The Stability of the Layer Nitrided in Low-Pressure Nitriding Process

The kinetics of the nitrided layer thickness growth and its structure depend on the nitrogen flux from the atmosphere to the nitrided surface. A nitrogen flux to the surface is more significant than a diffusion flux into the substrate, during forming surface iron nitrides and the internal nitriding zone. For pure iron, nitrided under low pressure, cutting off the nitriding atmosphere creates a flux from the subsurface layer of nitrides to the surface. The purpose of this paper is to determine the direction of the nitrogen flux in a similar situation for steels containing nitride-forming elements, thus answering the question of the stability of the layer nitrided under such conditions. The surface of X37CrMoV5-1 steel was nitrided under low pressure (of 24 hPa) and annealed in a vacuum or nitrogen. The microstructure, thickness of the nitride layers nitrided layers, the thickness of the internal nitriding zone, surface hardness and stresses were examined. The highest values of the nitrided layer properties were observed for the samples saturated only with nitrogen obtained from ammonia dissociation or additionally heated in nitrogen. It has been shown that using a pure vacuum during the annealing stage leads to unfavourable changes in the structure of the nitrided layer formed and, in particular, to the decomposition of the iron nitride layer formed at the saturation stage and occurrence of the tensile stresses—what excludes practical application of such layer. Ultimately, it has been shown that in the low-pressure nitriding process, the stability of the nitride layer of the nitrided surface strongly depends on the annealing atmosphere during the annealing stage, while the stability of the internal nitriding zone remains mainly at the same level.