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Unalloyed cast iron materials exhibit low tribological and corrosive resistance. In this respect, nitriding has a wide range of applications for steels. In the case of cast iron, the advantageous properties of nitrided layers are impaired by the presence of graphite. Electron beam remelting of cast iron surfaces prior to nitriding removes graphite. The homogeneous ledeburitic microstructure within the approx. 1 mm-thick remelted layer enables the formation of a dense compound layer during subsequent nitriding. The main objective of this study is to investigate the nitriding mechanism of unalloyed ledeburitic microstructures. Due to the complex relationships, investigations were carried out on both conventional ferritic and pearlitic cast irons and Fe-based model alloys containing one to four additional alloying elements, i.e., C, Si, Mn and Cu. The iron (carbo-)nitride composition (γ', ε) of this compound layer depends on the gas nitriding conditions, the chemical composition of the substrates and the microstructural constituents. As a result, a schematic model of the nitriding mechanism is developed that includes the effects of the nitriding parameters and alloy composition on the phase composition of the nitriding layer. These findings enable targeted parameter selection and a further optimization of both the process and the properties. [ABSTRACT FROM AUTHOR]

The microstructure formation during nitriding of white‐solidified Fe–C–Si alloys, relevant to advanced surface treatments for cast irons, is studied. The as‐cast microstructures mainly contain Si‐free, C‐rich eutectic cementite and Si‐enriched, C‐lean ferrite/pearlite. The spatial extent of these microstructure regions is significantly larger than the diffusion distance of Si during nitriding. The phases and microstructure forming during nitriding vary with the local Si content in the substrate. Nitriding conditions only allowing to form γ′–Fe4N from pure α‐Fe generate a compound layer composed of γ′–Fe4(C, N), ε–Fe3(C, N)1+x, and Si‐rich nitride, X. ε is the main constituent, whereas γ′ only forms from Si‐free eutectic cementite. Different nitriding experiments elucidate the interrelated thermodynamic and kinetic effects of C and Si on the phase evolution and the compound layer growth. A schematic model of the microstructure evolution is developed. Information on the topology of the largely unknown Fe–C–Si–N phase diagram is derived from the experimental observations. For this, the diffusion path concept is adapted to describe the microstructure formation during nitriding of compositionally inhomogeneous alloys, in which substitutional alloying elements are heterogeneously distributed and largely immobile. α + ε and α + ε + X phase fields are identified as particular features of the Fe–C–Si–N phase diagram at 540 °C. [ABSTRACT FROM AUTHOR]

Crystallographic orientation relationships (ORs) among α-Fe, cementite θ-Fe 3 C 1-z , and hexagonal close-packed iron carbide/(carbo)nitride ε-Fe 3 (N,C) 1+x are highly important to phase transformations and microstructure evolution during heat-treating of cast irons and steels, and during nitriding of these materials. Several α/θ, ε/θ, and α/ε two-phase ORs have been controversially discussed in literature. In that regard, an α/ε/θ three-phase crystallographic relation has been hypothesized, yet hitherto hardly verified, which justifies the existence of two distinct α/θ ORs by making use of an ε intermediate in the θ formation. In this study, ORs in a white-solidified and gas-nitrided hypoeutectic Fe-3.5 wt%C-1.5 wt%Si model alloy were investigated by electron backscatter diffraction (EBSD). The specific alloy composition and nitriding conditions allow simultaneously examining α/θ, ε/θ and α/ε ORs, and particularly the α/ε/θ three-phase relationship, which can be hardly assessed simultaneously in conventionally treated cast irons and steels. Eutectic cementite and pearlite constitute the white-solidified microstructure. Eutectic cementite and pearlitic α-Fe frequently fulfill the Isaichev OR and certain ORs, which are close to the Pitsch-Petch OR. Upon nitriding, ε forms from surface-adjacent cementite, whereas α has largely been retained due to the presence of Si. About 70% of the ε grains fulfill the OR 010 θ ∥ 11 2 ¯ 2 ¯ ε / 100 θ ∥ 11 2 ¯ 3 ε. The orientation of α adjacent to the newly formed ε determines which variant of that ε/θ OR is selected. Thereby, irrespective of the two main α/θ ORs, basically the same α/ε OR is established. The α-affected variant selection proves the existence of an α/ε/θ three-phase crystallographic relationship given by 03 1 ¯ θ ∥ 10 1 ¯ 1 ¯ ε ∥ 10 1 ¯ α / 100 θ ∥ 11 2 ¯ 3 ε ∥ 131 α or 1 ¯ 1 1 ¯ α. Image 1 [ABSTRACT FROM AUTHOR]

