Effect of titanium, niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys
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firing temperature of gas turbines is steadily increasing in o r d e r to improve the efficiency. The present first stage nozzles are exposed to temperature of 900 to 1000 ~ The conventional cobalt base superalloys used for the first stage nozzles have high strength and ductility at temperatures up to 900 ~ The present study was planned to develop a cobalt base casting alloy having superior strength and ductility at around 1000 ~ by adding Ti, Nb and Zr, as suggested by the known effects in high-Cr and high-Ni heat resistant alloys. ~-3 The effect of carbon up to the concentration o f 2.8 pct was investigated with a view to strengthen the cobalt base alloy containing Ti, Nb and Zr. EXPERIMENTAL METHOD A high-frequency furnace was used for melting. The alloys were air-melted and poured in sand molds under the atmosphere. Ingot size was 80 mm in length, 80 m m in width and 15 mm in thickness with a riser of 90 m m x 20 mm. The heat treatment of ingots is as follows; holding at 1150 ~ for 4 h, furnace-cooling to 982 ~ and then holding for 4 h, furnace-cooling to below 537 ~ and air-cooling to room temperature. Chemical compositions are shown in Table I. Heat No. l in Table I is the basis of comparison. Heats from No. 2 to 11 in the same table were made to study the effect of Ti, Zr and Nb on the high temperature strength and ductility. The effect of C was examined with heats from No. 12 to 17. Creep r u p t u r e testing was done at 982 ~ with specY. FUKUI, R. SASAKI, F. HATAYA, and T. KASHIMURA are SeniorResearcher, Chief Researcher, Senior Researcher, and Technician, respectively, Hitachi Research Laboratory, Hitachi Ltd., Ibaraki-ken, Japan. Manuscript submitted August l, 1980. METALLURGICALTRANSACTIONSA
imens having a parallel part of 6.0 mm diam by 30 mm in length. Metallographic samples were etched with a mixture of HCI, H20 and 3 pct H202 in the proportion of 50 : 50 : 10. Phase analysis was aided by a micro Auger electron spectroscopy, after the specimens were irradiated with argon ions for 20 min for the purpose of cleaning the surface. RESULTS A N D DISCUSSIONS Creep Rupture Strength Figure 1 shows the creep rupture strength at 982 ~ of Heats No. 1 to 11. Heat No. 2, containing 0.42 pct Nb, has a higher creep rupture strength and a less steep slope in the stress-rupture time curve (a - t) than No. 1. Heat No. 3, containing 0.19 pct Ti and 0.16 pct Nb, shows a tendency similar to No. 2. Heats No. 4, 5 and 6, containing Ti, Zr and Nb, have strength values which are close to each other and higher than No. 2 and 3. Heats No. 4, 5, 6, and 8, all containing Ti, Zr and Nb, may be grouped together as having the compositions optimum for creep rupture strength. Heats No. 10 and 11 with higher contents of Ti, Zr and Nb, have lower strength than the above group. Figure 2 shows the relationship between Zr contents and 103 h creep rupture strength. The strength increases with Zr contents up to 0.3 pct when Ti is from 0.13 to 0.19 pct. With higher Ti contents, 0.25 - 0.29 pct Ti, the strength decreases with Zr conten
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