IMPROVING C1023 MANUFACTURABILITY

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1 IMPROVING C1023 MANUFACTURABILITY USING TWO-STEP HEAT TREATMENT The inherent hightemperature strength of superalloys that makes these materials ideal for applications at elevated temperatures, such as gas turbine components in aero gas turbine engines, works against the fabricator in manufacturing the parts. Thermal processing is required that puts the material in a workable state and then reverts it back to its high strength condition for service. Iñigo Hernández*, Amaia Subinas, Dr. Iñaki Madariaga, and Dr. Koldo Ostolaza Industria de Turbo Propulsores (ITP) Materials & Processes Dept. Zamudio, Spain Nickel-base C1023 superalloy (Ni, 15.5% Cr, 9.7% Co, 8.3% Mo, 4.1% Al) is widely used in the manufacture of equiaxed nozzle guide vanes and seal segments for aero gas turbines, especially in low-pressure turbines (Fig. 1). However, in spite of its extensive applications in many different types of engines, this alloy is considered difficult to weld, repair, and machine (although it is considered weldable). The high strength of this alloy is mainly due to the presence of a fine distribution of (gamma prime) precipitates that are formed directly after casting. The quick formation of these precipitates does not allow processing of C1023 components in a soft condition, such as that of other superalloys in a solution annealed state. However, it is desirable to achieve some kind of temporary soft condition that could improve the manufacture of C1023 components. This article discusses this alternative with a detailed study on the response of C1023 material to different heat treatments. The focus of the Representative aero gas-turbine engine. investigation addressed heat treatments that would achieve opposite results. That is, first, it is necessary to achieve a heat treatment capable of softening the material to improve all the aspects related to its manufacturability. After this, the development of a second heat treatment that would be introduced at the end of the manufacturing process is necessary to recover the original high strength of the material. C1023 Microstructural Features C1023 is a face centered cubic (FCC) superalloy strengthened by a *Member of ASM International Fig. 1 Nozzle guide vanes. HEAT TREATING PROGRESS MAY/JUNE

2 Fig. 2 Typical C1023 microstructure containing Mo (white) and Ti and Mo (black) carbides; fine is distributed in the matrix Fig. 3 Sample microstructure in the as cast condition consists of a dendritic microstructure with fine precipitates ( is about 0.5 m in the interdendritic regions) Fig. 4 - Sample microstructure in 1190ºC heat treated and water ed condition. Nothing is resolvable with the SEM because has gone into solution. Secondary Primary Fig. 5 Sample microstructure in the as cast ºC/8 hours and slow cooled condition. A very fine distribution of secondary gamma prime exists between the primary precipitates that precipitate during slower cooling. fine distribution of precipitates Ni 3 (Al, Ti). Most of the precipitates have sizes around 1 m, but this material can also have submicron sized precipitates. MC, M 23 C 6, and MC 6 carbides are formed during the solidification along the grain boundaries with M being typically Mo and Ti. Figure 2 shows a representative C1023 microstructure. In some cases, phase, which is considered detrimental, can appear if the solidification of the casting is too slow in the temperature range of 800 and 900ºC (1470 and 1650ºF). The typical material features such as grain size, carbides, and hardness can be influenced by heat treatment. However, in the range of temperatures where this material has been studied in the present work, the main microstructural changes have been observed in the size and distribution and, to a lesser extent, in the volume fraction of carbides. The typical service condition of this material is as cast plus a stress relief treatment carried out at a temperature between 1150 and 1190ºC (2100 and 2175ºF). This heat treatment is usually conducted to relieve the stresses produced during the casting process. Heat Treatment Trials The high strength of this alloy is mainly due to the presence of a fine distribution of precipitates that are formed directly after casting (Fig. 3). Therefore, to achieve some kind of temporary soft condition that could improve the manufacture of C1023 components, it is necessary to achieve coarser precipitates by controlling the cooling rate or overaging in the formation temperature range. A second treatment that would be able to dissolve the and precipitate it again with a fine distribution to restore the original properties would also be necessary. With this purpose in mind, microstructural studies were carried out on specimens in the as cast condition and after various heat treatments. The different conditions included: As cast As cast + TT1190ºC/1h gas fan As cast + TT1190ºC/1h furnace cool As cast + TT1190ºC/1h water As cast + TT1190ºC/1h slow cool (5ºC/min) to 1000ºC and then gas fan As cast + TT1190ºC/1h gas fan + TT1100ºC/1h slow cool to 700ºC and then gas fan As cast ºC/8h furnace cooling As cast ºC/8h water A key factor in this study is that there are no visible precipitates in the microstructures of samples heated at 1190ºC/1 hour and water ed (Fig. 4). This means that complete solution of the occurs at this temperature. Therefore, it is possible to return to the standard microstructure if at the end of the manufacturing process a stress relief/ solution heat treatment is carried out. However none of the heat treatments carried out holding the material at 1190ºC and modifying the cooling rate was capable of producing the desired coarsening of the and then the intended softening of the material. The analysis of the samples heat treated at 1100ºC for 8 hours followed by slow cooling under high magnification (20,000 ) shows the formation of two different types of precipitates (Fig. 5). However, as shown in the sample ed after 8 hours at 1100ºC (Fig. 6), it is clear that the fine precipitates were not formed during the 8 hour hold at 1100ºC, and instead, must be the result of the pre- Fig. 6 Sample microstructure in the as cast ºC/8 hours and water ed condition. Quenching from 1100ºC avoids the precipitation of the secondary gamma prime, which means the precipitates of the sample in Fig. 5 formed during slow cooling from 1100ºC. 26 HEAT TREATING PROGRESS MAY/JUNE 2007

