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1. Department of Materials and Metallurgical Engineering, Tripoli University, Tripoli, Libya
2. School of Chemical Engineering and Advanced Materials, Newcastle University, UK
Received: November 17, 2011 / Accepted: December 01, 2011 / Published: February 25, 2012.
Abstract: The tribological performance of austenitic stainless steel is often limited by its relatively low hardness and tendency to suffer severe adhesive wear (galling) in sliding contacts. In this study, the wear resistance of austenitic stainless steel modified by two different surface treatments, namely colossal supersaturation case carburizing (known as Kolsterising) and sputtered TiN coating has been evaluated. As the colossal supersaturation process for surface carburizing of stainless steel is a relatively new surface treatment, the carburized case was characterized in some detail. Both treatments resulted in increase in surface hardness, however the surface properties produced by the two methods are different (i.e., the thickness, composition (and the hardness) of the surface layer) and they may be useful for different applications. For the purpose of comparison, both types of treatment were tested using the micro-abrasion test under identical conditions to determine the specific wear rate (k). This showed that both techniques had improved the wear resistance of austenitic stainless steel, however, the wear resistance of the TiN coated samples is about one order of magnitude higher than that of the carburized austenitic stainless steel.
Key words: Austenitic stainless steel, abrasive wear testing, case carburizing, TiN coating.
PACS: 68.35 Gy Mechanical Properties; Surface Strains.
1. Introduction
Various types of stainless steels find widespread use in many applications from the food industry to the petrochemical and petroleum industry. In general, the main selection criterion is corrosion resistance in various working environments, though mechanical and thermal properties may also be important. Austenitic and ferritic stainless steels are commonly used as they can be readily welded and posses adequate toughness and sufficient ductility to meet the requirements of resistance against catastrophic and premature failure. These grades however are soft and therefore their wear and erosion resistance are often points of concern.
Surface treatments aimed at improving the surface hardness without impairing the corrosion resistance of stainless steels would be very beneficial and attractive. Surface coating by CVD for example and plasma nitriding have been used to improve the surface hardness of stainless steels [1, 2]. Another relatively new treatment known as Kolsterising or colossal carbon supersaturation thermochemical treatment which is a low temperature carburizing of stainless steel aimed at improving surface hardness by carbon diffusion without impairing the corrosion resistance due to chromium carbide formation is now commercially available. The aim of this work is to evaluate the wear resistance of the carburized stainless steel produced by this process.
3. Characerization of the Carburized Layer
Fig. 1a shows a reflected light micrograph of the carburized layer and the substrate beneath. The thickness of the layer was found to be 35 μm as claimed. No structural details could be obtained by either the light microscope or the SEM as the carburized layer was metallographically featureless in contrast to the substrate material. The thickness of the layer is homogenous along the inspected cross section. Fig. 1b is a higher magnification scanning electron micrograph within the layer showing a de-cohesion of an inclusion within the hardened layer which was also observed by Jun Qu et al. [3].
The X-ray diffraction pattern obtained from the carburized layer is shown in Fig. 2. Comparing the X-ray profile for the carburized samples with normal 304SS revealed that peaks from the diffracting austenite planes marked A in Fig. 2 are shifted to
with the metallographic results. The surface hardness is almost three times the hardness of the uncarburized material. A similar trend in contact modulus was observed (Fig. 4b) but the increase was only about 20% and there was more scatter in the measured data. The thickness of the carburized layer is less clearcut in the contact modulus data which is not surprising given that elastic effects are more long range than plasticity and the uncarburized material properties will have an influence on the data obtained in the carburized region.
scars produced by 50 g load for 500 and 1000 revolutions respectively.
4.2 Results for TiN Coated Samples
TiN coated samples were tested with the same conditions as for the carburized samples. The summary of the test results and the plot of SN/Vc
austenite phase which should decline from the surface to the specimen interior. The increase in hardness is due to the supersaturation of the austenite phase with the interstitial carbon atoms which also resulted in a high surface compressive residual stresses as shown in Fig. 3. The high surface compressive residual stress is very beneficial to improve the fatigue strength of components subjected to cyclic stress loading.
The Young’s Modulus of austenitic stainless steel is typically 190-201 GPa [13] which is higher than the values measured here. This is due to the effect of pile-up which reduces the measured hardness and
modulus in materials with high E/H that do not significant strain hardening during indentation as in the uncarburized material here. Pile-up effects have been shown to be important in the assessment of carburized steels previously and correction factors can be determined by measuring the areas of the residual impressions by atomic force microscopy [14]. It is clear that the absolute values of the mechanical properties of steel obtained by nanoindentation should be treated with caution unless such corrections have been applied but never-the-less the trends can be assessed from the uncorrected data.
