New High Temperature Steels Seminar, IoM3 , Carlton … by P91 Users Group and European Technology...

New High Temperature Steels Seminar,IoM3 , Carlton House Terrace, London, 10-11 September 2008.

Organised by P91 Users Group and European Technology Development Ltd.Recent Developments in ASME codes concerning high Cr martensitic steels.

F. Masuyama1 and J Shingledecker2.1Kyushu Institute of Technology, Japan. [email protected], Palo Alto, Ca., USA.

Abstract.

On September 10 -11th. 2008, European Technology Development Ltd and the UK P91 Users Group organised a Seminar entitled "New High Temperature Materials. Use of 9 - 12 Cr Martensitic (P91, P92, P911, P122) and 2 Cr Bainitic (P23, P24) steels in thick and thin section components" at IoM3 in London, UK. As all users of these steels could obviously not be present, it was thought that readers of OMMI would be interested in the rapid dissemination of the major presentations that were made. The copies of the overheads are presented here and are self explanatory but for further information, the short abstract below highlights the work discussed in the presentations.

This PowerPoint presentation, comprising 27 overheads, is specifically concerned with the background, the development and the details of the ASME codes which are applicable to the high Cr ferritic martensitic steels under creep conditions. It considers an outline of the codes, applicable codes for this family of steels, the Creep Strength Enhanced Steels, the structure of the code committees and some practical problems. In addition, it also considers the history of changes to the stress allowables and of any changes in chemical composition, further improvements in microstructural control, heat treatment above and below the critical temperature, code rules for mitigation for specific heat treatments, Type IV cracking and weld stress reduction factors as well as cold forming limits and the use of hardness to identify “weak” Grade 91 steels and ASME limits.

Presentation published in OMMI, 2008, Volume 5, Issue 3, Dec. www.ommi.co.uk

Recent developments in ASME Codes concerning high Cr

martensitic steels

F. Masuyama,Kyushu Institute of Technology

J. ShingledeckerEPRI

ETD New High Temp. Materials’ SeminarSeptember 9-10, 2008

London, UK

Published in OMMI, 2008, Vol. 5, Issue 3, December. www.ommi.co.uk

Outline• ASME Code Committee Structure

– Groups & responsibilities• Current motivations• Current code requirements, recent changes, and

future considerations– Stress-allowables– Chemistry– Heat-treatment (including PWHT)– Weld Strength Reduction Factors (WSRFs)– Cold-forming– Hardness

• Summary


Definition• Creep Strength Enhanced Ferritic

(CSEF) Steels– Steels which high-temperature properties are

dependent on heat-treatment– Gr. 91, 92, 911, 122, 23, 24


Applicable ASME Codes for CSEF• I: Power Boilers

– Design by rule approach• Does not consider fatigue• ‘Safe design’ does not specify a design life

– Fabrication guidelines & rules– Does not address maintenance/inspection

• II: Materials– Approves material specifications

• Chemistry limits• Heat-treatment

– Allowable stress determination• Services other sections of the code

• III: Nuclear Power Components– Design by rule and by analysis– For high-temperature design: material data and rules are contained within Section III Subsection

NH (Considers fatigue, welds, etc.)• VIII: Pressure Vessels• IX: Welding & Brazing Qualifications• B31.1 Power Piping

– Covers piping from boiler (Section I) to steam turbines– Covers most aspects in design and fabrication

• B31.3 Process Piping– Similar to B31.1 for petrochemical industry


Section II Structure

• Actions are introduced and approved in various subgroups (SG) before passing to the Section II main committee

• These groups include:– Strength of Ferrous Alloys (SG-SFA)

• New Material Specifications• Setting of allowable stresses

– Strength of Weldments (SG-SW) joint with Section IX• Approves materials for welded construction• Can limit welding processes, filler metals, etc


ASME & the Industrial Application of CSEF Steels

• A majority of failures in CSEF steel components have been attributed to improper fabrication and heat-treatment

• The general consensus of users has been that the ASME Code does not adequately address these issues

• To address these issues, a Section II Task Group (TG) was formed: TG-CSEF

Failed Gr. 91 FeedwaterLine: Brittle failure due to untempered martensite from improper PWHT

Klueh & ShingledeckerORNL/TM-2006/1


History of Change to CSEF Stress Allowables (SG-SFA)

• Grade 91– Stresses set CC1943 year 1983, CC1992 year 1986– Stress at 1100F reduced due to long-term data, year 1993

• Grade 92– Stresses set CC2179 year 1994– Stresses reduced due to longer-term data and ASME SG-SFA re-

analysis was made, year 2006• Grade 122

– Stresses set CC2180 year 1994– Removed high Cr 122 due to new data with lower strength (delta-

ferrite containing), year 2006– Reduced strength due to longer-term data (SG-SFA), year 2006

• Incorporation?


