Usc Presentation

Super Critical Boiler Materials – Metallurgical Aspects

R N Mehrotra, GM Energy Technology

Boiler Materials a. Introduction b.Design Consideration c. Materials Consideration

Thermal Power Plant

Rece nt Desi gn of Therma l Pow er Pl ants Base d o n

 Hig her The rmal Ef fici en cy  Global Env ir onment al Concer n Req ui red Ma teri al Sub -Cr iti cal Sup er-Cri tical Ultra Su per Cri ti cal Boi ler

High Cr ee p Stre ngt h, oxid ation and corros ion re sis tance Stable Micros tru ct ure at High te mp er atu re

Ach iev ed By Precipitation strengthening (Nb, V,Ti, Mo, W etc. form stable carbide & inter-metallics) Solution Strengthening

Co ns id er at ions

(Ni, Cr, Mo, W gives Solution strengthening)

Tub e wal l th ick ne ss

Interstitial Element Strengthening

Wel da bili ty

(B & N gives interstitial Elemental strengthening)

What is Super critical Super Critical Fluid is defined as a substance: Above critical Temperature (Tc) & Above critical Pressure (Pc) At which Liquid & Gas states are in equilibrium For CO2 Tc= 31.1 deg C & Pc = 73.8 bar

Super critical and ultra supercritical conditions Critical Conditions

Ultra super critical conditions

•Temperature -374.150C

•Temperature above 5600C

•Pressure-225.56kg/cm2

•Pressure above 306kg/cm2

Improvement of thermal efficiency •Increasing the steam temperature (ή increases 0.31% every 100C of increase of main steam temperature & 0.24% every 100C of increase of reheat steam temperature ) •Increasing in the steam pressure (ή increases 0.1% increase with increase of 10 bar pressure)

Efficiency in USC Boiler

• European and Japanese USC PC Experience Base – 580-600°C high availability, good load followers In Development: – European Advanced 700°C (1292°F) PC plant stalled? – DOE EIO/ EPRI 760°C (1400° (1400 F) boiler materials program Improvement of power plant heat rate with increase in temp. and pressure to turbine

Efficiency in USC Boiler

34.5/760/760/760

Efficiency, %, HHV

47

34.5/704/704/704 45 34.5/648/648/648

Pressure- MPa Temperature-0C

24.1/565/565 24.1/565/565

40

16.5/537/537 37 537

648

Temperature, 0 C

760

Efficiency in USC Boiler

Plant Efficiency

Efficiency in USC Boiler

Improvement of steam conditions in Japan

b.Design Consideration

For super critical Boiler materials • Wall thickness – Heat transfer – Welding

• Corrosion (Oxidation) • Erosion – Gas Velocity

Consideration Of Boiler Tube Design t= (PD/2S+P) + 0.005D +e P=S [2t-0.01D-2e/{D-(t-0.005D-e)}] D= Original O.D t= Thickness of tube C= Minimum allowance for threading and structural stability P=Maxm. Allowable working pressure R=Inside radius S=Maxm. Allowable stress value at the design temp. of metal t will decreas e if S wi ll i ncre as e; S can be i ncr eas ed by ch angin g mate rial ch emi str y e.g by soli d so lution streng th ening and or precip ita ti on stren gtheni ng but we have to con sid er CE V also f or we ldab il ity .

