On the microstructure and phase identification of plasma nitrided 17-4PH precipitation hardening stainless steel

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Abstract

Systematic microstructure characterisation of plasma nitrided (350–500 °C for 10 to 30 h) 17-4PH alloy was carried out using SEM, XRD and TEM. Experimental results have shown that the microstructure and phase constituents of the plasma surface alloyed cases are highly treatment temperature dependent. When treated at low-temperatures (≤ 420 °C), the microstructure is dominated by nitrogen supersaturated martensite (α'N-expanded martensite); Nitrogen S-phase grains can be formed from the pre-existent retained austenite by converting the retained austenite grains in 17-4PH but no continuous S-phase layer was found. When treated at high-temperatures (above 420 °C), a surface γ′–Fe4N compound layer was formed, CrN precipitated and S-phase was decomposed.

Introduction

17-4PH martensitic precipitation-hardening stainless steel is attractive for many industrial sectors due to its desirable property combination of high strength, high toughness and good corrosion resistance. However, their wider applications are restricted by their poor tribological properties, which has necessitated the development of advanced surface engineering technologies to address the problem [1], [2].

Nitriding of PH stainless steels may present two challenges: (i) the nitriding temperature must be lower than the aging temperature of the steel and (ii) the surface oxide layer has to be removed before nitriding. These requirements, to a large extent, restrict the use of gas nitriding processes; however, these problems could be overcome by such surface alloying methods as plasma nitriding and ion implantation processes [3], [4], [5].

Plasma nitriding of 17-4PH steel can stem back to 1989 when Tesi et al. [6], [7], [8], [9] studied plasma nitriding of 17-4PH as a contemporary and complementary treatment to the age hardening. Significant surface hardening has been achieved, which is in line with most research on plasma nitriding of high chromium alloy steel as well as stainless steel [10], [11]. However, these investigations were limited to size and shape variation with hardness improvements in treated components and showed no detailed changes in microstructures or phases.

In 1993, Leyland et al. [1] reported plasma nitrided 17-4PH stainless steel at 420 °C for 30 h, and based on XRD depth profiling in conjunction with metallography they found a three-layer case structure. In particular, they suggested the formation of nitrogen-rich expanded austenite (i.e. S-phase) layer because of interstitial diffusion and conversion. However, it was noted that the investigation was focused on one treatment condition and their preliminary characterisation was mainly relied on XRD analysis without TEM studies.

After a decade, Sun and Bell [3] investigated low-temperature plasma nitriding characteristics of 17-4PH stainless steel at temperatures from 350–450 °C for 20 h. They found that when treated at temperatures below 425 °C, a thin, hard and featureless ‘white’ layer could be produced on 17-4PH, which was X-ray amorphous as derived from its lack of Bragg reflection peaks.

Recently, Manova et al. [12] have reported the formation of expanded martensite in nitrogen ion implanted 17-4PH steel at temperatures ranging from 380–400 °C; a transition to CrN and disappearance of expanded martensite took place at 420 °C. In addition to the formation of expanded martensite, Frandsen et al. [13] also claimed the formation of a surface nitrogen-expanded austenite (i.e. S-phase) layer by gaseous nitriding of two precipitation hardening stainless steels at 380 and 425 °C using a high nitrogen potential; however, it should be indicated that the existence of a S-phase layer was deduced from XRD analysis of highly overlapped Bragg reflection peaks as observed by Sun and Bell [3].

Clearly, all the previous phase identification work was based on XRD alone and the phases and microstructures formed in martensitic precipitation hardening stainless steels during nitriding is still a topic open to debate. Therefore, the present research was directed at systematic microstructural investigation and phase identification using both XRD and TEM to provide new insights into and thus advance scientific understanding of the microstructure of plasma nitrided 17-4PH martensitic precipitation hardening stainless steel developed during plasma nitriding of 17-4PH steel.

Section snippets

Material and treatments

The material used in the present work is 17–4PH precipitation hardening stainless steel with the following chemical compositions (wt.%): Fe–15.3Cr–4.55Ni–0.81Mn–0.02C–3.24Cu–0.19Nb–0.03Mo. Solution treatment was carried out at 1038 °C, followed by aging at 480 °C for 1 h and then air cooling. Coupon samples were cut from 25 mm diameter bars with a thickness of 8 mm. The flat surfaces of the disc sample were manually ground to 1200 grade to achieve a fine surface finish (Ra < 0.1 μm). Plasma

Metallography

It was observed that the microstructure produced during plasma nitriding of 17-4PH stainless steel varied with the treatment temperature and time. Optical microscopy and SEM observations showed that the nitrided layer produced below 420 °C appears not to be attacked by the reagent used, whereas the substrate was etched. A typical cross-sectional microstructure of 390 °C/20 h plasma nitrided specimens is depicted in Fig. 2 showing a thin bright layer on the substrate.

A typical SEM micrograph for

Discussion

As has been reviewed in Section 1, notwithstanding the fact that plasma nitriding of 17-4PH steel can date back to 1979, the phases and microstructure formed during plasma nitriding is still a topic open to debate [1], [3], [12], [13].

Examination by optical microscopy of the cross sections of the nitrided specimens has revealed that the morphology of the nitrided layer varied with nitriding temperature and time, and the former has played a more important role than the latter [16].

When treated

Conclusions

Plasma nitriding at temperatures between 350 and 500 °C for 10 to 30 h on 17-4PH martensitic precipitation-hardening stainless steels was investigated. Based on the experimental results and discussion, the following conclusions can be drawn from this research:

  • The microstructure and characteristics of nitrided 17-4PH precipitation hardening stainless steels are strongly affected by treatment temperature and time.

  • Low temperature (≤ 420 °C) plasma nitriding of 17-4PH stainless steel formed a

Acknowledgements

One of the authors (ME) would like to thank Universities UK for an Oversea Research Student (ORS) Award and Department of Metallurgy and Materials, University of Birmingham for a studentship. In addition, special thanks must go to their former colleague, Dr. C.X. Li (now with Smith & Nephew Orthopaedics, UK) for his technical assistance.

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