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Improvement of cavitation erosion resistance of AISI 316 stainless steel by laser surface alloying using fine WC powder
Surface and Coatings Technology 165 (2003) 258–267
Improvement of cavitation erosion resistance of AISI 316 stainless steel by laser surface alloying using fine WC powder K.H. Loa, F.T. Chenga,*, C.T. Kwoka,b, H.C. Manc a
b
Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, P.O. Box 3001, Taipa, Macau, China c Department Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 6 July 2002; accepted in revised form 25 September 2002
Abstract Fine WC powder of approximately 1 mm size was employed as a convenient source of tungsten and carbon in the laser surface alloying of AISI 316 stainless steel for improving the cavitation erosion resistance. A slurry containing WC powder was preplaced on the substrate by pasting and processed with a high-power CW Nd:YAG laser to achieve surface alloying. The composition and microstructure of the alloyed layer and the phases formed were investigated by energy-dispersive X-ray spectroscopy, optical microscopy, scanning electron microscopy, and X-ray diffractometry, respectively. The cavitation erosion behavior of the laser surface-alloyed samples in 3.5% NaCl solution was studied with a vibratory cavitation erosion tester. The microhardness of the alloyed layer increases with the total W content in the layer. By employing proper processing parameters, an alloyed layer that is hard but not too brittle can be formed, with a cavitation erosion resistance that may reach more than 30 times that of the asreceived 316. The improvement in cavitation erosion resistance may be attributed to the increase of W in solid solution and to the precipitation of dendritic carbides, both resulting from the dissociation of the fine WC powder during laser processing. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser surface alloying; AISI 316 stainless steel; Cavitation erosion; Tungsten carbide
1. Introduction Cavitation in a fluid is defined as the generation and collapse of cavities (i.e. bubbles) due to local pressure fluctuation arising from sudden change in flow, or from vibration. When bubbles collapse, shock waves and micro-jets are emitted, thus exerting pressure pulses on a solid surface near by. The repetitive attack by these pressure pulses on the solid surface leads to fatigue, fracture and loss of material, and such an erosion mechanism is known as cavitation erosion w1x. Cavitation erosion is a common cause of failure in engineering parts in hydraulic machinery and other liquid-handling systems. AISI 316 stainless steel is a popular engineering alloy because of its excellent corrosion resistance. However, owing to the relatively low hardness (e.g. a Vickers *Corresponding author. Tel.: q852-2766-5691; fax: q852-23337629. E-mail address: [email protected] (F.T. Cheng).
microhardness HV of only approximately 200 kgfy mm2), high stacking-fault energy and low tendency to stress-induced martensitic transformation, its cavitation erosion resistance is low w2,3x. As cavitation erosion occurs at the liquidysolid interface, and is thus related to surface properties rather than bulk properties, the cavitation erosion resistance of a material may be improved by surface modification. Surface modification has the advantage of consuming only a small amount of expensive material on the surface while using an inexpensive substrate for the bulk. It also allows a large number of combinations of surface and bulk properties, and thus significantly increases the number of options for the design engineer. Laser surface modification is a modern surfacing technique which is becoming more and more popular in engineering applications. Compared with other methods of surface modification, it possesses four important characteristics: (1) possibility of forming alloys of nonequilibrium compositions, (2) formation of a fine micro-
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 3 9 - 9
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Table 1 Laser processing parameters and sample designations Sample
Laser power P (kW)
Scanning speed v (mmys)
Spot diameter d (mm)
Fluence F (Jymm2)a
Overlapping ratio (%)
WC-316-1 WC-316-2 WC-316-3 WC-316-4 LSM-316
1.50 1.75 2.10 2.10 1.50
25 20 20 35 25
4 4 4 5 4
15.0 22.0 26.0 12.0 15.0
50 50 50 50 50
a
The values of fluence were calculated for a single track. Owing to overlapping, the actual fluence received by a sample was twice the value shown.
