EP0507131A1 - Turbine blade protected against water droplets abrasion and method of manufacture - Google Patents

Abstract

The invention relates to the surface-hardening of machine components. The object of the invention of indicating and producing a turbine blade which is better protected against erosion by water droplets, is achieved according to the invention, inter alia, in such a way that, in the turbine blade shown in the drawing, the constant surface harness of a very hard layer (2) corresponds to the maximum hardness of the particular steel, the depth of this very hard layer (2) is 0.1 to 0.9 mm, the total hardening depth is 0.7 to 3.5 mm and the very hard layer (2) extends at least up to that point (5) in the vicinity of the intake edge where the impingement direction (6) of the most damaging droplet size fraction is parallel to the tangent to the surface. According to the invention, this turbine blade is produced by means of a high-energy surface-hardening process. A main possible use of the solution according to the invention concerns the rotor blades, greatly stressed by droplet abrasion, of the last stages of steam turbines. <IMAGE>

Description

The invention relates to the surface hardening of machine components. Objects for which their application is possible and expedient are all machine components made of martensitic chrome steels which are subject to dripping or cavitation and which are used at operating temperatures below 250 ° C. The invention can be used particularly advantageously to protect end-stage rotor blades of steam turbines which are heavily erosively loaded by the impact of drops.

The blades of steam turbines are subject to a constant impact of water droplets during their operation, which leads to premature wear and thus destruction of the blades by drop impact erosion.
It is generally known in the specialist field to increase the wear resistance of blades made of martensitic chromium steels by means of flame-hardened leading edges (lit. ).
The lack of such hardened blades is that their wear resistance is too low. The reason for this results from the fact that the surface hardness is too low at 500 to 550 HV.
The flame hardening process is usually carried out at austenitizing temperatures of approx. 1000 to 1100 ° C. These austenitization temperatures only lead to a relatively low release of carbon from the (Fe, Cr) mixed carbides during the austenitization times that can be achieved in terms of process engineering. The lack of the method therefore consists in too little hardening on the surface.

Higher austenitization temperatures are prohibited because of the grain coarsening that occurs during carbide dissolution during the austenitization times that can be achieved and the hardening of the blade leading edge that increases rapidly with the peak temperature, which leads to a loss of toughness of the blade and a deterioration in the residual stress state at the leading edge. The cause of the defect is therefore the insufficient power density that can be entered.
According to CH-PS-564089, a turbine blade with an induction hardened leading edge is known. In accordance with the higher power density possible with induction hardening, higher austenitizing temperatures are possible without damaging consequences, but are still in the γ phase area. Leading edges hardened in this way accordingly have higher surface hardnesses. However, it has the disadvantage that these higher hardnesses of about 550 to 670 HV cannot be achieved over the entire zone at risk of erosion. The shortcoming of the method is that the power density entered cannot be adapted to the locally different heat dissipation conditions, as a result of which the local hardening temperature cannot be kept constant over the entire width of the zone at risk of erosion as required.
The reason for this is that the inductor can only be designed optimally for one contour of the zones near the leading edge, but the contour changes greatly over the length of the airfoil.
It is also known to use laser or electron beams to harden the leading edges (for example: V. Bedogni; M. Cantello; W. Cerri; D. Cruciani; R. Festa; G. Mor; F. Nenci; FP Vivoli: "Laser and Electron Beam Surface Hardening of Turbine Blades ", in: Proceedings of the International Conference." Laser Advanced Materials Processing 87 "; Osaka, May 1987, pp. 567-572). Although constant hardness values can be achieved over the entire back of the blade, it is disadvantageous that no hardnesses higher than 500-580 HVo.1 on samples (material X22 CrMoV 12.1. Cf.: M. Roth; M. Cantello: "Laser Hardening of a 12 ./. Cr-Steel ", Proceedings of 2nd ^and International Conference" Laser in Manufacturing "(ed by MF Kimmit) March 26-28, 1985, pp. 119-128) or 500-580 HV1 at blade leading edges (material AISI 403) can be achieved.
However, it is particularly disadvantageous that the hardness has dropped to 500-530 HVo, 1 or 480-560 HV1 at a depth of, for example, 0.2 mm. The disadvantageous effect of the drop in hardness results from the fact that, on the one hand, Drop impact load with the drop sizes and drop impact velocities occurring in the low-pressure part of steam turbines, the maximum of the reference stress lies at a fairly large depth of 0.05 mm to 0.2 mm and, on the other hand, the wear with intensive drop impact load only after a typical surface roughness with roughness depths of a few has developed 10 µm to a few 100 µm reaches a steady state. A relatively large hardness gradient that is too steep and also begins on the surface therefore prevents a steady state of wear even with sufficient surface hardness.
The cause of the drop in hardness is that too little carbon is released from the (Fe, Cr) mixed carbides in the depths in question.
The lack of the process is therefore that only in the immediate vicinity of the surface sufficient carbon is released from the (Fe, Cr) mixed carbides, which is necessary to achieve high hardness. The reason for this results from the fact that the temperature gradient of the indicated short-term curing is so steep that the peak temperature of the local temperature-time cycle is already too low at the depths in question.

