control of relaxation cracking in austenitic high

tTNO Industrial Technology

Control of relaxation cracking in austenitic high temperature

components

Hans van Wortel,EindhovenThe Netherlands

TNO Science and Industry

VeMet conference March 17th 2009,

Marknesse, the Netherlands

Control of Relaxation Cracking, March 2009. 2t

Crack tips of relaxation cracks in Alloy 800H (20Cr/32Ni) and Alloy 617 (Ni base)

- in 800H a metallic filament present in 617 not ! -

Alloy 617, no metallic filament on grain

boundaries

Alloy 800H, with metallic filament on

grain boundaries

Cr- rich oxide layer

Ni-rich filament

cavities

Composition:Location ↓ Cr Ni Fe OBase metal 20 32 48 ---Oxide layer >50 <20 <30 +++Metallic filament <5 >55 35 ---


DIFFERENT NAMES FOR THE SAME DEGRADATION MECHANISM IN AUSTENITIC MATERIALS? • Relaxation Cracking RC• Stress Induced Cracking SIC• Stress Induced Corrosion Cracking SICC• ReHeat Cracking RHC• Stress Oxidation Cracking SOC• Stress Assisted Grain Boundary Oxidation SAGBO• Stress Relief Cracking SRC• Post Weld Heat Treatment Cracking PWHTC• White Phase Fractures WPhF• Strain Age Cracking SAC

Yes it is the same degradation mechanism!


RELAXATION FAILURES IN THE CHEMICAL PROCESS INDUSTRIES

(Identified by the author)• Germany : 800H, 16Cr13NiNb, Alloy 617, Alloy 625• Canada : 800H(T)• US : 800H(T), 304H, 347H, Alloy 617, 25/35Nb, 601• Belgium : 800H, 321H, 304H, Alloy 601. Alloy 617• Norway : 321H, 347H • France : 800H, 347H, AISI 310, Alloy 601• UK : 800H, 316H, 304H• Netherlands: 800H, 304H, 316H, 321H, 20.32Nb, 617• Asia : 800H(T), 321H, 347H, Alloy 601. • Africa : 347H

Totally >55 failures identified during last decadeLast year 9 new failures: 4 in 347H, 2 in 321H, 2 in 800H,

and 1 in 617


Identification of Relaxation Failures

• Cracks always on grain boundaries, often with a metallic filament;

• Cracks in (repair) welded and cold formed areas;

• In front of cracks: cavities on grain boundaries;

• Vickers hardness > 200 HV5;

• Within 0.5 - 2 years service;

• Metal temperature between 500 and 750°C.


Relaxation cracking in austenitic materials- window of the degradation mechanism -

• In materials showing an age hardening behaviour:

- Precipitation of very fine carbides (50 nm) within the grains;

- Susceptibility significantly enhanced by a high dislocation density, introduced by welding, cold forming, cyclic loading.


Failed Alloy 304H reactor vessel,wall thickness between 50 and 75 mm


Main other degradation mechanism HT alloys- how to distinguish them from Relaxation Cracking ? -

• Hot cracks• High cycle / thermal / low cycle - fatigue• Polythionic Acid Stress Corrosion Cracking• Creep • Metal Dusting • Carburization / Sulphurisation / Nitriding• Corrosion


STARTING POINT JIP RESEARCH:Case 1:

- failure in a Alloy 800H reactor vessel after 6.000h service -

• Diameter vessel : 2.800 mm• Total height: 15 meter• Wall thickness shell between 35 and 80 mm. • Medium: hydro-carbons• Metal temperature 600-650°C • Leakage after 2.000h in the 80 mm material• After 6.000h a catastrophic failure. Brittle fracture in

HAZ of circumferential weld. Vessel broke in 2 parts resulting in a fire. However, no persons were killed


Detail of the reactor at the failed location

Failed location

Cross section failed location


Hardness and failed area of aAlloy 800H welded joint

100

125

150

175

200

225

250

275

300

-5 0 5 10 15 20 25Distance (mm)

Vic

kers

har

dnes

s (H

V1)

99-015661125-02-WOR

Heat Affected Zone base metalweld metal

after 6.000h service at 650°C

as welded

Failed area


Microstructure failed Alloy 800H base metal/HAZAs delivered: hardly carbides

Service exposed at 600-650°C: many very fine carbides


Mechanical properties of the brittle failed800H component

- at RT within requirements for new base metal ! -

Condition R0.2MPa

RM MPa

A %

Z %

ToughnessJoules

Required for new base metal

≥ 170

450 700 ≥ 30 -- ≥ 40

new base metal 195 545 53 65 235

failed base metal 285 625 35 44 115


Ductile bend behaviour of the brittle failedAlloy 800H reactor vessel!

