The notch toughness of the H.A.Z. of electrogas welded joints in mobil storage tanks at Wilhelmshafen

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LABORATORIUM VOOR

SCH EEPSCONSTRUCTI ES

TECHNISCHE HOGESCHOOL

-

DELFT

TH

RAPPORT Nr.

SSL 194

BETREFFENDE:

The notch toughness of the H.A.Z. of Electrogaswelded joints in

Mobil storage tanks at Wilhelmshafen. By prof.ir. J.J.W. Nibbering & Ing. R. Vonk.

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Ship Structures Laboratory, Delft University of Technology, Mekeiweg 2, Deift,

The Netherlands..:

March 1975.

THE NOTCH TOUGHNESS OF THE H.A. Z. OF ELECTROGAS,WELDED JOINTS IN MOBIL STORAGE TANKS AT WÍLHELNSHAFEN

by prof.ir. J.J.W. Nibbering and Ing. R. Vonk. (Purchase order q101-U.J..102.A Badger). .

Report no.

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1. Introduction.

Welding with.high heat-input in relatively thick mild steel plates causes extreme grain coarsening in a few mm's wide zone parallel to the fusion line.. The Charpy-V-notch energies are low arid do not meet existing specifications. For statically loaded structures like storage tanks, an impact test like the Charpy-test is not an appropriate method for a reliable prediction of service behaviour. The high speed of the impact loading constitutes an unfavorable

influence which will not be present in practice.

A purely static test like the C.O.D.-test (crack opening displacement) simul-ates service conditions much bettero 3 Specimens of that type have been used in the test program.

Another approach is advocated by the Ship Structures Laboratory., and consists of cyclic loading of notch-bending specimens at low temperature.

This procedure is more severe than C..O..D.-testing. It incorporates

fatigue-damage at the tip of the developing fatigue-crack In a single specimen

numer-ous points of the H.A.Z. are tested instead of one point as in a C.O.D.-test. For, during fatigue-bending the crack extends a little at each cycle, t.hus providing in principle as many initiation possibilities as there are numbers of cycles. This is particularly important for welded joints. For these .the

material properties differ largely from one point to another due to

hetero-geneity of the weld and H.A.Z. material, and deviations of the fusion surface from a plane surface.

In fatigue bending the chance that a crack, when travelling along the !-I.A.Z., meets the worst spot from a toughness point of view, is very great indeed. The result may be looked upon as a "lower limit" result.

The experience with the test is favorable, satisfactory correlations with re-sults for large size plate specimens., subjected to axial fatigue loading in the 1000 tons testing machine of the laboratory, have been. found /1/, I2I For coa.rse-grained materials (e.g. H.A.Z.) the test method has the additional advantage that the embrittlement of the crack tip material during cyclic

load-ing facilitates largely the initiation of a brittle crack. When such a crack is arrested outside the plastic zone the material proofs itself capable of arresting of a small brittle crack. (The same principle underlies Pellini's drop weight test; only the ernbrittied plastic zone takes th function .of Pellini's brittle weld).

The evaluation of the test results ïs discussed in sections 2and 3.

During the discussions with the representatives of Mobil and Badger, the need for even more realistic testing emerged, based on: the leak-before-break prin ciple. Specimens containing half-penny-shaped surface notches were subjected to tensile fatigue-loading (repeated loading) in order to investigate whether the surface-notch could develop into a through-the-thickness crack before fracturing completely. (Section 5).

All experiments have been carried out with 22,5 mm thick plates.. It wasof interest to know whether .30 mm plate. matérial would be influenced tó the same extent as 22,5 mm plate by heating at high temperature. This is discussed in section Lt _{together with the result of a chemical analysis with respect to} Al-and eventually Nb-contents.

2. C.O,D.-tests.

The static C.O.D. test is sufficiently well known. There aboUt the critical CO.D. to be required.

2ira .t

Wells' formula: COD 13/ gives for St. 52 and

e.

