METHOD FOR MANUFACTURING A TURBINE WHEEL

Abstract

A method for manufacturing a turbine wheel comprising casting the turbine wheel from an austenitic nickel-chromium-based superalloy, subjecting the cast turbine wheel to hot isostatic pressing and then subjecting a surface of the hot isostatically pressed turbine wheel to plastic deformation, wherein said hot isostatic pressing is effected at a pressure of 98 to 200 MPa and a temperature of 1160 to 1220 C. for a time period of 225 to 300 minutes. There is further described a hot isostatically pressed cast turbine wheel manufactured from an austenitic nickel-chromium-based superalloy, the turbine wheel having a plastically deformed surface; and a turbocharger incorporating such a turbine wheel.

Claims

1. A method for manufacturing a turbine wheel comprising casting the turbine wheel from an austenitic nickel-chromium-based superalloy, subjecting the cast turbine wheel to hot isostatic pressing and then subjecting a surface of the hot isostatically pressed turbine wheel to plastic deformation, wherein said hot isostatic pressing is effected at a pressure of 98 to 200 MPa and a temperature of 1160 to 1220 C. for a time period of 225 to 300 minutes.

2. A method according to claim 1, wherein said hot isostatic pressing is effected at a pressure of 98 to 108 MPa.

3. A method according to claim 1, wherein said hot isostatic pressing is effected at a temperature of 1190 to 1210 C.

4. A method according to claim 1, wherein said hot isostatic pressing is effected for a time period of 225 to 255 minutes.

5. A method according to claim 1, wherein said turbine wheel is cooled to a temperature of around 18 to 25 C. after said hot isostatic pressing and before said plastic deformation.

6. A method according to claim 5, wherein said turbine wheel is cooled at a rate of less than or equal to around 100 C. per minute after said hot isostatic pressing and before said plastic deformation.

7. A method according to claim 5, wherein said turbine wheel is cooled at a rate of less than or equal to around 10 C. per minute after said hot isostatic pressing and before said plastic deformation.

8. A method according to claim 1, wherein said plastic deformation is achieved using shot peening.

9. A method according to claim 8, wherein said shot peening employs high carbon cast steel shot conforming to SAEJ827.

10. A method according to claim 8, wherein said shot has a minimum size of 5070 to 5240 in accordance with SAEJ444.

11. A method according to claim 8, wherein said shot has a minimum size of S110 in accordance with SAEJ444.

12. A method according to claim 8, wherein said shot peening is effected at an intensity to achieve an Almen A strip arc height of 0.127 to 0.305 mm measured in accordance with SAEJ442.

13. A method according to claim 8, wherein said shot peening is effected at an intensity to achieve an Almen A strip arc height of 0.127 to 0.203 mm measured in accordance with SAEJ442.

14. A hot isostatically pressed cast turbine wheel manufactured from an austenitic nickel-chromium-based superalloy, the turbine wheel having a plastically deformed surface.

15. A turbine wheel according to claim 14, wherein the plastically deformed surface of the turbine wheel exhibits a residual compressive stress of 1000 to 1500 MPa at a depth of 25 to 90 microns below said surface of the turbine wheel.

16. A turbine wheel according to claim 14, wherein the plastically deformed surface of the turbine wheel exhibits a residual compressive stress of 1100 to 1500 MPa at a depth of 30 to 60 microns below said surface of the turbine wheel.

17. A turbine wheel according to claim 14, wherein the plastically deformed surface of the turbine wheel exhibits a residual compressive stress of 500 to 1200 MPa at a depth of 100 to 190 microns below said surface of the turbine wheel.

18. A turbine wheel according to claim 14, wherein the plastically deformed surface of the turbine wheel exhibits a residual compressive stress of 600 to 900 MPa at a depth of 112 to 160 microns below said surface of the turbine wheel.

19. A turbocharger comprising: a housing; a turbine wheel supported on a shaft within said housing for rotation about a turbine axis; and a compressor wheel supported on said shaft within said housing, wherein said turbine wheel is a hot isostatically pressed cast turbine wheel manufactured from an austenitic nickel-chromium-based superalloy, the turbine wheel having a plastically deformed surface.