Failed material

Failed material

New material

33

2

1


Outcome failure analysis:

• Relaxation cracking susceptibility can not be assessed or predicted with the standard mechanical tests:

• Room Temperature: Tensile, Charpy-V and bend tests do not indicate susceptibility

• Service temperature: Creep and L.C.F tests do not give information concerning susceptibility

CONCLUSION:Within the present codes the degradation mechanism: “Relaxation Cracking” is not tackled


Case 2:Alloy 800H header failed after 12.000h service

Through wall thickness cracks in the header - nozzle connections

Detail cracks in the area of the longitudinal welded joint

Inco A weld metal

HAZ: metallic filament present

Inco A weld metal: metallic filament present


STARTING POINT JIP Case 3

Catastrophic failure of a 617 reactor vessel in US

• Diameter vessel : 4.000 mm• Height: 8 meter• A number of welds present• Wall thickness shell 30 mm. • Metal temperature from 500 up to 800°C • Leakage after 7.000h service.

WHAT WAS THE ROOT CAUSE OF THE FAILURE?


Some facts concerning the failure• Cracked in the HAZ and in the weld metal

• Cracks along the grain boundaries

• Very high hardness values up to 380 HV10HV 380


Outcome failure analysis Alloy 617 vessel:

“Relaxation cracking”


Failed 800H header in the year 2005


RC failure in 4 mm 321H burner tube, year 2008

Failed location

321H tube: 60*4 mm (OD*WT)

Bulged materialCrack near tack

weld

Axial cracks in base material

fracture


Relaxation cracks in failed 321H burner tube


Header+ Support + weld metal: 347H

Failure in a 347H header after < 1 year service


Failed 347H welded joint, cracks in HAZ/BM

Metallic filament

cavities


False interpretation as a consequence of not correct preparation of the sample

Not correctly prepared: Conclusion: Corrosion

Correctly prepared: Conclusion: RC


Failed 347H welded joint, cracks in weld metal

Metallic filament

cavities


COMBINATION OF RELAXATION CRACKINGAND METAL DUSTING IN ALLOY 601

Relax. crack

Metal dusting

The degradation mechanism RC and Metal Dusting are acting in the same temperature regime


Example of a failure in a 347H weld not caused by relaxation cracking- operating temperature 600-650°C-

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40Distance (mm)

Har

dnes

s Vic

kers

(HV

5)

347 weld metal


Appearance of cracks and cavities in the 347 weld metal

No metallic filament


The root cause is not relaxation cracking: • Hardness is at the lower bound for susceptible

material.

• No metallic filament present

• The cracks are following the delta ferrite

• Lifetime > 5 years

• Cracks initiated at the inner surface, where the tensile stresses are acting at the outer surface

• No damage in the HAZ


Relaxation Cracks can be simulated in the lab- result of relaxation tests on 617 and 800H welds -

Alloy 617: 650°C Alloy 800: 600°C

Metallic filament in crackNo filament in crack


Example of a 3-point bending relaxation test

347H 800H

21/33 Mn

800H 800H

82

800H347H

21/33 Mn

Span 50 mm

t =15 mm


Registrations during a 2-step relaxation testPWHT: 3h/875°C

0

10

20

30

40

50

60

70

0 0,5 1 1,5 2Time (h)

Load

(kN

)

T=650°C

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300Time (h)

Load

(kN

)


Cross section after the 2 step relaxation test- no damage due to the PWHT -

21/33Mn

347H 800H

21/33Mn weld metal


EFFECT WELDING AND COLD DEFORMATION ON RELAXATION BEHAVIOUR

• Welding/cold deformation enhance dislocation density and results in hardness increase

100

125

150

175

200

225

250

275

300

-5 0 5 10 15 20 25Distance from fusion line in mm

Vick

ers

hard

ness

(HV1

0)


HV 210

HV 140


• During service this will result in:1. Rapid precipitation of very fine matrix carbides on

dislocation knots, blocking the dislocations and hardness increase.

100

125

150

175

200

225

250

275

300

-5 0 5 10 15 20 25Distance (mm)

Vic

kers

har

dnes

s (H

V1)

99-015661125-02-WOR


after 6.000h service at 650°C

as welded

HV 240

HV 210


2. Matrix can not deform, but stresses have to be reduced by creep deformation. All the deformation is concentrated nearby the grain boundaries.

