Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries
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INTRODUCTION
NITRIDING is a thermochemical surface treatment applied to improve the mechanical and chemical properties of ferritic steel components.[1–3] The nitrided zone can be subdivided into an outer iron-nitride compound layer, consisting of e-Fe3N1+x and/or c¢-Fe4N1y, on top of a diffusion zone of nitrogen in the ferrite matrix. The finite rate of nitrogen supply to the specimen surface (owing to the finite rate of dissociation of ammonia at the surface of the specimen, as holds for the usually applied gaseous nitriding processes) is in competition with its inward diffusion into the substrate. Consequently, the concentration of interstitially dissolved nitrogen at the surface, and also in the subsurface regions, of a nitrided pure iron specimen increases gradually as a function of nitriding time (at constant temperature).[4] If the ferrite is alloyed with nitride forming alloying elements, such as Al, V, and Cr, the inwardly diffusing nitrogen atoms become consumed by alloying-element nitride development, which then governs the shape of the developing nitrogen content depth profile. The precipitated (semi-) coherent nitrides cause B. SCHWARZ and H. GO¨HRING, Ph.D. Students, and S.R. MEKA, Research Scientist, are with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), Heisenbergstrasse 3, 70569 Stuttgart, Germany. Contact e-mails: s.meka@ is.mpg.de, [email protected] R.E. SCHACHERL, Research Scientist, and E.J. MITTEMEIJER, Professor, Head of Department Phase Transformations, are with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), and also with the Institute for Materials Science, University of Stuttgart, Heisenbergstrasse 3, 70569 Stuttgart, Germany Manuscript submitted January 6, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
a high hardness and a distinct residual stress-depth profile of compressive nature in the surface adjacent region, which effects are responsible for the considerable improvement of especially the mechanical properties, e.g., the fatigue resistance. If the nitriding potential is high enough, then after a certain time, the nitrogen solubility limit of the ferrite matrix is surpassed leading to the nucleation and growth of an iron-nitride compound layer at the surface, which can be associated with significant enhancement of the resistance against wear.[5] Formation of pores in the iron-nitride compound layer is a well-known phenomenon.[6–10] The pores can occur within the grains and along the grain boundaries of the iron-nitride layer. The coalescence of individual pores at the grain boundaries results in opened grain boundaries/ micro-cracks. Pores, in the form of ‘‘open’’ grain boundaries in contact with the outer surface, can be beneficial due to their function as reservoirs for lubricants to provide better tribological performance,[11] but in general pores are detrimental due to the associated mechanical weakening of the material. Different contradictory mechanisms have been presented i
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