revista tradusa

Analele Universitii Constantin Brncui din Trgu Jiu, Seria Inginerie, Nr. 3/2009

Annals of the Constantin Brncui University of Trgu Jiu, Engineering Series, No. 3/2009

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TEHNICA HVOF DE ACOPERIRE

CU STRATURI MCRALY A

COMPONENTELOR

TURBINELOR CU GAZE

Prof. Univ. Dr. Ing. Ion Mitelea ,

Universitatea Politehnica din Timioara ef Lucrri Marius-Eremia Vlaicu-Popa ,

Universitatea Constantin Brncui Tg-Jiu

Rezumat: Lucrarea analizeaz succint condiiile de exploatare a turbinelor cu gaz , materialele selectate la execuia acestora i oportunitiile oferite de tehnica pulverizrii HVOF pentru acoperirea suprafeelor cu straturi funcionale de tip MCrAlY. Cuvinte cheie: acoperire, turbinelor, gaze 1. Condiiile de exploatare ale turbinelor cu gaz Generatoarele de putere electric au utilizat i utilizeaz turbine cu abur, turbine cu gaz i turbine generator, tubulatur de oel, schimbtoare de cldur i boilere, fiecare ndeplinind o anumit funcie. Componentele lor sunt executate din metale sau aliaje metalice, de regul oeluri slab sau nalt aliate cu Cr, aliaje cu baz de Ni, aliaje cu baz de Co, alam sau Cu i, mai rar, aliaje cu baz de Ti [1]. Selecia acestor materiale se face lundu-se n considerare condiiile n care urmeaz a funciona componenta respectiv:

tensiuni mecanice, deformaii sub sarcini;

temperaturi de funcionare, modificri ale temperaturii;

natura mediului de lucru; durata operaional; costuri.

Condiiile de funcionare ale acestor componente pot conduce ns la discrepane ntre design-ul funcional i materialul

HVOF TECHNIQUE FOR

MCRALY LAYERS COATING OF

GAS TURBINES

University Professor Engineer Ion Mitelea

PhD, Polytechnic University from din Timioara

Lecturer Marius-Eremia Vlaicu-Popa , Constantin Brncui University Tg-Jiu

Abstract: The paper briefly analyzes gas turbines operation conditions, the materials selected for their execution and the opportunities provided by HVOF spraying technique for covering surfaces with MCrAlY functional layers.

Keywords: coating, gas turbines 1. Gas turbines operation conditions

Power generators have used steam turbines, gas turbines and generator turbines, steel piping, heat exchangers ad boilers, each with a certain function. Their components are executed of metals or metallic alloys, usually poorly or highly Cr alloyed steels, Ni based alloys, Co based alloys, brass or Cu based alloys and rarely Ti based alloys Ti [1]. The selection of these materials is made by considering the conditions for the operation of that component:

Mechanic strength, deformations below loads;

Operation temperatures, temperatures alteration;

Operation environment nature; Operational duration; costs.

The operation conditions of these components may lead to discrepancies between the functional design and the selected material. The requirements for



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selectat. Cerinele pentru maini performante conduc la necesitatea unor temperaturi de operare ridicate, ceea ce implic folosirea unor materiale speciale rezistente la aceste solicitri. Inginerii proiectani ncearc s tempereze impactul condiiilor de operare asupra integritii componentelor prin dou modaliti, fiecare dintre acestea fiind absolut esenial n proiectarea componentelor destinate funcionrii n seciunile fierbini. Prima modalitate se bazeaz direct pe tehnologia acoperirii suprafeelor pentru a mri durata de via a componentelor. O alt modalitate, n care straturile contribuie indirect dar semnificativ, se bazeaz pe reducerea temperaturii de funcionare prin rcirea componentei de baz, straturile jucnd i n acest caz un rol cheie. Turbinele staionare cu gaz sunt constituite de regul din 3 mari componente i anume: compresor, camera de combustie i turbin (fig. 1, 2). Aerul introdus este comprimat pn la presiuni de 30 bar, apoi este injectat combustibilul producndu-se combustia. Din camera de combustie, gazele fierbini sau cele de evacuare, cu temperaturi de cca. 1350C, sunt conduse n turbin, unde are loc o dilatare i apoi o rcire a gazului. Temperatura gazelor la evacuare este de cca. 640C [2, 3]. Atunci cnd este necesar o sporire a puterii, se poate injecta de asemenea n compresor sau n camera de combustie, pe lng combustibil i aer, ap sau aburi [1,4].