Hypoeutectic Fe-C and Fe-C-Si model alloys were produced at different solidification conditions. Copper mold casting yields low cooling rates promoting the formation of a eutectic microstructure, which is characterized by two morphologies: elongated cementite plates and a rod structure growing perpendicular to the plates, i.e. austenite rods in a cementite matrix. Electron beam surface remelting generates a mainly plate-like eutectic due to rapid solidification. The microstructures were characterized by light-optical microscopy and electron backscatter diffraction (EBSD). The latter allows for a spatially resolved investigation of the growth crystallography of the eutectic phases. Thereby, a possible existence of crystallographic orientations relationships between cementite and austenite within the plate-like eutectic was assessed experimentally. The eutectic phases were found to grow largely crystallographically independently. Moreover, ferrite and eutectic cementite within the decomposed eutectic microstructure frequently comply with the Bagaryatsky or the Pitsch-Petch orientation relationship. Complementary X-ray diffraction (XRD) analysis reveals a pronounced cementite {002} texture in the microstructure produced by mold casting. Characteristic changes in the lattice parameters indicate that as-cast cementite is non-stoichiometric. [ABSTRACT FROM AUTHOR]

Cementite (Fe3C) powder consisting of polycrystalline particles was investigated by synchrotron X-ray diffraction and neutron diffraction at temperatures between 10 and 973 K. The data reveal a pronouncedly anisotropic thermal expansion of the orthorhombic unit cell as well as microstrain broadening varying considerably with temperature. Using a theory for predicting thermal-microstress-induced microstrain already applied in previous work to ambient-temperature sealed-tube X-ray powder diffraction data from the same material [Leineweber (2012). J. Appl. Cryst. 45, 944-949], the temperature-dependent extent of the measured microstrain broadening could be quantitatively related to the measured temperature-dependent anisotropy of the thermal expansion. Thereby, the fact that the extent of the measured microstrain broadening is somewhat smaller than the predicted amount can be explained by the presence of the free surfaces of the powder particles reducing the level of microstress-induced microstrain. [ABSTRACT FROM AUTHOR]

Massive cementite layers with a time-dependent thickness were grown on ferrite substrates by nitrocarburising in a dedicated NH3/H2/CO/N2-containing gas atmosphere at 783 K, 823 K and 843 K. Nitrogen diffusion through the cementite layer into the ferrite substrate took place in conjunction with growth of the cementite layer; a significant, i.e. measurable, solubility of nitrogen in cementite was not observed. The nitrogen concentration-depth profiles in the substrate, underneath the growing cementite layer, were quantitatively determined using a calibrated microhardness measurement technique. The nitrogen concentration-depth profiles were simulated on the basis of a model using an implicit finite-difference method. The simulation yielded values for the diffusivity of nitrogen through cementite, including its activation energy. [ABSTRACT FROM AUTHOR]

Recently reported [Chihi T, Bouhemadou A, Bin-Omran S. High Press Res. 2013;33:572] results of density-functional theory calculations for a “martensitic” FeC with allegedly tetragonal body-centered ground state structure are shown to be compatible with a cubic rocksalt structure of that FeC. [ABSTRACT FROM AUTHOR]