structure, (3) presence of a metallurgical bond between the surface layer and the substrate, and (4) resulting in a small heat-affected zone w4x. Laser surface modification is especially suitable for local treatment at locations susceptible to erosion attack. Different types of laser surface modification have been attempted on AISI 316 stainless steel for improving the cavitation erosion resistance by the present authors, and varying degrees of improvement have been achieved w3,5,6x. The present study aims at laser surface alloying on AISI 316 stainless steel using fine WC powder for improving the cavitation erosion resistance. The incorporation of carbide particles in the laser surface modification of engineering alloys to enhance the wear resistance has been reported by a number of authors w7–9x. As coarse ceramic particles are beneficial to abrasive wear resistance, the WC used is coarse-grained and clad with a metal (Co or Ni) to minimize the degree of dissolution. Owing to a different mode of attack in cavitation erosion, coarse ceramic particles may not be advantageous as in the case of abrasive wear w10x. Rather, a uniform microstructure containing fine hard phases formed in situ may yield a high cavitation erosion resistance w11x. The present study is an attempt in this direction, with the use of fine unclad WC powder as a convenient source of alloying elements W and C. It has the additional advantage of avoiding the use of strategic or polluting elements like Co and Ni. 2. Experimental details 2.1. Materials and sample preparation As-received AISI 316 stainless steel (designated in this paper as As-316, with composition in wt.%: 18% Cr, 14% Ni, 3% Mo, 2% Mn, 1% Si, 0.08% C, 0.045% P, 0.03% S, balance Fe) was machined to discs of 12.7 mm diameter and 3.4 mm thickness. 2.2. Preplacement of alloying powder
220 grit SiC paper and then the slurry was painted on the sample and dried at 120 8C for 2 h. The painted layer was then polished with 1000 grit SiC to obtain a uniform preplaced coating of thickness of 0.3 mm. 2.3. Laser surface alloying Laser surface alloying was performed using a 2.5 kW CW Nd:YAG laser, with argon flowing at a rate of 20 lymin as the shielding gas. Preliminary trials with different values of laser power P, scanning speed v and laser spot diameter d were carried out to determine the feasible processing conditions. Four sets of processing parameters (shown in Table 1, with corresponding samples designated as WC-316-X) were chosen for further investigation of the alloyed layer. Laser surfacing was achieved by parallel tracks with 50% overlap. Such an overlapping ratio was chosen so as to compromise between surfacing efficiency and surface homogeneity. For comparison, laser surface-melted samples (designated in this paper as LSM-316) without the addition of WC were also prepared. 2.4. Metallographic and microstructural analysis After laser-treatment the samples were sectioned, polished, and etched with acidic chloride solution (25 g FeCl3, 25 ml HCl and 100 ml H2O). The average thickness of the alloyed layer was determined by image analysis. The microstructure of the alloyed layer was analyzed by scanning electron microscopy (SEM) and optical microscopy. The composition along the depth of the layer was determined by energy-dispersive X-ray spectroscopy (EDS) using a probing area of 30=30 mm2 at each point. The phases formed in the surface layer was determined by X-ray diffractometry (XRD) using Cu Ka as the radiation source (at 40 kV and 35 mA, with Ni filter). The Vickers microhardness at the surface of the samples was measured at a load of 200 g and a loading time of 15 s. 2.5. Cavitation erosion test
A slurry was prepared by mixing fine WC powder (particle size f1 mm) and a binder (4 wt.% polyvinyl alcohol, PVA). The sample surface was polished with
All the samples for the cavitation erosion test were polished with 1-mm diamond paste to ensure consistent
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260
Table 2 Properties of various laser surface-alloyed samples Sample
Melt depth D (mm)
Dilution ratio, h (%)a
Total W content (wt.%)
HV
Re (hymm)
Re*
WC-316-1 WC-316-2 WC-316-3 WC-316-4 LSM-316 As-316
0.35 0.40 0.47 0.31 0.60 –
14 25 36 3
70 52 44 Layer peeled off – –
1200 1000 900 Layer peeled off 220 200
320 1377 847 Layer peeled off 45 39
8.2 35.3 21.7 Layer peeled off 1.2 1
a
– –
The values of h were calculated based on the assumption that the preplaced layer was compact. Actual values should be higher.
surface roughness. Cavitation erosion tests were performed using an ultrasonic vibratory facility conforming to ASTM Standard G32-92 w12x, with the samples in the unattachment mode. The peak-to-peak amplitude and the vibration frequency used were 100 mm and 20 kHz, respectively, with a separation of 0.5 mm between the sample and the horn tip. The cavitating liquid was 3.5% NaCl solution kept at 23 8C. The sample was weighed at regular intervals of 30 min and converted to a cumulative mean depth of erosion (MDE) w6x. The mean erosion rate (MER) was calculated at the end of the test period, i.e. 240 min, and the reciprocal of MER was taken to be the cavitation erosion resistance Re. The surface morphology of the cavitated samples at the end of the test was investigated by SEM.
above, the preplaced layer was not a compact layer, and the actual values of h should be higher than the values obtained from this expression. With a suitable choice of processing parameters, a uniform alloyed layer free of cracks and pores was obtained, as shown in the optical
3. Results and discussion 3.1. Metallographic and microstructural analysis Under laser irradiation, the preplaced fine WC powder dissolved into the melt pool formed by melting a layer of the substrate. The melt pool rapidly solidified to form an alloyed layer. In the laser treatment, the laser power P, scanning speed v, and laser spot diameter d were varied in order to change the energy density (or fluence F) incident on the samples (FsPy(v=d)), and the interaction time (dyv). Although laser surface alloyingy cladding by powder injection is a one-step process and is also more versatile, laser alloying by powder preplacement is simple and the preplaced layer, which is inevitably porous, is expected to absorb laser energy more efficiently due to multiple reflections. Thus the fluence employed could be relatively low. The average thickness (D) of the alloyed layer for different samples is given in Table 2, together with the dilution ratio h calculated from hs(Dyt)yD, where t is the thickness of the preplaced layer. As pointed out
Fig. 1. Cross-sectional views of laser surface-alloyed samples (a) WC-316-2; (b) WC-316-4, an example of cracking.
