Post-Weld Heat Treatment on Dissimilar Metal Welds in the … Weld overlays Post Weld Heat Treatment Introduction 3 Material Composition Heat treatment Microstructure Surface condition

Junseog Yang*, Ho-Sub Kim**, Changheui Jang**,

and Kyung Soo Lee*

*KHNP-CRI(Korea Hydro & Nuclear Power Co., LTD - Central Research Institute)

**KAIST(Korea Advanced Institute of Science and Technology)

Nuclear & High Temperature Materials Laboratory

Post-Weld Heat Treatment on Dissimilar Metal Welds in

the Primary Piping System of PWRs to Mitigate PWSCC

Susceptibility

1st KEPIC/ASME Joint Seminar 2017.09.04-05

Sep. 4th

1

Contents

I. Introduction

II. Objective

III. Feasibility evaluation of PWHT application on DMW

Part I. Considerations for PWHT

Part II. Evaluation of PWHT effect on mechanical/corrosion properties of DMW

Part III. Evaluation of PWHT effect on PWSCC resistance of Ni-base weld

IV. Summary and further work

Introduction

PWR experience in dissimilar metal weld (DMW) Butt Welds : RPV and PZR nozzles

Weld residual stress and strain are the dominant mechanical driving force for

crack initiation and propagation within the DMW material due to PWSCC

2

Units Welds Date Operating time

VC SUMMER RPV Outlet Nozzle 2000 16 years

RINGHALS 3 and 4 RPV Outlet Nozzle 2000 19 and 17 years

TIHANGE 2 RPV Outlet Nozzle 2003

20 years PZR Surge Nozzle 2002

CALVERT CLIFFS 2 Piping Drain Nozzle 2005 28 years

*2. R.W. Staehle, Cold-work workshop, 2007

*1. EPRI, MRP -220, Technical report, 2007

*1. EPRI, MRP-220, Technical report, 2007

Remedial methods for mitigation of PWSCC Environment

Zn additions

Optimization of hydrogen partial pressure

Temperature reduction

Material Replace components

Use more Cr containing materials

Stress Peening of various types

Water jet, shot, ultrasonic and laser

Mechanical stress improvement

Weld overlays

Post Weld Heat Treatment

Introduction

3

Material Composition

Heat treatment

Microstructure

Surface condition

Environment Composition

Temperature

Corrosion potential

Flow rate

Stress / Strain Service stresses

Fit-up stresses

Residual stresses

Strain rate

SCC

Fatigue Corrosion

fatigue

Corrosion

*4. P.L. Andresen, Cold-work workshop, 2007

*3. P. Scott, INL Seminar on SCC in LWRs, 2013

Introduction

4

Limitations Needed to optimize PWHT*8

Limited temperature for application Up to < 650oC due to temper embrittlement of low alloy steels

→ Need to evaluate mechanical properties after PWHT A concern for sensitization of stainless steels

Similar temperature range : 500oC - 800oC

Cold Work enhanced sensitization kinetics*9

→ Need to evaluate sensitization behavior of SSs after PWHT

PWHT on DMW

Beneficial effects Reduce residual stress

Relaxation of residual strain

Relaxation of surface Cold Work

Grain boundary carbide precipitation

→ Improvement of PWSCC resistance of DMW*5,6,7

*5. S.L. Hong, et al, 10th EDM, 2001

*6. T.Cassagne, et al, 9th EDM, 1999

*7. C. Guerre, 15th EDM, 2011

*8. G. White, MRP-115, EPRI, 2004

*9. R. Singh, et. al., Corrosion, 61, 907, 2005

.