performant machines lead to the need for high operation temperatures, which involves using special resistant materials to this stress. Designer engineers try to balance the impact of operation conditions on components integrity by two methods, each of them being absolutely essential in designing the components meant for operation in hot areas. The first method is directly based on surfaces coating technologies in order to increase components operation time. Another method, where layers contribute indirectly, but significantly, is based on operation temperature decrease by cooling the main component, layers having a key role in this case. Standing gas turbines usually consist of 3 main components, namely: breaker, combustion chamber and turbine (fig. 1, 2). The introduced air is compressed up to 30 bar pressures, then fuel is injected causing combustion. From the combustions chamber hot or exhaust gases, with an approximate temperature of 1350C, are lead to the turbine, where gas is dilated and cooled. Exhaust gases temperatures is almost 640C [2, 3]. When power growth is required, along with fuel, air, water or steam may be injected in the compressor or combustion chamber [1,4].

Fig. 1. Turbin cu gaz (Siemens Wenstinghouse)

Gas turbine (Siemens Wenstinghouse)



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Materialele din care sunt executate componentele turbinelor cu gaz sunt supuse permanent unor mecanisme de degradare ca de exemplu oboseala mecano-termic, oxidare, coroziune etc. Degradarea datorat mediului cum este oxidarea sau coroziunea este dependent de natura i temperatura gazelor ce vin n contact cu materialul.

Materials included in gas turbines components permanently undergo degradation mechanisms like for instance, mechanic-thermal wear, oxidation, corrosion etc. Degradation due to environment like oxidation or corrosion depends on the nature and temperature of gases making contact to the material.

Fig. 2. Reprezentarea schematic a unei turbine staionare cu gaz

Graphic representation of a gas standing turbine

mbuntiri semnificative s-au realizat n domeniul straturilor de acoperire pentru a se asigura o bun rezisten la oxidare i coroziune la temperaturi nalte. Pentru un asemenea scop, la ora actual, foarte des utilizate sunt straturile de acoperire MCrAlY (n care M = Co, Ni sau Fe) sau aluminidele (TiAl, PtAl) [4]. Suplimentar, pentru a se asigura o izolare termic corespunztoare a componentelor turbinelor, se recurge la depunerea unui strat ceramic (ZrO2 Y2O3) peste startul MCrAlY, acesta din urm cu rol intermediar n sistemul TBC (Thermal Barrier Coatings straturi cu rol de barier termic). 2. Compatibilitatea strat substrat Straturile MCrAlY (M = Co, Ni, Fe, plus Cr, Al, Y i uneori alte elemente de aliere Ti, Hf) sunt aplicate ca straturi de acoperire ale componentelor ce lucreaz la temperaturi nalte i n medii corozive (de expl. palete de

Significant improvements have been made in the field of coating layers in order to provide oxidation and corrosion resistance at high temperatures. For such a purpose, MCrAlY coating layers are very used (where M = Co, Ni or Fe) or alluminides (TiAl, PtAl) [4]. In addition, in order to provide an accurate turbines thermal isolation, a ceramic layer (ZrO2 Y2O3) is laid over the MCrAlY layer, the latter having an intermediary role in TBC system (Thermal Barrier Coatings). 2. Layer-sub-layer compatibility MCrAlY layers (M = Co, Ni, Fe, plus Cr, Al, Y and sometimes other alloy elements Ti, Hf) are laid as elements coating layers operating at high temperatures and in corrosive environments (for instance turbine pallets). These layers have a good chemical and thermal compatibility with the sub-layer materials (usually Ni or Co based super-