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Fig. 2. Composition profiles of laser surface-alloyed samples (a) WC-316-1; (b) WC-316-2; (c) WC-316-3.
micrograph of the sample WC-316-2 in Fig. 1a. When F was too small, an insufficient amount of the substrate was melted and the resolidified layer cracked due to rapid cooling as shown in Fig. 1b, corresponding to the sample WC-316-4. On the other hand, when F was too large, an excessive amount of the substrate was melted, resulting in high dilution of the alloying elements. The composition profiles along the depth of the laser surface-alloyed samples are shown in Fig. 2. It can be seen that the elements were fairly evenly distributed,
with different relative amounts of the alloying elements corresponding to different processing conditions. In view of the fineness of the precipitates relative to the EDS probing area (30=30 mm2), the amount of an element shown in Fig. 2 represents the total amount of that element in solid solution and in the precipitates. The phases formed in the alloyed layer were identified using the XRD spectra shown in Fig. 3. As a result of the laser treatment, the fine WC powder dissolved in the melt pool and resolidified to form different types of
262
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Fig. 3. XRD spectra of laser surface-alloyed samples (a) WC-316-1; (b) WC-316-2; (c) WC-316-3.
carbides such as complex metal carbides M23C6, M7C3, M6C (MsFe, W, Cr), and tungsten carbides WC and W2C, together with the g-FeCrNiW phase. The SEM micrograph showing the microstructure of the laser surface-alloyed sample WC-316-2 is given in Fig. 4a. The microstructure, which is typical of one with high carbide content, is similar to the results reported by
Ayers and Gnanamuthu w13x and Riabkina-Fisherman w14x. Owing to the high concentration of C resulting from WC dissolution in the melt pool, metal carbides were first precipitated out as dendrites, with the interdendritic region composed of a eutectic of the gFeCrNiW phase and carbides. Absence of the original WC particles, which were angular in shape (Fig. 4b),
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263
3.2. Cavitation erosion resistance
Fig. 4. SEM micrographs of (a) laser surface-alloyed sample WC316-2; (b) original WC powder.
indicates complete dissolution of the preplaced WC. This is not unexpected in view of the fineness of the original WC particles, the high absorption coefficient of laser energy by carbides, and the low Gibbs free energy of 38.5 kJ moly1 for WC. The microhardness values (average of 5 measurements for each sample) of the laser surface-alloyed samples are given in Table 2. Owing to the presence of the carbides, the extended solid solubility of W in the g phase, and the refined microstructure due to the high cooling rate typical of laser treatment, the microhardness HV of the alloyed layer is significantly increased, from a value of 200 HV for as-received 316 to approximately 1000 HV for the laser-treated samples. The microhardness values in the present study are comparable to those reported by Choi and Mazumder w15x and Nagarathnam and Komvopoulos w16x in Fe–Cr–W–C coatings synthesized on steel.
Curves showing the cumulative MDE as a function of time for various samples in the cavitation erosion test are given in Fig. 5a. The corresponding cavitation erosion resistance Re and the normalized value Re* (relative to As-316) are given in Table 2 and shown in Fig. 5b. The cavitation erosion resistance is increased for all laser-treated samples, reaching a maximum of 35 times that of As-316 for sample WC-316-2. It is obvious from the discussion above that the presence of the element W plays an essential role in strengthening the alloyed layer via solid–solution hardening and the formation of complex carbides. Thus the total amount of W in the layer, as estimated from the composition profile in Fig. 2, is an important parameter and is included in Table 2. To ascertain this point, the relationship between HV and the total W content in the alloyed layer is shown in Fig. 6a. The microhardness of the alloyed layer increases monotonically with the W content, as is clear from Fig. 6a. On the other hand, the cavitation erosion resistance Re* is not a monotonic function of the total W content, as is depicted by Fig. 6b. The cavitation erosion resistance of sample WC316-1 is incommensurate with its high W content and hardness. The optical micrographs of the Vickers indentations in Fig. 7 provide some information on the fracture toughness of the alloyed layers w17,18x. The alloyed layer in sample WC-316-1 is too brittle, as is evidenced by the cracking around the indentation in Fig. 7a, while no cracks are present in the indentation for WC-316-2. This indicates that an appropriate compromise between hardness and fracture toughness or ductility (opposite to brittleness) would lead to high cavitation erosion resistance, as has been pointed out by Zum Gahr w19x and Wang et al. w20x. The higher brittleness of WC-316-1 could be attributed to a relatively higher W2C content as shown in the XRD spectra in Fig. 3. The SEM micrographs in Fig. 8a and b showing the cavitated surface of the as-received sample (As-316) and the laser surface-alloyed sample WC-316-2 reveal entirely different morphologies of damage. In As-316 sample, the surface was severely eroded after the 4-h test, with a morphology typical of ductile fracture. On the other hand, the damage was much milder in WC316-2 sample, with the interdendritic region preferentially eroded away, leaving behind a delineated dendritic microstructure. An additional point to note is that both the hardness and the cavitation erosion resistance of the laser surface-melted sample are only slightly higher than the as-received sample. This suggests that in the present case, the improvement in the laser surface-alloyed samples mainly results from W solid–solution strengthening and second-phase (carbides) strengthening, and not from grain refinement.
