Carbonitruration basse pression d'aciers et de pièces obtenues par la technologie MIM

Abstract : The carbonitriding treatment is now a widely accepted industrial process to improve the strength of treated steel parts. This type of solution has been proven in the case of massive parts, but internal oxidation of enriched layers remains a significant drawback of the treatment at atmospheric pressure.It is against this scientific backdrop that this project seeks to develop a carbonitriding treatment at low pressure for industrial application. Numerous experimental investigations carried out on the 16MnCr5 steel grade allowed the development of cycles answering two criteria of depth treatment: 0.25 - 0.4 mm 0.7 - 1.2 mm. Metallurgical observations show no form of precipitation, which however is no longer the case when the treatment is applied to a more highly alloyed steel grade (Fe - 0.18% C - 3.12% Cr - 0.7% V -0.45% W).From a simulation point of view, thermodynamic calculations confirm and clarify the precipitation phenomena observed. The more highly alloyed steel (which initially contains vanadium rich carbides of MC type) exhibits carbides of M23C6 and M7C3 type, and carbonitrides of M (C, N) type. To complement the determination of present phases and their composition, it is proposed that the diffusion of carbon and nitrogen be modeled. The model uses experimentally determined boundary conditions, and interdependent nitrogen and carbon taken from the literature. The kinetics of cooling during the quenching is determined to supply the calculation model of phase transformations and simulate the hardness profile. The coupling of developed models gives carbon, nitrogen and hardness profiles very similar to experimentally measured ones.The work also proposes the integration of the low-pressure carbonitriding treatment to the MIM (Metal Injection Moulding) process, allowing the production of complex shapes components. MIM parts obtained from the exploitation of the trade PolyMIM 16MnCr5 feedstock integrating a water-soluble binder system present 10% of porosity. Metallurgical results observed on MIM parts, carbonitrided consecutively to the sintering step are similar to those observed on massive wrought parts. Comparison of carbon and nitrogen profiles of carbonitrided MIM parts containing 10 and 20 % of porosity (obtained by reducing the length of the bearing sintering) shows that the porosity level influences the case depth of MIM parts.
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Tarek Marray. Carbonitruration basse pression d'aciers et de pièces obtenues par la technologie MIM. Autre. Ecole nationale supérieure d'arts et métiers - ENSAM, 2012. Français. ⟨NNT : 2012ENAM0056⟩. ⟨pastel-00840622⟩

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