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turbin). Aceste straturi prezint o bun compatibilitate chimic i termic cu materialul substratului (de regul superaliaj pe baz de Ni sau Co), avnd un efect minim asupra proprietilor acestuia, mbuntindu-i ns rezistena de oxidare i coroziune la temperaturi nalte. O atenie deosebit se acord compoziiei chimice a straturilor MCrAlY, n special n ceea ce privete coninutul de Cr i Al, aceste dou elemente ndeplinind rolul principal n asigurarea proteciei la oxidare (Al) i coroziune (Cr). Comportamentul bun la oxidare al straturilor se datoreaz abilitii lor de a forma la suprafa o pelicul rezistent de -Al2O3, care are ca scop mpiedicarea oricrei interaciuni ntre suprafaa materialului de baz i mediul exterior corosiv. Aceast pelicul de oxid reprezint unul dintre cele mai bune straturi protectoare mpotriva oxidrii n domeniul temperaturilor nalte, oxidul fiind compact i stabil chimic. Pelicula de -Al2O3 se consum n timp, refcndu-se ns continuu atta timp ct stratul MCrAlY dispune de rezerve suficiente de Al. n condiiile n care stratul nu mai poate furniza Al, ali oxizi se vor forma la suprafaa straturilor MCrAlY (Cr2O3, CoO, NiO sau oxizi micti spineli: Ni(Al,Cr)O, Co(Ni,Cr)O etc.) dar care nu asigur protecia dorit. La scderea coninutului de Al sub o anumit limit (n cazul straturilor Co/NiCrAlY de 5-6% Al), este necesar nlocuirea stratului. Straturile MCrAlY pot fi depuse prin numeroase procedee (depunerea fizic n stare de vapori cu fascicul de electroni: EB-PVD, depunere prin pulverizare termic cu jet de plasm la presiune joas LPPS sau n vid - VPS). n ultimii ani ns, procedeul de depunere prin pulverizare termic n flacr, cu vitez ultrasonic HVOF (High Velocity Oxygen Fuel), a fost adoptat pentru depunerea acestor straturi protectoare, tehnica respectiv devenind popular datorit

alloy), with a minimum effect on its characteristics, improving its oxidation and corrosion resistance at high temperatures. Special attention is paid to the chemical composition of MCrAlY layers, especially regarding the Cr and Al content, these two elements having the main role in providing protection to oxidation (Al) and corrosion (Cr). Layers good oxidation behaviour is due to their ability to form a resistance film of -Al2O3, in order to prevent any interaction between the main material basis and corrosive exterior environment. This oxide film is one of the best protective layers against oxidation in the field of high temperatures, oxide being compact and chemically stable. The -Al2O3 film is time consuming, continuously recovering as long as the MCrAlY layer has enough reserves of Al. If the layer cannot provide Al, other oxides will result at the surface MCrAlY layers (Cr2O3, CoO, NiO or mixed oxides spinels: Ni(Al,Cr)O, Co(Ni,Cr)O etc.) but which do not provide protection expected. When the Al content decreases below a certain limit (in case of Co/NiCrAlY layers of 5-6% Al), the layer needs replacing. MCrAlY layers may be laid through numerous procedures (physical steam deposit with electrons beam: EB-PVD, thermal spraying deposit with plasma jet at low pressure LPPS or VPS). For the last few years, thermal spraying flame coating procedure was used with HVOF ultrasound (High Velocity Oxygen Fuel), in order to lay down these protective layers, the technique becoming popular due to its flexibility, low costs, and especially due to the quality of resulting products, as they are dense and have low internal porosity and oxidation. In this paper, researches focused on the study of HVOF coating process of various layers of MCrAlY.



