CH654594A5 - TURBINE BLADE MATERIAL OF HIGH STRENGTH AGAINST CORROSION FATIGUE, METHOD FOR THE PRODUCTION THEREOF AND ITS USE. - Google Patents

Abstract

1. Turbine blade material of high corrosion fatigue strength with a two-phased mixed structure of austenite and ferrite or austenite and martensite, consisting of Carbon = 0.005 to 0.06% by weight Chromium = 18 to 30% by weight Molybdenum = 1 to 6% by weight Nickel = 2 to 6% by weight Manganese = 4 to 8% by weight at least one of the elements copper, titanium and aluminium, the content of each individual element being at least 0.2% by weight and the total content of copper, titanium and aluminium being 0.5 to 4% by weight, and optionally 0.2 to 1.5% by weight of nitrogen, the remainder being iron and unavoidable impurities, this material having a yield stress of at least 800 MPa, and elongation at break of at least 15% for a specimen length : specimen diameter ratio = 4.4 and a fatigue strength relative to alternating tensile/compressive stresses, under aerated 4N NaCl solution of pH = 5 at 80 degrees C, of at least 350 MPa without a static preload and of at least +- 250 MPa under a static preload of + 250 MPa relative to 10**8 load changes, and a notched impact strength of at least 50 J per cm**2 .

Description

The invention is based on a turbine blade material according to the preamble of claim 1, on a method for its production according to the preamble of claim 11 and on its use according to the preamble of claim 14.

Blade materials for steam turbines are subject to increased demands, particularly in the area of medium and low temperatures, in the course of many years of operating experience. At the same time, they should have high static strength, i.e. a high yield strength, a sufficient deformation reserve, i.e. have sufficiently high notch toughness and a high resistance to corrosion fatigue in the relevant temperature range in a possible aggressive atmosphere. In part, similar requirements are placed on the blade materials of turbo compressors in gas turbine plants.

It has been shown that blade damage has occurred in the low-pressure part of steam turbines, which is attributed to insufficient strength against corrosion fatigue (HJ Bohnstedt, P.-H. Effertz, P. Forchhammer and L. Hagn, Der Maschinenschaden, 51, 73, 1978; K. Yaeger, EPRI Journal, p. 44, April 1980). The ferritic or martensitic alloy steels commonly used here (13% Cr or 12% Cr / 1% Mo) probably have high static strength values (yield strength, 0.2% limit), their behavior towards dynamic stress and the simultaneous presence of aggressive media is obviously insufficient. The problem of corrosion fatigue arises in all turbomachinery, where condensation and consequently a concentration of the impurities present in the gas phase in the liquid phase must be expected as a result of water droplet formation.

Attempts have been made to solve the problem of corrosion fatigue in two ways. On the one hand, it is possible to reduce the dynamic stress (vibrations) of the blade by appropriate design. On the other hand, higher requirements must be placed on the degree of purity of the gaseous media in order to keep them as free as possible from pollutants. Such measures, however, proved to be very complex and expensive and often did not achieve the hoped-for success. In particular, even the purity of the steam or gas, however high it may be, is capable of local enrichment and thus saturation

5

10th

15

20th

25th

30th

35

40

45

50

55

bO

65

3rd

654 594

cannot be prevented with certainty in an aqueous solution, it is therefore practically always possible to expect an orrosive attack.

Another way of eliminating the above-mentioned difficulties is on the material technology side. Attempts have been made to combine good chemical resistance with sufficiently high mechanical strength (K.) etert, W. Bertram and H. Buhl, Werkstoffe und Korrosion, 1.439, 1980). However, the goal set to be able to recommend a material with high stability, high toughness and high fatigue strength in a corrosive environment was not achieved:

The corrosion-resistant steels can basically be divided into 3 groups: ferritic, ferritic-austenitic and austenitic. The first two generally have a maximum yield strength of 640 MPa, the last one a limit of only 400 MPa. There is therefore a need for blade materials that meet all three of the above conditions.

The invention is based on the object of specifying a turbine blade material and a corresponding manufacturing process which, in the finished product, guarantee a high level of strength against corrosion reduction with simplicity and avoidance of unusual, expensive starting materials with good ductility, high yield strength and toughness.

According to the invention, this object is achieved by the features of claims 1 and 11.

The essence of the invention is that as the turbine blade material, a stainless steel with a ferritic-austenitic mixture structure, which in itself has good strength: due to corrosion fatigue, is used, otherwise insufficient mechanical properties such as yield strength and notch toughness due to special choice the -governmental composition and by precipitation hardening. This is achieved through targeted heat treatment.