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Fig. 5. (a) Cumulative MDE as a function of time in cavitation erosion test in 3.5% NaCl solution at 23 8C; (b) Relative cavitation erosion resistance of various samples.
K.H. Lo et al. / Surface and Coatings Technology 165 (2003) 258–267
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Fig. 6. (a) Relationship between microhardness HV and total W content in alloyed layer; (b) Relationship between cavitation erosion resistance Re* and total W content in alloyed layer.
In a previous study, laser surfacing of AISI 316L stainless steel has been attempted using WC powder of approximately 40 mm diameter w6x. The use of coarsegrained WC powder as the added material led to the formation of an MMC layer with unmelted carbide particles in a metal matrix. An increase of 8.5 times in cavitation erosion resistance was achieved, which is lower than that in the present case. Thus, it seems that complete dissolution of WC followed by in situ formation of carbides by precipitation from the melt pool
would lead to a more homogeneous microstructure and a stronger interface at the carbides, both being responsible for a higher erosion resistance, in addition to the presence of W in solid solution. 4. Conclusions Laser surface alloying on AISI 316 stainless steel for enhancing cavitation erosion resistance by employing fine WC powder has been attempted and the following conclusions are drawn.
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Fig. 7. Optical micrographs of Vickers indentation (a) WC-316-1; (b) WC-316-2 (both at 500-g load).
(1) The cavitation erosion resistance of the laser surface-alloyed samples is significantly improved, and may reach more than 30 times that of as-received 316 in the most favorable case. (2) The high cavitation erosion resistance of the laser surface-alloyed samples could be attributed to a microstructure composed of carbide dendrites and interdendritic carbideyg-FeCrNiW eutectic. (3) The microhardness of the alloyed layer increases with the total W content in the layer. W plays an essential role in strengthening the alloyed layer via the formation of precipitated complex carbides and solution hardening. (4) The maximum cavitation erosion resistance occurs at a moderate microhardness of approximately 1000 HV and then decreases because the deleterious effect of brittleness becomes prominent at higher hardness. (5) Unlike the case of abrasive wear, a microstructure composed of fine precipitated carbides is more resistant
Fig. 8. SEM micrographs of cavitated surface of (a) As-316; (b) WC316-2, both after 240 min in cavitation erosion test.
to one containing coarse undissolved carbides. Such a microstructure can be achieved via laser surface alloying by employing fine WC powder as a convenient source of W and C, without involving strategic or polluting elements like Co or Ni. Acknowledgments The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5140y00E). Support from the infrastructure of the Hong Kong Polytechnic University is also acknowledged.
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References w1x A. Karimi, J.L. Martin, Int. Metals Rev. 31 (1) (1986) 1–26. w2x C.J. Heathcock, B.E. Protheroe, A. Ball, Wear 81 (1982) 311–327. w3x C.T. Kwok, H.C. Man, F.T. Cheng, Surf. Coat. Technol. 99 (1998) 295–304. w4x F.C.J. Fellowes, W.M. Steen, Laser surface treatment, in: D.S. Rickerby, A. Matthews (Eds.), Advanced Surface Coatings, Blackie, USA, 1991, pp. 244–277. w5x C.T. Kwok, F.T. Cheng, H.C. Man, Mater. Sci. Eng. A 290 (2000) 74–88. w6x F.T. Cheng, C.T. Kwok, H.C. Man, Surf. Coat. Technol. 139 (2001) 14–24. w7x R.C. Gassmann, Mater. Sci. Technol. 12 (1996) 691–696. w8x H. Wang, W. Xia, Y. Jin, Wear 195 (1996) 47–52. w9x C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 76–77 (1995) 450–455. w10x K.F. Tam, F.T. Cheng, H.C. Man, Mater. Res. Bull. 37 (2002) 1341–1351. w11x W.J. Tomlinson, R.T. Moule, J.H.P.C. Megaw, A.S. Bransden, Wear 117 (1987) 103–107.
267
w12x ASTM G32-92, Standard Method of Vibratory Cavitation Erosion Test, Annual Book of ASTM Standards, vol. 03.02, ASTM, Philadelphia, PA, 1994. w13x J.D. Ayers, D.S. Gnanamuthu, Metals Handbook, vol. 6, ninth ed., ASM, 1983, pp. 793–803. w14x M. Riabkina-Fisherman, E. Rabkin, P. Levin, et al., Mater. Sci. Eng. A 302 (2001) 106–114. w15x J. Choi, J. Mazumder, J. Mater. Sci. 29 (1994) 4460–4476. w16x K. Nagarathnam, K. Komvopoulos, Metall. Mater. Trans. A 26A (1995) 2131–2139. w17x G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533–538. w18x K. Komvopoulos, K. Nagarathnam, J. Eng. Mater. Technol. 112 (1990) 131–143. w19x K.H. Zum Gahr, Microstructure and Wear of Materials, Elsevier Science Publishers, 1987, Chapter 8. w20x B.Q. Wang, G.Q. Geng, A.V. Levy, Effect of microstructure on the erosion–corrosion of steels, Conference: Wear of Materials 1991, vol. 1, Orlando, Florida, USA, 7–11 April 1991, ASME, New York, USA, 1991, pp. 129–136.
