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Microstructure and Impact Toughness of
C-Mn Weld Meta ls
The formation of acicular ferrite in over half of the weld
appears to be the key to improv ing impact toughness
BY
L E .
S V E N S S O N A N D B . G R E T O F T
ABSTRACT. The effect of va r iat ion in car
bon and manganese contents on the mi
crostructure and impact propert ies of al l
we ld metal samples has been stud ied. The
welds w er e made using the shielded metal
arc welding technique. Four dif ferent car
bon levels , ranging f rom 0.03-0.12 wt -%
and four dif ferent manganese contents
(0.8-2.1 wt -%) were used.
I t was found that signif icant improve
ments in impact toughness at low t e m
peratures were achieved with increasing
amounts of acicular ferrite. High levels of
acicular ferr ite could be achieved with
several dif ferent combinat ions of carbon
and manganese. At excessive amounts of
alloying addit ions, the impact toughness
decreased. This is at t r ibuted to the pres
ence of bands of microphases being
aligned with the notch in the fracture sur
face.
For the lowest carbon content , un
expectedly low toughness was observed.
This may be due to the fact that these
metals contained a somewhat higher ni
t rogen content .
I n t roduct ion
The effect of carbon and manganese
on the microstructure and mechanical
prope rt ies of mild steel arc weld s has been
the subject of many invest igat ions. Vuik
(Ref. 1) has recent ly summarized the in
vest igat ions made concerning the ef fect
of carbon. Evans has published a number
of papers dealing with the ef fect of car
bon (Ref. 2), manganese (Ref. 3), silicon
(Ref. 4), interpass temperature (Ref. 5),
impurity elements (Ref. 6), molybdenum
(Ref. 7), heat input (Ref. 8) and heat treat
ment (Ref. 9) on the microstructure and
mechanical prope rt ies of m ild steel weld s.
The welds that Evans examined were of
the a l l -weld-meta l type, deposi ted wi th a
shielded metal arc m ultipass techniqu e. In
the work most comparable to the present
L-ESvenssona nd
B.
Gretoftare with theEsab
Group, Central
Laboratories,
Goteborg, Swe
den.
one ,
Evans found that the opt imum im
pact toughness propert ies were achieved
with an a l loy ing combinat ion of 0.07 wt -
%C-1.4 wt -% Mn, and at t r ibuted th is to
the compet i t ive act ion between the pro
gressively f iner ferrite grain sizes obtained
by increasing alloying addit ions and by the
simultaneously increasing yield strengths.
In this paper, an invest igat ion of the mi
crostructure and impact toughness of 16
di f ferent welds, wi th vary ing carbon and
manganese contents, is descr ibed. The
expe riment t o a large extent mirrors the
one of Evans (Refs. 2, 3), but the results
dif fer somewhat. The intent ion of this pa
per is to clarify the reason for the discrep
ancy between the dif ferent invest iga
t ions and to point out some addit ional
microstructural ef fects that might be of
importance for the impact toughness.
However, f irst a short descr ipt ion of the
role of carbon and manganese in control
l ing the microstructure and how this may
inf luence the impact propert ies wil l be
given.
B a c k g r o u n d
With the help of experimental (Refs. 1-
9) and theoret ical (Refs. 10-12) work, the
effects of var ious elements on the micro-
structure of the as-deposited area in a
weld is now re lat ive ly wel l understood.
The as-deposited microstructure of
C-Mn we ld meta ls is com mo nly descr ibed
with three major microst ructura l compo-
K E Y W O R D S
Microst ructure
Impact Toughness
C-Mn Weld Meta l
Acicular Ferrite
Mechanical Propert ies
Mild Steel
Arc We ld ing
Al loy ing Content
Microphases
nents:
grain boundary ferr ite, ferr ite with
aligned M-A-C (martensite-austenite-ce-
ment ite) and acicular ferr ite. The classif i
cat ion of the var ious microstructures is
based on the visual impression in the op
t ica l microscope. However , in the theo
ret ica l work based on thermodynamics,
the microst ructura l components are de
scr ibed f rom the mechanism of fo rmat ion
point of v iew. The components are then
called allotr iom orph ic ferr ite (sameasgrain
boundary fer r i te , but a more cor rect
name), Widmannstatten ferr ite side plates
(according to Dube classif icat ion) and ac
icular ferr ite. I t should be noted that the
mechanism of format ion of acicular
ferrite
is not yet kn ow n. In the fo l low ing text , the
last ment ioned denotat ion wil l be used.
