IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) Slurry Erosion Behavior of Thermal Sprayed Stellite-6 and Cr 3-25NiCr Coatings 1 Gurpreet Singh, 2 Sanjeev Bhandari 1,2 Dept. of Mechanical Engineering, Baba Banda Singh Bahadur Engineering College Fatehgarh Sahib, Punjab, India Abstract Slurry erosion performance of detonation gun (D-gun) sprayed ceramic coatings (Stellite-6 and Cr 3-25Ni4Cr) on 13Cr4Ni steel has been investigated. Commercially available silica sand was used as the abrasive media in a high speed erosion tester. Behavior of coated and uncoated steel has been observed at two different angles (30 and 90 ) under a set of significant factors such as slurry concentration, erodent size and rotational speed. The analysis of eroded samples was done using SEM. SEM of the eroded specimens showed mixed (brittle and ductile) mode of erosion mechanism. It has been observed that the Stellite-6 coating performed better than Cr 3-25Ni4Cr and uncoated steel when comparing them at 30. On the other hand, uncoated 13Cr4Ni showed good performance at 90 in comparison to coatings. Keywords Slurry Erosion, SEM, EDS, Detonation Gun, High Speed Erosion Tester I. Introduction Due to heavy silt content in water, the underwater parts of turbines like turbine blades, needles and nozzles get eroded and the turbine efficiency reduces and leads to high revenue loss in the hydro plant per year and such slurry wear leads to degradation of machinery performance and shortened service life [1]. Erosion is common phenomenon of underwater parts in turbines due to the impact of hard particles [2]. Silt problem occurs in mainly hilly areas, especially during the rainy seasons. Due to the increase in concentration of solid particles, filtration process is not possible [3]. Water contains Quartz, Tourmaline, Garnet, Zircon, etc of Hardness 7 on MHO scale [4]. These sediments are formed due to the fragmentation of rocks, erosion of land and land sliding because of heavy rains during the monsoon period in the Himalayan region of India [5]. Conventional steels used in hydro power plants are not able to sustain the slurry erosion problems that persist in hydro turbines [6-7]. To enhance the surface performance and durability of engineering components exposed to different forms of wear such as abrasion, erosion and corrosion thermal spray techniques are versatile means of developing a wide variety of coatings/protective layers [8 9]. Detonation gun (D-gun) spray process is a thermal spray coating process, which provides an extremely good adhesive strength, low porosity, coating surface with compressive residual stresses, low oxide contents and high intersplat strength [10-11]. The D-gun spray process involves the impingement of powdered materials through a water-cooled barrel with the supersonic speed on the surface of substrate. The two phase mixture of coating are heated to plasticity and impinges against a target substrate, where the high temperature, high velocity coating particles bond into the surface of substrate and a mechanical interlocking and microscopic welding may take place [12] To resist wear ceramic materials are now commonly employed in the form of coatings. Due to high melting point of the ceramic powders which require a high temperature jet to get deformed during coating formation, these coatings are usually deposited by atmospheric plasma spraying [13]. However, plasma sprayed coatings are possess more porous and brittle nature than high velocity thermal-sprayed coatings [14-15]. But due to close interlamellar contacts and small porosity, the high-velocity combustion spraying techniques provides greater hardness [16-17]. This is why several efforts have been undertaken to use these high-velocity spray techniques like to spray oxides Detonation gun spraying is mainly used [18]. Under actual service conditions it is often a difficult task to evaluate slurry erosion behavior of materials due to interactive effects of different parameters such as slurry concentration, velocity and properties of abrasive medium on wear rate. Accelerated erosion testing of materials can be performed by increasing load, velocity and other operational parameters in a laboratory test rig, where real contact conditions can be simulated. Objective of this study is to investigate the slurry erosion behavior of D-gun-sprayed Stellite-6 and Cr 3-25Ni4 Crcoatings on CF8M steel under the hydro accelerated conditions by using a high speed erosion test rig. II. Experimentation A. Substrate Material 13Cr4Ni stainless steels which are commonly used in hydropower plants were