Environmental influence on cracking susceptibilityand ageing on nuclear materials (ENVIS), specialarticle report: Low Temperature Crack Propagation(LTCP) susceptibility of nickel-based Alloy 182, 152,82 and 52 weld metals in PWR primary water

SAFIR 2014 seminar, 20.3.2015Matias Ahonen1, Ulla Ehrnstén1, Tapio Saukkonen2, Olga Todoshchenko2

and Hannu Hänninen2

1VTT Technical Research Centre of Finland2Aalto University School of Engineering

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Materials used in PWR plants

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Why is LTCP important?

Safe and economic NPP operation requires knowledge aboutpossible degradation modesDegradation modes are e.g. stress corrosion cracking, thermalageing, mechanical and environmentally assisted fatigue,radiation embrittlement of the pressure vessel, etc.Non-destructive inspection is used to monitor the integrity ofcomponentsStructural integrity assessment rules exist for many degradationmodesLow temperature crack propagation was recognized as onepotential degradation mode, first in X-750 and later in Ni-basedweld metalsIt is important to know which materials are prone to LTCP, inwhich conditions LTCP may occur, and what the possibleconsequences could be

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IntroductionLTCP (Low Temperature Crack Propagation)may occur in pre-cracked nickel-basedmaterials when following conditions are met:

hydrogenated waterlow temperatureslow loading ratehigh stress

A typical feature of LTCP is a transition in thecracking mechanism from ductile dimplefracture to intergranular (IG) / interdendritic(ID) crackingLTCP related studies are consistentlyshowing that increasing hydrogen content inwater decreases the fracture resistance ofnickel-based materials

Ductiledimplefracture

Intergranularfracture

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LTCP risk in PWRs

LTCP risk is a function of PWRshutdown procedures as it requires acertain amount of stress, hydrogenaddition and low temperaturesimultaneously. The hydrogen andtemperature conditions for LTCP tooccur can be present in some PWRshutdowns (example chart)

Crack growth rates in LTCPsusceptible materials may be veryfast in low temperaturehydrogenated waterNo LTCP incidents have beenreported in open literature thus far

Reference: Demma, A. et al. Low Temperature Crack PropagationEvaluation in Pressurized Water Reactor Service. In: Proceedings of the 12thInternational Symposium on Environmental Degradation of Materials inNuclear Power Systems – Water Reactors. 2005. USA: TMS, 2005.

Shutdown examples from some Americanand French plants:

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Aim of the study

The aim of this study wasto investigate the LTCP phenomenonin Alloy 182, 152, 82 and 52 weldmetalsto study the effect of pre-exposure tohigh-temperature (300 °C) water priorto loading of the specimens at 55 °Cto examine the microstructuralphenomena related to hydrogen-induced low-temperature fractureto evaluate the relations betweenresults from J-R tests, hydrogenthermal desorption measurementsand fractography

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Materials and specimensTest materials:

All-weld metal test blocks (Alloys 182, 82, 152and 52)Dissimilar metal weld (DMW) mock-ups(Alloys 182, 152 and 52)

SE(B) type pre-cracked specimens were used10 x 10 x 55 mm, 10% side groovesAll-weld metal specimens, L-S orientationDMW specimens, T-S orientation

All-weld metal test blocks

Aalto mock-up (Alloy 182)KAIST mock-up (Alloy 152)

NGW mock-up (Alloy 52)

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J-R test descriptionPneumatic servo-controlled loading in anautoclaveDC-PD measurement of crack growth,corrected after the testEnvironments:

Room temperature airHydrogenated low temperature (55 °C) waterin an autoclave

Hydrogen contents of 5, 30 and 100 cm3 H2/kgH2O were appliedBoric acid (H3BO3) 200 ppm and lithiumhydroxide (LiOH) 2.1 ppm

The effect of high (operation) temperaturewater was investigated by performing tests at55 °C after 24 hours or30 days pre-exposure at 300 °C

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Overview of the J-R test resultsClear and systematic reduction offracture resistance due to hydrogen

Alloy 182 exhibits the lowestaverage JQ values inhydrogenated water (30 cm3 H2/kgH2O and 100 cm3 H2/kg H2O)Addition of 30 cm3 H2/kg H2Ohydrogen lowers the fractureresistance of Alloys 152 and 82,although not dramaticallyAlloy 52 does not show ameasurable reduction of fractureresistance when tested with 30cm3 H2/kg H2O hydrogen100 cm3 H2/kg H2O hydrogen hasa clear effect in some Alloy 52specimens

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The effect of pre-exposure (300 °C) on thefracture resistance

The J-R –test results obtainedfor Alloys 182 and 152 after pre-exposure in 300 °C (24 h or 30days) hydrogenated water areconsistently higher than thoseobtained in the same testenvironment without pre-exposure.Alloy 52 does not showmeasurable susceptibility toLTCP with hydrogen content 30cm3 H2/kg H2O, neither with norwithout pre-exposure to hightemperature water.

