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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