Objective

5

Alloy 182 PWSCC

resistance properties

(initiation / CGR)

Evaluation of

PWHT effect on

mechanical/corrosion

properties of DMW

PWHT effect on

PWSCC resistance

of Ni-base weld

Feasibility evaluation

of PWHT application

on DMW

Mechanical

properties

PWHT effect on dissimilar metal weld

Corrosion

properties

Ni-base weld metal

PWSCC resistance

evaluation

Development of technical regulation for PWHT

Creep properties

PWHT condition

determination

Research on considerations

for PWHT condition

Sensitization evaluation

depending on PWHT

Creep properties of Alloy

182 depending on PWHT

Tensile properties,

micro-hardness

Considerations

for PWHT

Part. I

Part. II

Part. III

6

Feasibility evaluation of PWHT application on DMW

Part I. Considerations for PWHT

Considerations for PWHT

Sensitization of stainless steel Temperature occurring sensitization of stainless steels : 500-850oC

Depletion of Cr in the vicinity of Cr23C6 precipitation

L-grade SSs are resistant to sensitization due to low C content

0~5% residual ferrite contained in general

Effect of residual ferrite on sensitization of L-grade of SSs has not been well understood

US NRC Reg. guide 1.44 (Control of the processing and use of stainless steel) Material subjected to sensitizing temperature in the range of 427 to 816 oC should be L-grade

material (less than 0.03 wt% C)

Exceptions : Material exposed to PWR coolant with controlled dissolved oxygen

concentration during normal operation

7

*10. H. Sidhom, Metall. And Mater. Trans. 38A, 1269, 2007

*10TTP diagram of 316L SSs: non-CWed


8

IG carbide precipitation for PWSCC resistance of Ni-base weld Beneficial effect on CGR : decrease after PWHT

Improvement of initiation resistance after PWHT

NbC, Cr-rich (Cr23C6) carbide precipitation on GB

CEA, Alloy 82, 600oC*7h*12

PWHT

IG carbide NbC

Cr23C6

EDF, Alloy 182, 610oC*6h*13

PWHT

*11 P. Scott et al, MRP-215, EPRI, 2007 *12. C. Guerre, 15th EDM, 2011 *13. S.L. Hong, et al, 10th EDM, 2001

MRP-215, AREVA-NP, Alloy 82/182*11

Stress relief

(610oC*10h)

No

stress relief

(a)

(b)

U-bend test

PWSCC growth rate PWSCC initiation


9

IG carbide precipitation for PWSCC resistance of Ni-base weld Effect of carbon content (Alloy 182, 610oC*6h)*14

Low carbon content (< 0.03 wt%) – GBC increased

High carbon content (> 0.08 wt%) – No GBC change

Effect of ratio and size of GBC on CGR is in question

Temper embrittlement of low alloy steel Temperature for PWHT is limited below 700oC

Weld %C GBC(%) %GB carbides

size>1μm

D545 AR .029 28 3

D545 SR .029 38 .5

M1 AR .022 26 4

M1 SR .022 44 3

M2 AR .081 48 13.5

M2 SR .081 43 13

M4 AR .089 65 10

M4 SR .089 65 5

*14. T.Cassagne, et al, 9th EDM, 1999

PWHT condition should be determined to improve

PWSCC resistance while avoiding sensitization and

temper embrittlement

10


Part II. Evaluation of PWHT effect on

mechanical/corrosion properties of DMW

Test material and condition Dissimilar Metal Weld (DMW)

SA508 Gr.1a

Alloy 82/182

F316L & F316 stainless steel

PWHT Condition

10hours holding at 600oC, 650oC, 700oC

Heating rate : 38oC/h above 350oC

Furnace cooling

Determined from NESC-III project*15

Experimental

11

F316L or F316

Alloy 182

SA508 Gr.1a Alloy 82

Dissimilar Metal Weld

Materials C Mn P Si Ni Fe Cr Mo Nb

+Ta

F316L SS

(F316)

0.02

(0.05) 1.29 0.15 0.55 11.0 Bal. 16.7 2.59 -

Alloy 182 0.056 6.60 0.006 0.335 Bal. 3.25 14.06 0.008 1.50

SA508

Gr.1a 0.24 1.24 .009 0.25 0.29 Bal. 0.19 0.06 -

*15. N. Taylor et. al., NESC-III project, 2006

Experimental

Mechanical tensile test ASTM E8/E8M-13a

Standard test methods for tension testing of metallic materials

Round bar type Gauge length 24mm, dia. 4mm

Strain rate 5×10-4/s

Test condition : RT air

Strain measured by extensometer (epsilon 3555)

