Layer system, impeller, method to produce The invention refers to a layer system comprising a base material of a base element, wherein said layer system extends in a border area of said base element up to an outer surface, wherein said layer system comprises at least one layer of cladding material provided onto the base material. Further the invention relates to an impeller of a turbo-machine, in particular a turbo-compressor, in particular a centrifugal turbo-compressor, wherein said impeller comprises a base element of a base material comprising blades being arranged along a circumferential direction of a rotational axis, wherein said blades comprise a leading edge and a trailing edge. Further the invention relates to a method to produce a layer system and a method to produce an impeller respectively according to the before mentioned type. The specific focus of the invention is the avoidance of ero ^¬ sion in particular in turbo-machinery and in particular with regard to turbo-compressors. The rotating parts, especially the impellers of the turbo machine may suffer from a dramatic decrease in lifetime due to severe erosion wear. Dust parti- cles carries by a process fluid through the machine, for ex ^¬ ample so called ^x black dust' , may have hardnesses between 230 up to 600HV 0.01 (Vickers hardness test) . In combination with the high velocities of the rotating equipment especially the rotating parts of pipeline compressors are quickly damaged in particular at the leading edge and the trailing edge of impeller blades.

It is one object of the invention to reduce the erosion phe ^¬ nomenon in particular to rotating parts of turbo-machines.

In order to solve the problems outlined above the invention proposes a layer system, an impeller and a method to produce such a layer system or an impeller as according to the re- spective independent claims. The respective dependent claims refer to beneficial embodiments of the invention.

The layer system according to the invention comprises said base material and one or several of cladding material layers provided onto the base material. Further said layer system comprises a compressive residual stress layer extending from said outer surface into at least the outermost layer of clad ^¬ ding material. In case of several cladding material layers provided on a base material as part of the layer system said compressive residual stress layer may extend through several layers of cladding material from said outer surface and even extend into the base material as well. Preferably said com ^¬ pressive residual stress layer extends only in the outermost layer of cladding material.

A preferred embodiment of the invention is provided by gener ^¬ ating said layer or layers of cladding material by gas tung ^¬ sten arc welding (GTAW) .

Preferably a base material is X3CrNiMol3-4 (martensitic al ^¬ loy) . In detail the composition of X3CrNiMol3-4 (material number: 1.4313 according to EN10250) in weight percent

(wt% ) is :

Element Minimum Maximum

Carbon (C) 0 0.05

Silicon (Si) 0 0.7

Mangan (Mn) 0,5 1.5

Chromium (Cr) 12.0 14.0

Molybdenum (Mo) 0.3 0.7

Nickel (Ni) 3,5 4.5

Sulfur (S) 0 0.015

Phosphor (P) 0 0.04

Nitrogen (N) 0 0.02

Iron base Another preferred embodiment of the layer system provides that the material of at least one layer of cladding material is Stellite 21 (Stellite 2 l=commercial name (UNS=W73041, ASME/AWS= (SF) A5.21, ERCCoCr-E) , HRC=28-40)). In detail the composition of Stellite 21 in weight percent (wt% ) is

One preferred embodiment provides that at least two layers of cladding material are provided to the base material and most preferably three layers of cladding material are provided.

Preferably all three layers are of the same chemical composi ^¬ tion and most preferably all three layers are of Stellite 21.

Another preferred embodiment of the layer system is provided by giving a heat treatment of up to three hours at 570°C to the base element for at least one time to eliminate residual welding stresses. Surprisingly it was found that the subse ^¬ quent heat treatment as proposed does not impair the desira ^¬ ble high hardness of the cladding.

The most preferred application of the invention is the pro ^¬ duction of an impeller of a turbo-machine, in particular a turbo-compressor, in particular a centrifugal turbo- compressor, wherein said impeller comprises a base element of a base material comprising blades being arranged along a cir ^¬ cumferential direction of a rotational axis, wherein said blades comprise a leading edge and a trailing edge, wherein said leading edge and said trailing edge belong to a surface treatment zone, wherein at least part of said surface treat ^¬ ment zone is a layer system as described and defined in sev- eral embodiments above. Preferably these impeller blades are attached to a hub section of said base element and are ex ^¬ tending radially and/or axially from said hub section. Said impeller can be designed as a so called open type or closed type. In case of the closed type impeller flow channels are circumferentially delimited by said blades, a hub section and a shroud section being attached to the blades tips respec ^¬ tively defining the flow channels in axial-radial direction. In case of the open type impeller only the hub section defines the flow channels in axial-radial direction (see also figure 1, 2 regarding the difference) .

Preferably the layer system in particular the layer system of the impeller is produced applying the following steps:

1) machining a base element of a base material

2) defining a surface treatment zone,

3) transforming at least a part of said surface treat ^¬ ment zone into a layer system by the substeps of:

a) cladding at least one layer of cladding material to said base material of said surface treatment zone,

b) shot peening at least the area of said surface treatment zone in the area of said layer system.

It was found that the hardness of the Stellite cladding huge- ly increases after shot peening.

Preferably a further substep is conducted by:

c) heat treatment of said base element. The heat treatment of said base element after the shot peen- ing improves the residual stress in the base ele ^¬ ment down to an acceptable minimum of the one hand and on the other hand it does not impair the de ^¬ sired high hardness of the Stellite cladding. Since the heat treatment and the shot peening change the geometry of the base element a final ma ^¬ chining of the base element is preferably performed as a substep d) .

