Nov_Dec_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 6 5 trogen concentration, and nitriding potential (or ammonia dissociation rate). With the inputted process parameters, NitrideTool will determine if the compound layer forms, or not, based on the maximum nitrogen solubility in the fer- rite for the selected steel, which is determined by the alloy specific Lehrer diagram [4] . If the compound layer cannot form, NitrideTool will output the nitrogen concentration profile calculated from the mass transfer and diffusion model. If the compound layer forms, NitrideTool will cal- culate the nitrogen concentration profile using the com- pound layer growth model. NITRIDING POTENTIAL VS. AMMONIA DISSOCIATION RATE Nitriding potential is an important input parameter in NitrideTool. It is defined as: in which ρ NH 3 and ρ H 2 are the partial pressure of ammonia and hydrogen in nitriding atmosphere. In NitrideTool, ni- triding potential is used for the nitriding process simula- tion. However, the dissociation rate of am- monia is still being used for gas nitriding process control in some furnaces. There- fore, in NitrideTool, ammonia dissociation rate can be taken as input parameter and converted into nitriding potential for fur- ther simulation. The ammonia dissociation rate is measured by an ammonia dissociation burette shown in Fig. 3. Because ammonia can dissolve in water while hydrogen and nitrogen cannot, the burette can measure ammonia gas volume percentage in ex- haust gas. The dissociation rate is defined as the volume percentage of the dissociat- ed ammonia in exhaust gas, i.e., the vol- ume percentage of hydrogen and nitrogen gasmixture in exhaust gas. As it is shown in Fig. 3, the dissociation rate can be read from the burette as d / (r + d). CONCLUSION NitrideTool, based on the science of thermodynamics and kinetics, will help users in the heat treat industry simulate the gas nitriding process to meet pre-de- fined specifications of the nitride part. And all of this can now be done without the costly and time-intensive trial and error method used in the past. This software has the unique ability to allow users to simulate the gas nitrid- ing of steels so that they can better un- derstand nitriding potential, temperature, time, and surface condition on the gas ni- triding process. ~HTPro For more information: Mei Yang, Cen- ter for Heat Treating Excellence (CHTE), Fig. 2 — Flow chart of the algorithm of NitrideTool. Fig. 3 — Schematic of ammonia dissociation burette [5] . 11