selected for this research study. Chemical composition of 13Cr4Ni steel is given intable 1. Rectangular specimens of 10 mm X 10 mm were prepared from the steel. B. Coating Deposition The feedstock powders used in this study were commercially available Stellite-6 and Cr 3-25Ni4Crpowders, details of which are given in Table 2. The morphology of powder along with EDS has been studied by scanning electron microscopy (JEOL JSM- 6610LV). Samples were then coated by D-gun spray process available with SVX Powder M Surface Engineering Pvt. Ltd., Noida, India. The coatings were deposited using the standard process parameters reported in Table 3. Thereafter, the cut sections were mounted in transoptic powder using hydraulic hot mounting press (Model: Bainmount H, Chennai Metco Make). Subsequently, the mounted specimens were polished manually using SiC emery papers of 400, 600, and 1000 grit sizes. Finally, the specimens were mirror polished on a velvet cloth polishing machine with alumina powder suspension. Specimens were then washed and dried before being examined. Mirror-polished specimens were examined under the SEM. 94 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com
ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 Table 1: Chemical composition of 13Cr4Ni Stainless Steel (wt %) Steel C Si Mn Cr Ni N S Cu Co P Mo Fe 13Cr4Ni 0.06 0.74 1.16 13.14 3.9 -- 0.014 0.088 0.035 0.015 0.61 Bal. Table 2: Nominal Composition (Mass %) and Physical Properties of Coating Powders Powder (Commercial Code) / Make Stellite-6 / DeloroStellite Cr 3-25Ni4Cr, AMPERIT -584.054 / H.C. Starck Composition Co Base Cr 27-32% W 4-6 % C 0.9-1.4 % Others Ni, Fe, Si, Mn, Mo C 9-11% Cr 65.9 73% Ni 18-22% Fe max 0.5% O max 0.6% Table 3: Process parameters for D-gun spray process Morphology gas atomized powders Agglomerated sintered Parameters Stellite-6 Coating Cr 3-25NiCr Coating Oxygen flow rate (O 2 ) 3120 SLPH 2720 SLPH Pressure 0.2 MPa 0.2 MPa Acetylene flow rate ( H 2 ) 2400 SLPH 2400 SLPH Pressure 0.14 MPa 0.14 MPa Nitrogen flow rate (N 2 ) 1040 SLPH 960 SLPH Pressure 0.4 MPa 0.4 MPa Spray angle 90 90 Spray distance 150 mm 165 mm Power 450 VA 450 VA Particle Shape Size Spherical 53/10 µm Almost Spherical Hardness 36-45 HRC 380-490 HV 45/10 µm 775 1200 HV C. Slurry Erosion Testing To study the slurry erosion behavior of coated and uncoated steel, a high speed erosion tester was used (DUCOM TR401, Bangalore make). This set up consists of slurry abrasion chamber, slurry tank, rotor, control panel, 3 phase induction motor which is shown in fig. 1. Slurry abrasion chamber encloses the specimens and slurry. Slurry tank is cylindrical vessel of stainless steel in which slurry is prepared by mixing water and sand of required particle size and in required proportion. Rotor of the tester is used to mount specimen and it is enclosed in slurry chamber. 12 specimens can be placed on the rotor, which is main advantage of this tester. Control panel consists of controls for rotor speed, setting time for test, start and stop buttons. For recirculation of slurry 3 outlet pipes and 3 inlet pipes are provided between slurry chamber and tank. In the starting specially designed specimen holders are mounted on the appropriate place in chamber and adjusted at required angles for rectangular specimen. Then specimens are mounted in specimen holders. Now slurry chamber is closed by lifting the screw jack and tightening the nuts at four places to make it air tight. After setting rotational speed and time for a run, motor is started. Due to rotation of rotor vacuum is created and slurry enters the slurry chamber from slurry tank through the inlet pipes. And used slurry leaves the chambers and flow back to slurry tank through outlet pipes. This high speed erosion tester is capable of creating simulating environment similar to the hydro accelerated conditions to simulate the erosion of standard test specimens with water containing abrasive particles of controlled size and composition (slurry). Commercially used silica sand is used as slurry medium as silica is found to be main constituent of slurry as is evident from the literature; the slurry is found to be consisting of SiO 2, Al 2 O 3, CaO, and MgO in hydro power plant in northern India [2]. Tests were carried under concentration of 10000 ppm, with erodent particle size 600 µm and rotation speed of 1900 rpm to study the slurry erosion performance of Stellite-6, Cr 3-25Ni4Cr coated13cr4ni steel and 13Cr4Ni stainless steel which were placed at 2 different angles i.e. 30 and 90. These two angles were select to analyze the performance