advanced material castings for high temperature coal fired steam

1 Goodwin Steel Castings Ltd, Ivy House Rd, Hanley, Stoke-on-Trent, UK ST13NR

Telephone: +44(0)1782 220000

Europe’s foremost independent steel and nickel alloy foundry www.goodwinsteelcastings.com

Advanced Material Castings for High Temperature Coal Fired Steam Turbine Applications

Introduction: To facilitate increase power station efficiency, steam temperatures and pressures have to be increased. Efficiency is not only important in reducing the cost of energy production, but also of vital importance for the reduction of CO2. At present, current plant efficiencies range from 38% to 46% and operating temperature range is from 565‟C to 620‟C. Most UK coal fired power stations operate at the lower temperature spectrum, due mainly to fact that that when most were built back in the 1960‟s and 1970‟s, this was the current state of technology and material development. For this temperature range, around 570‟C, generally in the high-pressure region of the turbine, Cr-Mo and Cr-Mo-V steels were specified for operation. The majority of these early plants will reach the end of their life within the next few years unless they fit emission controls technologies as required by the EU‟s large Combustion Plant Directive. Since the last power stations were built in the UK over 20 years ago, material technologies have significantly advanced to allow increased steam pressure and temperature. Today, state off the art coal fired power plant in mainland Europe and Asia operate around 610 to 620‟C and at pressure up to 275Bar (27.5MPa). The benefit of this is plain to see with a reduction in carbon dioxide emissions of up to 20% per unit of electricity supplied. To enable operation at these temperatures and pressures, modified 9% Cr Steels are widely used for high pressure turbine casings, stop valves, control valves, and re heat steam valves. These components are manufactured from high integrity castings, can be fabricated from more than one castings, and are fully machined.


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Fig 1a: 600MW Stop Valve Assembly (weight: 23,000kg Cr/Mo/V Steel)

Produced by Goodwin Steel castings Ltd

Fig 1b - 1000MW MSV and CV RH and LH Assembly produced from 8 castings (49,895kg)

9.5% Cr Steel GX12CrMoWVNbN - Produced by Goodwin Steel Castings Ltd


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Fig 1c - Cast MSV Body produced from two casting 15,000Kg;

Material E911 (Goodwin Grade: D9TC) This short paper discusses the benefit of these steels in the turbine, the critical requirements that are required to produce high integrity 9% Cr steel castings, and future new advanced materials for 700„C+ steam operations. 9% Cr Steel – How does it work? By increasing Cr to over 7% in Cr-Mo Steel, a group of materials were produced having a fine martensitic lath microstructure characterised by high dislocation density and stabilised by M23C6 precipitates. Hardening of this material gave a large increase in strength over conventional 2.25%Cr-Mo steel. This group of materials are known as un modified 9% Cr Steel. Additional improvements, especially to creep strength, were achieved by alloying with niobium, vanadium, tungsten and later boron. Controlled tempering results in the precipitation of vanadium, niobium and other on ferritic carbo nitrides, which act as anchors to maintain strength at high temperature. These Steels are known as modified 9% Cr Steels. Modified 9% Cr Steel – Benefits over Conventional Steels:

1) Higher stress rupture strength of 9% Cr Steel facilitates increased steam temperature and pressures can be utilised.

2) 9% Cr Steels strength is greater than conventional 2.25%CrMo and 1CrMoV steels, and therefore permits increased safety margins than in existing units.

3) Significantly longer component life can be expected for a given creep and fatigue duty.


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4) Reduced wall thickness can be used for the same design conditions, leading to lower thermal storage and reduced thermal stress.

This last point is very important as thinner components require less time to reach thermal equilibrium within the station, and therefore, are less likely to be thermo-mechanically damaged during station cycling. The ability to cycle plant on a daily basis is becoming important to operators in order to follow load demand and maximise profitability.

