Microstructure analysis on some specimens of the fatigue bending at low temperature experiment in the NIL fracture programme

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SSL299

PG86-26

MICROSTRUCTURE ANALYSIS ON

SOME SPECIMENS. OF THE

FATIGUE BENDING AT Low.

TEMPERATURE EXPERIMENT IN

THE NIL FRACTURE PROGRAMME.

DeLft1 1986

TU Deift

Technical University floUt

ir. E. van Rietbergen ing. R. Vonk

Ship Structure Laboratory

-

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CONTENTS

INTRODUCTION

EXECUTION OF THE ANALYSIS 2

RESULTS .

. 2

DISCUSSION ANO, CONCLUSIONS . . . 3

LITERATURE

TABLE 1-4

FIGURE 1 - 9

1. INTRODUCTION

As a part o-F the NIL-Fracture

_{programme a seriesoffatigue}

bending tests at low temperature has been conducted .( 1] . The

purpose of this test method is to find _{a location of low f.ractur'e}

toughness by means of _fatiguing _(welded) _specimens _at _low

temperatures. ThIs experime.ntre.sulted _{i.ñ some rémaricable results}

which could not yet be fully _{explained. First} _{in the. transition}

from brittle to ductile _{behaviour the fracture toughness of} _the

70mm thickness series proofed to be betterthan _{the. 30mm series.}

Second one weidment o-P the _{30mm series showed a 20 OC .higher} transition teme:prature compared to the other 30mm Weldment.s.

In _{get on explanation for theeB phenomena it} _{was decided}

tope-r-form a -small :inastigaton

programme -o

analyse some

specimens on microstructure and chemical composition (2] This

report presents _{the results of the microstructure analysis} _The

aim of this investigation can be summarized _{as. follows:}

to investigate if the Initiation point of _{the final} _fracture

could be explained by abad spOt in _{t.he.microstrûcturá on that}

part icular location

if a difference in microstructur.e could ixplaln: .

a _{the 20 °C higher transition temperature of the} _8LTa

weidment of the 30mm series .

b. the better fracture toughness behavibur oft.he. 70mm specimens compared to the 30mm specimens. in, the

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EXECUTION OF THE ANALYSIS

For the analysis six 30mm specimens and three 70mm specimens were

chosen with similar fracture toughness behaviour at the

temperatures of interest. Fig la-9a give the fractùre surface of

the specimens, where the. most probable point of initiation of the final fracture is marked. The preparations were taken out with one edge at this initiation point, in ordèr to investigate the structure perpendicular to the fatigue crack. The preparations

were polished and etched (Nita].), and a photograph of the

microstruc'ture was taken with a magnification of 200x, showed in

fig. lb - 9b. For two specimens (1LT3 and 2LT15) the fine

microstructure of these specimens was investigated by making

photographs with larger magnification l000x and 2000x (fig. lc,d

and 2c,d) . At specimen 8L137 two loacationc of fracture

initiation can be distinguished, both were investigated.

RESULTS

All, preparations were taken out from welded material. The

composition of the used weld metal SW 60 is:

0.1% C, 1% Mn, 1% Ni, 0.15% Si, <0.015% P, <0.015% 8.

The structure ismainly ferritic, but consists of different kinds of ferrite. As can be seen on the microstructure photographs in fig. lb-9b, there are great differences in microstructure between

the different specimens. Because of the great

tempe'raturegrad'ient, the annealing 0f the last welding run to former welding runs, the microstructure is appa'i';antly strongly

dependent on thA location of the preparation.

Considering the grain size the specimens can be divided into two

parts:

- preparations with a fine grain size CASTM no. 5): 2LTIS, 8LT35, 5LT44, and

preparations ILT3 and 1ILTB. with a finer grain sze (>9).

- preparations. with an ,obviousiy coar.ser grain size' CASTM 4-5).: 2LT22, .8LT37 1l;LT2O, 11LT26.

The low toughne;ss.values of the spacimens.with a fine,grain.size

can be explained by precipitation of impurities at the grain

boundaries of problably carbides and perlite (fig. I c,d end'

2c,d)

In coarse grain structure's another brittle st'ructi.ire canap:pear,

caused by intragranular ferrite and acicular ferrite

(Widmannsttten-ferrjte)

Another phenomenon are seôondary crac;ks which can be seen on the microstructure photographs of specimens l:LT3, 2LTIB, BLT3S.,

8LT44, 11LT20, 11LT26..

