The development of nitrogen concentration profiles on nitriding iron
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INTRODUCTION
O N nitriding an a (bec)-iron specimen a nitrogenized zone develops which in general can be subdivided into a compound layer, adjacent to the surface, and a diffusion zone, beneath the compound layer, where at the nitriding temperature the nitrogen is dissolved interstitially in the octahedral interstices of the t~-iron matrix. Good tribological and anticorrosion properties are ascribed to the compound layer, whereas improvement of the fatigue resistance is due to the diffusion zone. z Therefore, it is of interest to know how the nitrogen concentration profile develops on nitriding. Standard analyrical solutions for this diffusion problem exist, if one assumes that the nitrogen surface concentration is constant during nitriding. 2 These analytical solutions are often used to estimate the case depth of nitrided workpieces.3 However, it will be shown below from numerical calculations and experimental results that the surface concentration is not constant during nitriding. In the field of carburizing it was observed previously that the surface carbon content changes with time, and various numerical procedures to evaluate the concentration profile were developed. 4'5,6 In the field of nitriding, the analogous phenomenon was not generally recognized before. On the basis of reaction-kinetical data 7 and experimental results it will be demonstrated in this paper that, on nitriding, the nitrogen surface concentration approaches relatively slowly the equilibrium value, which effect strongly influences the development of the concentration profile. As a result of the present examination it can also be explained why an incubation time exists for compound layer formation, as observed in nitriding practice. 8'9 The present numerical procedure provides estimates for such incubation times which correspond with the experimental results. As a further consequence it can be shown that the compressive residual surface stress, which is thought to possess a beneficial influence on the fatigue resistance,1'10 reaches a maximum value after some (optimum) time of nitriding.
CALCULATION OF NITROGEN CONCENTRATION P R O F I L E S
Consider an infinitely extended iron plate of finite thickness. On nitriding, nitrogen atoms diffuse from both outer surfaces into the specimen, and thus a symmetrical concentration profile is built up. The nitfiding process can be subdivided into three steps: (1) diffusion of ammonia in the gas phase toward the iron surface; (2) dissociation of ammonia at the iron surface; (3) diffusion of nitrogen into the iron specimen. When the gasflow is large enough to maintain a constant gas composition at the iron surface, step (1) can be ignored. The amount of nitrogen provided by the dissociation of ammonia at the surface of the specimen per unit of time and per unit of surface area, JN, can be described by (see Eq. [19] at p. 538 of Reference 7) JN = k(C,q - C,)
where k = reaction rate coefficient; C,q = nitrogen concentration in a-iron in equilibrium with the gas atmosphere, and C, = actual nitrog
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