Fig 2: Relative Rupture Strength of Ferritic Steels

Table 1: Typical Material Grades for SC and USC Applications

Steel Grade C Si Mn S P Ni Cr Mo V Nb Other

G17CrMoV5-10 CMV 0.17 0.45 0.70 0.015x 0.020x

0.4x 1.35 1.0 0.25 - Al <0.025

G17CrMo9-10 Cr-Mo 0.17 0.50 0.70 0.020x 0.020x

0.4X 2.25 1.0 0.02 - Al <0.025

G-X12CrMoVNbN9-1

C12A (P91)

0.12 0.30 0.60 0.010x 0.020x

0.4x 9.5 1.0 0.25 0.06 N2:0.05

G-X12CrMoWVNb10-

1-1

E911 0.12 0.25 0.75 0.010x 0.020x

0.55 10.0 1.0 0.25 0.06 W:1; N2:0.05

GX-

13CrMoCoVNbNB9

CB2

0.12 0.29 0.86 0.010x 0.020x

0.20 9.5 1.5 0.2 0.06 B:0.012 N20.02

Co: 1.0


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The production of 9% Cr Steel castings in many ways is a quantum leap in technology, from the production of the conventional Cr-Mo and Cr-Mo-V power generation steel grades. Melting Practice: The melting practice for 9% Cr Steel is far more stringent with regards to chemical composition control and gas level control than is required for conventional power generation steel grades. AOD or VOD secondary refinement is the preferred method of production to ensure consistent results. In my experience, Induction melting should be treated with trepidation as gas levels can be very difficult to control and results inconsistent with regards to casting quality.

Fig 3. 10ton Capacity AOD Vessel in operation at Goodwin Steel Castings Ltd


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Heat Treatment: Heat treatment of 9% Cr Steels is critical to produce the desired microstructure, room temperature mechanical properties and elevated temperature stress rupture strength. The high temperature properties of the material are essential for adequate in-service life. Quality heat treatment of the material consists of an austenitisation followed by a rapid air quench to room temperature. One peculiarity of the material is that the martensite finish temperature (Mf) is particularly low at around 100'C, and therefore, the material has to be properly cooled to below this temperature to ensure full transformation has taken place prior to tempering. Tempering is carried out typically between 700'C and 780'C depending on material grade, and transforms un-tempered martensite to ductile tempered martensite. Often with castings, a double temper treatment is applied. This is to ensure that the centre sections of large castings are completely tempered and any retained austenite after air quenching has been fully transformed to tempered martensite.

(Fig. 4)

Microstructure: The final 9% Cr Steel microstructure should be predominantly tempered martensite with less than 5% delta ferrite. Delta ferrite is controlled by chemical balance using Cr Equivalent values for differing grades, and the correct quality heat treatment. Castings cooled too slowly after austenitising will contain more free ferrite. The intermediate micro structures such as the as-cast condition, and after FAQ are particularly stressed structures and highly susceptible to cracking ("clinking"). Heavier sections increase the risk further and great care must be taken at these stages.


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Weld Repair of Castings: During the welding process, the casting, weld and heat effected zone (HAZ), are subject to temperature extremes which pass the material through various metallurgical phases. This is common with most materials, which are welded, but with 9% Cr Steels this is particularly relevant due to the high hardness seen in the as-welded condition of both the weld and HAZ .

Fig 5. Fabrication Welding of a 9%Cr Steel Reheat Steam Valve

at Goodwin Steel Castings Ltd Due to martensitic transformations and residual welding mechanical stresses, the weld and parent material are in a highly stressed condition prior to post weld heat treatment. Therefore, cooling and heat rates during post weld heat treatment must be carefully controlled to reduce the risk of stress cracking. Welding is more often carried out with close to matching parent filler materials. Castings are always pre heated prior to welding to reduce the thermal gradient between base materials and weld puddle. Interpass temperatures have to be maintained to prevent hydrogen pickup