On the preparations of 8LT44, 11LT8, 11LT2O, 2LT22 arid BLT37II

the initiation of the final fracture was preceded by deformation

of the microstruoture.

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4. DISCUSSION AND CONCLUSIONS

The results of the microstructura analysis _{are gathered in table}

1-4. These tables contain the same information, but are

differently arranged _{on particular variables, as plate thickness,}

temperature, ClOD-value and grain _size.

Low ClOD results of o series _{con not be explained} _{by just taking}

into account the grain size. It _{is however remarkable that for} the 30mm specimens se.cundary _{cracks occur} _{at finer} _{grain sizes} while for '70mm specimens these _{occur at the coarser grain sizs.}

However this combination of variables can be found at different

temperatures and ClOD values so that this phenomenon does not

give a solid explanation for low toughness béhaviour.

Grain deformation con be found at higher toughnass values, as

could be expected. That no deformation occurs at s.pec:imen 1LT3

with a relatively high ClOD value can be explained by the very

fine grain size. On the other hand grain _{deformation takes place}

at IILT2O with low toughness. However as can be seen from the

microstructure photograph of fig.5c, thies deformation is very

local.

The occure.nce o-P acicular.and intragranulor ferrite _{indicates low}

toughness results. This kind o-f microstructure is found at the

70mm specimens with relatively low ClOD results., whereas

comparable 30mm results are _{gained without} _{this structure. When}

this kind of microstructure occurs in 30mm specimens t.his' is

followed by a very low ClOD-value,, _{illustràted by} _{the result of} 8LT371. It i _marked _{that this} _{specimen is taken from the 8LT.a}

weldment with a 20 degrees C higher transition temperature.

However the _{low toughness result of specimen 8LT36: from} _{the same} weidment could not be explained by this phenomenon.

It is evident that in welded sections great _{differences in}

microstructure occur., although for all specimens the same welding

procedure was applied. From the present investigation no solid

explanation for the pósed _{questions in the first chapter can be}

found. No _{Ïearly nbtÏàb.le pherionomon -is presentto} _{e*p--ai-n--the}

differences between different series of plate thickness or

weldment. The onlyindicotion that con be found is that where

acicular and intragranular ferrite occur low toughness results

are found, but not all low results con be explained _{by this. For}

confirming the present results and to find ot!her.explonations a

larger number of specimBns hove tb bé investigated on

microstructure. Also an analysis of the chemical composition of

the material could contribute in explaining the differences in fracture toughness behaviour.

LI TERATURE

Scho,lte, H.G., Rietbergen, E.van,"Fatigue bendin;g tests at low temperature on welded specimens of 30mm and 70mm

thickness", Ship Structure Laboratory Report No.302, Daift 1966.

Vonk, R. ,"Structuuron.derz.oek t.b.v. "NIL-onderzoeIç

kerftaaiheid", Intern rapport nr.295, DelPt 1986.

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TABLE i SPECIMEN DATA AND MIC'ROSTRUCTURE ANALYSIS RESULTS

RANGED .ON PLATE THICKNESS ANO WELOMEN.T

specimen B t int. frac. A'STM SBCU.n- acicular matrix

number ClOD ClOD grain dary intragr. deform.

mm O _mm _mm

size cracks ''ferrite

1LT3 30. -50 .061.10 0.38 >9 x - -2LT15 30 -60 0.12 0.12 8 x. -2LT22 30 -50 .O6./.O9/.2 0.21 . 4-5 - x BLT44 30 -so 0.1.1 0.30 '8 x - x 8LT36 30 --50 .0.61.1 0.07 8 8LT371 30 -20. 0.11 0.11 4-6 X 8L13711 30 -20 0.11 1.61 4-5 x 11LTB. 70 -50 .081.24 0.25 >9 - X, 11LT2,0 70 -40 ..08/!18/.29 0.16 4-5 x x 11LT26 . 70. -60 .091.17 . 0.17 4-5 x x

TABLE 2 SPECIMEN. DATA AÑO MICROSTRUCTURE ANALtSIS RESULTS

RANGED ON TEMPERATURE . . ..