Improvement of cavitation erosion resistance of AISI 316 stainless steel by laser surface alloying using fine WC powder K.H. Loa, F.T. Chenga,*, C.T. Kwoka,b, H.C. Manc a
b
Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, P.O. Box 3001, Taipa, Macau, China c Department Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 6 July 2002; accepted in revised form 25 September 2002
Abstract Fine WC powder of approximately 1 mm size was employed as a convenient source of tungsten and carbon in the laser surface alloying of AISI 316 stainless steel for improving the cavitation erosion resistance. A slurry containing WC powder was preplaced on the substrate by pasting and processed with a high-power CW Nd:YAG laser to achieve surface alloying. The composition and microstructure of the alloyed layer and the phases formed were investigated by energy-dispersive X-ray spectroscopy, optical microscopy, scanning electron microscopy, and X-ray diffractometry, respectively. The cavitation erosion behavior of the laser surface-alloyed samples in 3.5% NaCl solution was studied with a vibratory cavitation erosion tester. The microhardness of the alloyed layer increases with the total W content in the layer. By employing proper processing parameters, an alloyed layer that is hard but not too brittle can be formed, with a cavitation erosion resistance that may reach more than 30 times that of the asreceived 316. The improvement in cavitation erosion resistance may be attributed to the increase of W in solid solution and to the precipitation of dendritic carbides, both resulting from the dissociation of the fine WC powder during laser processing. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser surface alloying; AISI 316 stainless steel; Cavitation erosion; Tungsten carbide
1. Introduction Cavitation in a fluid is defined as the generation and collapse of cavities (i.e. bubbles) due to local pressure fluctuation arising from sudden change in flow, or from vibration. When bubbles collapse, shock waves and micro-jets are emitted, thus exerting pressure pulses on a solid surface near by. The repetitive attack by these pressure pulses on the solid surface leads to fatigue, fracture and loss of material, and such an erosion mechanism is known as cavitation erosion w1x. Cavitation erosion is a common cause of failure in engineering parts in hydraulic machinery and other liquid-handling systems. AISI 316 stainless steel is a popular engineering alloy because of its excellent corrosion resistance. However, owing to the relatively low hardness (e.g. a Vickers *Corresponding author. Tel.: q852-2766-5691; fax: q852-23337629. E-mail address: [email protected] (F.T. Cheng).
microhardness HV of only approximately 200 kgfy mm2), high stacking-fault energy and low tendency to stress-induced martensitic transformation, its cavitation erosion resistance is low w2,3x. As cavitation erosion occurs at the liquidysolid interface, and is thus related to surface properties rather than bulk properties, the cavitation erosion resistance of a material may be improved by surface modification. Surface modification has the advantage of consuming only a small amount of expensive material on the surface while using an inexpensive substrate for the bulk. It also allows a large number of combinations of surface and bulk properties, and thus significantly increases the number of options for the design engineer. Laser surface modification is a modern surfacing technique which is becoming more and more popular in engineering applications. Compared with other methods of surface modification, it possesses four important characteristics: (1) possibility of forming alloys of nonequilibrium compositions, (2) formation of a fine micro-
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 7 3 9 - 9
K.H. Lo et al. / Surface and Coatings Technology 165 (2003) 258–267
259
Table 1 Laser processing parameters and sample designations Sample
Laser power P (kW)
Scanning speed v (mmys)
Spot diameter d (mm)
Fluence F (Jymm2)a
Overlapping ratio (%)
WC-316-1 WC-316-2 WC-316-3 WC-316-4 LSM-316
1.50 1.75 2.10 2.10 1.50
25 20 20 35 25
4 4 4 5 4
15.0 22.0 26.0 12.0 15.0
50 50 50 50 50
a
The values of fluence were calculated for a single track. Owing to overlapping, the actual fluence received by a sample was twice the value shown.
