toughnessof steel weldedjoint - j-stage

ISIJ International, Vol. 30 (1990), No. 5, pp. 390-396

The Effects of Grain

ment

Boundary PhosphorusSegregation

on Toughnessof 9•/• Ni Steel and

and Heat Treat-

Its WeldedJoint

OsamuFURUKIM11)and YOShiyuki SAIT02)

1) High Technology Research Laboratories, Technical Research Division, KawasakiSteel Corporation. Kawasaki-cho, Chiba, Chiba-ken,

260Japan. 2) Iron and Steel ResearchLaboratories, Technical ResearchDivision, KawasakiSteel Corporation, Kawasaki-cho, Chiba,

Chiba-ken, 260 Japan.

(Received on July 6, 1989; accepted in the final form on January 26. 1990)

Effects of phosphorus (P) and heat treatment on toughness have been investigated on a quenchedand tempered

9 '/• Ni steel base metal and its welded joint simulated by a thermal cycle simulator. The mechanismof toughnessimprovementby the diminution of Pcomponentwasexaminedby mechanical tests, microstructure observation andcomputer simulation of the grain boundary segregation. The critical CTODvalue and Charpy absorbed energy in the

base meta[ showedremarkable deterioration as increasing Pcontent, whenthe cooling rate after tempering was lower

than I x 10-2 K/s. Simulated bondsshowedreduction of toughness whenthe PIevel washigher than 0.008 •/• and/orthecooling rate after post weld heat treatment (PWHT)was lower than I x 10-2 K/s. TheCharpyabsorbed energy of basemetal washigher than that of PWHTwelded joint whenthey showedthe samecritical CTODvalue. Intergranular ductile

fracture wasoccurred in the base metal, while intergranular brittle fracture in PWHTwelded joint wasobserved.

The effect of cooling rate following the tempering on the grain boundary segregation of Pwasestimated by computersimulation, basedon Guttmann-McLeantheory. Thecomputedamountof Psegregation in grain boundarywasclosely

related to the toughness both for abasemetal and asimulated bond. It wasalso suggested thatwhen the grainboundarysegregation of Pwas lower than 0.1

,high toughness level wasobtained in the base metal and the simulated bond. The

ditference of toughness betweenthe base metal and the simulated bondwasattributed to the coarsening of grain size bythe welding thermal cycle. The9•/• Ni steel with PIevel lowerthan 0.005 "/• showsexcellenttoughness in the basemetal

and the welded joint.

KEYWORDS:9 '/• Ni steel; LNGstorage tank; fracture toughness; phosphorus; welded joint; computer simulation.

1. Introduction

With recent advances in steelmaking technology,impurities, such as Pand Sin steel have been re-duced to extremely low level.1) In 9o/o Ni steel for

large LNGstorage tanks, very high toughness level is

required to achieve very severe safety criterion. It is

knownthat Sreduces ductility of 9 o/o Ni steel, how-ever brittle fracture property is not severely dependenton SIevel.2) While the diminution of Pis one of theprominent ways to improve toughness in 9o/o Ni steel.

The production of 9 o/o Ni steels with low P Ievel

has been realized in the manufacturing process.3)

However, the mechanismof toughness improvementby the diminution of Phas not beencleared. For the

purpose of determinating the optimumPIevel to im-

prove the toughness of 9 o/o Ni steel, the effect of

Psegregation must be investigated. Oneof the ob-vious detrimental effects of Pon toughness is temperembrittlement, as shownin an extensive study in CrMosteel.4) The interaction between Ni and P has

an important role on properties of 9o/o Ni steel, butits mechanismhas not been clear.

Thepurpose of the present study is to examine theeffect of Pon toughness in both a base metal and awelded joint of 9 o/o Ni steel. The mechanismof theembrittlement, related P, wasinvestigated by scanningelectron microscope and Auger electron spectroscope.

Computer simulation was carried out to study theeffect of PIevel and thermal cycle on the grain bound-ary segregation of P.

