6 – Martensitic steels for rotors in ultra-supercritical power...

Materials for Ultra-Supercritical and Advanced Ultra-Supercritical Power Plants. http://dx.doi.org/10.1016/B978-0-08-100552-1.00006-3Copyright © 2017 Elsevier Ltd. All rights reserved.

Martensitic steels for rotors in ultra-supercritical power plantsG. ZeilerBohler Edelstahl GmbH & Co KG, Kapfenberg, Austria

6

6.1 Introduction

Energy is a basic necessity for a high standard of living in each country. To preserve the environment for future generations and to protect health of humanity and other living beings, energy production itself is faced with the introduction of stringent emis-sion regulations.

That is why in the next decades renewable energy resources such as wind and solar energies will have to be increased. Fossil fuels, such as coal, oil, and natural gas will nevertheless still play an important role as an energy provider in developed countries, also as a weather-independent basic supply and to cover periods where a maximum of demand is required.

Especially in large developing countries like India and China, the demand for coal-fired power plants will continue its expansion due to the regional deposits of coal in the near future.

This continuous trend toward more economic electricity production parallel to reduced environmental pollution can only be sustained by improving the thermal efficiency of power generation plants. The efficiency is increased by raising the temperature as well as the pressure of the steam, which finally results in the need for materials with improved high-temperature capability for the boiler and turbine design.

To minimize investment costs, which of course influences the effective cost of elec-tricity generation, martensitic steels for all major components in boiler and turbine have to be used as much as possible. Specifically, steels of the 9–12% Cr class are required with high long-term creep strength and oxidation resistance in steam, along with not too complex manufacturing processes.

During the last 25–30 years, enormous efforts, especially in development of new materials, were made to establish the technology for the new ultra-supercritical (USC) power plants, which are the standard of today’s fossil-fired power generation plants. The basic necessity for realization was to increase temperature and pressure of the inlet steam into the turbine combined with single and/or double overheating. Steam temperature of 600–610°C, 630°C overheating temperature, and overcritical steam pressures of about 300 bar could be realized [1–3].

As an example, the profit on thermal efficiency in a 600°C/300 bar power sta-tion with double overheating in comparison to the traditional 540°C/185 bar power

144 Materials for Ultra-Supercritical and Advanced Ultra-Supercritical Power Plants

plants with single overheating is more than 8%, and in parallel the CO2 emission can be reduced by about 20%, which has a significant impact on environmental aspects [4].

From this point of view, the USC power plants are playing an important role in power generation systems with high thermal performance.

Resulting from the oil crises in the 1970 years, the topic of reducing CO2 emis-sions, a careful dealing with resources and the more and more upcoming international competition led to the demand after flexible and cost-convenient fossil-fired steam power plants. To fulfil all these aspects, improved and advanced ferritic martensitic 9–12% Cr steels for all the key components in the hot sections of the power station became necessary, and research and development programs were initiated in the highly developed countries in Europe, United States of America, and Japan (Fig. 6.1) [4,5].

6.2 Common rotor material requirements

As in nearly all areas of technique, but especially in the field of energy machines operating at higher temperatures, the realization of these machines and their physical processes is strongly related to the availability of suitable materials. The improve-ment in efficiency and performance of modern steam power engines can only be achieved by raising the temperature as well as the pressure of the steam generated in the boiler. To withstand these advanced operating conditions, materials are required with improvements:

• high-temperature capability, • oxidation resistance in steam,

Figure 6.1 International R&D projects for development of improved creep resistant steels.

145Martensitic steels for rotors in ultra-supercritical power plants

• weldability, • low costs, • good availability and easy to fabricate.

To define the specific requirements/loads for steam turbine rotors, we have to consider several basic types of steam turbines. In general, we have to differentiate between high-pressure (HP), intermediate-pressure (IP), and low-pressure (LP) tur-bines, where each is operating in a specific temperature/pressure range, shown in Fig. 6.2, for power ranges up to 1000 MW [6] and a typical configuration of a turbine train (Fig. 6.3).

Further solutions for power generation with steam turbines are combined HP-IP and IP-LP turbines for the smaller power range for combined cycle power plants (CCPP). The rotors are hereby in monoblock or welded rotor design [2] (Fig. 6.4).

Figure 6.2 Typical temperature and pressure parameters for subcritical steam turbines.

Figure 6.3 Typical Siemens steam turbine arrangement with high-pressure (HP), intermedi-ate-pressure (IP), and low-pressure parts (LP).Siemens AG, Power Generation.


To fulfil the various customer needs for an advanced power plant, specific design requirements have to be met.

The most important components in turbo-sets like rotors, casings, valves, bolts, blades, and pipes are differently loaded, and as a consequence, specific property pro-files have to be considered.

Next, the basic material requirements for HP and IP steam turbine components are listed:

• static strength (tensile strength), • creep rupture strength, • fracture toughness, • low cycle fatigue (LCF), • high cycle fatigue (HCF), • creep crack growth, • fatigue crack growth, • erosion and oxidation behavior.

Today, advanced USC turbines are operating with temperatures from 580–600°C and a maximum steam pressure of 300 bar. However, the maximum possible operating temperatures of the developed materials are not fully used yet.

HP and IP rotor forgings, which can reach diameters of up to 1200 mm, need to be manufactured in well-selected materials to meet all property requirements. Creep behavior plays an important role for areas such as the rotor center and the blade attach-ment, whereas the fatigue resistance is mainly considered in the surface regions [6].

