Nitriding layer characteristics of modified AISI422 martensitic stainless steel in tempered state

Abstract

In this study, the nitriding layer of the modified AISI422 martensitic stainless steel was characterized by several method, including optical microscopy (OM), micro-hardness tester, X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), to study the phase transformation at the whole nitriding layer during the plasma nitriding. In order to investigate the evolution of the nitriding process the TEM samples of the nitriding layer has been prepared from the white layer to the un-nitriding zone by the focus ion beam method. With nearly 20 h nitriding process, the white layer, vein-like structure and the diffusion zone was clearly observed though the OM. Together, the TEM results show that the γ′-Fe4N and the micro holes existed in the white layer and the thick vein-like structure. And the morphology of the continuous precipitation of CrN was highly related with the depth of the nitriding layer. Excepted the normal coarsening mechanism, the EDS results indicate that the discontinuous precipitation of CrN may also originate from the transformation of the M23C6(M=Cr) during the nitriding process. Meanwhile, the residual stress value is not directly proportionate to the nitriding depth or the N contents. Furthermore, it could be affected by the N contents and the degree of the CrN spinodal decomposition, and these two factors lead the maximum value of the compress stress appears at the depth of 120μm.