IMPELLER, IMPELLER MANUFACTURING METHOD, AND ROTATING MACHINE

TECHNICAL FIELD

The present invention relates to an impeller used in a rotating machine.

BACKGROUND ART

A rotating machine such as an industrial compressor, a turbo refrigerator, and a small gas turbine includes an impeller in which a plurality of blades is attached to a disc fixed to a rotary shaft. The rotating machine applies pressure energy and speed energy to gas through rotation of the impeller.

Tensile stress caused by centrifugal force generated during rotation acts on the impeller. If the tensile stress exceeds material strength, the impeller is damaged. Therefore, to counter the tensile stress, for example, application of compressive stress to the impeller has been proposed, for example, as disclosed in Patent Literature 1 and Patent Literature 2.

In Patent Literature 1 and Patent Literature 2, compressive residual stress is applied to the impeller by pressing the impeller from inside of a fitting hole to which a rotary shaft is fitted, toward outside in a radial direction by a tool inserted into the fitting hole. In Patent Literature 1 and Patent Literature 2, the compressive residual stress is applied to a root of a part having the largest outer diameter because the root is most vulnerable to the tensile stress during rotation.

CITATION LIST
Patent Literature

Patent Literature 1: JP 2012-017713 A

Patent Literature 2: JP 2013-104306 A

SUMMARY OF INVENTION
Technical Problem

The impeller may be damaged due to fatigue even when the tensile stress exceeding the material strength is not applied.

Accordingly, an object of the present invention is to provide an impeller excellent in fatigue strength and a method of manufacturing the impeller.

Solution to Problem

An impeller according to the present invention includes a disc portion fixed to a rotary shaft rotating around an axis line, a cover portion disposed to face the disc portion, and a plurality of blade portions provided between the disc portion and the cover portion. In the impeller according to the present invention, a compressive residual stress layer is provided on a surface layer of a boundary with the disc portion and on a surface layer of a boundary with the cover portion, at a front end of each of the blade portions.

In the impeller according to the present invention, each of the blade portions and the disc portion may be integrally provided or joined at the boundary with the disc portion. The joining is performed on the premise that the disc portion and the blade portions are separately fabricated. Likewise, each of the blade portions and the cover portion may be integrally provided or joined at the boundary with the cover portion. The joining is performed on the premise that the blade portions and the cover portion are separately fabricated.

The present invention provides an impeller manufacturing method. The impeller includes a disc portion fixed to a rotary shaft rotating around an axis line, a cover portion disposed to face the disc portion, and a plurality of blade portions provided between the disc portion and the cover portion. The impeller manufacturing method according to the present invention includes applying, by shot peening, compressive residual stress to a surface layer of a boundary with the disc portion and to a surface layer of a boundary with the cover portion, at a front end of each of the blade portions.

The application of the compressive residual stress by the shot peening according to the present invention is preferably performed at least in two stages of a first step using a first shot having a first particle diameter and a second step using a second shot having a second particle diameter smaller than the first particle diameter.

Further, according to the present invention, a rotating machine including the above-described impeller is provided.

Advantageous Effects of Invention

The impeller according to the present invention includes the compressive residual stress layer at the front end of each of the blade portions at which breakage is easily caused by fatigue. This makes it possible to improve fatigue strength with respect to continuous operation of the impeller.

Further, the impeller manufacturing method according to the present invention applies the compressive residual stress by the shot peening. This makes it possible to easily apply the compressive residual stress only to the front end even in a closed impeller.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a centrifugal compressor according to an embodiment of the present invention.

FIG. 2 is a perspective view illustrating an impeller according to the present embodiment.

FIG. 3 is a cross-sectional view illustrating the impeller according to the present embodiment. [FIGS. 4A and 4B] FIGS. 4A and 4B are diagrams illustrating an impeller manufacturing method according to the present embodiment. [FIGS. 5A and 5B] FIGS. 5A and 5B are graphs each illustrating relationship between compressive residual stress applied to the impeller and a depth from a surface layer according to the present embodiment, FIG. 5A illustrating a case where one-stage shot peening is performed, and FIG. 5B illustrating a case where two-stage shot peening is performed.

DESCRIPTION OF EMBODIMENT

A centrifugal compressor 100 that is an example of a rotating machine according to an embodiment of the present invention is described below with reference to accompanying drawings.

[Configuration of Centrifugal Compressor 100]

As illustrated in FIG. 1, the centrifugal compressor 100 according to a first embodiment includes a casing 102 and a rotary shaft 101 that is supported by the casing 102 through a journal bearing 103 and a thrust bearing 104. The rotary shaft 101 is supported so as to be rotatable around an axis line O, and a plurality of impellers 1 that are arranged in the axis line O direction are attached to the rotary shaft 101.

