Surface Modification of 17-4PH Stainless Steel by Plasma Nitriding

Surface Modification of 17-4PH Stainless Steel by Plasma Nitriding

Sangita Jadeja

M tech (Industrial Metallurgy) Indus University, Rancharda Ahmedabad 382115, India

D. K. Basa

B. Ganguli

Senior Scientist

Institute for Plasma Research Bhat, Gandhinagar – 382428, India

Professor, Metallurgy Department Indus University, Rancharda Ahmedabad 382115, India

Abstract- Plasma nitriding of 17-4PH martensitic precipitation hardening stainless steels was carried out at 3500C, 3800C, 4000C, 4300C, 4500C and 5000C for 4 hours using a mixture ratio of N2:H2 = 1:4. The modified surface was evaluated for micro-hardness as well as characterization tests such as optical microscopy, SEM, and XRD. Corrosion tests were also performed on untreated as well as plasma nitrided samples.

The micro-hardness tests reveal increase in hardness by a factor of four compared to untreated sample at all temperatures. However, case depths varied from 17 microns (m) for 350oC to as high as 52 microns for 500oC. Optical microscopy and SEM confirm results obtained for micro-hardness, vis-à-vis thickness of the nitrided layer. XRD results show formation of expanded austenite phase (S-phase) below 400oC and predominantly CrN phase above 400oC. Corrosion rates evaluated in a electrochemical cell in 3.5% NaCl indicate increase in its value up to 380oC processing temperature, thereafter it decreases compared to untreated case. The best result is obtained for the case of 500oC processing temperature.

Keywords- 17-4PH stainless steel, plasma nitriding, microhardness, optical microscopy SEM, XRD, elctrochemical corrosion

1. INTRODUCTION

   17-4PH martensitic precipitation hardening stainless steel is attractive for many industrial application sectors (chemical, nuclear, aviation, etc.) due to its desirable property, i.e. combination of high strength, high toughness and good corrosion resistance [1,2,3]. However, their wider applications are restricted by their poor tribological property, which has necessitated the development of advanced surface engineering technologies to address the problem [4,5].

   Plasma nitriding is a diffusion process for surface hardening of steel substrates which generates an interface with a graded compositional and hardness profile. The nitrided layer reduces the difference of hardness between the substrate and the coating, improving in this way the tribological behavior and the load capability of the coating [6].

   The plasma nitriding method of surface hardening [7-13] uses d.c. glow discharge to impart elemental nitrogen to the surface of steel with subsequent diffusion into the bulk of material. Generally, two layers are created during plasma

   nitriding process. The compound layer is consisted of -Fe2- 3N and -Fe4N phases. This type of layer is very hard, but unfortunately brittle with good friction and anticorrosion properties. The thickness and hardness of ´- Fe4N depend on quantity and quality of nitride-forming elements. Parameters of plasma nitriding layer are not dependent only on process parameters of nitriding, such as duration, temperature, pressure, voltage and nitrogen potential, but also dependent upon the substrate alloy type as well as its microstructure [14].

   A few studies [1,2] have been undertaken to explore the possibility of enhancing the surface hardness of 17-4PH martensitic stainless steel by plasma nitriding; however little or no attention has been paid to the wear and corrosion of plasma nitrided material.

   In the present investigation the response of 17-4 PH stainless steel to plasma nitriding over a wide range of treatment temperature (350-5000C) and fixed duration (4 hours) has been investigated, and experimental results have shown that the nitrided layer characteristics are highly process-conditions dependent. Layer thickness varied from 17µm (3500C/4hr) to 52µm (5000C/4hr) in 17-4 PH stainless steels.

   Further the paper presents experimental results on the effect of plasma nitriding at 3500C, 3800C, 4000C, 4300C, 4500C and 5000C for process duration of 4 hours on the corrosion behavior of 17-4 PH stainless steel. Based on the experimental results, the mechanisms involved will be discussed.

2. EXPERIMENTAL

   1. Material and Treatments

      The material used in present work is 17-4PH stainless steel with the following composition (wt%): 0.041% C, 0.355% Si , 0.908% Mn, 0.019% S, 0.022% P, 15.395% Cr,

      4.354% Ni, 0.450% Mo, 3.328% Cu, 0.003% Ti, 0.043% V,

      0.183% Nb, 0.027% W and Balance Fe. Cylindrical samples were cut from 9.7mm diameter with thickness of 8mm. They were mirror polished by using SiC abrasive papers of different grit size and disc polishing machine. The samples were cleaned thoroughly with petroleum ether to remove the impurities prior to plasma nitriding. Plasma nitriding was

      carried out in a DC glow discharge reactor using a mixture of N2 and H2 (1:4) at a total pressure of 5 mbar at process temperatures of 3500C, 3800C, 4000C, 4300C, 4500C and 5000C for a fixed duration of 4 hours. Temperature was monitored using a K-type thermocouple and controlled by adjusting the bias voltage on the cathode (sample).

   2. Characterization

   After plasma nitriding, the specimens were characterized by a variety of analytical techniques, including metallography for transverse sections for layer morphology and X-ray diffraction for phase identification.

   The XRD of all the samples was carried out with the help of Bruker X-ray diffractometer with Cu-K radiation (=1.5406 Å), 40 kV, 40 mA with 0.05 step size using Bragg- Brentano powder mode. The 2 Theta scan range was from 30º to 90º.

   (a) 4000C

   SEM and optical microscopy were used to observe surface morphology of plasma nitrided samples. Thickness measurements were undertaken using SEM of LEO Corp., model s440i.