Gr. 91 >3′′Gr. 91 >3′′ (1993)Gr. 92Gr. 92 (2006)Gr. 122Gr. 122 (2006)

Evolution of Gr. 91, 92 122 ASME max. allowable stresses

3.5 TS Raised Allowable Stress

Long-term Creep Data Reduced Allowable Stresses

History of Chemical Composition• Combustion Engineering (CE) produced over 30 heats

and tested: tensile, impact, aging, microstructure, and creep to develop original aim chemistry (1977)

• ORNL continued work and defined a new aim and target (1980s)

• ASME Code Case approved the first Gr. 91 specifications (1983) by broadening the ORNL range

• ASTM limited Al, Ti, and Zr based on ASME recommendations and published results on Al/N ratios1

(2006)• ASME is currently proposing to ASTM additional chemical

compositional changes

(1) S.J. Brett, J.S. Bates, R.C. Thomson. “Aluminium Nitride Precipitation in Low Strength Grade 91 Power Plant Steels.” Proceedings to the Fourth International Conference on Advances in Materials Technology for Fossil Power Plants (Hilton Head, SC, Oct. 25-28, 2004). ASM-International, Materials Park, OH, 2005. 1183-1197.


Original ORNL research suggested on optimum chemical compositional window per the ‘butterfly’ diagram

Best combination of Creep Strength, Ductility, and Toughness

Source: ORNL-6303,Iron Age, June 25, 1982

Cr eq. of <10, δ is usually notpresent, 10-12 some δ may be present, >12 δ will be present(based on weld deposits)


Chemical Composition: Forgotten Lessons Remembered

• Deleterious effects of nitride formers were realized during alloy development but were not incorporated into the specification

ElementCE-Aim (1977)

ORNL Target (~1980)

ORNL Range (1984)

ASTM Spec (198X)

ASTM Spec (2006)

Future Considerations

C 0.09 0.10 0.08-0.12 0.08-0.12 0.08-0.12Mn 0.4 0.40 0.03-0.05 0.03-0.06 0.03-0.06Si 0.1 0.20 0.20-0.50 0.20-0.50 0.20-0.50P <0.01 0.02max 0.02max 0.02maxS <0.01 0.01max 0.010max 0.010maxCr 9 8.5 8.0-9.0 8.00-9.50 8.00-9.50 8.50-9.? ?Ni 0.1 <0.10 0.2max 0.40max 0.40max 0.20max ?Mo 1 0.95 0.85-1.05 0.85-1.05 0.85-1.05Cu <0.10 0.2 max ?V 0.2 0.21 0.18-0.25 0.18-0.25 0.18-0.25Nb 0.12 0.08 0.06-0.10 0.06-0.10 0.06-0.10N 0.05 0.05 0.03-0.07 0.03-0.07 0.03-0.07Al as low as possible <0.02 0.04max 0.04max 0.02max ?Ti 0.02 <0.01 0.01max 0.01max ?B 0.004 <0.001 <0.001W <0.01 <0.01Zr 0.004 <0.01 <0.01 0.01max ?O <0.02 <0.02Sb <0.001 <0.001


Recent research suggests improved heat-treatment and microstructural control can be achieved by further tightening of the

Specification using Computational Thermodynamics as a guide

Thermodynamic calculations of two extreme Gr. 91 compositions predict a composition with a very low A1 transformation temperature and the other with stable delta-ferrite

Both predictions were confirmed by experimental observations

100

80

60

40

20

0

Amou

nt (v

ol %

)

1600140012001000800600Temperature (°C)

100

80

60

40

20

0

Amou

nt (v

ol %

)

Liquid Ferrite Austenite

Source: J.P. Shingledecker, M.L. Santella, K.A. Wison. “Evaluation of Specification Ranges for Creep Strength Enhanced Ferritic Steels,”Proceedings to the conference on New Developments on Metallurgy and Application of High Strength Steels.” Buenos Aires, 2008.

Lower critical (A1 = ~775C)

Stable Ferrite

Heat-treatment • Optimum properties (including creep strength)

can only be achieved through proper normalization and tempering

• For ASME/ASTM: – Gr. 91 shall be normalized at 1040 to 1080˚C and

shall be tempered at 730 to 800˚C– After welding, a PWHT is required at the tempering

range 730 to 800˚C– 800˚C is NOT an aim it is a MAXIMUM NOT TO BE

EXCEEDED • Tempering should be conducted below the lower

critical (A1) temperature


For lower critical temperature depends on alloy composition

• A large review1 of Certified Material Test Reports (CMTR’s), the current specification range, and supplier melting practices showed some heats of Gr. 91 have a lower critical below 800˚C