2. Experimental Procedure

2. I .Materials

9o/o Ni steels with P Ievel ranging from 0.001 to

0.013 o/o were examined. Chemical composition ofthe steels is shownin Table I .

Thesematerials wereprepared by vacuum melting and cast into the

moulds with 150mmthickness. Then they were hotrolled to slabs of 110 mmthickness. The slabs wereaustenized at 1523 K for 60 min, rolled to a final

thickness of 20 mmat temperature range from 1373to 1223 K. Total reduction by 5 passes was 83 o/o'

The plates were heated at I 053K for 70 min, waterquenched and then tempered at 843K for 80 min.In order to examine the effect of cooling rate after

tempering on toughness, specimens were cooled bythree kinds of cooling rate ; water cooling (40 K/s), air

cooling (5x l0-1 K/s) and furnace cooling (lxl0-2K/s). Fig. I schematically shows the manufacturingprocess.

In order to simulate the bond of shielded metal

arc welding (SMAW)with heat input of 4.6xl03J/mmfor 16 mmplate, thermal cycle was applied to

the air cooled specimens by ainduction heating and

390 C1990 ISIJ

ISIJ International, Vol. 30 (1990), No. 5

Table I .Chemical composition of 9o/o Ni steels tested.

(wto/o)

Steel C Si Mn P S Ni

234

o.

o.

o.

o.

053 O.26

052 O.24

056 O.26

061 O.26

O.62 O.

OOl

O.67 O.

005

O.62 O.

0080.62 0.013

O,O009 9.

32

O,O009 9. 13

O,O009 9, 12

O,O008 9,

14

1523K, 60min

137SK(Thick.= 11Omm)

1223K(Thick.=20mm)

Fig.

2000

(5 passes)1053K, 70mirt

843K, 80min

1500~~

~:.

c!$ IOOO~(D,~E(D

h 500

Fig. 2.

o

40K/s5x lo ~1 Kls

1x l0-2 Klsl. Manufacturing process of 9o/o Ni steel.

Peak temperature : 1623K

If--30s--~'

O 20 40 60 1OO80

Time (s)

Thermal cycle simulating shielded metal arc weld-ing.

cooling device. Theemployedthermal cycle is shownin Fig. 2.

The specimens after the thermal cycle treatmentwere heated at 833 K for 120 min, and then cooledwith cooling rates of Ix l0-2 and 6x l0-2 K/s in orderto investigate the effect of post weld heat treatment(PWHT)on toughness in 9o/o Ni steel. It should benoted that the minimumcooling rate used for normalPWHTconditions corresponds to 6x 10-2 K/s.

2. 2. Experimental Method

The effect of P on toughness was examined byCharpy impact test and crack opening displacement(CTOD)test in accordance with British Standard BS5762.5) 10 mmthickness specimens with transversecrack propagation direction were machinedfrom theplate, in which both upper and bottom surfaces of theplate were eliminated.

Fractured surface was examinedby scanning elec-

tron microscope (SEM)with an accelerating voltageof 100 kV. Grain boundary segregation of P was

examinedby scanning micro Auger electron spectro-scope (AES) with beamdiameter of 500A. Thechange in P with sputtering time was also investi-gated.

2.3. Computer Silnulation of Grain Boundary Segregation

qf PAccurate quantitative evaluation of the grain

boundary segregation in 9 o/o Ni steel is very diflicult

because of the impossibility of brittle fracture of speci-

mensinside micro Auger equipment at low tempera-ture such as 108 K. So, grain boundary segregationsof the base metal and the simulated bond were esti-

mated by computer simulation using a model basedon Guttmann-McLeantheory.6)

Equilibrium concentration of Pon grain boundary,Xp(oQ), is given by the following equation :

Xp(oo) -X~exp (AG~IRT)

...(1 )~ l+X~[exp(AG~/RT)-l] ' """

where, R: gas constantT: temperature

X~: bulk atom fraction of Pand AG~is segregation energy of Pdescribed as fol-

lows7] :AG~= 10.5+(~NIP'XNi (kcal/mol), .........(2)

where, aNiP : interaction parameter between P andNi

XNi : bulk atom fraction of Ni.

