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

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

Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 2969 – 2975 www.elsevier.com/locate/surfcoat On the microstructur...

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Available online at www.sciencedirect.com

Surface & Coatings Technology 202 (2008) 2969 – 2975 www.elsevier.com/locate/surfcoat

On the microstructure and phase identification of plasma nitrided 17-4PH precipitation hardening stainless steel H. Dong, M. Esfandiari ⁎, X.Y. Li Department of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, UK Received 26 June 2007; accepted in revised form 30 October 2007 Available online 7 November 2007

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. © 2007 Elsevier B.V. All rights reserved. Keywords: 17-4PH; Precipitation hardening stainless steel; Plasma nitriding; XRD; TEM

1. 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 [3–5]. Plasma nitriding of 17-4PH steel can stem back to 1989 when Tesi et al. [6–9] studied plasma nitriding of 17-4PH as a ⁎ Corresponding author. TTI Group Ltd (Nitrotec Services), Units 2-8, Witton Park Industrial State, Birmingham, B6 7EF, UK. E-mail addresses: [email protected], [email protected] (M. Esfandiari). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.10.036

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 threelayer 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 lowtemperature 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.

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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 nitrogenexpanded 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. 2. Experimental 2.1. 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 b 0.1 μm). Plasma nitriding was carried out using a DC plasma nitriding unit (Klöckner, 40 kW) at 350, 370, 390, 420, 460 and 500 °C for 10, 20 and 30 h in a gas mixture of 25% N2 + 75% H2 at a pressure of 5 mbar (500 Pa).

Fig. 2. SEM micrograph of 17-4PH plasma nitrided sample at 390 °C for 20 h.

of the transverse sections, XRD phase analysis and TEM microstructure observation and phase identification. In order to avoid the complication caused by the edging effect associated with DC plasma treatment, only the centre areas of the treated samples were used for characterisation and property evaluation. Standard procedures were followed to prepare metallographic samples and Villela's Reagent was used as etchant solution. XRD measurements were performed to determine phase constituents in the as-nitrided surface using a Philips X-ray X'Pert Diffractometer with a Cu–Kα radiation (wavelength 0.154 nm) and Ni-filter from 30° to 80° at a scan step size of 0.02° and a scan step time of 1 s. Plan view TEM specimens were prepared by sectioning the plasma nitrided materials parallel to the surface into approximately 200 μm thickness sheets, trepanning into 3 mm diameter discs and manually grinding to about 100 μm from the back (substrate) side of surface treated specimens. Primary dimpling was carried out from back side of the surface treated specimens with the centre being at A (Fig. 1); secondary dimpling was conducted from the surface centred at B with an eccentricity of r relative to the centre of the primary dimple (A). Ion beam thinning was carefully controlled so that electron transparent areas can be produced both in the outmost layer (area near A) and the sublayer (area near B).

2.2. Characterisation The phase constituents and microstructure of plasma nitrided layers were characterised using optical and SEM metallography

Fig. 1. Schematic illustration of TEM specimen preparation.

Fig. 3. SEM micrograph of 17-4PH plasma nitrided sample at 420 °C for 20 h.

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conventional bright-field (BF) TEM and selected-area diffraction (SAD). 3. Results 3.1. Metallography

Fig. 4. SEM micrograph of 17-4PH plasma nitrided sample at 500 °C for 20 h.

TEM studies were carried out using a Philips CM20 microscope, and both the morphology and crystallography of plasma nitrided surface layers were investigated by means of

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 420 °C/20 h treated 17-4PH samples is shown in Fig. 3. It can be seen that the upper part of the 420 °C/20 h treated layer was slightly etched, implying

Fig. 5. XRD patterns of 17-4PH samples treated at 350–500 °C for (a) 10 h and (b) 30 h.

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that precipitation of nitrides may have occurred during the treatment. For samples plasma nitrided at 500 °C or above, their nitrided case became dark and grain boundaries in the treated case were preferentially etched. A typical cross-sectional microstructure of 500 °C/20 h treated specimen is exemplified in Fig. 4. 3.2. Phase analysis As depicted in Fig. 5, the phases present in the untreated 17-4PH are dominated by martensite (α') with retained austenite (γ). XRD analysis revealed the overlapped peaks and the gradual changes in phase constituents of nitrided layers as the nitriding temperature or time is increased. Fig. 5 shows the effect of treatment temperature on the evolution of the phase constituents in 10 h (Fig. 5a) and 30 h (Fig. 5b) treated 17-4PH samples. It can be seen from Fig. 5 that according to the characteristic peaks, these XRD charts could be divided into two groups: (1) low-temperature (≤ 420 °C) and (2) high-temperature (≥ 460 °C) treated samples. The high-temperature (≥ 460 °C) treated samples are characterized by peaks of chromium nitride (CrN) and γ′–Fe4N, which is in line with the observations by Sun and Bell [3]. On the other hand, no well-defined and sharp Bragg reflection peaks can be observed for low-temperature (≤ 420 °C) treated samples; instead, their XRD charts are characterised by highly overlapped peaks, which were described by Sun and Bell [3]

