CAVITATION EROSION RESISTANCE OF Co-ALLOYED STAINLE SS STEEL WELD CLADDINGS AS COMPARED TO THERMAL SPRAYED COATINGS

(1) Leonardo Boccanera

(2) Sérgio R. Barra

(3) Augusto J. A. Buschinelli

(4) Rainer Schwetzke (5) Heinrich Kreye

RESUMO Estudou-se a resistência à cavitação de vários revestimentos, depositados por soldagem e por aspersão térmica, através do ensaio vibratório ultrasônico. Tomando como referência o aço AISI 316, para cada revestimento, foram determinados o período de incubação e a taxa de erosão. Em relação a soldagem manual SMAW o processo GMAW automatizado, com pulsação térmica e oscilação da tocha, confere superior desempenho ao revestimento de aços inoxidáveis austeníticos ligados ao Co, sendo refino microestrutural, baixa diluição e ausência de defeitos estruturais os principais fatores a serem controlados. Esses resultados são comparados ao comportamento de camadas de X43Cr13 aspergidas por arco-elétrico e de Stellite 6 e MCrAlY pelo processo a chama de alta velocidade (HVOF). A resistência à erosão das camadas aspergidas é limitada pela presença de poros, microtrincas e óxidos, o melhor desempenho sendo alcançado com o processo HVOF. Palavras-chave: Cavitação, Aspersão térmica, Soldagem, Metalurgia. ABSTRACT The cavitation resistance of various welded and thermal sprayed coatings was investigated by using the vibratory ultrasonic test. Taking the AISI 316 steel as a reference material, for each coating the incubation period and the steady state erosion rate were determined. It was shown that the superior performance of Co- alloyed austenitic stainless steels can further be increased through the evolution from manual SMAW to automatic GMAW with thermal pulsation and oscillation of the torch, whereby grain refining, low dilution and absence of welding defects are the most relevant factors to be controlled. These results were compared to the behaviour of arc X43Cr13 and high-velocity oxy-fuel (HVOF) sprayed Stellite 6 and MCrAlY coatings. The erosion resistance of the thermal sprayed coatings is limited by pores, microcracks and oxides present in these coatings. Keys-words: Cavitation, Thermal spray, Welding, Metallurgy, (1) Eng. Labsolda /EMC/UFSC - Email: emc3leon@emc.ufsc.br (2) Eng. Labsolda /EMC/UFSC – Email: barra@emc.usfc.br (3) Prof. Dr.-Ing.- Labsolda/EMC/UFSC - Email: buschi@emc.ufsc.br

Cx. Postal 476 – Campus Universitário Trindade / CEP.: 88040-900 – Florianópolis –SC – Brasil (4) Eng. FB-MB - UniBw-Hamburg - Germany (5) Prof. Dr. rer. nat.- FB-MB - UniBw-Hamburg – Germany

1. INTRODUCTION Hydroturbine components such as runners, wicket gates and pump impellers are exposed to cavitation resulting from the collapse of vapor bubbles. In order to prevent damage by cavitation erosion, critical areas are cladded with materials possessing a high resistance to cavitation erosion. Among the coatings usually applied by arc welding techniques the cobalt-base alloys are known to have excellent cavitation resistance, but have been replaced for that application with newer cobalt-alloyed stainless steels that offer equivalent or better cavitation resistance and are significantly lower in cost. The deformation behaviour and fracture of these austenitic steels is typical of materials with low stacking fault energy, what in combination with the strain induced transformation of austenite into martensite results in high strain hardening rates. The long incubation period and low erosion rates of the alloy can be related to its high work hardening rate and high fatigue resistance. This relationship has been established previously from cavitation testing and microstructural analysis (1, 2). A refined microstructure is also a requisite for a good performance against cavitation, as the fine grain size produces smaller eroded particles and consequently results in a lower erosion rate. The higher resistance against cavitation associated to the finer grain size would be explained by an increase in the work necessary to remove the material, related to the expression from Hall-Petch (3, 4). Compared to arc welding, thermal spray processes are less cost effective and cause less distortion of the coated parts due to the lower heat imput. Therefore, thermal spraying is often considered as an attractive alternative to produce cavitation resistant coatings. In this study the application of modern and controlled arc welding and thermal spraying processes and their potential to produce highly protective coatings against cavitation erosion are investigated. 2. MATERIALS AND METHODS Weld Cladding: On a carbon steel as base material two buttering layers about 2.5 mm thick were deposited with AWS 309-L16 consumable as solid wire with 1.2 mm diameter. For hardfacing with cobalt containing austenitic stainless steel (SS) different welding processes were applied: (1) Shielded metal arc welding (SMAW) with a 3.25 mm diameter electrode (ER); (2) Gas metal arc welding (GMAW) with a flux-cored wire of 1.6 mm diameter, carried out in a robotic welding cell implemented in Labsolda/UFSC. Table 1 presents the chemical composition of the consumables and table 2 summarizes the welding parameters employed. The GMAW were made with pulsed arc combined with thermal pulsation, oscillation of the torch controlled by robot and checking of the metal transfer process and the interpass temperature. The weld positions were flat and

