Mitigation PWSCC by Ultrasonic Nanocrystal Surface Modification (UNSM) technology

International Light Water Reactor Materials Reliability Conference and Exhibition 2016

Hong-Seok Cho1, Young-Sik Kim2, Auezhan Amanov3, Young-Sik Pyun3, Hong-Seok Cho4, Vijay K. Vasudeva

n5, Seetha R. Mannava5 and Abhishek Telang5

1Technology Research and Development Institute, KEPCO KPS, Naju 58217, South Korea2Materials Research Center for Energy and Green Technology, Andong National University, Andong 36729, South Korea

3Department of Mechanical Engineering, Sun Moon University, Asan, 31460, South Korea4School of Materials Science and Engineering, Pusan National University, Pusan, 46241, South Korea

5Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0072, United States

Corresponding author: pyoun@sunmoon.ac.kr

Contents

I. Motivation and Background

IV. Concluding remarks

II. Microstructural Characterization and Mechanical Properties

of Stainless Steel Inlay Welded Dissimilar Materials by UNSM

III. Changes of Corrosion and SCC Characteristics of

Stainless Steel Inlay Welded Dissimilar Materials by UNSM

I. Motivation and Background

Could the UNSM technology become a Surface Stress

Improvement (SSI) technology which could mitigate

Primary Water Stress Corrosion Cracking (PWSCC)

complying with MRP-335 Revision 3 ?

U.S. NRC(the Nuclear Regulatory Commission) approved the industry’s topical report supporting

appropriate inspection relief with Surface Stress Improvement (SSI) technology (February 2016)

Primary Water Stress Corrosion (PWSCC) Cracking Mitigation by Surface Stress Improvement (SSI) (MRP-335 Revision 3, 3002007392 Final Report, February 2016)

▪ Surface stress improvement (SSI)mitigates PWSCC by inducing compressiveresidual stress at the surface exposed to reactor coolant

– Initiation of PWSCC flaws requires tensile stress at the surface

– Any existing flaws that are fully within the surface compressive stress zone cannot grow via PWSCC

▪ SSI provides an option to mitigate reactor vessel closure head penetration nozzles instead of replacing the entire head

▪ SSI provides an option to mitigate components that are not easily replaced or mitigated from outer surface using weld overlay or mechanical stress improvement (e.g., reactor vessel inlet/outlet nozzles)

SSI (Surface stress improvement) on DWMs (Dissimilar metal but welds)

ASME Code, Case N-766 ( N-516-3, N-770 )

J-Groove Head Vent Nozzles RPVHPNs

Output increased while decreasing operation cost of N-Power plant by “Acceptance of Inspection Relief ”

Asset Management

(10 CFR 50.59 Process)

Licensees may make changes to the facility, procedures and conduct tests and experiments without prior NRC approval

If change requires no license or technical specifications modifications

If change does not meet one or more of the eight criteria specified in 10 CFR 50.59(c)(2)

No license amendment request required

This process does not grant inspection relief

Optimized Inspection Intervals

(10 CFR 50.55a Process)

For NRC (the Nuclear Regulatory Commission) approved and regulation-mandated programs (e.g. in-service inspection programs) separate from the license

Licensees may submit “relief requests” to the NRC

NRC reviews and approves relief requests by Safety Evaluations (SE)

10 CFR 50.55a specifies the processes for requesting alternatives to, or relief from, the in-service inspection and testing requirements of the ASME Code

Code of Federal Regulations

Process for Acceptance of Inspection Relief

Development of ASME Code Requirements and U.S. NRC(the Nuclear Regulatory Commission) Review of Topical Report

Revised ASME Section XI Code Cases include performance criteria and inspection intervals for components mitigated by peening:– Alloy 82/182 DMWs per Code Case N-770-4, approved by ASME May 7, 2014– RPVHPNs per a revised version of Code Case N-729, expected to be approved soon by ASME

These revised code cases have not yet been approved by U.S. NRC and incorporated by reference in 10 CFR 50.55a

U.S. NRC approved the industry’s topical report supporting appropriate inspection relief with peening

– MRP-335 Revision 3 “Topical Report for Primary Water Stress Corrosion CrackingMitigation by Surface Stress Improvement (3002007392 Final Report, February 2016)