The Kolsterised layer on 304 SS samples in this work is almost identical to the Kolsterised layers on 316 SS reported previously [3, 11]. The ability to perform the treatment at moderate temperature retards the formation of chromium carbides due to the very low diffusion coefficient of substitutional atoms such as Cr in the austenite matrix thus preventing Cr depletion and preserving the corrosion resistance of the austenitic stainless steel [15]. No chromium carbides were detected by XRD here confirming this mechanism. The formation of Hag carbide or χ in the Kolsterised layer was also confirmed in this work but its size, distribution and amount should be determined to assess its effect on corrosion resistance and mechanical properties, this is the subject of further work.
The type of wear scars obtained for both types of specimens were groove scars or two body abrasion mode Figs. 5 and 7 [8]. The groove abrasion mode is expected under the applied testing conditions and the hardness level of the test samples and the testing ball as demonstrated by Adachi and Hutchings [8]. The condition of three body abrasion (or rolling particle motion) is less sensitive to test conditions and more reproducible test results could be obtained. However the criterion for three body abrasion as stated by Adachi and Hutchings [8] could not be met in this study due to the relative hardness of test sample to test ball.
6. Conclusions
The Kolsterising process has been suggested as a potentially useful treatment to develop a wear resistant surface on stainless steel without compromising its corrosion resistance. The work carried out here tends to support this assertion for 304 stainless steel. The low temperature carburizing process does not result in the formation of chromium carbides and leads to a high hardness, wear resistant supersaturated solid solution of carbon in austenite. Although the tribological properties of this layer are not as good as those of PVD titanium nitride which has been used to protect stainless steel components for some time, the fact that a thicker treated layer is possible suggests that the treatment may be used for components where high contact stresses and oscillating stresses are expected.
Acknowledgments
This work was supported by the University of Newcastle and the Libyan government through its visiting scholar scheme. V. Moorthy is thanked for help with the stress measurements.
References
[1] N. Yamauchi, A. Okamoto, H. Tukahara, K. Demizu, N. Ueda, T. Sone, et al., Diamond-like carbon films deposited on polymers by plasma-enhanced chemical vapor deposition, Surf. Coat. Technol. 174-175 (2003) 465-469.
[2] A. Recco, D. Lopez, A.F. Bevilacqua, F. Silva, A.P. Tschiptschin, Improvement of the slurry erosion resistance of an austenitic stainless steel with combination of surface treatment: nitriding and TiN coating, Surf. Coat. Technol. 202 (2007) 993-997.
[3] J. Qu, P.J. Blau, B.C. Jolly, Tribological properties of stainless steels treated by colossal carbon supersaturation, Wear 263 (2007) 719-726.
[4] B.D. Cullity, Elements of X-Ray Diffraction, 2nd Edition, Addison-Wesley, Reading, MA 1978.
[5] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564-1580.
[6] K.L. Rutherford, I.M. Hutchings, A micro-abrasive wear test with particular application to coated systems, Surf. Coat. Technol. 79 (1996) 231-239.
[7] M.G. Gee, A. Gant, I. Hutchings, R. Bethke, K. Schiffman, K. Acker, et al., Progress towards standardization of ball cratering, Wear 255 (2003) 1-13.
[8] K. Adachi, I.M. Hutchings, Wear-mode mapping for the micro-scale abrasion test, Wear 255 (2003) 23-29.
[9] K. Adachi, I.M. Hutchings, Sensitivity of wear rates in the micro-scale abrasion test to test conditions and material hardness, Wear 258 (2005) 318-321.
[10] Y. Kusano, K. Acker, I.M. Hutchings, Methods of data analysis for micro-scale abrasion test on coated substrates, Surf. Coat. Technol. 183 (2004) 312-327.
[11] Y. Cao, F. Ernst, G.M. Michal, Colossal carbon supersaturation in austenitic stainless steels carburized at low temperature, Acta Materialia 51 (2003) 4171-4181.
[12] K. Farrell, E.D. Specht, J. Pang, L.R. Walker, A. Rar, J.R. Mayotte, Characterization of carburized surface layer on an austenitic stainless steel, J. Nucl. Mat. 343 (2005) 123-133.
[13] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, Butterworth Heineman, Oxford, 1992.
[14] A. Oila, S.J. Bull, Nanoindentation testing of gear steels, Zeitschrift fur Metallkunde 94 (2003) 793-797.
[15] G.M. Michal, F. Ernst, H. Kahn, Y. Cao, F. Oba, N. Agarwal, et al., Carbon supersaturation due to paraequilibrium carburization: Stainless steels with greatly improved mechanical properties, Acta Materialia 54 (2006) 1597-1606.
[16] A. Leyland, A. Matthews, On the significance of the H/E ratio in wear control: A nanocomposite coating approach to optimized tribological behavior, Wear 246 (2000) 1-11.
[17] G.L. Sheldon, Galling resistant surfaces on stainless steel through electrospark alloying, J. Tribology 117 (1995) 343-349.