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and the risk of subsequent hydrogen cracking. After welding castings must be control cooled to below Mf to ensure full microstructural transformation prior to post weld heat treatment. Post Weld Heat Treatment: Correct tempering of 9% Cr steel weld is essential. The tempering is generally governed by the Larsen Miller parameter, where tempering temperature (P) should be 21 or higher to achieve adequate tempering. P= °C+273(20+log t) x1000 Where t = time in hours. Tempering at too low LMP will results in too higher hardness of the weld and HAZ and the toughness will be too low. However, too higher LMP value will cause the weld, HAZ and parent material to have too lower hardness and, therefore, low strength. Care should be taken when multiple PWHT conditions are applied to a component, as cumulative hours at temperature will also over soften the material. Over Tempering: This is where the PWHT temperature is too high and the material is heated to above the lower critical transformation temperature (AC1) and martensite starts to transform back to austenite. When 9% Cr steels are exposed this temperature, martensite partially re austenitise and the carbo nitride precipitates are coarsened but do not fully dissolve back into solution. The net effect is a part martensite part austenite material that lacks the pinning effect of the precipitates and therefore the creep rupture properties are substantially reduced.


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Fig.6: Electron backscatter diffraction grain boundary maps from P92 samples given post weld heat

treatment overshoot for 1 minute at (a) 800°C (b) 880°C (c) 900‟C (d) 960°C followed immediately by tempering at 750°C for 2hrs prior to subsequent cooling to room temperature.

“The effect of simulated post weld heat treatment temperature overshoot on microstructural evolution in P91 and P92 power plant steels”

Courtesy of Department of Materials, Loughborough University, Loughborough R.C. MacLachlan, J.J Sanchez-Hanton and R.C Thomson

Structure a) to d) shows a gradual progression from a fully martensitic microstructure to almost fully ferritic. Under tempering: This is where the PWHT temperature is too low, and results in materials precipitates not going to completion or being absent or insufficient in size to stabilise the structure. In addition to the loss of creep rupture strength that this condition will bring, the material is also at risk from brittle fracture and stress corrosion cracking. Cast Materials for 700°C+ Operation The drive for reduced carbon dioxide emissions and improved efficiency in coal fire power plant, has lead to much work being carried out around the world with regards to material development to enable 700°C + steam temperature operation with pressure of up to 100Mpa. At these elevated temperature and pressures steels just don‟t have enough strength, so instead material development has focused on Nickel alloys.


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Fig.7 shows the relative stress rupture strength of various Nickel Alloys put forward as potential candidate

alloys for 700‟C + plant operation.

Nickel alloy development has concentrated on two main groups of alloys, solution strengthened (SS) and precipitation strengthened (PR) alloys. The solid solution alloys are lower in strength than their precipitation hardened counter parts, and so have reduced stress rupture strength, but possess good ductility and weldability. However, for plant operation above 760°C, only precipitation strengthened Ni alloys look likely to have the require stress rupture strength. Table 2: Typical Chemical Composition of 700’C + Candidate Alloys

Material Grade

Ni Cr Mo Nb Co W Al Ti Strengthening Mechanism

Other

Alloy 625 BAL 20 9 3.5 - - 0.2 0.2 SS -

Alloy 263 BAL 20 6 - 20 - 0.6 2.4 PH Cu:0.20

Alloy 230 BAL 22 2 - 5 14 0.3 - SS B:0.015

Alloy 617 BAL 22 9 - 12 - 1 0.5 SS + PH B:0.006

Inconel 740®

BAL 25 - 2 20 - 1 1.8 PH -

Haynes 282®

BAL 20 8.5 - 10 - 1.5 2.1 PH B:0.005


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Solution Strengthened Ni Alloys: These alloys consist of an fcc matrix which is strengthened by the addition of solid solution

strengtheners such as Mo, Cr, Co and W. This method of hardening depends upon the difference in lattice parameters between the Ni based austenitic structure and the solute atoms, the strengthening potency being proportional to this difference. The greater the percentage of solute atoms the greater will be the hardening effect. However, the amount of alloying additions is limited by the possibility of σ-phase formation with its accompanying disadvantages.

