Treatment method for centrifugal compressor impeller capable of prolonging low-cycle fatigue life

Abstract

The invention relates to a treatment method for a centrifugal compressor impeller capable of prolonging the low-cycle fatigue life. A method for treating a centrifugal compressor impeller includes a combination of cold expansion of at least a portion of a bore of the impeller to induce residual compressive stress in a region around the bore and surface peening of at least a portion of the compressor impeller, such as multiple portions of an aft disc and blades.

Description

[0001] This application is a divisional application of application filed on November 12, 2022, with application number 202211469775.4, entitled "Method for treating centrifugal compressor impeller to extend low-cycle fatigue life". Technical Field

[0002] This application generally relates to centrifugal compressor impellers, such as those used in turbochargers for internal combustion engines. Background Technology

[0003] Typically, the life-limiting mode for centrifugal compressor impellers is low-cycle fatigue (LCF) failure. Various failure modes can occur. In one LCF failure mode, one or more cracks may initiate at or near the surface of the impeller's through-hole. In other modes, cracks may form on the impeller's back plate in a chordal or "pizza block" configuration. Summary of the Invention

[0004] The purpose of the current technological development leading to the invention described herein is to improve the LCF life of centrifugal compressor impellers.

[0005] This disclosure describes a treatment method for a centrifugal compressor impeller that significantly extends the impeller's LCF (Limited Compressive Force) life. The method involves applying two consecutive operations to the impeller. In one operation, at least a portion of the inner surface of the bore's length is cold-worked by forced expansion of the bore diameter, thereby creating a compressive residual circumferential stress zone in the metal surrounding the bore. In the second operation, surface shot peening is performed on at least a portion of the compressor impeller to induce compressive residual stress in the impeller surface. Advantageously, the bore cold working achieves a retention expansion of at least 1.2%, and the surface shot peening is performed at an intensity of 0.15 mm to 0.6 mm as measured in Almen N scale.

[0006] In one embodiment described herein, a processing method includes the following steps: A centrifugal compressor impeller made of a tough metal with tensile yield strength is provided. The compressor impeller includes a hub and a rear disc, and a plurality of blades connected to the hub and the rear disc. The plurality of blades define an inlet portion of the compressor impeller configured for axial entry of air into the compressor impeller and an outlet portion of the compressor impeller configured for radial discharge of air from the compressor impeller. Each blade has a root fillet at the blade-hull junction. The compressor impeller has a first face and an opposing second face. The hub of the compressor impeller defines an orifice extending centrally through the hub and the rear disc in a downstream axial direction. The metal is cold-worked at the inner surface of the hole to exceed the tensile yield strength in the circumferential direction around the hole, thereby inducing compressive residual circumferential stress in the metal adjacent to the inner surface of the hole; and At least a portion of the compressor impeller is shot-peened, excluding the inner surface of the bore.

[0007] In some embodiments, the cold working step includes applying radially outward pressure to the inner surface of the hole. This can be achieved mechanically, such as by applying radially outward pressure to the inner surface of the hole using a mechanical tool. For example, the cold working step may include axially pulling a tool through the hole in a downstream axial direction, the diameter of the tool exceeding the initial diameter of the hole before the cold working step. Various mechanical tools can be employed, non-limiting examples of which include: a radially expandable mandrel (or “slotted mandrel”) that can be triggered at a desired location along the hole, thereafter the expanded mandrel is then axially pulled along the hole to cold work the hole; or alternatively, a slotted sleeve and mandrel, wherein the slotted sleeve is located in the hole and the mandrel is pulled through the sleeve to cause the sleeve to expand radially outward, and thus cold work the hole. Ball pushing is another technique that can be employed. Cold working can be applied to the entire axial length of the hole, or alternatively, to only a portion of the length. When only a portion of the hole length is to be cold-worked, a countersunk hole of a larger diameter can be set in the area of ​​the hole that is not intended to be cold-worked, and the machine tool can be initially started in the countersunk hole before being pulled through the smaller diameter main hole portion.