Creep test ASTM E139-11

Standard test method for conducting creep, creep-rupture, and stress-rupture test of metallic

materials)

Test condition 600, 650, 700oC air

Primary creep behavior focused Tested during 30-48 hrs

Test load 150, 220, 270 MPa

12

PWHT effect on mechanical properties

Mechanical properties of DMW after PWHT Stainless steel (F316, F316L)

Yield strength decreased (max. 30MPa)

Changes of tensile strength and elongation are not

significant

SA508 Gr.1a No elongation change

Significant changes on tensile strength above

650oC due to tempering effect

Alloy 182 Yield strength decreased and tensile strength

increased above 650oC

No elongation change

13

316 SS 316L SS SA 508 Alloy 1820

100

200

300

400

500Test temperature: RT

As-received PWHT (600 oC)

PWHT (650 oC) PWHT (700

oC)

Yie

ld s

trength

(M

Pa)

316 SS 316L SS SA 508 Alloy 182400

450

500

550

600

650


Tensile

str

ength

(M

Pa)



oC)

316 SS 316L SS SA 508 Alloy 18220

40

60

80


Elo

ngation (

%)



oC)

100

150

200

250

300

700oC650

oC600

oCAR

Mic

ro-h

ard

ness (

HV

0.1

)

316 SS 316L SS

SA 508 Alloy 182

tempering effect

Micro-hardness

PWHT effect on mechanical properties

14

Creep properties of Alloy 182 Concerns for deformation by creep due to self-load during PWHT

Creep strain depending on load and temperature

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8Test condition: 600

oC, air

Alloy 182, 150 MPa

Alloy 182, 220 MPa

Alloy 182, 270 MPa

Cre

ep

str

ain

(%

)

Time (hr)0 5 10 15 20 25 30

0

5

10

15

20

25Test condition: 650

oC, air

Alloy 182, 150 MPa

Alloy 182, 220 MPa

Alloy 182, 260 MPa

Time (hr)

Cre

ep s

train

(%

)

0 5 10 15 20 25 300

5

10

15

20

25Test condition: 700

oC, air

Alloy 182, 150 MPa

Alloy 182, 220 MPa

Alloy 182, 250 MPa

Time (hr)C

reep s

train

(%

)

600oC 0.265% creep strain during 10hrs under 270MPa

(lower compared to 650, 700oC)

650, 700oC Creep strain rise sharply at 260, 250Mpa (near yield

stress) loads

Estimated deformation 600oC : 0.065~0.265% strain (10hr HT time)

650oC : 0.138~18.6% strain (10hr HT time)

700oC : 0.835~30.4% strain (1.5hr, 3.5hr HT time)

PWHT effect on microstructure

Microstructure characteristics after PWHT F316 SS

Grain boundary carbide (Cr-rich M23C6) precipitation

F316L SS No grain boundary precipitates due to low carbon content (0.03 wt%)

15

PWHT effect on microstructure

Microstructure characteristics after PWHT SA508 Gr.1a

Ferrite with pearlite microstructure for all condition

Alloy 182 Columnar dendritic structure

Cr-rich carbide and NbC precipitation after PWHT

16

Experimental

Sensitization test ASTM A262 practice A (Oxalic acid etching test)

Standard practices for detecting susceptibility to intergranular attack in stainless steel

ASTM G108 (DL-EPR) Standard test method for double loop electrochemical potentiokinetic reactivation test for

detecting sensitization of stainless steels

Degree of Sensitization (DOS) : Ir / Ia ×100 Ir : Reactivation current peak

Ia : Activation current peak

In typical 316L SS

Acceptable less than 0.2*16

Significant sensitization with greater than 7*17

17

Solution 0.5 M H2SO4 + 0.01 M KSCN

Starting voltage 50 mV below open circuit potential (OCP)

Scan rate 1.67 mV/s

Vertex potential 700 mV ▲ Diagram for the procedures of double loop EPR test method

*16. G.H.Aydogdu, Corr. Sci. 48, 3565, 2006

*17. T.K.Yeh et al., 15th EDM, 2011

PWHT effect on sensitization

Oxalic acid etching test (ASTM A262 practice A) F316L SS : No sensitization under PWHT cond.

