Characterization of the Cr C –NiAl coatings on sintered

100 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXIX

Characterization of the Cr3C2–NiAl coatings on sintered duplex stainless steels

Barbara Lisiecka, Agata Dudek*

Institute of Material Engineering, Czestochowa University of Technology, Czestochowa, Poland; *[email protected]

The main method for production of structural materials from metallic powders with addition or without addition of non-metallic powders is powder metal-lurgy (PM). This method is based on forming and sintering processes. The PM method consists of a small number of process steps and insignificant energy consumption, which allows to production of complex shape components, with great precision in dimensions and surface quality. Taking into account the above advantages, interest in sintered duplex stainless steels (SDSSs) is growing, which are characterized by two-phase microstructure consisting of ferrite and austenite. Such combination of properties makes the SDSS a very attractive material for numerous applications. As is well known, the chromium carbide coatings improve functional properties (e.g. hardness and wear resistance). The main purpose of this study was to examine the effect of chromium carbide coating on the properties of SDSSs. The multiphase sinters were prepared from two types of water-atomized steel powders: 316L and 409L. The technique of atmospheric plasma spraying (APS) was used to deposit Cr3C2–NiAl powder on SDSS surface. Additionally, it was carried out alloying the surface of the SDSS by gas tungsten arc welding (GTAW) process at current intensity 50 A. Light microscopy and scanning electron microscopy (SEM/EDS) were performed in order to determine the microstructure of SDSSs. The results of hardness and wear resistance measurements are also presented. The applied chromium carbide coatings cause hardening in SDSSs and modification of surface layer properties, such as coefficient of friction. Furthermore, the main assumption for this investigate was to analysis the microstructure and hardness of surface layer of SDSSs with different proportions of powders as a result the alloying process.

Key words: sintered duplex stainless steels (SDSSs), chromium carbide coating, surface alloying, gas tungsten arc welding (GTAW).

Inżynieria Materiałowa 3 (223) (2018) 100÷104DOI 10.15199/28.2018.3.2

MATERIALS ENGINEERING

1. INTRODUCTION

The extremely interesting technology used for production of struc-tural materials from metallic powders as a result of processes of forming and sintering is powder metallurgy (PM). This method al-lows to modify chemical composition in a very wide range which gives the possibility of production duplex steel with a varied mi-crostructure [1, 2]. In the case of sintered duplex stainless steel (SDSS), it is possible to obtain a structure with different proportions of the basic structural components, i.e. austenite and ferrite. Taking into account high mechanical strengths, high toughness and good corrosion resistance, the SDSSs are used in industry, especially in highly industrialized countries [3÷6].

One of the most commonly used techniques to improve surface properties of steel are the coating processes or surface treatments [7, 8]. An interesting surface modification is the formation of coating based on chromium carbide. Chromium carbides have three poly-morphic structures: the cubic (Cr23C6), the hexagonal (Cr7C3) and the orthorhombic (Cr3C2). In view of the best mechanical properties and appropriate adhesion to substrate, Cr3C2 is well-known chro-mium carbide [9, 10].

The chromium carbide coatings are widely used in high tempera-ture applications (i.e. shaft bearings, seals, high-temperature fur-naces, nozzles and metal machining moulds) because they present unique corrosion resistance and are characterized by higher hard-ness and strength than other carbides at such temperature. These coatings can be produced using electron beam physical vapour dep-osition (EB-PDV), atmospheric plasma spraying (APS) and high velocity oxy fuel (HVOF) [11÷13].

The method that enjoys unflagging interest is atmospheric plas-ma spraying (APS). In plasma spraying process, the powder (e.g. Cr3C2) is introduced into the plasma jet, which issue from a plasma torch. In the jet the powder is melted and propelled towards a sub-strate, where the formed a deposits are adherent to the substrate as coatings. The metallurgical properties of the substrate do not change after coating [14÷17].

