REVIEW OF SURFACE TREATMENTS ON TURBINE BLADE STEEL FOR SLURRY EROSION RESISTANCE

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1 REVIEW OF SURFACE TREATMENTS ON TURBINE BLADE STEEL FOR SLURRY EROSION RESISTANCE Rakesh Bhatia Yadavindra College of Engineering, Punjabi University, GK.Campus, Talwandi Sabo, Bathinda,Punjab , India ABSTRACT Slurry erosion of turbine components is a very serious problem in the most of the hydraulic power plants. Although there are also other factors that affect to the life of turbine parts like design of turbine, material used in hydraulic turbine in which condition we are using the hydro turbine. In all these effects the slurry erosion is main part of damage of hydro turbine material. More research is needed to improve the surface properties of hydro turbine material by laser hardening, boronizing, nitriding, laser cladding, laser surface melting, using different types of coating powders like WC-Co-Cr, WC+12Co,Wc636, satellite 6, colomonoy 88 etc and by using different types of coating deposition techniques like OF, D-gun, FS etc. In this review paper, we have studied different surface treatment and coating on 13Cr4Ni steel and compare their hardness and volume loss during slurry erosion. OF is comparable good technique for deposition of coating powder and T410 steel borided also shows better result than others. Keywords:- Slurry erosion, 13Cr4Ni steel, hardness, volume loss, surface treatments, coating powders. 1.Introduction Hydropower is currently the most important renewable source of the world s electricity supply and there is still a considerable untapped potential in many areas [1]. The basic principle of working of hydropower turbines is the fast moving water (kinetic energy) strikes the turbines and they start moving (mechanical energy) and then this energy is used to run electric generators. The transference of energy takes place to one form to another. Now instead of water wheels water falling from vertical height is used which has stored potential energy in it due to its position and then water is passed through the gate or vessels which take the water to turbines which converts potential energy into mechanical energy, these vessels are called penstocks[2]. Many parts of the hydro turbine components, namely blades, guide vanes, bearing bodies etc., experience silt erosion problems. Many researchers pointed out that even with many improvements in materials and in hydro turbine design; erosion remains an unsolved problem in hydro power plants. The loss of material from turbine components over a period of time implies a geometrical change in the turbine parts. This is caused by erosion. The eroding particles present in moving fluid possess high kinetic energy. When these particles strike the surface, wear of material takes place by cutting or deformation of the surface. Erosion depends on several factors such as (i) pro perties of erodent including average size, shape, hardness, material type; (ii) properties of substrate materials including chemistry, elastic properties, hardness, surface morphology; and (iii) operating environment such as velocity, impingement angle, fl ux rate or concentration, medium of flow, temperature etc.[1] Many researchers have worked on turbine materials, coating materials, surface treatments, velocity of slurry, size of particles, shape of particles, impact angles, etc. Yet researchers have not studied on common and mostly used turbine material with different coatings, different surface treatment, different velocity of slurry, different sizes of particles, different impingement angles etc. 2.Turbine Materials It is very important that to select the turbine materials. There are various types of turbine material available in the market. But the selection of turbine materials is based on the properties such as high fatigue, erosion, cavitations and corrosion resistance must be considered. Basically various steels are used for high head turbines, whereas cast iron and bronze are used for some low head small turbines. Many researchers have studied on different materials of turbine. Goyal et al [3] reviewed on carbon steel, AISI 1020 carbon steel and cast nickel aluminium bronze substrate, cr-1mo steel coupons, steel, base metal SCS6, Stainless steel, steel substrate (DIN ), HP heat resistant steel, carbon steel (AISI 1020), 0.2% carbon steel substrate (AISI 1020), BS 4360 steel substrate, carbon steel (BS970 EN8) and stainless steel (UNS

