US3148954A

US3148954A - Turbine blade construction

Info

Publication number: US3148954A
Application number: US35835A
Authority: US
Inventors: Haas Irene; Haas Renate; Haas Dieter; Haas Detlev
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-06-13
Filing date: 1960-06-13
Publication date: 1964-09-15
Anticipated expiration: 1981-09-15

Description

P 9 BQHAAS 3,148,954 nmsxus BLADE cons'mucnon Filed June 1 3. 1960 .INVENTOR Bows HAAS\ DECEASED BY IRENE HAAS BY W 9 M ATTORNEYS United States Patent 3,148,954 TURBINE BLADE CONSTRUCTION Boris Haas, deceased, late of Gaiglstrasse 20/1, Munich, Germany, by Irene Haas, Renate Haas, Dieter Haas, and Detlev Haas, heirs, all of Munich, Germany Filed June 13, 1960, Ser. No. 35,835 4 Claims. (Cl. 29-1961) The present invention relates to turbine guide and rotor blades made of a non-scaling metallic material with high creep strength which are exposed during operation thereof to fluctuating temperature primarily exceeding 600 C.

The present invention aims at an improved resistance of the blading material against cracks caused by temperature changes and against plastic deformations.

Cracks due to temperature changes arise in connection with turbine blades which are exposed frequently to abrupt heating and cooling, especially, however, to alternate rapid heating and cooling thereof. Such conditions occur, for example, with the guide and rotor blades of movable gas turbines which have to be started and stopped frequently, or which have to be accelerated and decelerated over very short periods of time.

Temperature-change cracks are caused by stresses which are caused by the uneven temperature distribution over the cross-section of the turbine blade. With a blade heated throughout, the outer zone thereof, especially during the initial period of heating thereof, has a higher temperature than the core thereof. The temperature difference reaches its maximum after a relatively short time. However, a complete temperature equalization takes place only very rarely so that almost always a continued residual temperature gradient from the surface toward the core of the blade remains in effect.

The outer zone which seeks to expand to a greater extent by reason of its higher temperature than the core zone is prevented from doing so by the molecular coherence of the material. The blade, instead, assumes a mean expansion over. the entire cross-section thereof whereby compression stresses are produced within the rim zones and tensional stresses within the core zone thereof. With a universal cooling of the blade body, the respective signs of the stresses change, i.e., the compression stresses of the outer zone now become tensional stresses, and the tensional stresses of the core zone now become compression stresses.

Especially with movable gas turbine in which sudden changes in load are unavoidable, the material of the turbine blading is endangered to a very high degree. By reason for the extraordinarily rapid heating during acceleration and of the abrupt cooling during deceleration of the turbine unit as well as by reason of the relatively poor thermal conductivity of the high-temperature resistant material for which only either high-grade austenitic steels or almost completely iron-free nickel and cobalt base alloys are normally considered, a relatively large temperature difference results between the core and surface of the blade, and a relatively large temperature gradient exists near the surface of the blade. Since the core by reason of its large cross-section fails to undergo, in practice, essentially any elastic deformation, the material disposed near the surface is forced to absorb practically the entire difference in expansion. As a result thereof, very high stresses are produced which almost always exceed to a large extent the elastic limit and lead to plastic deformations in the outermost layer of the material. Upon exceeding the elastic limit, there takes place a flow of the outer fibers of the material whereby the plastic deformation is the greater the higher the temperature stress. The amount of plastic deformation has to be 3,148,954 Patented Sept. 15,, 1964 ICC redeformed during cooling of the blade. Consequently, the outer fiber is plastically deformed back and forth until the separating strength of the material has been lowered to such an extent that it is exceeded during cooling by the tensional stresses and the incipient crack results therefrom.

The recognition that the temperature-change cracks are in effect endurance failures or fatigue fractures in the prolonged strength region and that the cracks occur the sooner the greater the alternate plastic deformations, leads to the inventive concept of the present application to reduce the magnitude thereof with the aid of a coating or layer.

