Nitrogen interstitial elements

Average interstitial element concentration in Cr-Re alloys with rare earth doping was below 200ppm oxygen and 50 ppm nitrogen independent of the Re concentration. 

This group of stainless steels (Table 1-3) has a chromium content of 11 to 30% and, in general, a low carbon content (<0.06%). Interstitial elements such as carbon and nitrogen are very poorly soluble in ferrite and so to avoid embrittlement, even at room temperature, the carbon and nitrogen content must be kept very low, especially at higher chromium levels. 

The first significant alloy developed commercially to meet these requirements contained 26% chromium and 1% molybdenum. To obtain the desired corrosion resistance and acceptable fabrication characteristics, the material had to have very low interstitial element contents. To achieve these levels, the material was electron-beam rerefined imder a vacuum. It was known as E-Brite alloy. Carbon plus nitrogen contents were maintained at levels below 0.02%. 

Thermochemical diffusion heat treatments that involve the introduction of interstitial elements, such as carbon, nitrogen, or boron, into a ferrous alloy surface at elevated temperatures... 

Tantalum reacts with oxygen, nitrogen, and hydrogen at elevated temperatures. The absorption of these interstitial elements, often called a gettering reaction, produces a sharp reduction in ductility... 

Failure of the nozzle weld was the result of intergranular corrosion caused by the pick-up of interstitial elements and snbseqnent precipitation of chromium carbides and nitrides. Carbon pick-up was believed to have been cansed by inadequate joint cleaning prior to welding. The increase in the weld nitrogen level was a direct resnlt of inadequate argon gas shielding of the molten weld puddle. Two areas of inadequate shielding were identified ... 

In order to preserve the structural integrity and corrosion performance of the new generation of ferritic stainless steels, it is important to avoid the pickup of the interstitial elements carbon, nitrogen, oxygen, and hydrogen. In this particular case, the vendor used a flow rate intended for a smaller welding torch nozzle. 

The same situation is met in R-M-N ternary nitrides in which the nature of the M element determines the dominating type of bond involved in the material. This is illustrated by the fact that with lithium (or barium) as a cationic element, the R-M-N corresponding nitride is essentially ionic in character, whereas with silicon, more covalent nitrido-silicates are formed. In addition, metallic nitrided alloys exist, with nitrogen located as an interstitial element in octahedral voids of the metal atom lattice. The presence of insertion nitrogen (as well as carbon) in such compounds is sometimes necessary for their existence, and can strongly modify the physical properties. 

The borderline between cluster and valence compounds is clearly crossed with the scandium nitride chlorides which are unambiguously cluster compounds. Scandium shows here its difference in behavior, with compounds such as SC4CI6N, SC7CI12N (Hwu and Corbett 1986) or ScsClgN (Hwu et al. 1987), where nitrogen is an interstitial element. 

Nitrogen forms binary compounds with almost all elements of the periodic table and for many elements several stoichiometries are observed, e.g. MnN, Mn Ns, Mn3N2, MniN, Mn4N and Mn tN (9.2 < jc < 25.3). Nitrides are frequently classified into 4 groups salt-like , covalent, diamond-like and metallic (or interstitial ). The remarks on p. 64 concerning the limitations of such classifications are relevant here. The two main methods of preparation are by direct reaction of the metal with Ni or NH3 (often at high temperatures) and the thermal decomposition of metal amides, e.g. ... 

The interstitial air trapped during this process preserves a largely unaltered record of the composition of past atmospheres on time scales as short as decades and as long as several hundred thousand years. Such records have provided critical information about past variations in carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), and the isotopic composition of some of these trace species. In addition, studies of the major elements of air nitrogen, oxygen, and argon, and their isotopic composition, have contributed... 

Nitrides. Among the elements of the 15th group, the particular behaviour of nitrogen is notable. Several are the analogies with carbon in the formation, for instance, of interstitial compounds. A number of these phases, such as the refractory solid solution MeN. phases, have been described in 3.8.4 ff. 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

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