The effect of ca rbon is mainly to l imit
the width of the coarse-grained allotr io
morphic ferr ite, formed at the pr ior aus
tenite grain boundaries, and in inf luencing
the rate of
Widmannstatten
fer r ite form a
t ion . Dur ing the t ransformat ion f rom aus
tenite to ferr ite, the carbon atoms dif fuse
into the remaining austenite and the
growth (or thickening) rate of the allotr io
morp hic ferr ite is con trolled b y the d i f fu
sion rate of carbon in austenite. A higher
carbon content g ives a s lower gro wth
rate of the ferr ite a nd, thus, a thinner layer
of ferr ite at the pr ior austenite grain
boundaries.
Increasing carbon content leads to
lower contents of both a l lo t r iomorphic
and
Widmannstatten
fer r i te , g iv ing ro om
for increasing contents of the f ine-grained
acicular ferr ite. However, i t is not known
whether the actual growth rate of ac icu
lar ferr ite is inf luenced by the carb on con
tent.
The manganese atoms, on the other
hand,
are not redistr ibuted dur ing the
transformat ion, but an increased manga
nese con tent reduces the dr iving force f or
the transformat ion. Thus, increasingman
ganese also leads to a thinner layer of al
lo t r iomorphic fer r ite .
In
a way, manganese
and carbon can be considered as com
plem entary e lements, and in pr inciple, the
same microstructure should be at tainable
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Ho we ve r, there is at least one point
martensite, degenerate pearl i te, bain
high. The term
It should b e no ted , that in pract ice there
f re
t ly used ones are Ni, M o and B. The
con
se the slag system in such a wa y as to
Turning to the mechanical propert ies
elationship b etw een microst ruc
we ld .
e
w e l d .
The rehea ted area unde r a bead
ained zo ne,
. The relat ive amounts of these zones
w i t h ,
am ong other th ings, chem
w i l l , of course,
luence the mechanical proper t ies. H ow
that the re is a re
zones and , the refor e, i t is possible to
When designing weld metals, the most
s to m eet requ iremen ts o n
at ional propert ies as good as possible.
Tensile properties are, at least for mild
steels, a smaller problem since usually the
strength of the weld metal is higher than
the strength of the steel.
Specif ications on impa ct toughness vary
substantially, but in many cases the re
quir em ent is 27 J (20 ft-lb) at a certain
temperature. For more advanced appl i
cat ions, higher toughness values are re
quired,
e.g., 34 or 40 ) (25 or 30 ft-lb).
These levels of toughness values are
achieved wi th on ly a relat ively small f rac
t ion of the fracture surface of an impact
toughness test bar having a duct i le, f i
brous fracture, while the remaining part is
a br it t le, cleavage type. To achieve ac
ceptable impact toughness at lower
t e m
peratures (which in many cases is the
t rend in development work today) i t is
necessary to a void cleavage fracturestart
ing too near the notch in the impact bar.
This can be achieved by control of the
microstructure.
To improve impact toughness, some
well-known physical metallurgy pr inciples
are used. First, increasing the amount of
acicular ferr ite by the control of al loying
elements gives a reduc ed grain size. Sec
ondly, use of basic-type consumables
gives a lo w amount of oxygen , wh ich
leads to a low volume fract ion of
inclu
sions. Finally, strict control of impurity el
eme nts like S, P, Sn, As, Sb and N helps to
prevent embr i t t lement of the s t ructure.
The applicat ion of the f irst of these
principles leads us back to the main ques
t ion of this paper: how can the micro-
structure be opt imized by changing car
bon and manganese contents?
As a contrast to this, Dolby (Ref. 13)
suggested that weld metals with a very
lean alloying content, having mainly a
coarse-grained structure and a low yield
strength,
could have good impact tough
ness.