The value of (rNiP in the present simulation was 6.2kcal/mol.7)

The time, required for the grain boundary to reachits equilibrium concentration, depends on diffusion

rate of segregation element through alloys. A solu-tion of the diffusion equation corresponds to segrega-tion to a planer interface for dilute alloys takes form8) :

Xp(t)-Xp(O) 4Dt 4Dt 1/2

=[( J,

Xp(co)-Xr(O) 1-exp a2d2 (r2d2erfc

.(3)

where, D: diffusivity of Pthrough steel

d: the thickness of the segregated interfaceregion (assumedto be one monolayer)

Xp(t), Xp(O): grain boundary atom fractionsof Pat times t and O, respectively

a : the ratio of equilibrium grain boundaryatom fraction of P, Xp(eo), to bulk atomfraction, X~

The diffusivity D is given by9) :

D= 1.58x 104.exp (-52 3001RT) (m2/s)

.(4)

With use of Eq. (3), P concentration on grainboundary during heat treatment is simulated.

3. Results

3.1. Effects of PLevel and Heat Treatment on Toughnessof 9o/o Ni Steel

Fig. 3 shows the effect of PJon ,C:harpy absorbed

391


250__o-- QT(40Kls)

-~~- QT(5xlO-~K/s)

~ -'H:h.- QT(1xl0-2K/s):(~ 200~ Ar ~r ~A\~; \:~ \~150 D\\D \\

(L, \\.

17 h\~~100\,

JD \o \:hcL*c'

50 D\~x: \.u '\.D

oO 0.005 aolO 0.01 5 0020

Pcontent ('/.)

Fig. 3. Effect of Pcontent on the Charpy absorbed energyat 77 K in quenchedand tempered 9o/o Ni steel,

where cooling rates after tempering were 40, 5X10-1 and I > l0-2 K/s.

energy at 77 Kin the quenchedand tempered9 o/o Nisteel. The effect of P Ievel on absorbed energy is

strongly related to the cooling rate after tempering.Whenthe cooling rate is high, the test material con-taining 0.013 o/o Pshowsgood impact properties. Inthe cooling rate of Ix l0-2 K/s, the Charpyabsorbedenergy of specimenswith PIevel higher than 0.008 o/o

decreases markedly.The CTODtest result is shownfor cooling rate of

1x l0-2 K/s in Fig. 4. The critical CTODvalue, ~.,

decreases with increase of PIevel. This result is con-sistent in that of Charpy test.

Fig. 5 shows the relation between P content andCharpy absorbed energy of specimens after the ther-

mal cycle simulating SMAWand PWHT.The Plevel dependenceon the absorbed energy at 108 K is

very small in the materials of as received condition.However, in PWHTthe energy decreases with the in-

crease of P. The effect of cooling rate on toughnessafter PWHTis also shown. The deterioration ofabsorbed energy by PWHTis observed in steel even if

Pcontent is 0.001 o/o for the cooling rate after PWHTis Ix l0-2 K/s. However, whenthe cooling rate after

PWHTis 6x 10-z K/s, the decrease of absorbed en-ergy is small below 0.005 o/o of Pcontent and service

temperature of 108 K is shownto be within the uppershelf energy region. Consequently, the welded jointof 9o/o Ni steel with ultra low P content shows ex-cellent toughness both in as welded condition andPWHTwith the cooling rate of 6x l0-2 K/s.