Table 3: The following elements are used to accomplish this strengthening:-

Element Wt % in Gamma Calc Increased in Flow Stress (Ksi)

Co 20% 2.56

Fe 10% 7.96

Cr 20% 22.8

Mo 4% 24.2

W 4% 25.5

V 1.5% 4.55

Al 6% 58.5

Ti 1% 5.69

Alloys 617 and 625 are alloys of this type. Although both may form intermetallic precipitates during service this is largely a by-product of their composition and not by design. Precipitation Strengthened Ni Alloys

Precipitation hardening is accomplished by the formation of three types of precipitate, gamma prime (γ/) gamma prime prime (γ//) (or double gamma prime),and grain boundary borides and carbides. γ/ is generally given the formula Ni3(Al,Ti), however, both the nickel and the aluminium and titanium can be substituted by other elements. The γ/ has the unusual property of increasing flow stress with temperature. The flow stress peaks at approximately 630°C but even at 830°C is still at least twice it's room temperature value and may be up to four times depending on the aluminium content. The flow stress of γ/ is also highly influenced by the aluminium content.

Thus at 1100K:- 23.5 At% Al the flow stress is 29 ksi 26.5 At% Al the flow stress is 87 ksi


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Manufacture of Ni Alloy Castings For the foundry, successfully producing these alloys as heavy section castings is a further

quantum leap in manufacturing complexity, especially for the gamma prime (γ/) strengthened alloys. With these alloys special consideration has to be given to the following:

Melting Practice: Melting practices for Ni Alloys are essentially different from conventional steel production in many ways. Specialised techniques are required to prevent oxidation of volatile alloying elements. VOD or AOD melting is employed for the production of large components. Pouring Technology: Ni Alloys that are alloyed with Titanium and Aluminium, and poured in air, have to be poured with great care to prevent oxidation defects on the surface of the castings. Special techniques have been developed in runner system design to prevent such defects.

Solidification: Ni alloy solidification characteristics are completely different from steels, and conventional methoding techniques are often not satisfactory alone in producing an acceptable quality casting.

Fig 8: Shows the X-Ray of two similar size feeders, one in Alloy 625 and the other in carbon steel

Goodwin Steel Castings Ltd: Technical Department


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Weld Repair and Fabrication: For Ni Alloy castings to be of any value in the power plant, their ability to be readily welded is essential, as any castings produced will have to be welded to inlet and outlet steam pipeline. To further complicate the matter, due to the high expense of nickel, these alloys will need to be welded to lower temperature material steels in lower temperature regions of the turbine. This arrangement will therefore necessitate a heavy section dissimilar weld. Weld repair of alloys like A625 is pretty weld established, however, heavy walled fabrication welding of one casting to another is not well established and takes special techniques and weld procedures to ensure success. Welding of heavy section age hardened cast alloys such as Inconel 740® and Haynes 282® are less well known, and are likely to be much more difficult due to the high strength of the material and relatively low ductility at room temperature.

NDE of Ni Alloy Castings: The power industry standard for many years for non destructive testing techniques has been magnetic particle inspection (MT) and ultrasonic inspection (UT). These techniques have been used for accessing the quality of the original steel castings during manufacture, and in line inspection during service. Unfortunately, neither of these inspection techniques are acceptable for Ni alloy casting due to the attenuation seen in Ni Alloys volumetric inspection using ultrasonics, and its poor magnetic permeability preventing MT from being a feasible technique for surface crack detection. The standard techniques for defect detection in Ni alloys are radiography (volumetric) and Dye Penetrant (surface). Radiography inspection, due to grain size and density of Ni alloy material, is performed using a linear accelerator for heavy sections castings.

