The “IMPACT” Project - Development of

“MARBN” Steel for Next Generation Steam Plant

New Materials Seminar, IOM3, London, 23-24 May 2013.

Authors

David Allen – E.ON New Build & Technology

Craig Degnan - E.ON New Build & Technology

Hailiang Du, Rod Vanstone - Alstom

Paul Moody, Peter Barnard - Doosan Power

Steve Roberts - Goodwin Steel Castings

Ryan Maclachlan, Prof. Rachel Thomson – Loughborough University

2

IMPACT – in outline

� UK Government (TSB) part-funded industry-led collaborative project

� 4 years – 2010 to end 2013 Budget £1.8M

� Development of advanced welded MARBN steels for USC power plant

� Improved design for welded components to reduce premature cracking

� Improved strain and materials monitoring to allow high temp operation

� Steel product – Shaped casting, ingot for forging, tube machining

� Demonstration plant component – welded boiler tube insert on plant

� Interaction with UK “Supergen” (university led) partnership

• Collaborative interchange with ongoing COST / KMM-VIN programme

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IMPACT - organisations

Partners

� E.ON New Build & Technology, Ratcliffe (Nottingham), UK – plant user

� Doosan Power, Renfrew (Glasgow), UK – boilers / welding

� Alstom, Rugby, UK – turbine and power plant technology

� Goodwin Steel Castings, Stoke-on-Trent, UK – cast materials supplier

� National Physical Laboratory, Teddington, UK – monitoring technology

� Loughborough Univ., UK – microstructural characterisation and modelling

Associates

� Tata (Corus), Rotherham, UK – steel development advice, supply

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IMPACT – the people

Partners

� E.ON – Craig Degnan (Project Manager), David Allen (Project Leader)

� Doosan – Peter Barnard, Paul Moody

� Alstom UK – Hailiang Du, Rod Vanstone

� Goodwin – Steve Roberts, Steve Birks

� NPL – Jerry Lord, Bryan Roebuck

� LU – Prof Rachel Thomson, Ryan Maclachlan, Letian Li, Mark Jepson

Associates

� Tata UK – Peter Morris, Richard Williams, Philip Clarke

� TSB contract Monitoring Officer – Jon Tenner

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IMPACT – the project roles

Partners

• E.ON – Project planning, management, testing, design, analysis

• Doosan – Weld manufacture, creep testing, design

• Alstom UK – Materials, mechanical & creep testing, European R&D liaison

• Goodwin – Materials, casting, development, 8 tonne “baseline” cast

• NPL – DIC strain monitoring, ETMT miniature testing

• LU – Electron metallography, modelling, materials development

Associates

• Tata – small scale pilot melts, steelmaking consultancy

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MARBN – A novel high alloy steel for power plant

1950s to date – Low alloy creep resisting steels – 2¼CrMo, CrMoV

Ferritic structure, limited carbide strengthening

Applications up to about 540 - 570°C maximum

1980s development – P91 or “Modified 9%Cr” steel

Introduced from early 1990s onwards – coal plant boiler headers and drums (UK first),

steam pipework and HRSG applications worldwide

Martensitic structure – fine scale lath structure for increased creep strength

Carbide precipitate chains on lath boundaries

Vanadium modified to add finer-scale network of vanadium nitride precipitates

Applications generally up to about 580°C (or higher if at low stress)

1990 - 2000 – P92 steel

Replace molybdenum with tungsten in P91 – Some strength increase

Applications – e.g. 600°C main steam, 620°C hot reheat

Today’s leading alloy – The end of the line for martensitics?