• Simple changes to the chemical compositional specification are being proposed to ASME to ensure the lower critical is above 800˚C

• These may include:– Lowering max Ni– Raising min Cr– Limiting Ni+Mn– Limiting Cr+Si

(1) J.P. Shingledecker, M.L. Santella, “The effect of composition within the specification range of CSEF Steels.” EPRI Workshop on Gr.91/92, Clearwater Beach, FL (April 7-9, 2008)

740

760

780

800

820

840

860

880

900

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

Ni+Mn Content (wt%)

Calc

ulat

ed A

1 Te

mpe

ratu

re (

o C)Specification Range (91, 911, 92)Gr. 91 Commercially Produced Heats


Is tempering or heat-treatment below the intercriticaltemperature sufficient to guarantee properties

• Heat-treatment below Ac1 may still induce a partial transformation to ferrite in Gr. 91 significantly reducing creep strengthMasuyama & Nishimura: ASME PVP, 2004

Test material: Gr.91


Rules for Mitigation• ASME requires mitigation steps if the intercritical

(800˚C) temperature is exceeded at any point during fabrication (not just PWHT)– Re-austenitize and temper the component in its

entirety– Remove, re-austenitize and temper, and re-install the

component – Allowable stress values can be reduced to Grade 9

values after proper tempering– If the part contains a weld, additional requirements

apply


Rules for Mitigation: Section IPG-5.6 For P No.5B, Group 2 materials, if during any phase of manufacturing or erection any

portion of the component is heated to a temperature greater than 1470°F (800°C), one of the following actions shall be performed:

a) The component must be re-austenitized and re-tempered in its entirety in accordance with the specification requirements

b) That portion of the component heated above 1470°F (800°C), including the Heat-Affected Zone created by the local heating, must be replaced, or must be removed, re-austenitized and re-tempered in accordance with the specification requirements, and then replaced in the component.

c) If the allowable stress values to be used are less than or equal to those provided in Table 1A of Section II, Part D for Grade 9 (SA-213 T9, SA335 P9 or equivalent product specifications) at the design temperature then the requirements stated above may be waived provided that the portion of the component heated above 1470F (800C) is re-tempered in accordance with the specification requirements. The use of this provision shall be noted on the manufactures data report.

If the portion of the component heated above 1470°F (800°C) does contain a weld, then the requirements of PW-39 for P No. 5B, Group 2 materials shall apply for re-heat treatment.


WSRF• CSEF steel weldments subcritically PWHTed are subject to Type IV

(Fine-grained heat affected zone) failure during service• This can be a large issue for long-seam welded components and

Section I, B31.1, and B31.3 had varying approaches to the issue• SG-SW reviewed all applicable data and made recommendations

for entire Code

Masuyama 2007,

Type IV fractures in Uniaxial and internal pressure creep tests on P91


WSRF• A large number of conflicting data were available on type IV failure in Gr. 91• All test data were to less than 20,000 hours and most data showed the weld

strength reduction was increasing (getting worse) with longer test times• SG-SW decided on a very conservative approach based on these data for

subcritically heat-treated Gr. 91– WSRF = 0.5 in creep regime for Gr. 91

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

400 450 500 550 600 650 700

SFsASME SWNew FitASME III-NHJAPEICSchubertTabuchi & Takahashi eq.WSRF T&T

TEMP. ( DEG C)

Gr 91 Weldments

SG-SW

After R.W. Swindeman, J.P. Shingledecker, ASME B&PV Code Meeting, SG-SW, Boston 2008

Study by Brett et al (2007) suggests WSF is approaching 0.5 at 600 to 650˚C


PG-26, Weld Strength Reduction Factors1,2,3,4,5,6 to be applied when calculating max. allowable working pressure or min. req. thickness of components fabricated with a long. seam weld

0.500.500.500.500.500.501.00--CSEF (Sub Crit)9

0.770.820.860.910.951.00---CSEF (N+T)9,10,11

0.640.680.730.770.820.860.910.951.00CrMo7,8,9

649621593566538510482454427Temperature (℃)

12001150110010501000950900850800Temperature (0F )


1. Cautionary Note: There are many factors that may affect the life of a welded joint at elevated temperature and all of those factors cannot be addressed in a table of weld strength reduction factors. For example, fabrication issues such as the deviation from a true circular form in pipe (e.g. ”peaking” at longitudinal weld seams) or offset at the weld joint can cause an increase in stress may result in reduced service life, and control of these deviations is recommended.

3. Carbon steel pipes and tubes are exempt from the requirements of PG-26 and Table PG-26.

4. Longitudinal seam welds in piping for materials not covered in this table operating in the creep regime are not permitted. For the purposes of thistable, the creep regime temperature range is defined to begin at temperature50˚F (25˚C) below the T-note temperature listed in Section II, Part D designproperty. tables for the base material involved.