Especially, the material class of the 9–12% CrMoV steels, used in power plants since the late 1950s, were identified with many advantageous properties and a big potential for improvement. Their mechanical and physical property profile made it possible to go for temperatures T > 600°C, and the target of material development in Europe was defined as follows [4,6–7]:

• 100,000 h creep rupture strength at 600–620°C and 100 MPa design stress, • good creep rupture ductility with > 10% elongation and no notch sensitivity,

Figure 6.4 High-pressure turbine with combined intermediate-low pressure in welded design.Siemens AG, Power Generation.


• good through-hardenability up to at least 1200 mm diameter, • 0.2% yield strength of at least 700 MPa, • other properties should not be worse than those of conventional 12% and 1% CrMoV rotor

steels.

The way and chronology of development for achieving this goal is now described in the following paragraphs.

6.3 Development of martensitic 9–12% Cr rotor steels for USC application

12% Cr steels, alloyed with Mo and V, have been used in power plants since the middle of the 20th century. The German alloy X21CrMoV12 1, strengthened by solid solution and precipitation of M23C6 and a small amount of V(C, N), was first used for blades, forgings, and castings in the 1950s, and after solving the weldability problems, the steel has been introduced successfully also for tubes and pipes in power plants. Since 1955, European turbine producers have manufactured numerous rotors of the material X21CrMoV 12 1, specified in SEW 555, with diameters up to 1300 mm and weights up to 35 metric tons. The 0.2% yield strength was in the range of 500–650 MPa at operating temperatures of maximum 580°C (560–600°C steam inlet temperature). In this temperature range, the X21 steel showed a clear advantage in comparison to 1% CrMoV steels [7,8].

At the end of the 1950s, two 12% CrMoV rotor steels were also developed in the United States for HP and IP rotors. One of them is alloyed with 1% tungsten and the other with additions of niobium and nitrogen (Fig. 6.5). At this time the big potential of the 12% Cr steel class for high long-term creep strength and oxidation resistance in steam was found, and a further milestone in development history was the introduction of the tube and pipe steel T/P91 in the late 1970s by Combustion Engineering and Oak Ridge National Laboratories in the United States.

At the same time, development work has been going on in Japan, led by Prof. T. Fujita at Tokyo University. Based on the highly creep-resistant 12% Cr steels, devel-oped in Europe and the United States, Fujita started to investigate to what extent alloying elements could improve the creep strength for 600°C application. This was found by alloying and balancing the steel composition with Nb, N, and W, whereas the strengthening was mainly pursued by solid solution with W. In the middle of the 1980s, this led to new alloys for turbine rotor forgings TR 1100, 1150, and 1200, as

Steel Source C Si Mn Cr Mo Ni W V Nb NCreep rupture

strength at600°C/105 h (MPa)

12%CrMoV SEW 555 0.22 0.25 0.50 12.0 1.0 0.50 – 0.30 – – 59

13%CrMoWV USA 0.22 0.40 0.80 13.0 1.0 0.75 1.0 0.25 – –

11%CrMoVNbN USA 0.18 0.25 0.70 10.5 1.0 0.70 – 0.20 0.08 0.06 85

(weight %)

Figure 6.5 Standard and developed 10–12% Cr steels for HP and IP steam turbine rotors.


well as to the pipe steel NF616 (P92 according to ASME code) developed together with Nippon Steel Corporation (Fig. 6.6) [7,8].

The compositions of the steels TR 1100, 1150, and 1200 were selected on the basis of short-term creep tests with less than 10,000 h and were supposed to have enough creep strength for the application at temperatures up to 593, 621, and 648°C according to 1100, 1150, and 1200°F.

Nevertheless, it has to be pointed out that extrapolation of short-term creep results to 100,000 h is very difficult and dangerous for these complex alloyed steels. Reliable data on the creep strength can only be provided after very long testing times. This was the conclusion of a critical evaluation of published creep results by Newhouse and Greenfield [7].

The steel TMK 1 or TR 1100 was then selected for the first big forging for indus-trial use for the HP rotor of the 50-MW demonstration plant at Wakamatsu [7]. The rotor weighed 28 metric tons with a barrel diameter of 980 mm. With the applied heat treatment of 1100°C/oil + 550°C/FC + 680°C/FC, a 0.2% yield strength of 745 MPa, and an impact energy of 32 J, respectively, +25°C FATT could be achieved in the center of the rotor [9].

In sequence, further optimizations for improving the creep strength were done by defining the optimum content of C, N, and Cr as well as a careful balancing of Mo and W, whereby the Mo equivalent = Mo + 0.5 W was determined to be the significant parameter [7].

The developments in the United States and Japan have been followed up in Europe in the COST (COST = cooperation in science and technology) programs: COST 501 (1983–1997), COST 522 (1997–2003), and COST 536 (2004–2009). In these programs, new ferritic creep resistant 9–12% CrMoV steels for turbine forg-ings, castings, and pipework were developed and characterized. Carried out under the frame of the European community, the participants consisted of power station manufacturers, manufacturers of forgings, castings, pipework, and welding consum-ables, power station operators, and research institutes as well as testing institutes and utilities.

