the low-temperature adsorption of excess nitrogen occurs, the initial ammonia content in the furnace atmosphere is much more important in the nitriding process than is the initial temperature. DOI: 10.1007/s11661-016-3705-2 Ó The Minerals, Metals & Materials Society and ASM International 2016
I.
INTRODUCTION
IN the nitriding process, the way that the surface reactions advance affects the microstructure of the parts in which the stratification of gamma prime (Fe4N) and epsilon (Fe3N) compounds[1,2] is vitally important to determine the nature of the solid solution. When the gamma prime compound dominates the outermost surface, a very brittle white layer forms, which is to be removed. Recent studies on the furnace flow[3,4] shows that the variation of the depth of the nitride layer in a given furnace operation can be controlled by various flow arrangements and load positioning. But this doesn’t eliminate the need for a precise control of the nitriding chemistry which is crucial not only for the characteristics of the surfaces achieving case standards but also for minimizing the processing costs and emissions.[5] Developing an intelligent process control with model-based algorithm[6,7] thus necessitates a priori knowledge of the temperature and composition of the gas in the furnace which must be based on a reliable estimation of the pathways of the chemical kinetics.[8,9] The nitriding chemistry can be described by local temperature and concentrations of gas phase species in the furnace atmosphere and coverages of adsorbed species on the surface of parts.[10] In the present work, the optimum composition and temperature in a nitriding _ BEDII O¨ZDEMIR, Professor, and FIRAT AKAR, Postdoctoral I. Student, are with the Fluids Group, Faculty of Mechanical Engineer_ ing, Istanbul Technical University, Gumussuyu 34437, Istanbul, Turkey. Contact email: [email protected] NILS LIPPMANN, Department Manager for Heat Treatment and Coating, is with Bosch Automotive Diesel Systems Co., Ltd., Heat Treatment and C-coating (RBCD/MOE9), 17 Xinhua Road, Wuxi 214028, Jiangsu, People’s Republic of China. Manuscript submitted March 31, 2016. Article published online September 16, 2016 METALLURGICAL
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