Consequently, the resistance of the blade material is to be increased in accordance with the present invention or an extension of the operating period up to the point of the occurrence of the aforementioned damages is to be attained in the alternative.

For that purpose, the present invention proposes to provide the inlet and outlet edge of the blade with an auxiliary metallic protective cover or coating adhering rigidly to the core material whereby the thickness of the metallic auxiliary coating amounts to between one percent and twelve percent of the blade cross-sectional length, the cocflicient of thermal expansion of which, within the temperature range above 600 C., is smaller than 0.8 times the coefiicient of expansion of the core material and the alternate plastic deformability of which is at least equal to that of the core material while the non-scaling characteristics thereof are also approximately equal to that of the core material.

As protective materials for the alloys are particularly suitable, for example, the known so-called non-scaling ferrite-austenitic or pure ferrite steels, such as, for example, those having approximately 25% chromium, 4% nickel and other additives or those with approximately 13% to 30% chromium and other additives or ductile chrome. These alloys have coefficients of expansion which have the desired and necessary relationship, as mentioned hereinabove, with respect to the alloys to be protected thereby, and which also have sufiicient plastic deformability. The non-sealing characteristics thereof are equal or better than those of the nickelor cobaltcontaining materials whereas the resistance against reducing and sulphur containing gases and against the vanadium pentoxide attacks is even better. Since the corrosion attacktakes place predominately along the thin edges of the blade, this property may be very valuable. It is true that the fatigue strength of the aforementioned protective materials is relatively slight, however, the load capacity or bearing strength of the protected blade cross-section is reduced thereby only slightly as com:

The formation of temperature-change cracks is effectively aided usually by corrosion of the material surface as a result of the heat-transferring medium in contact therewith. Most of the time, it occurs as grain-boundary oxidation and elfects, in addition to local stress increases as a result of the notch elfect, a strong decrease of the separating resistance, especially, if a longer operating period with relatively high temperatures lies between heating and cooling. Consequently, the protective surface layer, according to the present invention, is to have, as a further characteristic, a greater corrosion and nonscalingresistance than the core material. Possibly, an additional corrosion-resistant or non-scaling protective layer may be applied additionally over the layer with ing the corrosion resistance of the blade.

Protective surface layers in turbine blades are known per se. Protective layers, for example, made of oxidic, mineral, glass-like or ceramic materials have already been used heretofore in the prior art. However, these prior art protective layers had primarily the purpose to protect the core material against chemical and thermal influences. Additionally, metallic coatings are known in the prior art in connection with the steam turbine blades made of unalloyed or relatively low-alloy steel. For that purpose, a protective layer had been proposed in the prior art which contained nickel or cobalt or which consisted of a nickel steel. If a low-alloy steel alloyed with nickel or also Possibly pure nickel is used for achieving the protective effect, then such protective layer always has the same co efficient of expansion as the core material. Austenitic iron and nickel alloys with to 27% nickel thereby offer the best resistance. However, in particular the latter alloys have a coefficient of expansion that is approximately 50% larger than that of the non-alloyed or low-alloy steels. With cobalt, the conditions are essentially similar.

Consequently, whereas the prior art application of protective surface coatings served the purpose of protection against corrosion, according to the present invention, the stress peaks and/or plastic deformations which occur as a result of prevented deformation in the surface are reduced with turbine blades made of a non-scaling, metallic alloy with a high creep strength by the use of an adhering coating having a smaller coeflicient of expansion. For example, with an operating temperature of 800 C. and a temperature difference of 400 C., a decrease of the temperature stresses by about both during heating as well as during cooling is mathematically obtainable if a blade made of austenitic steel having a coefficient of expansion of l8 l() l./ C. is coated or covered with a corresponding layer of an alloy having a coefficient of expansion of 14 10 l/ C.

