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Newer Materials for Supercritical Power Plant Components – A

Manufacturability Study

Conference Paper · January 2015

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Newer Materials for Supercritical Power Plant Components – A Manufacturability Study

Anish Nair1,*, S.Kumanan2

1Production Engineering, NIT, Tiruchirappalli, 620015, anishn@live.com 2

Abstract

Production Engineering, NIT, Tiruchirappalli, 620015, kumanan@nitt.edu

The current trend in the energy sector is the development of advanced super critical power plants for operation at higher efficiency. But it is seen that the development of such plants are taking place at very slow pace especially in India. The rapid growth is hindered due to the material challenges which the engineers are facing. The supercritical power plants operate at a very high temperature and a lot of materials are being developed in order to suffice the high temperature requirements of the power plants. Although the materials are developed, the database regarding the different manufacturing operations that have to be carried out on such materials is still scarce. This paper gives a brief idea on the challenges faced in the manufacturability of supercritical power plant components. The manufacturing methods considered here are machining, welding and forming. Welding and forming are of prime importance in the boiler fabrication whereas machining plays a role in the fabrication of valves, turbine blades, vanes and other accessories. The paper also provides an insight into the different materials developed recently for supercritical applications.

Keywords: Supercritical, Welding, Forming, Nickel alloys

1 Introduction A thermal power plant converts heat energy into electrical energy and thermal power plants are the most common source of power generation. Almost 40% of the electricity need in the world is provided by coal (Source – IEA) and hence coal is the first source of energy generation. Although massive research is going on in renewable energy systems it has to be noted that it cannot be a quick replacement for thermal power plants. Hence it has to be ensured that the power plants run at maximum efficiency and also strive for greener energy.The critical temperature and pressure of water are 375 o

Table 1 Power Plant Classification

C and 22.06MPa respectively. The steam power plants are classified based on the outlet steam temperature and pressure values. The classification can be done in four different ways as shown in Table 1. Here SC stands for Super Critical; USC stands for Ultra Super Critical; AUSC stands for Advanced Ultra Super Critical.

Type Sub Critical SC USC AUSC

Temperature (o 565 C)

565-580

595-620 >705

Pressure (MPa) 16 - 17 22 -

24 25 - 34 >34

Plant Efficiency(%) 35 38 42 >45

CO21.12

Emissions (full load),

tonnes/MWh 1.09 1.05 <1.05

It can be noticed from the Table 1 that as the pressure and temperature of the inlet steam increases

there is a significant increase in the net power plant efficiency. The other factor to be noted is the reduction in the CO2 emissions. Hence a power plant operating at the ultra-supercritical condition will have maximum efficiency.

2 Material Selection The major challenge in the selection of materials for AUSC plants is the lack of creep resistance and corrosion resistance in conventionally used materials at the plants operating conditions.

Table 2 Material Types Material Advantages Disadvantages

Low alloy ferritic steels

Good weldability

Reduced creep strength

High strength, good steam side

oxidation resistance

upto 420 oC only

Enhanced Creep strength ferritic steels

Steam side oxidation resistance

Increased production times

upto 620 o Weaker weldment C

Advanced Austenitic Stainless steel

upto 680 o High thermal expansion C

High creep strength

Prone to Sensitization

High resistance to fireside

corrosion and steam side oxidation

Prone to stress corrosion cracking in wetted section

Nickel based alloys

Temperature above 680 o

High fabrication costs C

Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 326

A lot of research has been going on and a lot of materials have been developed specifically for the power plant applications. Ferritic steels are used for thick section pipes and headers but above 500o

Table 3 Material Selection

C they have very poor creep resistance. At higher temperatures austenitic steels or Nickel base alloys have to be used. Nickel improves the toughness but the amount has to be optimized for good weldability. Some of the requirements for material selection of boiler are sufficient creep rupture strength, low cycle fatigue strength, high resistance to fire side corrosion, steam side oxidation/exfoliation and easy machinability. Some of the commonly used materials and their characteristics are mentioned in the Table 2, Zhang (2013). Some of the recently developed materials which can be used for AUSC plants are CCA 617, Inconel 740, Inconel 625, Nimonic 105, Nimonic 115, Nimonic 263, U700, U701, U702, Waspaloy and Haynes 230. But these materials are also application specific in that they can be used for only certain specific components of the power plant. Table 3 shows the different power plant components and the candidate materials which can be used, Amir Shirzadi (2014).