2. School of Chemical Engineering and Advanced Materials, Newcastle University, UK
Received: November 17, 2011 / Accepted: December 01, 2011 / Published: February 25, 2012.
Abstract: The tribological performance of austenitic stainless steel is often limited by its relatively low hardness and tendency to suffer severe adhesive wear (galling) in sliding contacts. In this study, the wear resistance of austenitic stainless steel modified by two different surface treatments, namely colossal supersaturation case carburizing (known as Kolsterising) and sputtered TiN coating has been evaluated. As the colossal supersaturation process for surface carburizing of stainless steel is a relatively new surface treatment, the carburized case was characterized in some detail. Both treatments resulted in increase in surface hardness, however the surface properties produced by the two methods are different (i.e., the thickness, composition (and the hardness) of the surface layer) and they may be useful for different applications. For the purpose of comparison, both types of treatment were tested using the micro-abrasion test under identical conditions to determine the specific wear rate (k). This showed that both techniques had improved the wear resistance of austenitic stainless steel, however, the wear resistance of the TiN coated samples is about one order of magnitude higher than that of the carburized austenitic stainless steel.
Key words: Austenitic stainless steel, abrasive wear testing, case carburizing, TiN coating.
PACS: 68.35 Gy Mechanical Properties; Surface Strains.
1. Introduction
Various types of stainless steels find widespread use in many applications from the food industry to the petrochemical and petroleum industry. In general, the main selection criterion is corrosion resistance in various working environments, though mechanical and thermal properties may also be important. Austenitic and ferritic stainless steels are commonly used as they can be readily welded and posses adequate toughness and sufficient ductility to meet the requirements of resistance against catastrophic and premature failure. These grades however are soft and therefore their wear and erosion resistance are often points of concern.
Surface treatments aimed at improving the surface hardness without impairing the corrosion resistance of stainless steels would be very beneficial and attractive. Surface coating by CVD for example and plasma nitriding have been used to improve the surface hardness of stainless steels [1, 2]. Another relatively new treatment known as Kolsterising or colossal carbon supersaturation thermochemical treatment which is a low temperature carburizing of stainless steel aimed at improving surface hardness by carbon diffusion without impairing the corrosion resistance due to chromium carbide formation is now commercially available. The aim of this work is to evaluate the wear resistance of the carburized stainless steel produced by this process.
3. Characerization of the Carburized Layer
Fig. 1a shows a reflected light micrograph of the carburized layer and the substrate beneath. The thickness of the layer was found to be 35 μm as claimed. No structural details could be obtained by either the light microscope or the SEM as the carburized layer was metallographically featureless in contrast to the substrate material. The thickness of the layer is homogenous along the inspected cross section. Fig. 1b is a higher magnification scanning electron micrograph within the layer showing a de-cohesion of an inclusion within the hardened layer which was also observed by Jun Qu et al. [3].
The X-ray diffraction pattern obtained from the carburized layer is shown in Fig. 2. Comparing the X-ray profile for the carburized samples with normal 304SS revealed that peaks from the diffracting austenite planes marked A in Fig. 2 are shifted to
with the metallographic results. The surface hardness is almost three times the hardness of the uncarburized material. A similar trend in contact modulus was observed (Fig. 4b) but the increase was only about 20% and there was more scatter in the measured data. The thickness of the carburized layer is less clearcut in the contact modulus data which is not surprising given that elastic effects are more long range than plasticity and the uncarburized material properties will have an influence on the data obtained in the carburized region.
scars produced by 50 g load for 500 and 1000 revolutions respectively.
4.2 Results for TiN Coated Samples
TiN coated samples were tested with the same conditions as for the carburized samples. The summary of the test results and the plot of SN/Vc
austenite phase which should decline from the surface to the specimen interior. The increase in hardness is due to the supersaturation of the austenite phase with the interstitial carbon atoms which also resulted in a high surface compressive residual stresses as shown in Fig. 3. The high surface compressive residual stress is very beneficial to improve the fatigue strength of components subjected to cyclic stress loading.
The Young’s Modulus of austenitic stainless steel is typically 190-201 GPa [13] which is higher than the values measured here. This is due to the effect of pile-up which reduces the measured hardness and
modulus in materials with high E/H that do not significant strain hardening during indentation as in the uncarburized material here. Pile-up effects have been shown to be important in the assessment of carburized steels previously and correction factors can be determined by measuring the areas of the residual impressions by atomic force microscopy [14]. It is clear that the absolute values of the mechanical properties of steel obtained by nanoindentation should be treated with caution unless such corrections have been applied but never-the-less the trends can be assessed from the uncorrected data.