Materials Reliability Program

A novel SSI “UNSM” integration with Inlay Welding in Korea

KURIS : KPS Underwater Repair & Inspection System

UNSM: Compressive residual stress and improving Surface roughness

Fine or Micro Shot Peening (FMSP)

Ultrasonic Shot Peening (USP)

SMAT

Low Plasticity Burnishing (LPB)

Laser Shot Peening (LSP)

www.surfaceenhancement.com

http://www.lambdatechs.com/

www.lspt.com

www.metalimprovement.com

//www.toshiba.co.jp/nuclearenergy

www.sonats.com

www. fujiwpc.co.jp

Ultrasonic Nanocrystal Surface Modification(UNSM) imst.sunmoon.ac.kr/www.deignmecha.co.kr

Novel Surface Stress Improvement Technologies

Water Jet Peening/Cavitation Peeningwww.mhi.co.jp/ ww.mm.mech.tohoku.ac.jp

ST

USP

MSP

Cavitation

Peening

UNSM

LPB

LSP

Forge (Human) Forge ( Machine)

The Ultrasonic Nanocrystal Surface Modification (UNSM) technology

Conventional Forging

Less than

- 10 strikes / mm2

- 1,000 strikes / minute

- 3 Giga Pascal

N = 60f / SV - Contact count per unit area (mm2)

F = Fs+Fd =m(g + A/(2πf)2)Maximum total striking force

m – weight of static load onto the tool;

g – standard gravity; A – amplitude;

V - speed(m/min); S – feed (mm/rev); r - ball radius; R - specimen radius;

t – time (sec.); f- frequency

Generator

장치선단부분피가공물

장치선단부분피가공물

Wor

kpi

ece

UNSM Forging Mechanism

- 1,000 ~ 100,000 strikes/mm2

- Up to 2.4 million strikes/min

- 3 ~ 30 GPa contact pressure

UNSM schematics & Device

The UNSM device

Outer surfaceInner surface Horizontal Machining

Center

Vertical Machining

Center

UNSM device can be mounted on any machine,Even on robots and hand held manual operation

Nano scale roughness

Increased hardness

Compressive residual stress

Surface Modification by UNSM

UNSM Anticipated benefits

Deep compressive residual

stresses

(Greater than 1000MPa into

depths of more than 2000 ㎛ )

1) Improved LCF and HCF

endurance limit

2) Improved rolling contact fatigue

strength

3) Improved stress corrosion

cracking resistance

Micro dimples surface

(Dia. of area:1-2㎛2 ),

Depth : sub micron, Pattern pitch:

few ㎛)

1) Reduced surface roughness

2) Decreased friction coefficient

3) Reduced wear rate

Increased hardness

(into depths of more than 1500

㎛ )

1) Reduced wear rate

2) Improved LCF and HCF

endurance limit

Nanocrystalline structure

(Grain Sizes of 50-200 nm into

depths of 100 ㎛ )

1) Increased tensile strength and

hardness,

2) Increased fatigue strength,

3) Increased wear resistance

Top surface : Nano level surface roughness + texturing structureSubsurface : Nano crystal structure + improved hardness + compressive residual stress

~100nm

~1000nm

Microstructurally & Mechanistically Gradient Structure

LSP

UNSM

CSP

~50 nm sub-grains with

high dislocation

density and twins

50-100 nm

crystallites with

high dislocation

density and twins

30-100 nm

grains

UC + Tohoku

case: IN718 SPF MaterialsScience&EngineeringA576(2013)346–355C

DEPTH FROM

SURFACE(㎛)

GRAIN SIZE (㎚)

5 30

10 36

20 42

30 45

A. Cherif, Y. Pyoun and B. Scholtes, Journal of Materials Engineering and Performance,

Volume 19, Number 2, 282-286

Case: AISI 304

AfterBefore

Compressive residual stress and hardness of 3 SSI on IN718 SPF 718

MaterialsScience&EngineeringA576(2013)346–355

II. Microstructural Characterization and Mechanical Properties ofStainless Steel Inlay Welded Dissimilar Materials by UNSM

