The corrosion and corrosion–wear behaviour of plasma nitrided 17-4PH precipitation hardening stainless steel

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Abstract

Plasma surface nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 350 °C, 420 °C and 500 °C for 10 h using a DC plasma nitriding unit, and the surface properties of the plasma surface engineered samples were systematically evaluated. Experimental results have shown that the surface properties of the plasma nitrided layers in terms of hardness, wear resistance, corrosion behaviour and corrosion–wear resistance are highly process condition dependent, and it is feasible to provide considerable improvement in wear, corrosion and corrosion–wear resistance of 17-4PH steel using optimised plasma treatment conditions. All three treatments can effectively improve the surface hardness and the sliding wear resistance under unlubricated conditions; high temperature (420 °C and 500 °C) treated materials revealed improved corrosion and corrosion–wear properties due to the formation of surface compound layers.

Introduction

Stainless steel technology has played an important role in various industrial sectors to combat environmental and corrosive attack [1], [2]. Precipitation-hardening stainless steels have been developed to provide high strength and toughness while maintaining the corrosion resistance of stainless steels [3], [4]. 17-4 PH stainless steel has been called the “workhorse” of PH stainless steels by virtue of its high strength, excellent corrosion resistance and relatively simple heat treatment [5], [6]. However, the wider applications are restricted by their relatively low hardness and poor tribological properties. Therefore, they cannot meet challenging design requirements of high strength, high toughness and good resistance to both corrosion and wear (i.e. corrosion–wear) in some applications such as turbine blades, races, tools, bearings and even orthopaedic surgery [7], [8].

Corrosion–wear is the degradation of material under simultaneous removal by mechanical wear and electrochemical corrosion. These two mechanisms depend on each other in a complicated way and the total removal rate is usually not the sum of corrosion rate and wear rate measured in separate tests. In many cases, corrosion is accelerated by wear and similarly, wear may be accelerated (or slowed down) by corrosion [9]. Despite the importance of corrosion–wear, few investigations have been undertaken on the corrosion–wear of 17-4 PH stainless steel. Recent researches show that the corrosion–wear of surface engineered materials is very complex [10].

In practice, some 17-4 PH martensitic stainless steel components fail in service due to wear or corrosion–wear related degradation problems. The degradation of materials due to inter-action of corrosion and mechanical wear (tribocorrosion systems) is a major concern in processing conditions [11], [12]. In these situations, it is necessary to modify the structure and composition of the surface and subsurface of components to eliminate the adhesive wear problem, whilst it is equally important to maintain the anti-corrosion characteristics of the stainless steel surface.

This necessitates the development of advanced surface engineering techniques to address the problem. Some preliminary studies have been conducted to explore the possibility of enhancing the surface hardness of 17-4 PH martensitic stainless steels by plasma nitriding; however, little or no attention has been paid to the wear, corrosion and corrosion–wear behaviour of the plasma nitrided material [13], [14], [15].

The response of 17-4PH stainless steel to plasma nitriding over a wide range of treatment temperatures (350–500 °C) and time (5–30 h) have been investigated, and the experimental results have shown that the nitrided layer characteristics are highly process condition dependent. Layer thickness varied from 4 μ (350 °C/5 h) to 97 μ (500 °C/30 h) in 17-4 PH stainless steels (Fig. 1) [16].

This paper presents recent experimental results on the effect of plasma nitriding at 350, 420 and 500 °C for 10 h on the corrosion, wear and corrosion–wear behaviour of 17-4PH stainless steel. Based on the experimental results, the mechanisms involved will be discussed.

Section snippets

Material and treatments

The material used in the present work is 17-4PH precipitation hardening stainless steel (Table 1). Solution treatment was carried out at 1038 °C, followed by aging at 480 °C for 1 h and then air cooling. The coupon samples were cut from 25 mm diameter bars with a thickness of 8 mm. For corrosion–wear tests samples were cut to 70 mm × 12 mm × 10 mm size. The flat surfaces of the disc sample were manually ground to 1200 grade to achieve a fine surface finish (Ra < 0.1 μm). Plasma nitriding was carried

Layer morphology

It was observed that the microstructure generated during plasma nitriding varied with the treatment. As can be seen from the cross-sectional micrograph shown Fig. 3A, a white layer was formed for the 350 °C/10 h treated sample. When the treatment temperature was increased to 420 °C, a slightly dark sub-layer was formed (Fig. 3B). However, grain boundaries were revealed after etching the 500 °C/10 h sample, implying that precipitation of nitrides along the grain boundaries may have occurred

Corrosion performance

The corrosion behaviour of plasma nitrided 17-4PH precipitation hardening stainless steel in sodium chloride solutions has been examined. It has been found that the high temperature nitrided layer has much better pitting resistance than the untreated substrate material, and that low temperature nitriding significantly deteriorates the corrosion resistance.

These changes in corrosion behaviour can be explained by findings of the X-ray phase analysis and layer microstructure. The improved

Conclusions

Based on the experimental results, following conclusions can be drawn from the present investigation:

Plasma nitriding can be used to produce a variety of surface layer structures in 17-4PH precipitation hardening stainless steels, and the surface properties of the plasma nitrided layers in terms of hardness, wear resistance, corrosion behaviour and corrosion–wear resistance are highly process condition dependent.

All three plasma surface treatments can significantly improve the surface hardness

Acknowledgements

One of the authors (ME) would like to thank Universities UK for an Overseas Research Student (ORS) Award and Department of Metallurgy and Materials, the University of Birmingham for a studentship. In addition, special thanks must go to their colleagues, Prof. T. Bell, Dr. X. Y. Li and Dr. C. X. Li, for their assistance and discussions.

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