The invention is explained in more detail using the following exemplary embodiments:

Execution example I:

The starting point was a stainless steel (alloy Ij of the following composition:

C.

= 0.04% by weight

Cr

= 26

% By weight

Mn

= 6

% By weight

Ni

= 4

% By weight

Mon

= 2.5

% By weight

Cu

= 3

% By weight

N

= 0.4

% By weight

Fe

= Rest

The alloy was melted in a vacuum furnace and: u cast in a cast ingot. The ingot was rolled down to a thickness of 12 mm at a temperature of approximately 1050 ° C., the decrease in cross-section being at least> 0 "o and then quenched in water from the same temperature. By quenching the workpiece the possible formation of the brittle intermetallic iron / chromium compound, the so-called a-phase, is effectively suppressed. From the plate in question, test bar blanks were machined out, the longitudinal axes of which ran parallel to the rolling direction Heat treatment for 1 to 4 hours, precipitation hardened in the temperature range from 300 to 650 ° C. A total of several specimen bars were used for tensile specimens, Charpy and impact test specimens

Fatigue fatigue tests under axial load (tension / compression) with and without preload were investigated both in air and in an aerated 4 N NaCl solution with pH = 5 at 80 ° C. All fatigue strength determinations were made using a sinusoidal axial load. The state of stress, which is particularly important for the practical evaluation in operation, was realized by additionally applying a positive static preload (tension), which corresponded to a stress (mean value of the stress) of 250 MPa.

Working example II:

A stainless steel (alloy II) with the following composition was assumed:

= 0.04% by weight

= 26% by weight

= 8% by weight

= 2% by weight

= 1% by weight

= Rest

25 After melting under vacuum, the alloy was cast and the cast ingot produced in this way was further thermomechanically processed. In a first step of hot forming, the cross-section was reduced by 75%. At the beginning of this operation, the workpiece temperature was 1250 ° C, at the end of the same still 1050 ° C. This was immediately followed by the second deformation step, which was carried out isothermally at a temperature of 1050 ° C. During this step, the cross-section of the workpiece was totally reduced by a further 50%, based on the cross-section after the first operation. Now the semi-finished product thus produced was quenched in water at 1050 ° C. The test bar blanks were worked out in such a way that their longitudinal axes were parallel to the main direction of deformation of the workpiece. The precipitation hardening of the blanks was carried out for 1 to 4 hours in the temperature range from 300 to 650 ° C. Rods for tensile, notch impact and fatigue strength tests were produced which were tested under exactly the same conditions as given in example I.

Working example III:

A stainless steel (alloy 50 tion III) with the following composition was assumed:

= 0.04% by weight

= 25% by weight

= 8% by weight

= 2.5% by weight

= 0.4% by weight

= 0.5% by weight

= Rest

60 The manufacture, further shaping, heat treatment and testing of this turbine blade material were carried out in exactly the same way as given in Example II.

b5

Working example IV:

The starting point was a stainless steel (alloy IV) with the following composition:

20th

C.

Cr

Ni

Mon

Ti

Fe

C.

Cr

Ni

Mon

Ti

Al

Fe

654 594

4th

c

= 0.04% by weight

Fe = rest

Cr

= 22% by weight

Ni

= 10% by weight

Among the elements for precipitation hardening can

Mon

= 2.5% by weight

preferably at least one of the elements

Ti

= 0.2% by weight

5

Al

= 0.5% by weight

Cu

Fe

= Rest

Ti

The production, treatment and testing of this material was carried out exactly in accordance with working example II.

Test results:

The results of the tests are summarized in the table below. Alloys I to IV correspond to those in the exemplary embodiments. For comparison, the properties of a hardenable ferritic Cr-Mo steel of the standard X20 Cr Mo V 121 of the following composition, which is frequently used for turbine blades, are shown:

C.

=

0.20% by weight

Cr

=

12% by weight

Mon

=

1.0% by weight

Ni

=

0.7% by weight

V

=

0.3% by weight

Fe

=

rest

In addition, the well-known titanium alloy Ti 6 A14V with the following composition is used as a comparison:

AI V Ti

= 6% by weight = 4% by weight = rest

The table clearly shows that the turbine blade materials according to the invention are significantly superior to the two comparison materials under corrosive medium. This applies above all to Cr-Mo steel, which also has insufficient toughness. The titanium alloy can only show a higher static strength (yield strength), but drops considerably compared to the proposed alloys I to IV in terms of dynamic values. In view of the cost and difficult processability of the titanium alloy, this is all the more important.

