Keywords: cast Ni-base alloy, structure stability, high temperature annealing, carbide

Transcription

1 Materials Science Forum Online: ISSN: , Vol. 782, pp doi: / Trans Tech Publications, Switzerland Structure Stability of Ni-Base and Co-Base Alloys Božena Podhorná 1, a, Irena Andršová 1, b, Jana Dobrovská 2, c, Vlastimil Vodárek 2, d and Karel Hrbáček 3, e 1 UJP PRAHA a.s., Nad Kamínkou 1345, Praha 5 Zbraslav, CZ 2 FMMI VŠB Technická univerzita Ostrava, 17. listopadu 15, Ostrava Poruba, CZ 3 První brněnská strojírna Velká Bíteš, a.s., Vlkovská 279, Velká Bíteš, CZ a podhorna@ujp.cz, b andrsova@ujp.cz, c jana.dobrovska@vsb.cz, d vlastimil.vodarek@vsb.cz, e hrbacek.karel@pbsvb.cz Keywords: cast Ni-base alloy, structure stability, high temperature annealing, carbide Abstract. This article summarises results of structure stability investigation of cast Ni-base and Cobase alloys after prolonged high temperature exposure at C. Cast Ni(Co)-Cr-W-C alloys are resistant to high-temperature corrosion, due to high chromium content. Their heat resistance is caused by presence of carbides, which are stable at very high temperatures. Carbides precipitate in shape of large plate-like particles or carbide eutectics at casting cell boundaries, thus forming carbide skeleton of the alloy. Carbide morphology and temperature stability depends on chemical composition of the alloy, e.g. carbide content, type and content of carbide-forming elements. Microstructure changes were evaluated by stereological analysis and X ray-spectral microanalysis. Introduction Characteristics and behaviour of heat resistant materials that belong to a production programme implementing in Precision Casting Division is one of the issues handled by long-term cooperation between this organisation PBS Velká Bíteš, a.s. and UJP PRAHA a.s. [1]. Recent industrial development puts high demands on the creep and heat resistance of the applied materials. Nickelbase alloys meet these requirements due to their development with the aim of increasing the heatresistant properties together with a high-temperature corrosion resistance [2, 3]. A large group of heat resistant materials is formed by nickel-based alloys hardened by particles of γ phase, i.e. Ni 3 (Ti, Al). However, while maintaining the requirement of resistance to high-temperature corrosion it is necessary to alloy with high chromium concentrations above 25 wt. % which prevents hardening by γ phase in a sufficient amount. That is because of the inhibiting effect of high chromium content on the γ phase stability decreasing its melt temperature down to 900 C. Therefore, another type of nickel alloys has been developed hardened by carbides which are stable to very high temperatures. The desired hardening of material is achieved by increasing the content of carbon and alloying with elements forming thermally very stable carbides. Metal elements such as tungsten, niobium, tantalum, titanium and molybdenum have a high affinity for carbon and form carbides easily. These carbides precipitate either directly from melt (primary) or from solid solution (secondary) at high temperatures. The precipitation and temperature stability of these carbides is not conditioned by reducing the chromium content and their presence do not lead to reduction of high temperature oxidation resistance. Today, efforts of the engineering export to penetrate the world markets, where materials corresponding to the Europe Union and American standards are required, raise the need to develop new variants of these advanced alloys. For these reasons, the development of new variants of nickel alloys, working in liquid glass melt at temperatures of C, has begun in PBS Velká Bíteš, a.s. Considering the similarity of Ni and Co, a group of studied materials was extended of an experimental cobalt base alloy that can be melted and cast in air, which is a big advantage compared to most nickel alloys. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,14:58:22)