In order to modification of surface layer of SDSSs is the gas tungsten arc welding (GTAW) method. This method consists in melting and joining the surface of the welded metal by heating them with an electric arc, which is established between the non-consum-able tungsten electrode and the melted metal. The argon plays the role of shielding gas. The results of such modification is an increase in mechanical properties of SDSSs and their wear resistance. Fur-thermore, the GTAW method is easy to use and has a low cost of equipment [18÷20].

2. MATERIALS AND METHODS

The specimens for the investigations were made from water-atom-ized powders of 316L steel and ferritic 409L steel manufactured by Höganäs (Sweden). Table 1 presents chemical composition of steel powders. Both powders selected nominal particle size of 150 µm.

The different series of the samples (Tab. 2) was obtained of ferritic and austenitic steel powders mixed at various proportions. The powders were compacted uniaxially with addition of 1% Acrawax C lubricant at 720 MPa. The molded pieces were sin-tered at the temperature of 1250°C for 30 minutes and then cooled down with a rate of 0.5°C/s. The whole process was carried out in the reducing atmosphere using hydrogen in order to limit oxi-dation of the batch and protect from reduction of the chromium content.

In order to improve functional properties of SDSSs, a modified layer was formed by means of plasma spraying (APS method). The chromium carbide is mechanically mixed with nickel and alu-minium and the coating is performed at high temperature, therefore in this method, the particles of Cr3C2 are substituted in the Ni–Al layer (90 wt % Cr3C2–10 wt % NiAl). The chromium carbide coat-ing was prepared at the following parameters: voltage: Ur = 60 V, current intensity: Ir = 450÷500 A, distance of the plasmatron from the surface: 80 mm, plasma forming gases flow: argon ~45 l/min and hydrogen ~17 l/min. The resulting coating had a thickness of around 60 µm.

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The next step was alloying of the coatings by GTAW technol-ogy. The alloying treatment of sintered steel was carried out with constant surface scanning rate of 340 mm/min and welding at opti-mal current intensity of 50 A [21÷22] and voltage parameters. The shielding gas was argon, with the flow set at ~14 l/min.

The analysis of the microstructure after application of Cr3C2–NiAl powder and surface alloying was performed using the stereo microscope Olympus SZ61, light microscope Olympus GX41 and scanning electron microscope Jeol JSM-6610LV. The Vickers meth-od (with the load of 980.7 mN) was employed to measure micro-hardness of SDSSs obtained by Shimadzu HMV-G Series.

The scratch resistance test (Revetest XPress Plus using Rockwell indented tip) was performed in order to determine coefficient of friction for SDSSs obtained in the study. The following parameters were maintained during the test: permanent load of 10 N, scratch length 5 mm, scratch rate 5 mm/min.

3. RESULTS AND DISCUSSION

The grains of Cr3C2–NiAl powder observed by the scanning elec-tron microscope Jeol JSM-6610LV are presented in Figure 1.

The chromium carbide coating on the SDSS obtained using the APS method was of thickness approx. 60 µm. The microstructure of the coating surface before alloying process observed by a stereo microscope Olympus SZ61 is presented in Figure 2a. The micro-

Table 1. Chemical composition of steel powders, wt %Tabela 1. Skład chemiczny proszków stalowych, % mas.

ElementPowder grade

316L 409L

Cr 16.80 11.86Ni 12.00 0.14Mo 2.00 0.02Si 0.90 0.82

Mn 0.10 0.14C 0.022 0.020S 0.005 0.010Fe Balance Balance

Table 2. Fraction of individual powders in mixture, wt %Tabela 2. Udział poszczególnych proszków w mieszaninie, % mas.

Powder gradeSamples No.

1 2 3 4

316L 100 80 50 20409L 0 20 50 80

Fig. 2. Microstructure of the coating on SDSS samples No. 1 after APS process: a) surface; LM, b) cross-section of the layer; SEMRys. 2. Mikrostruktura powłoki na próbkach SDSS nr 1 po procesie APS: a) powierzchnia; LM, b) przekrój poprzeczny warstwy; SEM

Fig. 3. Morphology of SDSS surface after alloying 50 A, samples No. 4; LMRys. 3. Morfologia powierzchni SDSS po stopowaniu 50 A, próbki nr 4; LM

structure of coating cross-section examined by the scanning elec-tron microscope is presented in Figure 2b.