overhead with 45o to horizontal

(5).

The cladding SS-1 was deposited under the conditions: I = 120 A, Vs = 25 cm/min, heat input 17 kJ/cm. The SS-2 cladding was deposited under two different procedures: i) conventional pulsed arc with a heat input of 26 kJ/cm; ii) thermal pulsation where the heat input reached 14.5 and 9.5 kJ/cm, in the peak and base period respectively. A gas mixture of 99%Ar-1%O2 with a flow rate of 17 l/min was used.

TIG-remelting: was performed transversally to the welding direction, without previously grinding the surface, and with 3 different levels of heat input (6.0, 2.3 and 1.3 kJ/cm). A tungsten -2 % thoria electrode with a diameter of 2,4 mm and an angle of 60° at the tip was used. Thermal Spraying: The HVOF and arc sprayed coatings were applied to carbon steel at a thickness of 300 to 400 µm. For HVOF spraying a Diamond Jet Hybrid system, using hydrogen, and a JP-5000 system using kerosene as fuel were employed. Arc spraying was conducted with a OSU LD/U2 spray gun using compressed air for atomization. The composition of the materials sprayed is presented in table 3. Cavitation Erosion Tests: The cavitation erosion experiments were carried out by using a vibratory apparatus according to ASTM G 32(6). The standardized test basically consists of measuring the weight or volume loss of a 15.9 mm-diameter test specimen affixed to the tip of an ultrasonic horn that pulsates in a distilled water bath at a frequency of 20 ± 0.2 kHz over an amplitude of 50 µm (peak to peak). The temperature of the water bath is maintained at 22

o C. In the present investigations a modified procedure, illustrated in

figure 1, was used (7), which facilitates the sample preparation. The test specimen was placed in a distance of 0.5 mm opposite to the vibrating steel disc immersed 8 to 10 mm in distilled water. Specimen weight loss was periodically measured up to a total of 45 h. Cavitation rates obtained from this modified testing procedure usually show 40-50% of the rates determined with the standardized procedure. Microstruture: To study the erosion mechanism, the surface and cross sections of the samples were examined before and after erosion testing by optical and scanning electron microscopy (SEM). The grain size was determined according to ASTM E112 (8).

3. RESULTS AND DISCUSSION Cavitation Erosion Resistance: Weld claddings of conventional AWS 309 and 316 stainless steels show a relative short incubation period of 2 h, while in special resistant Co-alloyed weld claddings significant erosion started only after 8 h, achieving in the permanent period an almost constant rate in the range of 0.4-0.7 mg/h (table 4). This high resistance to cavitation erosion is comparable to that of Stellite 6, as the reported erosion rate of 1.2 mg/h for this material, determined with the standardized method (6) corresponds to about 0.5- 0.7 mg/h for the modified testing procedure used in this investigation. The use of a robotic cell for GMAW, as compared to manual SMAW, turned out as advantageous not only for the operational process, but also for the behaviour of the cladding against the cavitation erosion. Due to the automation of the pulsed arc GMAW a better control of possible welding defects and a good surface planicity could be achieved even in the overhead position, which reduced the time and costs for surface grinding and finishment work (5). The cladding deposited with thermal pulsation (PT) showed an increase in the incubation period (12 h) as compared to the conventional welding condition (PN). The waving of the torch brought an additional increase in the cavitation resistance of the weld cladding: a higher incubation period (20 h) and a lower erosion rate (0.3 mg/h) (figure 2). TIG-remelting of the weld cladding brought besides an improvement of the surface planicity the best results in the cavitation test: a grain refined layer about 1.5 to 2.0 mm thick showed a high incubation period and the lowest erosion rate (0.3 mg/h). By the other side a rough surface (as grinded condition) and porosity had a deleterious influence on the cavitation resistance, reducing and even suppressing the incubation period, which results in a higher loss of material, by an erosion rate in the permanent stage of approximately 0.5 mg/h (figure 2).