As illustrated in FIG. 2, each of the impellers 1 includes a substantially disc shape. Each of the impellers 1 is configured to discharge fluid sucked from an induction port 3 that opens on one side of the axis line O direction, from a discharge port 4 on outside in a radial direction through a flow path 105 that is provided inside each of the impellers 1.

The impellers 1 use centrifugal force derived from rotation of the rotary shaft 101 to stepwisely compress gas G supplied from the flow path 105 on upstream side provided in the casing 102, and cause the compressed gas G to flow toward the flow path 105 on downstream side.

As illustrated in FIG. 1, the casing 102 includes, on front side (F) in the axis line O direction of the rotary shaft 101, a suction port 106 to take in the gas G from the outside. Further, the casing 102 includes, on rear side (B) in the axis line O direction, a discharge port 107 that causes the gas G to flow out to the outside.

According to the centrifugal compressor 100, when the rotary shaft 101 rotates, the gas G flows into the flow path 105 from the suction port 106, the gas G is stepwisely compressed by the impellers 1, and the compressed gas G is discharged from the discharge port 107. FIG. 1 illustrates an example in which six impellers 1 are provided in series on the rotary shaft 101; however, it is sufficient to provide at least one impeller 1 on the rotary shaft 101. Note that, in the following description, a case where only one impeller 1 is provided on the rotary shaft 101 is described as an example, to simplify the description.

[Configuration of Impeller 1]

As illustrated in FIG. 2 and FIG. 3, the impeller 1 includes a disc portion 30, blade portions 40, and a cover portion 50.

The disc portion 30 is attached to the rotary shaft 101 through fitting from outside in the radial direction. As illustrated in FIG. 3, the disc portion 30 includes a first disc portion 31 and a second disc portion 35 that are divided in the axis line O direction at a joining layer CL orthogonal to the axis line O. The first disc portion 31 and the second disc portion 35 are joined with the joining layer CL. The joining layer CL is preferably configured of an adhesive or is formed by friction welding.

The first disc portion 31 includes a substantially cylindrical shape with the axis line O as a center. The first disc portion 31 includes, on front end part 33 side on the front side (F) of the axis line O, a grip part A that is fitted to the rotary shaft 101 with an interference. At this time, to fit the first disc portion 31 to the rotary shaft 101 with the interference at the grip part A, cold fitting or shrink fitting is adoptable. The impeller 1 according to the present embodiment is fixed to the rotary shaft 101 at only the grip part A.

The first disc portion 31 includes an outer peripheral surface 34 that is gradually increased in diameter toward the rear side (B) of the axis line O. The outer peripheral surface 34 has a curved surface recessed outward in a cross-section including the axis line O.

A rear end surface 32 of the first disc portion 31 on the rear side (B) of the axis line O is joined to the second disc portion 35 with the joining layer CL by friction welding.

The second disc portion 35 includes a disc shape that extends outward in the radial direction from a rear end part 36 side that is opposite to the front end part 33 side in the axis line O direction.

An inner-diameter-side region 38 on a front end surface 37 of the second disc portion 35 is joined to the rear end surface 32 of the first disc portion 31 with the joining layer CL. The rear end surface 32 and the inner-diameter-side region 38 of the front end surface 37 configure the joining layer CL orthogonal to the axis line O.

As illustrated in FIG. 2, the plurality of blade portions 40 are arranged at predetermined intervals in a circumferential direction of the disc portion 30.

As illustrated in FIG. 3, each of the blade portions 40 is formed to have a substantially constant plate thickness, and protrudes from the front end surface 37 of the disc portion 30 toward the front side (F) in the axis line O direction. Further, as illustrated in FIG. 3, each of the blade portions 40 has a slightly tapered shape toward the outside in the radial direction in a side view.

As illustrated in FIG. 2, each of the blade portions 40 is formed so as to be directed to rear side in a rotation direction R of the impeller 1 as going toward the outside in the radial direction of the disc portion 30 as viewed from the axis line O direction. Further, each of the blade portions 40 is formed so as to be curved in a recessed shape toward the rear side in the rotation direction R as viewed from the axis line O direction. The example in which each of the blade portions 40 is curved as viewed from the axis line O direction has been described here; however, it is sufficient for each of the blade portions 40 to extend to the rear side in the rotation direction R as being closer to the outside in the radial direction. For example, each of the blade portions 40 may be linearly provided as viewed from the axis line O direction.

As illustrated in FIG. 3, the cover portion 50 covers the blade portions 40 from the front end part 33 side in the axis line O direction.

