Effect of Carbon on Microstructure oc C-Mn Steel

8/19/2019 Effect of Carbon on Microstructure oc C-Mn Steel

1/8

The Effect of Carbon on the

Microstructure and Properties of C-Mn

Al l -Weld Metal Deposi ts

Carbon promotes acicular ferrite, at the expe nse of grain

boundary polygonal ferrite, and causes grain refinement of

the reheated regions

BY G. M. EVANS

SYNOPSIS. The effect of

0.05

to 0.15%

carbon on the microstructure and prop

ert ies of shielded metal arc welds con

taining 0.6 to 1.8% M n has bee n investi

ga ted .

It was found that carbon pro

moted acicular ferri te, at the expense of

grain boundary polygonal ferri te, and

caused grain refinement of the reheated

regions. The hardness of the deposits

increased,

and the tensi le properties

were defined by equations of the form:

a = a + b (C) + c (Mn) 4- d (C • Mn).

With regard to impact properties, i t

was fou nd tha t carbon t i l ted the Charpy-

V curves and substantia l ly reduced the

degree o f sca t te r ing . Opt imum tough

ness was achieved at a manganese level

of 1.4% when the carbon content was in

the intermediate range, i.e., 0.07 to

0.09%.

In t roduc t ion

Previous wo rk (Ref. 1), con duc ted as

part o f a jo in t p rogram wi th in Sub-Com

mission HA of the Inter national In stitute of

We lding, established, for low carb on

deposits, that manganese increasingly

refines weld metal microstructures and

gives rise to optimum impact properties

at a conc entrat ion of ab out 1.5%.

The present wo rk is a continu ation of

the p rog ram . Its main aim is to ascertain

whether the opt imum wi th regard to

manganese is displaced, depending on

the carbon level of the deposit.

Paper selected as an alternate for the 64th

AWS Annual Convention, Philadelphia, Penn

sylvania,

April

24-29,

1983.

G. M. EVANS is with Welding Industries Oerli

ko n Buehrle Ltd., Zurich,

Switzerland.

Experimental Procedure

Electrodes

Low hydrogen, i ron powder type e lec

t r o d e s - co d e d A , B, C a nd D-were

prepared as in previous work (Ref. 1).

The manganese content of the coverings

was varied to yie ld deposited metals

conta ining 0.6, 1.0, 1.4 and 1.8% M n ,

respectively.

At each of these manganese levels

different amounts of graphite were add

ed to the coatings to produce four nom

inal levels of carbon in the deposited

m e t a l s - n a m e l y ,

0.045, 0.065, 0.095 and

0.145% C. The core wire diameter of the

16 batches of experimental e lectrodes

thus prepared was 4 mm (0.16 in.), and

the coating factor (D/d) was 1.68.

Weld Preparation

The jo int geometry was that specif ied

in ISO 2560. Wel ding was do ne in the f lat

posit ion, and three weld beads per layer

we re d eposited (Ref. 1). The total num

ber of runs required to f i l l the individual

jo ints was 27. Direct c urrent (e lectrode

posi t ive) was employed, the amperage

being 170 A, the voltage 21 V, and the

heat- input was nomina lly 1 k j / m m (25

kj/ in.). The interpass temperature was

standardized at 200°C (392°F).

Mechanical Testing

Two subsize weld metal tensi le speci

mens were machined and tested for each

of the different deposits. Also, approxi

mately 35 Charpy-V notch specimens

were struck to obtain a fu l l transit ion

curve. The impact specimens were in the

as-we lded cond i t ion . On the o ther hand,

the tensi le specimens underwent hydro

gen removal treatment at 250°C (482°F)

for 14 hours (h).

Results

Chemical Composition

The chemical analyses of the weld

metal deposits are given in Table 1. The

composit ions were essentia l ly on target,

the nominal values for carbon being

0.045, 0.065, 0.095 and 0.145% at each

of the four manganese levels previously

(Ref. 1) designated as A, B, C and D. The

sil icon contents were relatively well bal

anced, the increase with increasing car

bo n being sl ight. Of note is the fact that

both su l fu r and phosphorus were low

throughout .

Metallographic Examination

General. Transverse sections were

prepared, and detai led examination was

carried out on the top weld beads and on

the adjacent super cri t ical ly heat-affected

zones as described previously (Ref. 1).