structure, (3) presence of a metallurgical bond between the surface layer and the substrate, and (4) resulting in a small heat-affected zone w4x. Laser surface modification is especially suitable for local treatment at locations susceptible to erosion attack. Different types of laser surface modification have been attempted on AISI 316 stainless steel for improving the cavitation erosion resistance by the present authors, and varying degrees of improvement have been achieved w3,5,6x. The present study aims at laser surface alloying on AISI 316 stainless steel using fine WC powder for improving the cavitation erosion resistance. The incorporation of carbide particles in the laser surface modification of engineering alloys to enhance the wear resistance has been reported by a number of authors w7–9x. As coarse ceramic particles are beneficial to abrasive wear resistance, the WC used is coarse-grained and clad with a metal (Co or Ni) to minimize the degree of dissolution. Owing to a different mode of attack in cavitation erosion, coarse ceramic particles may not be advantageous as in the case of abrasive wear w10x. Rather, a uniform microstructure containing fine hard phases formed in situ may yield a high cavitation erosion resistance w11x. The present study is an attempt in this direction, with the use of fine unclad WC powder as a convenient source of alloying elements W and C. It has the additional advantage of avoiding the use of strategic or polluting elements like Co and Ni. 2. Experimental details 2.1. Materials and sample preparation As-received AISI 316 stainless steel (designated in this paper as As-316, with composition in wt.%: 18% Cr, 14% Ni, 3% Mo, 2% Mn, 1% Si, 0.08% C, 0.045% P, 0.03% S, balance Fe) was machined to discs of 12.7 mm diameter and 3.4 mm thickness. 2.2. Preplacement of alloying powder
220 grit SiC paper and then the slurry was painted on the sample and dried at 120 8C for 2 h. The painted layer was then polished with 1000 grit SiC to obtain a uniform preplaced coating of thickness of 0.3 mm. 2.3. Laser surface alloying Laser surface alloying was performed using a 2.5 kW CW Nd:YAG laser, with argon flowing at a rate of 20 lymin as the shielding gas. Preliminary trials with different values of laser power P, scanning speed v and laser spot diameter d were carried out to determine the feasible processing conditions. Four sets of processing parameters (shown in Table 1, with corresponding samples designated as WC-316-X) were chosen for further investigation of the alloyed layer. Laser surfacing was achieved by parallel tracks with 50% overlap. Such an overlapping ratio was chosen so as to compromise between surfacing efficiency and surface homogeneity. For comparison, laser surface-melted samples (designated in this paper as LSM-316) without the addition of WC were also prepared. 2.4. Metallographic and microstructural analysis After laser-treatment the samples were sectioned, polished, and etched with acidic chloride solution (25 g FeCl3, 25 ml HCl and 100 ml H2O). The average thickness of the alloyed layer was determined by image analysis. The microstructure of the alloyed layer was analyzed by scanning electron microscopy (SEM) and optical microscopy. The composition along the depth of the layer was determined by energy-dispersive X-ray spectroscopy (EDS) using a probing area of 30=30 mm2 at each point. The phases formed in the surface layer was determined by X-ray diffractometry (XRD) using Cu Ka as the radiation source (at 40 kV and 35 mA, with Ni filter). The Vickers microhardness at the surface of the samples was measured at a load of 200 g and a loading time of 15 s. 2.5. Cavitation erosion test
A slurry was prepared by mixing fine WC powder (particle size f1 mm) and a binder (4 wt.% polyvinyl alcohol, PVA). The sample surface was polished with
All the samples for the cavitation erosion test were polished with 1-mm diamond paste to ensure consistent
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260
Table 2 Properties of various laser surface-alloyed samples Sample
Melt depth D (mm)
Dilution ratio, h (%)a
Total W content (wt.%)
HV
Re (hymm)
Re*
WC-316-1 WC-316-2 WC-316-3 WC-316-4 LSM-316 As-316
0.35 0.40 0.47 0.31 0.60 –
14 25 36 3
70 52 44 Layer peeled off – –
1200 1000 900 Layer peeled off 220 200
320 1377 847 Layer peeled off 45 39
8.2 35.3 21.7 Layer peeled off 1.2 1
a
– –
The values of h were calculated based on the assumption that the preplaced layer was compact. Actual values should be higher.
surface roughness. Cavitation erosion tests were performed using an ultrasonic vibratory facility conforming to ASTM Standard G32-92 w12x, with the samples in the unattachment mode. The peak-to-peak amplitude and the vibration frequency used were 100 mm and 20 kHz, respectively, with a separation of 0.5 mm between the sample and the horn tip. The cavitating liquid was 3.5% NaCl solution kept at 23 8C. The sample was weighed at regular intervals of 30 min and converted to a cumulative mean depth of erosion (MDE) w6x. The mean erosion rate (MER) was calculated at the end of the test period, i.e. 240 min, and the reciprocal of MER was taken to be the cavitation erosion resistance Re. The surface morphology of the cavitated samples at the end of the test was investigated by SEM.
above, the preplaced layer was not a compact layer, and the actual values of h should be higher than the values obtained from this expression. With a suitable choice of processing parameters, a uniform alloyed layer free of cracks and pores was obtained, as shown in the optical
3. Results and discussion 3.1. Metallographic and microstructural analysis Under laser irradiation, the preplaced fine WC powder dissolved into the melt pool formed by melting a layer of the substrate. The melt pool rapidly solidified to form an alloyed layer. In the laser treatment, the laser power P, scanning speed v, and laser spot diameter d were varied in order to change the energy density (or fluence F) incident on the samples (FsPy(v=d)), and the interaction time (dyv). Although laser surface alloyingy cladding by powder injection is a one-step process and is also more versatile, laser alloying by powder preplacement is simple and the preplaced layer, which is inevitably porous, is expected to absorb laser energy more efficiently due to multiple reflections. Thus the fluence employed could be relatively low. The average thickness (D) of the alloyed layer for different samples is given in Table 2, together with the dilution ratio h calculated from hs(Dyt)yD, where t is the thickness of the preplaced layer. As pointed out
Fig. 1. Cross-sectional views of laser surface-alloyed samples (a) WC-316-2; (b) WC-316-4, an example of cracking.