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flexibilitii, costurile reduse, dar n special datorit calitii acoperirilor rezultate, acestea fiind dense i prezentnd o porozitate i oxidare intern redus. n prezenta lucrare cercetrile au fost axate pe studiul particularitilor procesului de depunere HVOF a unor straturi diferite de MCrAlY. 3. Materialele utilizate pentru componentele turbinelor cu gaz Cerinele pentru maini performante au condus la necesitatea de cretere a temperaturii de operare, ceea ce implic folosirea unor materiale speciale, rezistente la aceste temperaturi. Materialele utilizate pentru componentele turbinelor cu gaz sunt: oeluri nealiniate, de cele mai multe ori acoperite cu un strat de vopsea pe baz de Zn pentru orificiile de admisie ale aerului; pentru paletele sau elicile compresorului oeluri aliate cu Cr n anumite condiii de operare, susceptibile la coroziune acvatic sau pitting, oeluri nalt aliate, aliaje de Ti sau protejarea acestor pri cu straturi de suprafa, prile rotative ale compresorului fiind de regul acoperite cu straturi rezistente la uzare prin abraziune [1]. Seciunile fierbini ale compresorului i turbinei sunt realizate din aliaje cu baz de Ni sau Co, aliaje complexe din punct de vedere al compoziiei chimice, ce se caracterizeaz prin proprieti superioare, o combinaie de rezisten i ductilitate la temperaturi relativ nalte (precipitarea fazei AlNi3 coerente cu reeaua matricei), rezisten proprie la mediu, tenacitate i rezisten la oboseal, ce nu sunt n prezent disponibile n cazul altor sisteme de aliaje. Atunci cnd problema rezistenei la coroziune sau oxidare devine important sau cnd apare necesitatea de izolare termic a componentelor, pentru protecia acestor superaliaje se utilizeaz straturi de suprafa, metalice sau ceramice (TBC). Difuzorul de evacuare este executat din oel placat sau pulverizat cu Zn, coloanele

3. Materials used for gas turbines components

The requirements for performant machines lead to the need for high operation temperatures, which involves using special resistant materials to these temperatures. Materials used for gas turbines components are: non-alloyed steels, often covered with Zn based paint layer for air inlet holes; for compressor pallets or propellers Cr alloyed steels in certain operation conditions, liable of aquatic erosion or pitting, highly alloyed steels, Ti alloys or by protecting these parts with surface layers, compressor rotating parts being usually coated with wear resistant layers through abrasion [1]. Compressor and turbine hot sections are made of Ni or Co based alloys, complex alloys from point of view of chemical composition, characterized by higher properties, a combination of resistance and ductility at relatively high temperatures ( phase precipitation AlNi3 coherent with network matrix), proper resistance to the environment, tenacity and wear resistance, which are not currently available in the case of other alloy systems. When the matter of corrosion or oxidation resistance becomes important or when components thermal isolation needs occurs, surface metallic or ceramic layers (TBC) are used in order to protect these super-alloys. The exhaust outlet is made of plated steel or Zn sprayed steel, rotor and stator columns are made of alloy steel, certain conditions requiring coating with protection layers, and rotor disks are made of Ni based alloys. At present, over 50% of gas stand turbines components are coated with protective layers [1]. If other methods have been used in order to achieve these layers, like galvanization or chemical steam deposit, at present thermal spraying is used. Table 1 describes gas turbines components that may be covered with protection layers, their main elements,



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rotorului i ale statorului sunt din oel aliat, n anumite condiii fiind necesar acoperirea cu straturi de protecie, iar discurile rotorului sunt produse din aliaje pe baz de Ni. Actualmente, peste 50% din componentele turbinelor staionare cu gaz sunt acoperite cu straturi protectoare [1]. Dac n trecut s-au utilizat pentru obinerea acestor straturi metode precum galvanizarea sau depunerea chimic n stare de vapori (CVD), n prezent se manifest tendina recurgerii la depunerile prin pulverizare termic. n tabelul 1 sunt prezentate componentele turbinelor cu gaz ce pot fi acoperite cu straturi de protecie, materialele de baz, straturile de protecie aferente precum i cerinele pe care acestea trebuie s le ndeplineasc.

their related protection layers as well as the requirements that they have to meet.