The Kolsterised layer on 304 SS samples in this work is almost identical to the Kolsterised layers on 316 SS reported previously [3, 11]. The ability to perform the treatment at moderate temperature retards the formation of chromium carbides due to the very low diffusion coefficient of substitutional atoms such as Cr in the austenite matrix thus preventing Cr depletion and preserving the corrosion resistance of the austenitic stainless steel [15]. No chromium carbides were detected by XRD here confirming this mechanism. The formation of Hag carbide or χ in the Kolsterised layer was also confirmed in this work but its size, distribution and amount should be determined to assess its effect on corrosion resistance and mechanical properties, this is the subject of further work.
The type of wear scars obtained for both types of specimens were groove scars or two body abrasion mode Figs. 5 and 7 [8]. The groove abrasion mode is expected under the applied testing conditions and the hardness level of the test samples and the testing ball as demonstrated by Adachi and Hutchings [8]. The condition of three body abrasion (or rolling particle motion) is less sensitive to test conditions and more reproducible test results could be obtained. However the criterion for three body abrasion as stated by Adachi and Hutchings [8] could not be met in this study due to the relative hardness of test sample to test ball.
6. Conclusions
The Kolsterising process has been suggested as a potentially useful treatment to develop a wear resistant surface on stainless steel without compromising its corrosion resistance. The work carried out here tends to support this assertion for 304 stainless steel. The low temperature carburizing process does not result in the formation of chromium carbides and leads to a high hardness, wear resistant supersaturated solid solution of carbon in austenite. Although the tribological properties of this layer are not as good as those of PVD titanium nitride which has been used to protect stainless steel components for some time, the fact that a thicker treated layer is possible suggests that the treatment may be used for components where high contact stresses and oscillating stresses are expected.
Acknowledgments
This work was supported by the University of Newcastle and the Libyan government through its visiting scholar scheme. V. Moorthy is thanked for help with the stress measurements.
References
[1] N. Yamauchi, A. Okamoto, H. Tukahara, K. Demizu, N. Ueda, T. Sone, et al., Diamond-like carbon films deposited on polymers by plasma-enhanced chemical vapor deposition, Surf. Coat. Technol. 174-175 (2003) 465-469.
[2] A. Recco, D. Lopez, A.F. Bevilacqua, F. Silva, A.P. Tschiptschin, Improvement of the slurry erosion resistance of an austenitic stainless steel with combination of surface treatment: nitriding and TiN coating, Surf. Coat. Technol. 202 (2007) 993-997.
[3] J. Qu, P.J. Blau, B.C. Jolly, Tribological properties of stainless steels treated by colossal carbon supersaturation, Wear 263 (2007) 719-726.
[4] B.D. Cullity, Elements of X-Ray Diffraction, 2nd Edition, Addison-Wesley, Reading, MA 1978.
[5] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564-1580.
[6] K.L. Rutherford, I.M. Hutchings, A micro-abrasive wear test with particular application to coated systems, Surf. Coat. Technol. 79 (1996) 231-239.
[7] M.G. Gee, A. Gant, I. Hutchings, R. Bethke, K. Schiffman, K. Acker, et al., Progress towards standardization of ball cratering, Wear 255 (2003) 1-13.
[8] K. Adachi, I.M. Hutchings, Wear-mode mapping for the micro-scale abrasion test, Wear 255 (2003) 23-29.
[9] K. Adachi, I.M. Hutchings, Sensitivity of wear rates in the micro-scale abrasion test to test conditions and material hardness, Wear 258 (2005) 318-321.
[10] Y. Kusano, K. Acker, I.M. Hutchings, Methods of data analysis for micro-scale abrasion test on coated substrates, Surf. Coat. Technol. 183 (2004) 312-327.
[11] Y. Cao, F. Ernst, G.M. Michal, Colossal carbon supersaturation in austenitic stainless steels carburized at low temperature, Acta Materialia 51 (2003) 4171-4181.
[12] K. Farrell, E.D. Specht, J. Pang, L.R. Walker, A. Rar, J.R. Mayotte, Characterization of carburized surface layer on an austenitic stainless steel, J. Nucl. Mat. 343 (2005) 123-133.
[13] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, Butterworth Heineman, Oxford, 1992.
[14] A. Oila, S.J. Bull, Nanoindentation testing of gear steels, Zeitschrift fur Metallkunde 94 (2003) 793-797.
[15] G.M. Michal, F. Ernst, H. Kahn, Y. Cao, F. Oba, N. Agarwal, et al., Carbon supersaturation due to paraequilibrium carburization: Stainless steels with greatly improved mechanical properties, Acta Materialia 54 (2006) 1597-1606.
[16] A. Leyland, A. Matthews, On the significance of the H/E ratio in wear control: A nanocomposite coating approach to optimized tribological behavior, Wear 246 (2000) 1-11.
[17] G.L. Sheldon, Galling resistant surfaces on stainless steel through electrospark alloying, J. Tribology 117 (1995) 343-349.