The chromium carbide coatings received in the investigation show microstructure typical for that deposition method — porosity, layers, heterogeneity. Furthermore, they have typical porous lamel-lar microstructure with cracks and non-melted particles.

Macroscopic examinations were used for evaluate the effect of alloying on surface quality. The effective macroscopic results were obtained for the bands alloyed using GTAW method at current in-tensity 50 A, which were characterized by smooth surface without defects (i.e. craters, cracks). The morphology of the surface after alloying process for SDSS obtained by stereo microscope Olympus SZ61 is presented in Figure 3.

Fig. 1. Morphology of the Cr3C2–NiAl powder used to the coating for-mation; SEMRys. 1. Morfologia proszku Cr3C2–NiAl użytego do wytworzenia powłoki; SEM


In order to observe the microstructure, the metallographic sec-tions were etched with aqua regia. The microstructure of the surface after alloying treatment obtained for SDSSs by the light microscope Olympus GX41 is presented in Figure 4.

Taking into account of microstructure analysis of the surface layers, it was revealed a homogeneous cellular-dendritic structure, which was formed after alloying process. The fast heat transfer and high gradient of temperature caused formation of columnar crystal oriented according to the direction of heat transfer. In the remelted zone was observed of epitaxial character of nucleation and growth of primary structure crystals.

The microstructure of SDSS surface after alloying process ob-served by the scanning electron microscope is presented in Figure 5.

The estimated chemical composition of the chromium carbide coating in SDSS is presented in Table 3. The content of carbon and oxygen were included with the error of the EDS method. The chem-ical composition of the surface layers in SDSSs after alloying using by GTAW method are presented in Table 4. The main aim was to compare areas obtained for SDSSs after surface treatment, determi-nation of the migration process of alloying elements (Cr, Ni) during crystallization process and determination of the chemical composi-tion homogeneity in surface layers.

Analysis of the elements content revealed that the content of Cr and Ni after boundary of the alloying zone and heat affected zone decreases linearly, while the Fe content increases. This is caused by the diffusion process of Cr and Ni during sintering. Chromium dif-fuses to ferrite, while nickel diffuses to austenite and that leads to different phase transformations.

Fig. 4. Microstructure of the entire alloyed zone, samples No. 2; LMRys. 4. Mikrostruktura strefy stopowanej, próbki nr 2; LM

Table 3. The chemical composition of the Cr3C2–NiAl coating for sam-ples No. 1, wt %Tabela 3. Skład chemiczny powłoki Cr3C2–NiAl dla próbek nr 1, % mas.

Cr Ni C O Al Fe Si

40.53 35.77 ~15 ~4 1.89 0.68 0.49

Table 4. The chemical composition of the SDSSs after alloying, wt %Tabela 4. Skład chemiczny spiekanych stali duplex po stopowaniu, % mas.

SDSSs Element Alloying zone

Heat affected zone

Native material

No. 1(100% 316L)

Fe 60.00 66.01 65.04

Cr 20.77 17.52 17.55

Ni 16.32 12.59 11.70

Mo 2.25 2.93 2.40

Si 0.66 0.96 2.98

Mn — — 0.32

Al — — —

No. 2(80% 316L

+ 20% 409L)

Fe 47.71 68.66 69.71

Cr 28.87 16.29 15.96

Ni 20.00 10.92 10.17

Mo 1.89 2.53 1.85

Si 0.73 1.28 1.44

Mn 0.49 0.25 0.62

Al 0.31 0.07 0.24

No. 3(50% 316L

+ 50% 409L)

Fe 70.45 73.14 75.24

Cr 18.77 16.01 15.36

Ni 8.96 8.12 6.69

Mo 0.95 1.63 1.51

Si 0.60 1.10 0.96

Mn 0.18 — 0.24

Al 0.09 — —

No. 4(20% 316 L

+ 80% 409L)