Relations betweenac and PIevel in specimensof aswelded condition and PWHTwith cooling rate of

l x l0=2 K/s are shownin Fig. 6. The effect of Pon~c of as welded condition is small, which is consistentin the Charpy impact test result. The decrease of~c in specimen after PWHTis remarkable except for

ultra low P Ievel less than 0.005 o/o' WhenP Ievelis as low as 0.005 o/o the decrease of ~c is not observed

even if the cooling rate is as slow as Ix l0-2 K/s.

Fig. 4

Fig. 5.

Fig. 6.

EEoo

c,

oo

QOuo

u

1. OO

o. 50

o. Io

0,05

0.0 1

QT(1 x 10-2 Kls)

O 0.005 aOIO ao15 0020P content ('/.)

Effect of' Pcontent on the critical CTODvalue atl08 K in quenched and tempered 9o/o Ni steel;

the cooling rate after tempering was I x l0-2 K/s.

250

:(co 200oIS

>e, 150*e,c:G,

~,un* 100ou'

J:)

o:hCL* 50oJ:u

O

-o--l~--•H~-

D'~*

\

Simulated bond

As received

PWHT(6xl0~2K/s)PWHT(1XI0-2K/s)

~~\

~O

\\\\~\

\ '\D\ '\A

'~D

o a005 OOIO OO15 OO20Pcontent ('/. )

Effect of Pcontent on the Charpy absorbed energyat 108K in simulated bondsof 9 '/* Ni steel.

~E_ 0.50

:(cooouo

0.10

ooHu8u

O O005 aolO OO15 OO20P content('/.)

Effect of Pcontent on the critical CTODvalue at

108 K in simulated bonds of 9o/o Ni steel.

3.2. Fractured Surface of Base Metal and Simulated Bondof 9o/o Ni Steel

Fig. 7showsscanning electron micrographs of frac-

tured surfaces of the base metal and the simulatedbond by Charpy tests. In the simulated bond after

PWHTwith the cooling rate of Ix 10-2 K/s, inter-

granular brittle fractured surface area increases withincreasing PIevel. In the base metal, however inter-

granular ductile fractured suti~;ce area increases with

392


(a), (d): P=0.001 o/o; (b) (e): P=0.008 olo

(c), (f): P=0.013 o/o

Fig. 7. Scanning electron micrographs offractured Charpyspecimens of quenched and tempered steels ((a)-(c)) and simulated bonds which were subjected to

PWHT((d)-(f)), where cooling rate was I X l0-2

K/s for tempering and PWHT.

increasing P Ievel. Similar results were obtained infractured surfaces of CTODtest specimens.

Fig. 8 shows a scanning electron micrographof fractured surface of the quenched and tempered0.0130/0P-90/0Ni steel after tempering with the coolingrate of I x 10-2 K/s. Identification of precipitates onthe surface wasperformed by X-ray energy dispersiveanalyser in SEM. This indicates that the precipitateis a kind of phosphide.

3. 3. Grain BoundaryPSegregation

Fig. 9shows surface condensations of P, C) and Oon intergranular fractured surface of the simulatedbond of 0.013 O/o PIevel subjected to PWHT.Thesevalues were calculated by the peak rate of those to Fe(703 eV) revealed by micro Auger analysis. It wasimpossible to fracture the specimen inside the microAuger equipment at low temperature. Therefore,specimenswere fractured outside the equipment priorto analysis at 108 K. Becauseof possible contamina-tion prior to the analysis, accurate quantitative analysisof segregation was difficult in these tests. However,the surface condensations ofp, C, and Oshowedrapiddecrease with sputter time as shownin Fig. 9. There-fore, intergranular segregation of P was clearly ob-

Fig.

Energy8. Scanning electron micrograph of Charpy specimen

of 0.0130/0P90/0Ni steel and X-ray dispersive analy-sis of precipitates.

a,LL

O

G,U!

O*

e)tL

O

~5LL~

~:

a)LL

O

a)L~

O

0.6

0.5

0.4

0.3

0.2

o. 1

o

(a)p=0.0 13%CR=6x10-2 Kls

~~

*QA.'Q\:~~\

~~\.D'A_~_A:~;*

o

--0=P--A- - C

- O-D .