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MARBN – A novel high alloy steel for power plant

European “COST” programme – Developing alternatives to P91 / P92

Some success – e.g. V&M VM12 alloy for better oxidation resistance (tubing)

FB2 and CB2 alloys for large turbine components - slightly better than P92

Experimental boron additions – Inconsistent results

Some materials showing promise – Free boron can segregate to interfaces and hence

stabilise the carbide structure - but this appeared hard to control

The breakthrough – By Fujio Abe, NIMS, Japan

Boron and nitrogen must be controlled together

(hence “MARBN” – MARtensite plus Boron plus Nitrogen)

When too much B and N are added – Boron nitride (BN) precipitates form

This removes free boron from solution and strengthening is lost

So – Careful microalloying with B and limited N achieves high creep strength

A winning combination? (but is this recipe too complex to be commercial?)

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Published MARBN creep data – NIMS and TU Graz

NIMS data – 160B 85N steel

Similar to Graz NPM1 data

(120B 130N)

Weld HAZ failure likely

at lower stresses

MARBN retains 30%+

creep strength advantage

over P92 in longer term

accelerated tests

A problem with creep testing at 650°C - Long term tests are needed

to show the weld strength reduction. In IMPACT, test at 675°C

NPM1 and Abe data, 650°C

0

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100 1000 10000 100000

Life h

Str

ess M

Pa

P92 mean

NPM1 parent

NPM cross-weld

Abe 160B 85N parent

Abe 160B 85N xweld

“VS4863” trial melt (rolled plate) – To match NPM1.

VS4863 normalised at 1150°C

Creep tests at 675°C – to 3000h

Broadly similar results

to NIMS / Graz

when expressed as % of

ECCC mean P92 creep strength

- Obtained in a shorter time

NPM1 and Abe data, 650°C, plus IMPACT data, 675C - Parent

80

100

120

140

160

60 80 100 120 140 160 180 200

Stress MPa

% P

92 m

ean

str

en

gth

NPM1

Abe 160B 85N

VS4863, N1150

MARBN shows greatest advantages over P92 at intermediate stress

High stress → Hot tensile test

Low stress → Long exposure, reduced precipitate strengthening (all alloys)

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“VS4863” trial melt (rolled plate) – To match NPM1.

Weld data now included.

IMPACT results at 675°C

again match Graz 650°C data

fairly well

Weld strength reduction may be

slightly less at 675°C than 650°C

IMPACT results confirm that

the MARBN HAZ is not immune

from “Type IV” creep failure

at lower stresses,

- but data still better than parent P92!

NPM1 and Abe data, 650°C, plus IMPACT data 675C

60

80

100

120

140

160

60 80 100 120 140 160 180 200

Stress MPa

% P

92 m

ean

str

en

gth

P92 meanNPM1 parentNPM1 cross-weldAbe 160B 85N parentAbe 160B 85N xweldNPM1 xweld fail parentVS4863N1150 parentVS4863N1150 xweld

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Project Summary – Alloy Development

� 11 small vacuum melts and 8 variant air melts made and creep

tested

� Variations in alloy content, B and N levels, processing, heat

treatment

� Vacuum melts tested after rolling to plate, normalising and tempering

� Air melts tested in the as-cast (N&Td) condition

� 4 air melt variants also tested after rolling to plate and N&T

� 8 variants (4 cast, 4 rolled) also welded (IN625) and cross-weld

creep tested

� Typically 4 creep tests per parent or cross-weld alloy variant

� Tested out to ~3,000h at 675°C – equivalent to 13Kh at 650°C on

LMP basis

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Alloy development – Key Results

� Simple air melting proves uncertain – hard to control N and deoxidation – not

recommended for MARBN production, but useful in alloy development

� Considerable variation in results – but a substantial range of variants show creep

strength at least 20-30% higher than P92 within test condition range

� HAZ strength reduction applies, but MARBN again much better than P92

� Rolled products show substantially poorer creep strength, but better ductility, than

corresponding as-cast materials

� Quality heat treatment and prior steel processing conditions substantially affect creep

