evaluation of craze cracking degradation · evaluation of craze cracking degradation –fyfitch,...

EVALUATION OF CRAZE CRACKING DEGRADATION

Steve Fyfitch (AREVA)

Mike McDevitt and Paul Crooker (EPRI)

EPRI International LWR Materials Reliability Conference and Exhibition

Chicago, Illinois

August 1-4, 2016

Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.2

Background

MRP Thermal Fatigue Guidelines (MRP-146 and MRP-192)

establish requirements in thermal fatigue examinations

Provide guidance to look for and report instances of craze

cracking

Currently no specific guidance for how to evaluate as-found craze

cracking condition

Overall objective of project to develop guidance for utility

engineers use in recognizing, understanding, and evaluating

as-found craze cracking conditions


Project Workscope (1/2)

Phase 1 Task 1

Literature search to identify state of knowledge and technical areas

to be addressed to develop consensus guidance for dispositioning

craze cracking indications

Phase 1 Task 2

Recommendations for resources (potential expert panel members

and research opportunities) to assess knowledge gaps


Project Workscope (2/2)

Phase 2 Task 1

Identify and assemble appropriate Expert Panel members

Discuss and develop guidance for disposition of craze cracking

Phase 2 Task 2

Conduct Expert Panel meeting

Discuss and develop utility engineer disposition guidance of craze

cracking identified during NDE examinations

Phase 2 Task 3

Prepare summary and guidance MRP report


Literature Search Results (1/10)

[Stress Corrosion Cracking Instances]

Craze cracking observed in reactor

vessel head control rod drive

mechanism (CRDM) nozzles

Identified at numerous units in

U.S. and Europe

Attributed to PWSCC

Flaws remained shallow

[< 2 mm (0.079 inch)]

Not associated with any known

through-wall flaws that caused

leakage

Fluorescent

Dye Penetrant

Results of

Ringhals Unit 2

CRDM Nozzle



[Thermally Driven – French Studies]

S. Taheri and others evaluated thermal fatigue craze cracking

observed in residual heat removal (RHR) systems of French 900

and 1300 MWe units

Provide explanation of differences between high cycle thermo-

mechanical and mechanical fatigue being related to difference

between spatial stress gradients

Stress gradients are higher near inner surface and negligible farther

away for high frequency thermal loading

Craze crack arrest explained as result of decreasing stress intensity

factor reaching threshold level




Craze cracking example

observed in French RHR

system piping

Operating temperature

between 20 and 300˚C (68

and 572˚F)

Temperature variation

around 140˚C (284˚F)

(a) Craze Cracking on a

French Residual Heat

Removal Pipe Inner

Surface

(b) Crack Depth Observed in

the Crazing at (a)




Necessary Conditions

for Crack Arrest

Parametric Study for Crack

Arrest at 2.5 mm Crack Depth

Effect of Tube Thickness

on Crack ArrestQualitative determination

of fatigue life using

assumptions that lead to

crack arrest at 2.5 mm

(0.098 inch) for RHR piping

in French 1300 MWe units

Crack arrest depth is

deeper for a thin tube

than a thick tube



[Thermally Driven – Japanese Studies]

Evaluations performed by Japanese in response to thermal

fatigue operating experience observed

If relatively high frequency fluid temperature fluctuation

sinusoidal waveform exists, possibility of crack arrest occurs at

less than 10 Hz

On contrary though, for cases where cracking propagates

through-wall, waveform deduced to be trapezoidal shape and

crack arrest frequency is relatively low



[Thermally Driven – Japanese Studies]

Results show that crack

could initiate at fluid

temperature range of more

than 120 K (i.e., 120C˚ or

216F˚) regardless of

magnitude of membrane

constraint

Growth of initiated crack

would be difficult to stopCalculated Fluid Temperature and Frequency

Conditions for Failure, Crack Arrest, and No Cracking

(Trapezoidal Temperature Fluctuation)



[Thermally Driven – ASME Code Fatigue Study]

Thermal fatigue testing and analyses

of stainless steel pipes performed by

Jones et alia

Purpose to provide data for

improvements in analytical methods

and ASME Code fatigue curves

Water chemistry controlled to very

low dissolved oxygen content,

consistent with PWR specifications

At periodic intervals, testing

interrupted to ultrasonically inspect

for cracking

Type 304 Stainless Steel Test Sample

(Dimensions in Inches)




Thermal cycling test performed as follows:

Pipes pressurized to 17.2 MPa (2500 psi) and temperature cycled

between 38˚C (100˚F) and 343˚C (650˚F) in three seconds

Followed by holding at 343˚C for 237 seconds and then quenching

to 38˚C in three seconds

Followed by another 237 second hold time




Liquid Penetrant Inspections for the Type 304

Stainless Steel Pipe Test Specimens

Craze cracking present at

all circumferential

locations in two thickest

sections with no preferred

orientation

No cracking seen in two

thinner sections

This cracking supports

assumption initial

cracking due to local peak

thermal stress on pipe

inner wall



[Thermally Driven – MRP Efforts]

MRP-23, Revision 1 provides

guidance for thermal fatigue

inspections to support

implementation of NEI 03-08

requirements

“Needed”

(MRP-146, Revision 1)

“Good Practice”

(MRP-192, Revision 1)

Dye Penetrant Photograph

of Typical Pipe Crazing

and Crack Patch

Dye Penetrant Photograph

of Typical Elbow Crazing

and Crack Patch


Key Drivers For Craze Cracking (1/2)

Material surface characteristics

Surface finish (roughness)

Cold-work (e.g., resulting from grinding, damage from a loose part,

or shot peening)

Metallurgical features (e.g., grain size tends to influence cyclic

plasticity and fatigue crack initiation)

Material cracking susceptibility (e.g., Alloy 600 is susceptible to

PWSCC)


Key Drivers For Craze Cracking (2/2)

Mixing of hot and cold fluids

Stratification of fluids with sufficiently high temperature changes (ΔT)

capable of exceeding the yield strength of the material

Thermal cycles exceeding the fatigue endurance limit of the material in

the environment of concern (i.e., PWR water conditions)

High residual stresses near welds

High levels of mean stress

Geometrical discontinuities (e.g., weld root and counterbore) near

mixing areas


Next Steps

MRP Technical Support Advisory Committee (TS-

TAC) to organize and convene Expert Panel to

investigate thermally-driven craze cracking

Critically evaluate state of knowledge regarding craze

cracking PWRs

Reach consensus for developing suggested guidance for

craze cracking disposition

Prepare and issue MRP report containing overall

summary and utility guidance

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