ISIJ International, AdvanceISIJ PublicationInternational, by AdvanceJ-STAGE, Publication DOI: 10.2355/isijinternational.ISIJINT-2020-050 by J-STAGE ISIJ International, J-Stage AdvancedISIJ International,Publication,ISIJ International, DOI: Advance http://dx.doi.org/10.2355/isijinternational.ISIJINT-2015-@@@ Vol. Publication 60 (2020), No.ISIJby J-Stage 9International, Vol. 60 (2020), No. 9, pp. 1–8 Development of a Prediction Method for Carbonitrided Surface Carbon and Nitrogen Contents by Computational Thermodynamics and Validation by Carbonitriding for Pure Iron Kenta TSUJII,1)* Marian Georg SKALECKI,2) Matthias STEINBACHER2) and Hans-Werner ZOCH2) 1) Corporate Research & Development Center, Daido Steel Co., Ltd, 30, 2-Chome, Minami-ku, Nagoya, Aichi, 457-8545 Japan. 2) Leibniz-Institut für Werkstofforientierte Technologien – IWT, Badgasteiner Straße 3, Bremen, 28359 Germany. (Received on January 31, 2020; accepted on March 23, 2020; J-STAGE Advance published date: May 25, 2020) In this study, a new method for predicting carbon and nitrogen contents of a carbonitrided surface using computational thermodynamics with Thermo-Calc was developed. The nitrogen content of alloyed steel, which is in equilibrium between the steel surface and the atmosphere, was predicted using the nitriding potentials and Thermo-Calc, and the experimental and calculated results were compared using pure iron. For lower nitrogen levels, the accuracy of prediction was sufficient. However, for higher nitrogen levels, the experimental nitrogen content was lower than the calculated value, which was attributed to pore formation. Through a comparison of the described method with the conventional one, it was confirmed that our novel prediction method exhibits sufficient accuracy to predict the nitrogen content following carbonitriding. KEY WORDS: carbonitriding; carbon; nitrogen; simulation; CALPHAD; pores. als, e.g. in contaminated lubricants.9) In contrast, an excess 1. Introduction of retained austenite results in decreased hardness and Carbonitriding is a thermochemical surface hardening strength.10) Therefore, in carbonitriding, control of the carbon treatment, which is similar to carburizing. In the gas car- and nitrogen contents is of particular importance, and thus, to burizing process, carbon is transferred from the atmosphere increase the application of this method, prediction techniques onto steel surfaces, and this reaction occurs in equilibrium for the resulting carbon and nitrogen contents are required. between the atmosphere and the steel surface. More specifi- Previously, hardware was developed at the IWT (Leibniz- cally, it relies on the so-called Boudouard reactions.1) Dur- Institut für Werkstofforientierte Technologien) for measure- ing this process, control of the carbon content is relatively ment of the nitriding potential for carbonitriding.4,5) More facile. In addition, through the carburization and hardening specifically, measurement of the nitrogen potential of the of steel components, certain properties, such as the wear atmosphere was performed by means of an ammonia sensor resistance, strength, and fatigue, can be altered, and thus, in the exhaust gas, and the carbon potential of the atmo- this technique is widely employed in industry. sphere was controlled using a conventional oxygen probe. In contrast, during the gas carbonitriding process, both The use of such devices therefore allowed simultaneous carbon and nitrogen are absorbed into steel surfaces simul- control of the nitrogen and carbon potentials of the atmo- taneously, and compared to carburizing, this process can sig- sphere during carbonitriding.4,5) nificantly increase wear resistance.2,3) As a result, although In addition, numerous prediction methods have been considerable research has been carried out in this field,1–8) reported for the carbon and nitrogen contents of alloyed the target carbon and nitrogen contents have been difficult steels through use of the carbon and nitrogen potentials of to achieve using controlled processes due to the interactions the atmosphere.11,12) In these methods, the effects of alloy- of carbon and nitrogen during the process, in addition to ing elements on the equilibrium contents were also taken the lack of a sufficient control system for automatic control into account, and these methods were proven to exhibit suf- of the nitrogen potential in the atmosphere. Carbonitrided ficient accuracy to predict the carbon and nitrogen contents components contain high amounts of both carbon and nitro- in the carbonitriding of alloyed steels under specific condi- gen, which leads to higher volume fractions of the retained tions.11,12) However, the influence of the alloying elements austenite.2) In this context, 20–35% of retained austenite has can vary with the temperature and the constituent phases; been proven to increase the wear properties of the materi- therefore, a more effective method is required. In contrast, computational thermodynamics has recently * Corresponding author: E-mail: [email protected] gained increasing attention in the context of nitriding, which DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2020-050 is also a commonly employed surface hardening treatment. 