2 S31603) AISI 304 steel and compare all these material with each other. Authors found that WC coatings applied by OF process have good erosion resistance. Grewal et al [4] studied 13Cr4Ni and 16Cr5Ni hydro turbine material and concluded that coatings of WC-10Co-4Cr could be deposited successfully on 13/4 and 16/5 steels using the detonation gun spray process. The deposited on the 13/4 steel showed a higher porosity along with a lower micro hardness compared with that on 16/5 steel. It was also found that the particle size was not the major factor to affect the erosion. In this case erosion was affected predominately by velocity alone. Bhandari et al [5] studied CF8M hydro turbine material. D-gun sprayed WC-10-Co-4Cr coatings can reduce the erosion rates of the CF8M steel. During the slurry erosion of CF8M steel, slurry concentration and rotation speed were found to more be dominant factors in comparison with average particle size. In the case of D-gun sprayed WC-10Co-4Cr coatings, average particle size was found to be more dominant factor in comparison with slurry concentration and rotational speed. Thus, D-gun sprayed WC-10Co-4Cr coatings can be used in high speed hydro turbines and with high concentration slurries. R Chattopadhyay [6] studied cast ferrite stainless steel of CA6NM type turbine runner material. Author found that the excellent erosion resistance of Stellite 6 plasma welded overlay on CA6NM can probably be explained as due to the slow removal of material through the closely spaced crystallographic axes of the h.c.p. matrix followed by the removal of dislodged carbides. Grewal et al [7] studied 13Cr4Ni hydro turbine material and evaluated liquid impingement erosion performance of thermal sprayed Ni-Al 2 O 3 coatings with varying content of Al 2 O 3. It was observed that content of alumina present in the coating significantly affected the erosion behaviour and mechanism of the coatings. Among the investigated Ni/ Al 2 O 3 coatings, a maximum resistance was provided by the coating containing 40 wt% Al 2 O 3. Various mechanisms responsible for the erosion of coatings were debonding of the splats, formation of pits in the softer matrix phase and fatigue induced cracks. Singh et al [8] studied 13Cr4Ni hydro turbine material and found that the modification of hydrodynamic design and surface engineering are the main approaches adopted to overcome cavitations erosion problem. The surface engineering including coating and micro structural modification has shown significant increase in the useful life components. Goyal et al [9] studied CF8M turbine steel and observed that the OF sprayed WC-10Co-4Cr coating was found to be useful to enhance the slurry erosion resistance of the given steel. This is mainly due to its higher hardness in comparison with its counterparts. The relatively low resistance of OF sprayed Al 2 O 3 + TiO 2 coating against slurry erosion may be due to its relatively low hardness, very high melting point of Al 2 O 3 particles which resulted in presence of large number of unmelted particles on the coating surface. This may have lead to spalling of unmelted and partially melted particles owing to their low adherence to substrates, thereby resulting in higher erosion rate. Kishor et al [10] studied 13Cr4Ni hydro turbine material and concluded that microstructure of the TMP steels evolved into finer martensite lath packets, which improve their slurry erosion resistance. Prior austenite grain size of 13/4 MSS reduced from 458μm to a much lower value in the range of 37-75μm in the TMP specimens. This was found to be responsible for the micro structural refinement of TMP specimens. TMP specimens (950º C, 0.001s -1 ) exhibited the least slurry erosion damage. It showed 78% more improvement in the slurry erosion resistance as compared to the ASR 13/4 MSS. This attributed to its higher tensile toughness, elongation, and strain hardening ability as evidenced by a greater depth of the strain hardened surface zone. Grewal et al [11] studied CA6NM hydro turbine material for Slurry erosion performance of Ni-Al 2 O 3 based coatings was investigated. Coatings were obtained by mixing alumina in various different proportions. It was found that Ni-Al 2 O 3 based coatings were effective in controlling the slurry erosion of CA6NM steel. Among the investigated Ni-Al 2 O 3 coating compositions, a highest resistance against slurry erosion was shown by coating containing 40 wt% Al 2 O 3. Brittle mechanism was found to be the primary mechanism responsible for the erosion of the coatings. Failure of coating was found to take place through cracking and spalling, fracture of alumina splats and removal of