HYDROGEN DIFFUSIVITY AND SOLUBILITY IN A PLASMA ION NITRIDED STEEL

The objective of the present study was to analyse the permeability of hydrogen through the nitrided case of a high strength low alloyed stee l. This type of thermochemical treatment is used to improve mechanical properties, such as tribological behaviour and fatigue resistance. Hy drogen contamination is intended to be avoided because of the risk of hydrogen embrittlement. This may be achieved by the formation of a com pact nitrided layer which plays the role of an efficient diffusion bar rier for hydrogen. The substrate was a high strength low alloyed steel , with a microstructure composed of banded pearlite in a ferrite matri x which was plasma ion nitrided at 500 degrees C during three differen t nitriding times : 0.5 h, 16 h and 28 h. Scanning Electron Microscopy and X-Ray analysis were used to characterize substrate and nitrided c ase microstructures. Microhardness profiles were obtained and then com pared to microdeformation profiles determined by X-Ray diffraction.For the sample nitrided for 0.5 h, the combination layer or white layer i s irregular, measuring 2 to 3 mu m in thickness. The white layer is co mposed of the gamma' (Fe4N) nitride. For the 16 h and 28 h of nitridin g, the white layer is homogeneous with a thickness of 5 mu m. The whit e layer is composed of gamma' nitride for 16 h of nitriding while in t he 28 h of nitriding epsilon (Fe2-3N) nitride is present in the outerm ost layer. The very high hardness attained in the combination layer is due to the intrinsic hardness of the nitrides, while the higher hardn ess in the diffusion layer is related to the presence of nitrides asso ciate to the high dislocation density, the pinning of the dislocations by precipitates as well as the saturation of the ferrite matrix. Elec trochemical tests were performed to obtain potentiodynamic polarizatio n curves and hydrogen permeation curves, at 50 degrees C, using a 0.1 N NaOH solution as electrolyte. This enabled the determination of hydr ogen permeability, solubility and diffusivity in the substrate and in the nitrided cases using De Miranda et al.'s methodology and Ash et al .'s model. The three types of ion nitrided cases present different com bination and diffusion layers as a function of the nitriding time. The calculation of hydrogen permeation parameters in the different layers allows a comparison of the behaviour of the cases which reference to the microstructure and the mechanical properties. It is shown that, de pending on the microstructure obtained after nitriding, the hydrogen p ermeability may be reduced (for 16 and 28 h nitriding) or increased (0 .5 h nitriding) compared to that of the substrate. Variations of the p ermeability are found to be essentially due to the presence and the th ickness of the combination layer. Comparison of the permeabilities for 0.5 h and 16 h nitriding times shows that a critical thickness exists for the white layer underneath which the role of barrier is not assum ed. For 28 h nitriding time, the presence of epsilon (Fe2-3N) nitrides as well as porosity into the white layer induce a decrease of hydroge n permeability.