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Int. J. Materials Engineering Innovation, Vol. 5, No. 1, 2014
Nanoindentation and microstructure of hybrid treated of AISI 316L at low temperature Askar Triwiyanto*, Patthi Hussain and Mokhtar Che Ismail Department of Mechanical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Bandar Seri Iskandar Perak, Malaysia Fax: +605-365-6461 E-mail:
[email protected] E-mail:
[email protected] E-mail:
[email protected] *Corresponding author Abstract: This paper presents the characterisation of hybrid treated layer on austenitic AISI 316L stainless steels using field emission scanning electron microscope (FESEM), universal scanning probe microscope (USPM) and nanoindentation after low temperature thermochemical treatments. By using these methods, the improvement of its mechanical properties due to the diffusion of carbon and nitrogen at low temperature treatments were confirmed. The hybrid treated layer has shown increment of hardness and elastic modulus compared to untreated sample. Based on the investigation, it is shown that all treated samples have enhanced E/H ratio which demonstrated the decreasing tendency to plastic deformation and reduced the disparity of properties, while keeping deformation within the elastic range. Keywords: nanoindentation; hybrid; characterisation; morphology; elastic modulus. Reference to this paper should be made as follows: Triwiyanto, A., Hussain, P. and Ismail, M.C. (2014) ‘Nanoindentation and microstructure of hybrid treated of AISI 316L at low temperature’, Int. J. Materials Engineering Innovation, Vol. 5, No. 1, pp.38–47. Biographical notes: Askar Triwiyanto is a Researcher in Surface Modification of Materials at Universiti Teknologi Petronas (UTP), Malaysia. He has expertise in the field of materials engineering, energy engineering, mechanical engineering, surface engineering, anti-corrosion materials, failure analysis, materials characterisation, and welding technology. He received silver medal awards on SEDEX 2011, Malaysia, and a research grant from Ministry of Higher Education of Malaysia and STIRF for low temperature thermochemical treatment on stainless steel to improve mechanical properties without impairing corrosion resistance. Currently, he has published more than ten international journals and a book chapter. He is also an active member in many professional institutions such as ASME, NACE, and ASM. Patthi Hussain completed his PhD and Master studies in the field of metal-ceramic joining from University of Strathclyde-Glasgow, UK. He has expertise in the field of powder metallurgy, field metallography, metallurgy, failure analysis, metal-ceramic joining, interface study between metal and ceramic, interface study between sialon and stainless steel, bonded NdFeB Copyright © 2014 Inderscience Enterprises Ltd.
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magnet. He received gold medal awards on INNOVA 2009, Brussels, Belgium and patent for a method for joining an engineering ceramic to metal, on 2011. Currently, he is working in Universiti Teknologi Petronas (UTP), Malaysia, as an Associate Professor in Department of Mechanical Engineering and Head of Centre for Corrosion Research (CCR-UTP). Mokhtar Che Ismail completed his PhD studies in the field of corrosion engineering from University of Manchester, UK. He has expertise in the field of CO2 corrosion, risk-based inspection, corrosion under insulation, plant corrosion, failure analysis, corrosion training in corrosion management, corrosion monitoring, quality control, materials selection and NDT. He completed his Masters of Science in Material Science and Engineering from National University of Singapore. Currently, he is working in Universiti Teknologi Petronas (UTP), Malaysia, as an Associate Professor in Department of Mechanical Engineering. This paper is a revised and expanded version of a paper entitled ‘Microstructure and nanoindentation characterization of low temperature hybrid treated layer on austenitic stainless steel’ presented at The International Conference on Manufacturing, Optimization, Industrial and Material Engineering (MOIME 2013), Bandung, Indonesia, 9–10 March 2013.