E

Lto

.t

/3

value of 0.23 mm.Nibbering prefers: /t/ whic

the case concerned 0,23 mm is chosen because the loading

may be Some argument

22 mm thickness a. h. gives 0,255. Yet for

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relative-ly low.

Because of the limited amount of material the smaller type of two possible specimens has been chosen. There is no reason to suppose that this will in-fluence the results in the direction of a more optimistic result.. The C.0.D. has been measured at half depth of the notch.' The notch tip' C.0.D. is atout equal to 0,6 times the measured one.

Figures 2 and 3 show the specimen and notch configuration. Table A summarizes theresults. It may be concluded that the joints test.ed will behave satisfac-torily at temperatures higher than -209C.

3. Fatigue-bend tests.

The specimen is shown in fig. 1. The, results aresummarized in table B and fig. 't.

When during testing a crack is formed one of three possibilities, dependent on the temperature takes place:

The crack grows as a pure fatigue-crack up to a length of 12 mm. This is an excellent result. (Specimen M o't). (At 12 mm length, the nominal edge

stresses over the notched section exceed o . , which leads to extreme

yield

cyclic strain hardening and iinevitalDle brittle fracturing).

After some fatigue-cracking the specImens fracture completely brittle. Whén the latter occurs before tie crack has obtained a length of 12 mm the result is wisatisfactory.

.3) One or more times the fatigue-cracking is interrupted by small brittle

steps. In specimen M 01L this can be seen clearly at 13 mm from. the notch tip. These brittle steps proof that the material In the HA.Z. is capable of arresting a

tiny

brittle crack which might have started ata' serious defect (notch length + crack length).

When such a brittle step occurs within the 12' mm zonend full fracturing does not happen before the crack has obtaned a length of 12 mm, such a result is called sufficient. It should be realized that the high-stress-low-cycle fatigue bending applied has a deteriorating influence on the material, which will not occUr in the storage tanks concerned and makes the experiments conservative.

From table B and fig. 't it can be seen that the results are favorable. Only specimen NO-1 has been classified as sufficient, but as the lead has been

in-creased during testing from 2/3 a to a., - in order to speed up this

ten-tative experiment - , the result may be as weil regardedasgood

It might be supposed that the good performanc.e of the joints is partlydue to the fact that the fusionline is not straight but more or less curved. The high quality weld metal may be able to protect the weaker H.A.Z.

This is true as far as the

arresting

of small brittle steps is concerned. It has no influence on the

initiation

of brittle cracks. As in three out of four specimens initiation has not taken place within the 12 mm zone,, the final con-clusion, that the joints behaved well, remains valid.

Apart from that the curved fusionlines provide of course an extra safety bar-rier, the presence of which should be taken into account when making final de-cisions.

The result of specimen MO-30 made of párent metal heated at 1200°C confirms that the late initiation of brittle cracks in the welded specimens was not due to the protection of weld metal at the edges of the specimens. For a large fatigue crack has developed.

't. Chemical anal s:is and comparison of 'behaviour ¿f 22,5 mm and 30 mm thick

material.

--High-heat input has proved to be particularly darnageing to steels containing

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-Lt

doubts about the presence of thèse elements in the steels used for the

Mobil-tanks. Thereforeboth. for 22,5 mm and 30 mm thick material the Niobium

contents have been estimated. The same has been done for the deoxidiser Alu-minium:, which has also grain-refining effects.

In none of three pieces analyzed Nb was found.. The amount of Al was 0,928% and 0,027% for two pieces of 22,5 mm plte and 0,032% for one piece of 30 mm plate.

From this, and the already known data about the steels it may be concluded that the sensitivity to high-heat input of 22,5 mm and 30 mm plate will be equal.

This was confirmed by the results of Charpy-V-notch tests with heat-treated 2.2,5 and 30 mm plate-material. The hat-treatment cnsisted of slowly heating of full-thickness pieces of plate up to. 1200 C and sbsequently cooling in quiet air. After that the Charpy-specimens were fabricated and tested.

In fig. 5 it can be seen that indeed 22,5 mm and 30 mm plate react similarly to the heat-treatment applied..

