Gsc Advanced Material Castings

8/13/2019 Gsc Advanced Material Castings

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1Goodwin 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 TemperatureCoal Fired Steam Turbine Applications

Introduction:

To facilitate increase power station efficiency, steam temperatures and pressures have to beincreased. Efficiency is not only important in reducing the cost of energy production, butalso of vital importance for the reduction of CO2. At present, current plant efficiencies rangefrom 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 tofact 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 werespecified for operation. The majority of these early plants will reach the end of their lifewithin the next few years unless they fit emission controls technologies as required by theEU‟s large Combustion Plant Directive.

Since the last power stations were built in the UK over 20 years ago, material technologieshave 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 areduction 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 widelyused 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 frommore than one castings, and are fully machined.




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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 requirementsthat are required to produce high integrity 9% Cr steel castings, and future new advancedmaterials 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 afine martensitic lath microstructure characterised by high dislocation density and stabilisedby M23C6 precipitates. Hardening of this material gave a large increase in strength overconventional 2.25%Cr-Mo steel. This group of materials are known as un modified 9% CrSteel.

Additional improvements, especially to creep strength, were achieved by alloying withniobium, vanadium, tungsten and later boron. Controlled tempering results in theprecipitation of vanadium, niobium and other on ferritic carbo nitrides, which act as anchorsto 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 lowerthermal storage and reduced thermal stress.

This last point is very important as thinner components require less time to reach thermalequilibrium within the station, and therefore, are less likely to be thermo-mechanicallydamaged during station cycling. The ability to cycle plant on a daily basis is becomingimportant 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 ApplicationsSteel 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 0.020

x

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

CB20.12 0.29 0.86 0.010x 0.020

x0.20 9.5 1.5 0.2 0.06 B:0.012

N20.02Co: 1.0




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The production of 9% Cr Steel castings in many ways is a quantum leap in technology, fromthe 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 chemicalcomposition control and gas level control than is required for conventional power generationsteel grades.

AOD or VOD secondary refinement is the preferred method of production to ensureconsistent results. In my experience, Induction melting should be treated with trepidation asgas levels can be very difficult to control and results inconsistent with regards to castingquality.

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




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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 isparticularly relevant due to the high hardness seen in the as-welded condition of both theweld and HAZ .

Fig 5. Fabrication Welding of a 9%Cr Steel Reheat Steam Valveat Goodwin Steel Castings Ltd

Due to martensitic transformations and residual welding mechanical stresses, the weld andparent 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 carefullycontrolled to reduce the risk of stress cracking.

Welding is more often carried out with close to matching parent filler materials. Castings arealways pre heated prior to welding to reduce the thermal gradient between base materialsand 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 cooledto 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 bythe Larsen Miller parameter, where tempering temperature (P) should be 21 or higher toachieve adequate tempering.

P= °C+273(20+log t) x1000Where t = time in hours.

Tempering at too low LMP will results in too higher hardness of the weld and HAZ and thetoughness will be too low. However, too higher LMP value will cause the weld, HAZ andparent 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 attemperature will also over soften the material.

Over Tempering:This is where the PWHT temperature is too high and the material is heated to above thelower critical transformation temperature (AC1) and martensite starts to transform back toaustenite. When 9% Cr steels are exposed this temperature, martensite partially reaustenitise and the carbo nitride precipitates are coarsened but do not fully dissolve backinto solution.

The net effect is a part martensite part austenite material that lacks the pinning effect of theprecipitates 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 heattreatment 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 notgoing to completion or being absent or insufficient in size to stabilise the structure. Inaddition to the loss of creep rupture strength that this condition will bring, the material isalso at risk from brittle fracture and stress corrosion cracking.

Cast Materials for 700°C+ OperationThe drive for reduced carbon dioxide emissions and improved efficiency in coal fire powerplant, has lead to much work being carried out around the world with regards to materialdevelopment 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, soinstead 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 candidatealloys 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 arelower in strength than their precipitation hardened counter parts, and so have reducedstress rupture strength, but possess good ductility and weldability. However, for plantoperation above 760°C, only precipitation strengthened Ni alloys look likely to have therequire stress rupture strength.

