111031 ccug p91 fabrication guidelines dave buzza-1
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111031 CCUG P91 Fabrication Guidelines Dave Buzza-1TRANSCRIPT
P91 Piping Fabrication Guidelines
David Buzza American Electric Power
Combined Cycle User’s Conference October 31, 2011
History
• Grade 91 was developed for high temperature service. Allowable stress of P91 is 2.5 times greater than P22 at 1080F.
• Grade 91 approved by ASME B&PV Code Section I and VIII in 1983-1984.
• Grade 91 has been in service since the 1990’s for retrofits of traditional fossil plants, new supercritical plants and new combined cycle plants.
Most Common Fabrication Issues with P91 Identified by Inspection
• Target Hardness – Components = 190-250 Hb – Welds = 190-280Hb
• Small Bore (</= 4 inch) – Hard Components – Hard Socket Welds – Soft Components
• Large Bore (>4 inch) – Hard Attachment Welds – Soft Components (mostly elbows)
P91 Fabrication Issues – Hard Components
• ASME A335 specifies max allowable hardness of 250Hb.
• Risks - Overload or creep failure due to improper metallurgy
• HP stop valve before seat drain line failure had hardness of 318-351Hb.
• Plant X - identified 4 small bore pipe sections (330-390Hb).
• Plant Y– identified 13 large and small bore components (316-366Hb) HP Stop Valve Before Seat
Drain Line Failure
Failure
Drain Line
P91 Fabrication Issues – Hard Socket Welds & Attachment Welds
• AEP P91 Fabrication specifies less than 280Hb weld hardness
• Risks - Cracking due to SCC or lack of toughness
• Plant X – 30 of 300 socket welds (300-417Hb)
• Plant Y – 76 of 773 socket welds (300-437Hb)
• Plant Y – 34 of 94 attachment welds (315-439Hb)
Socket Weld
Pipe
Elbow
P91 Fabrication Issues – Soft Components
• AEP P91 Fabrication Standard specifies hardness of >190Hb.
• Risks – shortened creep life due to improper metallurgy.
• Plant X and Y – identified numerous components with hardness of <165Hb
Metallurgy
P22 P91 Base Metal Ferrite / Pearlite Martensite
Welding Bainite microstructure. Hardness = 280Hb
Martensite microstructure. Hardness = 400Hb
PWHT Stress relieving (weld shrinkage) and tempering hardness. Bainite transforms to ferrite Hardness = 200Hb
Stress relieving, tempering hardness and formation of precipitates Martensite must be maintained Hardness = 250Hb
If PWHT is above LCT, then microstructure remains the same.
If PWHT is above the LCT, then microstructure changes drastically and component can become soft or hard.
LCT = Lower Critical Temperature
Improper PWHT is the Root Cause for P91 Fabrication Issues
Fabrication Issue Root Cause Hard Components PWHT temperature exceeded LCT
followed by rapid cooling.
Hard Welds PWHT temperature at the weld never reached hold temperature.
Soft Components PWHT temperature exceeded LCT followed by slow cooling. Or PWHT hold time is excessive.
PWHT Demonstration I Hold Temperature > LCT
2” Sch 160 P91 Pipe, Hold Time = 3 hours
Code Hold Temperature = 1350-1425F
Lower Critical Temperature = 1475-1575F
After PWHT target = 190-250Hb
PWHT Demonstration II Hold Temperature > LCT
Monitoring TC
Control TC
1. Thermocouple (TC)
2. Heating Beads
3. Insulation & Cook
PWHT Demonstration II Chart
1400F
1200F
1600 F Both socket welds hold at 1430 F
One Pipe T peaks at 1728 F
Other Pipe & Elbow remain under 1430 F
1800 F
2 Hr
PWHT Demonstration II Final Hardness, Hb
Low hardness - pipe temperature exceeded lower critical temperature
Good hardness - pipe & elbow temperature remained below lower critical temperature
Hardness Target = 190-250Hb
Waterford Drain Pipe Fabrication
• Material – 2 inch socket welded, P91 pipe spools.
• Pipe spools were fabricated in shop then shipped for install.
• Shop PWHT – Furnace – Induction
Spool installation
Drain Pipe Fabrication Furnace PWHT
• Temperature chart showed that target temperature and hold time were met.
• Final hardness check showed 100 of 170 welds to be hard.
• Conclusion – furnace temperature did not accurately reflect the actual weld temperature during heat treating.
Furnace layout of spools
Drain Pipe Fabrication Induction PWHT
• Additional thermocouples, TC’s were added.
• Temperature chart of pipe, elbow and weld showed that target temperature and hold time were met.
• Final hardness check showed that all 28 welds were properly heat treated.
• Conclusion – Proper heat treatment results in good hardness readings.
Keys to Proper Fabrication of P91
1. Check incoming material A. N:Al on the Material Test Report (MTR)
should be greater than 4:1 and under no circumstances less than 2.5:1
B. Hardness on the MTR and own testing should be 190-250Hb. A hardness of 220Hb is preferable because hardness will decrease with each PWHT
Keys to Proper Fabrication of P91 (con’t)
2. Improve heat treating procedures A. Follow AWS D10.10
i. Number of control zones based on pipe diameter ii. Heating blanket length based on pipe diameter and thickness iii. Monitoring thermocouples for soak zone and heat band zone
B. Use heat treating set-up sheets i. Procedure – ramp rates, hold times / temperatures ii. Wrap sheet
a. Heating pad size / placement for each heating circuit b. Control thermocouple placement for each heating circuit c. Monitoring thermocouple placements
C. Review heat treating set up sheets i. Check that hold time has been reduced for smaller welds. ii. Check that heating pads will contact the pipe as much as possible. iii. Check that thick sections and thin sections are on separate heating circuits. iv. Check that control thermocouples are towards center of pad and at the location that
you would expect to be the hottest. v. Check that monitoring thermocouples are where you want the heat (weld) and
where you do not want to overheat (base metal). vi. Check that someone will be monitoring the heat treating and can abort if needed.
Heat Treating Wrap Sheet that Shows Proper Control & Monitoring
• Heating blankets • Control
thermocouples – valve body – pipe soak band
• Monitoring thermocouples – weld – pipe heating band
Heat Treating Wrap Installed
TC’s
weld
Heating blankets
If in doubt, then add TC’s
Keys to Proper Fabrication of P91 (con’t)
3. Confirm proper heat treating by hardness testing
A. Weld hardness reduced. B. Base material hardness maintained within 0 to 20
Brinell hardness points of initial readings.
Note: Hardness testing should be performed on a grid by a qualified tester. Testers will think that they are qualified, but they are not. We use GE MIC10 and Pin brinell or telebrineller
Conclusions
• If your fabricator continues to treat grade 91 materials the same as they treat other traditional high temperature materials (i.e. grade 22), then you will continue to have quality issues.
• You must insist that your fabricator upgrades their heat treating (improved control and monitoring) and adds hardness testing to their procedures to successfully and consistently fabricate grade 91 materials.