Al though there have been major im
provements in the toughness levels that
can be achieved in weld metals dur ing the
last few decades, by applicat ion of the
principles ment ioned above, there is st i l l
room for fur ther improvement . A more
fundamenta l understanding of the mech
anisms controll ing the onset of cleavage
fracture and the complex interrelat ionship
between microst ructure and f racture
needs to be developed. Major advances
have indeed already been made in this
f ield by Knott and coworkers (Refs. 14-
16) who have studied the f racture beha v
ior of C-M n w elds in deta il and com bined
that with their ear l ier experience of f rac
ture in steels. They concluded that cleav
age fracture in welds of ten or iginated
from cracking of oxide inclusions, in par
t icular those situated in the coarse-grained
allotr iomorphic ferr ite, and that the size
distr ibut ion of these inclusions had a
sig
nif icant ef fect on the fracture toughness
results. In steels, where the volume frac
tion of oxide inclusions is much less, frac
ture toughness is l inked more to the car
bides precipitated along grain boundaries,
nucleating cleavage cracks (Ref. 17). How
ever, i t should be noted that in test ing
fracture toughness of weld metals, Knott
and coworkers used small size notched
bars and tested them in slow strain-rate
four-point bending, in a manner similar to
CTOD test ing. The observat ion of c leav
age cracks nuclea tingfrominclusions we re
numerous in these tests but similar obser
vat ions on impact specimens are, in fact ,
fairly rare.
Experimental
Laboratory-made shielded metal arc
electrodes , 4 mm (0.16 in.) in diame ter, of
E7018 type with basic coat ings were used
for the invest igat ion. The electrode coat
ings were varied to a systematic series of
four dif ferent manganese contents (0.8,
1.1,1.2
and2.1 wt-%)ateach carbon level
(0.03, 0.06, 0.09 and 0.12 w t-%). Allw e l d
ing was made in accordance with ISO
2560, with a current of 180 A, voltage 23
V and a maximum interpass temperature
of 250C (484F). A stringer bead tech
nique was used g iv ing a weld ing speed of
about 4 mm/s (9 in. /min). The heat input
then was around 1k j /mm (25 kj/ in.) .
The chemical composi t ion of the weld
deposits was measured using an opt ical
emission spectrometer (OES), except for
oxygen and n i t rogen, which were deter
mined using combust ion furnaces. The
OES analyses were made on the head of
the tensile specimen.
Two longitudinal al l-weld-metal tensile
specimens (10 mm/0.4 in. in diameter)
and 25 Charpy V-notch impact specimens
were taken f rom each w e l d . The speci
mens were taken f rom the middle of the
plate. The im pact toughness w as tested at
five differenttemperatures, wi thfivespec
imens tested at each temperature.
The microstructures of the weld metals
were examined by convent ional meta l
lography, using l ight opt ical microscopy.
The etching was m ade usingfirsta solut ion
of 4% picr ic acid in alcohol, fol lowed by
2.5% nitric acid in alcohol.
The quant itat ive assessment of the mi
crostructu re was m ade using a Swif t p oint
counter. At least 500 points were mea
sured on each specimen. The microstruc
ture const i tuents were ident i f ied accord
ing to the classif ication of the IIW (Ref. 18).
The austenite grain size was measured
normal to the length axis of the grains(i.e.,
the results are equal to L
tn
as denoted by
Bhadeshia, et al.-R e f . 19) .
T ofurtherstudy the microphases, t rans
mission electron microscopy (TEM) was
used.
Thin foi ls were prepared by polish
ing in a S truersTenupolin a 5% solut ion o f
perchlor ic acid in methanol.