It started in the 1980s with the first round and targeted a creep strength of 100 MPa for 100,000 h at 600°C. At the beginning, the approach was more or less empirical and concentrated at that time on the effect of boron additions and on a review of existing 9–12% Cr steel grades including worldwide development activities. In addition, the steelmaking developments in Europe and the forgemasters’ experiences on 9–12% Cr as well as the careful balancing of Ni and Cr equivalents to avoid δ-ferrite were of big

Figure 6.6 Newer developed 10–12% Cr steam turbine rotor steels in Japan.


importance. Furthermore, Al should be kept as low as possible, particularly due to the high affinity to N to enable a high amount of VN precipitates.

After this review, five grades of steel (called A to E) were identified as candidates for development, and small melts were produced for characterization of the materials. The alloy concept hereby was the addition of N, B, W, W + Mo, and Mo. Thereby the main requirements were to carry out long-term creep tests to avoid the uncertainty of extrapolation from short-time tests and to manufacture full-size rotors to get experi-ence in manufacturing of these new steels, as well as getting the knowledge about the properties of the inner and outer regions of a real component.

After careful analysis of all results the two most promising candidate alloys E (1%Mo and 1%W) and F (1.5% Mo) were selected for the manufacture of trial rotor forgings with barrel diameters up to 1200 mm, in addition to the already produced first trial rotor B2 (1.5% Mo, 100 ppm B) (Fig. 6.7 and 6.8). A lot of testing work was

performed on the materials, and they showed much improved creep strength and resis-tance to embrittlement in operation [10].

Meanwhile, these improved steels are in commercial operation in advanced European power stations and have made it possible to increase the operating steam temperatures from well-known 530–565°C up to 580–600°C with a corresponding increase in thermal efficiency.

The trend to even higher steam conditions was the subject of the COST 522 pro-gram, which explored the possibilities of stabilizing the tempered martensitic micro-structure through addition of small quantities of boron.

In addition, Co was added up to 1.3% to reduce Ni and Mn, due to their detrimental effect on creep rupture strength, and limiting the risk of δ-ferrite formation. Further-more, increased Cr levels up to 11% for better oxidation resistance were introduced, and a full program of screening tests was performed. From all these variants, only one alloy showed the best behavior and led to a new modified steel called “FB2”, first pro-duced as a trial melt (Fig. 6.9), and after getting very promising properties, a scale-up to industrial heat with the manufacture of the first trial rotor forging followed [11,12] (Fig. 6.10).

In total, three trial full-size rotor forgings have been manufactured applying three different melting routes.

The first, manufactured by Bohler Edelstahl/Austria in 2001, used a 29-metric ton ingot via the BEST (Bohler electro slag toping) melting route, Saarschmiede/Ger-many a 56-mt ESR (Electro slag remelting) ingot, and Società delle Fucine/Italy has

Figure 6.7 Within COST developed 10–12% Cr steels for steam turbine rotors.

150M

aterials for Ultra-Supercritical and A

dvanced Ultra-Supercritical Pow

er Plants

Dmax (mm)

Ingot weight (kg)

UTS (MPa)

0.2%YS (MPa)

Charpy (J)

FATT (°C)

UTS (MPa)

0.2%YS (MPa)

Charpy (J)

FATT (°C)

840

15,000

1200 914 801 27 +55 875 744 86 +5

42,000 783 647 76 +20 785 633 146 –10

1200 892 770 55 +30 855 729 42 +40

45,000 737 600 99 –2 764 609 75 +5

B2

E

F

Heat treatment

Saarschmiede Voelklingen (Ge)

Forgemaster Eng. Sheffield (UK)

ESR

VCD

1070°C/oil + 570°C + 690°C + 715°C

1070°C/oil + 570°C + 680°C + 710°C

Periphery, tangential, body top end Centre, radial, midsection of body Rotor

Bohler Kapfenberg (A)

Melting process

ESR

Manufacturer

1100°C/oil + 590°C + 700°C

801 651 33 +60642 22 +45 813

Figure 6.8 Basic data and mechanical properties at RT of COST rotors B2, E, and F (COST 501/II Program).


applied the steelmaking process VCD with an ingot weight of 53 mt [6,13,14] (Figs. 6.11 and 6.12).

The results of creep tests showed that all data lies in one narrow scatter band, con-firming the trial melt behavior of FB2. In Fig. 6.13 the values are all above the rotor B2 line, the boron containing steel from COST 501, and the basis for the development of FB2.

The very good creep behavior of steel FB2 can be attributed to characteristic micro-structural features and their stability under the influence of temperature and stress.

The microstructure in the quality heat-treated condition in general consists of tem-pered martensite and precipitates, mainly M23C6 carbides representing the dominant particle type.

During creep, in general, the dislocation density decreases, subgrains form and grow, M23C6 particles coarsen, the martensite laths become much wider, and also new

Steel C Si Mn Cr Mo W Ni Co V Nb N B

FB2 test melt 0.13 0.05 0.82 9.32 1.47 – 0.16 0.96 0.20 0.050 0.019 0.0085

FB2 trial rotor 0.13 0.09 0.33 9.08 1.43 – 0.16 1.26 0.22 0.054 0.022 0.0076

Figure 6.9 Chemical compositions of steel FB2 [wt%].