It should be added that the elongation at break of alloys I to IV based on a test bar with a length: diameter ratio of 4.4 was consistently over 15%, which speaks for the excellent ductility of this material. The fatigue strength values under 4N NaCI solution at pH = 5 and 80 ° C without static preload were in all cases over 350 MPa.

The invention is not restricted to the material compositions specified in the exemplary embodiments. Particularly suitable as turbine blade materials are steels with a two-phase mixture of the following general composition, consisting of ferrite or martensite on the one hand and austenite on the other hand:

C = 0.005 to 0.06% by weight

AI

"> are used, the total content of which should be at least 0.5% by weight, but the content of each individual element should be at least 0.2% by weight.

The following Mn-free alloys are preferred:

1.

20th

30th

C.

=

0.005

to 0.06

% By weight

Cr

=

18th

to 30

% By weight

Mon

=

1

until 6

% By weight

Ni

=

4th

until 11

% By weight

I (Cu, Ti, Al)

=

0.5

to 4

Wt%

for Cu, Ti, Al individually

> 0.2% by weight.

Fw

-

rest

Cu

0.005

to 0.06

% By weight

Cr

=

18th

to 30

% By weight

Mon

=

1

to 4

% By weight

Ni

=

4th

until 11

% By weight

plus a.Ti

=

0.5

to 3

% By weight

od.plusb.Al

=

0.5

to 3

% By weight

od.plusc.Ti

=

0.25

up to 2

% By weight

AI

=

0.25

up to 2

% By weight

Fe

=

rest

C.

0.01

to 0.06

% By weight

Cr

=

22

until 29

% By weight

Mon

=

2nd

to 3

% By weight

Ni

=

4th

until 11

% By weight

Ti

=

0.5

to 2.5

% By weight

Al

=

0.5

to 1.5

% By weight

Fe

=

rest

"Another selection of preferred alloys that contain both Ni and Mn is listed below:

4th

C.

= 0.005

to 0.06

% By weight

Cr

= 18

to 30

% By weight

Mon

= I

until 6

% By weight

Ni

= 2

until 6

% By weight

Mn

= 4

till 8

% By weight

Z (Cu, Ti, AI)

= 0.5

to 4

% By weight

55

where Cu, Ti, AI individually per Fe = rest

> 0.2% by weight

5. Composition as 4th, but additionally N = 0.2 to 1.5% by weight

Cr = 18

to 30

% By weight

6. C

=

0.005

to 0.06

% By weight

Mo = 1

until 6

% By weight

Cr

=

18th

to 30

% By weight

Ni + '/ îMn-

4th

% By weight

Mon

=

1

until 6

% By weight

65 'Ni

=

2nd

until 6

% By weight

Mn

=

4th

till 8

% By weight

+ at least one precipitation hardening element for Fe-

Cu

=

0.5

to 3

% By weight

Alloys

N

=

0.2

to 1.5

% By weight

654 594

7. C

=

0.01

to 0.06

% By weight

Cr

=

24th

to 28

% By weight

Mon

SS

1.5

to 3

% By weight

Ni

«

3rd

until 5

% By weight

Mn

=

4th

till 8

% By weight

Cu

SS

1.5

up to 3.5

% By weight

N

=

0.2

to 0.6

% By weight

table

Alloy:

Notched

Fatigue strength (tension / compression),

tion

MPa ability

108 Load change preload ±

J / cm:

Alternating load MPa

air

Air / 4N NaCl

80 ° C, pH 5

I.

800

50

250 ± 340

250 ± 270

11

800

50

250 ± 340

250 ± 260

III

800

50

250 ± 340

250 ± 260

IV

800

50

250 ± 340

250 ± 260

Cr-Mo-St

800

20th

250 ± 340

250 ± 40

TÌ6A14V

930

30th

250 ± 210

250 ± 180

The method according to the invention is not restricted to the steps indicated in the examples. The hot forming after the glazing can be carried out in the temperature range between 1000 ° C and 1250 ° C, the 5 cross-sectional decrease should be at least 50%. Depending on the alloy and workpiece size, the precipitation hardening can be carried out in the temperature range between 300 and 650 ° C for 1 to 8 hours.

The turbine blade material can preferably be used continuously as a steam turbine blade in the low-pressure part or as a turbo compressor blade up to temperatures of 350 ° C.