WELDING RESEARCH SUPPLEMENT
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Table
1Chemical
Compositions of the WeldMetals
Mn
Al
Sn
As
Sb
(a) Al l concen t ra t ions a re in wt -%, excep t fo r ox ygen and n i t roge n , wh ich a re g iven in we ig h t ppm
o
Sample No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.030
0.032
0.031
0.032
0.059
0.059
0.059
0.065
0.090
0.089
0.088
0.098
0.12
0.12
0.13
0.11
0.45
0.45
0.42
0.45
0.34
0.33
0.30
0.33
0.41
0.35
0.37
0.39
0.43
0.44
0.37
0.36
0.78
1.27
1.71
2.05
0.77
1.09
1.44
1.83
0.78
1.18
1.59
2.25
0.86
1.35
1.83
2.18
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.014
0.014
0.014
0.011
0.011
0.013
0.008
0.004
0.002
0.010
0.008
0.006
0.003
0.013
0.011
0.011
0.008
0.011
0.010
0.010
0.008
0.009
0.009
0.008
0.009
0.007
0.007
0.007
0.007
0.003
0.008
0.003
0.003
0.005
0.003
0.003
0.003
0.014
0.013
0.011
0.012
0.011
0.010
0.009
0.009
0.014
0.012
0.012
0.013
0.015
0.015
0.013
0.012
0.006
0.006
0.006
0.006
0.003
0.005
0.004
0.004
0.005
0.005
0.005
0.006
0.005
0.006
0.006
0.006
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.004
0.003
0.005
0.005
0.003
0.005
0.005
0.007
0.002
0.003
0.003
0.005
0.001
0.005
0.005
0.005
0.004
0.003
0.004
0.005
0.003
0.006
0.005
0.008
95
94
109
119
66
64
65
76
36
73
77
88
54
55
63
74
336
321
297
320
306
310
305
291
421
404
491
330
329
326
318
342
Results
Chemical Composition
The chemical composi t ions of the we ld
metals are given in Table 1. The carbo n
contents have
successfully
been kept close
to the nominal values. The manganese
content scat tered somewhat around the
nominal va lues, the maximum deviat ion
being around 0.15%.
The phosphorus content w as re lat ive ly
constant throughout the invest igat ion,
typically 0.010%. The sulfur content de
creased with increasing manganese co n
tent. This decrease was especially pro
nounced at the lower carbon contents .
The othe r im purity e lements (Sn, As and
Sb) were all on a low level, and their sum
did not exceed what is considered a safe
level (Ref. 20).
The n i t rogen content increased some
what with increasing manganese content,
being especia l ly pronounced for the low-
carbon e lect rodes, whi le the oxygen co n
tent on the whole was constant. I t should,
however , be noted that the three lower
manganese contents in the 0.09%C spec-
Fig.
1-Yield
strength
as
a
function of carbon
an dmanganese
content.
650
600
550
500
450
imens
had an oxygen content approx i
mately 100 ppm higher than in the other
specimens.
Mechanical Properties
The yield strength measured is shown
as a funct ion of
N4n-content
in Fig. 1. As
expected, the yield strength increased
with increasing carbon and Mn-content .
The inf luence of Mn is relat ively strong,
while the inf luence of carbon is quite
small,
except for the highest carbon con
tent.
The Charpy V-notch impact toughness
curves are p lot t ed in Figs. 2 A- D . First, it
can be noted that increasing manganese
content decreased the upper shelf ener
gies, probably simply due to an increased
yield strength of the matr ix. The impact
proper t ies at lowe r temperatures sho wed
mixed behavior , depending on the com
binat ion of C and Mn . For the lowe r
man
ganese contents, increasing carbon co n
tent led to signif icant improvement in im
pact toughness at lower temperatures. At
the higher manganese contents, the inter-
400
0 ,5 1,0 1,5 2,0
tVIANGANESE(%)
2,5
mediate carbon contents gave the best
impact values at the lower temperatures.
In
Fig.
3,
impact toughness at
60C
(76F)
is p lot ted as a funct ion of M n
content, for constant carbon levels. For
tw o of th e carb on levels (0.09 and 0.12%),
op t imum con ten ts o f Mn were f ound ,
whi le for the two lower carbon contents ,
the opt imum Mn content seemed to be
higher than the maxim um con tents used in
this invest igat ion.
The best impact toughness at
60C
was found for the combinat ion 0.12C-
1.35Mn, but also the intermediate carbon
levels, combined with a relat ively high
manganese level, sh ow ed g oo d results. At
- 4 0 C
(-40F)
almost the same pattern
was fo l lo we d. The best impact toughness
was achieved wi th the combinat ion
0.12%C-1.2%Mn. Also, the combinat ions
0.09%C-1.2%Mn and 0 .06%C-1.4-1 .8%
Mn gave sat isfactory toughness. Increas
ing the manganese content above 1.4%
gave a reduct ion in toughness for the tw o
highest carbon contents.