D~~=~:; :=

10 20Sputter time (min)

Sputter time (min)

Fig. 9. Surface condensations of P, Cand O revealed bymicro Auger analysis in simulated bonds of 9o/o Nisteel subjected to PWHT.

served, and it wasmoreobvious on lower cooling rate.Theeffect of Pcomposition on grain boundary seg-

regation of Pwas calculated by Eq. (3). The resultis shownin Fig. lO. This figure~als~shows the effects

393


:hCa1~r=O::

=,a

O,C:o

C:

OC~

C:(D

O::o

OQ*

0.3

0.2

o. 1

o

o

o

*~\~\~_

Calculated Actual (AES)

o 0.0 13~P0.008~,P0.0059ep0.001~,P

Fig.

>Ca1~

O!OC:

CO

O,

=OC:

OCa

C:

OO=0

O,L

10-2 1O210110-1

Cooling rate (Kls)

lO. Effect of cooling rate on grain boundary segrega-tion of Pestimated by computer simulation.

0.3

0.2

o. 1

lxl0-2 K/s 6xl0~2K/s

-o- -•- Base metal : QT

-~- -A- Simulated bond + PWHT

oo

llllll// ,il

d;

// /ll ~///

////IK /

ll//

// /JL

//////

Fig. Il.

0.005 0.0 1O 0.0 15Pcontent (9,,)

Comparisonof computedgrain boundarysegrega-tion of P in simulated bond with that in basemetal.

of the tempering cooling rate. Experimental data of

the Psegregation on grain boundary in materials with0.013 o/o P, were also plotted in the flgure.

The relation between P concentration on grain

boundary and Pcontent is shown for the simulatedbond after PWHTand the base metal in Fig. 11.

The difference between themwassmall, but, in boththe materials, Psegregation was relatively smaller in

6x l0-2 K/s than in I x 10-a K/s.

4. Discussion

4.1. Mechanismof Embrittlement Causedby PSegregation

In the case of slow cooling rate, Charpy absorbed

energy and ~* deteriorate with increase of P in boththe 9 o/o Ni steel base metal after the quenchedandtempered treatment and the simulated bond after

PWHT.This phenomenonknownas temper embrit-tlement is often observed in Cr-Mosteel.4) It is con-sidered that temper embrittlement results from inter-

granular fracture due to grain boundary segregationof P. Heat treatment conditions were evaluated bytempering parameter Tpro) as shownin Eq. (5).

T (20+10g t)• T• 103... . .. .. .

...(5)

where, t: holding time (h)

T: temperature (K).The calculated Tp are 16.97x 106 and 16.91 x 106 for

tempering at 843 K for 80 min and for PWHTat833 K for 120 min, respectively. No significant dif-

ference was obtained. Consequently, heat treatmentconditions for tempering of base metal and PWHTofsimulated bond are considered to be equal.

Fig. 12 shows relation between the ~c and theCharpyabsorbedenergy at 108Kin the quenchedandtempered9o/o Ni steels and the simulated bonds. Thelatter materials are subjected to PWHT.The rela-

tion between~. and absorbed energy in the simulatedbond is different from that in the base metal. Theabsorbed energy in the base metal is higher than thatin the PWHTsimulated bond, whenthey show the

same ~, value. As shown in Fig. 7, intergranularductile fracture wasobserved in the base metal, whileintergranular brittle fracture wasobserved in the simu-lated bond after PWHT.Therefore, the differenceof ~c~Charpyabsorbed energy relation is considered toresult from the difference offracture type.

Fig. 13 shol~'s the effect of Pon ~.. The effect of

PIevel on ~* is similar in both the base metal and the

PWHTsimulated bond. The crack initiation prop-erties of brittle fracture is still considerably influ-

enced by PIevel.

