Lesson 6 - Issues of Concern.ppt

Issues Of Concern To ASME Boiler & Pressure Vessel Committee TG On Creep-Strength

Enhanced Ferritic Steels, And Remedies Under Consideration

J. F. HenryIIW-AWS

Technical Lectures

The Cr-Mo Steels

January/February 2006

Lesson 6IIW-AWS


• Many Problems With Use Of Grade 91 With Existing ASME Rules

• With The Advent Of The Next Generation Of Creep-Strength Enhanced Ferritic Steels – Grades 23, 92, 911, 122, etc. – There Is A Clear Basis For Concern That Problems Will Be Compounded In The Absence Of Comprehensive, Technically Defensible And Widely Accepted Sets of Rules

A Need For Comprehensive Rules For This Class of Alloys

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• A Section II (Materials) Task Group Has Been Formed To Review Current Code Rules Governing The Use Of The Creep-Strength Enhanced Ferritic Steels And Make Recommendations For Changes That Will Control Their Use More Effectively

SCII Task Group Considering The Creep-Strength Enhanced Ferritic Steels

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Task Group Consists Of Industry Specialists In The Development And Use Of The CSEF Steels And Includes The Following:

• D. Canonico (Past Chmn - Main Committee, Exec. VP: Board of Pressure Vessel Technology • K. Coleman (EPRI)• J. Feldstein (Vice Chmn: Main Committee, Chmn:SC IX)• P. Flenner (Consultant)• D. Gandy (EPRI)• M. Gold (Chmn: SC II)• J. Henry (Alstom) - Chairman• F. Masuyama (Professor, Alloy Developer)• W. Newell (Euroweld)• M. Praeger (MPC)• B. Roberts (Consultant)• W. Sperko (Consultant)• R. Swindeman (ORNL)• J. Tanzosh (B&W)• J.C. Vaillant (V&M Tubing)

Lesson 6IIW-AWS


• Primary Focus Of Task Group, Consistent With Explicit Code Mission, Are Those Issues With Obvious Safety Implications

• Since One Important Use For These Alloys Is As Piping For The Main And Hot Reheat Steamlines In Power Plants, Anything That Potentially Affects Rupture Strength Or Weld Integrity Is An Issue

Principal Attention On Safety Issues

Lesson 6IIW-AWS


Quick Review of Basic Metallurgy

• CSEF Steels All Depend For Their Elevated Temperature Strength On A Specific Condition Of Microstructure

• The Precipitation Of Temper Resistant Carbides/Carbo-Nitrides At Crystalline Defect Sites Impedes Material Flow At Elevated Temperatures

• Anything That Disrupts This Structure, Reduces The Strength And Stability Of The Alloys

Lesson 6IIW-AWS


Issue 1: Intercritical Heat Treatment/Overtempering/Undertempering

Problem: (a) ICHT – Coarsens, but does not fully dissolve, precipitates; “pinning” effect is lost and “new” martensite has reduced high

temperature strength (strength drops to level of Grade 9)

b) Overtempering – Precipitates are coarsened, lath structure is destroyed, rupture strength drops to Grade 9 level

c) Undertempering – More rapid recovery, brittle structure, SCC susceptibility

Solution: (a) Impose Upper Temperature Limit On Tempering And PWHT To Avoid ICHT And Overtempering

b) Review Minimum Tempering Limitsc) Prohibit Localized Heat Treatments If Temperature Exceeds AC1

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The Effects Of Intercritical Heating

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Task Group Action

• Normalizing of Grades 91, 911, 23, 92, and 122: 1900-1975°F

• Tempering of Grades 91, 911, 23, 92, and 122: 1350-1470°F

• PWHT of Grades 91, 911, 23, 92, and 122: <1/2” 1325-1470°F>1/2” 1350-1470°F

• Note: For DMWs, if the Chromium content of the filler material < 3%, or if the filler material is an austenitic or nickel based material, then the minimum tempering temperature remains 1300°F.

• For any component in which a portion of the component is heated above 1470°, the component must be re-normalized and tempered in its entirety, or as an alternate, the heated portion can be removed from the component for re-normalizing and tempering and replaced into the component.

Lesson 6IIW-AWS


Issue 2: Post-Weld Heat Treatment

Problem: Some elements, such as Ni, depress both A1 and A3 temperatures, and Ms and Mf temperatures. Risk of either intercritical heat treat damage or presence of untempered martensite in weld metal (AWS allows up to 1% Ni in weld metal vs 0.4% max. in base metal specifications).