The aim of the invention is to provide a turbine blade that is better protected against drop impact erosion and to propose a method for its production.

The invention has for its object to provide a hardening zone formation for the leading edges of turbine blades, in which there is a sufficiently high surface hardness for the typical conditions of the drop impact load in the output stages of steam turbines and in which, even after the surface roughening typical of the load has been formed, the maximum of the reference stress is still in one sufficiently hard area.
According to the invention, this object is achieved with a turbine blade made of martensitic chromium steel which is protected against drip erosion and has a briefly hardened leading edge, the erosion-protected zone of which has a constant surface hardness over the entire area of the blade back side which is at risk of erosion, as shown in claims 1 to 4.
The expedient embodiment of the invention described in claim 2 makes advantageous use of the fact that usually the strength and area of the drop impact load increases sharply towards the tip of the blade, while the cyclic load decreases, so that with sufficient wear resistance the decrease in the hardening zone width improves the residual stress state at the leading edge and the toughness reserves of the blade can be increased.

In addition, the task is solved by a short-term hardening process for martensitic chromium-alloyed turbine blade steels, in which a peak temperature of the local temperature-time cycle is reached even at the required depth of 0.1 mm to approximately 0.9 mm, with which a complete temperature is achieved Carbide dissolution takes place without a property-deteriorating austenite grain coarsening already taking place on the surface.
The procedure according to the invention is as specified in claims 5 to 10.
When designing the method according to claim 6, the fact is advantageously taken into account that if the airfoil thickness falls below about 3 mm, the cooling rate already decreases in the temperature range in which austenite coarsening can take place due to poorer heat dissipation options.

It is advantageous in the design of the method according to claim 7 that an adjustment of the optimal exposure time τ _s to the necessary hardening zone width is achieved.
The advantage of the process design according to claim 8 is that this unusually small amplitude ratio allows the setting of an almost constant surface temperature T _s across the laser track within the very hard zone even under the conditions of very asymmetrical heat dissipation at the blade leading edge.
The invention is explained in more detail in the following embodiment.
In the accompanying drawing (Figure 1), a schematic representation of the location and structure of the edge protection according to the invention is shown.

Example 1:

The final stage blade of a 100 MW turbine, which is subject to dripping impact, is to be provided with a wear-resistant leading edge. The expected erosion zone width is 18 mm at the tip of the blade and decreases too little towards the blade root.

The hardening system used consists of a CO₂ cross-current laser with a nominal power of 5 kW, a motion machine that is used to realize the relative movement between the laser beam and the leading edge and whose control allows simultaneous movement in at least 4 coordinates. The beamforming system consists of an off-axis parabolic mirror with a focal length of f = 300 mm and a resonance scanner located in the partially focused beam, which allows the beam to oscillate at a frequency of 210 Hz perpendicular to the beam feed direction.
The area to be hardened is covered with an approximately 80 µm thick layer of black board paint or similar. provided that serves as an absorbent for the laser radiation.
Based on the previously measured thickness of the airfoil profile - depending on the distance to the blade tip - a suitable set of irradiation parameters (laser power, beam defocusing, oscillation amplitude, feed rate) is determined based on nomograms, which leads to the formation of the hardening zone according to the invention at every point of the leading edge.
The contour control program is then created by scanning the blade leading edge. The inclination of the area to be hardened relative to the laser beam is chosen so that, in cooperation with the set power density distribution of the laser beam across the feed direction, a constant temperature is established in the most wear-prone zone (the later very hard layer 2). The power density distribution in the laser beam can be varied sufficiently by the choice of the ratio of the oscillation amplitude of the beam A to the radius of the beam r.