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Fig. 2. Composition profiles of laser surface-alloyed samples (a) WC-316-1; (b) WC-316-2; (c) WC-316-3.
micrograph of the sample WC-316-2 in Fig. 1a. When F was too small, an insufficient amount of the substrate was melted and the resolidified layer cracked due to rapid cooling as shown in Fig. 1b, corresponding to the sample WC-316-4. On the other hand, when F was too large, an excessive amount of the substrate was melted, resulting in high dilution of the alloying elements. The composition profiles along the depth of the laser surface-alloyed samples are shown in Fig. 2. It can be seen that the elements were fairly evenly distributed,
with different relative amounts of the alloying elements corresponding to different processing conditions. In view of the fineness of the precipitates relative to the EDS probing area (30=30 mm2), the amount of an element shown in Fig. 2 represents the total amount of that element in solid solution and in the precipitates. The phases formed in the alloyed layer were identified using the XRD spectra shown in Fig. 3. As a result of the laser treatment, the fine WC powder dissolved in the melt pool and resolidified to form different types of
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Fig. 3. XRD spectra of laser surface-alloyed samples (a) WC-316-1; (b) WC-316-2; (c) WC-316-3.
carbides such as complex metal carbides M23C6, M7C3, M6C (MsFe, W, Cr), and tungsten carbides WC and W2C, together with the g-FeCrNiW phase. The SEM micrograph showing the microstructure of the laser surface-alloyed sample WC-316-2 is given in Fig. 4a. The microstructure, which is typical of one with high carbide content, is similar to the results reported by
Ayers and Gnanamuthu w13x and Riabkina-Fisherman w14x. Owing to the high concentration of C resulting from WC dissolution in the melt pool, metal carbides were first precipitated out as dendrites, with the interdendritic region composed of a eutectic of the gFeCrNiW phase and carbides. Absence of the original WC particles, which were angular in shape (Fig. 4b),
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3.2. Cavitation erosion resistance
Fig. 4. SEM micrographs of (a) laser surface-alloyed sample WC316-2; (b) original WC powder.
indicates complete dissolution of the preplaced WC. This is not unexpected in view of the fineness of the original WC particles, the high absorption coefficient of laser energy by carbides, and the low Gibbs free energy of 38.5 kJ moly1 for WC. The microhardness values (average of 5 measurements for each sample) of the laser surface-alloyed samples are given in Table 2. Owing to the presence of the carbides, the extended solid solubility of W in the g phase, and the refined microstructure due to the high cooling rate typical of laser treatment, the microhardness HV of the alloyed layer is significantly increased, from a value of 200 HV for as-received 316 to approximately 1000 HV for the laser-treated samples. The microhardness values in the present study are comparable to those reported by Choi and Mazumder w15x and Nagarathnam and Komvopoulos w16x in Fe–Cr–W–C coatings synthesized on steel.
Curves showing the cumulative MDE as a function of time for various samples in the cavitation erosion test are given in Fig. 5a. The corresponding cavitation erosion resistance Re and the normalized value Re* (relative to As-316) are given in Table 2 and shown in Fig. 5b. The cavitation erosion resistance is increased for all laser-treated samples, reaching a maximum of 35 times that of As-316 for sample WC-316-2. It is obvious from the discussion above that the presence of the element W plays an essential role in strengthening the alloyed layer via solid–solution hardening and the formation of complex carbides. Thus the total amount of W in the layer, as estimated from the composition profile in Fig. 2, is an important parameter and is included in Table 2. To ascertain this point, the relationship between HV and the total W content in the alloyed layer is shown in Fig. 6a. The microhardness of the alloyed layer increases monotonically with the W content, as is clear from Fig. 6a. On the other hand, the cavitation erosion resistance Re* is not a monotonic function of the total W content, as is depicted by Fig. 6b. The cavitation erosion resistance of sample WC316-1 is incommensurate with its high W content and hardness. The optical micrographs of the Vickers indentations in Fig. 7 provide some information on the fracture toughness of the alloyed layers w17,18x. The alloyed layer in sample WC-316-1 is too brittle, as is evidenced by the cracking around the indentation in Fig. 7a, while no cracks are present in the indentation for WC-316-2. This indicates that an appropriate compromise between hardness and fracture toughness or ductility (opposite to brittleness) would lead to high cavitation erosion resistance, as has been pointed out by Zum Gahr w19x and Wang et al. w20x. The higher brittleness of WC-316-1 could be attributed to a relatively higher W2C content as shown in the XRD spectra in Fig. 3. The SEM micrographs in Fig. 8a and b showing the cavitated surface of the as-received sample (As-316) and the laser surface-alloyed sample WC-316-2 reveal entirely different morphologies of damage. In As-316 sample, the surface was severely eroded after the 4-h test, with a morphology typical of ductile fracture. On the other hand, the damage was much milder in WC316-2 sample, with the interdendritic region preferentially eroded away, leaving behind a delineated dendritic microstructure. An additional point to note is that both the hardness and the cavitation erosion resistance of the laser surface-melted sample are only slightly higher than the as-received sample. This suggests that in the present case, the improvement in the laser surface-alloyed samples mainly results from W solid–solution strengthening and second-phase (carbides) strengthening, and not from grain refinement.
