EP0060577B1 - Turbine blade material with high fatigue-corrosion resistance, method of production and 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 relates to a turbine blade material according to the preamble of claim 1 and to a method for its production according to the preamble of claim 7.

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 a high static strength, i.e. H. a high yield strength, a sufficient deformation reserve, d. H. have sufficiently high notched toughness and a high resistance to corrosion fatigue in the relevant temperature range in a possible aggressive atmosphere. In some cases, 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 section of steam turbines, which is attributed to insufficient resistance to 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, no matter how high the purity of the steam or gas, local enrichment and thus saturation of an aqueous solution cannot be prevented with certainty. The possibility of a corrosive attack must therefore almost always be expected.

Another way of eliminating the above difficulties is on the material side. Attempts have already been made to combine good chemical resistance with sufficiently high mechanical strength (K. Detert, W. Bertram and H. Buhl, Materials and Corrosion, 31, pp. 439-446, 1980, in particular Table 1 Material 8: XCrNiMoCu 255 ; GB-A-1 456 634). These materials are basically ferritic-austenitic steels. However, the goal of being able to recommend a material with high static strength, high toughness and high fatigue strength in a corrosive environment has not been 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 latter 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 production method which ensure high strength against corrosion fatigue in the finished product with the simplest possible simplicity and avoidance of unusual, expensive starting materials with good ductility, high yield strength and notch toughness.

According to the invention, this object is achieved by the features of claims 1, 4, 7.

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

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

Example 1:

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

The alloy was melted in a vacuum furnace and cast into a cast ingot. The ingot was rolled down to a thickness of 12 mm at a temperature of approximately 1,050 ° C., the decrease in cross-section being at least 50% and then quenched in water from the same temperature. Quenching the workpiece effectively suppresses the possible formation of the brittle intermetallic iron / chromium compound, the so-called σ phase. Test bar blanks were machined out of the plate in question, the longitudinal axes of which were parallel to the rolling direction. The blanks were precipitation hardened by a heat treatment in the temperature range from 300 to 650 ° C. for 1 to 4 hours. A total of several test bars for tensile tests, Charpy impact tests and fatigue tests under axial load (tensile / compression) with and without preload were examined both in air and in an aerated 4 N NaCI solution with pH = 5 at 80 ° C. All fatigue strength determinations were made using a sinusoidal axial load. The stress state, which is particularly important for the practical evaluation in operation, was achieved by additionally applying a positive static preload (tension), which corresponded to a stress (mean value of the stress) of 250 MPa.

Example 11:

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

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 start of this operation, the workpiece temperature was 1 250 ° C, at the end of it still 1 050 ° C. This was immediately followed by the second deformation step, which was carried out isothermally at a temperature of 1,050 ° 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. The semi-finished product thus produced was then quenched in water at 1,050 _° 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 1.

Working example III:

A stainless steel (alloy 111) with the following composition was assumed:

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

Working example IV:

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

The production, treatment and testing of this material was carried out exactly in accordance with exemplary embodiment 11.

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 12 I of the following composition, which is frequently used for turbine blades, are shown:

In addition, the well-known titanium alloy Ti 6 AI4V with the following composition is used as a comparison:

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 values of the fatigue strength under 4N NaCI solution at p _H = 5 and 80 ° C without static preload were in all cases over 350 MPa.

+ mindestens ein Ausscheidungshärtungs-Element für Fe-Legierungen

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:

+ at least one precipitation hardening element for Fe alloys

herangezogen werden, wobei deren totaler Gehalt mindestens 0,5 Gew.-%, der Gehalt jedes einzelnen Elementes aber mindestens 0,2 Gew.-% betragen soll.At least one of the elements may preferably be among the elements for precipitation hardening

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.

wobei Ti, AI einzeln je ≥ 0,2 Gew.-%.The following Mn-free alloys are preferred:

where Ti, Al individually each ≥ 0.2 wt .-%.

Fe = rest

wobei Cu, Ti, AI einzeln je ≥ 0,2 Gew.-%A further selection of preferred alloys which contain both Ni and Mn is listed below:

where Cu, Ti, AI individually each ≥ 0.2% by weight

• 5) Zusammensetzung wie 4)., jedoch zusätzlich
  
  
  

Fe = rest

• 5) Composition as 4)., But additionally
  
  
  

According to the inventive method, the hot deformation after casting may be in the temperature range between 1000 ° C and 1250 ⁰ C are performed, wherein the reduction in cross section 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 ° C 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.

Claims (7)

1. Turbine blade material of high corrosionfatigue strength with a two-phased mixed structure of austenite and ferrite or austenite and martensite, consisting of

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 NaCI solution of pH = 5 at 80 °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 10⁸ load changes, and a notched impact strength of at least 50 J per cm².

2. Turbine blade material according to Claim 1, consisting of

the remainder being iron and impurities.

3. Turbine blade material according to Claim 1, consisting of

the remainder being iron and impurities.

4. Turbine blade material of high corrosionfatigue strength with a two-phase mixed structure of austenite and ferrite or austenite and martensite, consisting of :

the remainder being iron and unavoidable impurities, this material having a yield stress of at least 800 MPa, an 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 an aerated 4 N NaCI solution of 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 relative to 10⁸ load changes, and a notched impact strength of at least 50 J per cm².

5. Turbine blade material according to Claim 4, consisting of

the remainder being iron and impurities.

6. Turbine blade material according to Claim 4, consisting of

the remainder being iron and impurities.

7. Process for the manufacture of a turbine blade material according to Claims 1 and 4, characterised in that the components are melted and cast under vacuum, that the cast ingot produced in this way is subjected to hot-forming in a temperature range between 1 000 °C and 1 250 °C with a reduction in cross- section of at least 50 % and the workpiece produced in this way is quenched from the said temperature directly in water, is machined to the final form and then heat-treated in the temperature range between 300 °C and 650 °C for 1 to 8 hours for the purpose of precipitation hardening.