Unlike the weld overlays the HVOF and the arc sprayed coatings started to erode without previous incubation and after about 1 h cavitation time a constant erosion rate was observed. Erosion rates of arc spray coatings (Fe-Cr alloy, Co-alloyed stainless steel SS-2 and X43Cr13) were within the range from 9 to 21 mg/h, while with the HVOF coatings (stainless steel AISI 316L, Stellite 6 and MCrAlY) significant lower rates from 6.8 down to 1.3 mg/h could be measured. For the HVOF sprayed 316 L stainless steel a performance comparable to that of the welded or bulk material could be achieved. The coating MCrAlY showed the best performance, showing almost half the erosion rate of Stellite 6, a material known for its high cavitation resistance (figure 3). Coating Characteristics and Erosion Mechanism: The extremely high resistance to erosion of the cobalt-alloyed austenitic steel can be attributed to the deformation mechanisms of this alloy. Figure 4 shows early and advanced erosion stages of the GMAW overlay with this kind of material: in the initial stage the deformation is revealed by a twinning process, that subdivides the grains and the removal of material starts at slip steps and grain boundaries, where most of the deformation is concentrated. The advanced stage of erosion involves a predominantly ductile fracture mechanism. The process of phase transformation and twinning can be positively influenced by an initial fine grain size. Figure 5 represents the effect of grain size on the cavitation resistance of the austenitic steel SS-2, where through heat treatment a coarsening (ASTM 3) and TIG-remelting at a low heat input a grain refining (ASTM 6-7) of the original GMAW cladding microstructure (ASTM 4-5) was obtained. Since the deformation behavior of the cladding depends on the stacking fault energy and therefore on its chemical composition, a better control of the dilution in the automatic GMAW (20% against 30% in the SMAW) is followed by an optimized cavitation resistance already in the second welding layer. This was revealed by measuring the roughness of the eroded surfaces transversally to the weld layers (figure 6). Figure 7 shows that the concentration levels of Co, Mn, Cr and Si do not change significantly from the second to the third weld layer. Otherwise a rough surface finishment and pores are detrimental to the cavitation resistance. Figure 8 compares the erosion mechanisms for weld claddings of cobalt- alloyed stainless steel as grinded and in the presence of a pore. The grooves of the emery paper are preferential sites for the nucleation of cracks, that propagate transversal to and along the grooves. The pores are responsible for an elevated strain rate and a high loss of material along their boundary. Due to their form grooves and pores act as stress raisers, so that the strength of the material is exceeded rapidly and the erosion is nucleated. The cavitation resistance of thermal sprayed coatings is highly dependent on microstructural features such as pores, microcracks and oxide layers at the interface of individual splats. The arc sprayed coatings showed a microstructure with a high density of coarse oxides and pores (figure 9). In the HVOF process, low temperatures and high velocities of the particles, keeping the oxide and pore content of the coatings at a minimum, are a decisive factor for assuring good quality and better resistance to cavitation erosion of the coatings. The micrographs in figure 10 taken from the cross section of a partially eroded HVOF coating demonstrate that cavitation erosion takes place by the formation and propagation of fatigue cracks. The cracks originate from microcracks and pores present in the coating and propagate preferentially along interlamellar boundaries and the interface between individual particle splats in the HVOF and in the arc sprayed coatings as well.