2 432 Metallography XV Experimental materials Experimental alloys no. M2 and M3 were cast in PBS Velká Bíteš, a.s. - Precision Casting Division. The alloys were melted in an induction furnace and cast into ingots in the air, only test castings of cobalt-base alloy marked Stellite were cast on an open furnace. The design of chemical composition of new alloys M2, M3 was based on nickel alloy 141I, considering as the most suitable material used for the spinner disks, recently. So, 141I alloy is used as a comparative standard. To ensure high resistance to oxidation, an amount of chromium content in alloys M2 and M3 overreached the limit of 25 wt. %. In addition, the content of carbon and tungsten, which provide hardening by thermally stable carbides, and of niobium, which provides hardening by fine carbides at the grain boundaries, has been increased. All types of carbides of alloying elements take part in the formation of carbide skeleton at cell boundaries of casting structure that ensures strength at very high temperatures. Properties of as-cast alloys and after an isothermal annealing were studied. Chemical composition of experimental heats of the alloys is shown in Table 1. Table 1 Chemical composition of studied alloys Alloy Composition [wt. %] C Mn Si Cr Fe Nb W Co P S Ni M bal M bal Co bal Microstructure of as-cast alloys The microstructure of all as-cast alloys is similar, where the matrix is formed by solid solution of nickel and cobalt, respectively, with primary carbide precipitates in shape of large plate-like particles at casting cell boundaries. The morphology of those primary carbides differs only in details depending on individual alloys, see Figs. 1 to 3. Based on X-ray analysis, it was found that primary carbides are eutectic reaction products, which precipitate in the intercellular spaces of casting microstructure and are correlated with small amount of some other carbide phases. Because of niobium content in all the alloys, niobium carbide of MC type may be expected. Moreover, the eutectic skeleton contains particles corresponding to complex carbides of chromium and tungsten. Documentation and identification of the phases using scanning electron microscopy (scattered electrons mode - BEC) and X-ray spectral microanalysis were carried out at the Technical University of Ostrava [4]. Considering M2 alloy, two basic products of eutectic reactions with small amount of other two phases (eutectics) were identified, see Fig. 4. In this picture, while bright particles in the eutectic skeleton correspond to niobium carbides of MC type, dark particles correspond to carbides with high chromium content of M 7 C 3 or M 23 C 6 type. In case of M3 alloy, other two bright particles that formed by the complex carbides of chromium, niobium, and tungsten were identified, see Fig. 5. Also in cobalt alloy the same phases have been identified. As previously, the as-cast structure is formed by carbide skeleton of primary carbides (MC type rich mainly in niobium) and eutectic carbides, which have a double contrast, see Fig. 6. In this picture, white particles are rich in tungsten, gray ones in chromium.

3 Materials Science Forum Vol Fig. 1 Microstructure of as-cast M2 alloy Fig. 2 Microstructure of as-cast M3 alloy Fig. 3 Microstructure of as-cast Co-base alloy Fig. 4 Structure phases in as-cast M2 alloy Fig. 5 Structure phases in as-cast M3 alloy Fig. 6 Structure phases in as-cast Co-base alloy Microstructure of as-annealed alloys Structure stability at high temperatures of these alloys is crucial to their high temperature properties. All tested alloys were subjected to an isothermal annealing in an electric furnace at temperatures of 900, 1000 and 1100 C for 100 hours that represents the desired lifetime of casting glass disc [5]. Since the beginning of annealing, the precipitation of fine secondary carbides closed to eutectic carbide skeleton always takes place. With continuing annealing time, the amount of secondary carbides increases followed with the coarsening. However, temperatures of 1000 and 1100 C enable finer carbides to redissolve, so the achieved microstructure is similar to the as-cast condition. The microstructure evolution during annealing is very similar in all alloys, see Figs. 7 to 15. Fig. 7 Microstructure of as-annealed M2 alloy. Annealing: 900 C/100 hrs Fig. 8 Microstructure of as-annealed M2 alloy. Annealing: 1000 C/100 hrs Fig. 9 Microstructure of as-annealed M2 alloy. Annealing: 1100 C/100 hrs