M i c r o s t r u c t u r e
The austenite grain size, measured in
the last deposited bead, decreased with
increasing carbon and manganese
co n
tent , except for the 0.06% carbon welds,
w hic h all had a slightly larger austen ite
grain size. The austenite grain sizes are
given in Table 2.
There is no systematic var iat ion in aus
teni te gra in s ize wi th oxygen content .
However , i t should of course be noted
that the oxygen content var ies wi th in a
fair ly narrow range.
The results of the quantitative assess
ment of the microstructure are given in
Figs. 4 A- D . For a given ca rbo n c onte nt,
the am ount of acicular ferr ite increased at
the expense of both a l lo t r iomorphic fer
r ite and ferr ite side plates with increasing
manganese content . The maximum
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I
D
2 0 0
100
0 . 8 % M n
1.3 Mn
1.7 Mn
2.1% M n
8 0 - 4 0 - 2 0 0
T E M P E R A T U R E (C)
2 0 4 0
w
CO
C3
O
h-
O
-
v
' / ;
9-
-' -
v
- -
>
?
/
?
^ h ^ i
' ^ 5 7 ^ ^ ^
V
fl
',
IS. r
tr~^*r.Vv~
T
-
J
c^ **' / / / / /
^ s :
< Tfa
>-
-\. >./
^&&tf$&l& '')
.
^Aiakv.^j.'v.:-jf_v-,
';
V/y ..,
tf*
tax
4 j *
> - - * -
Fig.
5
Optical micrograph
of the
weld metal
with
0.03%C
0.8%Mn. The microstructure
consists mainly
of
a coarse allotriomorphic
fer
rite, ferrite side plates andcoarse acicular
fer
rite.
last
bead .
How ev er, since the microphases
appear as retained a usteniteinthe as-de
posited microstructure, they wil l be either
ferr ite+ carb ide aggregates, untemp ered
martensiteorstill retained austenite in the
reheated mater ia l , depending onreheat
ing temperature. Allthese phases c ould
induce br it t leness. Examinat ion of cross-
sections of impact specimens containing
segregated bandsof microphases shows
that in areas whe re the segregat ion bands
are parallel with then o t c h , the f racture
surface hasabr it t le appe arance Fig. 12.
In other cases,inadjacent beads, w here
the segregat ion bands are inclinedto the
notch, thef racture surface has a more
duct i le appearanceFig.13.
These observat ions cannot be taken as
str ict evidence for microphase- induced
cleavage cracking, but comb ined w i t h
many observat ions made earl ier,wecer
tainly feel conf ident that the segregated
microphases are responsibleforthe d rop
in toughness at higher manganese con
tents.
Discussion
It is com mo nly assumed that a high
amount of acicular ferr ite should be
present
in the
m icrost ructure
to
obta in
goo d impact toughness through the
effect
of thef ine g rain size. The increasein ac
icular ferrite is usually,butnot a lways,ac
compan ied by adecreasein theamount
of coarse-grained allotr iomorphic ferr ite
and, thus, asdiscussed previous ly, con
nected to the amounts of carbon and man
ganese.
In Fig. 14,theimpact toughness values
at
60C
isp lo t t ed as afunct ion of the
amountofacicular fer rite . Figure
15
shows
similar informat ion extracted from the
dataofEvans (Ref.2).
In bo th invest igat ions, the same general
t rend is fo un d, that increasing amountsof
acicular ferr ite improves toughness.In
deed, thereislarge scatter inthe relat ion
between acicular ferr ite andtoughness,
but as
a
rule
of
t h u m b ,
it
c an
be
said th at
more than 50% acicular ferr ite givesac
ceptable impact toughness. However ,in
Fig. 14itcanbenoted that the we ld m et
als with the lowestCcontents g ive m uch
lower impact toughness values thanex
pec ted f rom theacicular ferr ite con tent.
