i0. C. Zener, App. J. Phys., No. 20, 950 (1949). ii. H. K. D. H. Badeshia, Acta, Metall., No. 29, 1117 (1981). 12. R. Trivedi, Metall. Trans., No. i, 921 (1970). 12. M. Hillbert, Jernkontorets Ann., No. 141, 757. 14. N. Enomoto andH. I. Aaronson, Scr. Metall., No. 19, 1 (1985). 15. H. Ohtuka, M. Umemoto, and I. Tamura, Tetsu To Hagane, No. 73, 144 (1987)

DEVELOPMENT OF HIGH-TEMPERATURE CORROSION-RESISTANT ALLOYS AND HEAT-

TREATMENT REGIMES FOR COMPONENTS PLACED IN THE HOT SECTION OF STATIONARY

GAS TURBINES

Yu. I. Zvezdin, Yu. V. Kotov, E. L. Kats, V. P. Lubenets, E. V. Spiridonov, and M. L. Konter

UDC 669.14.018.44

The basic problem of the refinement of stationary power, gas-transfer, and commerical gas-turbine plants is the improvement of their fuel economy and reliability.

Elevation of the gas temperature at the inlet to the turbine is a promising trend in the improvement of turbine efficiency. Improvement in diesel economy is associated with the use of units with a turbo-supercharger.

The development of highly economical stationary gas-turbine plants and diesels is largely dependent on the use of new superhigh-temperature materials and commercial processes of precision casting in accordance with smeltable models.

At the present time, special cast high-temperature, corrosion-resistant alloys are used to fabricate comonents of the hot section in stationary turbines and diesels in connec- tion with the use of aggressive fuel containing S, Na, V, and C1 in these machines. In this case, the operating temperature of the metal is limited to 800-850°C; this is achieved by use of blades with complex internal cooling systems.

Cast high-temperature, corrosion-resistant alloys for stationary gas turbines and diesels should satisfy a set of requirements, basic among which are high resistance to sulfide cor- rosion, high-temperature strength, and plasticity with allowance for prolonged service (up to 50000 h).

The elevation of the gas temperature at the turbine inlet, the use of inexpensive grades of fuel, and an increase in service life of stationary turbines to 30000-50000 h has led to the need for the development of a new generation of high-temperature alloys for components of the hot section. In this case, the basic requirements for materials for the blades of the first stages are the high high-temperature strength, resistance to mechanical and thermal fatigue, and a sufficient level of plasticity, as well as resistance to sulfide corrosion, which ensures failure-free operation of a blade with a damaged protective coating for one year, i.e., the period between turbine inspections. Increased corrosion-resistant require- ments are presented for alloys used in the cast guide vanes and segments of the combustion chamber that opeate without coatings; in this case, these alloys should possess high resis- tance to thermal fatigue, adaptability to welding and other assembly operations, and suf- ficient high-temperature strength.

A system for the computer-assisted design of nickel alloys (Fig. i), which differs significantly from familiar computational systems, was developed to create a series of new materials. The system includes calculation of the volume percent and chemical composition of the y'-phase, carbides, and borides that are segregated after heat treatment and aging for a specific chemical composition of the matrix with allowance for liquation and homo- geneity. A number of physical characteristics of the alloy and its basic phases, which

Scientific-Production Union, Central Scientific-Research Institute of Technology and Machine Building. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 20-22, June, 1991.

0026-0673/91/0506-0443512.50 © 1991 Plenum Publishing Corporation 443

l Calc. of liquidus and, lelidus temp. Physical properties oJ ~lloy: density, coeff ~f thermal expansion~ lastio modulus

]orrosion reslstance: [) tests in oil~

losses with respecl to mass

~) during bench tests corrosion rate

~) parameters of noncatastrephic corrosion mechanisl

~nemical comp. of alloy (wt.%)

Chemical comp. of alloy (at.%) I

I l LAmount and temp. of carbides and betides I

"Chemical comp. o~ alloy without carbides | and borldes l

! Liquat ion coefficients I

I I ,,, I co~. of l lc°~" of i.- I I dendrite axes I I teraxial space I I "... I " I I

Volume percentage of y '-phase (%)

I ' 1 I, Dlstribution coeff, of elements between

~- and ~ i-phases

~a lc . Of chemical CO~. Of y - &~d ~ ' -p~ses I "|

I I I I I , . ~ -ph . . . . by Nvlmelt.hod .. sol,ubility index ~'

i I I I Calc" °f TPU-rlsk °f Y- I Lattlce p .... t .... f ~- and X '-phases I , , p h a s e by Ha method I I I

. I I I Calc. of risk ~ =,,WI Coeff. of thermal expansion of phases, l by Ha method a y, a ~ ,, and A a at working temp.

