Ni and H.V.O.F. Coatings

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1 e t International Journal on Eerging Technologies 3(1): (2012) ISSN No. (Print) : ISSN No. (Online) : Effect of Velocity on Erosion Perforance of 13Cr 4 Ni and H.V.O.F. Coatings Mithlesh Shara, D.K. Goyal*,Naresh Kuar** and Raan Shara** Departent of Mechanical Engineering, IET Bhaddal, (P.B.) *Departent of Mechanical Engineering, B.B.S.B.E.C. Fatehgarh, (PB) **Departent of Mechanical Engineering, SIET, Bilaspur, (H.P.) (Recieved 15 March, 2012 Accepted 10 April, 2012) ABSTRACT : The slurry erosion of two coatings applied by high velocity oxy fuel ( H.V.O.F.) processes onto shotblasted 13Cr 4 Ni steel was studied, and the results were copared to those obtained with 13Cr 4 Ni, which are coonly used for hydraulic turbines and accessories., while the icrostructure and worn surfaces were characterized by optical and scanning electron icroscopy. Slurry erosion tests were carried out in a odified ipact test rig, in which the saples were placed conveniently to ensure grazing incidence of the particles. the slurry was copose of distilled water and quartz sand particles with an average diaeter between 165 µ (AFS 50/70) and the slurry concentration content was pp in all the tests. the three different ranges of velocity is taken i.e. 25/s, 60/s and 80/s. at noral ipact angle and coparison regarding the erosion resistance was deterined fro the ass loss results. Higher erosion will be seen at ediu range of velocity i.e. at 60/s. the coated surfaces showed higher erosion resistance than the uncoated stainless steels, with the lower volue losses easured for the Cr 2 deposit. SEM analysis of the worn surfaces revealed intense plastic deforation in both coated and bare stainless steels, with little evidence of brittle fracture in the icrostructure. The easured adhesive strength of the coatings was considered acceptable for the processes eployed. Keyword : HVOF coatings, slurry erosion, icrostructure, theral coating. I. INTRODUCTION Stainless steels are widely used in hydroelectric power plants due to their good corrosion properties and acceptable resistance to solid particle erosion, since any coponents are in contact with aqueous solutions containing hard particles that ipact against the surface causing significant aterial loss (slurry erosion condition). The agnitude of the daage caused is a consequence of the aount, type and size of solid particles in the flow, together with the echanical properties of the surfaces, physical-cheical properties of the water and operating conditions [1, 2]. Slurry erosion probles are particularly iportant during rainy seasons due to the increase in the nuber of solid particles ipacting the surfaces, especially in systes where an exhaustive filtration process is not possible. This is the case of the Francis turbines installed in a hydroelectric power plant in north western Colobia, where intense erosive wear has led to changes in surface texture and loss of adjustent between the liners and the spiral case, as can be seen in Fig. 1. The angle of incidence of the particles is extreely iportant to deterine the ain wear echanis acting on the surface of the coponents subitted to erosion. It is well known that icro-cutting prevails for low ipact angles whereas for angles close to 90 the doinant effects are low-cycle fatigue and accuulation of plastic deforation up to a critical value that prootes aterial detaching [3, 4]. In addition, corrosive attack and boundary layer effects develop when the particles are carried by a liquid, configuring a uch ore intricate situation that is affected by the rheological properties of the carrying fluid such as its density and viscosity [5, 6]. A cost-effective way to iprove the slurry erosion resistance of the coponents is the application of therally sprayed coatings [7, 8]. The ter theral spray describes a faily of processes that use cheical or electrical energy to elt (or soften) and accelerate particles of a aterial which is then deposited on a surface [9]. The coatings ay have a good erosion resistance depending on the cheical and echanical properties of the aterial deposited, the surface preparation prior to application and the deposition conditions [7-9]. The high-velocity oxy-fuel (HVOF) process belongs to the faily of theral spraying techniques, and is widely used in any industries to protect the coponents against erosion, corrosion and wear. Particle degradation and open porosity are the two iportant factors that affect corrosion and erosion resistance. HVOF processing did not degrade significantly the coposition of the consuable and has been shown to produce coatings with low porosity, low oxide content, better density, better coating cohesive strength and bond strength than any theral spray processes [10, 11]. In this work, two therally sprayed coatings were studied in laboratory in order to evaluate their potential application in a particular coponent of a Francis hydraulic turbine. The evaluation included the analysis of the worn coponent and coparison between coated and uncoated saples subitted to controlled slurry erosion conditions.