SSRT specimen

U-bend test specimen

DMW test Specimens

DWM

DWM

Inlay Weld Mockup specimen

Welding & InspectionASME Section IX

UNSM Parameters

Condition 316L/alloy 82

Amplitude 60μm

Static load 30N

Pitch 0.03mm

Speed 1500mm/min

Tip diameter 2.38mm(WC)

UNSM treatment condition

Material composition

Composition, wt%

Ni Co Cr Fe C Ti Al Mo P Cu Mn Si S

316L 10 - 18 68.5 0.08 - - 2 0.045 - 0.07 1 0.03

Alloy 52 60 28 7 0.04 1 1.1 0.5 0.02 0.3 1 0.5 0.015

Alloy 82 74 18 3 0.1 0.75 - - 0.03 0.5 2.5 0.5 0.015

Effective Depth of UNSM (OM)

Microstructure of Weld Zone after UNSM

Effective Depth of UNSM (Based on Hardness)

Weld Zone Hardness Mapping

Current (A) Voltage

(V)

Wire speed (IPS) Travel

speed (IPS)

Gas flow rate

(L/min)PRI BKG PRI BKG

Cladding 180∼260 80∼160 9.2∼9.8 30∼70 30∼60 3.0∼3.8 15∼19

Buttering 180∼200 80∼100 9.5∼9.7 35∼42 35∼42 3.5∼4.2 15∼18

DM weld 160∼180 60∼90 9.3∼9.5 20∼40 20∼40 3.0∼3.5 15∼18

Weld process conditions and Heat treatment condition after welding

Inlay Weld Mockup specimen

Condition Specification Actual

Unloading temperature(℃) 425 375

Heating rate Max.56℃/Hr. 40℃/Hr.

Holding temperature(℃) 600∼620 608∼612

Holding time Min.2Hr 30min 2Hr 40min

Cooling rate Max.56℃/Hr. 40℃/Hr.

Unloading termperature(℃) 425 370

Vickers hardness(Hv)

XZ plane XY plane

1 2 3 4 1 2 3

Alloy 82

Before 139 211 173 186 140 230 152

After 159 240 229 238 160 302 250

Alloy 52

Before 138 210 175 173 106 253 138

After 154 243 230 237 161 262 263

Vickers hardness

before and after UNSM treatment

Before UNSM After UNSMRemarks

82 DWM 52 DWM 82 DWM 52 DWM

SS-XZ-20,50 -596.7 220 -918.2 -209.3

Line

1

SS/AL-XZ-40,50 64.6 -361.5 -916.5 -34.6

SS/AL-XZ-55,50 -656.6 181.3 -934.7 -228.5

AL-XZ-65,50 1505.6 -737.8 -365.1 -494.9

AL-XZ-70,50 -382.9 7 -1901.9 -14

AL-XZ-90,50 -356.6 -318.9 -205.9 -97.1

AL-XZ-110,50 -104.6 -304 -397.2 -580.6

CS-XY-104,24 271.5 272.2 -388.1 -427.4Line

3CS-XY-109,43 312.7 320.8 -387.4 -407.4

CS-XY-114,63 301.2 284.4 -390 -423.3

Residual stress of surface before and after UNSM

III. Changes of Corrosion and SCC Characteristics of

Stainless Steel Inlay Welded Dissimilar Materials by UNSM

- Improved resistance to Stress Corrosion Cracking(SCC) of Alloy 600

By ASTM A 262 Practice C/boiling 65% Nitric acid (HNO3)

Intergranular Corrosion Rate Results of 316L/Alloy 82

316L/Alloy 82(S2)

316L/Alloy 82U(S2)

Sodium sulfate (Na2S2O3) 0.1M

Anodic Polarization Test Results of 316L/Alloy 82

Stress Corrosion Cracking resistance by UNSM

↑

(Sodium hydroxide (NaOH) 40%

Improved resistance to Stress Corrosion Cracking(SCC) of Alloy 600Materials Science&EngineeringA648(2015)280–288

UNSM ParametersStatic load:20N, Amplitude 8 um, Scan speed 3000mm/min Scan interval 30 um