1
Introduction
The wide ranging application of austenitic stainless steel (ASS) is used due to its excellent corrosion resistance and good formability. However, some disadvantages of this material are poor wear and friction behaviour. This relatively low hardness as well as poor wear resistance of this material is derived from inherent austenitic structure which hinders a wider applicability. According this, surface engineering treatments for ASS stainless steel are alternative ways to increase the surface hardness and improving the wear resistance. The gaseous thermochemical treatments to improve surface properties of material are typically carried out in carbon and/or nitrogen bearing gases and usually associated with temperature above 500°C (Triwiyanto et al., 2012). On the other hand, there was a phenomena where sensitisation occurs which refers to the degradation in corrosion resistance where of exposing the stainless steels in the operating temperature range from 550°C to 850°C. This problem is occurs where precipitation of chromium carbides or nitrides at the grain boundaries at elevated temperatures, typically between 450 to 850°C; the formation of chromium nitride/carbide leads to the depletion of Cr from its matrix in austenitic solid solution and consequently unable to produce Cr2O3 passive layer to make stainless feature. As a result, it reduces the corrosion resistance property of the stainless steel as well as decreasing in ductility, toughness and aqueous corrosion resistance (Clark and Varney, 1962). The efforts have been made to modify the surfaces of these materials and improving their surface hardness, wear resistance as well as corrosion resistance by producing harden layer where the diffusion of nitrogen and carbon respectively in the austenite lattice were occur, this structure termed as expanded austenite is a supersaturated with nitrogen and carbon respectively (Lewis et al., 1993; Thaiwatthana et al., 2002a). More recently, a hybrid process has also been developed, which introducing both nitrogen and carbon gases in a single process cycle into the austenite lattice simultaneously to form a
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hardened zone comprising a nitrogen expanded austenite layer on top of a carbon expanded austenite layer (Tsujikawa et al., 2005; Sun and Haruman, 2008; Li et al., 2010). Recently, some investigators claim that low temperature hybrid process of nitriding-carburising of ASS is possible in conventional tube furnace (Triwiyanto et al., 2011). The low temperature treatments can be performed at temperatures up to 450°C to diffuse a large amount of nitrogen and/or carbon in the treated layer of up to 20 µm thick, to form an expanded austenite (γx) (X = N, C) layer without formation precipitation of nitride/carbide which related to excellent corrosion resistance as well as wear performances (Thaiwatthana et al., 2002b; Christiansen and Somers, 2006). So far, the surface and near-surface mechanical properties of thin films or coatings can be related to the final performance of materials. As a result, the need of better understanding in the field of depth-sensing nanoindentation would provides a quantitative method for mapping the mechanical properties, such as hardness and elastic modulus of the near-surface region (Pollock, 1992; Doerner and Nix, 1986). In order to investigate the high precision measurement of mechanical properties for thin layer nanomechanical characterisations were performed on all treated and substrate materials. Nanoindentation was carried out to measure the mechanical properties of the implanted surface layers using nano system 600, manufactured by Micromaterials UK, the schematic diagram for nano system is shown in Figure 1. Figure 1
A schematic diagram of the NanoTest instrument (see online version for colours)
Source: Tehrani et al. (2011)
Nanoindentation and microstructure of hybrid treated of AISI 316L
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The objective of present work is to characterise the expanded austenite layer on ASS substrates which representing a harden layer on a softer substrate respectively using the nanoindentation and its microstructure. The hardness of expanded austenite is determined by the Oliver and Pharr method. In order to achieve proper measurement the use of very high applied load is avoided to minimise substrate effect where higher penetration of indenter which probably overtook the diffusion zone and entering the substrate bulk region (Oliver and Pharr, 1992).