T

B

A

1350

Ø 7

70

Ø 1

180

Ø 8

65

300

930

Creep specimen,600ºC/160 MPa/1000h

D

C EG

G

F

B1110

RT:

625°C:

UTS (N/mm2)

UTS (N/mm2)

0,2YS (N/mm2)

0,2YS (N/mm2)

EL (%)

EL (%)

RA (%)

RA (%)

Charpy (J)FATT (°C)

835

A

7131556

30, 35, 30+52

Grain size (ASTM)

43738119622–0 1–0

7919323366

+6426, 28, 22

4414703838

B C

825 863700 73017 1361 40

35, 33, 31 27, 21, 23+36 +67

417361

368308

21 2970 793–1 1–0

824 838702 70816 12

325637, 40, 28 26, 24, 23

+49 +65

8347151656

31, 56, 35+48

429 416362 35920 1669 682–0 1–0 2–0

4503901970

QHT: 1100°C/spray quenching + 570°C/AC + 700°C/ACS, surface, tang.; C, center, transversal

D E F

Figure 6.10 Mechanical properties of first trial rotor FB2 by Bohler.


phases as the Laves phase appear. Not so in FB2, the good high temperature stability is caused by the following:

• a martensitic structure with narrow martensite laths, which are decorated with and therefore stabilized by M23C6 carbides, stabilized itself by boron [15],

• a high dislocation density, which is preserved for longer times, • and Nb, V carbonitrides (up to about 1 μm), which avoid grain growth during austenitization

and are stable during creep [16].

A representative TEM image of steel FB2 is shown in Fig. 6.14 [17].Under the influence of temperature, Laves phase also appears in FB2, but it is

very small (0.6–0.8 μm) and homogeneously distributed, and therefore has no negative effect on creep.

All in all, it can be stated that materials for USC power plants have been developed with the goal to provide environmentally friendly power plants with low consumption of primary energy and the lowest rate of emissions. These newly developed material grades are highly creep-resistant martensitic materials for the use of up to 625°C and

T

L1 L2 L3

B

Ø D1 Ø D2 Ø D3

Manufacturer Meltingprocess

Bohler Kapfenberg(Austria)

Societa delle FucineTerni(Italy)

SaarschmiedeVoelklingen(Germany)

BEST

EAF/VCD

ESR

D1(mm)

D2(mm)

D3(mm)

L1(mm)

L2(mm)

L3(mm)

770 1180 865

925 1110 790

800 1215 1050

1350 930 1110

800 2750 830

1085 2130 800

1.2/1.8/1.3

1.0/1.5/1.5

1.5/2.0/1.7

MDDS (mm)D1/D2/D3

MDDS, minimal detectable defect size

Figure 6.11 COST FB2 trial rotor forgings: dimensions and minimum detectable defect size (MDDS) results.

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artensitic steels for rotors in ultra-supercritical power plants

Figure 6.12 Basic data and mechanical properties at RT of COST rotors FB2 (COST 522 Program).


40

5060708090

100

150

200

300

400

24 25 26 27LMP = T (c + log(t)) *.001

Rotor E + FRotor B2Rotor FB2, BöhlerRotor FB2, Terni

Rotor FB2, Saarschmiede

Stre

ss (M

Pa)

Figure 6.13 Creep rupture strength of COST FB2 test rotors in comparison to other COST rotor alloys [14].

After 56.500 h(600°C/100 MPa)

After QHT

(a) (b)

Figure 6.14 Microstructure of FB2: after being quality heat treated (QHT) (a); gauge section (b) (600°C, 100 MPa) after 56.500 h [17].


steam pressures above 300 bar. Fig. 6.15 [18] shows the steps in the development of materials for USC power plants with respect to increasing steam temperatures.

Temppressureefficiency Actual market

540°C180 bar

42%

<600°C240 bar

47%

>700°C>300 bar

55%

600–650°C240–300 bar

50%

Conventionalplant

Currentadvanced

plant

DTI, cost522, 536

ThermieAD700

Low alloysteels

Nickelalloys

9–12% Crsteels

Improvedmaterials

+New

designs

State of technology

New material developmentprocess- and component-development

Figure 6.15 Steps in the development of the materials for USCs.Voest Alpine AG, Foundry Linz; C. Lochbichler, F. Füreder Kitzmüller, G. Zeiler, S. Paul, J. Klarner, T. Vogl, et al, Know how and process development for components used in (A) USC power plants, 10th Liege cost Conference on materials for advanced power Engineering, September 14th–17th 2014, 2014 [Liege, Belgium].

6.4 Common materials for steam turbine rotors in Europe: fabrication and basic properties

All the efforts undertaken in the European research activities COST 501/522 have been very successful and led to three new modified creep-resistant chromium steels: COST grade E (10% CrMoWVNbN), grade F (10% CrMoVNbN), followed by FB2 (9% CrMoVNbNB).

These improved steels have become well established and qualified and are now in commercial operation in advanced European fossil-fired power stations. They have made it possible to increase the operating steam temperatures from widely known 530–565°C up to 580–610°C with a corresponding increase in thermal efficiency from pre-vious 30–35% to now 42–47%, connected with approximately 30% CO2 reduction [19].

6.4.1 COST E and COST F rotor forgings for 600°C steam temperature

In the early 1990s, industrial production of these newly developed 9–10% Cr steels became ongoing in Europe and in Japan.


As described in Section 6.3, the development occurred very carefully, starting with investigation of several test melts and determination of candidate materials up to the manufacturing of pilot components with real dimensions followed by testing times up to 30,000 h. This succeeded the preconditions to realize 14 European steam power plants with advanced steam parameters and power ranges up to 950 MW [4]. The increase of the steam parameter from 540°C up to 610°C occurred systematically in a way that the property potential of the new steels did not need to be exploited for the first plants.

