Chemical Composition Variations in
Shielded Metal Arc Welds
Metal transfer droplet size, which changes with heating temperature,
is discovered to be a factor varying chemical composition
BY A. Q . BRACARENSE A N D S. LIU
ABSTRACT. The use of shielded metal
arc (SMA) welding can result in chemical composition variations along the
weld length. Manganese and silicon,
c o m m o n l y found in low-carbon steel
welds, change in composition with weld
position. This research was performed
to better characterize the composition
variations observed in structural steel
welds and to understand the controlling
factors that determine the extent of these
composition changes.
Single bead-on-plate and multipass
welds were performed and analyzed.
Manganese, silicon, and oxygen contents showed significant variation along
the weld length. Hardness measurements and microstructure confirmed the
strong effect of the composition change.
To determine the cause of such composition variations, additional experiments were carried out w i t h the w e l d ing arc established between the electrode and a water-cooled copper pipe.
The individual metal droplets were collected in water and processed using standard particulate materials processing
techniques to remove the slag covering.
The droplet size distribution was determined and related to the composition
variation and position along the weld
length. The results indicated that electrode preheating caused a change in the
size of the droplets transferred during
welding. At the beginning of welding,
the electrodes were not heated as much
and small size droplets predominated.
The fine droplets, with large surface areato-volume ratio, experienced more complete deoxidation reactions and large
losses in manganese and silicon. As elecA. Q. BRACARENSE and S. LIU are with the
Center for Welding and Joining Research,
Colorado School of Mines, Golden, Colo.
trade preheating becomes more intense,
globular transfer with large droplets replaced the small droplets. Chemical
analysis showed that more manganese
and silicon were transferred across the
arc to the weld pool.
Introduction
The shielded metal arc (SMA) welding process is probably one of the most
versatile methods for joining steels. It is
inexpensive, simple, and requires minimum welding skills in most applications.
An SMA electrode consists of a metal
core rod and a "clay-like" covering of
powdered minerals such as fluorides,
carbonates, oxides, organic materials,
and alloying additions. A silicate binder
is used to help extrude the flux ingredients onto the metal core rod. Subsequent
baking of the electrode removes the
moisture from the covering and forms a
hard covering over the metal rod.
During welding, both base metal and
electrode are melted by the heat gener-
KEY WORDS
SMAW
Chemical Comp. Varies
Covered Electrodes
Structural Steel
Electrode Heating
Chemical Analysis
Metal Transfer Mode
Weld Metal Manganese
Weld Metal Silicon
Arc Physics
ated from the arc. The transfer mode of
liquid metal from the electrode tip to the
weld pool in SMA welding is often difficult to establish without special experimental techniques because of the fume
and slag present (Ref. 1). However, it has
been shown that globular transfer occurs
in SMA w e l d i n g . Large droplets of l i q uid metal, at the size of the electrode diameter or bigger, grow at the tip of the
electrode, detach and fall to the molten
weld pool. Explosive transfer, before or
after short circuiting of the metal droplet
with the weld pool, was also observed
in SMA welding. A showery spray of
small droplets of liquid metal and slag
fly across the arc including many projected outside the weld zone (Refs. 1, 2).
Many factors are responsible for the
transfer mode in SMA welding. The
major ones are current, voltage, electrode diameter, melting temperature of
the core material, coating thickness, and
temperature of the electrode (Ref. 3). Few
studies (Refs. 3-6), however, provide insights on the effect of electrode temperature on metal transfer and weld deposit
properties.
During welding, an electric current
(I) passes from the electrode holder to
the electrode and through the electrode
to the arc column. As a result of the electrical resistance of the electrode, heating of the electrode occurs. Joule heating, which is given by the product of the
square of the current (I2) and the electrical resistance (R), causes the electrode
to heat up.
Additionally, part of the heat of the
plasma, w h i c h is given by the product
of the electric current (I) and the arc voltage (V 0 ), also raises the temperature of
the electrode. However, this contribution is minimum, because part of the arc
energy is used to melt the tip of the elec-
W E L D I N G RESEARCH SUPPLEMENT I 529-s
TR
=
HEAT FROM
JOULE EFFECT
HEAT FLOW
. THROUGH TIP
FLUX
COATING-
I
POWER
I SOURCE
I V0
Q,
LIQUID
=
DROPLET.