The turbine materials produced and proposed according to the invention combine high ductility and notch toughness with high static strength and high resistance to corrosion fatigue and thus ensure a long service life of the component.

20th

G

Claims (15)

654 594 2nd PATENT CLAIMS I. Tubular blade material of high strength against corrosion fatigue, consisting of a stainless steel, characterized in that it contains a two-phase mixed structure consisting of ferrite or martensite or an intermediate structure on the one hand and austenite on the other hand that it contains at least one additional suitable for precipitation hardening in iron alloys Element contains that it contains 0.005 to 0.06% by weight of carbon, 18 to 30% by weight of chromium, and 1 to 6% by weight of molybdenum as well as nickel or nickel and manganese, and its nickel plus Vi content Manganese content is at least 4% by weight. 2 Tnrhinenschaufelwerkstoff according to claim 1, characterized in that it contains as an element for precipitation hardening at least one of the elements copper, titanium, aluminum, such that the total content of elements at least 0.5 wt .-%, but that of each individual element at least 0 , 2% by weight. 3. Turbine blade material according to claim 2, characterized in that it contains 4 to 11 wt .-% nickel and totally 0.5 to 4 wt .-% at least one of the elements copper, titanium, aluminum, such that the content of each individual element is at least 0.2% by weight. 4. turbine blade material according to claim 3, characterized in that it 1 to 4 wt .-% molybdenum and 0.5 to 3 wt .-% titanium or aluminum or 1 to 4 wt .-% molybdenum, 0.25 to 2 wt .-% Titan and 0.25 to 2 wt .-% aluminum contains. 5. turbine blade material according to claim 3, characterized in that it 22 to 29 wt .-% chromium, 4 to 11 wt .-% nickel, 2 to 3 wt .-% molybdenum, 0.5 to 2.5 wt .-% Titanium, 0.5 to 1.5 wt .-% aluminum and 0.01 to 0.06 wt .-% carbon contains. 6. Turbine blade material according to claim 2, characterized in that it contains 2 to 6 wt .-% nickel, 4 to 8 wt .-% manganese and totally 0.5 to 4 wt .-% of at least one of the elements copper, titanium, aluminum , such that the content of each individual element is at least 0.2% by weight. 7. turbine blade material according to claim 6, characterized in that it contains 0.2 to 1.5 wt .-% nitrogen. 8. Turbine blade material according to claim 1, characterized in that it has 2 to 6 wt .-% nickel, 4 to 8 wt .-% manganese, 0.5 to 3 wt .-% copper and 0.2 to 1.5 wt. -% contains nitrogen. 9. turbine blade material according to claim 1, characterized in that it contains 24 to 28 wt .-% chromium, 3 to 5 wt .-% nickel, 4 to 8 wt .-% manganese, 1.5 to 3 wt .-% molybdenum, Contains 0.2 to 0.6 wt .-% nitrogen, 0.01 to 0.06 wt .-% carbon and 1.5 to 3.5% copper. 10. Turbine blade material according to claim 1 to 9, characterized in that it has a yield strength of at least 800 MPa, an elongation at break of at least 15% for a ratio of sample length: sample diameter = 4.4 and a fatigue strength in relation to tension / pressure under a ventilated 4 N NaCl solution with pH = 5 at 80 ° C, of at least 350 MPa without static preload and of at least ± 250 MPa under a static preload of + 250 MPa based on 108 load changes, as well as a notched impact strength of at least 50 J per cm2. II. A method for producing a turbine blade material according to claim 1, characterized in that the components are melted and cast under vacuum, that the cast ingot produced in this way in a temperature range between 1000 ° C and 1250 ° C of a hot deformation with a cross-sectional decrease of is subjected to at least 50% and the workpiece produced in this way is quenched from said temperature directly in water, machined to the final shape and then heat-treated in the temperature range between 300 and 650 ° C. for 1 to 8 hours for precipitation hardening. 12. The method according to claim 11, characterized in that the hot forming at a temperature of 1050 c C and the precipitation hardening is carried out for 1 to 4 h. 13. The method according to claim 11, characterized in that the hot deformation first up to a cross-sectional decrease of 75% at a falling temperature between 1250 ° C and 1050 ° C and then up to a further relative cross-sectional decrease of 50% based on the previous cross-section isothermal at a temperature of 1050 ° C and the precipitation hardening is carried out for 1 to 4 h. 14. Use of the turbine blade material according to claim 1 as a steam turbine blade. 15. Use of the turbine blade material according to claim 1 as a blade of a turbo compressor.