To i l lustrate the changes due to car

b o n ,

as obse rved in the l ight m icrosc ope,

typical micrographs for the extremes are

shown in Figs. 1-4 for the 1.4% Mn

level.

As-Deposi ted W eld Me ta l . The top

weld bead of each of the test weldments

was examined at the Welding Insti tute,

U.K., and the microstructural compo

nents were quantif ied according to the

scheme proposed by Abson and Do lby

(Ref. 2) and by Pargeter (Ref. 3).

Point counting was carried out at

X500, and the consti tuents were identi

f ied as fo l lows:

• Grain boun dary ferri te.

• Polygonal ferri te.

• Ferrite wi th aligned M-A-C.

• Acicular ferr ite.

WE LDIN G RESEARCH SUPPLEMENT

1

313-s


2/8

Table 1—Weld

Average

con ten t ,

0.045

Metal Chemical Compositions and Tensile Properties

designation

A

B

C

D

C

0.045

0.044

0.044

0.045

M n

0.65

0.98

1.32

1.72

Compos i t i on ,

Si

0.30

0.32

0.32

0.30

%

S

0.006

0.006

0.006

0.006

P

0.008

0.008

0.007

0.008

N/mm

2

*

406

432

451

488

Tensile properti

OR,

N / m m *

462

481

512

549

es

(a)

EL,

%

35.4

35.8

32.0

29.6

R.A., %

78.8

78.8

78.8

76.0

0.065

0.095

A

B

C

D

A

B

C

D

0.059

0.063

0.066

0.070

0.099

0.098

0.096

0.093

0.60

1.00

1.35

1.77

0.65

1.05

1.29

1.65

0.33

0.35

0.37

0.33

0.35

0.32

0.30

0.33

0.007

0.006

0.005

0.006

0.008

0.007

0.007

0.007

0,008

0.008

0.007

0.008

0.009

0.009

0.009

0.007

407

451

469

511

433

477

506

535

483

516

545

588

512

546

576

602

31.2

32.4

29.2

28.4

31.8

30.0

30.8

27.8

80.6

80.6

78.8

77.9

78.8

78.8

77.9

74.0

0.145

A

B

C

D

0.147

0.152

0.148

0.141

0 63

1 00

1 40

1 76

0 40

0 41

0 38

0 36

0 008

0 007

0 007

0 006

0 007

0 007

0 007

0 007

480

517

536

606

569

605

636

691

32 8

27 4

27 4

25 6

76 0

75 0

75 7

71 9

(a) o

E

= yield stress; •

-•

.... v A y ' - X ' ' ••


3/8

the prior austenite grain boundaries

decreased and thus enhanced the etch ing

response. The grain interiors tran sform ed

to a fine acicular structure, and the

changes were essentially a reflection of

those occurring in the as-deposited weld

metal.

Compar ison o f the two photomicro

graphs in Fig. 4 shows that grain refine

ment also occurred in the f ine grained

reheated regions. The results of linear

intercepts made at X630 are plotted in

Fig. 7. The effects of carbon and manga

nese were found to be approximately

equivalent over the experimental ranges

investigated. The microstructure became

more duplex with increasing carbon as

shown in Fig. 4, and the second phases

tended to separate, to an increasingly

greater extent, a long the primary segre

gation bands. The form and structure of

the second phase particles were revealed

by deep etching in a mixture of bromine

an d

me thano l ,

fo l lowed by examinat ion

in the SEM at X5000 magnif ication. The

phases were identified, by the British

Steel Corporation, as:

• Cemen tite f i lms.

: ® ^ A

&

Fig. 4 - Photomicrographs of fine grained

regions, 1.4 Mn. A-0.045 C; B-0.145

C. X630 (reduced 38 on reproduction)

1 0 0

9 0

° 8 0

l -

ITO

o

a

6 0

^ 5 0

<

cc

3

1-40

CJ

3

cn

H-30

IS

O

r

O20

10

0

1 0 0

^liiiiiiiiiiiliiiiiiiM

W M V A A M

• •: : : : : : : : ; ; : . : : ; : : : : : : : : : : : : : : : : : : : : : : : : : : :

-

i M M W i M m ^

0 0 5

0-10

C A R B O N IN WELD ,«

0 1 5

0

10

2 0

3 0

4 0

50

6 0

70

8 0

9 0

100

9 0 -

8 0 -

£ 7 0

O

a

§ 6 0

o

- i 5 0

<

cc

3

h-40

O

K

S

3 0

o

cc

O20

1 0 -

0 0 5

0 4 0

C A R B O N IN WELD , .