   A Vickers micro hardness tester (Leitz, model no. RZD-00) was used to measure the hardness on the as-treated surface and as a function of depth on cross-section of the treated specimen. Loads of 100gm and dwell times of 20s were employed.

   The corrosion properties of plasma nitrided layers were evaluated using Digi-ivy, model DY2300 Potentiostat in 3.5% NaCl to measure the corrosion rates with the help of Tafel plots.

3. RESULTS AND DISCUSSION

   1. Physical Appearance

      The difference between the untreated and nitrided samples can be seen from Fig 1. The untreated sample has a plain polished mirror like surface shown Fig. 1(a), whereas the nitrited sample has grey colour (Fig. 1 (b)).

      1. Untreated (b) Plasma nitrided sample

         Fig.1. Physical Appearance

   2. Optical microscopy

      We observed thick layer of nitrogen diffusion on the nitrided sample by optical microscopy. We can see thick layer shown in Fig.2(c). Maximum 52 micron thick nitrited layer can be seen between the mould material and core material of 17-4PH SS.

      1. 430oC

      2. At 5000C

      Fig.2. Comparison of nitrided layers for various process temperatures by optical microscopy

      Fig. 2 (a) & (b) shows the comparison of 4000C and 4300C treated 17-4PH after nital etching by optical microscopy. In this observation at 4000C temperature dark layer was not observed though high hardness was measured close to the surface. This hardness is due to S Phase and not by CrN. Nitrogen does not react with Cr at low temperature, so does not form CrN. At low temperature (350-3800C) corrosion resistances is decreased which implies that S Phase is detrimental for corrosion resistance. At 4300C S Phas disappears due to nitrogen reacting with Cr above 4000C temperature forming CrN.

   3. SEM Analysis

      In SEM analysis shown in Fig.3, nitrided layers are observed for various temperatures (350-5000C). Maximum thickness of nitrided layer is observed at the highest processing temperature (5000C). The microstructure of the modified surface is clearly visible. At lower temperatures (350oC and 380oC) it is due to S-phase while at higher temperatures (430oC and 500oC) it is due to CrN precipitates.

      a) 5000C

      (b) 430oC

   4. XRD analysis

      The phase composition in the nitrided layer was analyzed with XRD using Cu-K radiation. The XRD diffraction patterns of phase structure of the layers are shown in Fig.4. It can be clearly observed that peak for untreated sample is sharp. XRD analysis revealed the overlapped peaks and gradual changes in phase constituents of nitrided layers as the nitriding temperature is increased. High temperature (4300C) treated samples show peaks of CrN, , Fe4N. S phase is observed at low temperature (4000C). Therefore, at 3500C, 380oC and 4000C, a broad peak at the diffraction angle, i.e. 2 theta equaling 43.650 is observed, which has been associated with a metastable phase called S phase (expanded austenite). S phase peak disappears above 4000C temperature due to formation of CrN precipitates.

   5. Micro-Hardness

      Fig.4. XRD analysis

      (c) 3800C

      (d) 3500C

      Fig.3. Nitrided Layer Observation by SEM of a) 5000C

      (b) 4300C c) 3800C (d) 3500C temperatures

      The micro-hardness of nitrided layer as a function of depth from surface for various processing temperatures is shown in Fig.5. It is clearly observed that the thickness of the hard nitrided layer depends mainly on the nitriding temperature. The maximum thickness is observed at temperature of 5000C. Hardness increases above 4000C due to formation of CrN. Increasing the nitriding temperature increases the nitrided layer thickness.

      Fig.5. Micro-Hardness comparison

      6.00E+01

      5.00E+01

      Corrosion Rate (mmpy)

      4.00E+01

      3.00E+01

      2.00E+01

      1.00E+01

      0.00E+00

      Temprature (0C)

      Sample No.	1 (500oC)	2 (450oC)	3 (430oC)	4 (400oC)	5 (380oC)	6 (350oC)
      Case Depth (microns)	52	36	33	29	21	17

      Table 1. Nitrided case depths at different process temperatures

   6. Corrosion performance

   The corrosion behavior of plasma nitrided 17-4PH stainless steel in 3.5% NaCl solution has been examined using linear sweep voltammetry (log I vs. V) in an electrochemical cell. Tafel plots were obtained which provided the corrosion rates. Fig. 6 shows the results. At higher temperature corrosion performance is better than untreated, however at lower temperatures ( 380oC) corrosion resistance decreases. Above 4000C corrosion resistance is improved due to formation of Fe4N confirmed by X-ray diffraction analysis.

   Fig.6. Corrosion behavior of plasma nitrided 17-4PH stainless steel

4. CONCLUSIONS

Plasma nitriding of 17-4 PH stainless steel increases surface hardness by a factor of four (~1600HV compared to 370HV for the untreated sample) for all process temperatures starting from 350oC to as high as 500oC (for a fixed N2:H2=1:4, duration = 4 hours). However case depths range from a low of 17m for 350oC to as high as 52m for 500oC. XRD, optical microscopy and SEM confirm presence of S phase in the temperature range from 3500C to 4000C. Above 4000C S phase disappears. As nitriding temperature increases nitrogen diffusion also increases leading to more depth of the nitrided layer. From the XRD results it can be concluded that CrN and Fe4N form only at higher temperatures (430). Electrochemical corrosion rate measurements carried out in 3.5% NaCl reveal that the performance of plasma nitrided 17- 4 PH steel is better at higher temperature (400oC) leading us to conclude that it is due to formation of Fe4N at the top of the surface.

ACKNOWLEDGEMENTS

The authors are thankful to N. Kumar and M. Aslam for technical assistance during the course of the present investigation. Thanks are also due to A.B. Zala for the XRD work and to N.L.Chauhan for the SEM analysis.

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