A rear end surface 52 of the cover portion 50 in the axis line O direction is formed integrally with front side edges 41 of the respective blade portions 40. The cover portion 50 is formed in a plate shape in which a thickness on the outside in the radial direction is slightly thin, as with the disc portion 30 that is oppositely disposed. The cover portion 50 includes a bent part 51 that is bent toward the front side in the axis line O direction at positions of inside ends 42 of the respective blade portions 40.

The impeller 1 including the above-described configuration includes the joining layer CL that is disposed on the inside in the radial direction of the blade portions 40. Further, the front end part 33 of the first disc portion 31 is disposed to protrude toward the front side (F) in the axis line O direction more than a front end edge 53 of the bent part 51. Moreover, in the impeller 1, the flow path 105 through which the gas G flows is formed by the outer peripheral surface 34 of the first disc portion 31, the front end surface 37 of the second disc portion 35, side surfaces 43 of the blade portions 40, and the rear end surface 52 of the cover portion 50.

The above-described impeller 1 includes a compressive residual stress layer on the discharge port 107 side of the flow path 105, namely, on a surface layer of a boundary with the disc portion 30 (second disc portion 35) at a front end E of each of the blade portions 40. Further, the impeller 1 includes a compressive residual stress layer on a surface layer at a boundary with the cover portion 50 at the front end E of each of the blade portions 40. In FIG. 3 and FIG. 4, a region where the compressive residual stress layer is provided is illustrated as CRS.

[Compressive Residual Stress]

According to examination by the present inventors, when the impeller 1 is continuously operated, a crack C occurs at the boundary from the front end E, as illustrated in FIG. 2. FIG. 2 illustrates an example in which the crack C occurs at the boundary between each of the blade portions 40 and the disc portion 30. The inventors have found, from observation of a cross-section of the part where the crack C occurs, that the crack C is caused by fatigue due to repetitive application of tensile stress to the front end E with operation of the impeller 1. Accordingly, to prevent or reduce occurrence of the crack C, compressive residual stress CRS is applied to the part of the impeller 1.

The destruction due to fatigue is roughly classified into developing process and progressing process of the crack. The residual stress mainly largely influences the progressing process of the crack. A mechanism of the crack progression due to fatigue is derived from repetition of opening (blunting) of the crack with plastic slip at a crack end and re-sharpening at the crack end caused by stress in an opposite direction, and new plastic slip caused by next repeated stress. Accordingly, if new plastic slip does not occur at the crack end, the crack does not progress.

The compressive residual stress closes the crack to suppress progress of the crack, thereby improving fatigue strength.

As illustrated in FIG. 5A, the compressive residual stress shows a peak at a certain depth from the surface layer, a value of the compressive residual stress becomes small at a depth deeper than a position showing the peak, and the compressive residual stress is changed to tensile residual stress at a depth deeper than the certain depth. As described above, in the impeller 1, the compressive stress remains at the boundary between each of the blade portions 40 and the disc portion 30, and at the boundary between each of the blade portions 40 and the cover portion 50. This prevents or suppresses occurrence of the crack C due to fatigue.

Note that, in FIG. 4A, a positive value indicates the tensile residual stress, and a negative value indicates the compressive residual stress.

[Method of Applying Compressive Residual Stress]

In the present embodiment, the compressive residual stress is applied by shot peening. To apply the compressive residual stress, for example, surface treatment such as hardening and nitriding treatment may be used in addition to the shot peening. To apply the compressive residual stress only to the vicinity of the front end of each of the blade portions 40, the shot peening is effective.

The shot peening is a kind of machining to make a large amount of minute spherical particles, typically, metal balls collide with a metal surface at high speed, and the metal balls are referred to as shot. The shot is generally harder than a workpiece. Therefore, when the shot collides with a surface of the workpiece at high speed, the surface of the workpiece is dented, and circular dents remain. Accordingly, although a satin pattern is provided on the surface subjected to the shot peening, the compressive residual stress is applied to the surface, and hardness of the surface is increased as compared with hardness of the surface before the shot peening. In the present embodiment, providing a compressive residual stress layer at the front end of each of the blade portions 40 makes it possible to prevent occurrence or progress of the crack at the part.

The shot peening includes an impeller method and a compressed air nozzle method depending on means to blast the shot. To obtain high compressive residual stress, the compressed air nozzle method is suitable.

