METHOD OF INTRODUCING RESIDUAL COMPRESSIVE STRESSES INTO A SHAFT

Abstract

The invention relates to a method of introducing compressive residual stresses into shaft notches of a shaft which is configured as a stepped shaft having successive stages having a different diameter. Diameter transitions or notch regions are located between each two adjacent stages. The diameter transitions or notch regions are quenched in a controlled manner as part of a heat treatment of the shaft.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2007/051744, filed Feb. 23, 2007 and claims the benefit thereof. The International Application claims the benefits of European application No. 06011627.4, filed Jun. 6, 2006, both of the applications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method of introducing residual compressive stresses into shaft notches of a shaft which is configured as a stepped shaft having successive stages of a different diameter, wherein diameter transitions or notch regions are arranged between respectively adjacent stages.

BACKGROUND OF THE INVENTION

Shafts of this type are known and are used, for example, in turbomachines, for example steam turbines, which have a low-pressure part and can therefore also be referred to as low-pressure shafts. The shaft bears rotor blades which, together with associated guide vanes, form a blade cascade through which a flow medium, for example steam, flows.

Particularly in the low-pressure part of the turbomachine, the shaft consists of a base material which is tough at low temperatures; by way of example, 2-3.5-NiCrMoV steels are used for producing the low-pressure shaft.

The flow medium partly acts as a corrosive medium on the components of the turbomachine, for example on disks in disk rotors or on regions of the shaft, particularly of the shafts of low-pressure subturbines, which are close to the surface. The influence of these corrosive media can considerably reduce the fatigue strength of the base material. However, a reduction in the fatigue strength of the base material, for example of the shafts in low-pressure subturbines, also disadvantageously reduces the service life of the shaft.

In order to solve this problem, it is known to carry out fatigue strength tests under the influence of corrosive media, with appropriate design data being provided for use in the calculation (lowered with respect to ambient air). However, it is also known to reduce operating stresses by introducing, for example, residual compressive stresses into low-pressure drivers and groove regions by means of roller-burnishing or shot-peening in the finish-machined state or given the ultimate final contour of the shaft. However, it is also possible to introduce compressive stresses in the notch-free region of the shaft by means of a suitable heat treatment. During the production of the shafts, it is necessary to maintain the strictest tolerances, it being possible for the service life of the components, particularly of the shafts, to be reduced by cracks emanating from existing diameter transitions or notch regions. The crack sensitivity, particularly at diameter transitions or notch regions, affects the service life of the shafts extremely disadvantageously (component failure).

SUMMARY OF THE INVENTION

The invention is based on the object of improving a method of introducing residual compressive stresses into shaft notches of a shaft of the type mentioned in the introduction using simple means, to the effect that the resistance to component failure as a result of corrosion and dynamic loading is considerably improved.

According to the invention, the object is achieved by virtue of the fact that the diameter transitions or notch regions of the shaft are quenched in a controlled manner after a final tempering treatment, for example a hardening and tempering heat treatment, at tempering temperature and/or below the tempering temperature.

The shaft, particularly the shaft notches thereof, is thereby protected against a reduction in fatigue strength as a result of, for example, wet steam. In this case, in addition to the application of a protective layer, for example, a method of specifically increasing the residual compressive stresses in diameter transitions or notch regions is advantageously carried out according to the invention.

It is advantageously provided that the diameter transitions or notch regions are specifically sprayed with a cooling liquid or a quenching medium for quenching purposes. However, for the purposes of controlled quenching, it may also be provided that the shaft as a whole is transferred into a dipping bath.

It is also possible for the tempering treatment of the hardening and tempering to be followed by a separate heat treatment which has the sole aim of introducing residual compressive stresses. In order to avoid influencing the mechanical properties achieved, this expediently involves selecting a temperature which is sufficiently different from the final heat-treatment temperature but which is still high enough to achieve the desired effect.

It is possible to select any suitable medium, preferably water, as the cooling liquid or quenching medium for quenching purposes; however, it is also possible to use air/water mixtures, suitable polymers or oil and emulsions as the cooling liquid or quenching medium.

In order to ensure that a possible distortion of the component or of the shaft can be compensated for after the quenching (spraying or dipping), it is advantageous within the context of the invention if the diameter transitions or the notch regions in a heat-treatment contour are produced with an allowance provided in relation to an ultimate final contour, wherein the heat-treatment contour is removed during the production of the ultimate final contour after the quenching. After the finish-machining or the production of the ultimate final contour, the provided allowance means that sufficiently high residual compressive stresses are maintained in the shaft surface, specifically in the transition radii (diameter transitions or notch regions), with a defined action at depth.

It is expedient within the context of the invention if the allowance has a magnitude of at most from 10 to 40 mm on final use in relation to the final contour of the shaft.

It is advantageously provided that the heat-treatment contour of the diameter transitions or notch regions has a radius with a magnitude R of from 25 to 50 mm. The radii (diameter transitions or notch regions) of the heat-treatment contour of the shaft are accordingly configured specifically with a defined dimension as a function of the residual compressive stresses and depth distribution required in the final contour.

