Water Droplet Erosion Resistance Research Articles

Effect of graphene addition on the performance of in-situ (TiC+TiBx)/Ti composite coatings by laser cladding: Microstructure and water droplet erosion resistance

In this study, different amounts of graphene (Gr, 0–2 wt%) were added into Ni60/Ti6Al4V powder mixtures to prepare (TiC+TiBx)/Ti composite coatings on Ti6Al4V substrates by laser cladding. By means of XRD, SEM, EDS, and TEM, the effects of Gr content on the dilution rate, phase composition, microstructure, microhardness and water droplet erosion (WDE) resistance of the coatings were comprehensively analyzed. The results showed that the coatings comprised phases of Ti2Ni, TiC, TiB, TiB2, α-Ti and β-Ti. The increased Gr content precipitated more TiC, providing more heterogeneous nucleation sites for other grains and suppressing the grain growth. The TiC-TiB2 and TiB-TiC intergrowth structures exhibited high load-bearing capacity and contributed to the synergistic hardening and strengthening effects. When the Gr content was 1 wt%, the coating had the highest microhardness of 886.3 HV0.3 and showed the best WDE resistance. The excellent performance of the hard phases, the existence of a large number of grain boundaries and the effect of the skeleton improved the WDE resistance of the coatings.

Effect of CeO2 on the TiB2 + TiC evolution and water droplet erosion resistance of laser-clad Ni60A coatings

In this study, CeO2-doped Ni60A coating was fabricated on the surface of Ti6Al4V alloys. During the fabrication, heterogeneous nucleation occurred between TiB2 and TiC phases within Ni60A coatings, and the TiB2 + TiC eutectic structure was thereby generated. Ce2O3 phases were formed in-situ by the decomposition of CeO2 powder in the molten pool. On the grain boundary of the CeO2-doped Ni60A coating, Ce2O3 particles were distributed, which inhibited the growth of hard phases TiC and TiB2. Compared with the coating without adding CeO2 powder, the CeO2-doped Ni60A coating had significantly lower crack sensitivity, and the TiB2 phase in the CeO2/Ni60A coating had a far smaller grain size due to the grain refinement effect. In addition, the TiC + TiB2 eutectic structure acted as the skeleton in the coating, enabling the coating to effectively resist the shear effect caused by high-speed water particles. After coating Ni60A and CeO2/Ni60A on the Ti6Al4V alloy, the water droplet erosion depth of the alloy was reduced by 38.7% and 53.2%, respectively. Therefore, adding CeO2 powder into Ni60A coatings has great application value in improving the water droplet erosion resistance of steam turbine blades.

Water-droplet erosion behavior of high-velocity oxygen-fuel-sprayed coatings for steam turbine blades

Abstract The water-droplet erosion of low-pressure steam turbine blades under wet steam environments can alter the vibration characteristics of the blade, and lead to its premature failure. Using high-velocity oxygen-fuel (HVOF) sprayed water-droplet erosion resistant coating is beneficial in preventing the erosion failure, while the erosion behavior of such coatings is still not revealed so far. Here, we examined the water-droplet erosion resistance of Cr3C2–25NiCr and WC–10Co–4Cr HVOF sprayed coatings using a pulsed water jet device with different impingement angles. Combined with microscopic characterization, indentation, and adhesion tests, we found that: (1) both of the coatings exhibited a similar three-stage erosion behavior, from the formation of discrete erosion surface cavities and continuous grooves to the broadening and deepening of the groove, (2) the erosion rate accelerates with the increasing impingement angle of the water jet; besides, the impingement angle had a nonlinear effect on the cumulative mass loss, and 30° sample exhibited the smallest mass loss per unit area (3) an improvement in the interfacial adhesion strength, fracture toughness, and hardness of the coating enhanced the water-droplet erosion resistance. These results provide guidance pertaining to the engineering application of water erosion protective coatings on steam turbine blades.