Fig. 7. (a) BF TEM micrograph and (b) corresponding SAD pattern, b = [114], of untreated 17-4PH, showing retained austenite.

as so-called ‘amorphous-like features’. Clearly, it is difficult, if not impossible, to identify the phase constituents of the lowtemperature (350–420 °C) treated samples based only on XRD analysis. This is most probably caused by the overlapping of peaks of several potential phases (e.g. γ′–Fe4N, S-phase, α'N and CrN) and hence more detailed phase analysis work was conducted by TEM. 3.3. TEM characterisation

Fig. 6. (a) BF TEM micrograph and (b) corresponding SAD pattern, b = [100], of untreated 17-4PH.

TEM observation of untreated 17-4PH stainless steel revealed that the dominant phase of the material is lath martensite with the length and width about 2–2.5 µm and 0.3–0.9 µm, respectively. A typical microstructure and corresponding SAD of b = [100]α' pattern are shown in Fig. 6. Traces of retained austenite were found along the martensite lath boundaries, and their microstructure and SAD pattern are shown in Fig. 7. This phenomenon has also been reported by other investigators [13–15]. The microstructure of 350 °C/10 h treated 17-4PH stainless steel still maintained martensite characters. However, extensive micro twins and slip lines were observed across martensite laths (see Fig. 8), which contributed to the b110N streaks presented in the SAD pattern. In addition, the spots in the SAD pattern from the 350 °C/10 h treated sample are not as sharp as that from the untreated sample (Fig. 6), which is an indication of high residual stress; the calculated lattice parameter (a = 0.290 nm) is larger for the 350 °C/10 h treated than for the untreated samples

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According to the results of TEM investigation (Fig. 12), S-phase started to decompose into CrN and α–Fe and at the same time CrN also precipitated from α'N when 17-4PH were plasma nitrided at temperatures above 420 °C or at 420 °C for more than 10 h. 4. Discussion

Fig. 8. (a) BF TEM micrograph and (b) corresponding SAD pattern, b = [111], of 17-4PH treated at 350 °C/10 h.

(a = 0.287 nm), which implies the supersaturation of nitrogen in the martensite, i.e. α'N, during the treatment. In order to study the microstructure of the outermost layer and the sublayer beneath it, as marked A and B respectively in Fig. 1, the dimpling and final thinning of the TEM specimen was carefully designed and controlled to ensure penetration close to area A (see Fig. 1). Fig. 9a shows the BF TEM micrograph from the 420 °C/20 h treated sample with the outmost layer in the centre and the inner sublayer surrounded; Fig. 9b shows the corresponding SAD patterns taken from the circled area in Fig. 9a. Analysis of the SAD pattern revealed two sets of diffraction patterns corresponding to γ′–Fe4N and α'N; nitrogen supersaturated martensite, α'N (Fig. 10) which has the similar features, such as faint diffraction spots and streaks along b111N directions, as to that of 350 °C/10 h treated samples was identified from the areas away from the centre areas. According to Figs. 9 and 10, it is clear that a thin layer of γ′–Fe4N was formed on the outmost layer and beneath that is a nitrogen supersaturated martensite, α'N, sublayer. Isolated S-phase grains were observed in all the samples plasma nitrided at temperatures below 420 °C and 420 °C/10 h treated sample. A typical S-phase micrograph and the related SAD pattern are given in Fig. 11 for the 420 °C/10 h treated sample. It can be seen that the S-phase is surrounded by α'N and the size and distribution of the S-phase are similar to that of the retained austenite as shown in Fig. 7. Therefore, it can be deducted that the formation of S-phase in the martensitic 17-4PH stainless steel is related to the conversion of the retained austenite in the initial material.

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 at temperatures below 420 °C, the resultant nitrided layer was resistant to the etchant, such that a bright layer was produced (Fig. 2). TEM and X-ray results have demonstrated that the treated layer is free of precipitates and retains its martensitic structure (Fig. 8) but with a larger lattice parameter than the bulk. Indeed, GDS (Glow Discharge Spectoscopy) chemical composition analysis has revealed that the amount of nitrogen in the nitrided layer is much higher

Fig. 9. (a) BF TEM micrograph and (b) corresponding SAD pattern of outmost γ′–Fe4N and inner layer of α'N martensite from 420 °C/20 h treated sample.