The laser beam hardening is carried out under the following parameters: laser power P _L = 2.60 kW impinging on the turbine blade; laser power absorbed by the turbine blade P _a = 2.08 kW; Laser beam diameter on the blade surface 2.r = 9.6 mm; Vibration amplitude of the laser beam A = 8.9 mm, amplitude ratio A / r = 1.85; Initial speed of the laser beam at the blade tip V _BO = 242 mm / min, beam shape stretching $\text{(d ⊥ V) / (d ∥ V) = 2.85.}$

The laser beam hardening with these parameters leads to the following values of the temperature field in the vicinity of the leading edge 5: maximum temperature of the Temperature-time cycle on the back of the blade over a width of 16 mm: T _smax ≈ 1400 - 1440 ° C, laser beam exposure time: τ _s T = 2.38s.

The turbine blade hardened with these parameters has a hardening zone at the leading edge 5 of the following geometrical dimensions, hardness and hardness distribution: Width of the entire hardening zone 1 on the blade back (for the position of the hardening zones see FIG. 1) at the blade tip: 20.2 mm; Width of hardening zone 1 at a distance of 150 mm from the tip of the blade: 18.7 mm; Total hardening depth 4: 1.17 mm to 2.9 mm depending on the distance to the leading edge, width of the entire hardening zone 1 on the blade belly side 7: 2.8 mm near the blade tip; Surface hardness in the very hard layer 2: 700 HV _0.05 ± 35 HV _0.05 ; Depth of the very hard layer 2: 0.1 mm to 0.45 mm, decreasing with increasing distance from the leading edge, tapering at the leading edge approximately at the location of the greatest curvature of the airfoil profile. The hardness gradient in the very hard layer 2 is ≲ 30 HV / mm. The width of the very hard layer 2 in the vicinity of the blade tip is approximately 19 mm. The length of the entire hardening zone 1 amounts to 185 mm. It leaves the blade at an exit angle of 45 °.
The reproducibility of the desired hardness-depth curve according to the invention is very good. Corresponding to the decreasing wear intensity, both the depth of the very hard layer 2 and the total hardening depth 4 decrease with increasing distance from the leading edge. The location of the reference stress maximum is less than a third of the depth of the very hard layer 2. A property-worsening coarsening of the austenite grain size does not occur.
Compared to the prior art, at least 100 to 150 HV higher surface hardness is achieved. The difference in hardness is even greater at the depth of the reference stress maximum.
This ensures considerably better wear resistance of the turbine blades hardened according to the invention.
Another advantage over flame-hardened and, in a reduced form, also over induction-hardened blades is the more reproducible and more stress-resistant design of the hardening zone geometry along the leading edge. Among other things, it also leads to a much better reproducibility of the setting of a compressive residual stress state close to the leading edge along the leading edge over the entire hardened blade length.

List of the reference symbols and terms used

1

entire hardening zone

2nd

very hard layer

3rd

at 2 subsequent layers

4th

Total depth of hardening

5

Leading edge

6

Direction of impact of the most harmful droplet size fraction

7

Location on the bucket belly side

8th

Tangent to the surface at location 7

9

minimal bending moment of the blade cross section

10th

Blade cross section

Q _L

Cross section of the entire hardening zone 1

Q _s

Cross section of the hardened blade profile section

Claims (10)