Resulting in:RELAXATION CRACKING ALONG THE GRAIN BOUNDARIES

HV 240


TEM extraction replica’s of the failed service exposed 800H material

- matrix precipitates have been identified as M23C6

Grain boundary Grain boundary

Very small matrix precipitates (50 nm)


TEM thin foils of the failed service exposed 800H material

- many fine M23C6 carbides in the matrix -

Denuded zone

Grain boundary

Very small matrix carbidesand high dislocation density


Launch of a Joint Industrial Programme:- Control of Relaxation Cracking -

Consortium:* Shell * Exxon * Stoomwezen * Total * GEP

* Dow * BASF * Norsk Hydro * DSM * Nuclear Electric

* BP * Siemens * Air Products * Kema * Laborelec

* VDM * Manoir * Haynes * DMV * Special metals

* CLI * Paralloy * UTP * Thyssen * Böhler

* ABB * Technip * Uhde * Raytheon * TNO

* AKF * Stork * Verolme * Bodycote * Schielab* Industeel

* TNO


Objectives JIP:

• What are the determine factors concerning relaxation failures in austenitic materials?

• How to control the phenomenon ?


Extent of the programme• Assess the effect of:

• Chemical composition base materials (both Fe and Ni base). >20 base materials involved.

• Heat to heat variation.• Grain size.• Welding and type of consumables (> 60

conditions).• Cold deformation.• Operating temperature.• Pre- and Postweld / Postform heat treatments.

Reference materials: Alloy 800H and Alloy 617

AISI 347 was not included in the programme


800H plate materials and welding consumables involved within the programme

- Plate thickness from 10 up to 32 mm -

Base metal: C Ni Cr Fe Al

Ti

Al +Ti Mo Co Nb t

mmASTM

grain size 800 0.079 30.4 20.4 bal. 0.26 0.33 0.59 20 6-7 800L 0.02 32.2 19.9 bal. 0.27 0.49 0.46 18 6.5

800H 0.066 30.6 20.6 bal. 0.28 0.34 0.62 2020

3-4 1-2

800H 0.068 30.5 20.45 bal. 0.29 0.33 0.62 32 1.5 800HP 0.083 30.7 20.5 bal. 0.58 0.51 1.09 16 2 800HP 0.07 30.3 20.3 bal. 0.54 0.49 1.03 25 1.5

800HT 0.08 31.1 19.6 bal. 0.45 0.54 0.96 1012

4.5 2.5

Welds SMAW: 82 type 0.007 70.4 19.9 2.2 0.69 0.33 0.9 1.8 SMAW: 617 0.05 57.3 20.6 1 0.67 0.22 8.5 10.7 SMAW: 21/33Mn 0.13 33.3 21.6 bal. 1.25 GTAW: 21/33Mn 0.12 32.4 21.5 bal. 1.2


617 plate materials and welding consumables involved within the programme

- Plate thickness from 12 up to 25 mm -

Base metal: C Ni Cr Fe Al

Ti Mo Co

ASTM grain size

1 0.08 53.8 22.3 1.5 1.16 0.38 8.7 11.4 4 2 0.06 54.3 22.1 0.3 1.16 0.41 9.3 12.2 4-5 3 0.09 52.9 21.9 1.4 1.1 0.33 9.7 12.5 5 4 0.09 52.9 21.9 1.4 1.1 0.33 9.7 12.5 8 5 0.06 53.4 22.2 1.9 1.2 0.39 8.9 11.5 5 6 0.08 52.2 22.1 1.7 1.1 0.29 9.7 12.6 3-4

Welds SMAW 1 0.05 57.8 20.6 0.7 0.69 0.33 8.4 10.1 SMAW 2 0.09 52.4 23.1 1.2 0.18 0.12 9.3 12.1 SMAW 3 0.06 57.5 21.9 1.5 0.19 0.18 8.5 11 SMAW 4 0.06 55.2 21.1 1.1 1.1 0.29 8.9 11.6 GTAW 1 0.04 58.2 20.5 0.7 1.08 0.48 8.3 10.4 GTAW 2 0.09 53.5 22.2 1.5 1.1 0.34 9 12.6


Effect of material choice- some base materials involved and their

susceptibility for RC -Base metal: C Ni Cr Fe Al Ti Mo Co others 304H .06 10 17 bal. --- --- --- ---- 316H .07 11 17 bal. --- --- 2.5 --- 321H .07 11 18 bal. --- 0.4 --- --- 1.4910 .03 13 17 bal. --- --- 2.4 --- N, B NF 709 .08 25 20 bal. --- 0.1 1.5 --- N, B, Nb Alloy 800H .07 32 20 bal. 0.3 0.3 --- --- Alloy 803 .08 34 25 bal. 0.5 0.5 --- --- Nb AC66 .06 32 27 bal. --- --- --- --- Nb, Ce 602CA .17 63 25 9 2.1 .18 Alloy 617 .07 bal. 21 <2 0.9 0.4 9 12


• Mostly the welded joints are more susceptible than the base materials.