Tabel 1: Componente ale turbinelor staionare cu gaz, materiale de baz i pentru strat

(ZrO2-Y2O3 Yttria Stabilized Zirconia, strat ceramic cu rol de barier termic) Table 1: Gas standing turbines components, main materials and layer materials

(ZrO2-Y2O3 Yttria Stabilized Zirconia, ceramic layer with the role of thermal barrier)

Component Metal de baz Strat Proces de obinere a stratului

Cerine - rezisten

Orificii imersie aer Oel nealiat Zn, epoxy Vopsire Oxidare, eroziune, coroziune acvatic

Palete compresor Oel cu 12% Cr TiAl6V4

Al, ceramic Vopsire Eroziune, coroziune acvatic, stres, oboseal

Structuri asamblate Piese turnate din oel, n numeroase cazuri aliaje cu baz de Ni

CrC, WC+Ni, Co Depunere prin pulverizare cu arc, APS, HVOF

Uzur, friciune, sudur

Structuri turnate Piese turnate din oel slab aliat

NiCrxx, NiAlxx HVOF Oxidare

Alte pri ale compresorului

Superaliaje cu baz de Ni, Co

NiAl, MCrAlY APS, HVOF Strat de intermediar, coroziune i oxidare la temperaturi ridicate



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Pri de rcire ale combustorului


ZrO2-Y2O3 APS, HVOF Reducerea temperaturii suprafeei, TBC

Cr, Al CVD, cromizare, alitare

MCrAlY(+Re, Ta) LPPS, HVOF PtAl Galvanizare,

alitare AlSi Vopsire +

sintetizare

Oxidare, coroziune la temperaturi nalte, strat intermediar

Paletele turbinelor cu gaz


ZrO2-Y2O3 APS, HVOF TBC, reducerea temperaturii suprafeei

4. Procese de depunere prin nclzire cu flacr n cadrul acestui procedeu, materialul de depunere este trecut printr-o flacr de combustie, ajungnd astfel n stare topit. Particulele topite sunt accelerate spre substrat cu ajutorul unui curent de gaz sub presiune (aer sau azot, argon, heliu). Topirea cu flacr oxiacetilenic prezint avantajul c permite utilizarea ca material de aport att srm, pulbere metalic sau ceramic, ct i materiale plastice. n acest mod se poate realiza o mare varietate de straturi protectoare cu aceeai instalaie. n cadrul acestui procedeu exist trei principale variante ce utilizeaz flacra pentru topirea materialului de depus i anume:

a. Procesul de depunere prin pulverizare cu flacr;

b. Procesul de pulverizare i fuziune (retopire);

c. Procesul de pulverizare termic cu vitez ridicat HVOF.

a. Procesul de depunere prin pulverizare

cu flacr n acest caz, sursa de cldur necesar topirii materialului de depus (ce poate fi sub form de srm, vergele sau pulbere) este furnizat de ctre o flacr de combustie. n cazul sistemului cu srm sau vergele, mecanismul

4. Firing warming coating processes In this procedure, the coating material passes through combustion flame, reaching a melted state. Melted particles are accelerated towards the sub-layer with the help of gas current under pressure (air or nitrogen, argon, helium). Oxyacetylene firing melting has the benefit that it allows the use of additional material like wire, metallic or ceramic powder, as well as plastic materials. This allows to create many protection layers with the same plant. These procedure has three main variants using flame in order to melt coating material namely:

a. firing spraying coating process; b. spraying and fusion (re-melting)

process; c. HVOF high velocity thermal spraying

process. a. Firing spraying coating process

In this case, the heating source necessary for melting the coating material (either wire, rod or powder) is provided by a combustion flame. In the case of wire or rod system, the supply mechanism leads the stock of material inside the combustion chamber where the flame melts and carries the particles (due to high velocities of gases flowing) hence resulting a sprayed coating. In the case of powder variant, it is carried with the help of



