US2920007A

US2920007A - Elastic fluid blade with a finegrained surface

Info

Publication number: US2920007A
Application number: US709405A
Authority: US
Inventors: Bruce O Buckland
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1958-01-16
Filing date: 1958-01-16
Publication date: 1960-01-05
Anticipated expiration: 1977-01-05

Description

Jan. 5, 1960 PROTECTIVE LAYEI? (FINE GfiA/NED) B. o. BUCKLAND 2,920,007

ELASTIC FLUID BLADE WITH A FINE-GRAINED SURFACE Filed Jan. 16, 1958 BASE METAL (604 R85 saw/-50) INIT/AL BUCKET COLD WORK/N6 RECRKSTALL/ZA T/ON HEA T/NG COOL ING INVENTOR. BRUCE 0. BUCK LAND FIN/SHED BUCKET HIS ATTORNEYS United States Patent G ELASTIC FLUID BLADE WITH A FINE- GRAINED SURFACE Bruce 0. Buckland, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application January 16, 1958, Serial No. 709,405

7 Claims. (Cl. 148'-31) This invention relates to blades for elastic fluid turbines, compressors, superchargers and the like, and particularly to an improved blade having a surface layer which inhibits propagation of cracks resulting from nicks and flaws on the surface of the blade as well as from high stresses due to notches or similar shapes.

Modern steam and gas turbines and similar devices utilize turbine blades which are subjected in operation to high stresses, particularly in the form of centrifugal tension stresses and to high temperatures, and occasionally, high corrosive conditions. Superimposed on these conditions are relatively low vibrating stresses. Because of the high temperature under which the blades operate they are formed of alloy steels or special metal alloys and are preferably formed and treated to provide a large I metallic grain size in the base metal of the blade whereby to impart sufiicient strength in creep and rupture. Any small nicks or flaws in'the surface of the blade may be the starting point for serious rupture or fatigue cracks. Great care is therefore exercised in the manufacture of thebla'des to' eliminate as completely as possible surface flaws and nicks. However, even in the most carefully manufactured blade small imperfections propagate as rupture cracks after application of high stress for a long period of time. Furthermore, foreign particles going through the turbine will impinge upon the blade whereby to manthe finish and pr'o'duce nicks and flaws therein. These nicks and flaws grow into cracks and propagate 'from the surface into the'base' metal and can cause failure'of the blade.

Accordingly, it is an important object of the present invention to provide an improved elastic 'fluid blade'of the" type set forth which can withstand the rigorous operating conditions to which suchblades are subjected in modern machine constructions.

In'connection with the foregoing object, it is another object of the invention to provide an improved elastic fluid blade which will be resistant to failure by propagation of rupture cracks or fatigue cracks therethrough'resulting from nicks and flaws in the surface of the blade.

Still another object of the invention is to provide an improved elastic fluid blade which can withstand bombardment by foreign particles passing through the machine without the development and propagation of cracks in the blade. j 7

Yet another object ofthe invention is to provide a blade of the type set forth in which the surface layer of the blade can effectively stop propagation of cracks such as rupture cracks resulting from imperfections in the surface of the blade".

Still another object of the inventionis to provide a blade of the type set forth in'which a surface layer of metal is provided on'the blade which'can stop growth of cracks 'therein whereby to prevent formation of larger cracks in the base metal of the blade.

Yet'another object of the invention is to provide a blade ofthe type set forth in which a surface layer is provided thereon which is capable of reducing the stress- 2,920,007 Patented Jan. 5, 1960 These and other objects and advantagesof the invention will be better understood from the following description when taken in conjunction with the accompanying drawing. In the drawing wherein like reference numerals have been utilized to designate like parts:

Figure 1 is a perspective view of an elastic fluid blade embodying the present invention;

Fig. 2 is a greatly enlarged fragmentary sectional view of a portion of the blade shown inFig. 1, illustrating the base metal core thereof and the protective layer provided thereon; and i Fig. 3 is a schematic diagram illustrating the method of making the blade according to the present invention.