Steam Generating Tube Requirements for Steam generating Tube materials Creep properties and weldability Erosion resistance in context of Indian high ash coal

Materials

Chemical composition of candidate water wall materials for USC Boiler

Temp. (C )

Allowable stress(105hr )

T22

500

103 MPa

T23

500

111 MPa

T24

550

95 MPa

P92

550

103 MPa

Header and Steam Pipes Requirements Creep, thermal fatigue, weld ability Component

Phase 0 (31 MPa, 5650C)

Phase I (31MPa, 5930C)

Phase II (34.5MPa, 6500C)

Header and Pipes

P22, P23, P91, P92, P122

P91, P92, P122, E911

SAVE12, NF12

Evolution of Chromium steel Chemical composition of Candidate materials for Header

Thickness 820

Thickness, Cm

720 620

P22

520

P92

420

P122

320

NF709

220 120 20 400

450

500

550

600

650

700

Temperature, C

Thickness, Cm

800 700

P22

600

P92

500 400 300 200

Temperature Vs Thickness

100 0 400

450

500

550

600

650

Temperature Vs Allowable stress

700

Temperature,C

Allowable Stress and Thickness Requirement at three conditions of materials P22, and P92, P122, NF709 (with increase in Cr content) Conditions a. (172 kg/cm2, 4500C), b. (250 kg/cm2, 5500C), c. 306 kg/cm2, 6500C)

Contd. For Same Materials like P22 •Higher temp. and pressure thickness requirement is higher

Issue with Higher Thickness •Heat transfer affected •Chance of thermal Fatigue •Weld ability may be affected

Require •Lower thickness •Higher allowable stress •Materials of High Cr content like P92

c.Materials Property Considerations

Consideration Of Material Property  Cre ep  Fat igu e  Co rrosi on  Erosio n  Ox id at ion Wel da bi lit y Creep Oxidation

Remaining life due to change in microstructure due to creep

Erosion

Erosion For 200/210 MW unit

Issue Indian coal has higher ash content Ash is higher abrasive index For 500 MW unit

Tube failure & Loss of availability

Target 0% Current – 1.43% C-200- 1.02% C-500 – 1.62%

Creep and fatigue Enhancement of Creep strength by Decreasing stacking fault energy By stable precipitation By restricting dissolution and coarsening of precipitate By restricting grain boundary sliding Creep strength requirement with increase in temperature and pressure

By high dislocation density Delaying recovery of dislocation structure

Thermal fatigue Influence By Thermal conductivity of materials Thermal expansion co-efficient of materials Crack due to thermal fatigue

Strength of materials

Oxidation and Corrosion Oxidation is controlled by

Corrosion is controlled by

By formation of stable protective oxide layer

Formation of stable oxide layer, which will hinder diffusion of iron and electron

(By alloying addition like Cr, Al, Si)

Effective way to control By Chromising

Weight loss with chromium content

Boiler Tube Erosion

Tube failure analysis

Tube erosion Depend on •Fly ash particle size •Hardness •Velocity of propagation

Materials wear

Components •Steam generating tube •Header and Steam Pipe •Super-heater and Re-heater tube

Steam Generating Tube Requirements for Steam generating Tube materials Creep properties and weldability Erosion resistance in context of Indian high ash coal

Materials

Chemical composition of candidate water wall materials for USC Boiler

Temp. (C )

Allowable stress(105hr )

T22

500

103 MPa

T23

500

111MPa

T24

550

95MPa

P92

550

103 MPa

Header and Steam Pipes Requirements Creep, thermal fatigue, weld ability Component

Phase 0 (31 MPa, 5650C)

Phase I (31MPa, 5930C)

Phase II (34.5MPa, 6500C)

Header and Pipes

P22, P23, P91, P92, P122

P91, P92, P122, E911

SAVE12, NF12

Evolution of Chromium steel Chemical composition of Candidate materials for Header

Header and Steam Pipes

Thermal conductivity of some proposed header and steam separator materials

Thermal expansion coefficient of some proposed header and steam separator materials

Evolution of Cr-bearing steel

Super-Heater and Re-heater Tube Component

Super heater and Reheater tube

Phase 0 (31 MPa, 5650C)

Phase I (31MPa, 5930C)

Phase II (34.5MPa, 6500C)

 T91, 304H, 347 ( for non corrosive part) 310NbN (for corrosive)

TP347HFG,

NF709, Inconel 617

310NbN, SS347 (for corrosive)