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de alimentare conduce stocul de material n interiorul camerei de combustie unde flacra topete i propulseaz particulele (datorit vitezelor nalte de curgere a gazelor) rezultnd astfel o depunere pulverizat. n cazul variantei cu pulbere, aceasta este transportat cu ajutorul unui gaz adiional (n mod obinuit - azotat) ctre pistol, pentru ardere i depunere.

b. Procesul de pulverizare i fuziune (retopire)

Acest tip de proces este o modificare a procesului de depunere cu flacr menionat anterior, ce utilizeaz materialul sub form de pulbere. Procesul este o combinaie a 2 etape n cadrul crora, materialul de depus este mai nti pulverizat, apoi topit ntr-un arztor oxiacetilenic nclzit prin inducie sau tratat la cald ntr-un cuptor (n general n condiii de vacuum). Temperatura de fuziune este destul de nalt, adesea ajungnd la 1300C, producndu-se astfel straturi cu grosimi de cca. 2000 m, cu o duritate ridicat. Avantajul pe care l ofer acest proces este obinerea unei microstructuri dense i o bun aderen ntre strat i substrat. Principalul dezavantaj l reprezint temperatura nalt suportat de substrat n timpul etapei de fuziune.

c. Procesul de pulverizare termic cu vitez ridicat HVOF

Tehnica de pulverizare termic cu vitez ridicat HVOF (High Velocity Oxygen Fuel) intr de asemenea n categoria proceselor de depunere cu flacr, utiliznd material de strat numai sub form de pulbere. O larg utilizare a sa se remarc n industria aerospaial, unde straturi de suprafa sunt depuse n scopul de a reduce degradarea suprafeelor componentelor de baz, degradare ce survine datorit temperaturilor nalte i a mediilor de funcionare. Exist numeroase variante ale procesului,

an additional gas (usually nitrate) to the gun, for burning and coating.

b. Spraying and fusion process (re-melting)

This type of process is an alteration of the aforementioned firing coating process which uses material under the form of powder. The process is a combination of 2 stages in which the coating material is first of all sprayed in an oxyacetylene burner, warmed through induction or hot treated in a furnace (vacuum conditions generally) . The fusion temperature is relatively high, often reaching 1300C, resulting in layers of approximately 2000 m widths, with high roughness. This process advantage is the achievement of a dense adherent microstructure between layer and sub-layer. The main disadvantage is the high temperature faced by the sub-layer during the fusion stage.

c. HVOF high velocity thermal spraying process

High Velocity Oxygen Fuel is also included in the category of firing coating, using layer material only under the form of powder. Its wide use is revealed in spatial industry, where surface layers are deposited in order to reduce main components surfaces degradation, which occurs due to high temperatures and operation environments . There are various variants of the process, the widest known being: Detonation Gun HVOF System and Continuous Combustion HVOF System. The differences existing between these variants refer to the use of various combustion gases, various cooling systems etc. but, in general, for all process variants, the coating principle is the same. Detonation Gun The gun is easily recognized due to its front long rod (nozzle) (fig.3). When the gas mixture in general with acetylene and oxygen is fired through a firing



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cele mai cunoscute fiind: Detonation Gun HVOF System (Sistemul HVOF cu pistol de detonaie) i Continuous Combustion HVOF System (Sistemul HVOF cu combustie continu). Diferenele existente ntre aceste variante se refer la utilizarea unor diferite gaze combustibile, diferite sisteme de rcire etc. dar, n general, pentru toate variantele de proces, principiul de depunere rmne aceleai. Detonation Gun (Pulverizarea cu pistol de

detonaie) Pistolul poate fi uor de recunoscut datorit tijei lungi din partea din fa a acestuia (duza) (fig.3). Cnd amestecul de gaze, n general coninnd oxigen i acetilen, este aprins printr-o bujie de aprindere, o und de detonaie (explozie) controlat nclzete individual particulele pn la temperaturi de cca. 4500C, le accelereaz i apoi le expune la viteze subsonice de cca. 800 m/s. Structura lamelar, uniform i compact a straturilor astfel realizate ca urmare a temperaturii nalte i energiei cinetice a particulelor, asigur o duritate, densitate i for de legtur ridicate, comparabile cu cele ale straturilor depuse prin pulverizare n jet de plasm.

plug, a controlled detonation wave (explosion) individually heats particles up to temperatures of approximately 4500C, accelerates them and then it puts them through subsonic velocities of 800 m/s. Lamellar, uniform and compact structure of resulting layers due to high temperature and particles kinetic energy provides high connection roughness, density and strength, comparable to those of plasma jet spraying layers.

