Effect of Cooling Rate on The Plasma Nitriding Process of 304 Austenitic Stainless Steel


S.M. Khalil*

Citation: Effect of Cooling Rate on The Plasma Nitriding Process of 304 Austenitic Stainless Steel, American Research Journal of Physics, vol 4, no. 1, 2018, pp. 1-11

Copyright This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Abstract:

The aim of this work is to study the influence of the cooling rate on the properties of the modified surface layer of AISI 304 steel after rf plasma nitriding. The nitrided samples were characterized by glow discharge optical spectroscopy, x-ray diffraction, optical microscopy, scanning electron microscopy and Vickers microhardness measurements. The results revealed that microstructure, nitriding rate and surface microhardness values were found to be cooling rate dependent. The treated layer is mainly composed of nitrogen expanded austenite (γN), iron nitride (γ‘-Fe4 N) and chromium nitride (CrN). A maximum thickness of treated layer (19.9 µm) is achieved for sample treated at medium cooling rate of 900 Cm3 /min. It has a maximum surface hardness and nitriding rate of 1402 HV0.1 and 0.66 μm2 /s, respectively.

Keywords: AISI 304 stainless steel, rf plasma nitriding, cooling rate, surface morphology, nitriding rate, surface microhardness. 


Description:

INTRODUCTION

Plasma surface modifications have been used in various industrial applications over the past several decades [1,2]. One of these applications is the development of machinery tools and components. To increase productivity and efficiency, high speed and high load operating machines were used in industrial production. Components of such machines are exposed to harsher and harsher conditions. Therefore, they have to be tougher and more resistant to wear at high operating temperatures without cooling emulsion. Dry cutting and drilling are good examples. Surface modification of such machinery tools and components addresses those needs due to improving their mechanical and tribological properties which lead to increasing their service lifetime.  

Many tools and components are manufactured from different kinds of austenitic stainless steels due to their excellent corrosion resistance. However, the wider applications of austenitic stainless steels are restricted by their relatively poor mechanical and tribological properties. Plasma nitriding can be used as an effective surface modification technique to introduce nitrogen species into the surface of austenitic stainless steels to form various hard surfaces and sub surfaces composed of nitrides, keeping the bulk material without any modification [3, 6]. This technique has been intensively studied over several decades and its basic principles are well known [7-10]. 

During plasma nitriding process, the surface of the treated substrate is exposed to chemical and physical reactions. The properties of the nitrided layer are affected by these reactions depending on the optimization of many process parameters. Intensive research work have been done on the effect of treatment temperature, processing time, processing power and, adding different ratios of hydrogen or carbon gas to nitrogen even surface roughness on the properties of the modified surface layer of different kinds of stainless steel substrates [11-16]. Therefore, the present study focuses on the effect of cooling rate on the microstructure and mechanical properties of the treated layers.

EXPERIMENTAL DETAILS

AISI 304 austenitic stainless steel substrates were cut into coupons with dimensions 20 mm x 15 mm x 3 mm. The chemical composition of austenitic substrate is 0.5 wt.% Si, 1.2 wt.% Mn, 8.5 wt.% Ni, 19.1 wt.% Cr, 0.075 wt.% C and 69.95 wt.% Fe. The nitriding process was carried out using inductively coupled rf plasma with a continuous mode of operation. The discharge was generated by a three-turn copper induction coil energized from a 13.56 MHz rf power supply through a tunable matching network. The sample is mainly heated using the only source of rf plasma field. The treatment temperature is measured during the rf plasma process by a Chromel–Alumel thermocouple, which was lightly pressed on the surface of a blank sample. The untreated samples were only cleaned in acetone before entering the reactor tube of the rf plasma system. To meet the optimum condition of nitriding, the distance between the surface of the samples and rf coil was fixed at 21 mm. Nitrogen gas was introduced into the reactor tube to increase the base pressure from 7x10-3 to about 8x10-2 mbar. All samples were treated at a fixed input plasma processing power of 450 W for 10 min while the water flow rate is varied from 100 to 2000 cm3 /min. At the end of the process, the nitrided sample was allowed in the evacuated atmosphere of nitrogen in the reactor tube until it cooled down to the room temperature.

In order to obtain a highly reflective surface of the nitrided layers for cross-section morphology investigation, the specimens should be carefully cut, grinded and exposed to subsequent polishing operations before they can be examined under optical microscope. Through this work, low speed saw of ISOMETTM is used for precise and deformation-free cutting of the treated specimens into small work pieces. Grinding was accomplished by abrading the specimen surface through a sequence of operations using progressively finer abrasive grit (silicon carbide) sizes from 40 mesh through 150 mesh were used as coarse abrasives, and grit sizes from 180 mesh through 400 mesh as fine abrasives. Polishing process was started by the abrasive grit of 600 and 1200 mesh. After that, the micro polish of Alumina suspensions (0.3 and 0.1 micron) ware used on the top of the laps to achieve high quality polished surface mirror like. Finally, the specimens were washed and swabbed in warm running water. To show the morphology of the cross section of the treated samples, 2% Nital etcher was used for exposure time of 30 sec. Then, the layer thickness was measured by a micrometer scale attached with the optical microscope and visibly confirmed by the optical images.

The microstructure of the nitrided layers was characterized by Philips x-ray Diffractometer using MoKα radiation (λ = 0.70930 A˚ ). A glow discharge optical spectroscopy (GDOS) was utilized to measure the elemental concentration depth profiles. Scanning electron microscopy (SEM) was employed to study the microstructure of treated samples. Vickers microhardness measurements were taken, at room temperature, using 100 g load.