0 1 5

0

10

2 0

3 0

4 0

5 0

6 0

70

8 0

9 0

100

0 0 5

0-15

Fig. 5-

metals

0-10

CARBON IN WELD

• Effect of carbon on the microstructure of weld metals obtained using electrodes A ,

B,

C, and D with Mn in coverings varied to yield deposit

containing

Mn as follows: electrode A-0.6 Mn, electrode B- 1.0 Mn, electrode

C-1.4

Mn and electrode D- 1.8 Mn

WELDING RESEARCH SUPPLEMENT 1315-s


4/8

• Marten site / austenite (M / A).

• Bainite / Fine pea rlite (B / P).

The vo lume f ract ions were found to

increase with increasing carbon content

as shown in Figs. 8 and 9 for the 0.6 and

1.4% M n levels, respe ctively.

Hardness Testing

Average hardness values, obtained for

the last weld bead to be deposited in

each case, are plotted in Fig. 10. The

trends are essentially linear, the increase

over the range for manganese being 50

DP N (i.e., VHN) compared to approxi

mately 30 DPN (i.e., VHN) for the experi

mental range of carbon contents.

Hardness traverses along the center

l ine of deposits welded with electrode C,

at the two extremes of carbon, are plot

ted in Fig. 11 ; the difference of 30 DPN

(Le., VHN), as enc ounte red for the to p

beads, is reflected thr ough out most of

the we ldments.

Mechanical Properties

Tensile Results. The tensile test results

are prese nted in Table 1 . The yield

strengths and the ultimate tensile

E015

£

Q

| o - i o

or

O

cr

<

z

s

o

o

LU

o

<

cr

UJ

3

0 0 5

0

A - «

B - O

C - A

D-a

—

—

E

E

^ l ; ; - ^

B

-

-

FINE GRAINED REGION

j

i

0 0 5 0-10 0-15

CARBON IN WELD , .

Fig.

7 — Effect o f carbon on the grain intercept in the fine grained region

at different manganese levels as follows: A

—0.6 , B—

1.0 , C—

1.4

and

D-1.8

2 5

2 0

-15h

UJ

<

I

a.

§10

o

o

LU

co

5 -

I I

0-6%Mn

-

S^u

D yS

To ta l •

- A —

C e m . f i lm s A —

» ^ — -* ~~y

B/P

» ; ; '

i

_ — - A

__

_

_V

I

-

-

-

0 0 5

0-10

C A R B O N IN W E L D , %

0-15

Fig. 8 Effect of carbon

content

on percentage microphases in fine

grained regions, 0.6 Mn

2 5

2 0 -

1 5 -

LU

tfl

<

I

a

10

o

CJ

LU

tfl

5 -

I I

1-4°bMn

^ u

Total

n

.. . • —

_^-A

Cem.f i lms ^

A

-

I I

I

/

A

^

i

-

0 0 5 0-10

C A R B O N

IN

W E L D , %

0-15

Fig. 9

—Effect

of carbon co ntent on percentage microphases in fine

grained regions, 1.4 Mn (cementite films,

martensite/austenite,

bain-

ite/pearlite)

316-s |

N O V E M B E R 1 9 8 3


5/8

strengths are plot ted against carbon con

tent in Figs. 12 and 13, respe ctively. O n

assuming the tensile properties to be

linearly related to both carbon and

man

ganese, the fol lowing regressions were

obta ined:

1. For yield strength ( in N/mm

2

) :

o-

E

= 335 4- 439 C 4- 60

M n 4- 361 C • M n (1)

2. For ult imate tensile streng th (in N /

m m

2

):

O-R = 379 4- 754 C 4- 63

M n

4-

337 C • M n (2)

Impact Results. Charpy-V impact

curves, obtained from the average of the

scatter bands, are plotted in Fig. 14. It is

seen that the upper shelf was depressed

by the addit ion of carbon, whereas the

lower shelf tended to be raised.