Fe 51.10 75.64 81.53

Cr 30.86 16.49 14.14

Ni 16.95 6.68 1.40

Mo 0.22 0.26 0.80

Si 0.63 0.92 1.84

Mn 0.14 — 0.29

Al 0.10 — —

Fig. 5. Microstructure of the boundary alloying zone–heat affected zone for samples No. 3; SEMRys. 5. Mikrostruktura na granicy strefa stopowana–strefa wpływu cie-pła dla próbek nr 3; SEM

NR 3/2018 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 103

The results of hardness represent the mean of three measure-ments to chromium carbide coating, alloying zone, heat affected zone and native material obtained during after alloying at current intensity of 50 A (Tab. 5).

Mechanical properties of the whole sinter depend on contribu-tion of individual phases. Furthermore, application of Cr3C2–NiAl powder to the surface layers during alloying leads to increase in hardness and an increase in homogeneity of the layer. The hardness results show the improvement in strength properties after surface alloying treatment

In order to evaluate the coefficient of friction was performed scratch tests under constant load. The friction coefficient for the ma-terials obtained by the scratch test are presented in Table 6.

4. CONCLUSIONS

The main purpose of this study was the characteristic of mechani-cal properties, mainly hardness and wear resistance, of Cr3C2–NiAl coating and surface layers on SDSSs after alloying by GTAW meth-od. The first stage concerned on prepare of chromium carbide coat-ing by APS method, which have appropriate adhesion to substrate.

As demonstrated in the study, the alloying method used by the authors is a promising proposal for hardening of surface layer of SDSSs. The application of the chromium carbide coating itself leads to an increase in mechanical properties of SDSSs. Addition-ally, the results obtained in the study showed that alloying method causes a decrease in the coefficient of friction and increase in hard-ness, which consequently leads to the increased wear resistance. This allows for a wider use of these modern materials.

REFERENCES

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[8] Sarjasa H., Kulua P., Juhania K., Viljusb M., Matikainenc V., Vuoristoc P.: Wear resistance of HVOF sprayed coatings from mechanically activated thermally synthesized Cr3C2–Ni spray powder. Proc. Estonian Acad. Sci. 65 (2016) 101÷106.

[9] Korkmaz K.: Investigation and characterization of electrospark deposited chromium carbide-based coating on the steel. Surf. Coat. Technol. 272 (2015) 1÷7.

[10] Romero J., Lousa A., Marinez E., Esteve J.: Nanometric chromiumychro-mium carbide multilayers for tribological applications. Surf. Coat. Tech-nol. 163÷164 (2003) 392÷397.

[11] Zhao Z., Zheng H., Wang Y., Mao S., Niu J., Chen Y., Shang M.: Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the pre-cursor. Int. J. Refract. Metals Hard Mater. 29 (2011) 614÷617.

[12] Janka L., Berger L. M., Norpoth J., Trache R., Thiele S., Tomastik C., Matikainen V., Vuoristo P.: Improving the high temperature abrasion re-sistance of thermally sprayed Cr3C2–NiCr coatings by WC addition. Surf. Coat. Technol. 337 (2018) 296÷305.

[13] Matikainen V., Bolelli G., Koivuluoto H., Sassatelli P., Lusvarghi L., Vuo-risto P.: Sliding wear behaviour of HVOF and HVAF sprayed Cr3C2-based coatings. Wear 388–389 (2017) 57÷71.

Table 5. Results of hardness measurementsTabela 3. Wyniki pomiarów twardości

SDSSsHardness, HV0.1

Cr3C2–NiAlcoating

Alloying zone

Heat affected zone

Native material

No. 1(100% 316L) 375.00±53.75 282.67±11.59 198.33±4.64 92.77±10.94

No. 2(80% 316L

+ 20% 409L)239.33±65.95 376.33±19.33 207.67±12.76 122.33±6.34

No. 3(50% 316L

+ 50% 409L)344.00±14.35 344.67±47.61 302.33±2.62 123.33±10.08

No. 4(20% 316L

+ 80% 409L)268.67±28.08 368.00±18.40 274.33±27.35 140.33±11.09

Table 6. Results of friction coefficient measurementsTabela 6. Wyniki pomiarów współczynnika tarcia