I I I I ICalculatxon of risk of ~ T 1 1 .... I , OP (A~Z) i High-temp. strength of I

I I e a l o u l a t i o n of r i s k c~ base of 100-20000 h

111 o, , Be I I I Cost of charge I' materials

OP ~-i Multicriterial optimization 1~I- OP I

Fig. i. Flow chart of system for computer- assisted design of alloys (OP, optimization parameter; BC, boundary conditions).

8 ~ 6,;'oo: Nlmm 2 ,MARM2~6~ \ ----F-----]

3:0 I I oc'vig'~: I

blIARM2/+E ICNI~ONKB'~5" !

I :~v792 ekcN. ~ 1 YOO I \ ~, ".. I

::o - • ..:.<?,,,,, -+- INZI3LC C//K17 BC 5

CNIf 2I"

ZFO

/ 0 0

5 0 fO 1

CNif 2f ON/~ 2 ~ "<e

Fxx ~ Y4

,+0 -1 fB -z 10 - ' t A, xnm

Fig. 2. High-temperature strength of domestic and foreign alloys for gas-turbine blades and gears of supercharged diesels (metal losses during corrosion are laid off against the abscissa: testing at 900°C for 20 h in Na2SO 4 + 25% NaCI mixture).

can be determined by the computational method (density, liquids and solidus temperatures, temperature of complete dissolution of the 7'-phase, coefficients of thermal expansion, elastic modules, energy of stacking defects, and lattice parameters), serve as boundary con- ditions during development of the alloys, just as the phase stability, whose calculation is based on the Nv-PHACOMP and Md-PHACOMP methods for the TPU-phases and ~-W phase and on the original method for the ~-Cr phase. The high-temperature strength,corrosion resistance, structural stability (as determined by the difference in the lattice parameters of the ~- and ~'-phases), and cost of the charge materials are parameters of the multicritical opti- mization. It should be noted that the long-term strength of materials with oriented (DC) and single-crystal (SC) structures is calculated for the equiaxial state (ES) and the deter-

444

' - ~ - - - - " - ' " , L ~ ' ~ . . . . . . , , I t

. . .

. - , " . - : : -

• " . .-~, -" . ~ , - , > , - .~

." . , . , ~ . . g

l - ~ _ , . ..~.~',~. - - . . ,. , . _ _ ' . , . , ~ ; ~ , , ~ .

, , ~,, ',~: • b . - ,~? ' . t . - . '~ . . . . . .

Fig. 3. Microstructure of alloy CNK-7NK (×20000): a) after heat treatment (homo- genization with incomplete dissolution of primary ¥'-phase + aging); b) after hold- ing for 5000 h at 850°C; c) after service life of 6000 h in GTN-25 (PO NCL) gas- turbine plant.

mination of their actual high-temperature strength is an appraisable character. The computer aided design system is based primarily on equations obtained by the method of regression analysis of significant data banks (90-160 compositions per response) for alloys containing (percentage of total mass) 9-25% Cr, up to 20% Co, up to 5% Mo, up to 12% W, 2-5.5% AI, 2-5.5% Ti, up to 1.5% Nb, and up to 2% Ta.

The program for the nonlinear high-precision regression analysis called for possible use of the method of group accounting of arguments (MGAA); it was established during the study, however, that the models in which the signs before the coefficients and the powers of the terms were assigned in advance, proceeding from known physical effects, show better correlation than those obtained using the MGAA. Conclusions concerning the effect of ele- ments on the amount of ~'-phase in the alloy, which were thereafter confirmed experimentally, were drawn on the basis of analysis of the relationships obtained. The computational sys- tem, which is based on regression equations, exhibits high accuracy. For example, the high-temperature strength of the alloys in the 800-850°C interval is determined with an error to 3-4%. Use of this computational system by the Scientific-Production Union, Cen- tral Scientific-Research Institute of Technology and Machine Building promoted the develop- ment (jointly with other organizations) of new alloys of the TsNK (or CNK) series with an assigned set of operational properties in a short time and with minimal expenditures (Fig. 2). The first alloy of this series (CNK-7) was developed and implemented at the outset of the 1980s. At the present time, this universal alloy, which can be used for both equi- axial casting of guide vanes and blades, and the fabrication of highly stressed blades sub- jected to oriented crystallization (CNK-7NK), is one of the basic nickel superalloys of Soviet stationary gas-turbine building. It surpasses alloys IN738LC and GTDIII somewhat with respect to the set of operational properties, and in the DC-state, possesses high- temperature strength close to that of alloy IN792 with a significantly higher corrosion resistance. The CNK-7 alloy has an appreciable mean time to first failure and service life, and high phase stabilty during prolonged service. The typical microstructure of this alloy is showed in Fig. 3 (the ~'-phase amounted to approximately 48% of the total mass).