V0
Fig. I — Schematic illustration
HEAT FLOW TO FLUX
THE
of the thermal conditions
t r a d e a n d the base m e t a l , a n d part is lost
b y r a d i a t i o n a n d by e v a p o r a t i o n of m a terial f r o m the surface of the electrode
(Ref. 6). Figure 1 is a s c h e m a t i c illustration of the t h e r m a l c o n d i t i o n s e x p e r i e n c e d b y an S M A w e l d i n g e l e c t r o d e .
From the e s t a b l i s h m e n t of the arc, the
t e m p e r a t u r e of the e l e c t r o d e is e x p e c t e d
t o increase as s h o w n in Fig. 2 . It is clear
that d u r i n g n o r m a l w e l d i n g t h e t e m p e r ature of an e l e c t r o d e at a p o i n t r e m o v e d
f r o m the arc c a n vary s i g n i f i c a n t l y , f r o m
r o o m temperature to over 1000°C
(1 832°F) (Ref. 3). C o n s e q u e n t l y , the m e l t
rate of e l e c t r o d e a n d t h e m e t a l transfer
are e x p e c t e d to c h a n g e w i t h w e l d i n g
t i m e and position along the w e l d length.
D u r i n g w e l d i n g , the length of the
e l e c t r o d e , £, also d i m i n i s h e s , w h i c h d e -
=: ARC
FUSION
=
HEAT FROM
PLASMA
NEAR
BOUNDARY
VOLTAGE
in a covered electrode (Ref. 6).
creases t h e J o u l e e f f e c t . H o w e v e r , t h e
increase in t e m p e r a t u r e of t h e c o r e r o d
m a t e r i a l leads t o an i n c r e a s e o f t h e res i s t i v i t y , p (Ref. 7), w h i c h d e s p i t e t h e
e l e c t r o d e length decrease, the Joule
h e a t i n g c o n t i n u e s to be s i g n i f i c a n t . Figure 3 s h o w s the increase of resistivity of
s o m e c o m m o n steels w i t h t e m p e r a t u r e .
Based o n this fact, Fig. 4 s h o w s s c h e m a t i c a l l y t h e d i s t r i b u t i o n o f the t e m p e r a t u r e
of the metal c o r e rod a l o n g its l e n g t h , as
p r o p o s e d b y W a s z i n k , ef al. (Ref. 6).
Next to the electrode holder, the t e m perature of the c o r e r o d increases r a p i d l y
t o a steady t e m p e r a t u r e . A t a s h o r t d i s tance f r o m the arc, the t e m p e r a t u r e of
t h e e l e c t r o d e t i p increases r a p i d l y t o t h e
m e l t i n g t e m p e r a t u r e . W a s z i n k , et al.
(Ref. 6), e s t i m a t e d that the r a p i d t e m p e r -
Table 1 — Welding Conditions Used for the Three Electrodes
Core rod diameter = 3.2 mm (% in.)
Conditions
Current (amperes)
Voltage (volts)
Travel speed (mm/s)
Heat input (kj/mm)
E6013
| iA
23
2.05
1.5
E7018
El 2018
134
25
2.05
1.5
130
27
2.5
1.4
Table 2 — W e l d i n g Conditions Used in the Experiment w i t h E7018 Electrodes to Verify the
Composition Variation and Its Dependence of Welding Current
Core rod diameter = 3.2 mm (% in.)
Conditions
Current (amperes)
Voltage (volts)
Travel speed (mm/s)
Heat input (kj/mm)
Lower Current
100
25
1.69
1.5
Higher Current
150
25
2.54
1.5
Table 3 — C h e m i c a l C o m p o s i t i o n in w t - % of t h e A 3 6 Plate a n d E 7 0 1 8 Electrode C o r e Rod
Element
Carbon
Silicon
Manganese
530-s I DECEMBER 1993
A36 Steel
0.1282
0.2637
0.9688
E7018Core Rod
0.1136
0.0094
0.4957
ature increase occurred at about 1 mm
(0.039 in.) from the molten electrode tip.
At the melting front, the core rod is much
hotter than the surrounding covering. As
the electrode heats up by Joule effect
during welding, the portion of the electrode tip that experiences the transient
temperature increase, A9, w i l l also increase.