According to the present invention a protective surface layer, which may be from a few mils to & thick depending upon the size of the blade, of small grain recrystallized material is provided about the base metal of the blade which is formed of larger grain material. To provide such a protective layer about the base metal, a blade made of one of the usual high temperature resisting or corrosion resisting metal alloys which is workhardenable is cold-worked to harden the metal to a depth of several mils and the work-hardened layer is then recrystallized in the temperature range of stress relief or recovery for the metal alloy to relieve stresses in the layer whereby to avoid damage in fatigue and to improve the rupture and fatigue 'resistant'properties of the outer protective layer. In general, any of the metal alloys' utilized heretofore in high temperature turbine blades may be'utilized in'the present inventio'nprovided that the alloy can be hardened with cold-working.

In general, any of the austenitic base alloys and some of the ferritic alloys can be hardened by cold-working and are suitable for use in the present invention. For example, the alloy sold by the Haynes-Stellite Division of Union Carbide & Carbon Co. under the designation Low-Carbon N may be used. vThis alloy has the following'chemical composition, the amounts being indicated in percent by weight: C 0.15, Si 0.4, Mn 1.5, Cr 20, Ni 20, Co 20, M03, W 2, Cb l, N 0.15 and the balance Fe. A similar suitable'alloy' is manufactured by Alleghen'y-Lu'dlum Steel Co. under the designation 5-590 Alloy. This latter company also provides another suitable alloy in which the cobalt content is increased to 25%- and the iron content correspondingly reduced, this alloy being sold under the designation S 816 Alloy. The nickel base austenitic base alloys can also be utilized such as thattsold by the Haynes-Stellite Division of Union- Carbide & Carbon Co. under the designation Hastelloy B having the approximate composition: Ni 62, Mo 26-30, Fe 4-6, C 0.12 max. Also useful are the chrome irons such as crucible 422.

The age hardenable alloys which can be hardened by cold-working are also useful in the present invention. One of the preferred materials of construction is that sold by-the Mond Nickel Co.,*Ltd., of London, England, under the designation Nimonic 80. This alloy has the following approximate composition by weight: C 0.04, Si 0.47, Ni 75, Cr 21, Mn 0.56, Ti 2.45 and'Al 0.63.

- Other suitable age hardenable alloys includethose sold by the Timken' Roller Bearing Company under the designation 16 25-6 Steel Alloy can be utilized. These alloys are austenitic in character and have an approxi- 3 mate composition by weight as follows: C 0.12, N 0.15, Si 1.0, Mn 2.0, M 6.0, Cr 16.0, Ni 25.0, and the balance Fe.

Other equivalent and suitable materials can also be used as will be apparent to those skilled in the art provided the alloys have the general characteristic of the above specific examples.

In manufacturing a blade in accordance with the present invention, a blade is shaped and finished as has been customary heretofore. In general the blades are homogeneous throughout and have a relatively large grain structure to impart to the blade the necessary rupture strength. The shaped and finished blade has the surface thereof cold-worked according to the present invention to a depth varying from a few mils to or more depending upon the size of the part. As much small grained material as can reasonably be afforded is used. This is in the range of a thickness such that the cross section of the recrystallized layer is from to 20% of the total cross section. Thus not too much rupture strength is lost and as much surface protection is obtained as is commensurate with the size of the piece. A preferred depth of work-hardening is approximately 1 mil.

Any suitable method of cold-working may be used. Shot peening can be used for the most shallow depth of working but for not more than a few mils. Rolling, burnishing, or other methods will be more suitable for deeper working. In any case enough deformation of the material is obtained as required to result in recrystallization upon heating to the recrystallization temperature. After cold-working, the turbine blade is then heated to recrystallize the layer of metal which has been coldworked. The cold-working results in a distorted and disrupted crystalline structure in the worked area with a marked increase in both the hardness and the strength of the material in the worked layer. There also is produced by the cold-Working within the worked layer a plurality of small regions which may be termed strain recrystallization nuclei. These nuclei serve as centers of recrystallization of a new and unstrained structure upon heating of the bucket to a suitable temperature.