Root - GTAW SMAW SAW SAW

Thickness 0 to 5mm 5mm to 20mm 20mm to 100mm 100mm to 175mm

Material A625 mod A625 mod A625 mod A625 mod

Table 3: Weld Sequence Details


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Fig.9: Macro of a completed Alloy 625 Weld (largest cast Alloy fabrication weld ever

produced)

Goodwin Steel Castings Ltd: Technical Department

Machining of Ni Alloys: Ni alloys are more difficult to machine than conventional steels used in the steam turbine. This is not so surprising, as the same characteristics that ensure the Ni Alloy has good high temperature behaviour, is also responsible for the poor machinability of these alloys. Characteristics of Ni Alloys that make them difficult to machine are:

1) Retain high strength at elevated temperatures. 2)Contain hard intermetallic compounds that make some Ni Alloys abrasive to tooling. 4) Possess high dynamic shear stress. 5) Poor thermal conductivity at machining temperatures. 6) Work Harden during machining.


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Understandably, machining cost for these materials are far in excess of those for conventional CMV, Cr-Mo and 9% Cr steels.

Alloy Costs Ni Alloys are significantly more expensive then conventional power generation grades of steels.

This is compounded with the fact that major constituent elements in Ni alloys are traded on the stock market, so that prices can vary widely from one week to the next. The graphs below show

the variation in alloying element prices for Nickel 2005 - 2010.

Fig 10: Nickel Alloys price $/1000Kg

Table 4: Alloy material Costs in descending order (Hi to Low) – Sept 2010

Grade A740® A263 A617 A625 A230 A155 $/Kg $24.1 $24.1 $23.8 $22.2 $21.8 $18

£/Kg £15.11 £15.11 £14.92 £13.91 £13.67 £11.29

Grade MarbN E911 P92 P91 CMV Cr-Mo $/Kg $3.44 $2.19 $2.23 $1.73 $1.14 $1.13

£/Kg £2.16 £1.37 £1.39 £1.08 £0.71 £0.71

If we take P91 as being unity, then an easy way to look at the relevant Ni Alloy pricing is to

display the costs as a multiplying factor above that of P91.

Table 5: Cost Multiplier Table

Grade A740® A263 A617 A625 A230 A155 More expensive than P91

Multiplication Factor X15 X15 X15 X14 X13.5 X11


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The previous tables highlight the significant expense of Ni Alloys when compared to that of conventional power generation steels used for SC and USC steam valves and casing components.

For the power industry to justify the additional costs of the A-USC materials, the cost model for the operating life of the station, will have to out weight the initial capital outlay for such

advanced materials. It‟s also clear when studying the alloy costs, that the minimum amount of Ni alloys should be used in the design. This would only be in the hottest and highest temperature

zones, where steam temperatures are in excess of 650°C, and typically 700°C to 725°C.

References: 1)“Superalloys A Technical Guide – Second Addition” ASM Publication M.J.Donachie, S.J.Donachie ; March 2002 2)“High-Temperature Mechanical Properties and Microstructure of Cast Ni-based Superalloys for Steam Turbine Casing Applications” P.J.Maziaz, N.D.Evans, P.D. Jablonski, 2009 -Paper presented at EPRI conference 2010 (Santa Fe). 3)”Creep Resistant ferritic Steel for Power Plants” Ingo von Hagen and Walter Bendick 4)“The effect of simulated post weld heat treatment temperature overshoot on microstructural evolution in P91 and P92 power plant steels” R.C. MacLachlan, J.J Sanchez-Hanton and R.C Thomson (2009) -Paper presented at EPRI conference 2010 (Santa Fe). 5)”U.S Program for Advanced Ultra Supercritical (A-USC) Coal Fired Power Plants” Roberts Romanosky - presented at the “4th Symposium on Heat Resistant Steels and Alloys Used for High Efficiency USC Power Plants 2011”

Steve Roberts

Technical Director Goodwin Steel Castings Ltd

advanced material castings for high temperature coal fired steam

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