The water droplet erosion resistance of Ni-based composite coating through laser cladding

Water droplet erosion (WDE) usually occurs in TC4 turbine blades under actual working conditions, which seriously endangers the safe and stable operation of the turbine. To solve the problem of WDE in TC4 turbine blades under high-speed solid-liquid impact, CeO2/Ni60A composite coatings with different laser powers were successfully developed on the surface of TC4 by using laser cladding technology. The working environment of the turbine blade under high- speed solid-liquid impact was simulated by water jet technique, and the effect of laser power on the coating resistance to water droplet erosion was analyzed. It can be seen from the results that the coating was composed of α-Ti, Ti2Ni, TiB2 and TiC, and laser power had a significant influence on the growth of grain structure. At 6000 W laser power, the average microhardness of the coating reached 1105 HV0.3, i.e., 3.25 times of TC4 substrate. However, too high laser power made the hard phases of TiC and TiB2 brittle. In the water droplet erosion experiment, the high power resulted in the cracks in the coating. At the laser power of 5000 W, the grain refinement effect was the best. The width and depth of water erosion were 0.908 mm and 0.148 mm, respectively, which were lower than TC4 substrate by 27.5% and 41.5% times. The preparation of CeO2/Ni60A coatings on the surface of TC4 alloy was effective to solve the problem of WDE in TC4 turbine blades and prolong the service life of TC4 blades. Moreover, the finer the grain structure in the coating, the better the WDE resistance.

Experimental investigation on the water droplet erosion characteristics of blade materials for steam turbine

In this paper, the water droplet erosion ( WDE) characteristics of four blade materials under different high-speed liquid-solid impingement conditions are tested with the experimental steps and data processing methods in ASTM-G73. On the basis of cumulative erosion curves, the maximum erosion rate ( ERmax) is obtained to analyze the WDE resistance of the testing materials quantitatively. The effects of target surface roughness, impact angle and impact velocity on the WDE characteristics of blade materials under specific working conditions are studied. The velocity coefficients ( n) of the testing materials under high-speed liquid-solid impact are analyzed. In order to investigate the mechanism of WDE, the material failure characteristics and shallow layer hardness ( SLH) at three characteristic moments corresponding to different WDE periods, are investigated with the SEM morphology of erosion section and hardness distribution along the depth direction at the impact position. The research results can provide reference data and technical support for the structure design and material selection of steam turbine blades.

Experimental Research on Water Droplet Erosion Resistance Characteristics of Turbine Blade Substrate and Strengthened Layers Materials.

In this paper, the water droplet erosion (WDE) performance of typical martensitic precipitation substrate 0Cr17Ni4Cu4Nb in steam turbine final stage, laser solid solution strengthened sample, laser cladding sample and brazed stellite alloy samples have been studied based on a high-speed rotating waterjet test system. The WDE resistance of several materials from strong to weak is in sequence: Brazed stellite alloy > laser cladding sample > laser solid solution sample > martensitic substrate. Furthermore, the WDE resistance mechanism and the failure mode of brazed stellite alloy have been revealed. It is found that the hard carbide in the stellite alloy is the starting point of crack formation and propagation. Under the continuous droplet impact, cracks grow and connect into networks, resulting in the removal of carbide precipitates and WDE damage. It is proved that the properties of the Co-based material itself is the reason for its excellent WDE resistance. And the carbides have almost no positive contribution to its anti-erodibility. These new findings are of great significance to process methods and parameter selection of steam turbine blade materials and surface strengthened layers.

Experimental investigation on effect of surface strengthening process and roughness on water droplet erosion behavior in turbomachinery

In this paper, the water droplet erosion (WDE) performance of a typical martensitic blade material 17-4 PH (0Cr17Ni4Cu4Nb), surface strengthened laser cladding stellite and brazed stellite alloy is studied based on high-speed water jet test system. The WDE resistances from high to low are the brazed stellite alloy, laser cladding stellite alloy and 17-4 PH. For the same material, the greater the initial roughness of the target surface, the more severe is the WDE damage. Improving the surface finish of the target is conducive to the diversion and expansion of the liquid flow, prolonging the incubation period, and thus enhancing the WDE resistance.

Experimental study on water droplet erosion resistance of coatings (Ni60 and WC-17Co) sprayed by APS and HVOF

In order to simulate the actual operating environment of the last stage blades of steam turbines，especially under the condition of high-speed liquid-solid impact (over 600 m/s), a new experimental rig for testing the water droplet erosion (WDE) characteristics is built, and three kinds of specimen structure adapted to different impact angles are designed. The WDE resistance of the four kinds of thermal spraying coating, which are prepared using atmospheric plasma spraying (APS) and high velocity oxygen fuel spraying (HVOF) with two kinds of alloy powder (Ni60 and WC-17Co) on the substrate material (17-4PH stainless steel), are tested using the experimental rig. In this paper, three-dimensional optical microscopy is used to obtain the morphology of WDE grooves on the target surface of different coating specimens. During the measurement process, three characteristic sections are selected, and the average profile of erosion grooves at different times are given respectively. The average area loss of the erosion section is obtained with integrating method. The cumulative loss curves are analyzed and the WDE resistance of the present coatings are compared. It is found that the HVOF-WC17Co coating is of the best WDE resistance.

