HEAT TREATMENT AND PROPERTIES OF NICKEL SUPERALLOY 718PLUS

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1 HEAT TREATMENT AND PROPERTIES OF NICKEL SUPERALLOY 718PLUS Jiří ZÝKA a, Jan HLOUS b, Božena PODHORNÁ a, Jana DOBROVSKÁ c, Karel HRBÁČEK d a UJP PRAHA, a. s., Nad Kamínkou 1345, Praha - Zbraslav, Česká republika, zyka@ujp.cz b CTU in Prague, FJFI-KMAT, Trojanova 13, Praha, Česká republika c VŠB - TU, FMMI, 17. Listopadu 15, Ostrava - Poruba, Česká republika d PBS Velká Bíteš a.s, Vlkovská 279, Velká Bíteš, hrbacek.karel@pbsvb.cz Abstract Alloy 718Plus is one of newly developed nickel superalloys following the world most used nickel superalloy IN718. Alloy 718Plus was developed by the Allvac company in USA. Principal requirements for the alloy design were increasing the temperature capability 55 C higher than that of alloy 718, up to 700 C, and conservation of comparable processing characteristics (workability and weldability ) and economical characteristics (low cost of raw materials and production process) as alloy 718. Alloy 718Plus is mostly used in wrought state, but using of other production processes is also possible. This contribution deals with the investment cast alloy 718Plus. Analysis of heat treatment and alloy's microstructure and mechanical properties was performed. Keywords: Ni superalloys, investment casting, heat treatment, mechanical properties 1. INTRODUCTION Cooperation between the companies UJP PRAHA a.s and PBS Velká Bíteš a.s. in the study of mechanical properties of nickel alloys has a long tradition. The 718Plus alloy was included in the research programme in The alloy is considered as a candidate alloy for cast turbochargers [1]. Alloy 718Plus is one of newly developed nickel superalloys following the world most used nickel superalloy IN718. Alloy 718Plus was developed by the Allvac company in USA [2]. Principal requirements for the alloy design were increasing the temperature capability 55 C higher than that of alloy 718, up to 700 C, and conservation of comparable processing characteristics (workability and weldability) and economical characteristics (low cost of raw materials and production process) as alloy 718. Alloy 718Plus is mostly used in wrought state, but using of other production processes is also possible. Extensive investigation was performed into microstructure, the variants of heat treatment and long-term stability of the material s microstructure. Mechanical properties of the alloy were also examined. Test specimens were supplied by PBS Velká Bíteš. 2. EXPERIMENTAL The chemical composition of the 718Plus alloy melt used is given in Table 1. Tabulka 1 Chemické složení použité tavby slitiny 718Plus v hm.%. Table 1 Chemical composition (wt.%) of the 718Plus alloy C Cr Mo W Co Fe Ni Nb Ti Al Mn Si P S B 0,039 19,96 2,77 1,14 8,5 8,4 Zb. 5,5 0,77 1,35 <0,01 <0,035 0,002 0,002 0,004

2 Experimental work carried out in 2009 was focused in particular on understanding the structural processes occurring during heating of the alloy in as cast state. Kinetics of γ' phase precipitation during the subsequent hardening annealing was also investigated. The aim was to verify the mode of heat treatment of the investment-cast 718Plus alloy. Metallographic samples were subjected to different regimes of heat treatment. The structure of the samples was examined by light and electron microscopy and Vickers hardness HV30 was measured. This research was accompanied by DTA analysis of the 718Plus alloy carried externally on VŠB-TUO Ostrava. The next step was testing of the alloy's mechanical properties at room and higher temperatures. 3. RESULTS 3.1 Microstructure There are four principal intermetallic phases: Laves, γ', γ'' a δ, in γ matrix of the 718Plus alloy (Fig. 1). Volume fraction of γ' phase is usually 20%, γ'' phase up to 7% [3]. Particles of γ'' phase are too small to be viewed by light or scanning electron microscopy. Small amount of elongated δ phase particles is useful on grain boundaries. Stable MC type carbides of Nb and Ti are present in γ matrix too. γ' phase containing Nb and Ti is more stable than Ni 3 Al. γ phase Laves phase Obr. 1. Příklady struktur slitiny 718 Plus Fig. 1. Microstructure of the 718 Plus alloy 3.2 Heat treatment Solution annealing analysis was performed on vacuum melted investment-cast specimens. These specimens were annealed in air in different temperatures for different times. The structure of the samples was examined by light and electron microscopy and Vickers hardness HV30 was measured. Hardness values after 1 hour annealing at different temperatures are shown in Fig. 2. Annealing temperature of 1050 C for 1 hour and cooling in air results in minimum hardness values and good homogenization of the structure. There is no precipitation of another phases.