All the three European forgemasters involved in the COST 501 programs—Shef-field Forgemasters, Saarschmiede, and Bohler Edelstahl—were predestined for the manufacturing of the first rotor forgings, based on their knowledge and experiences gained in the COST action, when manufacturing the prototype forgings. Depending on the forgemasters specific equipment, the melting routes ranged from conventional ingot casting over the BEST process up to ESR with ingot weights up to 108 met-ric tons [20–22] at the beginning. To date, ingot weights up to 154 metric tons have already been realized [23].

Bohler Edelstahl, for many years a premium supplier of forged components for the power generation industry, mainly applied the melting route over the BEST process, a process with special measures to improve the ingot homogeneity in comparison to conventional casted ingots (Fig. 6.16).

Figure 6.16 Bohler electro slag topping (schematic).


A water-cooled ring is placed at the top end of a forging ingot mold. The mold is filled with liquid steel via a bottom pouring process. Afterward, the steel surface is covered with a slag of a special chemical composition. A consumable electrode is immersed into the slag, electrical energy is applied to the system, the slag is heated up, and the electrode begins to melt. Droplets of the melting electrode fall into the liquid pool of the solidifying ingot, compensating for shrinkage and influencing the solidification process in a positive way.

Ingots produced by this process are free from V-segregations, and the structure of A-segregations is similar to ESR-ingots. Homogeneity and cleanliness is better than in conventional casted ingots but not as good as ESR material.

The ingot weights were up to a maximum 48 metric tons, suitable for HP rotors in monoblock construction and shaft parts for welded rotor constructions.

A similar ingot-making process, developed by JCFC in Japan, is the ESHT (elec-tro slag hot topping) process. High-Cr steel rotor forgings with good homogeneity, sufficient cleanliness, and also good mechanical properties including creep rupture strength could be confirmed [22,24].

The second applied melting route used so far is the PESR (protective gas electro slag remelting) process, with a maximum possible ingot weight of 32 mt due to the equipment and possibilities at Bohler Edelstahl (Fig. 6.17).

Figure 6.17 Protective gas electro slag remelting (schematic).


Remelting takes place in a leak-proof chamber similar to the VAR process. Before starting remelting, the chamber is evacuated to a pressure lower than 10 mbar and flooded successively with protective gases argon and nitrogen. The reactive slag is first melted by the use of an electric arc. Once this has occurred the normal ESR process takes place with the advantage of the total absence of oxygen in the atmosphere above the slag surface and around the electrode. In that manner, no oxidation can take place, and this results in a very uniform quality of the remelted steel from bottom to top of the ingot. PESR is the desired remelting process for components made of super clean, creep-resistant 10% Cr steels with reduced Si and Al contents.

After heating the ingots to forging temperature, they are forged on a forging press, mainly by multiple upsetting operations followed by cogging and forging to the final shape. The number of upsetting operations is hereby strongly dependent on melting route, ingot weight, size, and final rotor forging dimensions. Especially in the case when large ingots are needed, the manufacturing parameters such as the forging tem-perature, soaking time, and deformation rate for the first forging steps are very import-ant to ensure a defect-free forging and suitable microstructure. To optimize the forging sequence and improve the reproducibility of the forging process, FEM modelling can be a very helpful instrument to develop the optimal forging technology. Achieving a largely uniform deformation resulting in uniform microstructure and grain size dis-tribution is one of the preconditions to achieve the high requirements on mechanical properties and ultrasonic detectability (Fig. 6.18).

Figure 6.18 FEM model for optimized forging steps (schematic).


When forged, the rotors are preliminary heat treated, which is a very important heat treatment step to allow optimum ultrasonic testing of the material/forgings in the quality heat-treated (QHT) condition. This can be carried out in different ways; mean-while, pearlitic phase transformation has become a common procedure [11,23,25,26]. After pre-machining, the quality heat treatment to adjust a yield strength ≥ 700 N/mm2 is performed, followed by final machining to delivery contour and testing including heat stability test prior to shipment.

The quality heat treatment (Fig. 6.19) consists of these steps:

austenitizing: 1050°C–1100°C/oil or polymer or water-spray quenchedfirst tempering: 570°C/air cooled, to complete the martensitic transformationsecond tempering: 690–700°C/FC, AC, to adjust the desired properties

The soaking times have to be defined according to the largest diameters of the rotor forgings.

Figure 6.19 Quality HT cycle (schematic).

Typical test results are summarized in Fig. 6.20.Beside the mechanical properties, which have to be fulfilled, the ultrasonic detect-

ability including sound attenuation is an important criterion for the quality of rotor forgings and therefore forms part of the specifications. Detectable flaw size values of minimum 1 mm for diameters ≤ 800 and 1.5 mm for diameters ≥ 800 mm are required and achievable (Fig. 6.21).

To date, these new steels have been introduced in highly efficient USC power plants with a total capacity of about 187 GW worldwide [IEA WEO 2013]. COST E and F are meanwhile specified in the German standard SEW 555 with the steel numbers 1.4906 (grade E) and 1.4902 (grade F). Bohler manufactured more than

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dvanced Ultra-Supercritical Pow

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Figure 6.20 Basic data and mechanical properties at RT of COST rotors E and F from different suppliers [23,25,26].