Overall, the method according to the invention reduces the local stress load during shutdown and during operation of the shaft. In addition, crack sensitivity is reduced at radii or diameter transitions, and this leads to an improved or increased service life of the shaft or of the component treated in accordance with the invention. As a result of specifically setting residual compressive stresses of from −100 to −400 MPa at the shaft surface, particularly in the diameter transitions or transition radii as a result of the specific quenching of the shaft, even relatively large defects which are close to the surface of the treated component or of the shaft may be admissible, and therefore the shaft as a whole can be produced at lower cost since stricter tolerances for defects which may arise from the production process no longer necessarily have to be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous refinements of the invention are disclosed in the dependent claims and the description of the FIGURE below:

FIG. 1, the only FIGURE, shows a basic illustration of a shaft for a low-pressure part of a turbomachine or of a steam turbine.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a detail of a shaft 1 which is configured as a stepped shaft having successive stages 2 of a different diameter D1 to D4 in relation to a mid-axis X, with four stages 2 being illustrated by way of example. The exemplary shaft 1 illustrated is a component of a low-pressure part of a turbomachine, for example of a low-pressure subturbine of a steam turbine. The shaft 1 is produced, for example, from a material which is tough at low temperatures; for example, 2-3.5-NiCrMoV steels are used for producing the low-pressure shaft. However, it is of course also possible for the shafts to be produced from other materials or material combinations.

Diameter transitions 3 or notch regions are arranged between each two adjacent stages 2. The diameter transitions 3 are designed in relation to the mid-axis X to be slightly curved or convex with a radius R in the direction toward the mid-axis X.

In order to achieve a specific setting of residual compressive stresses of from −100 to −400 MPa at the shaft surface, particularly at the diameter transitions 3 or the transition radii, said diameter transitions or the notch regions are quenched in a controlled manner as part of a heat treatment or after heating of the shaft.

The diameter transitions 3 or notch regions are preferably quenched in a controlled manner after a final tempering treatment at tempering temperature. A subsequent, separate heating and quenching after the tempering as a separate process step is of course also possible.

In the exemplary embodiment illustrated in FIG. 1, a cooling liquid or a quenching medium is sprayed onto the diameter transitions 3 for the purpose of controlled quenching after the final tempering treatment at tempering temperature, and this is illustrated by means of the fan-shaped spray jets 4. It is possible to select any suitable medium, preferably water, as the cooling liquid or quenching medium for quenching purposes; however, it is also possible to use air/water mixtures, suitable polymers or oil and emulsions. The shaft 1 can however also be dipped as a whole.

In the exemplary embodiment illustrated in principle in FIG. 1, the shaft 1 is configured with a heat-treatment contour 6. In relation to a final contour (dashed line 7, illustrated in exaggerated fashion for illustration purposes), that is to say of a finish-machined shaft 1 with its final contour finish-machined for installation in the low-pressure part of the steam turbine, the heat-treatment contour 6 has an allowance 8 of at most from 10 to 40 mm in the respective shaft radius r1 to r4 (r=D/2).

The allowance 8 of the shaft 1 in the heat-treatment contour 6 is therefore specifically increased for the hardening and tempering heat treatment (tempering treatment) by at most 10 to 40 mm, preferably in the respective shaft radius r or the respective diameter transition 3, with respect to the final end contour 7. This ensures that a possible distortion of the shaft 1 can still be compensated for after the quenching (spraying or dipping).

It is also possible to carry out machining, for example in the case of double tempering treatment. In this case, the allowances can then be adapted or machining may also be effected when a separate heat treatment for producing residual (compressive) stresses is carried out after the tempering treatment from the hardening and tempering.

After the finish-machining (production of the ultimate final contour), this maximum allowance 8 means that sufficiently high residual compressive stresses are maintained in the shaft surface and specifically in the transition radii or diameter transitions 3, with a defined action at depth. In the heat-treatment contour 6, the radii R have a magnitude of R of approximately equal to 25 to 50 mm.

The respective transitions from the diameter transitions 3 to the respective stages 2 are illustrated in exaggerated fashion in the exemplary embodiment and are, of course, correspondingly machined at least for producing the ultimate final contour.

Claims

1-6. (canceled)

7. A method for introducing a residual compressive stress into a shaft having successive stages of different diameters, comprising:

arranging a diameter transition between adjacent stages of the successive stages; and

controlled quenching the diameter transition after a final tempering treatment as part of a hardening and tempering heat treatment.

8. The method as claimed in claim 7, wherein the diameter transition is sprayed with a quenching medium for the quenching.

9. The method as claimed in claim 7, wherein the diameter transition is cooled by a dipping operation for the quenching.

10. The method as claimed in claim 7, wherein the diameter transition in a heat-treatment contour of the shaft is produced with an allowance to an ultimate final contour of the shaft.

11. The method as claimed in claim 10, wherein the heat-treatment contour is removed during producing the ultimate final contour of the shaft or after the quenching.

12. The method as claimed in claim 10, wherein the allowance is in a magnitude of at most 10 to 40 mm.

13. The method as claimed in claim 11, wherein the heat-treatment contour of the diameter transition has a radius with a magnitude of 25 to 50 mm.

14. The method as claimed in claim 7, wherein the residual compressive stress is introduced into a shaft notch of the shaft.

15. The method as claimed in claim 7, wherein the shaft is used in a turbomachine.

16. A method for introducing a residual compressive stress into a shaft notch of a shaft having successive stages of different diameters, comprising:

arranging a notch region between adjacent stages of the successive stages; and

controlled quenching the notch region after a final tempering treatment as part of a hardening and tempering heat treatment.

17. A turbomachine, comprising:

a shaft comprising: successive stages of different diameters; a diameter transition arranged between adjacent stages of the successive stages that is controlled quenched after a final tempering treatment as part of a hardening and tempering heat treatment.