Comparative erosion tests results of titanium alloy Ti-6Al-4V samples obtained by using 3D - printing technology and manufactured by the traditional technological method

Rotating blades of steam turbines and compressors operate in conditions of various damaging factors, including shock-dynamic impacts of liquid droplets contained in steam or in air. One of the most promising ways to reduce damage from water droplet erosion can be the application of the 3D - printing technology that will allow to replace damaged parts of structural elements, as well as cut repair costs and prolonging the service life of the equipment.The aim of present work was to determine the possibility of using the 3D - printing technology of the turbine blades for use in power engineering. The main task of the work was to check samples of the material obtained by this method for water droplet erosion resistance, which is one of the regulated tests type at the turbine blades design stage.This paper presented the results of water droplet erosion process experimental studies of the titanium alloy Ti-6Al-4V samples prepared by the method 3D - printing and by the traditional technological process at a collision velocity Cimp = 300 m/s with a monodisperse flow of liquid droplets with a diameter dd = 1000 μm. Erosion tests were carried out by using a set of research equipment URI «Hydroshock rig «Erosion-M» NRU «MPEI». It has been established that the erosion resistance for the incubation period duration of the Ti-6Al-4V titanium alloy samples manufactured by the 3D - printing method is 1.4 times less than for the samples manufactured by the traditional technological process. The data obtained can be used in the future in the development of complex passive methods for protection steam turbines and compressors last stages rotating blades: creation of replaceable erosion-resistant inserts by 3D - printing with a protective coating.

Water Droplet Erosion Resistance of Aluminizing Diffusion Coatings on Steel Tubes

Water Droplet Erosion Resistance of Aluminizing Diffusion Coatings on Steel Tubes

Plasma nitriding of Ti6Al4V alloy for improved water erosion resistance

Water droplet erosion (WDE) is a severe problem encountered in the operation of gas turbines. The damage is caused when small water droplets hit the surface of the compressor blades at very high velocities. In this work, RF plasma nitriding of Ti6Al4V alloy was performed with the aim of improving its resistance to the WDE. Process parameters, including temperature and gas atmosphere, were varied to evaluate the influence of nitriding conditions on the WDE resistance. Structural and morphological changes of the samples were investigated by scanning electron microscopy (SEM), phase composition by X-ray diffraction, and cross-sectional hardness distribution by nanoindentation. Water droplet erosion tests were performed using a rig that simulates WDE at working conditions of the compressor blades. Each sample was subjected to 6 min of WDE tests. After every minute of the experiment the erosion area of the sample was evaluated by the optical microscope and the specific mass loss was determined. Damage was analyzed using SEM and confocal laser scanning microscopy. The results show significantly higher resistance against WDE of Ti6Al4V nitrided at low temperature, especially in the case of hydrogen-free nitriding atmosphere. The incubation time for these samples was extended four times, as compared to the bare substrates or to the samples nitrided at higher temperatures.

HVOF and HVAF Coatings of Agglomerated Tungsten Carbide-Cobalt Powders for Water Droplet Erosion Application

Water droplet erosion (WDE) is a phenomenon caused by impingement of water droplets of several hundred microns to a few millimeters diameter at velocities of hundreds of meters per second on the edges and surfaces of the parts used in such services. The solution to this problem is sought especially for the moving compressor blades in gas turbines and those operating at the low-pressure end of steam turbines. Thermal-sprayed tungsten carbide-based coatings have been the focus of many studies and are industrially accepted for a multitude of wear and erosion resistance applications. In the present work, the microstructure, phase analysis and mechanical properties (micro-hardness and fracture toughness) of WC-Co coatings are studied in relation with their influence on the WDE resistance of such coatings. The coatings are deposited by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes. The agglomerated tungsten carbide-cobalt powders were in either sintered or non-sintered conditions. The WDE tests were performed using 0.4 mm water droplets at 300 m/s impact velocity. The study shows promising results for this cermet as WDE-resistant coating when the coating can reach its optimum quality using the right thermal spray process and parameters.