FIG. 4 illustrates a state where the shot peening is performed on the impeller 1 by the compressed air nozzle method. As illustrated in FIG. 4, an air nozzle 110 is disposed toward the impeller 1, and a shot S is blasted from the air nozzle 110. In the shot peening according to the present embodiment, the shot S is blasted while the air nozzle 110 is directed to the boundary with the disc portion 30 at the front end E of each of the blade portions 40. In addition, in the shot peening according to the present embodiment, the shot S is blasted while the air nozzle 110 is directed to the boundary with the cover portion 50 at the front end E of each of the blade portions 40.

As the shot S, a shot having a particle diameter of 0.2 mm to 1.2 mm is commonly used, and there is fine particle peening using a shot having a finer particle diameter of 0.04 mm to 0.2 mm. Note that the particle diameter indicates a diameter. In the present embodiment, one or both of the shot peening with the shot having the common particle diameter and the fine particle peening can be adopted.

Main difference between the common shot peening and the fine particle meaning appears on a depth at which the peak of the compressive residual stress appears. In other words, the position at which the peak of the compressive residual stress appears in the fine particle peening is close to the surface as compared with the shot peening with the shot having the common particle diameter. More specifically, in the case of the fine particle peening, the depth of the peak is, for example, about 0.01 mm. In contrast, in the case of the above-described shot peening with the shot having the common particle diameter, the depth of the peak is, for example, about 0.05 mm. In other words, the compressive residual stress by the fine particle peening is applied to a range relatively shallow from the surface layer, whereas the compressive residual stress by the shot peening with the shot having the common particle diameter is applied to a range relatively deep from the surface layer.

Therefore, in the present embodiment, two-stage shot peening in which shots with different particle diameters are blasted in two stages can be used. When the two-stage shot peening is performed, compressive residual stress obtained by superimposing the compressive residual stress by the shot having the common particle diameter and the compressive residual stress by the fine particle shot is applied, as illustrated by “two stages” in FIG. 5B. In other words, peaks by the compressive residual stress show at two positions different in a depth direction. This makes it possible to apply the compressive residual stress to the range in the wider depth direction.

In the two-stage shot peening, the shot peening with the shot having the common particle diameter (first particle diameter, first shot) is preferably performed as a first step, and the shot peening with the fine particle (second particle diameter, second shot) is then preferably performed as a second step.

Note that shot peening of three or more stages may be performed with use of shots having different particle diameters.

[Effects]

Effects achieved by the impeller 1 and the method of manufacturing the impeller 1 described above are described.

The impeller 1 according to the present embodiment includes the compressive residual stress layer at the boundary with the first disc portion 31 and at the boundary with the cover portion 50 at the front end E of each of the blade portions 40. Therefore, according to the impeller 1, the fatigue strength at the boundaries at which the crack is easily caused by fatigue is improved to prevent occurrence and progress of the crack. This makes it possible to prolong a life of the impeller 1. Further, since the compressive residual stress is applied to the boundaries, it is possible to improve stress corrosion cracking resistance at the boundaries.

In particular, since the compressive residual stress is applied only to each of the front ends at which the crack easily occurs in the impeller 1, it is possible to efficiently prevent occurrence and progress of the crack.

Further, according to the present embodiment, blasting the shot S from the air nozzle 110 toward the front end E of each of the blade portions 40 makes it possible to easily apply the compressive residual stress only to the front end even in a closed impeller.

Other than the above, the configurations described in the above-described embodiment may be selected or appropriately modified without departing from the scope of the present invention.

For example, the impeller 1 according to the present embodiment includes a structure in which the disc portion 30 is divided into the first disc portion 31 and the second disc portion 35, and the second disc portion 35, the blade portions 40, and the cover portion 50 are integrally formed. The impeller according to the present invention, however, is not limited thereto. For example, the present invention is applicable to an impeller in which the disc portion and the blade portions that are integrally formed is joined with the cover portion that is formed separately from the disc portion and the blade portions.

REFERENCE SIGNS LIST

1 Impeller

3 Induction port

4 Discharge port

30 Disc portion

31 First disc portion

32 Rear end surface

33 Front end part

34 Outer peripheral surface

35 Second disc portion

36 Rear end part

37 Front end surface

38 Inner-diameter-side region

40 Blade portion

41 Front side edge

42 Inside end

43 Side surface

50 Cover portion

51 Bent part

52 Rear end surface

53 Front end edge

100 Centrifugal compressor

101 Rotary shaft

102 Casing

103 Journal bearing

104 Thrust bearing

105 Flow path

106 Suction port

107 Discharge port

110 Air nozzle

A Grip part

C Crack

CL Joining layer

CRS Compressive residual stress

E Front end

G Gas

S Shot

IMPELLER, IMPELLER MANUFACTURING METHOD, AND ROTATING MACHINE

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