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50

100

150

200

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300

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0 10 20 30 40 50 60 70

Time (h)

Tem

pera

ture

(°C

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J-R -test

15 hours

24 hours

19 hours

High temperature pre-exposure

Cooling

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The effect of PWHT on the fracture resistance ofAlloys 182 and 52

The J-R –test results obtained for Alloys182 and 52 after PWHT are somewhatsimilar to the results obtained for AWspecimens

Alloy 182 shows slightly lower JQ buthigher J1mm and tearing resistanceT0.5mm values after PWHT

AWspecimens

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Activation energy for hydrogen desorptionThermal desorption spectra for DMW Alloy 182 samples in as-welded conditionand for DMW Alloy 52 samples in PWHT + 30 cc pre-exposure -conditionActivation energy determined applying a model by Lee & Lee (1984):

))

Different activation energies for hydrogen desorption for DMW Alloys 182 and52 indicate that the hydrogen trapping is caused by different carbides

Alloy 182 Alloy 52

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Microscopy

Studied materials: Alloy 182 and Alloy 52Applied techniques: SEM (including 3D), TEM, EDS(and EBSD)Mating fracture surfaces were investigated andcompared

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Alloy 182 fractographyThe fracture surfaces consisted of alternating areas of transgranular andintergranular crackingA few different types of precipitates were observed on the IG/ID surface

The larger precipitates, appearing light were found to contain niobium. Smallerprecipitates contain chromium, titanium, aluminum or silicon

The grain boundary is decorated by very small (< 100 nm) carbides

Pre-crack

J-R testTG IG/ID

151522/04/2015

Intergranular fracture surface morphology ofAlloy 52 specimens

Intergranular fracturesurfaces manifest a surfacelayer that contains a largeamount of M23C6 carbides.The fracture surface layerwas studied by using 3DSEM

The carbide-rich surfacelayer thickness (~100-300nm) correlates well with thecarbide size

Opposing sides of the mating fracture surfaces

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TEM examination of Alloys 182 and 52 (DMW)

Grain boundary carbides werepredominately Cr-rich carbides(M23C6)

In Alloy 52, the observed Cr-richcarbides were mostly tetrahedron-shaped. In addition, very small (~50nm) Ti-rich carbides wereobserved.In Alloy 182, the Cr-rich carbideswere sometimes tetrahedron-shaped, sometimes irregular-shaped. Also, some Nb-richcarbides were observed.The Cr-rich carbides were smallerin Alloy 182 (~30-70 nm) than inAlloy 52 (~100-300 nm).

Cr-rich

Cr-rich

Nb-rich

Ti-rich

Alloy 52

Alloy 182

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Conclusions (I)Hydrogen decreases the fracture resistance of Alloys 182, 152and 52 significantly in low-temperature water with high hydrogencontents (100 cm3 H2/kg H2O). Alloy 182 is the most susceptiblematerial to LTCP, and it exhibits a significant reduction of fractureresistance already with a hydrogen content of 30 cm3 H2/kg H2O.The high temperature pre-exposure decreases the amount oftrapped hydrogen in Alloys 182 and 152 and thus slightlyincreases the fracture resistance when compared to the resultsobtained for as-welded samples.Alloy 182 exhibits one type of a dominating trap whereas in Alloy52 there are at least two types of traps for hydrogen. Based onfractography and TEM the traps in Alloy 182 are probably Nb-richMC-carbides and in Alloy 52 Ti-rich MC-carbides and Cr-rich M23C6carbides.

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Conclusions (II)

Alloy 52 exhibits a fine carbide-rich fracture surfacestructure. Crack propagation deviates between two parallelplanes close to the grain boundary. The distance between thetwo planes correlates well with the grain boundary carbide size.Post-weld heat treatment slightly increases the J1mm valuesand tearing modulus of DMW Alloy 182 tested inhydrogenated low temperature water but does not have a cleareffect on JQ.LTCP risk can be minimized by avoiding a situation where thematerial is under stress, the amount of hydrogen is high and thetemperature is low. It must also be considered that the differentnickel-based weld metals exhibit a different LTCP behaviour.

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Acknowledgement

This work has been performed withinthe national nuclear safety program SAFIR2014, within the ENVISproject, which is financed by VYR (the State Nuclear WasteManagement Fund), VTT (Technical Research Centre of Finland),SSM (the Swedish Radiation Safety Authority) and OECD HaldenReactor Project andSINI (Structural Integrity of Ni-Base Alloy Welds) project, which isfinanced by TEKES (the Finnish Funding Agency for Technologyand Innovation), Teollisuuden Voima Oyj, Fortum Power and HeatOy, VTT Technical Research Centre of Finland, Fennovoima Oy,Vattenfall and OKG Aktiebolag.

Their support is highly appreciated.