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Hexagonal e-iron nitride and e-iron carbonitride phases are formed on nitriding and nitrocarburizing of iron and steel surfaces and can exist in broad compositional ranges. Long-range nitrogen ordering and magnetic properties for e-iron nitrides and their dependence on composition have been the focus of several studies. So far, limited attention has been paid to the carbonitrides. In the current work, the effects of substitution of nitrogen by carbon on the interstitial ordering and magnetic properties in Fe2(C,N)1-z are explored using neutron diffraction, Mossbauer spectroscopy and vibrating sample magnetometry. Neutron diffraction patterns showed 001 and 301 superstructure reflections, confirming a previously proposed structural model in space group P 3 ¯ 1 m (compared to P6322 for the pure nitrides). On partial substitution of nitrogen by carbon in e-iron nitride the Curie temperature, the saturation magnetization and the hyperfine fields of the iron atoms are increased, while isomer shifts are decreased. The effects on the a and c lattice parameters indicate a change in interstitial ordering, which is related to more favorable interactions between a nitrogen and carbon atom than among nitrogen atoms. This interaction leads to additional interstitial (short-range) ordering and a decrease in the c lattice parameter, while the a lattice parameter is largely unaffected.

The carbon solubility in expanded austenite is investigated by controlled low temperaturegaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hägg carbide (x-M5C2)develops when the carbon content in the expanded austenite exceeds the metastable solubility limit; the transformation of carbon expanded austenite into Hägg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in expanded austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the austenite lattice (i.e., 4.74wt%C). Decomposition of Hägg carbide into M7C3 occurs upon prolonged carburizing treatment orthermal exposure in inert atmosphere (in situ synchrotron experiments).

The crystal structure and magnetic properties of austenitic stainless steel with a colossal interstitial content, so-called expanded austenite, are currently not completely understood. In the present work, the magnetic properties of homogeneous samples of expanded austenite, as prepared by lowerature nitriding of thin foils, were investigated with magnetometry and Mössbauer spectroscopy. At room temperature, expanded austenite is paramagnetic for relatively low and for relatively high nitrogen contents (yN = 0.13 and 0.55, respectively, where yN is the interstitial nitrogen occupancy), while ferromagnetism is observed for intermediate nitrogen loads. Spontaneous volume magnetostriction was observed in the ferromagnetic state and the Curie temperature was found to depend strongly on the nitrogen content. For the first time, X-ray diffraction evidence for the occurrence of long-range interstitial order of nitrogen atoms in expanded austenite was observed for high nitrogen contents.

No consistent structural description exists for expanded austenite that accurately accounts for the hkl -dependent peak shifts and broadening observed in diffraction experiments. The best available description for homogeneous samples is a face-centered cubic lattice with stacking faults. Here Debye simulations of stacking fault effects were compared to experimental data for macro-stress free homogeneous expanded austenite to show that a faulted structure cannot explain the observed peak displacement anomalies. Instead it is argued that the shifts are the combined result of elastic and plastic anisotropy leading to (strongly) non-linear hkl -dependent elastic behavior during composition-induced plastic deformation on synthesis of expanded austenite.

Porous iron foils were used for synthesizing homogeneous samples of iron carbides and (carbo)nitrides. Homogeneous distributions of interstitial nitrogen and carbon were obtained without long treatment times due to limited required diffusion distances in the porous material. By adjustments of the nitriding and carburizing potentials, tailored nitrogen and carbon contents can be achieved, which allows assessment of a phase stability diagram for the Fe-N-C system, for which available experimental data is limited. Thermal decomposition sequences were established for the various iron carbides and (carbo)nitrides using in situ synchrotron X-ray diffraction. Hägg carbide (χ) and ε-carbonitride, Fe2(N,C)1-z, with high carbon content decompose to cementite (θ) above 850 K, while ferrite (α) forms above 950 K and austenite (γ) above 1025 K. For high nitrogen contents ζ- Fe2(N,C) is transformed to ε from 680 to 770 K, which decomposes to γ′-Fe4(N,C)1+x between 795 and 900 K as nitrogen is released as N2. Ferrite forms above 850 K while austenite may be briefly formed around 900 K.The two iron carbides, cementite and Hägg carbide, exhibit different coefficients of thermal expansion. Below approximately 480 K, cementite is ferromagnetic and a volumetric thermal expansion coefficient of αV = 1.5 × 10−5 K−1 is obtained. The average value in the paramagnetic state is αV = 4.3 (3) × 10−5 K−1. For Hägg carbide the average value is αV = 3.8 (5) × 10−5 K−1 and only a minor change in unit cell volume is observed at the magnetic transition temperature.