Hydrogen permeation modification of 4140 steel by ion nitriding with pulsed plasmas

It is widely known that the hydrogen in steel produces embrittlement. This effect may cause the failure of the elements (confining walls, mechanical p arts, etc.) whose surfaces are in contact with this gas or with processes i n which hydrogen is continuously generated. In this work it is shown that t he ion nitriding of the surface of AISI 4140 is a good mechanism to act as a barrier against hydrogen permeation in its bulk. The ion nitriding was pe rformed using a square wave DC glow discharge. The development of a compoun d layer of iron nitrides was observed as the cause of the hydrogen permeati on reduction. For equal duration of treatment, thicker compound layers were developed in higher discharge/post-discharge ratios in the square wave of the applied voltage onto the sample (cathode), with a greater reduction of hydrogen permeation coefficient as a consequence. Nevertheless, the permeat ion was not reduced to zero in any of the treatment conditions used. The re sults of the analysis of the permeation tests and the image of the photomic rographs showed that the existence of cracks, fractures, failures, etc. in the compound layer (pre-existing in the AISI 4140 steel) could be the cause of the residual hydrogen permeation. This can be attributed to the movemen t of the hydrogen through these defects diffusing through the original alph a-Fe phase of the non-treated steel. (C) 1998 Elsevier Science S.A. All rig hts reserved.