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than the solubility of nitrogen in the martensite lattice [16]. Accordingly, it can be concluded that the nitrided layer formed at temperatures below 420 °C is a nitrogen supersaturated martensite layer rather than a compound layer. This layer is very similar to the S-phase (i.e. expanded austenite) obtained with austenitic stainless steel, and the analogy between them makes it reasonable to term that newly identified phase as “expanded martensite” [17]. The formation of expanded martensite has also been found recently by Manova et al. [12] and Frandsen et al. [13] in plasma nitrided precipitation hardening stainless steels. When the nitriding temperature was increased to about 420 °C or above, some dark phases began to form in the outer part of the nitrided layer (e.g. 420 °C/20 h, Fig. 3). These dark phases were preferentially developed along the original austenite grain boundaries. Further increasing the nitriding temperature or time resulted in the development of the dark layer towards the layer/core interface (Fig. 4). As has shown in Fig. 5, no clear and sharp reflection peaks but rather a very broad complex peak can be observed for samples treated at temperatures below 460 °C. Sun and Bell [13] claimed that the plasma nitrided layers are amorphous-like because of the shape of the XRD charts. However, detailed TEM results from the present investigation have clearly indicated that depending on the treatment conditions, the plasma nitrided layers may consist of α'N-martensite, γ′–Fe4N and S-phase (expanded austenite) or CrN and all of theses phases are not amorphous but crystalline. Therefore, it is the

Fig. 10. (a) BF TEM micrograph and (b) corresponding α'N SAD pattern, b = [111], from 420 °C/20 h treateated sample.

Fig. 11. (a) BF TEM micrograph and (b) corresponding SAD pattern from the S-phase in 420 °C/10 h treated sample.

coexistence of these crystalline phases, supersaturation and the strong compressive residual stresses that may have caused the strong overlapping of their XRD peaks. When treated at temperatures below 420 °C or at 420 °C for 10 h, the microstructure is dominated by α'N-martensite in conjunction with a small amount of S-phase; when further increasing the treatment temperature or time, the S-phase decomposed, CrN precipitated and γ′–Fe4N started to form, thus leading to the formation of dark areas in the treated layers (Fig. 4). In addition, it is also of great interest to find from the present studies that no continuous S-phase layers were formed in plasma nitrided layers of 17-4PH steel although Leyland et al. [1] suggested the formation of nitrogen-rich austenite (i.e. S-phase) layer because of interstitial diffusion and conversion. Frandsen et al. [13] also claimed merely based on XRD analysis that an outer expanded austenite and an inner expanded martensite were formed during gaseous nitriding of martensitic precipitation hardening stainless steels when using a very high nitrogen potential. The discrepancies between Frandsen et al. [13], Leyland et al. [1] and the present authors might be attributed to different nitriding methods (i.e. gaseous, triode discharge and diode discharge, respectively) and nitrogen potentials. Indeed, Frandsen et al. [13] found that when treated using a relatively low nitrogen potential, no S-phase layer can be formed. However, it should be indicated that XRD analysis is unable to provide depth information on layer structure. This is mainly because the penetration depth of x-ray is estimated to be 8 microns for Cu-Kα in 17-4PH steel [18]. Therefore, the above different views are most probably stemmed from the different

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• Low temperature (≤ 420 °C) plasma nitriding of 17-4PH stainless steel formed a nitrogen supersaturated martensite layer, which may be called “expanded martensite”. • Nitrogen S-phase grains can be formed at temperatures below 420 °C or at 420 °C for a short time (≤10 h) from the pre-existent retained austenite in martensitic 17-4PH stainless steel. • A γ′–Fe4N surface layer and CrN precipitates form during plasma nitriding of 17-4PH steel at temperatures above 420 °C or at 420 °C for a long time (N 10 h). • High-temperature plasma nitrided 17-4PH steel is dominated by a γ′–Fe4N surface layer, CrN precipitates and α–Fe. 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. References Fig. 12. (a) BF TEM micrograph and (b) corresponding SAD pattern from the CrN-phase in 420 °C/20 h treated sample.

characterisation methods used; whereas Leyland et al. [1] and Frandsen et al. [13] used XRD alone, the present investigation employed both XRD and TEM. As has been shown in Fig. 11, an S-phase grain (expanded austenite) is isolated by the surrounding expanded martensite; the formation of S-phase is related to the conversion of existing retained austenite grains as is evidenced by comparing Figs. 7 and 11, which is in line with the findings made by Leyland et al. [1] that the S-phase was formed from the pre-existent retained austenite. When the nitriding temperature was increased to 460 °C or above, the majority of the resultant nitrided layer appeared dark after etching (Fig. 4), which is the typical microstructure of conventional high-temperature plasma nitrided 17-4PH steel. Well-defined and sharp XRD peaks of CrN and γ′–Fe4N dominate the XRD charts. This indicates that when treated at high-temperatures precipitation of CrN occurred and a compound layer of γ′–Fe4N was formed. 5. 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.

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