Samples Zone Coefficient of friction

No. 1 (100% 316L)

Cr3C2–NiAl coating 0.22

Alloying zone 0.20

Native material 0.16

No. 2(80% 316L

+ 20% 409L)


Alloying zone 0.19


No. 3(50% 316L

+ 50% 409L)


Alloying zone 0.13


No. 4(20% 316L

+ 80% 409L)


Alloying zone 0.12


[14] Yaghtin A. H., Salahinejad E., Khosravifard A., Araghi A., Akhbarizadeh A.: Corrosive wear behavior of chromium carbide coatings deposited by air plasma spraying. Ceram. Int. 41 (2015) 7916÷7920.

[15] Brupbacher M. C., Zhang D., Buchta W. M., Graybeal M. L., Rhim Y. R., Nagle D. C., Spicer J. B.: Synthesis and characterization of binder-free Cr3C2 coatings on nickel-based alloys for molten fluoride salt corrosion resistance. J. Nucl. Mater. 461 (2015) 215÷220.

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[21] Dudek A., Lisiecka B.: The effect of alloying method on the microstruc-ture and properties of the PM stainless steel. 25th Anniversary International Conference on Metallurgy and Materials. TANGER Ltd., Ostrava (2016) 1045÷1050.

[22] Dudek A., Lisiecka B., Ulewicz R.: The effect of alloying method on the structure and properties of sintered stainless steel. Arch. Metall. Mater. 62 (2017) 281÷287.


Charakterystyka powłoki Cr3C2–NiAl na spiekanych stalach duplex

Barbara Lisiecka, Agata Dudek*

Instytut Inżynierii Materiałowej, Politechnika Częstochowska, Polska; *[email protected]

Inżynieria Materiałowa 3 (223) (2018) 100÷104DOI 10.15199/28.2018.3.2

MATERIALS ENGINEERING

Słowa kluczowe: spiekane stale duplex (SDSSs), powłoka Cr3C2–NiAl, stopowanie powierzchni, spawanie łukowe (GTAW).

1. CEL PRACY

Powłoki z węglika chromu poprawiają właściwości funkcjonalne materiałów takie jak twardość i odporność na zużycie. Głównym celem pracy była analiza wpływu powłoki z węglika chromu oraz stopowanej warstwy wierzchniej na właściwości spiekanych stali nierdzewnych typu duplex (SDSSs).

W celu wytworzenia powłoki z proszku Cr3C2–NiAl na po-wierzchni SDSS zastosowano natryskiwanie plazmowe w warun-kach atmosferycznych (APS). Dodatkowo przeprowadzono stopo-wanie powierzchni SDSS łukową metodą spawalniczą GTAW przy natężeniu prądu o wartości 50 A.

W celu określenia mikrostruktury SDSS wykonano badania za pomocą mikroskopii świetlnej i skaningowej mikroskopii elektro-nowej (SEM/EDS). Przeprowadzono analizę wyników pomiarów twardości i odporności na zużycie w powiązaniu z analizą mikro-struktury i warstwy powierzchniowej SDSS powstałej w wyniku procesu stopowania.

2. MATERIAŁ I METODYKA BADAŃ

Spieki otrzymano z rozpylanych wodą komercyjnych proszków sta-lowych 316L i 409L szwedzkiej firmy Höganäs. Skład chemiczny proszków przedstawiono w tabeli 1.

Badano spieki otrzymane z mieszanin proszków stali austeni-tycznej i ferrytycznej w różnych proporcjach (tab. 2). Zastosowano prasowanie dwustronne z dodatkiem środka poślizgowego (Acra-wax C) pod ciśnieniem 720 MPa. Spiekanie prowadzono w tem-peraturze 1250°C przez 30 minut, a następnie spieki chłodzono z szybkością 0,5°C/s. Cały proces przeprowadzono w atmosferze redukującej (H2) w celu ograniczenia utleniania wsadu oraz zabez-pieczenia przed zubożeniem w chrom.