Solution: Modify PWHT requirement based on Ni + Mn Content.

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Effect of Ni and Mn on A1 Temperature

Lesson 6IIW-AWS


• For P 5B, Group 2 Material (Only Grade 91, at Present)• PWHT Temperature Range: 1350-1425 °F (730-775 °C)• < 5“ thick: 1 hr/in, 30 min. minimum• > 5” thick: 5 hr + 15 min for each inch over 5”• For weld thickness < 0.5” , minimum PWHT temperature is 1325 °F • If chemical composition of matching filler metal is known; the

maximum PWHT temperature can be increase as follows:– If Ni + Mn < 1.50%, but > 1.0%, the max. PWHT temp. = 1450 °F (790 °C)– If Ni + Mn < 1.0%, the max. PWHT temperature = 1470 °F (800 °C)

Task Group Action

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Task Group Action - PWHT

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Issue 3: Code Acceptance Of New Materials

Problem: The Code Does Not Specify Any Control Of The Chemistry Of The Minimum of 3 Qualifying Heats Relative To The Supplier’s Recommended Ranges. For CSEF Steels, The Level Of Precipitate-Forming Elements Is Critical To Material Performance (Example of Grade 23)

Solution: (a) Require That Qualifying Heats Include A “Rich” And “Lean” Heat(b) Approve Chemistry Ranges Based Strictly On Chemistries Of Qualifying Heats(c) Insure Careful Review Of All Intentionally Added Elements (e.g., Aluminum)

Lesson 6IIW-AWS


Element Compositional Limits, %

Carbon 0.04-0.10 Manganese 0.10-0.60

Phosphorus, max. 0.030 Sulfur, max. 0.010 Silicon, max. 0.50 Chromium 1.90-2.60

Molybdenum 0.05-0.30 Tungsten 1.45-1.75 Vanadium 0.20-0.30

Columbium 0.02-0.08 Nitrogen, max. 0.030

Aluminum, max. 0.030 Boron 0.0005-0.006

Grade 23 Chemistry Requirements

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Issue 4: Cold Work Effects On Creep-Rupture Strength

Problem: The energy induced by cold work can destabilize the structure, triggering more rapid recovery/recrystallization, with loss of the desired microstructure. Different alloys do not respond in the same manner to the same level of cold work, according to limited studies performed to date (e.g. Grade 91 vs Grade 92).

Solution: Impose requirements for each alloy so that above a certain level of cold strain, renormalizing and tempering of the component is required. For Grades 23 and 91, the level probably will be near 20% cold strain. (R/D 2.5)

Lesson 6IIW-AWS


Effect of Cold-Work On Stress-Rupture Behavior Of Grade 91 Material

1.0

10.0

100.0

57.00 58.00 59.00 60.00 61.00 62.00 63.00 64.00 65.00 66.00 67.00 68.00 69.00

LMP=(T+460)(36+logt)/1000

Stre

ss (k

si)

Imputed Mean For Grade 91 (<3 in.)

Imputed Mean For Grade 91 (>3 in.)

Imputed Minimum For Grade 91 (<3 in.)Imputed Minimum For Grade 91 (>3 in.)

Grade 91-Base Metal (0%CW)

Grade 91-30%CW

Grade 91-20%CWGrade 91-10%CW

Note: Imputed Mean And Minimum Properties Are Calculated Based on ASME Maximum Allowable Stress

Lesson 6IIW-AWS


Issue 5: Control of Properties Through Hardness Testing

Problem: A quick and inexpensive method for evaluating process integrity is needed, and hardness testing is an obvious tool that may provide an indication of the condition of the material. However, there can be substantial variability in portable hardness test results. Variables include type of tester (e.g., rebound vs penetration), skill of tester, surface decarburization, surface cold work, intercritical heat treatment effects.

Solution: Impose “recommended” hardness limits that, if exceeded, require additional testing (e.g., replication, destructive sampling) to demonstrate integrity of the processing. (Note that this does not address the issue of intercritical heat treatment effects.)

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Issue 6: Stress-Corrosion Cracking

Problem: The 9-12% Cr CSEF Steels can be susceptible to SCC in the fully hardened condition – a kind of sensitization. Environment and composition are factors of unknown (at this time) significance.