Recrystallization is carried out at a temperature within the range of temperatures designated by the term region of stress relief or recovery. Heating to such a temperature will cause recrystallization of the material in the cold-worked layer to provide a small grain structure. This recrystallized layer will have the stresses imparted thereto by work-hardening removed. Care is exercised to maintain the temperature of recrystallization below that effective to alter the basic large grain characteristic of the basic metal. In the case of alloy Nimonic 80 a suitable recrystallization temperature is 1250 F. and the bucket should be held in this temperature for 8 or 10 hours. In the case of steels such as 16-25-6 Steel Alloy the blade may be heated to a temperature of 1250 degrees F. for recrystallization and held at that temperature for the period of 8 to 10 hours. After heating to recrystallize, the blade is cooled.

Referring now to the drawing, the blade 10, as shown in Fig. 1, and embodying the features of the present invention, essentially comprises a radially extending body portion 11 provided with a base 12 that is suitably attached to an associated rotatable disc 13.

The method of making the blade 10 of the present invention is diagrammatically illustrated in Fig. 3; wherein the initial blade is cold-worked, as indicated at 20, to produce a surface layer thereon which is under stress, the surface layer having, for example, a thickness of 20 mils. The cold-worked blade is then placed in a suitable furnace where it is heated to achieve recrystallization, as indicated at 21; and thereafter the blade is cooled, as indicated at 22, to produce the finished blade.

Fig. 2 comprises a greatly enlarged diagrammatic illustration of a cross-section taken through the surface of thebody 11 of the blade 10 and made in accordance 4 with the present method. The body 11 comprises a core 11a formed of base metal and having a coarse grain. The coarse-grained structure of the base metal of the core Ha imparts the necessary rupture strength thereto. Integral with and intimately adhered to the core or base metal 11a is a protective layer 11b. The layer 111) has been illustrated as being of 20'mils thickness and is of fine grain structure. The fine-grained structure of layer 111) does not possess the structural strength and properties of the coarse-grained base metal 11a, but layer 11b represents such a small fraction of the total cross sectional area of the blade that the useful load carrying ca acity of the blade is not impaired. Actually the base metal of the core 11a extending between the junctions thereof with the protective layer 1112 on each side thereof is substantially unchanged by the cold-working and the subsequent heating to recrystallize layer 11b which have been discussed above.

By way of illustration, it is noted that the crystal grain size in the layer 11:: might be in the general range 0.100- 0200 mm., while the crystal grain size in the layer 11b might be in the general range 0010-0030 mm.

The layer 11b, due to cold-working and subsequent recrystallization thereof, is relatively weak in rupture and creeps more readily than does the base metal of core 11a. Because of this character of protective layer 11b, the layer 11b is subjected to relaxation or creep whenever stresses are produced on the surface of the bucket. Accordingly, any stress concentrations produced by nicks or fiaws in the surface of the blade, and particularly in protective layer 11b thereof, will be neutralized. This will occur without regard to the origin of the surface imperfections. For example, any nicks or flaws originally in the surface of the blade 10 will have the stress concentrations produced thereby neutralized. Similarly, small mechanical imperfections resulting from foreign particles passing through the turbine will not develop any substantial stress concentrations within surface layer 11b. More specifically, when the blade 10 is subjected to high centrifugal stresses in use, stress concentrations which tend to build up around nicks or flaws in layer 11b will be relaxed by a slight flow within the protective layer due to the relaxation or creep thereof. This relief of stress concentrations will inhibit or eliminate the formation of cracks in surface layer 11b. In the event that cracks such as rupture cracks are formed, the propagation of these cracks is halted by the compensating flow of material within protective layer 11b. Accordingly, the cracks cannot grow into larger cracks and cannot penetrate into the base metal of body 11a whereby to impair the mechanical strength under stress of body 11a.

In addition to cold-working by shot peening, rolling or burnishing as described above, the protective layer 11b can be formed on the blade 10 by other methods of cold-working. For example, grinding, milling, sand blasting, and other suitable material hardening coldworking processes may be used.

The method described above has been found to be equally applicable to all forms of elastic fluid blades. More specifically, the invention is useful when applied to turbine blades or buckets, compressor blades, supercharger blades and the like.

Although in the foregoing, the fine grain layer 11b is described as being produced on the vane section 11, such a layer can be used on other portions of the blade or all over its surface. For example it might at times be useful to produce the fine grain layer on the hooks and in the relatively sharp corners of the dove tail portions. Present here are often high steady stresses due to the centrifugal load and the stress concentration resulting from the small radii of the dove tail fastening. The fine grain material can creep and relax these high steady stresses and thus make the blade suitable to stand higher vibrating stress without a failure.