Energy based approach for understanding water droplet erosion

Water droplet erosion (WDE) is a complex wear phenomenon with many interacting parameters. For decades, many test rigs and instruments have been developed to study it, producing a vast amount of useful data pertinent to WDE resistance of different structural materials. Comparing test results produced by different test rigs has always been a challenge, since test conditions used by each rig were difficult to replicate by other test setups. In this work, a new method of representing WDE results in terms of the applied energy intensity is proposed. This method is used to report the WDE test results of three structural materials (12% Cr stainless steel, Ti6Al4V and TiAl) tested at various conditions. The new representation enables better comparison between test results. A new coefficient (ξ) is introduced as a measure of how representative the applied energy intensity is for WDE tests. The proposed severity coefficient (ξ) captures the variation in the absorbed energy by the sample's surface due to test conditions change. This is achieved by quantifying the materials response to the change in WDE test parameters. (ξ) is then used to compare the results of WDE experiments done at various erosion conditions or even on different test rigs.

Influence of impact speed on water droplet erosion of TiAl compared with Ti6Al4V.

Water Droplet Erosion (WDE) as a material degradation phenomenon has been a concern in power generation industries for decades. Steam turbine blades and the compressor blades of gas turbines that use water injection usually suffer from WDE. The present work focuses on studying erosion resistance of TiAl as a potential alloy for turbine blades compared to Ti6Al4V, a frequently used blade alloy. Their erosion behaviour is investigated at different droplet impact speeds to determine the relation between erosion performance and impact speed. It is found that the relationship is governed by a power law equation, ER ~ Vn, where the speed exponent is 7–9 for Ti6Al4V and 11–13 for TiAl. There is a contrast between the observed speed exponent in this work and the ones reported in the literature for Ti6Al4V. It is attributed to the different erosion setups and impingement conditions such as different droplet sizes. To verify this, the erosion experiments were performed at two different droplet sizes, 464 and 603 μm. TiAl showed superior erosion resistance in all erosion conditions; however, its erosion performance exhibits higher sensitivity to the impact speed compared to Ti6Al4V. It means that aggressive erosion conditions decrease the WDE resistance superiority of TiAl.

The effect of initial surface roughness on water droplet erosion behaviour

Water droplet erosion (WDE) is defined as the progressive loss of original material from a solid surface due to continuous impingements of water droplets or jets. Several factors are known to influence the WDE process, such as impact speed and water droplet size. The initial surface roughness was not given enough attention in the literature as a factor that may influence the WDE behaviour of materials. In this work the effect of initial surface roughness on the WDE of a special martensitic stainless steel (12%Cr-steel) and Ti6Al4V is investigated. Experiments were done by varying three parameters: initial surface roughness, test speed, droplet size. It was concluded that the initial surface quality influences the length of the incubation stage, and may influence the maximum erosion rate. The amount of asperities and irregularities on the surface of samples was found to be the main reason for the difference in the WDE erosion behaviour. Moreover, the importance of reporting the initial surface of tested samples was emphasised, especially when comparisons between the WDE resistance of different substrate materials and/or treatments are held.

Water Droplet Erosion Behavior of High-Power Diode Laser Treated 17Cr4Ni PH Stainless Steel

This article deals with water droplet erosion (WDE) behavior of high-power diode laser (HPDL) treated 17Cr4Ni PH stainless steel. After HPDL treatment, the water droplet erosion resistance (WDER) of 17Cr4Ni PH stainless steel has not improved. The main reason is the surface hardness, which has not improved after HPDL treatment though the microstructure has become much finer. On the other hand, precipitation hardening of the alloy at 490°C for 3 h has resulted in improved WDER more than twice. This is because of its increased microhardness and improved modified ultimate resilience (MUR), and formation of fine grained microstructure. The WDER has been correlated with MUR, a single mechanical property, based upon microhardness, ultimate tensile strength, and Young’s modulus. WDERs of HPDL treated, untreated, and precipitation hardened 17Cr4Ni PH stainless steel samples were determined using a WDE test facility as per ASTM G73-1978. The WDE damage mechanism, compared on the basis of MUR and scanning electron micrographs, is discussed and reported in this article.