RESULTS AND DISCUSSIONS

 Surface temperature and temperature gradient

Table (1) shows surface temperature and cooling rates values of austenitic stainless steel samples. It has been found that, the surface temperature was found to decrease continuously from 590 to 490 o C with increasing the cooling rate from 100 to 2000 Cm3 /min. 

Glow discharge optical spectroscopy (GDOS) analysis

Fig.1 (a-c) shows the elemental concentration depth profiles of the constitutive elements iron (Fe), chromium (Cr), nickel (Ni), oxygen (O), carbon (C), and nitrogen (N) for samples treated at different cooling rates of 100, 900 and 2000 Cm3 /min. The concentration of nitrogen has approximately the same value of 25±1 at.% in the near surface region for all nitrided substrates. The low variation of nitrogen concentration, at different cooling rates, is ascribed to that the austenitic substrates were treated at fixed plasma processing power using the same flow rate of nitrogen. The substrates were achieving approximately the same plasma density of reactive nitrogen species which mainly controls nitrogen concentration gradient. In addition, the thermal contact between the bottom of the substrate and the top surface of the sample holder can be considered as a significant experimental factor, affecting treatment temperature and temperature gradient at fixed other processing parameters. This factor can be neglected in the present study by using austenitic substrates in a sheet form with the same dimensions positioned at the same location on the surface of the sample holder, besides cleaning the sample holder surface after each experiment to avoid any undesired coating might affect the thermal contact. However, it has observed that the amount of nitrogen diffused into the bulk substrate is a little bit low (20%) for the sample treated at low cooling rate of 100 Cm3 /min with respect to that ones were treated at relatively medium and high cooling rate of 900 and 2000 Cm3 /min, respectively.

The amount of nitrogen diffused underneath the surface is calculated by integrating the area under nitrogen depth profiles. Although the narrow variation of surface treatment temperature (490-590 oC), the nitrogen depth distribution has been found to be cooling rate dependent. Furthermore, the thickness of nitrogen saturation region and the total thickness of the nitrided layer have maximum values at a relatively moderate cooling rate of 900 Cm3 /min. This might leads to that the diffusion process of reactive nitrogen species toward the bulk substrate can be partially controlled by adjusting the cooling rate. 
Moreover, a small amount about 5 at % of carbon has found to be diffused underneath the surface for all treated samples due to the existence of hydrocarbon gases and carbon contamination in the reactor tube while the nitriding is processed at relatively high base pressure of 7x10-3 mbar [17].
Microstructural Analysis

The cooling rate and the plasma density can be considered as main parameters; control the microstructure of the nitrided layer. Therefore, it is more appropriate to demonstrate the microstructure of the treated samples as a function of cooling rate. Fig. 2(a-d) indicates the microstructure of the untreated austenitic substrate and the nitrided sample at different cooling rates of 100, 900 and 2000 Cm3 /min. A typical pattern of the untreated substrate is examined and shown here for comparison (Fig. 2-a). The microstructure of the nitrided layer for all treated samples shows the existence of solid solution phase of expanded austenite (γN), chemical compound phases of chromium nitrided (CrN) and iron nitride (Fe4 N). It is observed that the supersaturation phase of γN is detected with much higher diffraction intensity than other nitride phases overall cooling rates. Furthermore, the chemical compound phases such as Fe4 N has been detected with lower intensity in the nitrided layer were created out at low surface temperature (high cooling rate) compared to that ones were created out at medium and high surface temperatures (low cooling rate). These results are in a good agreement with previous work of Williamson et al. [18], who reported that γN can be formed near 500˚C or even 600˚C provided that the plasma processing time at these treatment temperatures is not too long. Other research groups concluded that the formation of γN is surface temperature dependent [18-21]. However, at surface temperature of 500˚C and below, chromium nitrides was found to be barely observed and decreased with decreasing the surface temperature [22- 23]. From figure 2 (b-d) that nitrogen expanded austenite phase achieves low lattice expansion of 1.34%, 2%, and 1.5% for samples nitrided at cooling rates of 100, 900 and 2000 Cm3 /min, respectively.


Surface characterization and cross-section morphology
Fig.3 (a-d) shows the surface morphology of untreated and treated samples at different cooling rates. As one can see from Fig.3 (a) the untreated surface seems microcracks, non-uniform and randomly packed grains with thin boundaries. Furthermore, undistributed micro-porous, some dislocations and cavities have been found on the surface. At low cooling rate (Fig. 3(b)) the nitrided sample is characterized by smaller and less linked grains. Moreover, the dark precipitations of CrN are still observed in the microstructure. At medium cooling rate (Fig. 3(c)), heavy precipitation of chromium nitride and relatively small grains are observed for nitrided sample. At high cooling rate, as shown in Fig.3d, the nitrided surface is characterized by a relatively smaller grain size compared to that one of the untreated substrate. Moreover, few dark precipitations are observed on the grain boundaries. It was previously [24] found that the dark precipitation is related to the chemical compound phase of CrN. Similar surface morphology has been observed for nitrided stainless steel at treatment temperature ranging from 490 to 590 o C [25]. The surface morphology we have seen is in a good agreement with the reflected pattern obtained from XRD where the intensity of CrN is found to be depended on the cooling rate.