1 © 2020 ISIJ ISIJ International, Advance Publication by J-STAGE ISIJ International,ISIJ International, Advance Vol. Publication 60 (2020), No.by J-Stage9 More specifically, a prediction method for the constituent where the unit is Pa −1/2 or atm −1/2. phase was developed by Yang and Hiraoka’s groups using Thermodynamic calculations for predicting the nitro- computational dynamics.13,14) In their study, the Lehrer dia- gen content were carried out according to the CALPHAD grams of pure iron and alloyed steels were calculated using methodology,16) which has been under development since the CALPHAD methodology, and the calculation results the early 1970s. In this study, Thermo-Calc, which is a were proven to agree with the experimental results.13,14) This commercially available software, was used in combination method therefore also exhibited potential to be an effective with the TCFE7 database to calculate the thermodynamic tool for the prediction of carbon and nitrogen contents dur- properties.17) ing carbonitriding. Through the combined use of Thermo-Calc and TCFE7, However, to date, the use of CALPHAD to predict the the thermodynamic parameters of the solid solutions (e.g., carbon and nitrogen contents during carbonitriding has yet the Gibbs energy) were calculated using the sub-lattice to be reported. Thus, we herein report the application of model between the FCC phase and the M (C,N) carbonitride computational thermodynamics in the carbonitriding pro- phases, based on the amounts of carbon and nitrogen.17) cess for prediction of the nitrogen content. The prediction During the carbonitriding process, samples are heated to accuracy is also considered and compared with experimental 1 123 K, resulting in the absorption of carbon and nitrogen, observations. Moreover, the validity of this method is exam- and adoption of the FCC phase. The relationship between ined by comparison with the conventional method. the nitrogen content and the thermodynamic properties (e.g., the chemical potential and activity) can be calculated using these methods. Furthermore, we note that the nitrid- 2. Prediction Method for the Nitrogen Content Based ing potential, K , during the carbonitriding process can be on Computational Thermodynamics N controlled using hardware such as sensors;4,5,12) hence, the During carbonitriding, both the carburizing reaction and nitrogen content can be accurately predicted. the nitriding reaction occur simultaneously. In terms of the nitriding reaction in the nitriding atmosphere, the chemi- 3. Experimental Procedure cal potential of nitrogen, μN, thermodynamically defines the nitridability.15) At the thermodynamic equilibrium, the 3.1. Sample Material chemical potential in the steel surface, μN,s, equals that in Table 1 presents the chemical composition of the foil 1 employed for this study. The foil, which is essentially pure the nitriding atmosphere, µNg2 , , as described by Eq. (1): 2 iron, was used to prepare test samples of 50 μm thickness 1 to ensure that the samples reached their equilibrium carbon Ng,, Ns ............................... (1) 2 2 and nitrogen contents within a short period of treatment in the carbonitriding atmosphere. This set-up allowed the The chemical potential of nitrogen in the steel can there- equilibrium carbon and nitrogen contents to be measured. fore be related to the nitrogen activity, aN, by: C S 3.2. Heat Treatment Conditions 1 0 1 pN2 0 RT lnD T Ns, RT ln aN ......... (2) Figure 1 presents the carbonitriding conditions employed 2 Ng2 , 2 D p0 T E N2 U herein, and further details are given in Table 2. For the purpose of this study, the gaseous carburizing atmosphere where R is the gas constant; T is the temperature; pN2 is the 0 furnace employed herein had dimensions of 300 mm × 300 partial pressure of nitrogen; and p is the partial pressure N2 mm × 300 mm. The inlet nozzle was located on the upper of nitrogen in the standard state. wall, and the analyzed gas was pumped upwards from Since the chemical potential of nitrogen is extremely low in N2 and relatively high in ammonia, ammonia is used as the principal constituent of the nitriding atmosphere. The nitriding reaction by ammonia can be represented by: Table 1. Chemical compositions of the foil samples employed herein. 3 NH32[]NH...........................