of specimen when sand particles strike the surface tangentially and normally. The interaction of slurry particles with rectangular specimen in slurry chamber is shown in fig. 2. A typical erosion test cycle began with mounting of specimen holder at the proper place at correct angle. Then specimen was placed in the holders in the slurry chamber. After fixing the specimens in holders every time chamber was made air tight by tightening the nuts at four places so that proper vacuum can be created. The water was filled in stainless steel water tank and silica sand of appropriate particle size was added to the water for preparing required concentration. Then rotational speed was set and test was started. After completing the slurry erosion testing cycle of 1 h, specimens were removed from the rotor assembly, brushed gently and cleaned with acetone to remove attached sand particles if any. The specimens were weighed before and after each slurry erosion cycle. The loss in mass of each specimen was recorded with the help of precision micro weighing scale www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 95
IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) Fig. 1: Experimental Setup of High Speed Erosion Tester (DUCOM TR401) Fig. 2: Schematic Diagram Showing Interactions of a Rectangular Specimen With the Slurry Particles in the Slurry Chamber at 30 and 90 96 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com
ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) having an accuracy of 0.1 mg. As erosion is a surface phenomenon so the surface area of each specimen was calculated by measuring the length and width at two places, taking their mean for getting average length and width of specimen with the help of digital vernier caliper of least count 0.01 mm. Specific mass loss was calculated using Specific mass loss = mass loss (g) X 10 6 / Exposed surface area (m 2 ) The slurry erosion process was repeated for six cycles, each cycle of duration 1 hour. The results have been plotted as cumulative mass loss per unit area in (g/m 2 ) versus time of exposure (h) to ascertain the kinetics of slurry erosion behavior of coatings and substrate. III. Results and Discussion IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 A. SEM and EDS Analysis of Coating Powders and Coating as Sprayed SEM and EDS analysis of Stellite-6 powder is shown in fig. 3. From SEM analysis it can be observed that the particles are of spherical shape. From EDS analysis it is clear that it is Cobalt based alloy contacting chromium content also. SEM and EDS analysis of Cr3C2-25Ni4Cr is shown in figure 4. It can be seen that the particles are almost spherical and they are Agglomerated sintered and powder containing the chromium, carbon as well as nickel content. Fig. 3: SEM and EDS Analysis of Stellite-6 Fig. 4: SEM and EDS Analysis of Cr 3-25Ni4Cr www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 97
IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) In fig. 5, surface morphology of Stellite-6 is shown. The coating seems to contain fine grains dispersed in the matrix. The grains appear as round white particles, whereas very dark small round spots are the micro-pores. The cross-sectional view of the coated sample of Stellite-6 (Fig. 6) shows distinctly the coating layer, interface, and the substrate. The interface, by and large, seems to be defect free showing nearly a continuous contact of coating and substrate. The coating thickness is found to be 100-150 µm. Some micro-voids along the splat boundaries of the coating are also present. Fig. 10 shows Cumulative mass loss versus time plots for D-gun sprayed Stellite-6, Cr 3-25Ni4Cr and uncoated 13Cr4Ni at 90. At this angle cumulative mass loss also increases for coatings as well as for uncoated steel. But cumulative mass loss for uncoated steel is less as compared to both coatings. After a run of 6 hours overall specific weight loss for 13Cr4Ni is 84.94 g/m2, for Stellite-6 is 283.01 g/m2, and Cr3C2-25Ni4Crfor 520.62 g/m2. This indicates that coating is not much useful on steel at 90. Fig. 5: Surface SEM Analysis of the D-Gun-Sprayed Stellite-6 Fig. 7: Surface SEM Analysis of the D-Gun-Sprayed Cr 3-25Ni4Cr Fig. 6:Cross-Sectional SEM of Stellite-6 on 13Cr4Ni Surface morphology of Cr 3-25Ni4Cr is shown in fig. 7. And the cross-sectional view of Cr 3-25Ni4Cr is shown in figure 8. Similar to Stellite-6 coating, this coating is also found to be defect free except few micro pores. The thickness of coating is found to be 150 to 200 µm. B. Slurry-Erosion Testing Cumulative mass loss versus time plots for