Table 2: Typical Chemical Composition of 700’C + Candidate Alloys

MaterialGrade

Ni Cr Mo Nb Co W Al Ti StrengtheningMechanism

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:0.006Inconel

740®BAL 25 - 2 20 - 1 1.8 PH -

Haynes282®

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 thedifference in lattice parameters between the Ni based austenitic structure andthe solute atoms, the strengthening potency being proportional to thisdifference. The greater the percentage of solute atoms the greater will be thehardening effect. However, the amount of alloying additions is limited by thepossibility of σ-phase formation with its accompanying disadvantages.

Table 3: The following elements are used to accomplish thisstrengthening:-

Element Wt % in Gamma Calc Increased in FlowStress (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 intermetallicprecipitates during service this is largely a by-product of their composition andnot by design.

Precipitation Strengthened Ni Alloys

Precipitation hardening is accomplished by the formation of three types ofprecipitate, gamma prime (γ /) gamma prime prime (γ //) (or double gamma prime ),andgrain 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 byother elements.The γ / has the unusual property of increasing flow stress with temperature. Theflow 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 thealuminium content. The flow stress of γ / is also highly influenced by thealuminium content.

Thus at 1100K:-23.5 At% Al the flow stress is 29 ksi26.5 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 furtherquantum 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 fromconventional steel production in many ways. Specialised techniques are required toprevent oxidation of volatile alloying elements. VOD or AOD melting is employed forthe production of large components.

Pouring Technology: Ni Alloys that are alloyed with Titanium and Aluminium, andpoured in air, have to be poured with great care to prevent oxidation defects on thesurface of the castings. Special techniques have been developed in runner system

design to prevent such defects.

Solidification: Ni alloy solidification characteristics are completely different fromsteels, and conventional methoding techniques are often not satisfactory alone inproducing 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 readilywelded is essential, as any castings produced will have to be welded to inlet andoutlet steam pipeline. To further complicate the matter, due to the high expense ofnickel, these alloys will need to be welded to lower temperature material steels inlower temperature regions of the turbine. This arrangement will thereforenecessitate a heavy section dissimilar weld.

Weld repair of alloys like A625 is pretty weld established, however, heavy walledfabrication welding of one casting to another is not well established and takes specialtechniques 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 andrelatively low ductility at room temperature.

NDE of Ni Alloy Castings:

The power industry standard for many years for non destructive testing techniques has beenmagnetic particle inspection (MT) and ultrasonic inspection (UT). These techniques havebeen used for accessing the quality of the original steel castings during manufacture, and inline inspection during service.

Unfortunately, neither of these inspection techniques are acceptable for Ni alloy casting dueto the attenuation seen in Ni Alloys volumetric inspection using ultrasonics, and its poormagnetic permeability preventing MT from being a feasible technique for surface crackdetection.

The standard techniques for defect detection in Ni alloys are radiography (volumetric) andDye Penetrant (surface). Radiography inspection, due to grain size and density of Ni alloymaterial, 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 175mmMaterial A625 mod A625 mod A625 mod A625 mod

Table 3: Weld Se uence 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 steamturbine. This is not so surprising, as the same characteristics that ensure the Ni Alloyhas good high temperature behaviour, is also responsible for the poor machinabilityof 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 abrasiveto 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 forconventional CMV, Cr-Mo and 9% Cr steels.

Alloy CostsNi 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 thestock market, so that prices can vary widely from one week to the next. The graphs below showthe 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 todisplay the costs as a multiplying factor above that of P91.

Table 5: Cost Multiplier Table

Grade A740® A263 A617 A625 A230 A155More expensive than P91

Multiplication Factor X 15 X 15 X 15 X 14 X 13.5 X 11




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The previous tables highlight the significant expense of Ni Alloys when compared to that ofconventional 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 forthe 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 Nialloys should be used in the design. This would only be in the hottest and highest temperaturezones, 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-basedSuperalloys 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 onmicrostructural evolution in P91 and P92 power plant steels” R.C. MacLachlan, J.J Sanchez-Hanton and R.C Thomson (2009) -Paper presented at EPRIconference 2010 (Santa Fe).

5)” U.S Program for Advanced Ultra Supercritical (A-USC) Coal Fired Power Plants”Roberts Romanosky - presented at the “4

th Symposium on Heat Resistant Steels and Alloys

Used for High Efficiency USC Power Plants 2011”

Steve RobertsTechnical Director

Goodwin Steel Castings Ltd

Gsc Advanced Material Castings

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