Component Upto700 o Upto760 C oC Casings (shells, valves, nozzles)

CCA617 Inconel 625 Nimonic 263

CCA 617 Inconel 740

Bolting Nimonic 105 Nimonic 115

Waspaloy

Nimonic 105 Nimonic 115 U700 U710

U720

Rotors/Discs Inconel 625 Inconel 740 Haynes 230

CCA 617 Inconel 740

Nozzles / Blades

Wrought Ni-based

Wrought Ni-based

Piping CCA 617 Inconel 740

3 Boiler Components The first generation of super critical power plants were faced with a variety of problems in design, operations and material related problem that resulted in reduced availability and reliability. The maintenance costs also drastically increased resulting in power industry reverting back to subcritical units. The boiler was designed to remain at constant pressure throughout the working and hence the startup became complicated leading to large pressure differentials in valves resulting in their corrosion. For AUSC boilers water enters at 320oC and steam leaves at around 480oC with metal temperatures around 540o

Creep strength has to be much greater for superheater and reheat tubing. For the secondary superheater the material will be austenitic stainless steel for supercritical boilers, advanced austenitic stainless steel for USC and Ni alloys for AUSC. As reheat temperatures are often higher than those of superheat the grades of material used are slightly higher than for the superheater.

C. T22 has sufficient strength but the fireside corrosion resistance and steam side oxidation resistance is very low for AUSC conditions. In

general higher chromium content materials are more effective but the effect minimizes beyond 30%. Some suggestions have been made to construct the water wall from T91 or T92 which have the strength and corrosion resistance, but require careful heat treatment. Inconel 617 satisfies all the requirements but its cost is a serious drawback.

High fireside corrosion is required by all the materials used in AUSC boilers. Different materials were tested for their corrosion resistance and they are indicated in Figure 1.

Figure 1 Corrosion Rate of Materials

Steam side oxidation is another major issue which can be reduced by efficient water treatment techniques. But beyond a particular operating condition the water treatment will not be effective. The main scope of welding is in the boiler fabrication. The boiler shells are welded together using Submerged arc welding (SAW).

Figure 2 Boiler Header Fabrication

0

1

2

3

4

Inconel 740

Haynes 230

Inconel 617

Super 304 H

Cor

rosi

on R

ate

Material

Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 327

A lot of welding is involved in the fabrication of the headers for the boiler and also in the other accessories. The headers comprise of different pipes of different materials joining the same tank and hence the scope of dissimilar metal welding is very high here. Also as discussed before the failure of components at high temperature mainly tend to occur at the welded joints due to various microstructure phenomena. Figure 2 shows a specimen of a header used in boilers where there is lot of dissimilar metal welding, (Shingledecker, 2010).The different welding processes are Submerged Arc welding (SAW) for header and pipe longitudinal and circumferential seam, Gas tungsten arc welding (GTAW) for tube to tube joints, Shielded metal arc (SMAW) and Gas tungsten arc welding for tube to header socket joints.Viswanathan et al reported that SAW which is used conventionally cannot be used with Nickel based alloys. Gas metal arc welding (GMAW) also had been successfully used for welding. In the case of casings there is the scenario of dissimilar metal welding hence the materials are selected such that their thermal expansion coefficients are similar. Inconel 718 and Nimonic 263 are good materials for that purpose.

Figure 3 Thermal Expansion Coefficients

Alloys such as Inconel 740, Haynes 230 and Haynes 282 satisfy the conditions for super heater tube materials. Austenitic nickel based materials have significantly less complicated metallurgy so post weld heat treatment is not necessary. Welding these alloys is much more difficult when compared to austenitic or ferritic alloy steels. Additional protective measures such as weld overlay, plasma spray coatings and shielding can be applied where there is higher corrosion. Creating welded joints from nickel based alloys due to hot tearing in weld metal and fusion zone. Joining temperature of 1200o

Pipes can be prepared using the EngerhaltPushbench method. The tubes of Alloy 617 made in such a way are shown in Figure 4. These

pipes were further subjected to other welding and forming tests. Tubes can also be produced by drilling forged bar steel.

C with holding time of 10min is optimal for welding of Ni based alloys. Micro fissures were noticed in the weld joints in thick sections of the headers when welding Inconel 740.