Fig. 3. Reprezentarea schematic a unui pistol de detonaie Graphic representation of a detonation gun

Grosimile straturilor (de pn la 0,5 mm) sunt Layers thickness (up to 0,5 mm) are achieved



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obinute prin detonaii multiple. n timpul procesului se produc nivele sonore nalte de cca. 150 decibeli din acest motiv sunt necesare incinte cu perei rezisteni, procesul fiind controlat din exterior. Sistemul HVOF cu combustie continu Sistemul de pulverizare termic DJ-HVOF este o tehnic de depunere cu flacr n cadrul creia materialul sub form de pulbere este topit prin utilizarea combustiei oxigenului i a gazului combustibil, fiind apoi propulsat cu o vitez foarte mare, prin utilizarea aerului comprimat, deasupra suprafeei ce urmeaz s fie acoperit (fig. 4). n zona de combustie, materialul sub form de pulbere este introdus n zona flcrii unde trece n stare topit sau semitopit, n funcie de temperatura de topire i de viteza cu care se alimenteaz materialul.

through multiple detonations. During the process, high sounds levels occur of approximately 150 decibels and this is why resistant walls buildings are required, the process being controlled from the outside. Continuous combustion HVOF system DJ-HVOF thermal spraying system is a firing coating technique in which the powder material is melted by using oxygen and combustible fuel and then rapidly propelled by using compressed air, above the surface o be covered (fig. 4). In the combustion area, the powder material is introduced in the firing area where it passes in melted or semi-melted state, depending on the melting temperature and on the material supply velocity.

Fig. 4. Reprezentarea schematic a unui pistol de pulverizare Diamond Jet

Graphic representation of a Diamond Jet spraying gun

Temperatura flcrii este cuprins n intervalul 2300-3000C. Particulele topite sau semitopite sunt apoi propulsate spre exterior prin duza pistolului, la viteze superioare de cca. 1350 m/s, deasupra substratului. Datorit flexibilitii sale i a costurilor efective reduse, procedeul HVOF a fost adoptat pentru depuneri de straturi n numeroase industrii, fiind eficient n producerea de straturi dense, cu porozitate redus, putere de stratificare ridicat i o suprafa relativ fin a stratului depus.

Flame temperature is included in 2300-3000C range. Melted or semi-melted particles are then propelled towards the exterior, through the gun nozzle, at higher 1350 m/s, above the sub-layer. Due to its flexibility and low actual costs. HVOF procedure was adopted for layer coating in various industries, being also efficient in producing dense layers, with low porosity, high stratification power and a relatively fine surface of the layer.



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5. Avantajele tehnicii de depunere HVOF Viteza particulelor reprezint o caracteristic important n procesul de pulverizare termic, valorile mari ale acesteia avnd ca rezultat o putere de stratificare nalt i porozitate redus, ntruct particulele au un timp redus pentru a se rci, ele lovind substratul n stare topit sau semitopit. Procesul HVOF este proiectat astfel nct s produc viteze nalte, aceasta contribuind la avantajele pe care le ofer HVOF n comparaie cu alte procese de pulverizare termic. Pentru acestea se menioneaz: nclzirea uniform i eficient a

particulelor ca urmare a turbulenei lor ridicate n interiorul camerei de combustie;

Expunere de scurt durat n aer n timpul deplasrii spre substrat datorit vitezelor mari cu care sunt proiectate particulele ceea ce conduce la o oxidare redus a suprafeei particulelor;