2
Experimental
The substrate material used throughout the experimental series with standard metallographical surface preparation was to be made prior to the treatment were AISI 316L type ASS of following chemical compositions (in wt.%): 17.018 Cr, 10.045 Ni, 2.00 Mo, 1.53 Mn, 0.03 C, 0.048 Si, 0.084 P, 0.03 S and balance Fe. This steel was supplied in the form of 2 mm thick hot-rolled plate. Samples of 20 mm × 70 mm size rectangular coupon were cut from the plate. The sample surface was ground on 320, 600, 800, 1,000, 1,200 grit SiC papers, and then polished using 1 μm Al2O3 pastes to the mirror finish. Before treating, the polished samples need to be wash with detergent and degreased in acetone and thoroughly wash in distilled water and further subject to ultrasonic cleaning in acetone for 10 min. Hybrid treatments were performed at 400 and 450°C in a horizontal tube furnace which involving reactive gases both NH3 (for nitriding) and CH4 (for carburising) for a total duration of 8 h. The gases flow were controlled by Aalborg flowmeter and the linear flow rate of gas mixture through the alumina retort were conditioned by gas mixing tank. In this system the sample were be placed in the quartz boat of about 12 cm in the centre of isothermal zone of electric resistance tube furnaces. Prior to treating, the specimens were soaked in concentrated HCl (2 M) solution for 15 minutes duration with the purpose to remove the native oxide film that commonly forms on stainless steel and protects the metal matrix from corrosion. This oxide layer is believed to act as a barrier for diffusional nitrogen transport (Yamauchi et al., 2001; ASM Committee on Gas Carburizing, 1991). After thermochemical treatments, the specimens were quenched in water. The cross sectioned treated specimens were first characterised by metallographic examination. To reveal the microstructure, the polished surfaces were etched in Marble’s solution (4 g CuSO4 + 20 ml HCl + 20 ml distilled water). The structure and morphology of specimens were characterised using field emission scanning electron microscope (FESEM) (Zeiss Supra 55VP) and universal scanning probe microscope (USPM) (NanoNavi: E-sweep) to reveal 3D surface topographical profile at higher resolutions. Indentation measurements were performed by using rigid indenters. This nanoindentation also known as depth-sensing indentation is a nanomechanical property testing system with a Berkovich tip to measure force versus displacement data quantitatively and determines the elastic modulus E and hardness H of materials at nano and micrometer scales.
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Table 1
Treatment conditions and their corresponding layer thicknesses
Treatment
Time (h)
Temp (°C)
Hybrid process 316L
8
Hybrid process 316L
8
Gas (%)
Layer thickness (μm)
CH4
NH3
N2
400
5
20
75
3.421
450
5
20
75
6.425
In the NanoTest unit as shown in Figure 1, forces are generated by means of a coil and magnet system located at the top of a pendulum arrangement and displacements of the probe into the surface are monitored with a sensitive capacitor plate arrangement. In the recent measurement, the nanoindentation techniques were set as described in Table 2. Table 2
Instrumented hardness settings
Variable Type of experiment
Value Depth vs. load
Type of indentor
Berkovich pyramid
Maximum load
300 mN
Maximum depth
200 nm
Dwelling time at maximum load Loading rate
3
5s 3.99 mN/s
Results and discussion
3.1 Layer morphology and surface topography analysis Different morphologies of harden layers as describes in Table 1 were observed as a result of the various treatment conditions and the thicknesses of the layers produced in different specimens. According to the micrograph in Figure 2, expanded austenite layer is recognised as a featureless surface layer. For a similar treatment duration, the plasma process is reported (Li et al., 2010; Michalski et al., 2007) to produce about 18 μm thick layer which is much higher compared to that of the present conventional hybrid treatment in horizontal tube furnace. In plasma process, the native oxide layer was removed mostly by bombardment of the plasma gas which is completely absent in conventional process. This is one of the reasons why conventional horizontal tube furnace produced small layer thickness compared to the corresponding plasma nitriding. Previous investigation revealed that nitriding at 450°C became effective after treatment for 6 h where a continuous treated layer was produced (Haruman et al., 2006). The two specimens processed under hybrid treatment conditions as shown in Figure 2, actually produced duplex layers although not clearly revealed in the present micrograph, this separation of dual structures are observable under SEM.