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Fig. 5. (a) Cumulative MDE as a function of time in cavitation erosion test in 3.5% NaCl solution at 23 8C; (b) Relative cavitation erosion resistance of various samples.
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Fig. 6. (a) Relationship between microhardness HV and total W content in alloyed layer; (b) Relationship between cavitation erosion resistance Re* and total W content in alloyed layer.
In a previous study, laser surfacing of AISI 316L stainless steel has been attempted using WC powder of approximately 40 mm diameter w6x. The use of coarsegrained WC powder as the added material led to the formation of an MMC layer with unmelted carbide particles in a metal matrix. An increase of 8.5 times in cavitation erosion resistance was achieved, which is lower than that in the present case. Thus, it seems that complete dissolution of WC followed by in situ formation of carbides by precipitation from the melt pool
would lead to a more homogeneous microstructure and a stronger interface at the carbides, both being responsible for a higher erosion resistance, in addition to the presence of W in solid solution. 4. Conclusions Laser surface alloying on AISI 316 stainless steel for enhancing cavitation erosion resistance by employing fine WC powder has been attempted and the following conclusions are drawn.
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Fig. 7. Optical micrographs of Vickers indentation (a) WC-316-1; (b) WC-316-2 (both at 500-g load).
(1) The cavitation erosion resistance of the laser surface-alloyed samples is significantly improved, and may reach more than 30 times that of as-received 316 in the most favorable case. (2) The high cavitation erosion resistance of the laser surface-alloyed samples could be attributed to a microstructure composed of carbide dendrites and interdendritic carbideyg-FeCrNiW eutectic. (3) The microhardness of the alloyed layer increases with the total W content in the layer. W plays an essential role in strengthening the alloyed layer via the formation of precipitated complex carbides and solution hardening. (4) The maximum cavitation erosion resistance occurs at a moderate microhardness of approximately 1000 HV and then decreases because the deleterious effect of brittleness becomes prominent at higher hardness. (5) Unlike the case of abrasive wear, a microstructure composed of fine precipitated carbides is more resistant
Fig. 8. SEM micrographs of cavitated surface of (a) As-316; (b) WC316-2, both after 240 min in cavitation erosion test.
to one containing coarse undissolved carbides. Such a microstructure can be achieved via laser surface alloying by employing fine WC powder as a convenient source of W and C, without involving strategic or polluting elements like Co or Ni. Acknowledgments The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5140y00E). Support from the infrastructure of the Hong Kong Polytechnic University is also acknowledged.
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References w1x A. Karimi, J.L. Martin, Int. Metals Rev. 31 (1) (1986) 1–26. w2x C.J. Heathcock, B.E. Protheroe, A. Ball, Wear 81 (1982) 311–327. w3x C.T. Kwok, H.C. Man, F.T. Cheng, Surf. Coat. Technol. 99 (1998) 295–304. w4x F.C.J. Fellowes, W.M. Steen, Laser surface treatment, in: D.S. Rickerby, A. Matthews (Eds.), Advanced Surface Coatings, Blackie, USA, 1991, pp. 244–277. w5x C.T. Kwok, F.T. Cheng, H.C. Man, Mater. Sci. Eng. A 290 (2000) 74–88. w6x F.T. Cheng, C.T. Kwok, H.C. Man, Surf. Coat. Technol. 139 (2001) 14–24. w7x R.C. Gassmann, Mater. Sci. Technol. 12 (1996) 691–696. w8x H. Wang, W. Xia, Y. Jin, Wear 195 (1996) 47–52. w9x C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 76–77 (1995) 450–455. w10x K.F. Tam, F.T. Cheng, H.C. Man, Mater. Res. Bull. 37 (2002) 1341–1351. w11x W.J. Tomlinson, R.T. Moule, J.H.P.C. Megaw, A.S. Bransden, Wear 117 (1987) 103–107.
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w12x ASTM G32-92, Standard Method of Vibratory Cavitation Erosion Test, Annual Book of ASTM Standards, vol. 03.02, ASTM, Philadelphia, PA, 1994. w13x J.D. Ayers, D.S. Gnanamuthu, Metals Handbook, vol. 6, ninth ed., ASM, 1983, pp. 793–803. w14x M. Riabkina-Fisherman, E. Rabkin, P. Levin, et al., Mater. Sci. Eng. A 302 (2001) 106–114. w15x J. Choi, J. Mazumder, J. Mater. Sci. 29 (1994) 4460–4476. w16x K. Nagarathnam, K. Komvopoulos, Metall. Mater. Trans. A 26A (1995) 2131–2139. w17x G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (1981) 533–538. w18x K. Komvopoulos, K. Nagarathnam, J. Eng. Mater. Technol. 112 (1990) 131–143. w19x K.H. Zum Gahr, Microstructure and Wear of Materials, Elsevier Science Publishers, 1987, Chapter 8. w20x B.Q. Wang, G.Q. Geng, A.V. Levy, Effect of microstructure on the erosion–corrosion of steels, Conference: Wear of Materials 1991, vol. 1, Orlando, Florida, USA, 7–11 April 1991, ASME, New York, USA, 1991, pp. 129–136.