Drop impact-resistant turbine blade made of martensitic chrome steel with a short-term hardened leading edge, whose erosion-protected zone has a constant surface hardness over the entire area of the blade back side, which is at risk of erosion,
characterized in that this constant surface hardness corresponds to the maximum hardness that can be achieved with the respective steel with optimal parameters of the short-term hardness cycle, - This hardness depending on the carbon content and increasingly with a carbon content of 0.1 - 0.13% C a hardness of 580 - 620 HV _0.05 and at 0.18 - 0.24% C a hardness of 600 - 750 HV reached _0.05 , - The entire hardening zone (1) from a very hard layer (2) with a first hardness plateau, which reaches this maximum hardness and with a depth of 0.1 mm to 0.9 mm chosen according to the intensity of wear and the depth of the reference stress maximum the hardness gradient is around 0 to 100 HV / mm and at its lower edge, depending on the carbon content of 0.1 to 0.13% C, at least hardnesses of 550 - 600 HV _0.05 and with a carbon content of 0.18 to 0 , 24% still has hardnesses of 580 to 680 HV _0.05 - And there is an adjoining layer (3), which reaches a thickness of 0.4 mm to 2.0 mm and with a significantly larger hardness gradient near the surface and at least a hint of a second hardness plateau with a hardness of 350-540 HV _0.05 having, - The total hardening depth (4) is 0.7 mm to 3.5 mm depending on the wear intensity, the very hard layer (2) extends at least to the location (5) in the vicinity of the leading edge where the direction of impact (6) of the most harmful droplet size fraction lies parallel to the tangent to the surface, - The hardening zone (1) extends from the blade tip in the direction of the blade root over the entire length of the leading edge (5), which is at risk of erosion, to a point of minimal or at least non-critical cyclical loading of the blade and the blade there leaves at an exit angle to the leading edge of 30 to 65 °. Drop impact erosion-protected turbine blade according to claim 1,
characterized in that the width of the hardening zone (1) is adjusted according to the width of the erosion zone which changes along the leading edge. Drop impact erosion-protected turbine blade according to claim 1,
characterized in that the total hardening depth (4) in the case of blades which are subject to severe cyclical or stress corrosion cracking at least reaches the location (7) on the blade belly side where the tangent (8) to the surface perpendicular to the direction of the minimum bending moment (9) of the Blade cross section (10). Drop impact erosion-protected turbine blade according to claim 1,
characterized in that the edge of the hardening zone (1) on the blade rear side around a damper wire hole with the radius R _{D describes} an arc of a circle with the radius R _S , where:

${\text{R}}_{\text{D}}{\text{+ 3 mm ≲ R}}_{\text{S}}{\text{≲ R}}_{\text{D}}\text{+ 10 mm}$

A process for the production of the turbine rotor blade protected against drip erosion of claims 1 to 4 by means of a high-energy surface hardening process
characterized in that - The temperature-time cycle of short-term hardening is carried out so that at the end of an energy exposure period τ _s of 0.3 s ≲ τ _s ≲ 3 s a surface temperature T _smax considerably above the γ area of 1380 ° C ≲ T _{s max} < 1430 ° C is reached, via a corresponding combination of power density, feed rate and beam dimension in the feed direction, the surface temperature gradient is set so that at a depth t of 0.1 mm ≲ t ≲ 0.9 mm still a maximum temperature of the local temperature-time -Cycle T _L of T _L ≈ T _{s max} - (B.60 K + C.τ _s .36 K / s), whereby the constants B and C reach about the value 1, but the exact value depends on the chemical composition and the starting structure of the steel is selected, - And that the power density distribution across the beam feed direction is set so that the local surface temperature T _s at no point on the surface of the very hard layer (2) falls below the value T _{s max} by more than 50 K. Method according to claim 5,
characterized in that, for hardening of airfoil sections, the mean thickness of which is less than 3 mm, at a constant surface temperature T _{s max} the maximum possible energy exposure time τ _s max corresponding to the ratio of the desired cross section Q _{L of} the hardening zone (1) to the cross section Q _{S of} the airfoil section according to τ _{s max} = 3 s. L - KQ _L / Q _{s is} reduced, where K and L are constants depending on the selected steel with a value close to 1. Method according to claim 5,
characterized in that when a laser beam or electron beam hardening process is used, the beam is shaped such that the ratio of the expansion of the beam perpendicular to the beam feed direction d ⊥v to the expansion of the beam in the beam feed direction d ∥v in the range 2 ≲ (d ⊥v) / ( d ∥v) ≲ 8 is selected. Method according to claims 5 and 7,
characterized in that the beam formation is produced by the rapid oscillation of the laser beam or electron beam transversely to the feed direction, and in the case of the laser beam with a sinusoidal law of motion, depending on the power density distribution of the non-deformed laser beam, a ratio of the oscillation amplitude of the beam A to the radius r of the beam from

1.3 ≲ A / r ≲ 2.1 is selected. Method according to claims 5, 7 and 8,
characterized in that the ratio A / r of the oscillation amplitude to the beam radius r during surface hardening is continuously changed in accordance with the change in the airfoil profile close to the leading edge. Method according to claims 5 and 9,
characterized in that the beam shaping is generated by the rapid oscillation of the laser beam or electron beam transversely to the feed direction, in both cases the instantaneous beam power or the law of motion of the beam oscillation according to the inclination of the beam axis to the local surface normal and the heat dissipation conditions of the blade edge profile close to the leading edge and selected accordingly Change in the surface normals and the heat dissipation conditions along the leading edge is changed during the short-term hardening.

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