performance

� Thermodynamic modelling usefully clarifies roles of BN, borides, etc

� Optimised composition developed for pilot scale manufacture

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GA

M5C

GA

M1C

GA

M6C

GA

M4C

GA

M7C

8T s

tandar

d HT

GA

M8C

GA

M3C

8T H

T A8T

HT B

GA

M2C

8T H

T C

Goodwin cast material

Avera

ge %

P92 c

reep

str

en

gth

8 tonne melt – Cast plate – Comparative short term creep

data – Averaged values for each material variant

GAM = Goodwin air melt: marks 8T melt

Outside

developed

specification

Inside

developed

specification

Improved

heat

treatment

Improved heat treatment – The IMPACT way forward

Too little B and N – Not enough carbide stabilisation, and not enough VN formed

Too much B and N – All the B and N vanish into useless large BN inclusions

Also – BN tends to form at temperatures around 1050-1100°C in casting and processing

– So conventional “normalising” heat treatment temperatures (e.g. 1040-1100°C for

P91) are not well optimised.

TU Graz development – Normalise at 1150°C

IMPACT (Loughborough University) showed: This still does not dissolve all BN

Recommendation: Normalise at 1200°C to redissolve all BN (and control alloy

balance to prevent delta ferrite formation at that temperature)

IMPACT data show – This can add ~ 10-15% to creep strength

and – The range of acceptable B and N levels also widens

How much stronger is MARBN?

Mean data analysis: Typical result is 20-40% stronger than mean P92

(short term data)

Hence – take MARBN as 30% stronger than P92

Possible 10%+ further improvement – optimise HT

Possible reduction – in long term lower stress data

Lower bound analysis: Poorest MARBN cast - 120% mean P92 strength

Poorest P92 material – 80% mean P92 strength

So – Design value is based on lower bound:

Design strength [MARBN / P92] = (120 / 80)

So – arguably - MARBN is 50% stronger than P92!

MARBN – Narrow range in strength, wide range in

composition

Element Cr Co W Mo V Nb B N Al B+N

wt % wt % wt % wt % wt % wt % ppm ppm ppm ppm

Maximum 8.9 3.2 3.1 0.1 0.22 0.11 195 210 200 380

Minimum 8.6 2.8 2.5 0.01 0.19 0.06 90 150 50 285

The range of creep strength values –

commonly assumed to be typically +/- 20%

for a specified alloy, is quite narrow.

However, the range of compositions

included in the corresponding six variant

materials is quite wide.

So, MARBN properties are not critically dependent on precise control of

chemistry. On the contrary, quite wide ranges are acceptable.

Also please note, the above may not always be the widest acceptable ranges –

they are merely the limiting values applying to the variants we made!

Why is MARBN mean creep strength 30–40% > P92?

Two main possibilities:

(1) – “9Cr3W3Co” base alloy is intrinsically stronger than P92, 9Cr0.5Mo1.8W

(2) - Control of B and N content is crucial strengthening factor

Which is dominant?

IMPACT also developed trial alloys based on P92 but with controlled B and N

Best result was – 13% stronger than the mean for normal P92

However, the normalising temperature was not optimised in this development

8 tonne melt data show – 1200°C normalising can add further 10%+ strength

So – A tentative, approximate estimate might be:

Control of B and N may account for some ~20-25% strengthening

Intrinsic advantages of 9Cr3W3Co may account for some ~10-15%

- but more work is needed to clarify these factors

Trial melts – To match NPM1, and lower N alternative

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80 100 120 140 160

Stress MPa

Str

en

gth

%P

92

140B 85N

140B 85N w eld

VS4863 140B 150N

VS4863 140B 150N w eld

Aim Actual

B 160 140

N 85 85

B 120 140

N 130 150

Abe lower N composition

Graz NPM1 composition

Weld strength reduction – some initial comments

� As shown by TU Graz, MARBN is not immune to weld HAZ cracking

� Reducing nitrogen to 85ppm does not avoid the problem

� Note that Abe has shown that if N is reduced to the ~10 ppm level, weld

HAZ cracking is certainly eliminated – but the parent steel is then weaker

� Failure in welds with nickel alloy fillers is often partially along the fusion line,

then partially through the HAZ. This increases the stress acting on the HAZ

and may produce pessimistic data on weld performance. A matching ferritic

filler could therefore probably produce a better result

� MARBN shows a substantial weld strength reduction, but it may well be a

lesser percentage than is applicable e.g. to P91 or P92

� In any case, MARBN is a much stronger steel than P92!