F316 SS : Not significant until 10hrs at 600, 650oC

18

HT 0 h 5 h 10 h 30 h 50 h 100 h

F316L SS

(650oC)

Step

structure

Step

structure

Step

Structure

Dual

Structure

Dual

Structure -

F316L SS

(700oC), SA

Step

structure

Step

structure

Step

structure

Dual

structure

Ditch

Structure

Ditch

Structure

Sensitized

Step structure

Dual structure

Ditch structure

HT 1.5h 5h 10h 20h 30h 50h 100h

F316 SS

(550oC)

Step

structure

Step

structure

Step

structure

Step

structure

Step

structure

Step

structure

Step

structure

F316 SS

(600oC)

Step

structure

Step

structure

Step

structure

Dual

structure

Dual

Structure

Ditch

Structure

Ditch

Structure

F316 SS

(650oC)

Step

structure

Step

structure

Step

structure

Dual

structure

Ditch

Structure

Ditch

Structure

Ditch

Structure

PWHT effect on sensitization

DL-EPR test (ASTM G108) F316L stainless steel

No sensitization under PWHT condition

F316 stainless steel (0.05 wt% C) Sensitization could be accelerated due to higher carbon content than F316L SS

No sensitization until 100hrs at 550oC

Not significant until 10hrs at 600, 650oC

19

TTS diagram of 316 SS*18 (C: 0.066 wt%)

*18. B. Weiss, et al., Metallurgical Transactions, 851-866, 1972

PWHT

condition

20


Part III. Evaluation of PWHT effect on PWSCC

resistance of Ni-base weld in DMW

Experimental

21

PWSCC test for Alloy 182 PWSCC Initiation test

Using spring-loaded U-bend specimen (t=3mm) (ASTM G30) Standard practice for making and using U-bend stress-corrosion test specimens

Pre-formed to 3% => Flattening

=> bending them to 11.3% in the reverse direction

13.0%

Sample machined L-S orientation

As-welded / 600oC*10h condition

Crack observation

Exposure time

100, 500, 800, 1000h

*19. EPRI, MRP-215, Technical report, 2007

Test system Water loop to simulate PWR environment

Recirculation system

2 Autoclave in parallel

Experimental

22

Test time 1000 hours

Water temperature & pressure 325oC, 15.5MPa

Water

Chemistry

pH 6.3 at room temperature

DO < 5 ppb

DH 2.2 ppm (30 cc/kg)

Conductivity ≈ 22 S/cm (pure water + 1200 ppm H3BO3 + 2.2 ppm LiOH)

PWSCC initiation test Applied 13% strain

130% yield stress from Alloy 182 stress-strain behavior

Cracking behavior As-welded condition

All specimen cracked at 1000h

PWHT (600oC, 10h) condition

GB carbide observed

Improved SCC initiation resistance until 500hr

Cracking tendency increased after 500hr

Additional test needed 620, 650oC condition – under test

PWHT effect on PWSCC resistance

Alloy 182

PWSCC

initiation

time

As-welded

(pre-strained)

PWHTed

(pre-strained)

PWHTed

(single bent)

Cracked specimen ratio

100h 4/20 2/10 0/10

500h 11/20 3/10 2/10

805h 17/20 8/10 7/10

1000h 20/20 9/10 8/10

23

Summary and further work

Summary Mechanical properties

No significant changes at 600oC*10h condition

Mechanical property changes for 182, SA508 above 650oC

Sensitization Not sensitized for F316 SS(0.05 wt% C) at 600oC*10h condition

No sensitization for F316L SS under all PWHT conditions

PWSCC Improved initiation resistance for Alloy 182 (600oC*10h)

Higher temperature condition (620, 650oC) test needed for detail

analysis

Further work PWSCC growth rate test to confirm previous research results

Application to Mock-up nozzle and properties evaluation

Development of technical regulation for PWHT

24

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Post-Weld Heat Treatment on Dissimilar Metal Welds in the … Weld overlays Post Weld Heat Treatment Introduction 3 Material Composition Heat treatment Microstructure Surface condition

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