20. A turbocharger according to claim 19, wherein plastically deformed surface of the turbine wheel exhibits at least one of a residual compressive stress of 1000 to 1500 MPa at a depth of 25 to 90 microns below said surface of the turbine wheel, a residual compressive stress of 1100 to 1500 MPa at a depth of 30 to 60 microns below said surface of the turbine wheel, a residual compressive stress of 500 to 1200 MPa at a depth of 100 to 190 microns below said surface of the turbine wheel, or a residual compressive stress of 600 to 900 MPa at a depth of 112 to 160 microns below said surface of the turbine wheel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Specific embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0031] FIG. 1 is an axial cross-section through a variable geometry turbocharger incorporating a turbine wheel according to a first aspect of the present disclosure;

[0032] FIG. 2 is a flow diagram illustrating steps involved in the manufacture of a turbine wheel by a method in accordance with an embodiment of the present disclosure; and

[0033] FIG. 3 is a graph of residual compressive stress against depth below a surface of turbine wheels subjected to (a) HIP alone, (b) HIP and shot peen combined in accordance with the present disclosure, (c) shot blasting alone, and (d) shot and sand blasting.

DETAILED DESCRIPTION OF EMBODIMENTS

[0034] FIG. 1 illustrates a variable geometry turbocharger comprising a housing incorporating a variable geometry turbine housing 1 and a compressor housing 2 interconnected by a central bearing housing 3. A turbocharger shaft 4 extends from the turbine housing 1 to the compressor housing 2 through the bearing housing 3. A turbine wheel 5 is mounted on one end of the shaft 4 for rotation within the turbine housing 1, and a compressor wheel 6 is mounted on the other end of the shaft 4 for rotation within the compressor housing 2. The shaft 4 rotates about turbocharger axis 4a on bearing assemblies located in the bearing housing 3.

[0035] The turbine housing 1 defines an inlet volute 7 to which gas from an internal combustion engine (not shown) is delivered. The exhaust gas flows from the inlet volute 7 to an axial outlet passage 8 via an annular inlet passage 9 and the turbine wheel 5. The inlet passage 9 is defined on one side by a face 10 of a radial wall of a movable annular wall member 11, commonly referred to as a nozzle ring, and on the opposite side by an annular shroud 12 which forms the wall of the inlet passage 9 facing the nozzle ring 11. The shroud 12 covers the opening of an annular recess 13 in the turbine housing 1.

[0036] The nozzle ring 11 supports an array of circumferentially and equally spaced inlet vanes 14 each of which extends across the inlet passage 9. The vanes 14 are orientated to deflect gas flowing through the inlet passage 9 towards the direction of rotation of the turbine wheel 5. When the nozzle ring 11 is proximate to the annular shroud 12, the vanes 14 project through suitably configured slots in the shroud 12, into the recess 13.

[0037] The position of the nozzle ring 11 is controlled by an actuator assembly of the type disclosed in U.S. Pat. No. 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring 11 via an actuator output shaft (not shown), which is linked to a yoke 15. The yoke 15 in turn engages axially extending actuating rods 16 that support the nozzle ring 11. Accordingly, by appropriate control of the actuator (which may for instance be pneumatic or electric), the axial position of the rods 16 and thus of the nozzle ring 11 can be controlled. The speed of the turbine wheel 5 is dependent upon the velocity of the gas passing through the annular inlet passage 9. For a fixed rate of mass of gas flowing into the inlet passage 9, the gas velocity is a function of the width of the inlet passage 9, the width being adjustable by controlling the axial position of the nozzle ring 11. FIG. 1 shows the annular inlet passage 9 fully open. The inlet passage 9 may be closed to a minimum by moving the face 10 of the nozzle ring 11 towards the shroud 12.

[0038] The nozzle ring 11 has axially extending radially inner and outer annular flanges 17 and 18 that extend into an annular cavity 19 provided in the turbine housing 1. Inner and outer sealing rings 20 and 21 are provided to seal the nozzle ring 11 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the nozzle ring 11 to slide within the annular cavity 19. The inner sealing ring 20 is supported within an annular groove formed in the radially inner annular surface of the cavity 19 and bears against the inner annular flange 17 of the nozzle ring 11. The outer sealing ring 21 is supported within an annular groove formed in the radially outer annular surface of the cavity 19 and bears against the outer annular flange 18 of the nozzle ring 11.