By reason of the decrease in the temperature stresses, the amount of plastic alternate deformation, the so-called alternate sliding, is correspondingly decreased in the rim or edge fibers of the blade and therewith the life-length of the turbine blade is considerably increased. The coefficient of thermal expansion of the protective layer can be adjusted to the required value by any known suitable change in the chemical composition thereof.

If the protective surface layer at the same time has a better plastic deformability than the core material, then the protective layer is capable of absorbing more frequently the amount of plastic deformation, i.e., the alternate sliding.

By reason of the increased corrosion resistance of the coating material, the unfavorable influence of the corrosion of the contacting medium on the life-length of the particular part is thereby effectively eliminated.

The surface layer according to the present invention may be formed either by coating or by diffusion. The coating may be made, for example, by dipping or immersion in a fused mass, by casting it around the blade, by build-up welding, by plating with rollers or by extrusion presses, by vaporization, by spraying or by galvanic deposition.

Accordingly, it is an object of the present invention to effectively eliminate the disadvantages and shortcomings of the turbine blade constructions used heretofore in the prior art.

Another object of the present invention resides in the provision of a protective layer for a turbine blade which effectively prevents the formation of cracks along the edges thereof normally caused by large changes in the operating temperatures.

Still another object of the present invention resides in the provision of a protective layer for a turbine blade which makes possible improved use of the blade in connection with gas turbine engines exposed to frequent accelerations and decelerations.

A further object of the present invention resides in the provision of a protective layer having such a coeflicient of expansion as to minimize peak stresses throughout the blade cross-section while at the same time increasing the corrosion resistance of the blade.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in accordance with the present invention, and wherein FIGURE 1 is a cross-sectional view of a blade in accordance with the present invention,

FIGURE 2 is a diagram showing the temperature distribution along the blade of FIGURE 1, and

FIGURE 3 is a diagram showing the stress distribution within the blade illustrated in FIGURE 1.

Referring now to the drawing, FIGURE 1 illustrates a blade cross-section of a turbine blade constructed in accordance with the present invention, the core 1 of which consists of a non-scaling metallic material with high creep strength and the inlet and outlet edges of which are protected by layers or coatings 2. The metallic materials for the protective layers in accordance with the present invention has a coefiicient of expansion within the temperature range above 600 C. which is smaller than 0.8 times the coefficient of expansion of the core material while the resistance to alternate deformations is at least equal to that of the core material whereas its non-scaling property is approximately equal to that of the core material.

During acceleration of the turbine, a very pronounced differing temperature distribution occurs in the longitudinal direction of the blade profile as shown by curve 3 of FIGURE 2. If the blade body is made of only one and the same material, then this temperature distribution produces a stress distribution illustrated in FIGURE 3 by full line curve 4.

The relatively high peak stresses along the edges of the blade are quite noticeable in FIGURE 3.

The blade in accordance with the present invention, the edges of which are provided with a protective layer of thickness 5, exhibits with the same temperature distribution as illustrated in FIGURE 2, a stress distribution illushated by the dash curve 6 in FIGURE 3 from which it is clearly noticeable that a decrease in the stresses along the edges results therefrom.

With relatively rapid cooling, the same curves will be obtained for the temperature and stress distributions, however, of mirror image-like configuration with respect to the X-axis. The X-axis of the temperature diagram of FIGURE 2 corresponds in that case to the operating temperature instead of to the initial temperature While the rim and core stresses change the signs thereof.

It is clearly visible from the curve 6 of FIGURE 3 that that thickness of the protective layer 2 has to be so matched and selected that the stress at the transition between protective and core material is not significantly greater than the reduced rim stress. This effect is obtained by determining the thickness of the protective material 2 in such a manner that, depending on the core material and on the protective material and on the existing temperature gradient, it lies between 1% and 12% of that cross-sectional dimension in the direction of which the largest temperature differences occur. Consequently, in the illustrated example, the thickness 5 has to amount to about 0.01 to 0.12 times the width 7 of the blade.