individual alumina splats. Lin et al [12] studied high speed slurry erosion characteristics of NiCrBSi sprayed coating deposited on SUS304 Stainless steel. The post thermal treated NiCrBSi coating have an extra high hardness and small quantity of porosity. They can exhibit a much better slurry erosion resistance than the SUS304 stainless steel, especially at lower impinged angles and higher impinged speeds. A preliminary test has revealed that the NiCrBSi spray coating can increase significantly the using life, 3-4 times of needles and nozzles in hydraulic machinery. Mann and Arya [13] studied plasma nitride 12Cr and 13Cr4Ni steels andfound that the OF coated steel performed much better than plasma nitrided 12Cr and 13Cr-4Ni steels. The microstructure of OF sprayed tungsten carbide shows a few voids as defects. Plasma nitride 12Cr steel performed much better than

3 plasma nitride 13Cr4Ni steels. This is due to higher micro hardness and its ability to absorb more nitrogen under identical plasma nitriding experimental condition. Mann [14] studied 13Cr4Ni, 16Cr5Ni and 17Cr4Ni hydro turbine materials and found that HDPL treated tungsten carbide based HP-OF coatings is approximately 41.5 times better than 13Cr4Ni stainless steel in SE and is an excellent erosion resistant shield. It can be used for all those components which are severely affected due to SE having silt concentration 2500ppm. Shivamurthy et al [15] have studied on 13Cr4Ni hydro turbine material. It was found that material removal mechanism appears to be a function of microstructure and hardness. Eroded surface of 13Cr4Ni steels show cutting with deep crater formation, where as Stellite 6 deposited shows lesser damage at the eroded surfaces due to micro cutting action and shallow crater formation. Colmonoy 88 deposites show very few shallow craters formation along with fracturing and intact of secondary phase. The slurry erosive wear rate is directly proportional to slurry velocity. Hadad et al [16] have studied on Ni-Cr hydro turbine materials. In this work adhesion and wear behaviour of cermet and different intermediate coatings within sandwich structure based cermet coatings were studied. Cermet combinations with Ni-Cr OF deposited and Ni platting electrochemically deposited coating showed high adhesive strength value. The blasted interface of Ni plating did not show higher adhesion value rather a difference in wear performance. Ramesh et al [17] have studied on mild steel substrate hydro turbine materials. The authors found that Inconel- 718 coatings were developed on mild steel substrate by Air plasma spray technique. Increased coating thickness decreases the porosity and increase the hardness significantly. Under all the test conditions studied, Inconel 718 coatings exhibited improved mild steel. Slurry erosive wear resistance of developed coatings increases with increase in coating thickness. By the study of all above research papers, we conclude that 13Cr4Ni hydro turbine material is good because of having excellent cavitations erosion, corrosion resistance and slurry erosion. We also conclude that on 13Cr4Ni material, researchers do not have compare this material by different coatings, different surface treatment, different velocities, and different silt particles size. In this paper we have studied this material by comparing of different coatings and methods and also fine the hardness and slurry erosion volume of the material. Table-1 Summary of Studies taken up for Literature Surve Author Coating Technique Erodent Micro hardness Grewal et Ni+20%Al2O3 FS Tap water is used, 563 al.[6] Ni+40%Al2O3 velocity is 147m/s, 714 Ni+60%Al2O3 pressure 14MPa, ph 1151 value is 7.2 Grewal al.[2] et Sigh et al.[8] Kishor al[10] et WC-Co-Cr D-Gun spray process WC+12Co Cr3C2+25NiCr T410 Steel Plasma nitriding Hard chrome plated D-Gun spray D-gun spray Thermo mechanical processed at Sand particles collected from Jhakri Hydro Power Plant HP, size 53 to 500 m, concentration is 5000ppm Water with Mineral sand of hardness 1100hv, irregular shape, size is 135 m,concentrati on is 2000rpm, flow rate is 40g/min, velocity is 7.5m/s Tap water of 7.0 ph value of 10:1 ratio of water and silica to to to to to to VHN 428 VHN 412 VHN Volume loss 0.5mm 3 /s 0.2mm 3 /s 0.3mm 3 /s 2.5 to 3.5 mm 3 /s 0.8 to 1.8 mm 3 /s Other Coating thickness:- 684,628,583 m respectively Slurry erosion increases with increase in velocity Little effect of change in angle Little effect of particle concentration Little effect of particle size Coating thickness: , 50-60, 50-60, , , m respectively.