Requirements Creep resistance Corrosion resistance Oxidation resistance

Austenitic steel are candidate materials for final stages, Nickel base super alloy can be used at still higher temperature

Allowable stress value

Evolution of austenitic steel

State of the Art Materials

Welding Aspects

Weldability Weld ability Require •Crack free weld •Achieve adequate mechanical property

Issue •Type IV Cracking •SCC of weldment

•Weld resistance to service degradation

PWHT is always required for advanced high chromium alloy Weded joint creep rupture strength should be considered

Welding Require • Proper welding process for joining of materials of different Cr content •Proper Choice of filler materials •Minimum Hardness requirement of HAZ

Issue •Micro structural degradation •Type IV cracking •Over tempering of base materials during PWHT

Cause of Type IV cracking •Undissolved Precipitates •Grain-boundary sliding •Impurity segregation •High stress in weldment

Type IV cracking

Welding

Welding Processes for Chromium steel GTAW SMAW

Consideration Pre-heat temperature Post weld heat-treatment temperature & Time

FCAW SAW Why Pre-heat? To resist hydrogen assisted cold cracking

Why PWHT?  To improve toughness of HAZ Lower the hardness of HAZ

Welding

Postweld heat treatment requires controlling temperature in four phases to

relieve the stress caused by welding for P91 steel

Why PWHT?

Welding Welding parameter Material: P92 Welding process: SMAW, SAW, GTAW Pre-heat treatment: For 350mm dia & 50mm. thickness 1500C for SMAW and 1000C for GMAW process, for thickness upto 6-8mm GMAW process and no pre-heat treatment PWHT: 7500 C -7600C for 2-4 hrs. for 50mm & above thick Material: P23 Welding Process: SMAW, SAW, GTAW Pre-heat treatment: 1500C for higher thickness, PWHT: 7150C for 2hrs for 50mm thick

Welding (P-92) 600

Ms

400

T E M P.

Mf

300

(0C)

200

Hardness vs cooling time

HAZ microstructure is martensitic at all cooling rate,

500

Hardness, HV

400

HAZ hardness is higher than 350HV

300

HAZ have lower impact toughness

200 100 0 0

50

100

150

Cooling time, second

200

250

It indicates PWHT is required for all cooling rate as HAZ has higher hardness and lower toughness

Welding Reported welding Parameter for SMAW process for P92 grade Welding Electrode: Composition almost similar to base metal Welding current: 140-180A Welding voltage: 18-26V Travel speed: 4-15cm/min Pre-heat and interpass Temperature: 200-3000C Diameter of Electrode: 4.0mm Heat input: 40-54kJ/cm Welding Pass: 30 PWHT: 7600C for 5hrs As transformed hardness of martensite in weld metal and HAZ in P92 is 350- 450HV & Higher tensile strength than acceptable value

Welding 600 400

T E M P.

300 200

(0C)

Hardness, HV

Hardness vs cooling time

CCT diagram of T24 steel

400 350 300 250 200 150 100 50 0

HAZ microstructure is Bainitic 0

50

100

150

Cooling time, second

200

250

Welding

Silent feature of Weding Parameter for P23/T23 steel Bainitic transformation takes place in HAZ Hardness of HAZ is <350HV Tube of smaller thickness not required Pre-heat-treatment PWHT is also not required for small thickness some time Good brittle fracture resistance of HAZ For higher thickness a PWHT at 7400C for 2 hrs in SMAW and 4 hrs for SAW process

Welding

Welding consumable for X20 & P91

CCT diagram of X-20 and P91 steel

Conclusions Higher steam temperature and pressure require materials having higher allowable stress at higher temperature High Chromium ferritic steel is used header and steam pipes Proper welding flux selection is required for welding of materials of dissimilar Chromium content High Chromium Austenitic steel is used for super-heater and reheater tubes Higher temperature of operation beyond 7500C may require Ni-base alloy