3 cipitation occurring during the slow cooling from 1100ºC. Thus, it appears that a partial dissolution of the occurs at 1100ºC, but no coarsening effect occurs. Therefore, none of these heat treatments is capable of producing the desired softening of the material. This was confirmed not only by means of microstructural analysis but also with the microindentation hardness results, which are similar in all the samples except the material ed in water (425 HV1). The hardness value of the rest of the samples is about 380 HV1. The results of the first trials led to the exploration of using HIP (hot isostatic pressing, or sometimes processing) cycles, taking into account that the cooling rate of the HIP process is very slow from a very high temperature. HIP Study During this heat treatment study, C1023 material was also the focus of an investigation directed toward the reduction of the shrinkage formed on C1023 components during the casting process. The material was processed by hot isostatic pressing (HIP), a process used to help close the internal porosity of castings, and widely used in aircraft gas turbine industry. The cooling rate of the process is very slow from the very high temperatures of the HIP cycles, in this case 1200ºC (2190ºF), and such a cooling rate would most likely lead to a slight modification of the microstructure and the properties of C1023. Based on the results of the heat treatment studies, it was believed that a holding period at 1200ºC would be able to fully dissolve the. Afterward, a very slow cooling rate such as that associated with HIP would be able to achieve the coarsening of the that was not achieved with the heat treatments investigated. Using as a basis the standard HIP conditions of 1200ºC for 4 hours and 100 MPa (14,500 psi) pressure, laboratory samples were processed using a variety of thermal cycle combinations including: 1. As cast 2. As cast + 2HT 3. As cast + HIP without pressure (NP) 4. As cast + HIP (NP) + HT 5. As cast + HT + HIP (NP) 6. As cast + HT + HIP (NP) +HT 7. As cast + HIP 8. As cast + HT + HIP 9. As cast + HT + HIP +HT The first part of the study covered the cycles 1 to 6 described. At this 1. As cast 2. As cast + 2 HT 3. As cast + HIP 4. As cast + HIP + HT 5. As cast + HT + HIP 6. As cast + HT + HIP + HT 7. As cast + HIP 8. As cast + HT + HIP 9. As cast + HT + HIP + HT Fig. 7 SEM photomicrographs of samples in various conditions show little difference in microstructure (samples 3-6 HIPed without pressure; samples 7-9 HIPed with pressure). There are differences in hardness among the samples possibly due to the presence of very fine precipitates that are not resolvable at the SEM magnification of 5,000. HEAT TREATING PROGRESS MAY/JUNE

