Super Critical Boiler Materials – Metallurgical Aspects

R N Mehrotra,

GM

Energy Technology

Boiler Materials

a. Introduction

b.Design Consideration

c. Materials Consideration

Recent Design of Thermal Power Plants

Based on

Higher Thermal Efficiency

Global Environmental Concern

Sub-Critical

Super-Critical

Ultra Super Critical Boiler

Required Material

High Creep Strength, oxidation and corrosion resistance

Stable Microstructure at High temperature

Achieved By

Precipitation strengthening

(Nb, V,Ti, Mo, W etc. form stable carbide & inter-metallics)

Solution Strengthening

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

Interstitial Element Strengthening

(B & N gives interstitial Elemental strengthening)

ConsiderationsTube wall thickness

Weldability

Thermal Power Plant

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

•Temperature -374.150C

•Pressure-225.56kg/cm2

Ultra super critical conditions

•Temperature above 5600C

•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)

• 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 760DOE EIO/ EPRI 760°C °C (1400(1400°F) boiler F) boiler materials programmaterials program

Improvement of power plant heat rate with increase in temp. and pressure to turbine

Efficiency in USC Boiler

Eff

icie

nc

y,

%,

HH

V

34.5/760/760/760

34.5/704/704/704

34.5/648/648/648

24.1/565/565

24.1/565/565

16.5/537/537

47

45

40

37

537 648 760

Temperature, 0 C

Efficiency in USC Boiler

Pressure- MPa

Temperature-0C

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

t= (PD/2S+P) + 0.005D +eP=S [2t-0.01D-2e/{D-(t-0.005D-e)}]

D= Original O.Dt= Thickness of tube

C= Minimum allowance for threading and structural stabilityP=Maxm. Allowable working pressure

R=Inside radiusS=Maxm. Allowable stress value at the design temp. of metal

t will decrease if S will increase; S can be increased by changing material chemistry e.g by solid solution strengthening and or precipitation strengthening but we have to consider CEV also for weldability.

Consideration Of Boiler Tube Design

Requirements for Steam generating Tube materials

Creep properties and weldability

Erosion resistance in context of Indian high ash coal

Chemical composition of candidate water wall materials for USC Boiler

Materials Temp. (C ) Allowable stress(105hr )

T22 500 103 MPa

T23 500 111 MPa

T24 550 95 MPa

P92 550 103 MPa

Steam Generating Tube

Header and Steam Pipes

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

Chemical composition of Candidate materials for Header

Evolution of Chromium steel

RequirementsCreep, thermal fatigue, weld ability

Temperature Vs Thickness Temperature Vs Allowable stress

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)

0

100

200

300

400

500

600

700

800

400 450 500 550 600 650 700

Temperature,C

Thic

knes

s, C

m

P22

P92

Thickness

20

120

220

320

420

520

620

720

820

400 450 500 550 600 650 700

Temperature, C

Th

ickn

ess,

Cm P22

P92

P122

NF709

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

Contd.

c.Materials Property Considerations

Creep Fatigue Corrosion Erosion OxidationWeldability

CreepOxidation

Consideration Of Material Property

Remaining life due to change in microstructure due to creep

Erosion

IssueIndian coal has higher ash content

Ash is higher abrasive index

Erosion

For 200/210 MW unit

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

Creep strength requirement with increase in temperature and pressure

Enhancement of Creep strength by

Decreasing stacking fault energy

By stable precipitation

By restricting dissolution and coarsening of precipitate

By restricting grain boundary sliding

By high dislocation density

Delaying recovery of dislocation structure

Thermal fatigue Influence By

Thermal conductivity of materials

Thermal expansion co-efficient of materials

Strength of materialsCrack due to thermal fatigue

Oxidation and Corrosion

Weight loss with chromium content

Oxidation is controlled byBy formation of stable protective oxide layer

(By alloying addition like Cr, Al, Si)

Corrosion is controlled byFormation of stable oxide layer, which will hinder diffusion of iron and electron

Effective way to control

By

Chromising

Boiler Tube Erosion

Tube failure analysis Tube erosion

Materials wear

Depend on

•Fly ash particle size

•Hardness

•Velocity of propagation

Components

•Steam generating tube

•Header and Steam Pipe

•Super-heater and Re-heater tube

Requirements for Steam generating Tube materials

Creep properties and weldability

Erosion resistance in context of Indian high ash coal

Chemical composition of candidate water wall materials for USC Boiler

Materials Temp. (C ) Allowable stress(105hr )

T22 500 103 MPa

T23 500 111MPa

T24 550 95MPa

P92 550 103 MPa

Steam Generating Tube

Header and Steam Pipes

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

Chemical composition of Candidate materials for Header

Evolution of Chromium steel

RequirementsCreep, thermal fatigue, weld ability

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

Component Phase 0(31 MPa, 5650C)

Phase I(31MPa, 5930C)

Phase II(34.5MPa, 6500C)

Super heater and Reheater tube

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

TP347HFG, 310NbN, SS347 (for corrosive)

NF709, Inconel 617

Super-Heater and Re-heater Tube

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

Evolution of austenitic steel

Allowable stress value

Requirements

Creep resistance

Corrosion resistance

Oxidation resistance

State of the Art Materials

Welding Aspects

Weldability

Weld ability Require

•Crack free weld

•Achieve adequate mechanical property

•Weld resistance to service degradation

Issue

•Type IV Cracking

•SCC of weldment

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

Type IV cracking

Cause of Type IV cracking•Undissolved Precipitates

•Grain-boundary sliding

•Impurity segregation

•High stress in weldment

Welding

Welding Processes for Chromium steel

GTAW

SMAW

FCAW

SAW

ConsiderationPre-heat temperature

Post weld heat-treatment temperature & Time

Why Pre-heat?

To resist hydrogen assisted cold cracking

Why PWHT?

To improve toughness of HAZ

Lower the hardness of HAZ

Welding

Why PWHT?

Postweld heat treatment requires controlling temperature in four phases to

relieve the stress caused by welding for P91 steel

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)

Hardness vs cooling time

0

100

200

300

400

500

0 50 100 150 200 250

Cooling time, second

Har

dnes

s, H

V

TEMP.(0C)

600

400

300

200

Ms

Mf

HAZ microstructure is martensitic at all cooling rate,

HAZ hardness is higher than 350HV

HAZ have lower impact toughness

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

Hardness vs cooling time

050

100150200250300350400

0 50 100 150 200 250

Cooling time, second

Har

dn

ess,

HV

TEMP.(0C)

600

400

300

200

HAZ microstructure is Bainitic

Welding

CCT diagram of T24 steel

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

CCT diagram of X-20 and P91 steel

Welding consumable for X20 & P91

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