Figure 4 Alloy 617Tube

Microstructural, mechanical, and creep properties of seamless tubes and pipes afternormalising and tempering heat treatment are compared with those obtained after coldbending and hot induction bending. Since during boiler fabrication, tubes and pipes are often subjected to many formingoperations such as cold bending, swaging or hot induction bending, the effect of plasticdeformation on microstructural, mechanical and creep properties has been widelyinvestigated. Cold bends were produced starting from small diameter tubes, whilehot induction bends were produced from big diameter pipes. The effect of plastic deformation on the creep properties has also been investigated.Creep resistance after cold deformation lies in the lower scatter band of base material,while creep resistance of hot induction bends is in line with that of unbent material. The major shop floor problem when welding the nickel-based alloys isone of cleanliness, the austenitic alloys being very sensitive to weldmentsolidification and liquation cracking, with sulphur being the most commoncontaminant. There are, however, other fabrication issues with alloys such asInconel 740, since they require solution treatment and precipitation hardeningheat treatments during fabrication to develop their optimum properties. Theheat of welding will have an adverse effect on strength, and filler metalswill be in the ‘as-cast’ condition. Hot and cold bending operations will alsorequire a more complicated heat treatment regime than would be requiredfor the solid solution strengthened alloys such as alloy 617. When usinghigh sulphur bituminous coals at temperatures of 750 °C/760 °C, fire-sidecorrosion will also restrict the use of lower chromium nickel-based alloys containing less than 20%Cr) and alloys containing molybdenum.In circumstances such as this, it may be necessary to use co-extruded tubecomprising a Ni-base alloy protective layer and a low alloy steel substrate to protect the tubing with a suitable corrosion-resistant overlay.

4 Turbine Components

0

5

10

15

20

P91

AISI

316

Allo

y 71

8

Allo

y 74

0

Allo

y 28

2

Allo

y 26

3

Allo

y 61

7

Allo

y 23

0

Ther

mal

Exp

ansi

on C

oeff

icie

nt

(mm

/K)

Material

Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 328

The main components considered here are : the turbine casing/shell, steam cylinders and valve bodies; bolting; turbine rotors and discs; vanes and blades.

4.1 Casings/Shells The casings of steam turbines typically are large structures with complex shapes that must provide thepressure retainment for the steam turbine. Design of the turbine includes an inner casing orcylinder and is employed to enclose the hot gas path, so that the main steam from the steam generatorfirst flows into the steam chest, through the inner cylinder, over the vanes and blades, and then returnsthrough the annulus between the inner cylinder and the outer casing before being sent to the reheater.For higher temperatures, Ni-based alloys will be required, and the question will be whether adequatestrengthening can be developed in cast alloys, or whether wrought alloys will be needed. The candidatealloys chosen for evaluation in the AD700 program goals included both Fe-based superalloys and Ni-basealloys: 155, 230, 263, 617, 625, 706, 718, 901, and Waspaloy39a modified version of 617 (CCA617), and a new alloy,Inconel 740, appear to meet the strength and creep-rupture criteria for the 760°C goal of the U.S. USCsteam program. An extensive data generation effort for wrought versions of these alloys is in progress in the U.S. The major materials needs are for Ni-based alloys for operation at 760°C with (i) adequate creep rupturestrength; (ii) abilities to cast them into the required size and shape, and to inspect for defects42; and (iii)ability to perform initial fabrication welding (on cast or wrought forms, including dissimilar metal welds),and to make repair welds on aged material. The effort required is considerable, and involves thedevelopment of rupture, creep, and rupture ductility relationships for these materials, Maile (2013). 4.2 Bolting The major requirements for bolting materials are high resistance to stress relaxation (ageingcharacteristics) at temperatures that can range up to the maximum steam temperature experienced by thecasing for the hot gas path; thermal expansion characteristics compatible with those of the structure to bebolted; and low notch sensitivity.Long-termcreep data are available for a number of these alloys, including U-700, U-710, U720 variants, Nimonicalloys 105 and 115. Overall, for bolting, the choice of materials appears to be relatively straightforward. There do not appearto be significant manufacturing issues, since these alloys are available as bar stock suitable for rolling orgrinding to shape. Similar requirements exist for gas turbines, although there may be some scale-upissues to be addressed.

4.3 Rotors/discs The HP rotor/discs will have to handle the highest steam conditions, so that a Ni-based alloy will berequired for temperatures greater than 620°C but it can be noted that the size of the blades is significantly less depending on the overall steam turbine design. The IP rotor handles steam at the maximumsystem temperature, but at reduced pressure; while the strength requirement may be relaxed compared tothe HP rotor, the issue of oxidation in steam remains. The manufacture of large nickel base alloy components such as turbine rotorshas been recognized as a key issue for a 700°C power plant. As far as joining is concerned, all the alloys can be fusion welded. Oneoutstanding issue concerns dissimilar metal welds, in which the nickel- base alloysshould be welded to chromium steels. In order to reduce alloy costs, somecomponents may be manufactured by welding the nickel alloy to steel, so thatthe nickel alloy is only used for the parts of the component subjected to the highesttemperatures. Such welds need to be investigated and qualified. There are also requirements for cast components for valve bodies and turbinecasings. It will therefore be necessary to examine the properties of cast versionsof the candidate alloys. The higher operation temperatures of a 700°C power plantmean that some of the ancillary components, such as turbine shaft sealsand their retaining springs, erosion shields and wear- resistant valve stem bushes,will require alloys capable of higher temperature operation than the currentlyused alloys.