[0008] As an alternative to using mechanical tools, cold working of holes can be accomplished by non-mechanical means (non-limiting examples of which may include laser treatment or chemical treatment of the inner hole surface). Attached Figure Description

[0009] This disclosure has already been described in general terms; reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and in the drawings: Figure 1 This is a cross-sectional view through the compressor impeller prior to the processing method according to an embodiment of the present invention; Figure 2 This is a side view of a mandrel that can be used to perform cold expansion of a hole according to the processing method of the present invention; Figure 3 Is it possible to... Figure 2 A side view of a slotted sleeve used together with a mandrel to perform cold expansion of a hole; Figure 3A Is it through Figure 3 A sectional view along line 3A-3A; Figure 4 This is a side view of the front cover, which can be connected to... Figure 2 and Figure 3 The mandrel and the slotted sleeve are used together to perform the cold expansion step of the processing method according to an embodiment of the present invention; Figure 5It is an exploded view of a compressor impeller and a mechanical tool, which includes a mandrel for cold expansion of the bore for the impeller, a slotted sleeve, and a front end cap; Figure 6 This is a cross-sectional view through the compressor impeller after the slotted sleeve and mandrel have been inserted into the bore of the compressor impeller, showing the large diameter of the mandrel located in the countersunk bore before it is pulled through the bore to allow for its cold expansion; Figure 7 This is a representative example of the relationship between the pressure applied by the slotted sleeve and the axial position of the large diameter of the mandrel within the hole; Figure 8 It is a graphical depiction of two types of artifacts caused by the cold expansion of the slotted sleeve of the bore, including an axially extending ridge on the inner surface of the bore corresponding to a crack in the sleeve, and a protrusion on the back of the compressor impeller at the outlet of the bore due to the cold expansion via the slotted sleeve. Figure 9 This is a partial cross-sectional side view showing the expanded and reamed bore of the compressor impeller to eliminate any artificial defects caused by the cold expansion step and to make the bore conform to the desired final inner diameter; Figure 10 yes Figure 9 The enlarged portion is shown to illustrate the details of the reaming step after expansion; Figure 11 This is a representative graph showing the residual stress induced on the surfaces of two compressor impellers shot-peened with intensities of 0.2 mm and 0.3 mm at the Almen N scale, respectively, illustrating the relationship between residual stress and depth below the surface; Figure 12 An Almen test strip holding fixture for calibrating a shot peening machine, which is specifically configured to shot peen a compressor impeller, is an embodiment of the processing method according to the present invention. Figure 13 The illustration shows how the Almen strip holding fixture is positioned to hold two test strips at a position approximately corresponding to the surface of the compressor impeller to be shot-peened; Figure 14 This is a schematic side view of a shot peening machine with two nozzles for directing two streams of shot peening material toward the compressor impeller as it rotates about its axis. Figure 15 yes Figure 14 A schematic top view of a shot peening machine; Figure 16 This is a schematic side view of a shot peening machine configured for shot peening the back surface of a compressor impeller; and Figure 17 This is a flowchart illustrating the steps of a processing method according to an embodiment of the present invention. Detailed Implementation

[0010] This disclosure will now be described in more detail with reference to the accompanying drawings, which depict some, but not all, embodiments of the invention to which this disclosure relates. These inventions may be embodied in various forms, including those not expressly described herein, and should not be construed as limited to the specific exemplary embodiments described herein. In the following description, the same numerals consistently denote the same elements.

[0011] Figure 1 The illustration shows a compressor impeller 20 to which the processing method according to the invention can be applied. The impeller is manufactured from a tough metal such as aluminum alloy (suitable examples include grades 354, 355, or 2618) by a casting process or by machining from a billet. A cylindrical hole, or “starting hole” 22, with an initial hole diameter SHD, is then drilled through the center of the impeller. A countersunk hole 23 with a countersunk hole diameter CBD larger than the starting hole diameter is formed at the end of the starting hole adjacent to the front end N of the impeller.

[0012] The impeller includes a hub 24 and a rear disc 25, and a plurality of blades 26 connecting the hub and the rear disc, the blades extending generally radially outward from the hub. At the point where the blades 26 engage with the hub 24, there is a blade root fillet 28, which achieves a blended or smooth transition between the generally radially extending airfoil surface of the blade and the generally circumferentially extending outer surface of the hub. The blades define an inlet portion 20i of the compressor impeller configured for axial air entry into the compressor impeller and an outlet portion 20e of the compressor impeller configured for radially outward air discharge from the compressor impeller.

[0013] The shape of the hub 24 in the centrifugal compressor impeller makes it possible for the axial distance from the front end of the impeller to increase (in Figure 1 (From left to right) The radius of hub 24 increases, and therefore the mass per unit length of impeller increases toward the rear plate 25 of the impeller. The relatively higher mass per unit axial length near the rear plate 25 of the impeller, relative to the smaller mass per unit length of the impeller front end region, causes a larger value of cyclic tensile stress in the bore 22 at that location. The life-limiting mode of a centrifugal compressor impeller is typically one of two possible modes. The first common failure mode is low-cycle fatigue (LCF) failure, which originates from a crack originating at or near the inner surface of the bore of the impeller, generally at a point of relatively high mass concentration in the longitudinal portion of the bore. The second common failure mode is one or more cracks due to a chord or pizza block structure in the rear plate. This invention aims to reduce LCF stress in centrifugal compressor impellers, thereby extending the LCF life of the impeller.

[0014] This document describes an embodiment of a treatment method for centrifugal compressor impellers that significantly extends the impeller's LCF (Limited-Fold Life) of the impeller. The treatment method involves two operations that induce residual compressive stress in the impeller. In the first operation, cold expansion of a bore to tension the material beyond its elastic yield strength induces residual compressive circumferential stress in the impeller material in a radially outward region extending from the inner surface of the bore. This radial range depends on various factors such as the amount of plastic deformation induced, the bore size, and material properties. In the second operation, at least some surfaces of the impeller are shot-peened to induce residual compressive stress in the surfaces.

[0015] Figures 2 to 5 An exemplary tool for cold expansion of a bore in an impeller is illustrated. The tool comprises an assembly of a tapered mandrel M, a slotted sleeve SS, and a front end cap NC. The tapered mandrel is made of a high-strength metal such as tool steel and includes a rod-shaped member having a small diameter along most of its length. D min It also has a tapered section whose diameter increases to a large diameter at the distal end toward the spindle. D maj This large diameter is the maximum diameter of the rod portion. The proximal end of the mandrel defines an enlarged head, which is configured to engage with a hydraulic cylinder assembly for applying axial movement toward the mandrel.

[0016] A slotted sleeve (SS) is a cylindrical tube of a high-strength metal such as tool steel, with an axially extending slot along its entire length. The wall thickness of the slotted sleeve is... t ( Figure 3A The proximal end of the slotted sleeve defines a flared section for the purposes described below. The relaxed outer diameter and wall thickness of the slotted sleeve are specifically designed to be sized relative to the inner diameter of the initial cavity in the compressor impeller to be treated. Conversely, the small and large diameters of the mandrel are specifically designed to be sized relative to the dimensions of the slotted sleeve to be used.

[0017] The front cover NC is a conical member that attaches to the hydraulic cylinder assembly and centers on the mandrel, while also providing a reference surface for positioning the slotted sleeve onto the mandrel. A hole extends through the front cover to allow passage of the rod portion of the mandrel.

[0018] Reference Figure 6The diagram shows an impeller with a tool mounted within bore 22, but prior to the start of the cold expansion process. In this example, the slotted sleeve SS extends only partially along the bore or starting hole 22, terminating at a point axially spaced downstream of the impeller's front end N. The slotted sleeve is internally lubricated to allow free movement of the mandrel M within the sleeve. The flared section of the slotted sleeve provides a surface for engagement with a reference surface defined by the front end cap and centers the slotted sleeve within the starting hole. Prior to the start of the cold expansion process, the large diameter of the mandrel is located within the countersunk bore 23. To allow the starting hole 22 to cold expand, the mandrel is moved towards the starting hole via a hydraulic cylinder assembly (not shown). Figure 6 The right side of the mandrel is axially pulled through the slotted sleeve. As the tapered portion of the mandrel begins to enter the interior of the slotted sleeve, the wall of the slotted sleeve expands radially outward, which in turn expands the inner diameter of the bore. This causes the ductile metal of the impeller to yield in the circumferential direction beyond its elastic tensile yield strength. This plastic yielding occurs in the region surrounding the bore. The plastic yielding zone essentially occupies only the portion of bore 22 in which the slotted sleeve is located. Therefore, Figure 6 The portion of hole 22 on the left side of the slotted sleeve SS was not cold-worked and does not have a plastic yield zone.

[0019] The total radial expansion of the hole consists of elastic and plastic components. Once the mandrel leaves the hole, the elastic component disappears, and the hole partially shrinks towards its original diameter, but the residual or retained expansion of the hole diameter retains the plastic component corresponding to the expansion. Therefore, the expansion process can be characterized by “applied expansion” and “retained expansion” as given by the following formulas: , in D maj It is the large diameter of the spindle. t It is the wall thickness of the slotted sleeve. SHD It is the diameter of the initial hole in the impeller, and THD It is the diameter of the hole after the cold expansion process.

[0020] Figure 7This is a representative graph showing the relationship between the pressure applied by the mandrel to the slotted sleeve and the axial position of the mandrel's large diameter as it travels from the impeller's front end to the rear disc. The relatively low-pressure plateau on the left side of the graph represents the travel of the large diameter through the countersunk orifice. The pressure increases rapidly, reaching peak pressure, as the tapered section begins to move through the slotted sleeve; the pressure then gradually decreases as the large diameter passes through and eventually exits the sleeve. By testing a batch of compressor impellers with a certain range of SHD values ​​and measuring the retention expansion % and peak pressure of each impeller, the relationship between retention expansion % and peak pressure can be derived, along with an equation for peak pressure variation with retention expansion %. This allows for setting a minimum acceptable peak pressure limit for the process.

[0021] Cold expansion of the hole typically leads to the generation of man-made defects. Figure 8 Two types of man-made defects are illustrated in the diagram. The first type is an axially extending ridge R on the inner surface of the cold expansion hole, corresponding to the location of a crack in the slotted sleeve. The second type is a surface protrusion SU on the back of the compressor impeller, which includes a thin annular region around the outlet of the hole and extends slightly beyond the originally flat surface.

[0022] Advantageously, the impeller processed according to embodiments of the invention can undergo an expansion operation to remove any such human-made defects. Thus, as, for example, in Figure 9 and Figure 10 As depicted, axial ridges are removed through a reaming operation, in which a reaming tool RT is used to ream the treated hole from the treated hole diameter THD to a slightly larger final hole diameter FHD. The reaming operation removes a “reaming allowance” RA, which is large enough to remove ridge-like artificial defects and ensure uniform hole diameter and controlled surface finish along the hole length to help minimize crack initiation. The second post-expansion operation is scraping the back of the impeller to remove surface protrusions SU. After completing the cold expansion and post-expansion operations, the impeller is machined in a second operation of the processing method according to an embodiment of the invention. The second operation includes a surface shot peening operation, as now referred to... Figures 11 to 16 As stated above.

[0023] Surface peening is a process used to induce residual compressive stress in the surface of a treated part. Various types of surface peening processes are known, including shot peening, ultrasonic peening, and laser peening. The selection of a suitable peening operation considers the depth to which residual stress is applied below the part surface and the process's impact on the surface finish of the part. For current purposes, these considerations tend to favor shot peening as the preferred method because compressive stress can extend to a relatively large depth and produce a relatively smooth surface finish.

[0024] Shot peening typically takes place in a chamber to confine the blast and facilitate its collection for reuse. The workpiece to be shot-peened is typically introduced into the blast stream by a mechanical device designed to expose critical areas to the blast according to a predetermined program. The blast consists of hard particles categorized by size, typically ranging in size from 1 / 16 to 3 / 32 of an inch in diameter. Various types and varieties of blasts are available. The blast can be propelled by air or by an impeller at a speed of approximately 200 feet per second. The area covered by the blast stream is called the "blast pattern." In the case of pneumatic machines, the blast is delivered from a nozzle, and the blast pattern covers a circular area approximately 2 or 3 inches in diameter, depending on the nozzle size and distance from the workpiece. Shot peening induces plastic flow on the surface of the part, subjecting the material to stresses exceeding its yield strength, resulting in residual compressive stress. The depth to which this compressive stress extends depends on the properties of the material, the characteristics of the blast, and the amount of blast hitting the peened area. The properties of the blast are defined by the blast velocity and its size and type. The amount of shot hitting the peened area is a function of the amount of the jet stream, the jet pattern, the relative movement between the part and the jet stream, and the time of exposure to the jet stream.

[0025] In contrast to manually operated devices, shot peening of the compressor impeller is advantageously performed by an automatic shot peening machine, such that each shot peening medium flow is propelled on the impeller at a controlled rate, in a controlled direction, and for a controlled shot peening duration. Therefore, the shot peening machine provides a means to move the impeller through the medium flow or move the medium flow on the impeller at a uniform speed in translation, rotation, or both, as needed. The nozzle and impeller should be mechanically held and moved. During this process, the impeller should not experience any random movement. The shot peening machine should be able to consistently reproduce the desired shot peening intensity.

[0026] Shot peening intensity and coverage are two important parameters of this process. Standard test procedures and materials have been developed for measuring the intensity of shot peening operations and the percentage of coverage. These test procedures involve the use of metal test strips made to specified standard length, width, and thickness dimensions, and are commonly referred to as Almen test strips made of carbon steel SAE 1070. Almen test strips are available in "A", "C", and "N" types. Almen "N" strips are used for relatively low-intensity shot peening applications, have a thickness of 0.76-0.81 mm and a hardness of approximately 72.5 HRa, and were used to calibrate shot peening equipment during the development of this invention.

[0027] A standard test strip holder is used for Almen test strips and typically consists of four steel balls fixed in a rectangular array on a heavy-duty steel base, providing point contact with the underside of the test strip at four locations near the four corners. Mounting screws clamp the test strip onto the top of the balls. When the top side of the test strip is exposed to the shot peening stream, the residual stress generated at the top surface causes the strip to bend upwards in an arc shape. The height of the arc is related to the intensity of the shot peening; higher intensity results in a higher arc height, and lower intensity results in a lower arc height. The exposure time also affects the arc height; longer exposure times result in a greater arc height. Therefore, a graph of arc height versus exposure time is typically plotted by shot peening a series of identical test strips at various exposure times and measuring the arc height of each strip. As the exposure time increases, the slope of the arc height versus time curve decreases. The saturation point on the curve is defined as a point (at exposure time T), at which point, by doubling the exposure time (2T), the arc height will increase further by 10%. In other words, saturation time is essentially the point of diminishing returns, where further exposure time yields almost no further increase in arc height. Therefore, the "intensity" of shot peening is defined as the arc height at the saturation exposure time T.

[0028] Shot peening coverage is defined as the percentage of the surface that has been dented at least once by the shot peening medium. Coverage is considered "full" when at least 98% of the surface is dented (according to SAE International Standard J2277, "Shot Peening Coverage Determination"). For the purposes of this invention, the shot peening time or duration is the shortest time required to achieve 98% coverage of the shot-peened surface.

[0029] Selecting a suitable shot peening medium is also an important consideration in this invention. For aluminum alloy compressor impellers, glass beads are preferred. Generally, for aluminum alloy turbocharger compressor impellers, glass peening with a diameter of not less than 0.05 mm is suitable. The size of the peening should ensure that the nominal peening diameter does not exceed half the minimum fillet radius of the material to be peened.

[0030] According to the invention, after the cold expansion operation and post-expansion finishing steps have been completed, the compressor impeller is subjected to shot peening at a specified intensity, which is selected to obtain the desired LCF life characteristics of the impeller. The selected intensity can be obtained through empirical evaluation on a series of identical impeller specimens, which are shot peened at various intensities and then subjected to stress cycling to determine the optimal intensity for LCF life. For the purposes of the invention, the shot peening intensity is advantageously 0.15 mm to 0.60 mm (Almen N) as measured according to the standard test procedure SAE International AMS2432 “Shot Peening, Computer Monitored”. Preferably, the intensity is from 0.2 mm to 0.4 mm, and more preferably from 0.25 mm to 0.35 mm.

[0031] Figure 11 This is a graph illustrating the effect of shot peening intensity on residual stress at and below the surface of the compressor impeller. Residual stress versus depth below the surface was plotted for 0.2 mm Almen N intensity and 0.3 mm Almen N intensity. It can be seen that the peak residual compressive stress for both intensities is approximately 260 MPa, occurring at approximately 13 μm below the surface. More generally, for the purposes of this invention, it is desirable that the peak residual stress generated by shot peening is at least 200 MPa.

[0032] For shot peening the hub, blades, and blade root fillet surfaces of the compressor impeller according to the invention, it is advantageous to use two nozzles that guide two jet streams toward the impeller at two different locations and orientations, such as... Figure 14 and Figure 15 As illustrated in the diagram. It can be done by using, for example... Figure 12 The test fixture 100 shown can calibrate the machine to achieve the desired strength. The test fixture 100 holds two Almen N test strips 110 and 120 in a selected orientation to reasonably and accurately simulate the orientation of the impeller surface to be shot peened. Figure 13 The image depicts how fixture 100 positions two test strips 110 and 120 at a critical location (shown in dashed lines) on compressor impeller 20, to which two jet streams will be directed. During shot peening, the compressor impeller is oriented so that its central axis is vertical and it will rotate about its axis. Therefore, the test fixture is able to rotate about its central axis.

[0033] Before starting shot peening on the compressor impeller, the machine setup and process parameters can be calibrated and verified by installing a test fixture at the location where the compressor impeller will be installed, and the test strip can be shot peened to verify that the desired strength has been achieved. Once the parameters are verified, shot peening of the compressor impeller can be performed.

[0034] Figure 14 and Figure 15 The diagram illustrates a shot peening machine arrangement for shot peening the blades and hub of an impeller. The arrangement includes a first nozzle 210 and a second nozzle 220 for directing two streams of jet media onto the compressor impeller 20. The streams are directed to impact both the suction surface 26s and the pressure surface 26p of the blades 26, as well as the hub line 24 and the blade root fillet 28. Figure 16 The diagram illustrates the arrangement for shot peening the rear disc 25 of the impeller.

[0035] Figure 17 This is a flowchart of a processing method according to an embodiment of the present invention. In step 300, a compressor impeller is manufactured from a suitable tough metal such as aluminum alloys (non-limiting examples of which include grades 354, 355, and 2618). The impeller can be machined from a blank of material using a CNC milling machine, or it can be cast using a suitable casting process. Next, in step 310, a starting hole is machined through the center of the impeller. If the impeller is to include a countersunk hole (e.g., as shown in the diagram), the impeller can be further modified accordingly. Figure 1 As shown in the diagram, a countersunk hole is drilled in the front end of the impeller. However, it should be noted that the invention is not limited to the machining of impellers with countersunk holes. The cold expansion process can also be applied to impellers without countersunk holes, in which case the entire length of the hole is cold expanded in a manner substantially as described herein, with the large diameter of the mandrel starting outside the initial hole at the front end of the impeller.

[0036] In step 320, cold expansion of the initial bore is performed according to the process parameters and characteristics described herein. Then, in step 330, the cold-expanded bore is reamed to the precise inner diameter according to the impeller specifications, which also removes any artificial defects such as axial ridges caused by the cold expansion operation, producing a desirable smooth surface finish on the inner surface of the bore. In step 340, the back of the impeller adjacent to the bore is scraped to remove any surface protrusions caused by the cold expansion operation.

[0037] Then, in step 350, the impeller is shot-peened according to the process parameters and characteristics described herein. Finally, in step 360, the impeller is balanced.

[0038] Tests conducted by the applicant indicate that minimal retention expansion of the orifice is required to significantly improve LCF lifetime. Therefore, for the purposes of this invention, the cold expansion of the orifice should achieve a retention expansion of at least 1.2%.

[0039] Based on this disclosure, those skilled in the art will recognize that modifications and other embodiments of the invention described herein can be made without departing from the inventive concept described herein. For example, although cold expansion is described as being produced by a mandrel and a slotted sleeve, the invention is not limited to using this particular type of tool and other tool types can be used instead, such as a slotted mandrel capable of radially outward expansion to expand the hole. Specific terminology used herein is for illustrative purposes and not for limiting purposes. Therefore, the invention is not limited to the specific embodiments disclosed, and modifications and other embodiments are intended to be included within the scope of the appended claims.

Claims

1. A method for producing a centrifugal compressor impeller, comprising the following steps: A manufactured centrifugal compressor impeller is provided, made of a tough metal having tensile yield strength. The compressor impeller includes a hub and a rear disc, and a plurality of blades connected to the hub and the rear disc. The plurality of blades define an inlet portion of the compressor impeller configured for axial air entry into the compressor impeller and an outlet portion of the compressor impeller configured for radial air discharge from the compressor impeller. Each blade has a radius at the blade root where it connects to the hub. The compressor impeller has a first face and an opposing second face. The compressor impeller defines an orifice extending centrally through the hub and the rear disc in a downstream axial direction. and The manufactured compressor impeller is processed through the following steps: (a) A hole processing step comprising forcibly expanding the diameter of the hole to cold work the metal at the inner surface of the hole to exceed the tensile yield strength in the circumferential direction around the hole, so as to induce compressive residual circumferential stress in the metal adjacent to the inner surface of the hole; and (b) A rear plate treatment step, which includes surface shot peening of at least a portion of the rear plate of the compressor impeller.

2. The method for producing centrifugal compressor impellers according to claim 1, characterized in that, The hole processing steps are performed only along the incomplete portion of the hole, and the hole is not forcibly expanded outside the incomplete portion.

3. The method for producing centrifugal compressor impellers according to claim 2, characterized in that, The starting point of the incomplete portion of the hole is axially spaced downstream from the leading edge of the root fillet of the blade.

4. The method for producing centrifugal compressor impellers according to claim 1, characterized in that, It also includes surface shot peening of at least some areas of the blade.

5. The method for producing centrifugal compressor impellers according to claim 4, characterized in that, The area of ​​the blade that has been shot-peened includes the inhalation surface and the pressure surface of the blade.

6. The method for producing a centrifugal compressor impeller according to claim 5, characterized in that, It also includes surface shot peening of the blade root radius and the hub.

7. The method for producing a centrifugal compressor impeller according to claim 6, characterized in that, The surface shot peening is performed at an intensity ranging from 0.15 mm to 0.6 mm, measured in Almen N scale.

8. The method for producing a centrifugal compressor impeller according to claim 6, characterized in that... The surface shot peening is performed at an intensity ranging from 0.2 mm to 0.35 mm, measured in Almen N scale.

9. The method for producing a centrifugal compressor impeller according to claim 6, characterized in that, The surface shot peening is performed at an intensity ranging from 0.25 mm to 0.35 mm, measured in Almen N scale.

10. The method for producing a centrifugal compressor impeller according to claim 1, characterized in that, The hole processing step includes setting a slotted bushing around the mandrel inside the hole, and axially pulling the mandrel through the slotted bushing to cause the slotted bushing to expand radially outward.

11. The method for producing a centrifugal compressor impeller according to claim 1, characterized in that, The hole treatment step produces at least 1.2% retention expansion at the inner surface of the hole.

12. The method for producing a centrifugal compressor impeller according to claim 1, characterized in that, It also includes a step of machining the inner surface of the hole to remove surface irregularities generated during the hole processing step.

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