4 950 0 C 0.001s C 10s C 0.001s C 10s -1 sand, size of sand particles is 270 m 414 VHN Mann et al [13] Mann et al [14] Shivamurthy et al [15]. WC636 Plasma nitriding OF... High power diode laser treated Satellite 6 Colmonoy 88 Laser hardening Two stage laser surface hardening Mineral sand of hardness 1100, silt concentration is 2350 ppm, water velocity and flow rate is 66.3 m/s, pressure is 330 mm of water column Water slurry erosion test with mineral sand of 1100 hardness, jet velocity 29.0 m/s and silt concentration is 2500 ppm SiO 2 mixed with water of ratio 1:10, erodent size is 100 m, velocity is 8 to 20 m/s mm 3 /kg 0.54 mm 3 /kg times more than base material (10-3 g/kg-hour) (10-3 g/kg-hour) (10-3 g/kghour) Erosive wear rate is directly proportional to slurry velocity. Surface cracks and voids are also a little problem with these coatings. 3. Conclusion In this review, slurry erosion performance of 13Cr4Ni was investigated. Also technique, erodent, micro hardness and volume loss is compared with tabular form. Following results were obtained from the present study: OF coated steel perform much better for slurry erosion because of high micro hardness and low volume loss during silt erosion test and also surface is defect free. 13Cr4Ni has shown better result when using coating powder of WC-Co-Cr with D-gun spray process. Further studies can be carried out on 13Cr4Ni using different coating powder, different technologies like OF and D-gun spray process and at maximum concentration of silt particles at maximum velocities and at different angles. REFERENCES 1. R.C. Shivamurthy, M. Kamraj and R. Nagarajan., Laser surface modification of steel for slurry erosion resistance in power plants., Woodhead publishing limited, B.S. Mann., High energy particle impact wear resistance of hard coatings and their application in hydro tubines., wear 237 (2000) D.K.Goyal, H. Singh and H. Kumar, An overview of slurry erosion control by the application of high velocity oxy fuel sprayed coatings. J. Engineering Tribology, Harpreet Singh Grewal, Sanjeev Bhandari and Harpreet Singh, Parametric study of slurry erosion of hydrotubine steels with and without detonation gun spray coatings using Taguchi technique, The Materials, Metals and Material Society and ASM International, volume 43A, sep Sanjeev Bhandari, Harpreet Singh, Harmesh Kumar and Vikas Rastogi, Slurry erosion performance study of detonation gun sprayed WC-10Co-4Cr coatings on CF8M steel under Hydro-Accelerated conditions, Journal of thermal spray technology, 1054-volume 21(5) sep R. Chattopadhyay, Case study, Wear, (1993) H.S.Grewal, H. Singh and Anupam Agarwal, Understanding liquid impingement erosion behaviour of nickel-alumina based thermal spray coatings, wear 301 (2013) Raghuvir Singh, S. K. Tiwari and Suman K. Mishra, Cavitation erosion in hydraulic turbine components and mitigation by coatings: current status and future needs, journal of materials engineering and performance, volume 21(17) July Deepak Kumar Goyal, Harpreet Singh, Harmesh Kumar and Varinder Sahni, Slurry erosion behaviour of OF sprayed WC-10Co- 4Cr and Al 2O 3+13TiO 2 coatings on a tubine steel, wear 289 (2012) Brij Kishor, G.P. Chaudhari, and S.K. Nath. Slurry eosion of thermo-mechanically processed 13Cr4Ni stainless steel, tribology International 93 (2016) H.S. Grewal, Anupam Agarwal, H. Singh and B.A. Shollocck. Slurry erosion performance of Ni-Al 2O 3 based thermal sprayed coatings: effect of angle of impingement., journal of thermal spray technology, volume 23(3) Feb M.C. Lin, L.S. Chang, H.C. Lin, C.H. Yang and K.M. Lin., A study of high speed slurry erosion of NiCrBSi thermal sprayed coating., Surface and coatings technology 201 (2006)

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