Yutaka Fukui

Effect of titanium, niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.

Development of 12CR Steel Disc for Heavy Duty Gas Turbines

This paper describes research and development of 12Cr steel for heavy duty gas trubine disc. The 1300°C class gas turbine developed by Hitachi, Ltd. (H25) is commercially operated for land-based power generator (25MVA) which is realized after five years designing.The quantitative study of alloying elements, which increase mechanical properties such as creep rupture strength, tensile strength, and toughness, is discussed. The sixteen pieces of real scale disc for 1300°C class gas turbine are examined to evaluate mechanical properties in terms of real operation. The recent study of 12Cr steel disc for 1500°C class gas turbine is also discussed.© 1994 ASME

Development of a Cobalt Base Superalloy for Heavy Duty Gas Turbine Nozzles

This paper describes an improvement in creep strength and thermal fatigue and hot corrosion resistance of a cobalt base superalloy developed for the applications of heavy duty gas turbine nozzles. The developed alloy has superior resistance to creep, thermal fatigue and hot corrosion over that of conventional alloys.The optimization of alloying elements which improve the creep properties and restrain the coarsening of carbides is discussed. A reduction in the volume fraction of eutectic carbides promoted the prevention of fast thermal fatigue cracking and also increased the hot corrosion resistance of the developed alloy.The mechanical properties were evaluated by general method for gas turbine materials. Thermal fatigue property was examined by introducing cyclic thermal stress into test pieces. During the test, crack propagation of the test pieces was observed. Hot corrosion resistance was evaluated by molten salt corrosion tests. After tests, mass loss of specimens was measured and penetration depth of sulfidized scale was observed. The developed alloy showed good properties which can allow a wide temperature margin for high temperature gas turbine nozzle designing.The developed alloy can be applicable to other gas turbine hot sections. Application of the developed alloy could realize an increase in gas firing temperature or extension of lifetime for current conditions.© 1996 ASME

Development of Materials and Coating Processes for High Temperature Heavy Duty Gas Turbines

This paper describes materials developed for the first stage nozzle and all stages turbine disc, and the new coating processes of the first stage bucket for high temperature, heavy duty gas turbines.A cobalt base casting superalloy (30Cr-10Ni-7W-0.2Nb-0.15Zr-0.2Ti-bal.Co), having high creep rupture strength and ductility, was used for the first stage nozzle and 12Cr forging material, having high creep rupture strength, was used for all stages turbine disc of the high temperature, heavy duty gas turbine, the H25. To increase hot corrosion properties of the first stage bucket, MCrAlY was applied on the bucket external surfaces by a low-pressure plasma-spray coating technique. A procedure of chemical vapor deposition for Al coating on the bucket internal cooling paths was also established for future high temperature gas turbines.Copyright

CHARACTERISTICS OF TITANIUM ALLOY 40 INCH LAST STAGE BLADE OF STEAM TURBINE

ABSTRACT Enlargement of the last stage blade (LSB) is one of the most effective ways to improve thermal efficiency. This requires development or selection of the materials which have a high tensile strength-to-weight ratio suitable for long low pressure LSBs of the steam turbine. It was confirmed that Ti-6Al-4V alloy was suited to a 40 inch last stage blade for 3600 rpm turbine instead of 12Cr materials. Shot peening was effective to improve high cycle fatigue strength. A trial 40 inch blade which was manufactured from Ti-6Al-4V alloy had offered excellent characteristics to satisfy the specified requirements.