The superhigh-temperature alloy CNK-8 has been developed and implemented for use in the blades of modern high-temperature turbines (see Fig. 2). Complex alloying of this alloy makes it possible to ensure the most effective resistance to cooperative creep of disloca- tions through particles of ~'-phase due to the optimal combination of elements forming to y'-phase and the hardening matrix. Alloys with CD- and SC-structures possess a greater tendency to dendritic hardening and adaptability to manufacture than the CNK-SM alloy. The volume of ~'-phase in the alloy is 52-55%; the high-temperature strength of the equiaxial

445

TABLE 1

Element content, %

A11oy crlCoI oI IA I INblTalCl B

IN738LC IN792 IN939 INTI3LC MM246 F S X414

16,0 8,5 1,75 2.5 3,4 3,4 0,9 1.75 0,45 0,01 12,7 9.0 2,0 3.9 3,2 4,2 - - 3.9 0,2 0.02 22,4 19.0 - 2.0 1,9 a.7 1.0 1.4 0.15 0.008 ,2.0 4,~ 50 o6 2,0 o.o5 o o i 9 , o , o ~ o 2 , 5 1~o 5 . 5 1 . s - 12 o , 5 o.o15

29,5 t ~ . - 7,o 0,25 o.o12

W + M o Alq-Ti

TsNK-7 14,7 9,0 7,2 8,0 0,1 0,01 T s N K - 8 M 12,5 9.0 6,7 8,7 1,0 B T s - 5 19,5 9,0 3,1 7.5 0,02 0,2 T s N K - 2 1 21,0 I0,5 3.8 5,8 0,7 - - 0,08 0.01 T s N K - 2 4 23.0 10,5 3.4 4,3 0.6 - - 0,08 0,01 T s N K - 1 6 14,0 4,0 5,6 8,7 0.8 -- 0,13 0,01 T s N K - 1 7 14,0 -- 5.6 8,7 0,8 -- 0,13 001

Note. Nickel is the base in all alloys, with the exception of alloy FSX414, the base of which is cobalt and the nickel content 10.5%.

alloy corresponds to that of alloy IN792, while that of the alloy subject to oriented crystallization corresponds to the high-temperature strength of alloy MARM-246 (SS) with a significantly higher corrosion resistance. A further increase in high-temperature strength (to the level of the MARM-246DS alloy) with retention of satisfactory corrosion resistance is characteristic for alloy CNK-9, which contains approximately 60% of ~'-phase and which has an appreciably hardened matrix (Fig. 2).

Alloy BTs-5, which is hardened with 35-38% of ~'-phase in which the carbon is replaced by boron, is recommended for highly stressed guide vanes capable of operating without coat- ings. This alloy is superior to the Udimet720 alloy in terms of corrosion resistance with- out being inferior to it in terms of high-temperature strength. Alloy CNK-21, whose pro- perties are close to those of alloy IN939, and whose cost (in terms of charge materials) is 60% lower, possesses even higher corrosion resistance. Alloy CNK-21 is intended for the casting of guide vanes of imported series MS3000 and MSS000 turbines, in lieu of alloy FSX414.

The combustion chambers of gas-turbine plants are usually fabricated from sheet materials with solid-solution hardening of a type X Hastelloy. Ruggedization of the temperature regime of turbine operation results, however, in the need to develop coolable combustion chambers consisting of cast segments of complex shape. The requirements for high-temperature strength, which are set forth for materials used in combustion chambers exceed the maximum attainable level of high-temperature strength for this case of alloys by several times. The cast nickel alloy CNK-24, which contains 24-28% of ~'-phase, exhibits not only increased corrosion resis- tance (on a level with alloy IN935), but also high phase stability, adaptability to welding and straightening, and resistance of thermal fatigue. As compared with alloy IN935, alloy CNK-24 contains half the cobalt and niobium.