4 434 Metallography XV Fig. 10 Microstructure of as-annealed M3 alloy. Annealing: 900 C/100 hrs Fig. 11 Microstructure of as-annealed M3 alloy. Annealing: 1000 C/100 hrs Fig. 12 Microstructure of as-annealed M3 alloy. Annealing: 1100 C/100 hrs Fig. 13 Microstructure of as-annealed Co-base alloy. Annealing: 900 C/100 hrs Fig. 14 Microstructure of as-annealed Co-base alloy. Annealing: 1000 C/100 hrs Fig. 15 Microstructure of as-annealed Co-base alloy. Annealing: 1100 C/100 hrs Stereological quantitative analysis Changes in microstructure during the annealing were also determined by quantitative stereological analysis. The volume fractions of primary phases precipitating in studied alloys were evaluated with automatic image analyzer NIS-Elements (Laboratory Imaging s.r.o.) from images taken by an optical microscope Nikon MA 500. In contrary, the images for an evaluation of amount of secondary phase were taken by a scanning electron microscope JEOL 5510LV because of the phase dimensions. Nevertheless, the all data, i.e. corresponding to primary and secondary precipitates as well, were processed using the private software STRPAR, which is based on the Saltykov s method [6] modified with Saxl s [7] corrections by Kudrman [1]. High alloying of new alloys M2 and M3 has led to the precipitation of primary phases with 1214 % of volume fraction. Comparing to M group of alloys, cobalt-base alloy has the volume fraction of primary particles about 10 % higher. At the beginning of annealing, a slight decline in the amount of primary phases occurs, but after 50 hours of annealing time a peak was measured. This phenomenon is probably associated with a balancing of initial chemical inhomogeneity caused during solidification of alloy. Finally, after 100 hours holding at annealing temperature a slight decrease in volume fraction of primary phase in the matrix of alloy takes place. The precipitation of secondary phases was already observed after 5 hours of annealing, but most particles were formed after 100 hour of annealing time. The most intensive changes have occurred in nickel-base alloys at 900 C. Also, the cobalt-base alloy has shown a systematic increase in the volume fraction of secondary phases at all experimental temperatures, while the peak was observed after annealing at 1000 C for 100 hrs. Although, a mean volume fraction of precipitates of all alloys oscillated around 5 % in an average that should not significantly affect the functional characteristics of these alloys.

5 Materials Science Forum Vol Fig. 16 Time dependence of volume fraction of primary (left) and secondary (right) phases precipitating in M2 alloy Fig. 17 Time dependence of volume fraction of primary (left) and secondary (right) phases precipitating in M3 alloy Summary Fig. 18 Time dependence of volume fraction of primary (left) and secondary (right) phases precipitating in Co-base alloy Structure stability of the nickel-base alloys M2, M3 and cobalt-base alloy even after prolonged annealing at C is sufficient. During the annealing, a chemical inhomogeneity occurring in the as-cast alloys turns to equilibrium and a slight dissolution of primary phase accompanied with a precipitation of secondary carbides is observed. With increasing exposure time the secondary precipitates grow and their number decreases. No undesirable intermetallic phases were found in any studied alloy.

6 436 Metallography XV Acknowledgements This work was financially supported by Ministry of Industry and Trade of the Czech Republic within the framework TIP in a project no. FR-TI1/095. References [1] Kudrman J., Čmakal J., Nedbal I., Siegl J., Kunz J., Lauschman H., Karlík M. Role of Carbon in precipitation of high-temperature phases in Ni-Cr-(heavy melting metal) alloys. [Report no. 852/99], Project reg. no. 106/96/0150 by Czech Science Foundation [in Czech]. [2] Nickel, Cobalt and Their Alloys, ASM SPECIALITY HANDBOOK, ASM International Materials Park, OH , December [3] Superalloys, Source Book, ASM, Metals Park, Ohio, [4] Dobrovská J., Vodárek V., Jonšta Z., Konečná K. Progress report on project no. FR-TI1/095 by the Ministry of Trade and Industry of the Czech Republic. VŠB-Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 2012 [in Czech]. [5] Podhorná B., Zýka J., Andršová I. Research and development of materials suitable for use in liquid glass environment, development of technologies of precision casting of new types of highly thermally and mechanically stressed castings. Project no. FR-TI1/095 by the Ministry of Trade and Industry of the Czech Republic. [Final report no. UJP 1500], UJP PRAHA a.s., 2012 [in Czech]. [6] Saltykov S.A. Stereometric Metallografy, Metallurgy, Moskva, 1970 [in Russian]. [7] Saxl I., Sklenička V., Čadek J. Proceedings of Quantitative Methods of Precipitation Processes Study, Praha, 1977, pp. 12 [in Czech].