This beha vior is in contradic t iontothe re
sultsofEvans (Ref. 2), w h o fou ndamuch
faster improvement inimpact toughness
with increasing the a m o u n t of acicular
ferr ite.
The only element which clear ly isdif
f e ren t be tween the low-carbon we lds
and the other weldsinthis s tudy isn i t ro
gen;being much h igher in the low -carbo n
welds. Ni t rogeniswe l l k n o w n to
embr i t -
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Fig. 6 - Optical m icrograph ot the weld metal
with0.032.1%Mn. Mainly a cicular ferrite and
thin rims of allotriomorphic ferrite.
.-.'
* . : > :
\ss*-.
f
\>
*ft,
;r. ;
.
ft;
fTv77. -vjr ; < ,
V ^ ' * > V ' - - ^ '-* *
2
- ..- * '? - V * .v
^ . , - , V K . ^ - < . :
, j .
i . a * . .
, 2 0 p m
fig. 7 Optical microg raph of the weld metal
withO
12%C-0.8%Mn. The rim of allotriomor
phic ferrite is quite thin. The amo unt of ferrite
side plates is quite high.
r
-r
2 0 um
I
H
I
Fig. 8Optical microg raph of the weld m etal
withO. 12 iC
2.2%Mn. Shown
is
the extremely
fine microstructure with a high amo unt o f acic
ular ferrite.
tie w e l d m e t a l s , a l t h o u g h t h e v a l u e s f o u n d
h e r e w o u l d n o t b e c o n s i d e r e d a s d a n g e r
o u s .
U n f o r t u n a t e l y , E va n s d i d n o t g i v e
va l u e s o f n i t r o g e n co n t e n t i n h is r e p o r t .
T h e a g r e e m e n t f o r l o w v a l u e s o f a c i c u la r
f e r r i t e i s no t su rp r i s ing , s ince th i s is de te r
m i n e d b y t h e o v e r a l l b r i t t l e m i c r o s t r u c
t u r e .
T h e d e v i a t i o n a t t h e h i g h a m o u n t s o f
ac i cu la r f e r r i t e i s , f o r l ack o f a b e t t e r
e x p l a n a t i o n , a s s u m e d t o b e d u e t o n i t r o
g e n . H o w e v e r , t h is is p u r e l y s p e c u l a t i v e
a n d n e e d s m o r e i n v e s t i g a t i o n .
A n o t h e r o b s e r v a t i o n th a t c a n b e m a d e
f r o m bo th F igs . 14 and 15 is t ha t t h e imp ac t
t o u g h n e ss sh o w s a s l ig h t d e c r e a se f o r t h e
h i g h e s t a m o u n t s o f a c i cu l a r f e r r i t e . T h i s
d e c r e a s e i n t o u g h n e s s s e e m s t o o c c u r f o r
a c i cu l a r f e r r i t e co n t e n t s i n e xce ss o f a b o u t
7 0 % . C o m p a r i s o n w i t h F ig . 1 2 s h o w s t h a t
t h e l o w e r t o u g h n e s s is d u e t o i n c r e a s e d
a m o u n t s o f b r i t t l e c l e a v a g e f r a c t u r e .
Eva n s ( R e f . 2) a r g u e d t h a t t h e d e c r e a s
i n g t o u g h n e ss o f h i g h a l l o y i n g co n t e n t
w e l d s w a s d u e t o i n c r e a s i n g y i e l d s t r e n g t h
w i t h o u t a c o r r e s p o n d i n g d e c r e a s e in g r a i n
s i ze .
A s e x p l a i n e d i n t h e b a c k g r o u n d s e c
t i o n ,
t h e m e c h a n i c a l p r o p e r t i e s a r e a
f u n c t i o n o f a m i x t u r e o f m i c r o s t r u c t u r e s .
T o a ssess t h e i n f l u e n ce o f e a ch t yp e o f
m i c r o s t r u c t u r e o n t h e p r o p e r t i e s is a c o m
p lex t ask . Eve n if i t is a g re a t o ve rs imp l i f i
c a t i o n t o r e la t e th e m e c h a n i c a l p r o p e r t i e s
t o t h e a s - d e p o s i t e d m i c r o s t r u c t u r e s , t h i s
a p p r o a c h s h o u l d g i v e g u i d a n c e t o t h e o p
e r a t i n g m e c h a n i s m s .