5. All weld filler metal shall have a minimum carbon content of 0.05% for theCrMo and CSEF materials and a minimum carbon content of 0.04% for theaustenitic stainless steels.

9. Basicity index of SAW flux ≧1.0.

Notes:Published in OMMI, 2008, Vol. 5, Issue 3, December. www.ommi.co.uk

Cold-forming limits

No HTPBHTN+TN+T600<T

No HTPBHTPBHTN+T538<T<600

No HTNo HTNo HTNo HTT<538Service Temp., T (C)

ε <520> ε >530> ε >20ε >30Cold Forming Strain, ε (%)

No HTN+TN+T705<T (Ex)

No HTNo HTN+TT<705 (Ex)

No HTNo HTNo HTT<480 (S)Service orExposed Temp., T (C)

ε <520> ε >5ε >20Cold Forming Strain, ε (%)

PG-20.2 Grade 91 Cold Forming Rule

PG-20.1 Grade 23 Cold Forming Rule

PBHT: 732 – 774C


10

100

1 000

17 000 18 000 19 000 20 000 21 000 22 000

Larson Miller Parameter (C=20)

Stre

ss (M

Pa)

T/P23 V&M mean curveNot deformedCold rolled 20%Cold rolled 20%+780°C 1440°F/30 mnCold rolled 20% + PBHT 1440°F + Nor + tempered 1440°F

10

100

1 000

17 000 18 000 19 000 20 000 21 000 22 000


Stre

ss (M

Pa)

T/P23 V&M mean curveNot rolledCold rolled 35%Cold rolled 35%+780°C 1440°F/30 mn

10

100

1,000

30,000 31,000 32,000 33,000 34,000 35,000 36,000 37,000Larson Miller Parameter (C=35)

Stre

ss (M

Pa)

T/P91 V&M mean valuesNot deformedCold rolled 20%Cold rolled 20%+780°C (1440°F)/30 mnCold rolled 20%+750°C (1380°F)/30 mn

10

100

1 000

30 000 31 000 32 000 33 000 34 000 35 000 36 000 37 000


Stre

ss (M

Pa)

T/P91 V&M mean values

Not deformed

Cold rolled 35%Cold rolled 35%+780°C (1440°F)/30 mn

Creep Rupture Properties of Cold Formed Grades 91 and 23

20% Strained Gr.91

35% Strained Gr.91

20% Strained Gr.23

35% Strained Gr.23

Gabrel et al. Creep 2007, July 22-26, 2007


Studies have shown hardness can identify creep ‘weak’ Gr. 91

100

1000

22,000 24,000 26,000 28,000 30,000 32,000

Gr. 91 DatabaseGr. 91 -20%Normal Hardness (this study)Soft-Zone (this study)Ryu et al., 2002: 155-180HVBrett, 2004: Low N, High Al

Stre

ss (M

Pa)

LMP (C=28)

>195HV

~175HV

J.P. Shingledecker, M.L. Santella, R.L. Klueh. “Evaluation of heat-treatment temperatures and properties of improperly heat-treated grade 91.” ETD Seminar on New Steels, 2007.

Masuyama. “Transformation Behavior of CSEF Steels.” ETD Seminar on New Steels, 2007.

220HV168HV


Hardness Drop with Creep Life Fraction

200

Vick

ers

Har

dnes

s (H

V)

0

Life Fraction (t/tr)

230

190

180

170

160

210

220Crept at 650℃ and 71MPa for 770℃-Tempered Gr.91

168HV (Tempered at 832℃)

0.2 0.4 0.6 0.8 1.0


ASME Hardness Limits• Currently there are hardness maximums for CSEF steels• No hardness minimums are currently in the Code

although tensile minimums do apply• Although hardness can identify material with poor creep

strength (studies suggest this lower limit is 190 to 195HV), it cannot guarantee ‘good’ creep properties.– Field hardness is subject to a number of quality issues including

• Type of testing apparatus• Skill of user• Decarburization

– For these reasons, ASME has been hesitant to issue an acceptable hardness range

– ASME is in the process of issuing a white paper on the subject of hardness


Summary• The ASME B&PV code has addressed a number of

issues with CSEF Steels through its various subcommittees, mainly section II– Allowable stress values have changed for some materials based

on new data– Chemical compositions have been refined in ASTM through

ASME and more changes are being considered– Heat-treatment and mitigation are addressed but implementation

appears to be the governing issue– WSRF have been conservatively addressed for seam welded

components but not for other weld geometries– Cold forming limits and hardness rules are under consideration

• Finally, ASME is a group of volunteers, the meetings are open to the public and anyone (US or International) are welcome to attend and join.


New High Temperature Steels Seminar, IoM3 , Carlton … by P91 Users Group and European Technology...

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