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8 Tonne AOD melt successfully produced

� Optimised IMPACT MARBN steel composition, including practical

provisions for control of minor elements and impurities, successfully applied

in scaling-up to 8 tonne AOD melt production

� Goodwin 8T melt poured in May 2012

� Products produced:

- Small 150x150mm ingots for forging / rolling to a trial wrought product

- Larger O2 ingot available - potential manufacture of thick (pipe?) product

- 3 tonne “bonnet” shaped casting to simulate a typical turbine component

- Cast material test plates and weld plates

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8 Tonne Melt Production – May 2012

Courtesy of Goodwin Steel Castings LtdPhotographs by Ryan McLachlan and Letian Li, Loughborough University

3,500Kg Ingot Cast Test Plates

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Next Steps – Testing the 8 Tonne Melt

� Short term creep tests at 675°C –conditions to match devt. programme �

� Investigate effect of product form – casting, forging, section size – to follow

� Investigate effects of heat treatment variables, optimise tempering �

� Develop manufacturing, welding procedures for tube plant trial - ongoing

� Medium term creep tests at 675°C, 650°C and 625°C - Planned

� Parent and cross-weld

� Critical evaluation and data comparison with alternative MARBN products,

which have mainly been tested at 650°C

� Long term tests beyond 2013 – Required for MARBN design code approval

� UK – Aim primarily to test forged and welded material

� Forging of ingot to bar for (machined) tube manufacture – under way

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Next Steps – MARBN Steel

� A real advantage over P92 – Potential for 20°C+ plant uprating?

� Evolution versus revolution – Why seek more novel ideas, when we already

have a substantially proven concept which we have not yet implemented?

� Needed – Long term design data

� Needed – Realistic data for long term, lower stress, welded applications

� Needed – Characterisation and quality assessment of large shaped casting

� Needed – Development of pipe manufacture

� Needed – Development of matching welding consumables

� Needed – Modelling of long term degradation mechanisms (e.g. Z, Laves)

� Needed – European collaboration into the future

� IMPACT has extensive data and experience to offer - for the next steps!

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Next Steps – IMPACT and KMM-VIN

� KMM-VIN – The successor organisation to COST in European collaboration

� IMPACT steel - Potentially available free to KMM-VIN partner/s and others

� Subject to suitable collaboration agreement

� Cast material (either plate or shaped casting) available now for creep testing

� Large “bonnet” casting available for product characterisation and high temperature

mechanical testing (e.g. stress rupture, LCF, tensile, impact, welding and weld

performance,etc) (30° segment removed in as-cast condition: main casting available

for full-scale quality heat treatment)

� Coupons etc available for steam oxidation testing

� 150x150mm ingot/s available for tube manufacture

� Large “O2” octagon ingot available to develop pipe manufacture

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Next Steps – IMPACT and beyond

� Cast material testing – Plans now being worked up for long term creep

testing and fatigue / creep-fatigue testing at three European laboratories

� Coupons for steam oxidation testing – Multi-laboratory project being

worked up – MARBN may have abnormally good oxidation resistance

enabling high temperature tubing application, but not yet well understood.

� 150x150mm ingot/s for tube manufacture – Interest in European

development of conventional (pierced and drawn) tube making process

� O2 octagonal ingot for development of pipe manufacture – Under

discussion with a leading European pipe manufacturer

� Wrought material testing – Long term creep testing planned in UK

� Under discussion – Welding development using IMPACT material