Residual stresses and (b)FWHMafterUNSMinAlloy600

Slow strain rate tests

Sodium sulfate

Materials

316L/alloy 82, 316L/alloy 52, 316L

SCC tests

SSRT in simulated 1’ry water(320OC, strain rate: 8.7x10-8/sec)

U-bend in 320OC, 40% NaOH

U-bend in 340OC, 0.01M Na2S4O6

Corrosion tests

Intergranular corrosion test

•A262 pr. A

•DL-EPR test

•A262 pr. C

Anodic polarization test

•0.1M Na2SO4, 0.1M Na2S2O3, 3.5% NaCl

AC Impedance measurement, Mott-Schottky plot

Ongoing test

Sodium hydroxide: NaOH, Sodium Sulfate: Na2SO4, sodium thiosulphate: Na2S2O3, Sodium chloride:NaCL

IV. Concluding remarks

▪ UNSM, a novel surface stress improvement (SSI), could also

mitigate PWSCC by inducing compressive residual stress at the

surface exposed to reactor coolant as like LSP and WJP.

Every technology has pros and cons due to it’s unique

characteristics, but UNSM could improve surface roughness

only among three technologies. UNSM can treat ceramics,

TSC and even Si Wafer for solar cell, etc.

The answer to “ Could the UNSM technology comply with the re

quirement of “MRP-335 Revision 3 “Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement” is Yes,

but ……

Any requests for collaboration are welcome !

Questions and comments !!!

IV. Concluding Remarks

THANK YOU FOR YOUR ATTENTION!!!

Before UNSM After UNSMRemarks

82 DWM 52 DWM 82 DWM 52 DWM

SS-XZ-20,50 -596.7 220 -918.2 -209.3

Line

1

SS/AL-XZ-40,50 64.6 -361.5 -916.5 -34.6

SS/AL-XZ-55,50 -656.6 181.3 -934.7 -228.5

AL-XZ-65,50 1505.6 -737.8 -365.1 -494.9

AL-XZ-70,50 -382.9 7 -1901.9 -14

AL-XZ-90,50 -356.6 -318.9 -205.9 -97.1

AL-XZ-110,50 -104.6 -304 -397.2 -580.6

CS-XY-104,24 271.5 272.2 -388.1 -427.4Line

3CS-XY-109,43 312.7 320.8 -387.4 -407.4

CS-XY-114,63 301.2 284.4 -390 -423.3

Residual stress of surface before and after UNSM

Microstructure Analysis after UNSM

316L 316L/52 316L/82 Alloy600 Alloy690

- Deformation bands near

the UNSM-treated

surface

- Austenite matrix

- Twin boundaries

- Presence of delta ferrite

- Effective depth: 212.5

μm

- Alloy 52: intragranular

networks and

coarse/equiaxed grains

- Uneven bond line

- Effective depth: 102.1

μm

- Alloy82: intragranular

networks and columnar

grains

- Uneven bond line

- Effective depth: 126.9

μm

- Relatively large grain

size

- Severe plastic

deformation near the

UNSM-treated surface

- Effective depth: 237.9

μm

- Deformation bands near

the UNSM-treated

surface

- Effective depth: 52.7 μm

Materials Microstructure analysis Hardness test Residual stress

calculation

- Ferrous Alloy : 316L stainless steel

- Welding : 316L/Alloy52, 16L/Alloy82

- Nickel-based alloy : Alloy 600 & 690

- Optical microscopy & electron microscopy

- Effective depth measurement

- Through-thickness mapping & profiling

- Effective depth measurement

- Using X-ray technique

Hardness and Residual Stress Measurement

316L 316L/52 316L/82 Alloy600 Alloy690

- Effective depth : 400μm - Near-surface hardness is

similar between 316L and

Alloy 52.

- Matrix hardness is lower in

Alloy 52.

- Effective depth: 275μm

- Near-surface hardness is a

little bit higher in Alloy 82.

- Matrix hardness is lower in

Alloy 82.

- Effective depth: 225μm

- Effective depth: 325μm - Effective depth : 150μm

Residual stress measurement using X-ray technique

Residual Stress, MPa

316L -508.8

Alloy600 -1020.6

Effective depths measured from the microstructure analysis and the micro-hardness profiles