Although certain preferred materials of construction of the blade of the present invention have been given for purposes of illustration, it is to be understood that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Accordingly, the invention is to be limited only as set forth in the following claims.

What is claimed is:

1. An elastic fluid blade comprising a core of base metal formed of material having high mechanical strength and having a large grain size in the general range 0.100 to 0.200 mm., and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size in the general range 0.010 to 0.030 mm., the thickness of said protective surface layer being such that its cross section is to of the total cross section of the portion of the blade on which it is used, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in the protective surface layer, whereby substantially to prevent propagation of cracks 'from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

2. An elastic fiuid blade comprising a core of base metal formed of large grain size material having high mechanical strength, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in the protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent the formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said face layer.

3. An elastic fluid blade comprising a core of base metal formed of large grain size material having high mechanical strength and hardenable by cold-working, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in said protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

4. An elastic fluid blade comprising a core of base metal formed of large grain size material having high mechanical strength, said material being austenitic in form and hardenable by cold-Working, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of protective surbase metal and being subject to creep to reduce stress concentrations in said protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

5. An elastic fluid blade comprising a core of base metal formed of large grain size material having high mechanical strength, said material being an age hardenable alloy that is also hardenable by cold-working, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in said protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

6. An elastic fluid blade comprising a core of base metal formed of large grain size material having high mechanical strength, said material being a metal base alloy hardenable by cold-working, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in said protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

7. An elastic fluid blade comprising a core of base metal formed of large grain size material having high mechanical strength, said material being a stainless steel hardenable by cold-working, and a protective surface layer intimately adhered to said core and extending therearound, said protective surface layer being formed of the same material as said core and having a small grain size, the cross section of said protective surface layer being small compared to the cross section of said core of base metal, said protective surface layer having a high relaxation rate compared to said core of base metal and being subject to creep to reduce stress concentrations in said protective surface layer, whereby substantially to prevent propagation of cracks from nicks and flaws in said protective surface layer and thus substantially to prevent formation of rupture cracks and fatigue cracks in said core due to nicks and flaws in said protective surface layer.

References Cited in the file of this patent UNITED STATES PATENTS Scott et al. Aug. 22, 1950 OTHER REFERENCES Preprint from Transactions of ASM, vol; 40, 1948, Number 24.

Claims

1. AN ELASTIC FLUID BLADE COMPRISING A CORE OF BASE METAL FORMED OF MATERIAL HAVING HIGH MECHANICAL STRENGTH AND HAVING A LARGE GRAIN SIZE IN THE GENERAL RANGE 0.100 TO 0.200 MM., AND A PROTECTIVE SURFACE LAYER INTIMATELY ADHERED TO SAID CORE AND EXTENDING THEREAROUND, SAID PROTECTIVE SURFACE LAYER BEING FORMED OF THE SAME MATERIAL AS SAID CORE AND HAVING A SMALL GRAIN SIZE IN THE GENERAL RANGE 0.010 TO 0.030 MM., THE THICKNESS OF SAID PROTECTIVE SURFACE LAYER BEING SUCH THAT ITS CROSS SECTION IS 10 TO 25% OF THE TOTAL CROSS SECTION OF THE PORTION OF THE BLADE ON WHICH IT IS USED, SAID PROTECTIVE SURFACE LAYER HAVING A HIGH RELAXATION RATE COMPARED TO SAID CORE OF BASE METAL AND BEING SUBJECT TO CREEP TO REDUCE STRESS CONCENTRATIONS IN THE PROTECTIVE SURFACE LAYER, WHEREBY SUBSTANTIALLY TO PREVENT PROPAGATION OF CRACKS FROM NICKS AND FLAWS IN SAID PROTECTIVE SURFACE LAYER AND THUS SUBSTANTIALLY TO PREVENT FORMATION OF RUPTURE CRACKS AND FATIGUE CRACKS IN SAID CORE DUE TO NICKS AND FLAWS IN SAID PROTECTIVE SURFACE LAYER.