On reconsider ing the absorbed energy

as a function of manganese (Fig. 15), the

opt imum composi t ion for the t rans i t ion

range was found to occur at about 1.4%

M n ,

independent ly of the carbon level of

the deposits.

The relat ive ef fects of carbon and

manganese on lateral shift are depicted in

Fig.

16. Here the Charpy-V temperatures

corresp onding to an arbit rary level of 100

) are plot ted against composit ion. At the

low manganese level (A), carbon was

found to be marginally benef icial, where

as at the high level (D) carbon was

deleter ious. For the intermediate manga

nese content (C), opt imum toughness

was achieved at an intermediate carbon

content of 0 .09%. Compar ison of the

tw o graphs in Fig. 16 show s, for the

specific ranges, that manganese had a far

greater influence on lateral shift than

carbon.

An addit ional feature to the t i l t ing of

the average Charpy-V curves was the

observed fact that the degree of scatter

ing decreased as the carbon content

increased.

The phenomenon is i l lustrated

in Fig. 17 for the two extremes of carbon

at the 1.4% Mn level. The situat ion for the

low carbon level is seen to be undesir

able,

the transition being extremely steep

and such that ful l bi-modal f racture

occur red at -40°C (-40°F).

Discussion

It is generally accepted (Refs. 4-6) that

the addit ion of carbon to low strength

ferrit ic weld metal causes the yield and

tensile strengths to increase and ductility

to decrease. Furthermore, the hardness

increases an d, at a constant g rain size, the

yield-to-tensile strength ratio decreases.

The role of dif ferent al loying elements is

known to be complex; carbon in isola

t ion ,

for example, behaves dif ferent ly

o

150

5 K g L o a d .

TO P B E A D

0 0 5

0-10 0-15

C A R B O N IN W E L D . \

.

Fig. 10—Effect of carbon on hardness of

as-deposited weld metal at different mang a

nese levels as follows: A—0 .6 , B—1.0 ,

C-1.4 andD-

1.8

0 5 10 15

D I S T A N C E F R O M T O P S U R F A C E O F P L A T E , m m .

Fig.

11.-Effect

of carbon on yield stress at different manga nese levels as follows:

A-0.6 ,

B-

1.0 ,

C- 1.4 and D-

1.8

7 0 0

7 0 0

0O5

0-10 0-15

C A R B O N IN W E L D , wt .%.

Fig. 12 — Effect of carbon on yield stress at different manga nese levels as

follows: A -0.6 , B- 1.0 , C- 1.4 andD- 1.8

$ 6 0 0

LU

cr

H

i7 5

z


6/8

than when in the presence of manga

nese, silicon and oxygen (Ref. 7).

The present metal lographic studies

have shown tha t the microstructure o f

as-deposited we ld metal is mo dif ied by

the add i t ion o f carb on. On a macroscale ,

the main observation was that the prior

austenite grain width decreased, due

possibly to a change in the solidification

sub-structure . Carbon was more e f fec

tive in this respect than manganese, and it

is presumed that the relative effects on

dendrite spacing are different.

On a microsca le , carbon was found to

increase the amount of acicular ferrite at

the expense of the proeutectoid ferri te

occurring at the boundaries of the colum

nar grains. For an increm ent of 0 .1 % C,

however, the overa l l change in micro-

structure was far less than that encoun

tered when varying the manganese

con

tent over the range from 0.6 to 1.8%. On

the other hand, increasing carbon did

lead to the precipitat ion of increasing

amounts of carbide with in the acicular

ferri te.

The microstructure of the reheated

zones was also modif ied by the addit ion

of carbon. In the case of the coarse

grained regions, the ferri te envelopes

tended to be el iminated, and in the f ine

grained regions the grain size decreased

appreciably. The degree of grain refine

ment induced by the increase in carbon

was essentially the same as that encoun

tered for the experimental range of man

ganese contents. As expected, the

amount of second phase carbides in the

fine grained regions increased as the

carbon level was raised.

The tensi le properties achieved in the

present instance varied linearly with

respect to both carbon and manganese,

the regression equations being in the

form:

a

= a + b (C) 4- c (Mn) 4- d (C • Mn).

Interaction occurred as indicated by

the lines in Figs. 12 and 13 which are not

parallel. This is as expected, since it is

known, fo r wrought mater ia ls, tha t bo th

elements have an effect on sol id solution

hardening, grain size and the percentage

amount of pearlite (Ref. 8).