4. CONCLUSIONS High velocity oxy-fuel flame spraying (HVOF) can produce coatings of various materials (stainless steel 316L, Stellite 6 and MCrAlY) that exhibit a resistance to cavitation erosion similar or even lower than bulk samples of stainless steel 316 L. The steady state erosion rates of 1.3 to 7 mg/h, as determined with the modified vibratory test (indirect method) are considerably lower as compared to the cavitation erosion rates of arc sprayed coatings (9 to 22 mg/h) or plasma sprayed coatings (40 to 70 mg/h) (11). The cavitation erosion resistance of thermal spray coatings - arc and HVOF as well - is limited by pores, microcracks and oxides present in these coatings. Therefore, even HVOF coatings of Stellite 6 could not provide a higher cavitation erosion resistance. Weld claddings of cobalt-alloyed stainless steel produced by SMAW and GMAW can provide higher cavitation resistance, characterized by erosion rates of 0.5 mg/h and less as compared to HVOF coatings. The use of a robotic cell for GMAW with an improved pulsed power source can produce, by controlled metal transfer and waving of the torch, weld overlays with excellent planicity in the overhead position. With the low dilution levels, of base metal erosion rates, as low as 0.3 mg/h, can be achieved in the second layer. TIG-remelting of the weld cladding allows an additional improvement of the surface planicity and the best results in the cavitation test: a grain refined layer, about 1.5 to 2.0 mm thick, showed a high incubation period and the lowest erosion rate (0.3 mg/h). Also a rough surface (as grinded condition) and the porosity had a deleterious influence on the cavitation resistance, reducing and even suppressing the incubation period, which results in a higher loss of material, by an erosion rate in the permanent stage of approximately 0.5 mg/h 5. ACKNOWLEDGMENTS The support of Professor Jair Dutra, UFSC Florianopolis, Mr Rybak and Mr Höschele from RHV Technik, Waiblingen (Germany), and Mr Krömmer from Linde AG, Höllriegelskreuth (Germany), for providing the welding and spraying equipment, and the financial support by Volkswagen Foundation and CNPq are greatly acknowledged. 6. REFERENCES (1) HEATHOCOCK, C. J., PROTHEROE, B. E. and BALL, A. Cavitation Erosion of Stainless Steels. Wear 81 No. 2, p 311-327. 1982 (2) SIMONEAU R., LAMBERT P., SIMONEAU M., DICKSON J. I., and L’ESPERANCE G. L.. Cavitation Erosion and Deformation Mechanisms of Ni and Co Austenitic Stainless Steels. IREQ. 1987 (3) GDYNIA-ZYLLA, I. M. D. Gefügeoptimierung von metastabilen austenitischen Cr- Mn-Stählen zur Erhöhung der Kavitationsbeständigkeit durch verformungs-induzierte martensitische Umwandlung. VDI Verlag. Germany. Nr. 217. 1991 (4) DUBÉ, D., M. FISET, R. LALIBERTÉ and R. SIMONEAU. Cavitation resistance improvement of IRECA steel via laser processing. Materials Letters No. 28, p.93-99. 1996

(5) BARRA, S. R.. Influência dos procedimentos de soldagem sobre a resistência à cavitação de depósitos obtidos com a utilização de arames tubulares de aços inoxidáveis austeníticos ligados ao Cobalto. Dissertação de Mestrado / UFSC– Florianópolis - Brasil. 1998 (6) ANNUAL BOOK OF ASTM STANDARDS-G32. Standard Test Method for Cavitation Erosion Using Vibratory Apparatus. p 97-110. 1992 (7) POHL, M und M. FEYER. Prüfung von Schichtverbunden durch Kavitation. Journal für Oberflächentechnik 5, p 72-74. 1995 (8) ANNUAL BOOK OF ASTM STANDARDS-E 112. Estimating the average grain size of metals, p 207-243. 1974 (9) KREYE H. Vergleich der HVOF-Systeme - Werkstoffverhalten und Schicht- eingenschaften. 4. Kolloquium Hochgeschwindigkeits-Flamspritzen, GTS-Germany, p 13-21. 1997 (10) AKHTAR, A. Plasma Sprayed Coatings for cavitation Protection in Hydraulic Turbines. Materials Performance, p 15-18. 1982 (11) SCHWETZKE, R. and H. KREYE. Cavitation Erosion of HVOF Coatings. Thermal Spray: Practical Solutions for Engineering Problems, Ed. by C.C. Berndt, ASM International, Materials Park, OH, p 153-158. 1996 7. TABLES

Table 1: Chemical composition (wt %) of weld consum ables /////////// C N Ni Cr Co Mn Si Mo E 309L 0.03 - 13 23.5 - 0.9 0.9 - SS-1 0.17 0.13 - 17.8 9.28 9.08 0.89 - SS-2 0.17 0.18 - 17 9.5 10 3.5 -

Table 2: Welding parameters for GMAW Pulsed Arc

Material Ip (A) Ib (A) tp (ms) tb (ms) va (m/min) vs(m/min) T1(s) E309 L 300 55 3.6 3.2 7 20 0.5 SS-2 300 30 5.3 8 8 20 0.5