[0039] Gas flowing from the inlet volute 7 to the outlet passage 8 passes over the turbine wheel 5 and as a result torque is applied to the shaft 4 to drive the compressor wheel 6. Rotation of the compressor wheel 6 within the compressor housing 2 pressurises ambient air present in an air inlet 22 and delivers the pressurised air to an air outlet volute 23 from which it is fed to an internal combustion engine (not shown).

[0040] The turbine wheel 5 shown in FIG. 1 was manufactured as explained below with reference to FIG. 2. In step 201 the turbine wheel is cast, using a conventional investment casting process, from a suitable austenitic nickel-chromium-based superalloy, such as Inconel 713C. In step 202, the cast turbine wheel is subjected to hot isostatic pressing at a pressure of 1035 MPa and a temperature of 120010 C. for a time period of 24015 minutes. In step 203, the turbine wheel is cooled to 18 to 25 C. at a rate of less than 10 C. per minute. In step 204, a surface of the turbine wheel is shot peened using high carbon cast steel shot conforming to SAEJ827, having a minimum size of S110 in accordance with SAEJ444 and at an intensity to achieve an Almen A strip arc height of 0.127 to 0.203 mm measured in accordance with SAEJ442. In step 204, it is preferred that as close as possible to 100% of the external surface of the turbine wheel is subjected to shot peening, except for the back face weld boss area of the turbine wheel, which ideally is not shot peened, for example by some form of suitable masking applied to that region of the turbine wheel prior to shot peening.

[0041] SAE J827 is the international standard which describes the chemical composition, hardness, microstructure and physical characteristic requirements for high carbon steel shot to be used for shot peening applications. The properties of shot conforming to SAEJ827 and having a minimum size of S110 in line with SAEJ444 are set out below.

[0042] Chemical Composition:

TABLE-US-00001 Element % Carbon 0.8-1.2 Manganese 0.6-1.2 Silicon 0.4 minimum Sulphur 0.05 maximum Phosphorous 0.05 maximum

[0043] Microstructure: Uniform tempered martensite.

[0044] Hardness; SAE J827 specification. 40 to 51 HRC.

[0045] Apparent Density; 7 glee minimum

[0046] Defects: To meet the requirement of ISO 11124/3 and SAEJ827.

[0047] Nominal Size: 0.30 mm

TABLE-US-00002 Tolerance Screen Number mm All pass 30 screen 0.600 10% min retained 35 screen 0.500 80% min retained 50 screen 0.300 90% min retained 80 screen 0.180

[0048] The residual stress at different depths below the surface of turbine wheels subjected to (a) HIP alone, (b) HIP and shot peening combined in accordance with the present disclosure, (c) shot blasting alone, and (d) shot and sand blasting, is illustrated in FIG. 3. As can be seen, across a wide range of depths, from 16 to 224 microns (m), the turbine wheel manufactured according to the present disclosure exhibited a residual stress greater in magnitude than the three turbine wheels manufactured using alternative methods involving just HIP or plastic deformation alone.

[0049] A comparative test of turbine wheels manufactured using different methods was carried out to investigate the approximate high cycle fatigue (HCF) life. The results are presented below. The results for the turbine wheel manufactured according to the method of the present disclosure are underlined and clearly demonstrate an improvement in fatigue durability.

TABLE-US-00003 ~Mean Manufacturing ~Min life ~Max life life Method (hrs) (hrs) (hrs) No. data points No HIP or Shot Peen 1.5 18 10 30 HIP Alone 2 9 5.5 10 Shot Peen Alone <1 12 6.5 10 HIP&ShotPeen 4 40 22 20

[0050] It will be appreciated that numerous modifications may be made to the preferred embodiments described above without departing from the underlying inventive concepts defined in the various aspects of the present disclosure. Moreover, any one or more of the above described preferred embodiments could be combined with one or more of the other preferred embodiments to suit a particular application.

[0051] The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred in the description suggest that a feature so described may be desirable, it may nevertheless not be necessary and embodiments lacking such a feature may be contemplated as within the scope of the invention as defined in the appended claims. In relation to the claims, it is intended that when words such as a, an, at least one, or at least one portion are used to preface a feature there is no intention to limit the claim to only one such feature unless specifically stated to the contrary in the claim. When the language at least a portion and/or a portion is used the item can include a portion and/or the entire item unless specifically stated to the contrary.