F.O. de Araújo

Ionic nitriding in cathodic cage of AISI 420 martensitic stainless steel

Abstract Cylindrical shape samples of AISI 420 martensitic stainless steel were nitrided by cathodic cage plasma nitriding technique at temperatures of 623, 673 and 773 K for 5 h. In this technique the samples are placed on an insulating plate inside a cage that shields the cathodic potential. A systematic study was made to test the efficiency of this technique, when compared with conventional ionic nitriding, related to the elimination of defects such as edge effect. The process present a bigger nitriding rate and the samples nitrided using this new technique displayed crystalline phases and hardness comparable with those obtained using conventional ionic nitriding. Moreover, it was possible to eliminate completely the erosion rings often present in conventionally ionic nitrided samples due to edger effects.

Uniformity of temperature in cathodic cage technique in nitriding of austenitic stainless steel AISI 316

Abstract A series of austenitic stainless steel AISI 316 cylindrical samples with different heights and samples with the same height placed at different positions were simultaneously nitrided using the cathodic cage technique. In this technique the samples are placed on an insulate plate inside a metallic cage which shield the cathodic potential, therefore it stays under a floating potential. A systematic study of the nitriding temperature variation effects was carried out to evaluate the temperature uniformity inside the cage. The samples were characterised through optical microscopy, X-ray diffraction and microhardness measurement. The results were compared with those obtained through conventional ionic nitriding, and it was verified that the samples nitrided by conventional technique present similar surface hardness, less uniformity and lower nitrided layer thickness than the samples treated under similar conditions through this new technique.

Cathodic cage plasma nitriding of austenitic stainless steel (AISI 316): influence of the working pressure on the nitrided layers properties

In this work, the influence of working pressure on the characteristics of the layers produced by the nitriding treatment on the AISI 316 austenitic stainless steel surface using the Cathodic Cage Plasma Nitriding technique (CCPN) is assessed. The treatments were carried out at a temperature of 723 K for 5 hours under working pressures of 120, 250 and 500 Pa. The morphology, microstructure and corrosion resistance were studied through optical microscopy, x-ray diffraction, and electrochemical potential curves. We analyzed the effects of working pressure on the layer thickness, microhardness and corrosion resistance. The E×i curves and the electrochemical impedance of polarized samples were measured in order to investigate the effect of the ion transfer inside the produced film placed into the anodic solution. The electrochemical impedance of the films were measured, and showed an apparent capacitive behavior. For treatment under a pressure of 120 Pa we found the nitrides Fe3N, Fe4N and CrN. For treatment at a pressure of 250 Pa, the CrN disappears and the observed phases are the nitrides Fe3N, Fe4N and the S-phase. For a treatment pressure of 500 Pa there is an increase in the corrosion resistance of nitrided layers, confirmed by the predominance of S-Phase in the x-ray diffraction pattern. On the other hand, the sample nitrided under the pressure of 250 Pa showed very unstable impedance behavior when immersed in NaCl solution, indicating that the treatment at this specific working pressure has a negative effect on the corrosion resistance of plasma nitrided AISI 316 stainless steel surfaces.

Cathodic Cage Plasma Nitriding: An Innovative Technique

Cylindrical samples of AISI 1020, AISI 316, and AISI 420 steels, with different heights, were simultaneously treated by a new technique of ionic nitriding, entitled cathodic cage plasma nitriding (CCPN), in order to evaluate the efficiency of this technique to produce nitrided layers with better properties compared with those obtained using conventional ionic nitriding technique. This method is able to eliminate the edge effect in the samples, promoting a better uniformity of temperature, and consequently, a smaller variation of the thickness/height relation can be obtained. The compound layers were characterized by X-ray diffraction, optical microscopy, and microhardness test profile. The results were compared with the properties of samples obtained with the conventional nitriding, for the three steel types. It was verified that samples treated by CCPN process presented, at the same temperature, a better uniformity in the thickness and absence of the edge effect.

Nitriding using cathodic cage technique of martensitic stainless steel AISI 420 with addition of CH4

AISI 420 martensitic stainless steel samples were nitrided by cathodic cage technique with addition of methane in the atmosphere aiming to reduce chromium nitride precipitation, to increase hardness and wear resistance without the presence of characteristic defects inherent to the ionic nitriding process. Microhardness measurements and X-ray analysis confirm the formation of a high hardness double-layer constituted by two regions: one internal region composed of carbon and another composed of nitrogen.

Nitretação em gaiola catódica: influência do tempo de tratamento

AISI 316 austenitic stainless steel samples have been nitrided by cathodic cage technique in same treatment conditions: pressure 250 Pa, temperature 500 oC and nitriding mixtures 80%N2/H2. The variance is limited just to process duration, from 1 to 5 hours. In this work the objective is to observe the increase of nitrided layer. It has been seen that there was layer formation to all times of nitriding. They are confirmed through optical microscopy and microhardness analysis. The X-ray difratogram presents similar phases proving that is possible obtain nitrided layer using cathodic cage technique even to short during treatment.

NITRETAÇÃO A PLASMA DE AÇO INOXIDÁVEL AUSTENITICO AISI 316: UNIFORMIDADE DA CAMADA NITRETADA

Plasma nitriding is a metal surface hardening technique based on the interaction between plasma and the exposed metal surface. Although commercially established, further attention concerning the understanding of nitriding of samples of complex geometry is required, because of formed non-uniform nitrided layers. In this work, cylindrical stain-less steel AISI 316 samples with different height (1, 3, 5, 8 and 10 mm) were plasma nitrided at pressures of 360 Pa, and at temperature of 500 oC. Top surface nitriding layer uniformity were studied and related to the nitriding condi-tions and plasma parameters above the top surface. The top surface nitrided layer thickness increases with height of the samples. Erosion rings are formed. Their distance from the edges decreases with height. The layer thickness and uniformity was caracterized by optical microscopy and for measurements of microhardness to evaluate the profile of hardness in the area of the rings.