Effect of welding and consumable selection- some results in as welded condition -

• Not susceptible base metal:

– AC66– Alloy 800– Alloy 803– 1.4910

• Weldment, no PWHT:– WM not susceptible– WM is susceptible– WM is susceptible– WM not susceptible


Summary relaxation cracking susceptibility Alloy 800 H,T and Alloy 617

• All the as delivered 800HT and 617 base materials were susceptible. No significant effect of:• grain size• chemical composition• supplier

• The welded joints were in as welded conditionmost susceptible. No significant effect of:•Welding procedure

•chemical composition consumables


Outcome JIP• In age hardened condition relaxation cracks can be

expected at <0.2% relaxation strain.

• Welding and some cold deformation (<2%) can already produce these low relaxation strains during high temperature service.

• After additional heat treatments the components can withstand >2% relaxation strain without cracking. This is far beyond the relaxation strains in the field.


Effect of heat treatments• Stabilising heat treatment base materials:

• Effective to avoid Relaxation Cracking, both in as delivered condition as after cold forming

• Postweld heat treatments welded joints:• Effective to avoid Relaxation Cracking in the welds


Effect PWHT on relaxation cracking susceptibility of a Alloy 617 welded joint

As welded:cracked in WM+HAZ

After PWHT at 980°C:no RC cracks


Effect heat treatment on relaxation performance weld- welded joint is after heat treatment not susceptible for RC -

Severe cracking in weld metal

020406080

100120

0 50 100 150 200 250 300Time (h)

crack initiation

As welded condition, T=650°C Weld stabilised (980°C/3hr), T=650°C

020406080

100120

0 50 100 150 200 250 300Time (h)

no crack initiation

No damage at all


Effect of a stabilizing heat treatment on creep performance at 600°C of Alloy 800H base metal

10

100

1000

1000 10000 100000time to rupture (h)

cree

p ru

ptur

e st

ress

(MPa

)

Actual data 800H base metal,stabilised 980°C/3hActual data 800H base metal, as delivered

10

102

103

103 104 105

600°C

+20%

-20%


Effect of a stabilizing heat treatment on creep performance at 700°C of Alloy 800H base metal

10

100

1000

1000 10000 100000time to rupture (h)

cree

p ru

ptur

e st

ress

(MPa

)

Actual data 800H base metal, stabilised 980°C/3hActual data base metal as delivered

10

102

103

103104 105

700°C

+20%

-20%


Deliverables Joint Industrial Programme

• Recommended Practice for the identification and prevention of Relaxation Cracking

• Equipment Degradation Document (EDD)


The control of the degradation mechanism“RELAXATION CRACKING”

- Implementation results in the field and outcome -

• Companies within the JIP consortium do not have failures anymore for > 6 years. They all are using the Recommended Practice (mostly heat treatments).

• Still failures are encountered at companies who were not involved in the programme.


General final conclusion

• To date Relaxation Cracking is under control by a correct selection of:

• base materials

• welding consumables

• heat treatments

• Equipment manufacturer


OUTLOOK• A material and welding consumables have been

developed:

• Not susceptible for relaxation cracking, where a PWHT can be avoided with:

• A high creep strength

• For operating temperatures from 600 up to 900°C

• Cost effective where the Ni content is significantly lower than for Alloy 800H

- A patent is pending

- Draft agreement how to share profits is available


105h creep rupture strength new developed base material, not susceptible for relaxation cracking compared to 800H

0

40

80

120

160

200

240

600 650 700 750 800 850 900 950operating temperature, °C

105 h

cree

p st

reng

thnew base metalbenchmark Alloy 800H: 32%Ni+20%Cr


Cost effectiveness new base material not susceptible for relaxation cracking

• Based on a Alloy 800H heat exchanger,• Weight 86 tonnes• Diameter 2.2 meter• Height 8 meter• Shell thickness from

25 up to 57 mm• 2 tube sheets, 178 mm

each• Several forgings• A number of tubing• Design temperature

between 625 and 825°CdT=625°C

dT=825°C

dT=680°C

dT=825°C

dT=670°C

dT=680°C

dT=760°C

dT=760°C

Ø 2200mm

1800mm

5265mm

1000mm

178

178

33mm

46mm

57mm

33mm

25mm

Alloy 800H


Calculations performed by anequipment manufacturer

Cost item

Relative cost Alloy 800H

Relative cost new material

Cost benefit new material

relative to Alloy 800H

Material 100 % 69 % 31 % Labour 100 % 98 % 2 % Various 100 % 90 % 10 %

Total 100 % 75 % 25 %

control of relaxation cracking in austenitic high

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