200 300 400 500 600 700 800 900 1000Sound path (mm)

0,0

0,5

1,0

2,0

3,0

3,5

2,5

1,5

Ultr

ason

ic d

etec

tabi

lity

(mm

) Cost F / QHT

Cost F / PHTStraight beam probe

frequency 2 MHz

Figure 6.21 Ultrasonic detectability versus sound path (COST F steel).

Figure 6.22 Shaft component at QHT, COST steel F.

Figure 6.23 Shaft component machined, COST steel E.


Figure 6.24 HP rotor at final forging, COST steel E.

B2 > 600 131,000 122

E > 700 106,000 95

F > 700 115,000 95

FB2 > 700 60,000 125

Trial rotor 0.2%YS(MPa)

Max. testing time (h)

CRS at600°C/10 5 h

(MPa)

Figure 6.25 100,000 h creep rupture strength of COST type E, F, B2, and FB2 steels.

480 rotor forgings out of COST steels type E and F, namely high-pressure shafts as well as shaft components for welded rotor constructions with diameters from 700 to 1180 mm and shaft ends with flange diameters of up to 1800 mm (Figs. 6.22–6.24).

6.4.2 COST FB2 for up to 620°C

After the successful development of COST type E, F, and B2 material within COST 501 (1983–1997), the subsequent COST 522 action (1998–2003) followed the trend to develop materials for even higher steam temperatures (Fig. 6.25).

Based on the composition of the boron-alloyed B2 steel, which showed the high-est creep resistance of the three steels [19], a new modified steel called “FB2” with the addition of Co has been developed and first produced as a trial melt by Bohler Edelstahl. The very promising properties of this FB2 test material led to an upscale to industrial heat and the manufacture of a full-size trial rotor forging with a final weight of 17,000 kg, again manufactured by Bohler Edelstahl. Then forgings from Società delle Fucine Terni and Saarschmiede followed [6,12–14].

The excellent creep resistance of FB2 and the successful transformation of trial melt behavior to large components led to the start of industrial production for new USC power plant projects in Germany, the United States, and Asia, starting in 2007 (Fig. 6.26).


Again, as already described in the previous chapter for the 600°C materials COST type E and F, the melting route for FB2 steel is also ranging from conventional ingot casting with ingot weights of up to approximately 30 metric tons for smaller shaft dimensions over BEST process [11,12,17,21] or equivalent ESHT [27–29] up to ESR with ingot weights up to 90 metric tons [30–32]. Therefore the melting/remelting know-how in combination with the specific steelmaking equipment of the forgemas-ters are of great importance to fabricate a highly homogeneous and uniform ingot with a good boron distribution from top to bottom and also over the whole cross-section of the ingot. This is not very easy to adjust for FB2 type steel, but these are basic neces-sities to gain the excellent creep rupture strength properties for the forging.

The basic manufacturing steps for FB2 rotor forgings are quite similar to those for type E or F forgings and are roughly described for a conventional ingot as follows [17]:

Melting the steel in an electric arc furnace (EAF), then ladle furnace (LF) followed by argon oxygen decarburization (AOD) and LF with vacuum degassing (VD), casting the ingot by bottom pouring process, hot forming, preliminary heat treatment, pre-ma-chining, ultrasonic testing, QHT, machining, and final testing (Fig. 6.27).

Considering the fabrication of larger forgings with the need of heavier ingots, the use of BEST or even better remelted (PESR) materials due to a better consolidation of the ingots combined with less segregations and a better homogeneity of the later on product is advisable.

The ingot is then heated up to forging temperature and forged on a forging press by multiple upsetting operations followed by cogging and forging to the final shape. All the considerations about deformation rates—described under Section 6.4.1—are also fully valid for FB2 forgings, whereby forging temperatures have to be set much more carefully to not damage the microstructure. It is of great importance to avoid delta ferrite, which can occur depending on the actual chemical composition beginning at 1180°C and above [31]. Forging temperatures and soaking times have to be tuned very well and adapted to each deformation step.

When forging is finished, the preliminary heat treatment (PHT) has to be carried out to cool down the forging to room temperature. Meanwhile, it is well known that

Figure 6.26 First USC power plants using FB2 in turbines.


boron-alloyed steels have to be austenitized at higher temperatures to generate the expected high creep resistance. The disadvantage, when all precipitates are in solid solution at austenitizing temperature, is grain coarsening and later on a bad ultra-sonic detectability, worse than COST E or F materials. A simple martensitic trans-formation is only sufficient in case of smaller cross-sections because there is not so much improvement in grain size expected. For large forgings, a pearlitic transforma-tion, similar to that applied on the 600°C class 10% Cr rotor forgings, has meanwhile become evident and is much more efficient to get a finer grain after quality heat treat-ment followed by an improvement of the minimum detectable defect size (MDDS) [27,28,31–33].

The microstructure obtained after pearlitic transformation consists of ferrite, fine M23C(B)6, and eutectoid structure at the prior austenite grain boundaries.

After pre-machining the rotors to achieve a defect-free surface, ultrasonic tests are performed to confirm the internal quality of the forgings. Then the quality heat treatment (Fig. 6.28) for adjusting the mechanical properties has to be carried out by austenitizing at 1100°C/rapid cooling (oil or polymer or spray quenching, depending on the facilities) and double tempering to the target 0.2% yield strength of ≥ 650 N/mm2. Double anneal-ing has to be performed to ensure a totally annealed martensitic microstructure.