D-gun sprayed Stellite-6, Cr 3-25Ni4Cr and uncoated 13Cr4Ni at 30 is shown in figure 9. It has been observed that cumulative mass loss for uncoated steel is higher than the coatings at 30. It can be observed that as the time increases the cumulative weight loss per unit area increases for coatings as well as for uncoated steel. After a run of 6 hours overall specific weight loss for Stellite-6, Cr 3-25Ni4Cr and uncoated 13Cr4Ni at 30 is 187.43 g/m2, 386.13 g/m2 and 699.06 g/m2 respectively. This indicated that coating is useful to develop erosion-resistance in the base steel at 30. Fig. 8: Cross-Sectional SEM of Cr 3-25Ni4Cr on 13Cr4Ni Fig. 11 shows the comparison between Stellite-6 at 30 and Stellite-6 at 90 on cumulative mass loss per unit area versus time graph. With the increase in time weight loss per unit area of Stellite-6 increases for both angles. After 6 hour run overall specific weight loss for Stellite-6 at 30 is 187.43 g/m2 and for Stellite-6 at 90 coating it is 283.01 g/m2. Specific weight loss for Stellite-6 at 90 is 1.5 times specific weight loss for Stellite-6 at 30. Cumulative mass loss per unit area of Stellite-6 at 30 and 90 is shown in fig. 12. From the graph it can be seen that Stellite-6 performed better at 30 as the maximum weight loss for Stellite-6 at 30 is less than the Stellite-6 at 90. The maximum weight loss for Stellite-6 at 30 is 187.43 g/m2 and at 90 it is 283.01 g/m2. 98 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com
ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 Fig. 9: Cumulative Mass Loss Versus Time Plots for D-Gun Sprayed Stellite-6, Cr 3-25Ni4Cr and uncoated 13Cr4Ni at 30 Fig. 12: Comparison Between Cumulative Mass Loss Per Unit Area of Stellite-6 at 30 and 90 In fig. 13 there is a comparison between cumulative mass loss per unit area of Cr 3-25NiCr at 30 and 90. More erosion of Cr 3-25NiCr takes place at 90. The maximum weight loss for Cr 3-25NiCr is 386.13 and 520.62 g/m2 at 30 and 90 respectively after a 6 hour run in slurry erosion chamber of high speed tester. The ratio of maximum weight loss at 30 to 90 comes out 1.34. Fig. 10: Cumulative Mass Loss Versus Time Plots for D-Gun Sprayed Stellite-6, Cr 3-25Ni4Cr and Uncoated 13Cr4Ni at 90 C. Material Removal Mechanism In the present study rectangular specimens were rotated in high speed tester containing the slurry. This slurry may cause erosion and abrasion damage of the specimen surfaces. In the present study the specimen was placed at two different angles i.e. 30 and 90. When specimen is at 90 there will be only normal interaction of sand particles with surface of specimen therefore this interaction can make the material to fail by fatigue or under plastic deformation. Whereas when specimen is at 30 then sand particles will hit surface tangentially. Hitting force can be divided into two components i.e. Tangential component which is parallel to the surface of specimen has a cutting effect at the surface. Whereas, the perpendicular component may be the reason of fatigue failure or may cause plastic deformation. Therefore it can be thought that slurry erosion of specimen at 90 will be due to the fatigue alone whereas at 30 it can be thought that slurry erosion of the specimens is occurring simultaneously by cutting abrasion and deforming abrasion. To investigate the material removal mechanism, all eroded specimen were examined under SEM. Figure 14 and 15 shows the SEM analysis of eroded 13Cr4Ni at 30 and at 90 respectively. SEM analysis of eroded Stellite-6 is shown in figure 16 and 17 at 30 and at 90 respectively. SEM analysis of eroded Cr 3-25Ni4Crat 30 and 90 is shown in fig. 18 and 19 respectively. Fig. 11: Comparison Between Cumulative Mass Loss Per Unit area of 13Cr4Ni at 30 and 90 www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 99
IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) Fig. 13: Comparison Between Cumulative Mass Loss Per Unit Area of Cr 3-25Ni4Crat 30 and 90 Fig. 16: SEM Analysis of Eroded Stellite-6 at 30 It can be observed from SEM analysis of 13Cr4Ni at 30 that only ploughing effect is dominating in this case. At 30 in coatings there are the evidence of removal of grains, pit formation and brittle signatures. At 90 in coatings pit formations are more as compared to 30. Also in Cr 3-25Ni4Cr there are more pits as compared to Stellite-6 and 13Cr4Ni at 30 and 90 due to the more hardness of Cr 3-25Ni4Cr. Due to more hardness of Cr 3-25Ni4Cr the material is removed in the shape of flakes. Fig. 17: SEM Analysis of Eroded Stellite-6 at 90 Fig. 14: SEM Analysis of Eroded 13Cr4Ni at 30 Fig. 18: SEM Analysis of Eroded Cr 3-25Ni4Crat 30 Fig. 15: SEM Analysis of Eroded 13Cr4Ni at 90 Fig. 19: SEM Analysis of Eroded Cr 3-25Ni4Crat 90 100 International Journal of Research in Mechanical Engineering & Technology www.ijrmet.com