It can also be seen that the heat-treatment had practically the same effect

as has welding with high heat input on the H A Z material near the

fusion-line. (Curve hlMobil?! result). All Charpy-results correspond closely.

In conclusion it can be said that all results obtained for\22,5 nm plate mate-rial may be applied to 30 mm plate, provided that, a correction is made on the

safe temperature of some 5 C. In other words, when -20°C is a safe temperature for 22,5 mm plates in welded condition, -15 C will be safe for 30 mm plates. This correction is based on former experience obtained. with 3It and 46 mm plates.

(See /1/ and /2/). . .

5. Axial c. clic badin: with s.ecmens coftaining half-.ienny-shaped surface

. notches. , . .

Two plates have been welded, one with normal pre-welding gap, and the other with the greatest gap which may have been used during the construction of the tanks.

It was the intention to select with the aid of Charpy-specimens the worst H.A..Z. from both plates.

However for plate I the results of the Charpy-bars at half plate thickness were superior to those of plate II, while the results for bars situated at theplate surface were opposite, (figures 6 and 7).

I-twas-decided_to_testthree specimens, two of plate Ii and one of plate I,

because the n.id-thickness properties were thttbeofgrea-te-rimpor-tance

than the surface propertiès.. At mid-thickness the stress-state is triaxial and therefore most critical.

In fig. 8 the specimen is shown. The loading consisted of constant-net-stress axial loading.

In table.0 a summary of the resUlts is presented. in all three cases a clear' leak-before-break condition was obtained: extensive through-the-thickness

cracks had developed prior to omplete fracturing.

Fig, 9 shows the fracture surfaces. The result may be called extremely good and confirms the conclusions given in the foregoing sections.

Final conclusion: Static C.0.D.-, fatigue-bending- and leak-before-break-experiments permit.all three the ccin.ciusion that. the 22,5 mm E.G.-joints are safe at temperatures higher than -20 C.

For 30 mm E.G.-joints the safe temperature will be somewhat higher, but most probably not more than -5 C.

Two t:hings should be emphasized:

The fact that every kind of experiment yielded a satisfactory result is remarkable and of particular value, (high reliability of the conclusions). Every type of experiment was over-realistic, in the sense that the

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Literature.

/1/ J.J.W. Nibbering &-A.W. Laileman:

"Low-cycle fatigue tests at low temperature with E.G.-welded 34 mm plates of St. 52 Nb".

S.S.L.-report no. 143a, May 1970. /2/ J.J.W. Nibbering g A.W. Lalleman:

"Brittle fracture in the H.A.Z. of E.S.-welded '46 mm plates of St. 52 Nb subjected to low cycle fatigue".

S.S.L.-report no. 16'4a, AprIl 1972. /3/ A.A. Wells:

"Application of fracture mechanics at and beyond general yielding". British Welding Journal, Nov. 1963.

/i/ J.J.W. Nibbering: "COD-beproeving".

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TABLE. A

Static C.O.D.tests.

F1G3

Fusion line

ICenter

Lire of notch

Specimen

N°

Distance to

fusion line

Test

temperature

C.O.D.

measured

C.O.D.

corr. at crack ti:p

MOlO

1mm

20 oc

0,38mm

0,228 mm.

MOli

1mm

30 oc

0,265mm

0,159mm

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TABLE. B

Specimen

N°

Distance.

from

fusion

line

Test

temp.

Nominal strss

over notched

section (at start

of test)

/ .

Number of cycles at, brittle

steps or fracture

.

Results

Remarks

MO-i

1mm

-2O

2-cT

_{(after ,,}

3 y

14000 increased

to -Gy (for

reasons of time)

Initiation approx 11000 cycLes

Fatigue crack of 20mm at.

25000 cycles.

Very rough surface of

g .

Sufficient

)mm

.we1d . .. . .

..

I-tnotch

Fusion line

MO-2

'

1mm

-20°C

1 Initiation approx at 8000 cycles

Fatigue crack of 15mm at

88000 cycles.

.

Very little brittle steps

'start-¡ng at 63900 cycles.