2 II. MATERIAL AND METHODS A. Selection of Substrate Material Steel 13 Cr 4 Ni steel which is used as aterial for Hydro power plants in soe plants in northern part of India has been used as a substrate in the study. The speciens with approxiate diensions of were cut fro the turbine aterial for erosion studies. Saples were grinded with SiC papers down to 180 grit and Shot -blasted with SiO 2 before being HVOF sprayed to develop better adhesion between the substrate and the coating. stainless steels coonly used for turbines and hydraulic accessories were used, naely 16 Cr 5 Ni steel, whose noinal cheical copositions are shown in Table1 Also, two coercial powders, Cr 2 and CrC + NiCr were deposited onto 13 Cr 4 Ni steel by High Velocity oxy fuel (HVOF) processes, respectively spraying was carried out using a HIPOJET 2100 equipent (M/S Metallizing Equipent Co. Pvt. Ltd., Jodhpur, India), which utilize the supersonic jet generated by the cobustion of liquid petroleu gas (LPG) and oxygen ixture. LPG fuel gas is cheap and readily available as copared to other fuels used for HVOF spraying. The spraying paraeters eployed during HVOF deposition are listed in Table 1. All the process paraeters, including the spray distance were kept constant throughout the coating process. Table 1: Noinal cheical coposition of the substrate aterials (wt %). identify the various phases present on their surfaces. Table 3 : Spray paraeters eployed for HVOF spray process. Oxygen flow rate Fuel (LPG) flow rate Air-flow rate Spray distance Powder feed rate Fuel pressure Oxygen pressure Air pressure 250 l/in 60 l/in 700 l/in g/in 588 kpa 883 kpa 588 K The icrostructure characterization was done in a JEOL 5910LV SEM at IIT, ROPAR. The porosity of the coatings was easured by digital iage analysis. Vickers hardness and icro-hardness easureents were perfored by using a Wolpert hardness tester (HV62.5 kg f) and a Shiadzu icro-hardness tester (HV0 g, 15 s), respectively. Localized cheical analyses of the speciens were done with an EDS spectroeter coupled to the SEM. The 13Cr 4 Ni stainless steel saples were anufactured fro the ithla castings. The aterial taken fro the larji power station presented accelerated wear daage which we can easily see with the help of Fig. 1. Materi- C Mn Si Cr Ni P S als 13Cr 4 Ni Cr 2 90 CrC-NiCr B. Apparatus Required Measureent of Coating Thickness 1. The coating thickness was easured during spraying with a Minitest-2000 Thin Fil Thickness Gauge (precision ± 1 µ),to obtain coating of unifor thickness. For verification of thickness of deposited coating, the as sprayed specien was cut across the cross-section with a diaond cutter. 2. X-Ray Diffraction (XRD) Analysis the XRD analysis was perfored on the coated and uncoated speciens to Fig. 3. Microstructure of 13CR4NI stainless steel, 500, SEM The arrow shows a chroiu carbide precipitated at the grain boundary of prior austenite. C. Slurry erosion tests The slurry erosion testing is conducted out in a odified test rig which is known as JET IMPACT TEST RIG in which the speciens were subjected to the erosion conditions which are very uch siilar to those of the hydro power plant Fig. 2 shows the configuration of the testing achine, which is coposed of a coercial centrifugal pup connected to an electrical otor, a flow discharging apparatus, flow eter and an isotheral bath to control the slurry teperature. The flow eter is used to vary the flow rate as we have to study the erosion conditions at different velocity ranges. The saples were located at the outlet of

3 Shara, Kuar and Shara 120 the nozzle which is carrying out the water coing out fro the outlet of centrifugal pup to ensure grazing incidence of the particles (Fig. 2). The slurry was coposed of distilled water and quartz particles with a particle size distribution which will lead to average diaeter of 155 icroneter and the solids content was 000 pp. The ean ipact velocity of the slurry was 35 /s, 55/s and 70/s and the erosion resistance was deterined fro the ass loss results. Mass losses were easured every in by using easuring balance which is having a accuracy a scale with 0.01 g resolution. The total duration of each test was 120 in, and after that period both the saple and the slurry were replaced. Fig. 4. Microstructure of ASTM 13Cr 4 Ni Cr 2. Analysis of worn surfaces steel, after coating The worn surface were analyzed in stereoscopic and