1st Ukrainian-Hungarian Seminar “Safety, Reliability and Risk of Engineering Plants and Components”, Miskolc 11-12 April 2006

Intergranular stress corrosion cracking of the piping Dy300 welding joints of

unit #3 before and after inactivation of Chernobyl NPP

A.J. Krasowsky

Pisarenko Institute for Problems of Strength,

National Academy of Sciences of Ukraine

 

 

“Materials like persons are interesting with their imperfections” (F.C. Frank, W.T. Read) 

 

1.      Introduction2.      Main factors governing the IGSCC of welding joints        Chemical composition of water        Sensibilisation of austenitic steel 08X18H10T        Tensile stress (residual + service)3.      Main features of IGSCC        Metallurgical aspects        Fracture mechanics aspects4.      Statistics of crack evolution before and after inactivation5.      Entire stress at welding joint before and after inactivation        Software complex “StrengthMaster”        Flaw assessment module “WFMaster”        Crack shape prediction6. Remaining life of WJ prediction

General view of unit #3 RBMK-1000, Chernobyl NPP

3D model of Dy300 down-flow piping

Chemical composition and mechanical properties of steel 08X18H10T (similar to steel 321) and weld metal

As received (base metal)Т,0С RT

m, МPа RTp0,2, МPа А,% Z, % RT

p0,2/ RT

m

20 510 216 35 55 0,42

350 412 177 26 51 0,43

After 18 years (specimen with welding)20 618 320 46 72 0,52

350 432 250 32 77 0,58

After 18 years (weld metal)20 654 422 44 63 0,65

350 465 303 22 43 0,65

Element C Mn Si Cr Ni S P Ti Cu Fe Mo

Steel  08X18H10T (accord. ГОСТ 5632-72)

 < 0.08

 < 1.5

 < 0.8

 17-19 10.0 -

11.0

  0.0

2

  0.0

35

 5C-0.06

  0.3

0 bal-

weld metal (accord. ГОСТ 5632-72)

  0.0

6

 0.9-2.0

  0.6

017.8- 20.0

9.8 - 12.0

 -

 -

 -

 -

bal1.88-

3.0

History: Gamma-NDT results of pipes Dy300 welding joints revealed on unit No 3 ChNPP during 1997

History: Ultrasonic-NDT results of pipes Dy300 welding joints revealed on unit No 3 ChNPP during 1997-1998.

Chemical composition of water before and after inactivation of Chernobil NPP

Parameter, (1988 – 1997)

 

Maximum observed

 

Averaged for 10 years

 

Allowed 

Minimum observed

 pH 

7.6 

7.2 

6.5-8.0 

6.7 

Conductivity, S/cm 

0.35 

0.21 

<1.0 

0.1 

Chloride-ion 3.7 

<2.1 

<100 

<2.0 

CaCO3-equivalent, g/kg

 3.0 

3.0 

<5.0 

3.0 

Copper, Cu, g/kg 

8.7 

5.1 

<20.0 

2.6 

Iron, Fe, g/kg 

36.0 

13.3 

<50 

5.0 

Oil, g/kg 

<100 

<100 

<200 

<100 

Conductivity after inactivation:

2001 – 2002: 0.55 – 1.2, average 0.88 micro-S/cm allowed 2.0 micro-S/cm2004 – 2005: 0.53 – 1.01, average 0.56 micro-S/cm

Metalography features of IGSCC process

Axial stress distribution at the root of welding joint for different service stress (courtesy Prof. Makhnenko)

Templets of welding joint with the “bending opened” crack

Metalography features of IGSCC process

IGSCC rate versus SIF of steel 10X18H10T at different corrosive environment (courtesy Prof. M. Speidel)

Predicted regions of IGSCC (courtesy Prof. M. Speidel)

Intergranuliar cracks length evolution during 1997 - 2005

Pipeline type

Year of NDT 1998 1999 1999 2001 2002 2003 2004 2005

Average current month of NDT

2 14 20 40 58 70 82 93

PressurizedOverall crack length, mm

1065 1250 1565 2520 3825 3755 4735 4865

Relative crack length, %3.62 4.24 5.31 8.56 12.99 12.7 16.0 16.5

Down-flowOverall crack length, mm

2385 2345 2385 2755 3115 4685 5280 6060

Relative crack length, %4,89 4,81 4,89 5,71 6,39 9,6 10,8 12,4

Emergency cooling

Overall crack length, mm70 70 110 110 175 185 225 225

Relative crack length, %1,9 1,9 2,99 2,99 4,75 5,02 6,11 6,11

AltogetherOverall crack length, mm

3520 3665 4060 5415 7115 8625 10240 11150

Relative crack length, %4,3 4,47 4,96 6,61 8,68 10,5 12,5 13,6

Intergranular cracks length evolution during 1997 – 2005:

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100months

Presurized

down-flowed

emergency

all

Relative crack length , l % :

N

ill1inDnL %100

L

ll

Schematic of welding joint containing seven IGSCC cracks

Actual measured crack depth versus crack length (1998)

3D calculation model of Dy300 down-flow piping

Computer version of national Code of Ukraine ВБН В.2.3...

Window of input data for remaining life calculation, «WFMaster» software

Axial stress distribution along the wall thickness from the root of welding joint (courtesy Prof. Makhnenko)

Axial stress distribution along the wall thickness, H, at different service conditions

0.0 0.2 0.4 0.6 0.8 1.0 -150

-100

-50

0

50

100

150

200

250

300

350

400 Res. stress 40 [МPа] 50 55 60 65 70 80 90 100 110

, МPа

x/H

Stabilized crack shape (a – crack depth, L – crack half length) (single nucleus approximation)

0 2 4 6 8 10 12 14 160.0

0.5

1.0

1.5

2.0

a0/L0

2

0.666

0.2

0.1

Уст. форма

a/L,

a, мм

Stabilized crack depth versus crack length evolution

+12 МПа

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120 140

L, мм

a, м

м

нелинейный закон линейный закон

Remaining life of welding joint versus half-length of crack

+ 12 МПа

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100 120 140

L, мм

T,

лет

нелинейный закон линейный закон

CONCLUSIONS1.  Three main factors are responsible for IGSCC process in welding joins of pipeline Dy300:a.  Tensile stress of necessary levelb. Corrosion environment (oxygen in cooling water)c.  Grain boundaries sensibilisation of steel at HAZ2.  Effect of these factors was considered at remaining life prediction.3.  Evolution of IGSCC cracks relative length within 89 welding joints during 1997 – 2005 demonstrates similar rate before and after inactivation of unit #3. These data of in-service NDT are unique.4.  Due to the fact that crack depth is governing parameter of IGSCC process whereas NDT inspection provides the crack length, the conservative model of single nucleus was developed for remaining life assessment of welding joints.