The series of low-cost alloys CNK-16, CNK-17, and CNK-18, which are in no way inferior to alloys B1914, 713C, and GMR235D in terms of high-temperature strength, and which surpass them in terms of corrosion resistance have been developed for the wholly cast gears of turbo- supercharging systems (see Fig. 2). These alloys possess high adaptability for the fabrica- tion of gears with equiaxial and radially oriented structures.

The production of the highly loaded gears of a turbo-supercharger formed from alloy CNK-SM with an MC structure has been mastered.

Special heat-treatment regimes, which make it possible match heat treatment with the production cycle of the application of plasma protective coatings, have been developed as applies to the critical components located in the hot section of stationary gas-turbine plants formed from CNK alloys.

The heat treatment includeshomogenization annealing at I140-1260°C (depending on the alloy, the annealing temperature should be above that of complete dissolution of the pri- mary 7'-phase and below the melting point, primarily of the y/y' eutectic). The optimum~ homogenization temperature can be determined using the above-described computational system

446

on a computer. Controlled cool down to 800-I000°C and holding at these temperatures with subsequent cooling in air and aging (to improve the stability of the alloy at 800-900°C) are called for to obtain the optimal partial size of the ¥'-phase (0.2-0.5 Dm) in the alloy, complete segregation of the y'-phase to attain the alloy's maximum level of high-tempera- ture strength, and the formation of serrated crystallite boundaries, which promote an im- provement in the high-temperature strength and resistance to mechanical and thermal fatigue and corrosion resistance in the presence of chlorine ions in a medium of fuel-combustion products.

For single-crystal alloys and alloys subjected to oriented crystallization, the forma- tion of the optimal structure is ensured without holding at 1000°C. To produce the re- quired strength in combination with the high plasticity of the alloys in the initial state and during the service life, regimes for their heat-treatment, which contribute to incomplete dissolution of the ~'-phase and to the formation of a structure during cooling, which con- sists of coarse particles (larger than 0.5 ~m) and rather fine (approximately 0.i ~m) ~t- phase, are developed.

CONCLUSIONS

New single-crystal alloys for the blades of gas turbines, highly corrosion-resistant alloys for guide vanes and combustion chambers, and low-cost alloys for the gears of turbine compressors have been developed and implemented. In term sof the set of properties, the new alloys are superior to foreign alloys for stationary turbines. A computer-aided design system for alloys with a given level of properties has been created for the development of a new generation of high-temperature nickel alloys. Special heat-treatment regimes, which make it possible to combine heat treatment with the production cycle involving the application of plasmas protective coatings and to achieve the combination of basic mechanical properties that is optimal for a specific component have been developed as applies to specific operating conditions of turbine components.

CHANGE OF PROPERTIES OF THE SURFACE OF MATERIALS BY CONCENTRATED

ENERGY FLUXES

V.D. Kal'ner, Yu. V. Kal'ner, and A. K. Verner

UDC 621.791.72:669.14

Concentrated energy fluxes (CEF) as heating sources for heat treatment are finding ever wider application in industry. However, together with the known advantages opening up broad technological possibilities, there are certain difficulties hindering the wider use of this technology. Since the temperature field, the stress field, and their changes in time unambiguously determine the structure, and consequently also the properties of the material, inspection of these parameters is very important. However, present-day technology cannot ensure checks of temperature with sufficient accuracy at its rapid changes, this process therefore must be mathematically modeled.

The previously developed mathematical models of processes of treatment with CEF are based, as a rule, on the solution of the problem of heat conduction (numerical methods') or on semiempirical dependences (analytical methods). It should be noted that the hitherto existing models make it possible to evaluate a temperature field qualitiatively only be- cause they use the semiinfinite statement of the problem and do not take into account the changes in the physical properties of the material in dependence on the temperature. Such a qualitative determination of the temperature field had the effect that the optimization of the technology of surface treatment by CEF in each actual case turned into an indepen- dent investigation involving large material expenditures and much time.

Production Association of the V. I. Lenina Plant NITsTL, Academy of Sciences of the USSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 22-24, June, 1991.

0026-0673/91/0506-0447512.50 © 1991 Plenum Publishing Corporation 447