The present data serve to confirm the

statement made by Heuschkel (Ref. 9)

that, for all practical purposes, there is

little error in assuming 0.04 to 0.14%

250

2 0 0

-.

$150

cc

LU

z

U

a

100

LU

CO

t r

O

ID

CQ 5 0

<

0

c, .'

0-065

0095

I

I

.-•

,

i

i i i i

0-6% Mn

ytiZZZ—

II

» *

i y

• • •

r

i

ii

ii

/

// /

/

/

A.W.

i I 1 1

-

2 5 0

- 30

- 20

E

\

a

- 1 0

-80 -60 -40 -20 0 20

T E S T T E M P E R A T U R E .

C

0

- 20

A.W.

3 0

E

\

Q.

10

-80 -60 -40 -20 0 20

T E S T T E M P E R A T U R E ,

°C

.

2 5 0

2 0 0 -

>

O

cc

LU

z

UJ

a

LU

CD

tr

O

cn

co

<

1 5 0 -

1 0 0 -

5 0 -

c. .

1

0-065

—

0-095

0-145

1

.ijl

1

I 1 l

1-4%Mn

A^A

A

A

4

in

I'l

••

r

i i i

i i

-

A.W.

I i

2 5 0

3 0

- 20

E

a

- 1 0

Fig-

SO

-60 -4 0 -20 0 20

T E S T T E M P E R A T U R E

,°C

.

14 — Charpy

V-notch

test results, as-welded

3 0

2 0

E

f

a

1 0

-80 -60 -40 -20 0 20

T E S T T E M P E R A T U R E , °C .

318-s

| NOVEM BER 1983


7/8

carbon increases yield strength in a linear

manner. Straight line relationships have

been obtained previously for manga

nese, the specif ic constants being depen

dent on process variables, such as inter

pass temper ature (Ref. 10) and heat-input

(Ref. 11).

The add i t ion o f carbon to the we ld

deposits modif ied the shape of the Char

py-V curves by lowering the upper shelf

energy values and making the transit ion

between the duct i le and the br i t t le mode

of fracture more gradual. Thus, with

increasing hardenability, the critical inclu

sion distance decreased and the second

phases increasingly served as sites for

micro-vo id coa lescence.

Allen,

et al.

(Ref. 12), have suggested

that carbide f i lms contribute to the

change in slope of the transit ion curves.

A lso , den Ouden, e f al. (Ref. 7), proposed

that f lattening occurs to a greater extent

when carbon is p resent together w i th a

certa in amount o f oxygen. At the lower

end of the transit ion range, carbon was

beneficia l to an extent that depended on

manganese; also, the detrimental inf lu

ence of carbides on cleavage (Ref. 12)

was evident ly compensated fo r by the

reduction in grain size. The overall situa

t ion was such tha t the opt imum wi th

regard to manganese remained at

approximately 1.4%, independent of the

carbon content — Fig. 15. This finding is

contrary to that expected and indicates

that d i lut ion wit h a high carbon base

materia l cannot be compensated for by

lowering the manganese level.

Mo ll and Stout (Ref. 13) and den O u

d e n ,

et al.

(Ref. 7), have shown that

commercia l we ldments have comparab le

if not bette r transit ion characterist ics than

deposi ts syn thesized f rom pure raw

materia ls. In addit ion, Sagan and Camp

bell (Ref. 4) refer to an instance where a

low ca rbon co ntent is by no means

desirable. In that case, an extra low car

bon E7018 e lectrode, p roduced wi th an

ingot- i ron core wire , gave a room

tem

perature upper shelf value in excess of

360 | but was inferior to a commercial

p roduct a t -20°C

(-4°F).

The require

ment, therefore, is for an intermediate

carbon level so that the up per shelf is not

depressed too much while st i l l t i l t ing the

curve sufficiently and limiting the scatter

b a n d - F i g . 1 7 .

2 5 0

2 0 0

>-

(D

cc

UJ

^ 1 5 0 -

Q

UJ

CQ

O100

co

<

5 0

0 0 4 5 % C

°0-5

- 2 0

A.W.

_L

1 0

1-5

2 0

M A N G A N E S E

IN

W E L D ,

.

