Microstructure and dry-sliding wear properties of DC plasma nitrided 17-4 PH stainless steel

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Abstract

An attempt that the precipitation hardening steel 17-4PH was conducted by DC plasma nitriding (DCPN) is made to develop a kind of candidate material for nuclear reactor. Nitriding process performed at temperature  400 °C takes effect on creation of the layers composed of S-phase (expanded austenite) and αN (expanded martensite). Up to the temperature of 420 °C, the S-phase peaks disappear due to the transformation occurrence (S-phase → αN + CrN). For the samples nitrided at temperature  450 °C, no evidence of αN is found owing to a precipitation (αNα+CrN) taking place. For the 480 °C/4 h treated sample, it is the surface microhardness that plays the lead role in the wear rate reduction but the surface roughness; while for the 400 °C/4 h treated sample, it is both of the surface roughness and the S-phase formation. Dry sliding wear of the untreated 17-4PH is mainly characterized by strong adhesion, abrasion and oxidation mechanism. Samples nitrided at 400 °C which is dominated by slight abrasion and plastic deformation exhibit the best dry sliding wear resistance compared to the samples nitrided at other temperatures.

Introduction

It is reported that 17-4 precipitation hardening stainless steel (17-4 PH) has been increasingly used in a variety of applications including aircraft fittings [1], gears, fasteners, compressor impeller [2], [3], nuclear reactor components [4], [5], [6]. However, the wider applications are restricted by their relatively low hardness and poor tribological properties, for example, the components made from 17-4PH stainless steel for nuclear reactor are unable to service due to the mild wear failure. Under this background, it is required to try to take advantage of a certain surface engineering technique to address the problem in order to improve the anti-wear property of 17-4 PH. Plasma nitriding is established to be an effective way for the wear resistance improvement of stainless steel [7], [8], consequently, it can be employed to make an attempt to develop a kind of candidate material for nuclear reactor. It is found that some preliminary studies have been conducted to explore the possibility of enhancing the surface hardness of 17-4 PH stainless steels by plasma nitriding; however, little or no attention has been paid to the correlation of microstructure with dry-sliding wear behavior of the plasma nitrided material [9], [10], [11]. Probably, new insights into the dry-sliding mechanism of the plasma nitrided 17-4 PH stainless steel can be probably provided by the ring-on-block contact configuration used in the test. Therefore, the aim of this paper is to investigate the influence of microstructure on the dry-sliding wear resistance of 17-4 PH stainless steel by DC plasma nitriding temperature, as well as the wear mechanism.

Section snippets

Material and treatments

The samples were prepared from grade 17-4 PH stainless steel with the following composition (wt.%): 0.04% C, 16.39% Cr, 4.32% Ni, 3.40% Cu, 0.30% Mn, 0.60% Si, 0.023% P, 0.36% Nb, trace Mo and balance Fe. The samples were cut from a hot rolling bar and then machined into a size of 10 mm × 10 mm × 10 mm. The flat surfaces of the block sample were manually grounded, using sandpapers (180, 320, 400, 600 and 1200 mesh), polished with diamond slurry in a polishing machine to achieve a fine finish (Ra  0.1 

Microstructure analysis

SEM micrographs of cross-section of the nitrided layers of the sample are shown in Fig. 2. In order to evaluate the effect of temperature on the resulting microstructure, plasma nitriding of the grade 17-4 PH was carried out at various temperatures. As can be seen from the cross-sectional micrograph shown in Fig. 2(a), a bright layer of approximately 5μm is observed for the 350 °C/4 h treated sample. However, for both of the 420 °C/4 h and 480 °C/4 h treated samples, a relatively dark layer of

Conclusion

Microstructure and dry sliding wear properties of the DCPN nitrided layers are investigated in this paper. It is certain that layers of different structures are produced. Nitriding process performed at temperature  400 °C take effect on creation of the layers composed of S-phase and αN. Up to the temperature of 420 °C, the S-phase peaks disappear due to the transformation occurrence (S-phase → αN + CrN). For the samples nitrided at temperature  450 °C, no evidence of αN is found owing to a

Acknowledgement

The authors gratefully acknowledge support of this work by Nuclear Power Institute of China and Coating Technology development Department of Chengdu Tool Institute.

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