Figure 5 Types of Turbine blades

Fritz Klocke et al. have discussed the fabrication operation of turbine blades as shown in Figure 5 using Pulsed electrochemical machining.Ni-based alloys will be required for the highertemperatures, and candidates include Inconel alloys 617, 625, and the new 740, and Haynes 230. Exceptfor 740, these alloys are approved by the ASME Boiler and Pressure Vessel Code so that a significant design database exists for them, although this does not include fatigue data. 4.4 Blading The current supercritical steam plants in the U.S. typically use vanes and blades made from 12 Cr ferriticsteels such as type 422, or proprietary alloys of

Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 329

similar composition. For higher temperatures there isavailable a wide choice of wrought Ni-based alloys, for which a substantial design database exists fromtheir application in gas turbines. For operation with steam at 760°C, it is considered likely that materialsnew to steam use will be necessary for at least four stages in the HP turbine, and probably also in the IPturbine47. The choice of blading material will depend on (i) the temperature of the rotor, hence on thethermal expansion characteristics of the material from which it is made, and (ii) the size and shape of theblade, which will be designed using computational fluid dynamics modeling. For rotors and discs, modernsecondary steel making practices enable large rotors to be produced from the Cr-Mo-V and 12 Cr alloysused up to 620°C, and the European programs have explored the capabilities for Ni-based alloys.However, since there is a limited range of alloys from which to choose for HP rotors for operation at 700and 760°C, investment in resources needed to process these alloys may be a major factor. The IP rotor isa critical item because of the size, but it is expected that the turbine can be designed so that thiscomponent can use ferritic steels. For the airfoils, while there is a range of wrought (and cast) Ni-basealloys suitable for use in the higher-temperature HP turbine, there appears to be a need to generate theproperty data necessary for processing these components, which may be large, and involve complexshapes to maximize efficiency. For higher-temperature bolting materials, the issue also is one of selectingfrom a range of materials; this will be driven by the need to match the thermal expansion of the alloysused for the major components, and the generation of creep/relaxation data at the higher temperatures. Finally, there exists a major need to demonstrate that available materials can be made into actualcomponents that work as intended, and to obtain property data for design purposes and service lifeprediction.

Figure 6 Valve synthesized by Powder Metallurgy

Study has been going on in the field of powder metallurgy for producing the components for AUSC plants such that the disadvantages associated with welding and forging can be reduced. The production of large, complex components can be done using the near net shape technology. Components thus produced will require only minimal machining and the weldability is also enhanced due to the homogeneity of the microstructure. A valve manufactured using the powder metallurgy technique is shown in the Figure 6. The feasibility study of the powder metallurgy process was carried out and it was concluded that the parts produced had acceptable metallurgical and mechanical properties compared to the existing cast/forged products. 5 Conclusion A brief overview of different supercritical boiler components is given in the paper. Rapid research has been going on in the development of different materials which can bear the very high temperature operating conditions. It can be concluded that only Nickel based alloys are suitable for operation at temperature above 760o

Machining of super alloys has always been an challenge for engineers. Hence better and efficient machining techniques have to be developed for machining of these super alloys. The edge preparation of forged boiler components is done using milling process and extensive machining is used in the manufacture of high temperature valve components. Electrochemical Machining has been found as a very good alternative for machining of turbine blades and vanes. It produces components with minimal stresses and high finishing. Cryogenic machining has also been investigated and found to give good results. The scope of turbine blade cooling is still being investigated and if it is feasible then high precision drilling of turbine blades is needed. Forming processes have also been discussed and currently induction heating bending is followed for better precision of the bend. Hence it can be seen that with the evolution of newer materials the industry faces more challenges in manufacturing these materials into finished products. Thus there exists a lot of scope in studying the different manufacturing operations for the different advanced super critical power plant components.

C. Different Nickel based alloys have been synthesized and continually studies have been going on in different manufacturing operations of these materials. Welding is one of the key processes due to the tough weld ability of Nickel based alloys and weak weldments. Further the failure analysis results show that failure is predominant in the weld zones and hence further studies have to be conducted in welding of such alloys.

Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 330

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Bilgi, D.S. et al., 2004. Electrochemical deep hole drilling in super alloy for turbine application. Journal of Materials Processing Technology, 149(1-3), pp.445–452.

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Proceedings of the International Conference on Advances in Production and Industrial Engineering 2015 331

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