ISSN : 2249-5762 (Online) ISSN : 2249-5770 (Print) IV. Conclusion Based on the results obtained from this study, the following conclusions may be drawn: 1. The erosion resistance of coatings and substrate steel at 30 is found to be in following manner- Stellite-6 >Cr 3-25Ni4Cr>13Cr4Ni 2. The erosion resistance of coatings and substrate steel at 90 is found to be in following manner- 13Cr4Ni >Cr 3-25Ni4Cr >Stellite-6 3. Performance of substrate 13Cr4Ni steel is better at 90 when comparing with at 30. 4. Stellite-6 at 30 is having better erosion resistance as compared to at 90. 5. Cr C -25Ni4Cr also performed better at 90 when comparing 3 2 with at 30. 6. Dominating material removal mechanism for the material at 90 was found to be Fatigue and brittle failure whereas ploughing and fatigue failure is dominated for the material at 30. References [1] Shengcal L.I., Proceedings of the 5th International Symposium on Cavitation, CAV03-GS-11-008, Osaka. [2] Chauhan A. K., Goel D. B., Prakash S., Erosion behaviour of hydro turbine steels, Bull. Material Science, Vol. 31, No. 2, April 2008, pp. 115 120. [3] Santa J. F., Baena J. C., Toto A., Slurry erosion of thermal spray coatings and stainless steels for hydraulic machinery, Wear, Vol. 263. pp. 258-264, 2007. [4] Singh R.P., Silt damage control measures for underwater parts-nathpajhakri Hydro Power Station, Case study of a success story, Vol. 66, No. 1, January-March, 2009, pp. 36-42. [5] Naidu B.S.K., Renovation and Modernization of Silt Prone Hydropower Stations in India, Workshop on Silting Problems in Hydroelectric Power Stations, June 25-26, 1987 (New Delhi), pp. V-13-V-16. [6] Mann B.S., Erosion Visualization and Characteristics of Two Dimensional Diffusion Treated Martensitic Stainless Steel Hydrofoils, Wear, 1998, 217, pp. 56-61 [7] Hattori S., Mechanism of Erosion,"Fundamentals of Cavitation and Slurry Erosion, Turbomachinery, 2002, 30(11), pp. 644-651 [8] Joshi, S.V., Sivakumar, R., Protective coatings by plasma spraying, Trans. Indian Ceram. Soc. 50, pp. 50 59, 1991. [9] Budinski, K.G., Surface engineering for wear resistance, Prentice Hall, Englewood Cliffs, 1988. [10] Rastegar, F., Richardson, D.E., Alternative to chrome: HVOF cermet coatings for high horse power diesel engines, Surf. Coat.Technol. 90, pp. 156 193, 1997. [11] Rajasekaran, B., Raman, S.G.S., Joshi, S., Sundararajan, G. Influence of detonation gun sprayed alumina coating on AA 6063 samples under cyclic loading with and without fretting, Tribol. Int. 41(4), pp. 315 322, 2008. [12] Vuoristo, P.M.J., Niemi, K., Mantyala, T., On the properties of detonation gun sprayed and plasma sprayed ceramic coatings, In: Berndt, C. (ed.) Thermal Spray: International Advances in Coatings Technology, pp. 171 175. ASM International, Metals Park, OH, 1992. [13] Wolentarski W., Material coating by the Detonation Gun process, Proceedings of the eleventh turbomachinery symposium, Newyork. IJRMET Vo l. 3, Is s u e 2, Ma y - Oc t 2013 [14] Liu Y., Fischer T.E., Dent A., Comparison of HVOF and plasma-sprayed alumina/titania coatings microstructure, mechanical properties and abrasion behavior, Surf. Coat. Technol. 167, pp. 68 76, 2003. [15] Psyllaki P.P., Jeandin M., Pantelis D.I., Microstructure and wear mechanisms of thermal-sprayed alumina, Coat. Mater. Lett. 47, pp. 77 82, 2001. [16] Voyer J., Marple B.R., Sliding wear behavior of high velocity oxy-fuel and high power plasma spray-processed tungsten carbide- based cermet coatings, Wear 225 229, pp. 135 145, 1999. [17] Wang Y., Friction and wear performances of detonation-gun and plasma-sprayed ceramic and cermet hard coatings under dry friction, Wear 161, pp. 69 78, 1993. [18] Pawlowski L., The Science and Engineering of Thermal Spray Coatings, Wiley, West Susseex P109, IUD (1995). Gurpreet Singh is working as Lecturer at Gian Jyoti Institute of Engineering and Technology, Shambhu Kalan, Banur, Punjab. He is pursuing his M. Tech (Part Time) in Mech. Engg. & Machine Design from BBSBEC, Fatehgarh Sahib, Punjab, India. Sanjeev Bhandari is working as an Assistant Professor at Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab, India. His area of interest is Tribology, Surface Engineering etc. He is working on his Ph.D. in the field of slurry erosion of turbine materials. www.ijrmet.com International Journal of Research in Mechanical Engineering & Technology 101