3 Annealing time second step [h] Obr. 2. Tvrdost po 1 hodině žíhání na uvedených teplotách Fig. 2. Hardness values after 1 hour annealing at different temperatures Obr. 3. Tvrdost po rozpouštěcím žíhání 1050 C/1h a různých režimech vytvrzovacího žíhání Fig. 3. Hardness values after 1050 C/1h solution annealing and different precipitation anneling regimes Precipitation annealing regimes were also investigated. Two step annealing: 790 C/8h/Furnace cooled C/8h/Air cooled is recommended by producer. Similar procedure as in former case was performed. Metallographic specimen were subjected to 1050 C/1h solution treatment and then to different combination of time and temperatures of precipitation annealing. Hardness values are given in Fig. 3. It was found that hardness increases with annealing time in case of one step 790 C annealing. The second step annealing is useful after first step 790 C/8h only. In case of shorter first annealing step the second step results in slight decrease of hardness. Furnace cooling between the two annealing steps results in higher hardness values than air cooling. This investigation confirms the recommended precipitation annealing regime 790 C/8h/FC C/8h/AC. 3.3 DTA Study of microstructure and its transformation during heat treatment was accompanied by a DTA analysis carried by VŠB-TUO in Ostrava [4]. Selected results from heating regime are shown here (Fig. 4, Tab. 2).

4 Obr. 4 Vyhodnocené DTA křivky slitiny 718Plus Fig. 4 Evaluated DTA curves of 718 Plus alloy Tabulka 2 Transformační teploty slitiny 718Plus pro různé rychlosti ohřevu Table 2 Transformation temperatures of 718 Plus alloy at different rating rates Heating rate [ C/min] T γ,s T γ,e T δ, S T δ, E (T Laves,S ) Phase transformation temperature - heating T Laves, E T S, 1 T S, 2 T MC,1 T MC,2 T MC,3 T L (calc.) [ C] Phase transformation temperatures for different rates of heating are shown in Tab. 2. T γ,s incipient solution temperature of γ phase, T γ,e final solution temperature of γ phase, (T δ,s ) incipient solution temperature of δ phase, T δ,e (T Laves,S ) final solution temperature of δ phase (incipient solution temperature of Laves phase), T Laves,E final solution temperature of Laves phase, T S, 1 solidus start temperature, T S, 2 - solidus finish temperature, T MC,1, T MC,2, T MC,3 solution temperature of MC-type carbides, T L liquidus temperature. Results of DTA in heating regime could be compared to microstructure observation during solution annealing analysis. Temperature interval of γ solution is similar C. More processes are to be considered in this temperature region: γ solution and transformation to δ phase, γ phase precipitation and γ phase solution. It is important to note that γ'' phase transformations were not indicated directly by DTA. It is caused by low amount and low dimensions of γ'' phase particles.