sa.cun.- aci'c:ular matrix

dary' intrag'r. de,form,

cracks ferrite x X X specimen number B mm ' t °C'.

mt,

ClOD . mm frac. ClOD mm . AST,M grain size. 2L116 3 -60 0.12 .. . 0.12. 8« 11LT26 70 -60 .09/.. 17 .0.17. 4-5 8LT35 30 . -6.b .06/.1 0.07 8 2LT22 30 5Q _.061.091.2 0.21 45, 11LT8 7Ó -SO .0.81.24 0.25 >9 BL 144 30 -60 0. 11 Q .30 . 8 1LT3 .30 . -50 .051.10 0.36' >9 11LT2O 70 -40 .081.151.29 0.16 4-5 8LT371 30 -20 0.11 0.11 4-5 8LT3711 30 . -20 0.11 1.61 4-5

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1 1LT26 2LT22 1 1LT2O 8 L T 371 BL T.37I I 2LT LB 8LT35 8 L 144 11LT8 1LT.3

TABLE 4 SPECIMEN DATA AND MICAOSTAUCTURE RESULTS RANGED ON' GAAIN SIZE

specimen B t

mt.

frc. ASTM

_secun- _{aojo u lar} _matri

number clOD ClOD 'grain dary' intrag:r. deforn

mm mm mm size cracks f er r i. te specimen n umber B mm t O$

mt.

CloD mm f p': CT 00 mm ASTM grain size seoun-d:or'.y cracks acicular int.ragr. ferrite mat.ri def'orm 8LT36 30 -50

.O6/1

0.0.7. 8 X' 8LT3'?I 30. -20 0.11 4-5 X 2LT 16 30.. -60 0.12 0', 12: B X' l'1LT2O 70 -40 . 08,1. 18/ 29 0. 16. 4-5 X. X i 1LT26 70 -.60

.09/V?

4-5 X 2LT22 30 -50 .06/.09/.2 0.21... 4-5 X i 1LT8 70 ' -50' .08/.24 0.25 >9. X 8LT44 30 . -50 0. 11 0 . 30 .8 X 1LT3 30 -:5Q _.05/.10 _0.35 _>9

8LT37I1. 30

-20: 0.11 1.61 4-5 X 70 30 -60 -50 .09/.1.7 .06/. 09/ .2 0.1? 0.21 4.-5 .4-5 X,

-

X 70 -40 .08/.18/.2.9 0.16 4-5 X X 30' -2.

011

0.11' 4-5 - X 30 _-20

a .11

_1.61

_4-5 - X 30- -60 O1-2 30 -50 .06/. 1 0.12. 0.0.7 8 -8 X, X. 30 -50 O. t i 0.30 8 X 70 -50 .08/.24 0.25 >9

30 -50

05/. 10

0.38 >9

TABLE 3 SPECIMEN DATA AND MICROSTRUCTURE. RESULTS

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fig. la

location of preparation

fig. lb

final fracture point of initiation fatigue flaw I LT 3 50x i LT 3 10 Ox

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secundar.y cracks

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fatigue ftaw f 1g. 2 h secundary cracks fina t fracture

fig. 2a

Location of preparation 2 LT 15 200 2 LT 15 50

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il.

_{f 1g. 3}

ocation of preparation

fig. 3b

finaL fracture point of initiation

fatigue flaw

2 LT22 200x

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8LT371 lOOx

fig. 4a

tocation of preparation B LT 37 II location of preparation 8LT37 I

fig. 4b

fatigue fLaw

point of Initiation

final fracture

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fig

. 4b fatigue f law

of inttiatlon

final fracture point of initiation 8 LT 37 II 200

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fIg. Sa

fig. 5b

final fracture secundary. crack

point of initiation

fatigue fLaw 8 LT 35 200

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200x

fig. 6o

fig. 6b

secundary crack final fracture point of initiation fatigue flaw

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fig.

1ocation of preparation

fig. 7b

final fracture point of Initiation fatigue fLaw 11118 200

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11 LT 20

fig. Ba

location of preparation 8b final fracture point of initiation secundary cracks fatigue flaw 11 LT 20 200x

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L sS'. 11 LT 26

t

200 x

fig.

Location of preparation

fig. 9b

secundairy cracks

final fracture

point of inifion fatigue flaw