scanning electron icroscopes in order to identify the wear echanis and relate the to the ass loss results III. RESULTS AND DISCUSSION A. Microstructure (i) 13 Cr4Ni steel The icrostructure of this steel is coposed of austenietic-artensitic steel with delta ferrite and about 20 to 25 stable austenite. Chroiu carbides precipitated at the prior austenite grain boundaries, as can be seen in Fig. 3. The average hardness easured was 20 HV62.5 kg f. Micro-hardness easureents reported 332 HV25 g f, 15 s. in the artensitic atrix. Fig. 5. Microstructure of 13Cr4Ni steel after coating with with CrC-NiCr. (ii) Cr 2 Coating Fig. 5 shows the icrostructure of a typical coating with a thickness of coating 200µ. The coating layer is coposed by a soft, nickel-rich atrix (191 HV average hardness) containing elongated chroiu oxide particles (1120 HV25 g f, 15 s). The easured average volue fractions of Cr 2 particles and pores were 15% and 9%, respectively. The wear-resistant Cr 2 coating is coposed of hard, Cr particles (1120 HV average hardness) and softer Ni-Cr regions (639 HV average hardness), together with a nuber of unelted particles and pores. The volue fraction of pores was estiated to 15% by digital iage processing of SEM iages. This porosity aount is acceptable for HVOF coatings [10]. This average value is in agreeent with literature for the HVOF process [9, 10] and it is an indication of acceptable quality of the coating. (iii) CrC-NiCr coating The icrostructure of CrC-NiCr coating can be seen in Fig. 5. The thickness of the coating was circa average µ. The icrostructure of the wear-resistant coating CrC-NiCr (690 HV0 g, 15 s) is a distribution of chroiu carbides in a high carbon steel atrix. The easured volue fraction of chroiu carbides and porosity were 12% and 15%, respectively. B. Exaination of worn surfaces The typical aspect of the worn surfaces as seen in stereographic icroscope is shown in Fig. 6. Detailed analysis of the worn surfaces revealed wear arks typical of erosion at grazing incidence, with icro-cutting and icroploughing as the ain wear echaniss observed at the surface of all the stainless steels tested and the Cr 2 coating (Fig. 7), being these arks ore evident and evenly distributed in the stainless steel saples. The SEM iage on icrographs tells us about the surface orphology of the various saples. The 13Cr4Ni steel has shown platelet echanis which is being operationalized in erosion. The foration of crater and lips can be viewed. These are fored due direct ipact and can be reoved by ipact of slurry.fig shows splat by splat laellar structure foration in both the coatings with the presence of partially elted region of the nano particles in Cr 2 coatings. On the other hand, the worn surfaces of the CrC-NICR coatings showed a differential response as a function of the phases present in the icrostructure, as shown in Fig. 9. Chroiu and oxides in Cr 2 and Cr/Co areas in CrC-NICR coatings contributed to increase the wear resistance due to their high hardness and Young odulus. As the testing tie increased the hard phases were gradually exposed to the erosive particles and the ain wear echanis changed fro icro cutting of atrix to spalling of hard phases. Evidences of brittle fracture were observed in the CrC-NiCr coating, as can be seen in Fig. 9 (arrow in left upper corner). Nevertheless, the analysis of the coatings before the slurry erosion tests reveals that siilar cracks are fored as a consequence of the theral spray process eployed, due to the high cooling speeds and the theral coefficient isatch between crc/co

4 Shara, Kuar and Shara 121 particles and Ni-Cr regions (Fig. 10). Unelted particles and droplets can also be observed before the surface is subitted to the slurry wear tests, but these features are reoved during the tests due to their low adherence to the substrate. A significant increase in icro-hardness was observed in the stainless steels surfaces after the slurry erosion tests, probably as a consequence of both artensitic transforation of retained austenite and work hardening effect. Spalling of hard phases Fig. 6. specien after conducting 2hrs erosion testing coated with CrC-NiCr Fig. 7 specien after conducting 2Hr erosion testing coated with Cr 2. Fig. 7(a) Morphology of the sand particle used in the tests Fig. 7(b) EDS of sand grains ) 60 (% 50 cy n e40 u q F re Plastically defored unused used Fig.c 0 40(414µ 50(298µ 