While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of many changes and modifications within the spirit and scope of the present invention, and I, therefore, do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

What is claimed is:

1. A turbine blade for use at temperatures above 600 C. comprising a core essentially consisting of austenitic steel having a coefiicient of expansion of about 18 X l./ C., said blade being provided at the leading and trailing edges with a protective layer of metallic material adhering to the austenitic steel of said core body, said metallic material of said protective layer comprising a steel alloy selected from the group consisting of chromium nickel steel of approximately 25% and 4% nickel and chromium steels having 1330% chromium, said alloy having a coeflicient of expansion of about 14 10- l./ C., said protective layer having a thickness of between 1 to 12% of the cross-sectional length of the blade taken in the direction of maximum difference of temperature in said blade, the alternate plastic deformability of said protective layer being at least equal to that of said core.

2. A turbine blade for use at temperatures about 600 C., comprising a core essentially consisting of a metallic material with a high creep strength selected from the group consisting of high-grade austenitic steels and almost completely iron-free nickel and cobalt base alloys and provided at the leading and trailing edge thereof with a metallio protective layer adhering to the blade core material and selected from the group consisting of chromium nickel steel of approximately 25% and 4% nickel and chromium steels having 13-30% chromium, the thickness of said protective layer amounting to between 1 to 12% of the cross-sectional length of the blade taken in the direction of maximum difierence of temperature in said blade, and the coefiicient of thermal expansion of said metallic protective layer Within the temperature range above 600 C. being smaller than approximately 0.8 times the coefiicient of expansion of the core material, and the alternate deformability of said protective layer being at least equal to that of the core material.

3. A turbine blade according to claim 2, wherein said metallic protective layer has a greater resistance to scaling than said blade core material.

4. A turbine blade according to claim 2, wherein said metallic protective layer has a greater corrosion resistance than said blade core material.

References Cited in the file of this patent UNITED STATES PATENTS 2,034,278 Becket et a1 Mar. 17, 1936 2,447,896 Clarke Aug. 24, 1948 2,497,151 Clark et al Feb, 14, 1950 2,586,100 Schultz Feb. 19, 1952 2,606,741 Howard Aug. 12, 1952 2,763,919 Kempe et al Sept. 25, 1956 2,861,327 Bechtold Nov. 25, 1958 2,946,681 Probst et al July 26, 1960 FOREIGN PATENTS 309,235 Great Britain Apr. 11, 1929

Claims

1. A TURBINE BLADE FOR USE AT TEMPERATURES ABOVE 600* C. COMPRISING A CORE ESSENTIALLY CONSISTING OF AUSTENITIC STEEL HAVING A COEFFICIENT OF EXPANSION OF ABOUT 18X10**-6 1./C., SAID BLADE BEING PROVIDED AT THE LEADING AND TRAILING EDGES WITH A PROTECTIVE LAYER OF METALLIC MATERIAL ADHERING TO THE AUSTENITIC STEEL OF SAID CORE BODY, SAID METALLIC MATERIAL OF SAID PROTECTIVE LAYER COMPRISING A STEEL ALLOY SELECTED FROM THE GROUP CONSISTING OF CHROMIUM NICKEL STEEL OF APPROXIMATELY 25% AND 4% NICKEL AND CHROMIUM STEELS HAVING 13-30% CHROMIUM, SAID ALLOY HAVING A COEFFICIENT OF EXPANSION OF ABOUT 14X10**-6 1./ *C., SAID PROTECTIVE LAYER HAVING A THICKNESS OF BETWEEN 1 TO 12% OF THE CROSS-SECTIONAL LENGTH OF THE BLADE TAKEN IN THE DIRECTION OF MAXIMUM DIFFERENCE OF TEMPERATURE IN SAID BLADE, THE ALTERNATE PLASTIC DEFORMABILITY OF SAID PROTECTIVE LAYER BEING AT EQUAL TO THAT OF SAID CORE.