Elsevier

Materials Letters

Volume 181, 15 October 2016, Pages 78-81
Materials Letters

Effect of cathodic cage size on plasma nitriding of AISI 304 steel

https://doi.org/10.1016/j.matlet.2016.05.144Get rights and content

Highlights

  • Studied the influence of cathodic cage size on active screen plasma nitriding.

  • High surface hardness for smaller cathodic cage size.

  • Phase transformation from γN to Fe2–3N, Fe4N at 400 °C, 4 h; without CrN.

  • Enhanced corrosion resistance for smaller cathodic cage size.

Abstract

The aim of this work is to investigate the influence of cathodic cage size on active screen plasma nitriding of AISI 304 stainless steels. The specimens are nitrided in fixed processing conditions while using CC with diameters of 13, 15, 17, 19 and 21 cm. The surface morphology, compound layer thickness and elemental composition, phase composition, hardness profile and corrosion resistance of the processed specimens is assessed. The results show that when the CC diameter is reduced, surface hardness profile and corrosion resistance significantly improve, while iron nitrides (Fe23N,Fe4N) are formed along with the expanded austenite phase (γN).

Introduction

The AISI 304 is one of the family of austenitic stainless steels (ASSs), widely employed in food, chemicals, biomedical and nuclear industries [1], due to their inherent superior corrosion resistance. However, their mechanical properties such as wear resistance and surface hardness are not outstanding [2].

Plasma nitriding is a well know technique to improve the surface properties of various metals including ASSs. In this process, nitrogen atoms are introduced into the surface of the steel specimens at elevated temperatures 400–650 °C [3]. The process transforms the phase composition of a thin layer, at the surface of the specimen. The effectiveness of the process depends on parameters such as gas composition [4], pressure [1], current density [5], cathodic cage (CC) configuration [6] and specimen's temperature [7], geometry [8] and composition [9]. In most studies, metastable nitrogen supersaturated austenite phase (γN) has been reported if the specimen temperature is kept below 450 °C [10]. When higher temperature up to 600 °C [11], [12] or longer processing time of 12 h [13] are used, the γN transforms to iron and chromium nitrides (Fe23N,Fe4N,CrN,Cr2N). The formation of nitrides results in much higher surface hardness, but the presence of chromium nitrides significantly deteriorates the corrosion resistance of specimens.

In this work, active screen plasma nitriding is employed at a relatively low temperature of 400 °C for the nitriding of AISI 304 steel, as a function of cathodic cage (CC) diameter.

Section snippets

Experimental details

The mirror polished AISI 304 specimens (square geometry with size 10×10×2mm) are processed in a 40 kHz pulsed DC active screen plasma nitriding reactor (similar to the setup used by [9], except dimensional difference; 33.5 cm height and 31.5 cm diameter) under fixed processing conditions. Five CC's (made up of 2 mm thick AISI 304 sheet) of varying diameters from 13 cm to 21 cm are used in this experiment. The diameter of a hole in each CC is 8 mm. The height of CC and CC-specimens vertical separation

Results and discussion

Fig. 1 shows the hardness profile and XRD analysis of base and processed specimens. It shows that the surface hardness increases with the decrease in cathodic cage (CC) diameter. For the specimens processed with the smallest CC diameters, A and B, the surface hardness increases even at higher loads. Such improvement in hardness even at higher loads indicates the in-depth penetration of nitrogen in the steel surface. The change in the surface hardness of specimens can also be assessed by

Conclusion

The influence of cathodic cage size on cathodic cage plasma nitriding of AISI 304 stainless steel is investigated. The results showed that when cathodic cage diameter is decreased; the surface hardness increases even at higher loads and corrosion resistance improves more than two orders of magnitude. These beneficial improvements are caused by phase transformation from conventionally reported γN-phase to iron nitrides (Fe23N,Fe4N) without chromium nitrides. The OES showed that the reduction in

Acknowledgment

This work is partially supported by Higher Education Commission of Pakistan Research Project no. 20-2002 (R&D) and the Quaid-i-Azam University Research Fund for Plasma Physics Laboratory.

References (22)

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