Solution: Impose limits on permissible time between completion of welding or normalizing and completion of PWHT or tempering. Or, require that hardened component be maintained dry until tempering/PWHT. Or, require NDE after completion of tempering/PWHT to demonstrate freedom from cracking (both OD and ID surfaces).

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SCC in Grade 91 at Safe End Welds.

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Issue 7: Use of Tempering Parameter To Control Processing of CSEF Steels

Problem: Accurate control of final properties can only be achieved through the use of a tempering parameter. The final microstructure is a function of total time at temperatures (unless the critical limits are exceeded.) However, optimum range of parameters for each material have not been definitively established.

Solution: Commission additional testing to identify optimum parameter range for each material, and then impose restrictions so that results of total processing fall within that range for each material

Lesson 6IIW-AWS


Issue 8: Integrity of Long-Term Creep-Rupture Data Extrapolations for CSEF Steels

Problem: The creep-rupture behavior of the CSEF Steels appears to be more sensitive to the effects of temperature and stress within certain operating ranges then “traditional” Cr-Mo steels, such as A1 Grade 22.– Question of validity of the LM constant of 30-35 at

lower stresses– Lowering of allowable stresses for certain grades

Solution: Continual re-evaluation of data as longer-term tests are concluded to verify reasonableness of extrapolations.

Lesson 6IIW-AWS


Major Reduction in Allowable Stresses for Grade 122 Based on Test Data Misinterpretation

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Issue 9: Elastic Follow-Up

Problem: The Mixing of CSEF Steels and “Traditional” Low Alloy Steels in a Piping System Can Result in the Application of a Non-Diminishing Secondary Stress (sic) at Dissimilar Metal Joints, i.e. Elastic Follow-Up.

Solution: a). Control Relative Proportion of CSEF Steels and “Traditional” Low Alloy Steels That Can Be Used in a Given Piping System.b). Require that Secondary Stress Resulting from Elastic Follow-Up Be Treated as a Primary Stress.

Lesson 6IIW-AWS


Background Information

Main Steamline Piping: 18” (457mm) OD, Sch. 140 (1.562” (40mm) NWT); SA-335, Grade 91 Material

Stop/Control Valve: 1.25Cr/1.0Mo/0.25V Material; Thickness at Connection ~ 3” (~75 mm)

Filler Metal: 2-1/4Cr-1Mo (B3)

Design Outlet Steam Temp: 1050°F (565 °C)

Design Outlet Steam Press: 1800 psi (12.4 Mpa)

Total Hours of Service: < 5000

No Cold Spring Incorporated Into Piping System During Erection

Lesson 6IIW-AWS


Through-Wall Cracking Appearance on the OD Surface of the Joint

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Illustrating the Path of Fracture Along the Weld Fusion Boundary

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The Path of Fracture Through the Decarburized Zone in the Weld Metal

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Creep-Induced Cavitation and Microfissuring Ahead of the Main Fracture

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Leitz Miniload Hardness Tester – 500 g

HARDNESS VALUES - HVQuarter Point HAZ Carbon Depleted Zone Weld Metal

12 o’clock 296, 301, 301, 307 324, 324, 336 216, 219, 230

6 o’clock 290, 307, 301, 312, 318 336, 356 223, 237, 230

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Original Joint Geometry

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Illustrating Differences In The Rate Of Secondary Creep Strain Accumulation Between Grades 91 And 22 At 1050ºF (565 ºC) (Curve For Grade 91 Developed At Temperature of 1067ºF (575ºC) - All Others At 1050ºF (565 ºC))

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1 10 100

Stress (ksi)

Cre

ep ra

te (a

bs/h

)

Grade 91

Grade 22 & 1.25Cr-Mo-V

Grade 22, Decarb HAZ

Grade 22 - Exp Data

Grade 91 - Exp Data

Secondary Creep Strain Accumulation

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Results Of Limited Structural Analysis

1. Root cause of cracking unidentified

2. Axial stress across decarburized zone a significant factor

3. Piping support system satisfactory – primary and secondary stresses below Code limits

4. Thermal transients played no significant role in the failure

5. Effect of elastic follow-up (i.e., lack of significant creep relaxation in Grade 91, following start-up) likely critical in creating highly axial stress, and requires further investigation

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Lesson 6 - Issues of Concern.ppt