Nanoindentation and microstructure of hybrid treated of AISI 316L Figure 2
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(a) Top view image from USPM, cross-sectional optical micrograph (b) Hybrid 400°C and (c) hybrid 450°C sample (see online version for colours)
10 μm
(a)
10 μm
(b)
(c)
3.2 Nanoindentation measurement profile The nanoindentation tests on AISI 316L untreated and hybrid treated samples has confirmed a considerable increase in hardness 4 to 5 order and a small rise in the elastic modulus of the material after the hybrid thermochemical treatments as describe in Table 3. The curves in Figure 4 has shown that there was an agreement with previous investigation which explained the resilience and toughness of the material are of significant importance, and the ratio of hardness to elastic modulus has been reported to be a more appropriate index than the mere hardness, to rank the wear resistance (Leyland and Matthews, 2000, 2004). Meanwhile, the best tribological performances have been reported for combinations of high surface hardness and relatively low elastic modulus, to reduce the tendency to plastic deformation and reduce the mismatch of properties, while keeping deformation within the elastic range which is an agreement with recent findings as describes in Table 3 due to the high nitrogen and/or carbon contents in the layers resulting from deposition process as describes elsewhere (Tsujikawa et al., 2005), a characteristics relief is usually observed which results in an increase in the surface roughness. Table 3
Roughness, indentation hardness and elastic modulus for each hybrid treatment
Treatment (°C)
Roughness, Ra (μm)
Hardness (GPa)
Elastic modulus (GPa)
E/H
Untreated
0.12
2 ± 0.2
210 ± 2
97.22
Hybrid 400°C
0.22
7.892 ± 0.7
167.518 ± 4.2
21.226
Hybrid 450°C
0.28
8.471 ± 0.5
173.64 ± 3.1
20.50
Hardness and elastic modulus values were determined by nanoindentation test using a nano test 600 apparatus from micromaterials instruments (Wrexham, UK) equipped with Berkovich indenter. The hardness and elastic modulus values have been extracted from the elastic unloading curves according to the equivalent indenter method. Performed in the same conditions, regular arrays of 6 × 6 indentations covering a 24.32 × 21.89 μm2 area were realised in order to probe about three areas which consist of substrate, interface and harden layer respectively. Observation on Figure 4 shows that indentation size on substrate is bigger than indentation on hardened layer which indicating that the hardened
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layer has higher hardness level than substrate which shows an agreement with previous work (Mridha, 2007; Oumarou et al., 2010). Figure 3
Depth-sensing indentations performed on (a) hybrid treated layer, (b) hybrid 400°C and (c) hybrid 450°C (see online version for colours)
(a)
(b) Figure 4
(c)
Depth-sensing indentations performed on hardened layer and substrate (white circles) Indentation on HARDENED LAYER with 5mN indentation load
Indentation on SUBSTRATE with 5mN indentation load
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3.3 Elemental profile of nitrogen and carbon on surface treated layer From the micrographs, it can be obviously observed that the typical nitrogen and carbon profiles measured by EDS with line scan mode for hybrid at different temperatures such as 400, and 450°C. It can be seen from Figure 5 that hybrid layers at higher temperature gave more carbon beyond nitrided layer, but some carbon remains in the sub-surface layer. This figure shows that very high amount of nitrogen near the surfaces, while maximum carbon concentration appears beneath as if carbon was ‘pushed’ to the middle of the layer by incoming nitrogen. A high nitrogen peak was detected at the surface, thus proving the push-in effect of dissolved carbon by nitrogen during the hybrid process for all specimens have also been reported in the literature (Oliver, 1992). Figure 5
Elemental profile of carbon and nitrogen on (a) hybrid 400°C and (b) hybrid 450°C (see online version for colours)
(a)
4
(b)
Conclusions
The microstructure of layer produced in horizontal tube process is not uniform in thickness under the same treatment conditions according to micrograph from FESEM. The layer thickness of hybrid treated at 450°C is 6.425 μm while at 400°C gave much smaller thicknesses for the same processing conditions. The elemental compositions on the surface had also changed after the hybrid process where nitrogen concentration is gradually decreasing from surface to the substrate core with distance increasing due to a low diffusion rate in the substrate at low temperature. However, some carbon remains in the sub-surface layer. USPM observation of all treated surfaces shows a higher surface roughness after treatments. Nanoindentation tests reveal a higher elastic modulus and hardness for all treated samples compared to the untreated. It is found that all treated samples have enhance E/H ratio exhibit the decreasing tendency to plastic deformation and reduce the mismatch of properties, while keeping deformation within the elastic range.
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Acknowledgements The authors would like to express their gratitude to Universiti Teknologi PETRONAS for supporting this research under STIRF Grant No. 49/2011.
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