' Good

tweld

'1mm

-.. ,.

J::

r-notch

Fusion line

MO-3

1mm

-10°C

a

. .

Initiation approx at 10000 cycles

Fatigue crack of 15mm at

49700 cycLes.

.

No brittle steps in fatigue

surface.

. ExceLLent

''

. . . .weLd 1 mm '

A

.

h-4.notch

Fusion line

MO-4

1,5mm

-20°C

-j-ai

2

Initiation approx at 14000 cycles

Fatigue crack of 15mm at

'

68600 cycles.

. .

Excellent

We Ç

_Id

1,5mm

.'.

I. '

Ç.notch

Fusion line

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TABLE.0 RESULTS OF CONSTANT-NET STRESS FATIGUE EXPERIMENTS WITH SURFACE NOTCHES.

Specimen

N2

Distance

from

fusion

line

Test

temp.

Nominal strss

over the notched

section (axialI

repeated loaing)

Number of cycles at brittle

steps or fracture

Results

Remarks

MO-90-A

1

i,.,

-20°C

0,7 Gy

Initiation after approx.

8000 cycles.

Total fatigue surface at the

end of the test was 47% at

52000 cycles.

Good 51 i 87

MO-80-A

.

Ifl ri.p..

-20°C

0,7 cT

Initiation after approx.

17000 cycles.

Total fatigue surface at the

end of the test was 86% at

110000 cycLes.

Excellent

5 14

16

MO-90-B

_{in HAZ}

-20°C

0,7 Gy

Initiation after approx.

7500 cyçles.

Total fatiue surface at the

end of the test was 96% at

132000 cycles.

Note: over the last 15000

cyc-Excellent

,20

f

6

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Fatigue specimen.

FIG.1

0:0specimen..

FIG.2

375

thickness: 22,5

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FIG. 4

rrbçp -V,°c

rr LoAc

rt

Mo.

rç -rp

_-

ooc

rct LoAD

TE'r PC.MO,O

1 rr Lot, 1G'

Fracture ürfaces of notched specimens subjected to

fatigue-bending at indicated temperatures

Mc4

rT 1AAP.

-

_Qo°c

TTLOAD

r

.

MP,

2o'c

1T LOAD

_C?

f ' /

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5 3 C4 E E E -X

>2

L dl C Ql Q-L ru -C C-) o 22,5 mm materiaL "MOBIL" resuLt

(see Letter of 6Nov.)

o ₊₁₀

Test temperature (°C),

FJG.5

_{COMPARISON BETWEEN 22,5}

_{mm PLATE AND}

30mm PLATE AFTER WELDSIMULATING

HEATTREATMENT:

I I J I J +15 +20 +25 130mm material /(Heated at 1200°C)

(NO-30)

22,5mm materiaL 4 (Heated at 1200°C) 3 E >' L Ql C Q' > Q-_L ru -C C-) O 5 7 6 5 E

E,

-x > dl C Ql. Q-L. ru -C C-, O

/

Ptatefl

/

r.

I I. I I 0 +5 +10 +15 +20

25 Test temperature (°C)

. Plate I /(MO-96) Plate I (Mo - B3) Plate I (Mo-so)

FIG.7

_{CHARPYVALUES AT PLATE SURFACE}

_AND

HALFPLATE THICKNESS OF TWO E.G.WELDED

PLATES.

6 5 4 6 6

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MO-83(93) I

I '1Ì1 MO-84(94) ' MO-85(95)

t. \

MO-86(96)

MO -87 (97) Mû-88 (98)

iLI

FIG.6

SITUATION OF CHARPV-V SPECIMENS.

j MO-80(90)

j MO-Bl (91)

MO -82 (92)

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1000

t1iikhess: 22,5

Fatigue specimen

30

(axial repeated loading)

AA

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TEV

i

j TETAP. -cÇ.

TEV CC

N MOSc..

4

TsTT/P.

-2o°c

TV SPEC.N

MOoQ

TE

Fig. 9. Fracture surfaces of surface-notched specimens subjected to axial cyclic loading with constant net stress.