Ca urmare a vitezei mari de proiectare a particulelor, materialul depus va fi mai puin supranclzit rezultnd temperaturi finale ale particulelor mai reduse, comparativ cu a altor procese (de exemplu pulverizare cu plasm sau cu arc opereaz la temperaturi de cca. 16000 respectiv 6000C, n opoziie, procesul HVOF (amestec oxigen/propilen) opereaz la cca. 3000C);

Permite depunerea unei mari varieti de pulberi (ca mrime, densitate etc.);

Reprezint o variant atractiv de obinere a straturilor de suprafa din punct de vedere economic: depunerea se realizeaz n aer deci nu necesit o camer vidat, diferite componente pot fi acoperite cu straturi de protecie chiar la locul de operare nemaifiind astfel necesar dezansamblarea lor.

5. HVOF coating technique advantages Particles velocity is a significant characteristic in the thermal spraying process, its high values having high stratification power and low porosity, because particles have little time to cool, by touching the sub-layer in melted or semi-melted state. The HVOF process is designed in order to cause high velocities, contributing to the advantages that HVOF has in comparison to other thermal spraying processes. Among them we mention: Uniform and efficient heating of particles

due to their high turbulence in the combustion chamber;

Short exposure in the air during its movement to sub-layer due to high velocities resulting in little oxidation of particles surface;

Due to particles high velocity of design, the deposited material will be overheated leading to lower particles final temperatures, in comparison to other processes (for instance plasma or arch spraying operate at temperatures of approximately 16000 and 6000C respectively, as opposed to the HVOF process (oxygen/polypropylene mixture) operating at about 3000C);

It allows a great range of powders to deposit (as far as size, density are concerned etc.);

Has an attractive variety of surface layers achievement from economic point of view: depositing is made in the air but it does not require a vacuum chamber, as various components being able to be covered with protection layers at the operation site without having to disassemble them.



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CONCLUZII Acoperirea cu straturi MCrAlY a suprafeelor componentelor turbinelor cu gaz asigur mbuntirea durabilitii i fiabilitii acestora. Creterea rezistenei la oxidare i coroziune la temperaturi nalte se justific prin formarea la suprafa a unei pelicule aderente i compacte de oxid -Al2O3 care mpiedic contactul cu mediul exterior agresiv. Procesul de depunere HVOF a straturilor MCrAlY ofer avantajul obinerii unor straturi depuse de calitate, la preuri de cost competitive. BIBLIOGRAFIE 1. K.E. Schneider a.o: Thermal spraying for power generation components. Wiley VCH GmbH&Co, KgaA Weinhrim, 2006. 2. D. Eckhardt, P. Rufli: Advanced gas turbine technology ABB/BBC historical firsts. Proceeding of ASME Turboexpo 2001,New Orleans, Lousiana, 2001 3. S. Ingistov: For system performance in power augementation Of heavy duty power generating gas turbines model 7EA. Proceeding of ASMkTurboexpo 2000, Munich, Germany. 4. P. Kopfstad: High temperature corrosion. Elsevier Applied Science Publishers LTD, 1998.

CONCLUSIONS MCrAlY layer coating of gas turbines components provides improvement of their durability and reliability. Oxidation and corrosion resistance increase at high temperatures is justified through the formation of an oxide adherent and compact film -Al2O3 which prevents contact with the exterior aggressive environment. MCrAlY layers HVOF depositing process has the advantage of achieving quality layers, at competitive cost prices. REFERENCE NOTES 1. K.E. Schneider a.o: Thermal spraying for power generation components. Wiley VCH GmbH&Co, KgaA Weinhrim, 2006. 2. D. Eckhardt, P. Rufli: Advanced gas turbine technology ABB/BBC historical firsts. Proceeding of ASME Turboexpo 2001,New Orleans, Lousiana, 2001 3. S. Ingistov: For system performance in power augementation Of heavy duty power generating gas turbines model 7EA. Proceeding of ASMkTurboexpo 2000, Munich, Germany. 4. P. Kopfstad: High temperature corrosion. Elsevier Applied Science Publishers LTD, 1998.

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