2 5 0

3 0

E

u

CL

10

0-5

0 0 6 5 % C

D

A.W.

T O T 5 2 0

M A N G A N E S E

IN WELD

, % .

3 0

2 0

E

u

a

10

2 5 0

0-5

1 0

M A N G A N E S E

1-5

2 0

IN W E L D , %.

2 5 0

2 0 0

>

a

cc

UJ

z

m

150

Q

LLI

CQ

O100

ca

<

5 0

B

0-145 C

j _

+

20 C

- 2 0 °

- 4 0 °

- 5 0 °

- 6 0 °

-7 0 °

A.W.

2 0

CN

-

b

E

a

10

_L

3 0

0-5 TO 1-5

2 0

M A N G A N E S E

IN WELD

,

.

0

Fig. 15 — Effect of mangan ese on energy absorbed at different temperatures for different carbon levels

WELDING RESEARCH SUPPLEMENT

1319-s


8/8

A S W E L D E D

0 0 5 0-10 0-15

C A R B O N

IN

W E L D .

- 2 0

- 3 0

' - 4 0

o

w-50

rr

3

A S

l

_^

D

P

-

0

W E L D E D

0-5

1 0

1-5

M A N G A N E S E

IN

W E L D .

Fig.

16 —

Effects of carbon and m anganese on

test temperatures correspond ing to

1001

F o r E 7 0 1 8 e l e c t r o d e s , w h i c h h a v e

b e e n p e r m i t t e d t o y i e l d u p t o 1 .6 % M n

s i n ce t h e i n t r o d u c t i o n o f A W S A 5 . 1 - 7 8

(Ref . 14) , t he mo st su i t ab le range sugge st

e d b y t h e r e su l ts o f t h e p r e s e n t w o r k i s

b e t w e e n a p p r o x i m a t e l y 0 . 0 7 a n d 0 . 0 9 %

c a r b o n .

C o n c l u s i o n

F o r I SO 2 5 6 0 t yp e d e p o s i t e d m e t a l ,

w e l d e d w i t h b a s ic i r o n p o w d e r e l e c

t r o d e s o f a sp e c i f i c s la g b a se t yp e , t h e

f o l l o w i n g o c c u r r e d o n i n c r e a s i n g t h e c a r

b o n c o n t e n t :

1. T h e a v e r a g e w i d t h o f t h e p r i o r

a u s t e n i t e g r a i n s d e c r e a se d .

2 . T h e a m o u n t o f a c i cu l a r f e r r i t e

i n c r e a se d a t t h e e xp e n se o f t h e p r o

e u t e c t o i d f e r r i t e .

3 . Th e aspe ct ra t i o o f t h e ac i cu la r

f e r r i t e ch a n g e d , i n c r e a s i n g t h e a m o u n t o f

c a r b i d e f o r m e d b e t w e e n t h e l a t h s .

4 . G r a i n r e f i n e m e n t o c c u r r e d in t h e

h i g h t e m p e r a t u r e r e h e a t e d r e g i o n s .

5 . G r a i n r e f i n e m e n t o c c u r r e d in t h e

l o w t e m p e r a t u r e r e h e a t e d r e g i o n s .

6 . I n c r e a s i n g a m o u n t s o f se co n d

p h a se s w e r e p r e c i p i t a t e d i n t h e f i n e

g r a i n e d r e g i o n s .

7 . T h e h a r d n e ss i n c r e a se d .

8 . Th e y ie ld a nd tens i l e s t r eng ths

i n c r e a se d li n e a r l y , b o t h p a r a m e t e r s b e i n g

d e f i n e d b y e q u a t i o n s o f t h e f o r m :

/ D U

2 0 0

-

^150

LU

z

100

G

U J

CQ

a

O

co

50

<

0

I I

T

i

0-045

C

\

i l

•v j l l l l

I I

1-4%Mn

i

i

^ J P

jL*

N

0-147%C

-

i

|

A.W.

I I

- 3 0

- 2 0

E

a

- 1 0

- 1 0 0

- 8 0

- 60

- 4 0 - 2 0 0 2 0

TEST TEMPERATURE

,°C

.

4 0

0

Fig. 17 —Charpy

V-notch

impact curves showing scatter bands for low an d high carbon levels at

1.4

Mn

Effect of Carbon on Microstructure oc C-Mn Steel

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