Subsequently to step 3) a further forth step can be conducted as :

4) mounting a shroud section to said base element. The shroud section is preferably welded to said base ele ^¬ ment, in particular to the blade tips of said base ele ^¬ ment. To further improve mechanical properties of the combination of the base element with the shroud section a heat treatment may be performed afterwards . Due to the changes in geometry expected from welding and heat treatment a final machining may preferably be done af ^¬ terwards. In case of the base element being an impeller of a turbo-machine preferably balancing of the impeller and overspeeding may be performed to improve operational behavior and minimize risk of any damage.

A preferred embodiment of the impeller is provided by machin ^¬ ing the base element with additional grooves respectively re ^¬ cesses compared to the ordinary impeller in the area of the surface treatment zone where the layer system according to the invention is provided. Theses recesses may be provided cumulatively or alternatively like the following:

• In particular the respective leading edges of the blades should be provided with recesses in the ar ^¬ ea, where the layer system is to be provided and the layer system should be provide there.

• In particular, in the area of the trailing edges, the trailing edge should be provided at least part ^¬ ly with a recess to fit in the layer system and the layer system should be provide there.

• In particular, in a transition area between the

blade and said hub section in proximity to the blade's trailing edge a recess to fit in the layer system should be provided to also improve the hub section erosion resistance and the layer system should be provide there.

The above mentioned attributes and other features and ad ^¬ vantages of this invention and the manner of attaining them will become more apparent and the invention itself will be better understood with reference to the following description of the currently best mode of carrying out the invention tak ^¬ en in conjunction with the accompanying drawings, wherein:

Figure 1 shows a schematic depiction of a longitudinal sec ^¬ tion through an impeller according to the invention including a shroud section,

Figure 2 shows a schematic depiction of a longitudinal sec ^¬ tion of an impeller as according to the invention without a shroud section,

Figure 3 shows a schematic flow diagram to illustrate the method according to the invention,

Figure 4 shows a longitudinal section through a layer system according to the invention.

The same reference signs are used in different embodiments of the invention in the detailed description to identify ele ^¬ ments of identical function. Terms like axial, radial, cir ^¬ cumferential or tangential always refer to a central rota ^¬ tional axis X if not indicated otherwise.

Figure 1 and figure 2 respectively show a 2-dimensional lon ^¬ gitudinal section through an impeller according to the invention along a rotational axis X. The impeller IMP rotates dur ^¬ ing operation in a turbo-machine, in particular in a turbo- compressor TC around the rotational axis X. Said impeller IMP comprises a based element BE of a base material BM comprising blades BLA being arranged along a circumferential direc ^¬ tion CDR of said rotational axis X. These blades BLA comprise a leading edge LE and a trailing edge TE . The terms ^x leading edge LE' and ^x trailing edge TE' refer to a process fluid flow direction during operation for which the impeller IPM is fluid dynamically designed. Said leading edge LE and said trail- ing edge TE belong to a surface treatment zone STZ. A layer system LSY is provided to said surface treatment zone STZ. This layer system LSY is shown in figure 4 schematically in a longitudinal section in detail in a border area (shown as a detail in Figure 4) of said base element BE. Said layer sys- tern LSY comprises said base material BM of said base ele ^¬ ment BE and extends in said border area of said base ele ^¬ ment BE up to an outer surface OSF. In the preferred embodi ^¬ ment shown in figure 4 said layer system LSY comprises three layers of cladding material, a first layer LCM1, a second layer LCM2 and a third layer LCM3. This number of layers is an example which what was found to be advantageous.

Figure 3 shows schematically the steps of the method accord ^¬ ing to the invention to produce a layer system LSY as part of a base element BE here an impeller IMP. In particular this example of figure 3 refers to the impeller IMP but basically includes the generation of a layer system according to the invention to other parts as well preferably rotating parts of turbo-machines .

In step 0) a raw part is provided, which is subsequently ma ^¬ chined into the basic shape of an impeller IMP during steps 1), 2) . In step 2) a surface treatment zone STZ is defined. The impeller IMP is machined with additional grooves in the area of the leading edge LE and the trailing edge TE belonging to the surface treatment zone STZ of the base element BE. These additional grooves respectively recesses RE are provid ^¬ ed to avoid any protrusion due to the provision of the layer system LSY in this areas. The final impeller IMP is meant to have the same fluid dynamic properties as any conventional impeller IMP. In step 3) a) the surface treatment zone STZ defined during step 2) is transformed at least partly into said layer sys ^¬ tem LSY by the substeps of:

3) a) cladding at least one layer of cladding material LCM1- LCMn to said base material BM of said surface treatment zone STZ .

In subsequent step 3) a) a) a heat treatment is performed at 570°C for 2 hours to reduce stresses caused by the welding procedure of the cladding.

Any deformations are eliminated during machining in step 3) a) b) .

During subsequent step 3) b) shot peening in the area of said layer system LSY is performed to improve surface hardness. In case of producing a closed impeller configuration including a shroud section SRS said shroud section SRS is mounted to the blade's tips of the base element BE preferably by welding. Subsequently a heat treatment and final machining is per ^¬ formed during step 4) a) .

After not illustrated optional steps of balancing and

overspeeding the impeller IMP is mounted during assembly in a turbo-compressor TC in step 5) .

Between all these steps of the method several examinations may be performed to detect any material defects like cracks, in particular magnetic particle examinations can be done as non-destructive examination procedures.

The resulting impeller IMP including the layer system LSY applied by gas tungsten arc welding cladding of Stellite 21 powder onto the base element BE or base material X3CrNiMol3-4 results in an erosion resistant rotating part having partly a high hardness of approximately 690HV0.01 after heat treat ^¬ ment. In these critical areas of the surface treatment zones STZ the impeller surface is therefore harder than the maximum particle hardness of approximately 600hv0.01.