H o w e v e r , as n o t e d a b o v e , t h e h i g h e s t
a l l o y e d w e l d s c o n t a i n e d h i g h e r a m o u n t s
o f a c i cu l a r f e r r i t e t h a n t h e l o w e r a l l o y e d
w e l d s . T h e y i e l d s t r e n g t h o f t h e se a l l o ys
a ls o w a s h i g h e r t h a n t h e l o w e r a l l o y e d
m e t a l s . If t h e a b o v e w a y o f r e a s o n i n g is
a c c e p t e d , t h e n a h i g h e r a m o u n t o f a c ic u
l a r f e r r i t e i s e q u i va l e n t t o a d e c r e a s i n g
g r a i n s i ze i n t h e w h o l e w e l d m e t a l . T h e
h i g h e r y i e l d s t r e n g t h i s , t h u s , p a r t l y a g r a i n
s i ze e f f e c t . F i n e r g ra i n s sh o u l d a l so l e a d t o
b e t t e r t o u g h n e s s , c o n t r a r y t o w h a t is o b
s e r v e d .
T h e c la ss ica l m o d e l o f c l e a v a g e f r a c t u r e
is t h a t th i s o c c u r s a t a t e m p e r a t u r e w h e r e
t h e y i e l d s t r e n g t h e x c e e d s t h e f r a c t u r e
s tr e s s. H o w e v e r , b o t h t h e y i e l d s t r e n g t h
a n d t h e f r a c t u r e s t re ss a r e g r a i n - s ize d e
p e n d e n t i n su ch a w a y t h a t f i n e r g r a i n s
l e a d t o b o t h h i g h e r y i e l d a n d f r a c t u r e
s t r e n g t h . T h u s , t h e a m o u n t o f c l e a v a g e
f r a c t u r e is n o t e xp e c t e d t o i n c r e a se i n t h e
h i g h e s t a l l o ye d w e l d m e t a l s , b u t t h is is
o b v i o u s l y w h a t h a p p e n s w h e n t h e t o u g h
n e ss f a l l s . O b v i o u s l y , so m e t h i n g i n t h e m i
c r o s t r u c t u r e o f f se t s t h e b e n e f i c i a l e f f e c t
o f f i n e r g r a i n s . T h e m o s t l i ke l y f a c t o r
r e sp o n s i b l e f o r t h is is t h e se g r e g a t e d m i
c r o p h a s e s , w h i c h is i n l in e w i t h t h e o b s e r
va t ions in F igs . 12 and 13 .
**
-
k
i~ S
K,.. *;
~^pt^\
. ' ' > ' , ? . ' ' ''. ' ' - * ' - . -
/ C , 4 '-i''-' ' *
:
5 . . 20 pin
A - v . . I tlI
Fig.
9A
Optical micrograph showing how the microphases (white small grains) are randomly
dispersed; Bwith higher alloying content, the micropha ses are becom ing more se gregated. Etch
ing was made, usingKlemms reagent (R ef. 21), to more easily distinguish the micropha ses.
WELDING RESEARCH SUPPLEMENT
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Fig.10 TEM micrographshowingisolated islandsofretained austenite
(arrowed) andsomegrain boundarycarbides.
Fig.11
Centereddark fieldTEMmicrograph of weld metal0.12%C-
2.2%M n, showing an almost continuous layer of retained
austenite.
m * '
:
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.
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ij^i^BK*-y^i.
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.
50um
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i
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F/g. 12 Optical micrograph
of a
polished
cross-section through
an
impact specimen,
showing that the britt le appearanceofthe
fracture surface is parallelwith thesegregation
band ofmicrophases.
Fig.
4 Impact
toughnessat60C 3
asa function of
nm
v
: v * :
:>
?,
?-ig
50 pm
F/g. 7 i
Opticalmicrographshowinga ductile
appearance of the fracturesurface in an area
where thebands ofmicrophasesappear at an
angle to the fracturesurface.
acicular ferrite
o
content, from this
investigation.
C O
C O
a
o