Especially, in the case of slow cooling rate after thetempering of base metal or PWHTof simulated bondof9 o/o Ni steel, Pdecreases the crack initiation energyin intergranular fracture. In the case of the basemetal with relatively fine grain size, intergranularductile fracture occurs in crack propagation stage.

Onthe other hand, in the simulated bond, intergran-ular brittle fracture occurs in coarse grain structure.Therefore, the difference of fracture toughness be-

tween both materials are due to the difference offracture process causedby PIevel and grain size effect.

4.2. Relation betzg'een Grain Boundary Segregation andToughness

The Charpy absorbed energy decreases linearly

with increasing grain boundary segregation. Rela-tion between the computedPconcentration on grainboundary and the Charpy absorbed energy is shownfor the base metal after tempering at 77 K in Fig. 14.

This figure suggests that the deterioration of impactproperties in the base metal is attributed to the grain

boundary segregation of P. This conclusion is also

supported by the result of ac and computedP seg-regation on grain boundary.

Fig. 15 shows the effect of Pgrain boundary seg-regation computedby Eq. (3) on ~.. The ~c simplydecreases with decreasing P concentration of grain

boundary in both the base metal and the simulatedbond. Howeverthe difference of ~* between them is

not due to the grain boundary segregation of P, be-

cause the computed grain boundary P segregationafter the PWHTsimulated bond is nearly equal to

that of the base metal. Th~rl'efore, the deterioration

394

ISIJ rnternational, Vol. 30 (1990), No. 5

1.00

EE

0.50

:(coooto 0.10

oOH 0.05

oou

u0.0 1

-O- Basemetal : QT(1xl0~2K/s)

-~~--Simulated bond + pWHT(1xl0-2 Kls)

f (67)

/0.001'/.

l~ (75)/ 0.00 '/.

/(81)

(65)0.013///~j0.00

8V.A: (go)

OO13•!.

(20)0.008•/.

(5)0.00 1'/.

(10)

0.005 V.

( )Crystallinity

%Pcontent

O 50 250100 150 200Charpy absorbed energy at 108K(J)

Relation between the critical CTODvalue andthe Charpyabsorbed energy at I08 Kin quenchedand tempered 9o/o Ni steels and simulated bondswhich were subjected to PWHT.

1~:

Nhce

>o,(D

o1:,

oJ:~

oJDCQ

;~CL

caJ~:

O

300

200

1oO

Fig. 12.

WCACFCo o e

AAAD 91 IQ ~ C

P=0.013%P=0.008%P=0,005%P=0.00 i %

c DA1' CI A

,o

l

Fig.

o

A

e

Fig.

~E_ 0.50:(cooa

(JQ 0.1 Ocro~u8u

0.005 ao10 OO15 a020Pcontent ('/.)

13. Effect of Pcontent on the critical CTODvalue of

base metals and simulated bonds of quenchedandtempered9o/o Ni steel.

of thoughness in the simulated bond is attributed tothe coarsening of grain size by welding,

4.3. Application of the ComputerSimulation for Conirolling

Pand Heat Treatment

As shownin previous sections, the effect of Pcom-position and thermal cycle on concentration of Pongrain boundarycan be estimated by the computersim-ulation. Charpy absorbed energy and ~, of the baseplate were closely related to computedPconcentration

on the grain boundary. If the P concentration ongrain boundary is less than O. l, high toughness level in

9o/o Ni steel is obtained as shownin Figs. 14 and 15.

With use of the computer simulation, the PIevel andheat treatment condition to satisfy the above condi-tion can be determined. In the case of the P Ievel

lower than 0.005 o/o' the P grain boundary concen-tration does not exceed 0.1 for the cooling rate of aslow as 2x 10-2 K/s, which is lower than the minimumcooling rate of 6x 10-2 K/s used for normal PWHTcondition.