5 3.4 Mechanical properties Rp0,2, Rm [MPa] Rp0,2 Rm A, Z[%] A Z Teplota [C] Teplota [C] Obr. 5. Mez kluzu a mez pevnosti v závislosti na teplotě zkoušky Fig. 5. Yield strength and rupture strentgth vs. Test temperature Obr. 6. Hodnoty tažnosti a kontrakce v závislosti na teplotě zkoušky Fig. 6. Elongation and contraction vs. Test temperature Results of tensile tests of heat treated 718Plus alloy are depicted in Fig. 5 a 6. As can be seen, strength values decreases linearly with rising test temperature. The decrease is sharper above approximately 700 C. Measured strength values are smaller than of 718Plus alloy in a worked state, but are comparable to those of 718Plus alloy in an investment cast state [5]. Our values of elongation A and contraction Z are rather smaller and exhibit quite large scatter. Specimens of investment-cast 718 Plus alloy were subjected also to Charpy toughness tests. Results are shown in Table 3. Measured values exhibit large scatter. Maximum value is KCU = 59,6 J/cm 2 at 500 C, minimum value is KCU = 20,2 J/cm 2 at 600 C. It seems that KCU values are almost temperature independent. The test temperature is probably not origin of the data scatter. Tabulka 3 Hodnoty vrubové houževnatosti Table 3 Results of Charpy impact toughness tests Test temperature Specimen Charpy toughness KCU Specimen Charpy toughness KCU ( C) (J/cm 2 ) (J/cm 2 ) 20 I1 25,6 I2 49,2 400 I3 44,2 I4 42,0 500 I5 24,1 I6 59,6 600 I7 22,2 I8 20,2 700 I9 31,4 I10 51,4 800 I11 45,7 I12 38,7 3.5 Fractographic analysis Fracture surfaces of selected specimens were subjected to fractographic analysis by means of scanning electron microscopy. The goal was to find out origin of low plastic properties and large scatter of KCU and plastic properties data. Fracture surfaces of specimens from tensile test and Charpy toughness test are very similar. Fracture is predominantly ductile interdendritic (Fig. 7). Large

6 intergranular casting defects were found on specimens exhibiting low values of A, Z and KCU. Dimensions of these defects can be of order hundreds of microns (Fig. 7). Obr. 7. Pohled na lomovou plochu a licí vady Fig. 7. Fracture surface and casting defects 4. SUMMARY The 718Plus alloy research programme has started. Microstructure investigation, heat treatment analysis and short term mechanical properties tests were performed. The microstructure is not simple and is a bit different to nickel superalloys tested previously [6]. Heat treatment is rather complicated. Possibility of its refinement can be investigated. Mechanical properties of the 718Plus alloy are promising. In case of 718Plus alloy production, reduction of casting defects could be necessary. It can be done by means of casting modification or by HIP treatment. Recently, long-term high-temperature microstructure stability is investigated as well as degradation of mechanical properties after long-run high-temperature degradation. Creep properties of the alloy will be also measured. ACKNOWLEDGMENT This investigation into the properties of the 718Plus heat-resistant alloy was a sponsored by the Czech Ministry of Industry and Trade, programme Tandem, project FT-TA4/023. LITERATURE [1] PODHORNÁ, B. ZÝKA, J HLOUS, J.: Výzkum a vývoj mechanických vlastností materiálů použitých pro nové typy turbodmychadel, spojený s vývojem nové progresivnější technologie přesného lití těchto částí. [Výzkumná zpráva UJP 1356]. Praha, UJP PRAHA a.s. 2009, 58s. [2] JENISKI, R. A. KENNEDY, R. L.: Development of ATI Allvac 718Plus Alloy and Applications [online]. [cit ]. Dostupné v ATI Allvac, Monroe (NC USA), [3] CAO, W.: Solidification and Solid State Phase Transformation of ALLVAC 718Plus Alloy [online]. [cit ]. Dostupné v ATI Allvac, Monroe (NC USA), [4] Zlá, S., Smetana, B., Dobrovská, J. DTA - analýza slitiny 718Plus. Výzkumná zpráva, FMMI VŠB-TUO Ostrava, listopad 2009

7 [5] BAYHA LU, M. KLOSKE, K. E.: Investment Casting of ALLVAC 718PLUS ALLOY [online]. [cit ]. Dostupné v ATI Allvac, Monroe (NC USA), [6] ZÝKA, J. HRBÁČEK, K. SKLENIČKA, V., Analysis of Creep Tests of the IN 792-5A Alloy. In. Proceedings of METAL 2009 [CD-ROM], Hradec nad Moravicí, Tanger, 2009.