4 Table 1 Hardness comparison after different HIP cycle combinations Trial Condition of sample Microindentation hardness, HV1 Sample Ref. No. 1 As cast ±6.4 S A 2 As cast + 2 HT ±6.3 S B 3 As cast + HIP (no pressure) ±6.4 S C 4 As cast + HIP (no pressure) + HT ±6.2 S D 5 As cast + HT + HIP (no pressure) ±7.4 S E 6 As cast + HT + HIP (no pressure) + HT ±6.3 S F 7 As cast + HIP (with pressure) ±6.5 S G 8 As cast + HT + HIP (with pressure) ±6.5 S H 9 As cast + HT + HIP + HT ±5.7 PCB ref 05/0637 stage of the study, it was considered that the influence of the HIP pressure on the microstructural evolution of the material would be negligible and all the cycles were conducted without pressure. The solution treatment at 1190ºC/1h/slow cooling to 1000ºC and then gas fan is defined as HT. Arbitrary units Arbitrary units Fig. 8 Relative stress rupture time of HIP and non-hip material. Fig. 9 Relative ultimate tensile strength of HIP and non-hip material. SR 850 C no HIP SR 850 C HIP + HT SR 850 C HT + HIP + HT SR 850 C database typical SR 850 C database minimum ,000 1,200 Time to rupture, h No HIP HIP + HT HT + HIP + HT Database typ. Database minimum ,000 1,200 Temperature, C Cycles 7 to 9 were conducted in the second part of the study. These cycles were carried out on samples extracted from components that had been HIPed in production furnaces, in an attempt to determine if the microstructural changes observed on laboratory scale furnaces and samples could also be observed on production parts. The heat treatments were analyzed by comparing the microstructures obtained after each cycle and also by measuring microindentation hardness. The results of all combinations of heat treatments and HIP cycles are presented in Fig.7, and the corresponding hardness values are listed in Table 1. There is not a great deal of variation in the microstructures of the different treatments, but hardness decreases for samples that were processed using HIP cycles (with or without pressure), and that did not receive a final heat treatment at 1190ºC. In the particular case of samples 3 and 5, the obtained hardness value is below 350 HV compared with the original values of 380 HV for the as cast material and the as cast + stress relieved material. The results also demonstrate that although HIP produces a decrease in hardness, the original hardness of the as cast material could be recovered by treating the material at 1190ºC. The differences between the dissimilar hardness values can not be explained on the basis of the microstructural features observed on the material at SEM resolution. For example, Fig. 7 does not show any clear microstructural differences between any of the samples analyzed. However, apart from the micronsized precipitates that can be seen using SEM, this material also has much smaller precipitates that can not be resolved using SEM, and that can explain the differences among the samples. Mechanical Testing The results above show it is pos- 30 HEAT TREATING PROGRESS MAY/JUNE 2007

5 sible to achieve a softening of C1023 by heat treatment and then recover the original properties by performing a second heat treatment. To verify these results, mechanical tests were conducted, including tensile, stress rupture, low cycle fatigue (LCF), and impact tests. Figs 8 and 9 show the result of the stress rupture and tensile tests conducted on samples processed with and without the HIP cycle. In both cases, the results obtained with and without HIP are comparable. LCF and impact tests also show the same trend observed in the stress rupture and tensile tests with very little differences among sample processed with and without HIP, with all the results within the typical scatter of properties of this material. Therefore, while it seems the material is softer immediately after the HIP treatment (as indicated by the reduced hardness), the final heat treatment is capable of fully dissolving the and producing an improved precipitation pattern upon cooling, which is able to restore its original properties. Conclusions Test results show that it is possible to achieve a significant reduction in the hardness of C1023 after 4 hours at 1200ºC followed by controlled slow cooling. A starting microindentation hardness of 375 HV was reduced to 341 HV on laboratory samples and to 356 HV on real components. This suggests it could be possible to improve the machinability and weldability of this alloy after HIP. The application of an 1190ºC heat treatment for 1 hour is able to fully dissolve the coarse precipitates formed during HIP, and it is possible subsequently to form a fine distribution of the precipitates, which restores the original properties. Results of tensile, stress rupture, impact, and hardness tests are better after HIP + HT than those in the as cast + HT condition. These results confirm the restoring effect of the 1190ºC heat treatment. For more information: Iñigo Hernández Industria de Turbo Propulsores, ITP, S.A., Parque Tecnológico Edificio Zamudio, Spain tel: inigo.hernandez@itp.es Internet: Proven and Reliable Combustion Solutions for Heat Treating i As the North American source for Kromschroder burners and recuperative burners like the Ecomax from LBE, in addition to our own SVG line, Hauck has an extensive array of burners, control strategies, and total system solutions for the heat treating industry. Our extensive product lines can be used for direct and indirect fired applications. Our high velocity burners combined with pulse firing control technologies will improve your product quality, reduce emissions and optimize fuel efficiency. Hauck Manufacturing Company, PO Box 90, Lebanon, PA Phone: Fax: To Earn our Customer s Loyalty - Every Day HEAT TREATING PROGRESS MAY/JUNE