In the present simulation, the thermal cycle is de-vided into infinitesimal time steps and the finite_ dif-

ference equation derived from Guttmann-McLeanequation (3) is utilized. By which, P concentration

~~~e

CO

O

~D

OOHO

O

OO O. 1 0.2 0.3

ComputedP concentration on grain boundary14. Relation between computedgrain boundary seg-

regation of P and Charpy absorbed energy in

quenchedand tempered9o/o Ni steel.

1,oo

0.50

o. Io

0,05

0,0 1

~)- QT(1xi0~2K/s)

-~- Simulated bond + pWHT(1xl0~2K/s)

A- -

\A\

+1~

'~ " "A

o

Fig. 15.

O. I 0.30.2

ComputedP concentration on grain boundaryEffect of computedgrain boundary segregation on5c of base metal and simulated bond.

was computed at each steps for the purpose of ap-plying to complex heat cycles. If the interaction pa-rameter between impurity and metal and diffusivity

of impurity in steel are properly determined, the pres-ent simulation method can be applicable to variousalloy steels.

Theaccurate quantitative analysis of concentrationof impurities such as Pon grain boundary is some-times difficult, because breaking the specimen inside

the AESis not easy. In such cases, the computersimulation becomesa useful tool to understand theacceptable impurity level and the required heat treat-

ment to provide a good toughness value.

5. Conclusion

Effects of Pon toughness were examinedin the basemetal and the welded joint simulated by thermal cy-cle of quenchedand tempered 9 o/o Ni steel for LNGstorage tanks. Mechanismof to~u~hrress improvement

395


by the diminution of Pin steel wasalso investigated.(1) Critical CTODvalues and Charpy absorbed

energy deteriorate accompanyingintergranular duc-tile fracture in base metal with P Ievel higher thanO008 o/o' whena cooling rate after tempering is aslow as I x 10-z K/s.

(2) Critical CTODvalues and Charpy absorbedenergy deteriorate by intergranular brittle fracture insimulated bonds subjected by post weld heat treat-

ment (PWHT)with the cooling rate of I x l0-2 K/s,whenPIevel exceeds 0.008 o/o'

(3) Although similar CTODvalues were obtainedin both materials, the Charpyabsorbed energy of thebase metal is higher than that of PWHTsimulatedbond. Intergranular ductile fracture was observedin a base metal, while a PWHTsimulated bondshowedintergranular brittle fracture. The transitionof ductile to brittle fracture modeis attributed to thecoarsening of grain size by welding thermal cycle.

(4) Crack initiation properties of 9o/o Ni of basemetal and welded joint are influenced by P Ievel.

The critical CTODvalue decreases with the increaseof PIevel.

(5) By using computer simulation based on Gutt-mann-McLeantheory, the effect of cooling rate after

tempering of steel on the grain boundary segregationof P in the base metal was estimated for the basemetal. Thecomputedgrain boundary segregation of

P is closely related to toughness of the base metal.The Charpy absorbed energy of the base metal de-

creases with the computedgrain boundary segrega-tion.

(6) The toughness of PWHTsimulated bond is

also dependent on the computedgrain boundary seg-

regation of P. The difference of toughness betweenthe base metal and the simulated bond is attributed tothe coarsening of grain size by the welding thermalcycle.

(7) Computersimulation su~gests that whenthe

gramboundary segregatlon of Prs less than 0.1 hightoughness level is obtained in both the base metal andthe welded joint. WhenP is lower than 0.005 olo'

high toughness is expected at the cooling rate as lowas 2x 10-2 K/s.

1)

2)

3)

4)

5)

6)

7)

8)

9)

lO)

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D. McLean: Grain Boundarics in Metals, ClarendonPress, Oxford, (1957), I15.

P. L. Gruzin and V. V. Mural: Fiz. Metal. Metalloved, 17(1964), 62.

J. H. Hollomon and L. D. Jaffe: Trans. Am. Inst. MinMettal. Pet. Eng., 162 (1945), 223.

"~ *1"

396

toughnessof steel weldedjoint - j-stage

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