Laser Treatment of Textured X20Cr13 Stainless Steel to Improve Water Droplet Erosion Resistance of LPST Blades and LP Bypass Valves

X20Cr13, a martensitic stainless steel, is commonly used for the manufacture of low pressure steam turbine (LPST) moving blades and LP bypass valves of fossil fuel and nuclear power plants. The LPST blades, at present, are laser surface treated to improve their water droplet erosion (WDE) resistance. The laser-treated X20Cr13 stainless steel has improved the water droplet resistance (WDER) several times compared to untreated ones. Further improvements are being carried out by providing a carbide-based HVOF coating having appropriate surface roughness or by creating textured surfaces and treating with a high power diode laser. The surfaces, having appropriate roughness, absorb more laser energy, resulting in improved microstructure, microhardness, modified ultimate resilience, and thicker hardened layer. The WDER of laser-treated textured X20Cr13 stainless steel has improved significantly compared to the untextured ones. The WDE test results of laser-treated textured and untextured X20Cr13 stainless steel along with their microhardness, modified ultimate resilience, microstructure, SEM, and XRD analysis are discussed and reported in this paper. The laser-treated textured X20Cr13 stainless steel is highly suitable for LP bypass valves and LPST blades for achieving a thicker hardened layer with lesser heat input to the components.

Cavitation Erosion Behavior of HPDL-Treated TWAS-Coated Ti6Al4V Alloy and Its Similarity with Water Droplet Erosion

Twin wire arc-sprayed (TWAS) coating of commercially available SHS 7170-cored wire was obtained on Ti6AL4V alloy, and to improve its properties, it was further surface treated with high-power diode laser (HPDL). The cavitation erosion (CE) resistance of TWAS-coated samples was evaluated as per ASTM G-32-2003 and it was compared with laser-treated and untreated Ti6Al4V alloys. The CE resistance of TWAS-coated SHS 7170 samples after HPDL treatment has improved significantly. The main reasons for its improvement are elimination of pores, increased fracture toughness, reduced hardness, and brittleness. The CE resistance of HPDL-treated TWAS coating is compared with water droplet erosion resistance. It is observed that there is a similarity in the both the phenomenon.

Water droplet erosion and microstructure of laser-nitrided Ti6A14V

The ability of laser nitriding to improve water droplet erosion resistance of Ti6Al4V alloy was studied. Using a CO 2 continuous laser, a layer of about 400 μm thickness was nitrided and another layer of 400–500 μm was only heat affected. Electron microscopy observations showed that the microstructure of nitrided layers consists essentially of TiN compounds which are embedded in Ti(α) matrix. Depending on the nitrogen concentrations within the feeding gas, the titanium nitrides exhibited plate-like shape or dendritic morphologies. Below the nitrided layer a thickness of 50–100 μm of samples underwent martensitic structure which in its turn gives rise progressively to bimodal (α + β) base material. Laser nitriding increased microhardness from 370–400 to 650–800kgf mm −2, and enhanced erosion resistance significantly compared with untreated Ti6A14V and hardened 12% Cr stainless steel. The mechanism of material removal by erosion was changed from work hardening and platelets detachment in untreated samples to brittle fracture by formation of large flakes and spalling in nitrided layers. Advanced stages of erosion are accompanied by the appearance of macrocracks of ten in the nitrided zone, but some of them propagate even into heat-affected area. The annealing at 650 and 700°C of the laser-nitrided samples results in precipitation of β phase rich in vanadium.

Water droplet erosion of laser surface treated Ti6A14V

The work reports on the effect of laser surface treatments on the water droplet erosion resistance of a commercial alloy of nominal composition Ti6A14V. Use is made of a continuous wave CO 2 laser to melt the Ti6A14V in both inert and dilute nitrogen atmospheres. The gas composition of the atmosphere is set at either 100% Ar, 90% Ar + 10% N 2 or 80% Ar + 20% N 2 by volume. The successive intertrack overlap is set at either 50% or 75% of the surface width of a single melt track. The cumulative mass loss during water droplet erosion of the laser surface melted material decreases substantially relative to the untreated material for all processing conditions examined. The enhanced erosion resistance is attributed to increases in the surface layer microhardness as well as the resistance of the martensitic microstructure to the hydraulic penetration mechanism of erosion. The benefits of nitrogen alloying over surface melting in an inert environment are not substantial, but this may be attributed to pre-existing micro-cracks in the nitrogen alloyed surface layers.