The rotors are then machined and checked (Fig. 6.29) using ultrasonic testing to determine the MDDS. Usual measured MDDS values range from 1.5 mm for a smaller part with ∅ 860 mm up to 2.2 mm for parts with a diameter of 1120 mm up to 1200 mm, respectively, when applying martensitic transformation as a PHT. There is only a small improvement in MDDS of approximately 0.8 mm when comparing with the preliminary heat-treated condition [17,33,34].

Figure 6.27 Principal flow chart of FB2 rotor manufacturing route.


It is reported [27,32,33,35] that by applying (isothermal) pearlitic transformation the grain transformation process results in a significant improvement of the mini-mum detectable defect size of 1.1–1.3 mm for a rotor barrel diameter of 1200 mm (Fig. 6.30).

All turbine rotors are subjected to mechanical technological testing to ensure their suitability for use. Usually the properties are checked at different test positions, depending on the shape of the rotor forging; specimens in tangential direction from the outer segments on both ends, from the barrel diameter, from the core when taking a core sample, and in some cases specimens are taken from a near-center test ring from the face end. Typical test results of basic strength and toughness properties are summarized in Fig. 6.31.

Figure 6.28 Quality HT cycle (schematic).

Figure 6.29 Quality heat-treated and machined FB2 forging; final weight 9.2 mt.


The microstructure, checked at the edge as well as center and/or near-center posi-tions, consisted of tempered martensite with grain sizes of 0–2 according to ASTM E112 in the edge and about 00 in the near-center areas [17,27,33,34]. No δ-ferrite could be observed, but the microstructure contained some small isolated boron nitrides, which are not avoidable and expected not to be detrimental.

It has to be taken into consideration that adjusting the correct chemical composition is not enough to gain the excellent creep rupture strength of the steel. Boron has to be in solid solution to stabilize the M23C6 carbides for a high level of creep resistance.

Figure 6.30 Achievable detectable flaw sizes.

Figure 6.31 Mechanical properties of an FB2 rotor forging.


In addition, nitrogen is necessary for the formation of the MX precipitates, which are basically Nb, V-carbo nitrides. If there are too many boron nitrides (BN) in the microstructure, especially clusters, the creep properties will not achieve the expected level due to a lack of B and N in the matrix. B and N have therefore to be balanced very carefully.

All the results in Fig. 6.32 have been achieved by industrial melts based on 9–12% Cr steels. Larger BN are present among and above the red line; smaller iso-lated ones among the blue line. Having N contents below the blue dotted line, there is no BN formation observed.

To check and roughly verify the creep behavior, short-term creep tests and observ-ing the creep strain is a good tool to do so. Fig. 6.33 shows the results achieved on

µµ

Figure 6.32 Influence of boron and nitrogen on BN generation.

0 200 400 600 800 1000 1200 1400 1600

FB2 test melt

FB2 trial rotor

A1, FB2 production rotor

B1, FB2 production rotor

Time (h)

0,45

0,40

0,35

0,30

0,25

0,20

0,15

0,10

0,05

0,00

Stress 160 MPaT = 600°C

Cre

ep s

train

(%)

Figure 6.33 Short-term creep testing in comparison to FB2 trial melt and trial rotor forging.


the shaft in Fig. 6.31 and compared with the creep behavior achieved on the FB2 trial melt, respectively, the FB2 trial rotor forging from the COST program. Test specimens were taken from the edge and near-center location and tested at 600°C and 160 MPa load for 1300 h. The results, plotted as a creep strain versus time curve, are good in line with the FB2 trial rotor.

All in all, great efforts have been made to increase the thermal efficiency by increas-ing the operating temperature of steam power engines to currently 620°C. Research work as well as first industrial manufacturing showed and confirmed the suitability of these steels for use at high temperatures, and FB2 is now going to be the material for the next turbine generation.

6.5 Present material development for application temperatures above 620°C

Over the past 50 years, new, improved creep-resistant martensitic 9% Cr steels have been developed and successfully introduced in power plants. The maximum achiev-able steam conditions could be improved from subcritical 180 bar and 540°C up to ultra-supercritical values of 300 bar and 600–620°C, now being introduced worldwide with unit sizes of up to 1100 MW. In comparison to the X21 material, the creep rupture strength of the last generation of 9–10% Cr steels were nearly doubled [19], and cur-rently the best and most stable alloy for rotor forgings is the FB2 material (Fig. 6.34).

A further increase of the steam parameters up to the aim of 325 bar and 650°C needs again to double the creep strength of the currently best martensitic Cr steel FB2. In parallel, the resistance against steam oxidation at such high temperatures has

500 550 600 650 700

Cost 501

Cost 522

ThermieAlloy 617, 625, 263

9%CrMoCoVNbNB (FB2)

9%CrMoVNbNB (Cost B2)10%CrMoWVNbN (Cost E)10%CrMoVNbN (Cost F)

9%CrWCoVNbNB (MARBN)(Cost 536) KMM-VIN

Temperature (°C)

12%CrMoV

1%CrMoV

0

100

200

c. 30°C c. 25°C

Cre

ep ru

ptur

e st

reng

th(1

05 h) (

MP

a)

Figure 6.34 100,000 h creep rupture strength of the newly developed European steels.


to be taken already into consideration, and 9% Cr steels have to be surface coated. Within the last COST action, COST 536 (2004–2009), all attempts to make stronger 9–12% Cr steels have led to breakdowns in long-term creep strength properties [6]. However, achievements have been made in better understanding of microstructural stability; especially the effect of B and N on long-term creep properties, and these two elements have to be balanced very well to avoid BNs (Fig. 6.35). This was the basis for a series of new, stronger 9% Cr test alloys for predicted improved creep strength, developed at the end of the COST program and now being investigated and optimized in the current KMM-VIN action, the continuation of the COST program [19].