The electrode covering plays an important role by keeping the generated
heat from the Joule effect and the heat
conducted from the plasma inside the
core rod. Since the electrical resistance
of the electrode covering is several orders of magnitude higher than the metal
core rod, Joule heating in the electrode
covering is negligible because it can be
considered that no current flows through
the covering material (Ref. 6). In summary, with the melting of the electrode,
more heat is generated and the electrode
becomes hotter.
The effects of electrode temperature
increases during welding are various. It
has been suggested (Ref. 3) that when
an electrode is heated to high temperatures, the specific melting rate may experience a five-fold increase, w h i c h is
the ratio between the latent heat of the
hot metal and its heat of fusion. An extension of this observation may be the
effect of electrode temperature on metal
transfer mode, w h i c h may strongly influence the weld chemical composition.
Additionally, it was found (Ref. 5) that
oxygen content decreased w h i l e manganese and silicon contents increased
as a function of the droplet growth time,
as shown in Fig. 5. These observations
seem to indicate that larger droplets will
exhibit higher manganese and silicon
content. This phenomenon must be associated with the deoxidation of the liquid metal droplet, w h i c h is controlled
by kinetic factors such as temperature
and droplet surface area.
Experimental Procedure
Bead-on-plate welds on A36 steel
plates were conducted using E6013,
E701 8, and E1 201 8 electrodes to verify
the composition variations along the
weld length. The welds were made with
a linear heat input approximately equal
to 1.5 kj/mm. The welding conditions for
each electrode are shown in Table 1.
Four additional sets of welding experiments were performed using E7018
electrodes to investigate the influence
of Joule heating of the electrode on weld
metal chemical composition.
In the first one, bead-on-plate welds
were prepared to verify the composition
variation in the weld metal along the
weld length at two levels of current.
Composition changes were also correlated with d i l u t i o n , hardness and m i -
O
115 AMP
1200
/
O 1000
or:
m o AMF
/
/
/
/
/
/
.
y
<
/
o_
5
/
V
y
EL s,MP
/j
/
/
A
A
/
/.
SAE-1008
SAE-1025
SAE-1042
/ '*
A
0
20
40
60
80
TIME OF WELDING
100
120
(S)
Fig. 2 — Increase in temperature in SMA electrodes with time for different welding currents (Ref. 3).
Fig. 3 — Resistivity of low-carbon
crostructure. The welds were performed
using a constant current p o w e r source
w i t h d i r e c t c u r r e n t e l e c t r o d e positive (reversed polarity). The w e l d i n g c o n d i t i o n s , s h o w n in T a b l e 2, w e r e kept c o n stant b y u s i n g an a u t o m a t i c v o l t a g e c o n trol apparatus.
2 . Samples w e r e p r e p a r e d for c h e m i c a l
a n a l y s i s as s h o w n i n F i g . 9 . W i t h f o u r
layers o f w e l d b e a d s , t h e effect of base
m e t a l d i l u t i o n is c o m p l e t e l y r e m o v e d .
T h e results of this set of e x p e r i m e n t s w i l l
b e c o m p a r e d w i t h t h e results o f t h e
metal droplet experiments.
T o investigate the possible reactions
b e t w e e n t h e e l e c t r o d e c o v e r i n g and the
metal core r o d , the remaining electrode
tips after w e l d i n g w e r e r e t a i n e d for further analysis.
In t h e t h i r d set o f w e l d i n g e x p e r i ments, multipass welds w e r e prepared
w i t h the h i g h c u r r e n t w e l d i n g c o n d i t i o n s
s h o w n in T a b l e 2. The start of the w e l d s
was staggered to p r o d u c e a long w e l d
as in a c t u a l s t r u c t u r a l f a b r i c a t i o n . W i t h
this p r e p a r a t i o n , it is e x p e c t e d that t h e
o v e r l a p p i n g w e l d s w i l l m i n i m i z e t h e eff e c t of c o m p o s i t i o n c h a n g e s b y m i x i n g
m a t e r i a l o f t h e e n d of w e l d s ( h i g h - a l l o y
contents) w i t h material of the b e g i n n i n g
of welds ( l o w - a l l o y contents). