Method for producing a compressive residual hoop stress zone on an incomplete portion of a central hole of a centrifugal compressor disk and a compressor disk

Abstract

The present invention relates to a method for producing a compressive residual hoop stress zone on an incomplete portion of the central hole of a centrifugal compressor disk and a compressor disk. A method is provided for cold working the inner surface of a hole in a centrifugal compressor disk along only an incomplete portion of the hole length (i.e., along less than the entire axial length of the hole) to produce a compressive residual hoop stress zone in the metal around the hole where the disk requires beneficial residual stress. The method is intended to avoid cold working the hole at locations adjacent to high stress areas and features of the disk where cold working would negatively affect the overall life of the disk.

Description

Technical Field

[0001] The present invention relates generally to centrifugal compressor impellers, such as turbochargers for internal combustion engines, and in particular to methods of treating a central bore of a centrifugal compressor impeller to produce a compressive residual hoop stress zone over an incomplete portion of the bore length, and the compressor impeller produced thereby. Background Art

[0002] Typically, the life limiting mode of a centrifugal compressor disk is a low cycle fatigue (LCF) failure mode resulting from cracks that initiate at or near the through-hole surface of the disk. Summary of the invention

[0003] One object of the current technical development leading to the invention described herein is to increase the LCF life of centrifugal compressor impellers having a through hole, ie a central hole extending completely through the impeller along the axis of rotation.

[0004] Processing methods according to embodiments of the present invention require a deep understanding of the stresses in the compressor impeller to pre-create conditions for achieving localized hole cold working processing. Embodiments of the present invention described herein relate to high speed centrifugal impellers or impellers having high stresses on the impeller hole, impeller blade root fillet, and impeller rear disk. In many such high speed impellers, the shape of the hub line (from the start to the end of the hole) presents additional challenges to the application of the present invention because of the variable wall thickness around the impeller hole and the presence of high stress features near the wall area around the hole. At these joints, if cold working is applied without the accommodation provided by the present invention, harmful overlapping stresses can greatly reduce the benefits of cold working.

[0005] The present disclosure describes a method that involves cold working the inner surface of a hole along only a non-complete portion of the hole length (i.e., along less than the entire axial length of the hole) to create a compressive residual hoop stress zone in the metal surrounding the hole where the wheel disc requires beneficial residual stresses. The method purposefully avoids cold working the hole at locations adjacent to high stress areas and features of the wheel disc where cold working would negatively impact the overall life of the wheel disc.

[0006] In one embodiment described herein, the cold working method comprises the following steps:

[0007] A centrifugal compressor wheel made of a ductile metal having a tensile yield strength is provided, the compressor wheel including a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a bore extending centrally through the hub in a downstream axial direction, the bore having an axial length;

[0008] selecting an incomplete portion defining an axial length of the bore between a starting point and an end point of the incomplete portion, wherein the starting point is axially spaced downstream from the first face of the compressor wheel and the end point is axially spaced downstream from the starting point; and

[0009] The metal is cold worked on the inner surface of the hole only along the incomplete portion of the hole, exceeding the tensile yield strength in the hoop direction around the hole, thereby inducing compressive residual hoop stresses in the metal near the inner surface along the incomplete portion.

[0010] In some embodiments, the cold working step includes applying radially outward pressure on the inner surface of the hole along the incomplete portion only. In some embodiments, this can be achieved mechanically, such as by using a mechanical tool to apply radially outward pressure on the inner surface of the incomplete portion of the hole. For example, the cold working step can include pulling the tool axially through the hole in a downstream axial direction, the diameter of the tool exceeding the initial diameter of the incomplete portion of the hole before the cold working step. Various mechanical tools can be used, non-limiting examples of which include: a radially expandable mandrel that can be triggered at a desired position along the hole, after which the expanded mandrel is then axially pulled along the hole to cold work a selected longitudinal portion of the hole; or alternatively an open sleeve and mandrel, wherein the open sleeve is located in a selected hole portion for processing, and the mandrel is pulled through the sleeve to expand the sleeve radially outward, thereby cold working the hole portion.

[0011] In some embodiments, the hole may have a counterbore extending from the first face of the compressor wheel to the start of the incomplete portion of the hole, the diameter of the counterbore exceeding the initial diameter of the incomplete portion of the hole and exceeding the diameter of the tool. The cold working step may then include positioning a tool (e.g., an expandable mandrel) in the counterbore and then pulling the tool through the incomplete portion of the hole in a downstream axial direction.

[0012] As an alternative to using mechanical tools, cold working of selected bore portions may be accomplished by non-mechanical means (non-limiting examples of which may include laser treatment of the inner bore surface or chemical treatment of the inner bore surface).

[0013] The present invention may be applied to centrifugal compressor disks of various types and configurations, including both single-impeller disks and dual-impeller disks. In the case of a dual-impeller disk, the disk may include a first impeller and a second impeller arranged back-to-back, the first impeller defining a first face, and the second impeller defining a second face of the compressor disk. In this case, the hole may include a first counterbore extending from the first face of the compressor disk to the beginning of the incomplete portion of the hole, and may include a second counterbore extending from the end of the incomplete portion of the hole to the second face of the compressor disk. The first and second counterbores may each have a diameter that exceeds the initial diameter of the incomplete portion of the hole prior to the cold working step.

[0014] The present disclosure also describes a compressor wheel produced by a cold working method. According to one embodiment, a centrifugal compressor wheel made of a ductile metal having a tensile yield strength includes a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a hole extending centrally through the hub in a downstream axial direction, the hole having an axial length. The compressor wheel includes a cold worked zone having a residual compressive hoop stress in a relaxed state of the compressor wheel, the cold worked zone extending radially outward from an inner surface of the hole, and wherein the cold worked zone extends axially only along an incomplete portion of the axial length of the hole between a start point and an end point of the incomplete portion, wherein the start point is axially spaced downstream from the first face of the compressor wheel, and the end point is axially spaced downstream from the start point. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Having described the present disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and in which:

[0016] Figure 1 is a cross-sectional view of a compressor wheel before a cold working method according to an embodiment of the present invention;

[0017] Figure 2 is an exploded view of a compressor wheel and a machine tool according to an embodiment of the present invention, the machine tool including a mandrel surrounded by an open sleeve for cold working a portion of a bore of the wheel;

[0018] Figure 3 After the tool has been inserted into the hole of the wheel, but before the tool is activated to cold machine a portion of the hole Figure 2 A cross-sectional view of a wheel disc and a machine tool;

[0019] Figure 4 is similar to Figure 3 sectional view showing a mandrel being pulled axially through a split sleeve, the mandrel being disposed within only a portion of a bore in which it is desired to cold work the bore;

[0020] Figure 5 is similar to the Figure 4 A cross-sectional view of

[0021] Figure 6 After the split sleeve has been removed to complete the cold working method Figures 1 to 5 A cross-sectional view of a compressor wheel;

[0022] Figure 7is a cross-sectional view of a dual impeller compressor wheel having a machine tool disposed therein in preparation for performing a cold working operation over a portion of the length of a bore according to another embodiment of the present invention; and

[0023] Figure 8 is a cross-sectional view of a compressor disk before the cold working method according to an embodiment of the present invention, the disk is generally similar to Figure 1 As shown, but including counterbores. DETAILED DESCRIPTION

[0024] The present disclosure will now be described in more detail with reference to the above-mentioned drawings, which depict some but not all embodiments of the inventions to which the present disclosure relates. These inventions may be implemented in various forms, including forms not explicitly described herein, and should not be construed as being limited to the specific exemplary embodiments described herein. In the following description, the same numerals always represent the same elements.

[0025] Figure 1 A first type of single-impeller compressor disk 20 is shown to which the method according to the invention may be applied. The disk includes a cylindrical hole 22 extending centrally through the disk. The disk includes a hub 24 to which a plurality of blades 26 are joined, the blades extending generally radially outwardly from the hub. Where the blades 26 join the hub 24, there is a blade root fillet 28 (see also FIG. 2 ). Figure 2 ), which achieves a blend or smooth transition between the generally radially extending airfoil surface of the blade and the generally circumferentially extending outer surface of the hub. The hub 24 in a centrifugal compressor disk is shaped so that as the axial distance from the nose of the disk increases (in Figure 1 As shown in the figure (from left to right in the figure), the radius of the hub increases and therefore the mass of the wheel disc per unit length increases towards the rear disc 30 of the wheel disc.

[0026] The mass per unit axial length of the disc 30 towards the rear of the disc is relatively high relative to the disc nose region where the mass per unit length is low, which induces a larger magnitude of cyclic tensile stresses in the bore 22 at this location. The life limiting mode of a centrifugal compressor disc is typically a low cycle fatigue (LCF) failure mode that originates from cracks that initiate at or near the inner surface of the bore of the disc, generally somewhere in the longitudinal portion of the bore where the mass is relatively concentrated. The present invention is directed to alleviating the LCF stresses in the bore, thereby extending the LCF life of a centrifugal compressor disc.

[0027] refer to Figures 2 to 6 ,against Figure 1A single impeller disk of the type shown describes a first embodiment of a method according to the invention. The method entails cold working the inner surface of the bore to stress the material beyond its elastic yield strength, thereby inducing a residual compressive hoop stress in the material of the disk in a zone extending from the inner surface of the bore and radially outwardly for a distance that depends on various factors such as the amount of plastic deformation induced, the size of the bore, the material properties, etc.

[0028] Therefore, if Figure 2 As shown, the method in the illustrated embodiment uses a mechanical tool T to achieve cold working. The tool includes an open sleeve SS and a mandrel M disposed in the sleeve and axially movable therein. According to the present invention, cold working is not performed along the entire axial length of the hole 22. Instead, an incomplete portion FP of the length of the hole is selected for cold working; cold working is not performed on any area outside the incomplete portion. Therefore, the incomplete portion FP for cold working is defined between a starting point A and an end point Z. The starting point A is axially spaced downstream from the first (upstream) face F1 of the wheel disc, and the end point is axially spaced downstream from the starting point. The end point may be at the opposite second (downstream) face F2 of the wheel disc (i.e., the rear disc 30), or it may be spaced a certain distance upstream of the second face. Therefore, the mechanical tool T is constructed and operated so that it will only cold work a selected incomplete portion FP of the hole.

[0029] refer to Figure 3 , shows a wheel disc with tool T mounted in bore 22, but before the cold working process begins. Split sleeve SS extends only partially along bore 22, terminating at a point spaced axially downstream of first face F1 of the wheel disc. Figure 4 Next, a step of the method is shown in which the mandrel M is pulled axially downstream within the split sleeve SS. The mandrel diameter is larger than the relaxed inner diameter of the split sleeve, so when the enlarged mandrel begins to enter the end of the split sleeve, the sleeve is forced to expand in diameter, which in turn forces the inner surface diameter of the hole 22 to expand. This causes the ductile metal of the wheel to yield in the annular direction beyond its elastic tensile yield strength. This plastic yielding occurs in the zone surrounding the hole. The plastic yielding zone essentially occupies only the non-complete portion FP of the hole 22 in which the split sleeve is disposed. Therefore, Figure 4 The portion of the hole 22 on the left side of the center split sleeve SS is not cold worked and does not have a plastic yield zone.

[0030] Figure 5 depicts the wheel disc 20 after the mandrel M has been completely pulled out along the split sleeve SS, and Figure 62 shows the result of the next step, in which the split sleeve is removed from the hole. The resulting wheel disc 20 has a residual compressive hoop stress zone extending from the inner surface of the hole 22 and extending a radial distance outward from the hole, but only along the incomplete portion FP of the hole that was cold worked. More specifically, as Figure 6 As shown, the hoop stress contours are depicted τ h2 , which represents a contour of constant compressive (negative sign) hoop stress with relatively high magnitude. Radially further outward from this contour are contours of hoop stress τ h1 , which represents contours with constant compressive hoop stress, whose magnitude is less than τ h2 The value of the contour line. τ h1 The contour lines radially farther out are marked τ h0 Because along this contour line, the hoop stress is equal to zero, which means that there is neither compressive hoop stress nor tensile hoop stress. τ h0 As the line moves radially outward, the hoop stress becomes positive, that is, tensile. τ h0 The wheel disk area radially inside the contour line is an area with residual compressive hoop stress. The compressive hoop stress reaches a maximum value at the inner surface of the hole 22, and the value decreases with increasing radial distance from the inner surface of the hole.

[0031] An important aspect of the method of the invention is the selection of the non-complete portion FP of the hole 22 for cold working. The method for making such selection requires the following aspects: the starting point A of the cold worked non-complete portion cannot be too close to the first face F1 of the disk, because if the cold worked portion of the hole extends too far towards the first face, the cold working will generate harmful residual tensile stresses in the leading edge blade root fillet. On the other hand, the cold worked portion FP of the hole should cover the entire area of ​​potential LCF hub type failure of the disk. Therefore, there are two opposite constraints: a longer processed portion of the hole will have the advantage of ensuring that the entire area of ​​potential LCF hub type failure is covered; however, a shorter processed portion will have the advantage of avoiding the risk of causing harmful residual tensile stresses in the leading edge blade root fillet. According to the invention, at least in some embodiments, the non-complete portion FP for cold working is selected by taking into account the maximum von Mises stress of the disk hub generated by the blade leading edge fillet load and ensuring that it is sufficiently below the material yield strength. Basically, the cold worked portion FP is selected so that it covers the portion of the hole length that is susceptible to LCF hub type failure while satisfying this maximum von Mises stress criterion.

[0032] like Figure 7 As depicted, another embodiment of the invention may include a dual impeller disk 120 including a first impeller Im1 and a second impeller Im2 arranged in a back-to-back configuration and sharing the same bore 122, and having a clearance counterbore 123 adjacent to each nose for a cold working process. This embodiment may advantageously be performed with an expandable open mandrel SM. In each counterbore 123, the expanded mandrel does not perform any cold working of the hole. After the cold working has been performed on the incomplete portion FP of the hole 122, a final reaming to the final hole size may end the method.

[0033] according to Figure 8 In another embodiment of the present invention shown, a single impeller compressor wheel 20' is similar to Figure 1 The wheel disc is different in that the hole 22' includes the "nose" side of the hole adjacent to the wheel disc ( Figure 8 A clearance hole 23' is formed in the figure (to the left in the figure). The clearance hole has a hole diameter that is larger than the incomplete portion FP of the hole to be cold worked. The clearance hole has a defined length. The expandable mandrel appears and is triggered at a position within the clearance hole where cold working is not required. In this step, the mandrel is pulled to the right through the section of the hole designed to receive cold working. The result is that cold working is performed in a portion FP of the hole that has the highest hole stress and can therefore positively affect the hole life while avoiding harmful overlapping stresses at the blade root fillet closest to the nose of the impeller. The overlapping stresses described herein are related to the leading edge root fillets of the impeller blades and the tensile stresses generated by the cold working method near these leading edge root fillets. In general, such harmful overlapping stresses can be avoided by spacing the starting point of the treated hole portion axially downstream from the leading edge blade root fillet.

[0034] Advantageously, a wheel disc machined in accordance with an embodiment of the present invention may also undergo a post-reaming operation to remove axial ridges (residual artifacts resulting from the cold working process) to ensure uniformity of hole diameter and controlled surface finish along the length of the hole to help minimize the initiation of cracks.

[0035] There is a trade-off between cold working using the mandrel and open sleeve technique and cold working using the expandable mandrel technique without a sleeve. However, of the two cold working methods, the present invention meets the primary need of the present invention: introducing beneficial residual compressive hoop stresses in the hole by the cold working technique and localizing the treated area to the portion of the length of the hole with the highest hoop stress.

[0036] Another (not shown) embodiment may include a finished hole with a threaded section at the nose area of ​​the hole for direct attachment to the shaft of the rotor if the nose is long enough to allow sufficient distance from the processed hole area to the threaded area of ​​the hole at the nose.

[0037] Aspects of the invention described herein include, but are not limited to:

[0038] Aspect 1: A cold working method comprising the following steps:

[0039] A centrifugal compressor wheel made of a ductile metal having a tensile yield strength is provided, the compressor wheel including a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a bore extending centrally through the hub in a downstream axial direction, the bore having an axial length;

[0040] selecting an incomplete portion defining an axial length of the bore between a starting point and an end point of the incomplete portion, wherein the starting point is axially spaced downstream from the first face of the compressor wheel and the end point is axially spaced downstream from the starting point; and

[0041] Cold working the metal on the inner surface of the hole only along the incomplete portion of the hole, exceeding the tensile yield strength in the circumferential direction around the hole, the hole not being cold worked outside the incomplete portion, thereby inducing compressive residual hoop stress in the metal near the inner surface along the incomplete portion.

[0042] Aspect 2: The cold working method according to aspect 1, wherein the starting point is axially spaced downstream from the leading edge of the blade root fillet.

[0043] Aspect 3: The cold working method according to aspect 1, wherein the cold working step includes applying radially outward pressure on the inner surface of the hole only along the non-intact portion.

[0044] Aspect 4: The cold working method according to aspect 3, wherein the cold working step includes applying radially outward pressure on the inner surface of the non-intact portion of the hole using a mechanical tool.

[0045] Aspect 5: The cold working method of aspect 4, wherein the cold working step comprises pulling a tool axially through the hole in a downstream axial direction, the diameter of the tool exceeding an initial diameter of the non-intact portion of the hole prior to the cold working step.

[0046] Aspect 6: A cold working method according to Aspect 5, wherein the providing step also includes providing the hole to have a counterbore located upstream of the starting point of the incomplete portion of the hole, the diameter of the counterbore exceeds the initial diameter of the incomplete portion of the hole and exceeds the diameter of the tool, and wherein the cold working step includes setting the tool in the counterbore and then pulling the tool through the incomplete portion of the hole in a downstream axial direction.

[0047] Aspect 7: A cold working method according to Aspect 4, wherein the machine tool includes an open sleeve surrounding the mandrel, the axial length of the open sleeve corresponding to the axial length of the incomplete portion of the hole, wherein the cold working step includes positioning the open sleeve in the incomplete portion of the hole, and axially pulling the mandrel through the open sleeve to expand the open sleeve radially outward.

[0048] Aspect 8: The cold working method according to Aspect 1, wherein the providing step further includes: providing the compressor wheel as a double impeller having a first impeller and a second impeller arranged back to back, the first impeller defining a first surface, and the second impeller defining a second surface of the compressor wheel; and providing the hole to have a first counterbore adjacent to the first surface of the compressor wheel and extending to the starting point of the incomplete portion of the hole, and having a second counterbore extending from the end point of the incomplete portion of the hole toward the second surface of the compressor wheel, the first and second counterbores each having a diameter that exceeds the initial diameter of the incomplete portion of the hole before the cold working step.

[0049] Aspect 9: The cold working method according to aspect 8, wherein the cold working step includes applying radially outward pressure on the inner surface of the non-intact portion of the hole using a mechanical tool.

[0050] Aspect 10: The cold working method of aspect 9, wherein the cold working step comprises pulling a tool axially through the hole, the diameter of the tool exceeding an initial diameter of the incomplete portion of the hole prior to the cold working step.

[0051] Aspect 11: According to the cold working method of Aspect 9, the first and second counterbores each have a diameter that exceeds the initial diameter of the incomplete portion of the hole and exceeds the diameter of the tool, and wherein the cold working step includes: setting the tool in one of the first and second counterbores, and then pulling the tool through the incomplete portion of the hole in a downstream axial direction until the tool reaches the other of the first and second counterbores.

[0052] Aspect 12: A centrifugal compressor wheel made of a ductile metal having a tensile yield strength, the compressor wheel comprising a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a hole extending centrally through the hub in a downstream axial direction, the hole having an axial length,

[0053] The compressor wheel includes a cold worked zone having a residual compressive hoop stress in a relaxed state of the compressor wheel, the cold worked zone extending radially outward from the inner surface of the hole, and wherein the cold worked zone extends axially only along an incomplete portion of the axial length of the hole between a start point and an end point of the incomplete portion, wherein the start point is axially spaced downstream from the first face of the compressor wheel, and the end point is axially spaced downstream from the start point.

[0054] Aspect 13: The centrifugal compressor wheel of aspect 12, wherein the starting point is axially spaced downstream from a leading edge of the blade root fillet.

[0055] Aspect 14: The centrifugal compressor wheel of aspect 12, wherein the wheel comprises a counterbore located upstream of the start of the incomplete portion of the bore.

[0056] Aspect 15: A centrifugal compressor wheel according to Aspect 12, comprising a dual-impeller wheel having a first impeller and a second impeller arranged back to back, the first impeller defining a first face, and the second impeller defining a second face of the compressor wheel, wherein the hole comprises a first counterbore adjacent to the first face of the compressor wheel and extending to the starting point of the incomplete portion of the hole, and a second counterbore extending from the end point of the incomplete portion of the hole toward the second face of the compressor wheel.

[0057] Aspect 16: An article produced by a method comprising the steps of:

[0058] A centrifugal compressor wheel made of a ductile metal having a tensile yield strength is provided, the compressor wheel including a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a bore extending centrally through the hub in a downstream axial direction, the bore having an axial length;

[0059] selecting an incomplete portion defining an axial length of the bore between a starting point and an end point of the incomplete portion, wherein the starting point is axially spaced downstream from the first face of the compressor wheel and the end point is axially spaced downstream from the starting point; and

[0060] Cold working metal on the inner surface of the hole only along the incomplete portion of the hole, exceeding the tensile yield strength in the circumferential direction around the hole, the hole not being cold worked outside the incomplete portion, thereby inducing compressive residual hoop stress in the metal near the inner surface along the incomplete portion, so that the compressor wheel includes a cold worked zone with residual compressive hoop stress in a relaxed state of the compressor wheel, the cold worked zone extending radially outward from the inner surface of the hole and axially extending along the incomplete portion of the axial length of the hole.

[0061] Based on this disclosure, those skilled in the art will recognize that modifications and other embodiments of the invention described herein can be accomplished without departing from the inventive concepts described herein. The specific terms used herein are for the purpose of explanation, not for the purpose of limitation. Therefore, the present 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 cold working method comprising the following steps: Providing a centrifugal compressor wheel made of a ductile metal having a tensile yield strength, the compressor wheel including a hub and a plurality of blades joined to the hub and extending radially outward from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a bore extending centrally through the hub in a downstream axial direction, the bore having an axial length; selecting the incomplete portion of the axial length of the bore defined between a start point and an end point of the incomplete portion, wherein the start point is axially spaced downstream from a leading edge of the blade root fillet and the end point is axially spaced downstream from the start point; providing the bore to have a counterbore located upstream of the start of the incomplete portion of the bore, the counterbore having a diameter exceeding an initial diameter of the incomplete portion of the bore; placing a machine tool in the counterbore, the machine tool having a diameter exceeding the initial diameter of the incomplete portion of the bore; and The compressive residual hoop stress is induced in the ductile metal near the inner surface of the hole along the incomplete portion by pulling the machine tool through the incomplete portion of the hole in the downstream axial direction, cold working the ductile metal at the inner surface of the hole only along the incomplete portion of the hole so that the tensile yield strength is exceeded in the circumferential direction around the hole, and the hole is not cold worked outside the incomplete portion.

2. The cold working method according to claim 1, wherein: The machine tool comprises an open sleeve surrounding a mandrel, the open sleeve having an axial length corresponding to the axial length of the incomplete portion of the hole, wherein the cold working step comprises positioning the open sleeve within the incomplete portion of the hole and axially pulling the mandrel through the open sleeve to expand the open sleeve radially outward.

3. The cold working method according to claim 1, wherein: The providing step also includes: providing the compressor wheel as a dual impeller, the dual impeller having a first impeller and a second impeller arranged back to back, the first impeller defining the first face, and the second impeller defining the second face of the compressor wheel; wherein the counterbore is positioned adjacent to the first face of the compressor wheel and extends to the starting point of the incomplete portion of the hole, and the providing step also includes further providing the hole to have a second counterbore extending from the end point of the incomplete portion of the hole toward the second face of the compressor wheel, the second counterbore having a diameter exceeding the initial diameter of the incomplete portion of the hole before the cold working step.

4. A centrifugal compressor wheel made of a ductile metal having a tensile yield strength, the compressor wheel comprising a hub and a plurality of blades joined to the hub and extending radially outwardly from the hub, each blade having a blade root fillet where the blade joins the hub, the compressor wheel having a first face and an opposing second face, the hub of the compressor wheel defining a bore extending centrally through the hub in a downstream axial direction, the bore having an axial length, The compressor wheel includes a cold worked zone having a residual compressive hoop stress in a relaxed state of the compressor wheel, the cold worked zone extending radially outward from an inner surface of the bore, and wherein, the cold worked zone extends axially only along an incomplete portion of the axial length of the bore between a start point and an end point of the incomplete portion, wherein the start point is axially spaced downstream from a leading edge of the blade root fillet and the end point is axially spaced downstream from the start point, wherein the hole has a counterbore located upstream of the start point of the incomplete portion of the hole, the counterbore having a diameter that exceeds an initial diameter of the incomplete portion of the hole; The cold working zone is obtained by the following steps: placing a machine tool in the counterbore, the machine tool having a diameter exceeding the initial diameter of the incomplete portion of the bore; and The compressive residual hoop stress is induced in the ductile metal near the inner surface of the hole along the incomplete portion by pulling the machine tool through the incomplete portion of the hole in the downstream axial direction, cold working the ductile metal at the inner surface of the hole only along the incomplete portion of the hole so that the tensile yield strength is exceeded in the circumferential direction around the hole, and the hole is not cold worked outside the incomplete portion.

5. The centrifugal compressor wheel of claim 4, comprising a dual impeller wheel having a first impeller and a second impeller arranged back to back, the first impeller defining the first face and the second impeller defining the second face of the compressor wheel, wherein The hole includes a first counterbore adjacent the first face of the compressor wheel and extending to the start point of the incomplete portion of the hole and a second counterbore extending from the end point of the incomplete portion of the hole toward the second face of the compressor wheel.

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