W celu poprawy właściwości funkcjonalnych SDSS została utworzona za pomocą natrysku plazmowego (metodą APS) war-stwa. Cząstki Cr3C2 natryskiwano z dodatkiem NiAl (90% mas. Cr3C2, 10% mas. NiAl), a otrzymana powłoka miała grubość około 60 μm.

Drugim zastosowanym sposobem modyfikacji mikrostruktury warstwy wierzchniej było przetopienie powierzchniowe próbek przeprowadzone spawalniczą metodą łukową GTAW (gas tungsten arc welding), wykonane przy optymalnym natężeniu prądu 50 A. Gazem osłonowym był argon o ustalonym przepływie ~14 l/min. Obróbkę stopową spieków stalowych przeprowadzono przy stałej prędkości skanowania 340 m/min.

Analizę mikrostruktury po natryśnięciu proszku Cr3C2–NiAl i stopowaniu powierzchni przeprowadzono za pomocą mikroskopu

stereoskopowego Olympus SZ61, mikroskopu świetlnego Olym-pus GX41 i skaningowego mikroskopu elektronowego Jeol JSM-6610LV.

Pomiar twardości SDSS wykonano sposobem Vickersa (przy obciążeniu 980,7 mN). Współczynnik tarcia wyznaczono na pod-stawie próby odporności na zarysowanie (Revitest XPress Plus z użyciem końcówki Rockwell).

3. WYNIKI I ICH DYSKUSJA

W wyniku zastosowanej metody APS otrzymano powłoki o grubo-ści około 60 μm. Topografię powierzchni powłoki przed procesem stopowania przedstawiono na rysunkach 2a, b.

Otrzymane powłoki charakteryzują się porowatością i jednorod-nością typową dla zastosowanej metody APS. Obserwację mikro-struktury obszarów przetopionych łukowo SDSS przeprowadzono na zgładach metalograficznych trawionych wodą królewską (aqua reggia). Mikrostrukturę powierzchni po procesie przetopienia po-szczególnych spieków przedstawiono na rysunkach 3 oraz 4. Zaob-serwowano komórkowo-dendrytyczny front krystalizacji z epitak-sjalnym wzrostem kryształu. Wyróżniono strefę przetopioną, strefę wpływu ciepła i materiał rodzimy.

Analizę składu chemicznego powłoki Cr3C2–NiAl przedstawio-no w tabeli 3, natomiast przetopionych warstw powierzchniowych dla poszczególnych spieków w tabeli 4. Głównym celem anali-zy było porównanie składu chemicznego obszarów uzyskanych w wyniku stopowania SDSS, określenie migracji pierwiastków stopowych (Cr, Ni) w trakcie krystalizacji i określenie stopnia jed-norodności przetopionych warstw. Badania przetopionej warstwy powierzchniowej wykazały migrację Cr i Ni podczas spiekania wy-wołaną dyfuzją.

Wyniki pomiarów twardości zamieszczone w tabeli 5 wskazu-ją na poprawę własności wytrzymałościowych po przetopieniowej obróbce powierzchniowej. Zmiany wartości współczynnika tarcia warstw wierzchnich spieków (tab. 6) nie były znaczące.

4. PODSUMOWANIE

Przygotowane metodą APS powłoki z węglika chromu mają odpo-wiednią przyczepność do podłoża. Jak wykazano w pracy, metoda stopowania stosowana przez autorów jest dobrym sposobem pro-wadzącym do utwardzenia warstwy powierzchniowej SDSS. Zasto-sowanie samej powłoki z węglika chromu prowadzi do zwiększenia twardości SDSS. Wyniki badań wykazały, że metoda stopowania powoduje zmniejszenie współczynnika tarcia i wzrost twardości, co w konsekwencji prowadzi do zwiększonej odporności na zużycie.

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Characterization of the Cr C –NiAl coatings on sintered