70(155µ AFSNUMBER 100(1µ140(100µ 200(74µ [OPENINGSIZE(µ) Fig. 7(c)grain size distribution before and after the test in icroeters. C. Degradation of abrasive particles The typical orphology of abrasive particles before the tests and the change in size distribution as a consequence of the erosive process are presented in Fig. 7. Note that after the tests the distribution is shifted to saller grain sizes, which reveals fragentation of the particles and subsequent loss of their ability to erode the surface of the saples. D. Mass loss The ass loss of all the saples in the slurry erosion tests is shown in Fig. 8. The reported values were calculated fro the easured cuulative ass losses and tie of ipacting of particles of each of the aterials studied, naely 1.Generally speaking, in all the cases the uncoated steels reported higher volue losses than the theral sprayed coatings. The Cr 2 coating showed the best erosion résistance in all the velocity ranges, while the CrC-NiCr steel reported the higher ass losses of the tested aterials with coating. It is worth noticing that the uncoated stainless steels presented siilar volue losses during the first stages of the tests (Fig. 8). Nevertheless, after 90 in testing the 13Cr 4 Ni steel saples undoubtedly showed better erosion resistance, probably due to the differences in icrostructure such as the presence of hard chroiu carbides precipitated at grain boundaries. The one another iportant thing to be noted down that as the velocity ranges increases the aount of erosion also increase up to the 55 /s after going higher range of velocity the erosion is not linear with respect to other two velocities. This ay be due to that as the velocities is increasing the kinetic energy of the particle also increases as a result of which the particles are rebounded back and they interrupt the path of incoing particles. The axiu erosion will takes place at 55 /s. IV. CONCLUSIONS (i) The Cr 2 coating applied by HVOF process onto 13Cr4Ni stainless steel reported the best slurry erosion resistance of the studied aterials, in all the cases ainly as a consequence of the cobined properties of hard, wear-resistant particles and a ductile etallic atrix. (ii) The study shows the perforance of the steel and coating with arrange of velocity. (iii) The effect of increasing the velocity is also studied with the help of this study. (iv) The studied coatings showed ability to defor plastically when subitted to slurry erosion conditions, with little evidence of ass reoval by brittle fracture echaniss. Unelted particles and droplets are easily reoved fro the surface, but this does not affect the overall perforance of the coatings in ters of volue loss and ain wear echaniss.

5 (v) The applied coatings are an interesting alternative to enhance the wear resistance of coponents used in hydraulic achines, in particular under grazing incidence conditions and oderate-to-low ean ipact velocities. 20 s 2 Cr2O a g / c 3 10e tiv ( Ạ ) la 0u S s/ c u o L 60 Tie(in) Fig. 8. cuulative ass loss after slurry erosion tests at 40/s. e g 20 tiv 10 la u sin 0 lo cu a s tie in ins 13Cr4Ni Cr2O3 CrC-NiCr Shara, Kuar and Shara 122 REFERENCES [1] J. Hengyun, et al., The role of sand particles on the rapid destruction of the cavitation zone of hydraulic turbines, Wear 112, (1986). [2] T.H. Kosel, Solid particle erosion, in: ASM Handbook, vol. 18, Friction, Lubrication and Wear Technology, ASM International, pp (1992). [3] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, Cabridge, (1992). [4] K.H. Zu Gahr, Microstructure and Wear of Materials, Elsevier, Asterda, (1987). [5] G. Sundararajan, A coprehensive odel for the solid particle erosion of ductile Materials, Wear 149, (1995). [6] A.V. Levy, P. Yau, Erosion of steels in liquid slurries, Wear 98, (1984). [7] K. Sugiyaa, et al., Slurry wear and cavitation erosion of theral-sprayed cerets, Wear 258, (2005). [8] P. Kulu, I. Hussainova, R. Veinthal, Solid particle erosion of theral sprayed coatings, Wear 258, (2005). [9] D. Crawner, Theral spray processes, in: Handbook of Theral Spray Technologies, ASM International, (2004). [10] B.K. Prasad, Slurry wear characteristics of zinc based alloy effect of sand content of slurry silicon. Fig. 9. cuulative ass loss after slurry erosion tests at 80/s. 50 in s s 0 lo s g a tieinins 13Cr4Ni Cr2O3 CrC-NiCr Fig. 10. cuulative ass loss after slurry erosion tests at 60/s.