Thermal based

E309L 300 55 3.6 10 3.5 20 0.5 SS-2 300 30 5.3 18 3.5 20 0.5

Table 3 : Chemical composition (wt %) of the therma l sprayed coatings. Process Material C Si Mn Cr Other Rest

Arc(1) Stelloy 60 G 0.55 4.97 0.17 14.2 2.73 B, 4.76 Fe Ni

Arc(1) Corodur 35 0,1 1.3 1 29 3 B Fe

Arc(1) X43Cr13 0.46 - - 13 - Fe

HVOF(2) Stellite 6 1 1 - 28.5 4 W, 1.8 Fe Co

HVOF(3) MCrAlY * - 2.8 - 25 5.5 Al, 1 Ta, 0.6 Y Ni

(1) wire Ø = 1,2 mm; (2) powder particle size range -45+16 µm; (3) -45+11 µm.

Table 4: Cavitation erosion rates of the coatings i nvestigated

Coating Process Material Hardness Cavitation

Erosion Rate

VHN (300g)

(mg/h)

Welding SMAW Stainless Steel 309L 250 7.5

Welding SMAW Stainless Steel 316 L 200 6.0

Welding SMAW Co-alloyed SS-1 350 0.4 – 0.7

Welding GMAW Co-alloyed SS-2 340 0.3 – 0.5

HVOF JP 5000 Stainless Steel 316 L 260 6.8

HVOF DJ 2600 Stellite 6 650 2.3

HVOF DJ 2600 MCrAlY 460 1.3

Arc X43Cr13 400 21.7

Arc Fe-Cr-alloyed 350 9

Arc Co-alloyed SS-2 350 18

Bulk material Stainless Steel 316 L 165 6.0

8. FIGURES

Transducer

Vibratory Horn

Test specimen Destilled water

Support

Cooling bath

Figure 1: Modified cavitation erosion test (7)

Cav

itatio

n er

osio

n ra

te (m

g/h)

Cavitation erosion rate Mass loss Incubation period

8

40

GMAW - Co alloyed stainless steel

7

Incubation

30

6 Mass

0,5

20

loss

0,4 period

0,3

(mg)

0,2

10 (h)

0,1

0

0,0

316 309 Co-alloy PN PT Waving TIG-remelting Pores

SMAW-Stainless steel torch: 2 Hz 1,3 kJ/cm

Figure 2: Cavitation erosion performance of various weld overlays.

Cav

itatio

n er

osio

n ra

te (m

g/h)

25 Bulk Thermal spraying

20Material ASP

15 HVOF

10

56 6,8

2,3

0 1,3

MCrAlY FeCr alloy X43Cr13

AISI 316

Stellite 6 Co alloyed

Stainless steel

Figure 3: Cavitation erosion performance of various thermal sprayed coatings.

a) b) c) Figure 4: (a) Twinning in the incubation period; (b ) material loss along twins and

grain boundaries; (c) final fracture.

TT 1100 oC As welded

TIG remelting

Cav

itatio

n er

osio

n ra

te (

mg/

h)

0,45

0,40

0,35

0,30 Cavitation erosion rate

0,25 Incubation period

3 4 5 6 7

Grain size (ASTME 112)

25

20

Incubatioperiod(h)

15

10

5 0

Figure 5: Effect of grain size on the cavitation pe rformance of the Co-alloyed

welded coatings

Rou

ghne

ss R

y (

µ m

)

Layers

15 3

2

10 1

5

0

0 5 10 15

Time (h)

Che

mic

al c

ompo

sitio

n (%

wt)

25

20 Cr

15

Mn

10 Co

5 Si

0 Ni

Buttering 1 2 3

Layers

Figure 6: Roughness variations (Ry) in Figure 7: Chemical composition in the

butterings (309L) and three layers of

three layers of the Co-alloyed cladding.

the Co-alloyed cladding.

a) b) c) Figure 8: (a) Erosion in the risks of the emery pap ers; (b) twinning in the

contour of the pore; (c) eroded pore

a) b) Figure 9: Oxide and pore in thermal spraying coatin gs; (a) X43Cr13, arc

sprayed; (b) MCrAlY, HVOF sprayed.

a) b) Figure 10: Surface (a) and cross section (b) of the HVOF sprayed coating of

stainless steel after cavitation testing.