Influences of plasma nitriding edge effect on properties of 316 L stainless steel

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Abstract

Some rings of different colors are formed when cylindrical stainless steel samples are plasma nitrided. They appear because of a phenomenon known as the edge effect associated with this technique. In this paper, we discuss some of the characteristics of these rings formed in nitrided AISI 316 L austenitic stainless steel: dimension, microhardness, composition, and corrosion resistance. Two cylindrical samples with different diameters were plasma nitrided in a 80% H2–N2 gas mixture, at a pressure of about 4 torr, for 4 h, at TN = 400 °C. One of the samples was surrounded by another ring-shaped sample, whose dimensions were chosen to contain the rings formed during the nitriding process. Consequently, rings of different dimensions were produced. The composition of samples was identified by conversion electron Mössbauer spectroscopy (CEMS) and Glancing angle X-ray diffraction (GAXRD). Regarding their relative dimensions, the rings showed similar characteristics, in spite of their different diameters. The samples displayed higher values of hardness in their rings than in their central regions. GAXRD detected peaks of Cr2N on the rings and of the expanded austenitic γN phase in the central region. CEMS results demonstrated the presence of the paramagnetic γN and ε phases. The proportion of these phases was consistent among the samples, regardless of their diameters. Concerning corrosion, the samples with a ring showed a poorer corrosion resistance than those without a ring. All results are in agreement with the presence of chromium nitride Cr2N on the ring of each sample.

Introduction

Austenitic stainless steels offer good resistance to general corrosion due to the formation of a passive surface film. For this reason, they are widely used in many industrial fields. However, the low hardness and poor tribological properties of these materials can shorten the life of components subject to wear. Some techniques, such as plasma nitriding [1], [2], [3], [4], [5], form a surface coating which improves the characteristics of stainless steel components.

Plasma nitriding improves various physical properties of metallic surfaces, such as hardness, wear and corrosion resistance, promoting the use of nitrided samples. This process also has advantages when compared with conventional nitriding processes, for example: non-emission of pollutants, energy saving, and shorter treatment time, despite some inconvenience when components with complex geometry are treated. For instance, plasma nitriding has an associated phenomenon known as the edge effect, which produces a non-uniform surface on cylindrical samples with different colors in the central and peripheral regions. The edge effect occurs because treated samples are submitted to a high cathodic potential to produce plasma directly on their surface. As a result of distortions in the electric field around the corners and edges, the shape of the plasma sheath, which is associated to the shape of samples, determines ion flux distribution, creating erosion rings characterized by different colors and discontinuous hardness. This effect occurs mainly in the treatment of materials containing a high percentage of alloy elements, such as chromium, which produces nitrides, as in the case of AISI 316 L [6], [7].

The aim of this research is to investigate the following characteristics of the rings formed in nitrided AISI 316 L austenitic stainless steel: dimension, microhardness, composition, and corrosion resistance.

Section snippets

Materials and methods

In this study, we used the AISI 316 L stainless steel produced by Villares Metals. Its chemical composition (in wt.%) is: Cr (17.03), Ni (10.16), Mo (2.16), C (0.03), and Fe balance. Three disks, named here samples A, B, and C, and a ring, named sample D, were prepared. Their dimensions are reported in Table 1.

All samples were ground using sandpaper from 220 to 1200 mesh, polished with alumina (0.3 and 1 μm), cleaned ultrasonically in an acetone bath, and finally air-dried. Next, they were plasma

Results and discussion

Fig. 1 shows pictures of the nitrided samples. An initial visual analysis suggests that regions can be classified according to their color. Thus, the borders of samples B and D, named here as rings, are light gray, and were labeled 1 and 4, respectively. Towards the center, there is a region in shades of blue and brown, labeled 2 (sample B) and 5 (sample D). The central region of sample B is dark yellow, labeled as number 3. Sample C, which was nitrided inside ring D shows the same yellow shade

Conclusion

This paper presents a study of some of the characteristics of the rings formed in nitrided AISI 316 L samples, which were prepared with two different diameters. The following conclusions were reached:

  • (1)

    Both samples showed the same ring characteristics regarding dimensions and hardness.

  • (2)

    XRD results showed the γN phase. In the ring, the N concentration is less than in the central region. Besides this phase, GAXRD also showed the presence of Cr2N in the ring.

  • (3)

    CEMS results showed γN and ε phases,

Acknowledgment

We thank Dr. J.M.D.A.Rollo, for support in the hardness measurements. We are also grateful to the technical collaborator Natália A. Zanardi. This work was partly supported by the Brazilian research funding agencies CAPES, CNPq and FAPESP.

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