Steel

MARBN

MARN

C Si Mn Cr Mo Ni Co W V Nb N B

0.078

0.002

0.31 0.49 8.88

0.29 0.51 9.19

– –

– –

3.00 2.85 0.20 0.051 0.0079 0.0135

0.00700.04900.0600.203.09 2.96

Figure 6.35 Chemical compositions of newly developed 9% Cr steels [wt%].

923K, 150 MPa30

25

20

15

10

5

00

PAGB 50 µm

PAGB 100 µm

PAGB 300 µm

PAGB 50 µm IsoMar

PAGB 100 µm IsoMar

PAGB 300 µm IsoMar

200 400 600 800 1000 1200 1400 1600 1800 2000 2200Time (h)

Stra

in (%

)

Figure 6.36 Creep curves of FB2-2 LN steel for three different prior austenite grain boundar-ies (PAGB) and two types of microstructure [36].

These new low-carbon martensitic steels, alloyed with boron and nitrogen, so-called MARBN steels, are one of the preferred materials showing a big potential for further increase in creep resistance and having good chances for reaching the target of 650°C. However, the development and long-term testing takes at least another 10 years to be ready for their introduction into industrial fabrication.

Two test alloys with 3.5 metric tons each, FB2-2LN and NPM1, have been manufac-tured by PESR process at Bohler Edelstahl, forged to square 250 mm and preliminarily heat treated. These alloys are now in the phase of investigation and optimization; run-ning research activities have the goal to create improved microstructures, influencing the mechanical properties at high temperatures. One important tool for that is an optimized heat treatment, to adjust a specific prior austenite grain size and in sequence cooling to a temperature between Ms and Mf to get about 80% martensite and the remaining aus-tenite transforms into 20% Bainite, a heat treatment called IsoMar [36,37] (Fig. 6.36).


As comparative creep test results have shown, the time to rupture of the selected MARBN steels could be increased by more than 10% when having adjusted this advanced microstructure.

One of the new alloy concepts is Z-phase (Cr (V, Nb) N)-strengthened steel. Z-phase precipitation is normally detrimental to the creep strength as it dissolves finely distrib-uted MX particles. The precondition for a Z-phase-strengthened steel would be very finely distributed Z-phase, which should be stable and showing no or only a small coarsening effect during operating time [38].

Also, investigations on experimental alloys based on 15% Cr (Japan) with dual phases of ferrite and martensite and precipitation hardened by intermetallic phases like Laves phase and carbide Cr23C6 as well as the 22% Cr class (Germany) are currently being carried out [39,40] (Fig. 6.37). Nevertheless, these materials are at the begin-ning of development on a laboratory scale and still have to be tested long-term and, if successful, finally transferred into industrial production and tested on real components again. This will need at least a further 15–20 years, but there are good prospects for the realization of 650°C steam power plants, and these martensitic alloys could be an alternative to the very expensive and possibly risky application of Ni-based alloys.

6.6 Summary and conclusions

The improvement in efficiency and performance of modern steam power engines can only be achieved by raising the temperature as well as the pressure of the steam gen-erated in the boiler.

During the last four decades, enormous efforts, especially in development of new materials, were made to establish the technology for the new ultra-supercritical (USC) power plants. The basic necessity for realization was to increase temperature and pres-sure of the inlet steam into the turbine, which called for the need of new materials with improved high-temperature capability.

Specifically, the 9–12% Cr grades steels have the potential for high long-term creep strength and oxidation resistance in steam. To develop improved and advanced ferritic martensitic 9–12% Cr steels for all the key components in the hot sections of the power station, research and development programs were initiated in Europe, United States, and Japan.

In Europe, the main efforts to improve this steel class were concentrated in the COST programs and led to the development of new ferritic steels for forgings, cast-ing, and pipework. For steam turbine rotors, three modified creep resistant chromium steels—COST grade E (10% CrMoWVNbN), grade F (10% CrMoVNbN) and FB2

Steel Source C Si Mn Cr Mo Ni Co W V Nb N B Ti La

Japan

Japan

Germany

15Cr–3Co–1.2Ni

15Cr–3Co–2.0Ni

Crofer 22H

0.049

0.048

0.007

– –

– –

– – –0.21 0.43

15.02

14.96

22.93

1.00 1.21

1.00 2.00

2.96

2.98

6.04

6.07

1.94

0.20

0.20

–

0.051

0.050

0.510

0.042 0.0028

0.00290.036

0.015 –

– –

––

0.07 0.08

Figure 6.37 Experimental steels based on 15/22% Cr in Japan/Europe [wt%].


(9% CrMoVNbNB)—were developed and characterized. These improved steels have become well established and qualified and are now in commercial operation in advanced fossil-fired power stations. They have made it possible to increase the operating steam temperatures from the supercritical plant with 540°C steam temperature and 250 bar steam pressure to the first generation of ultra-supercritical with 580°C/270 bar and finally to the second generation of USC plants with 600–620°C and 300 bar steam pressure.

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6 – Martensitic steels for rotors in ultra-supercritical power...

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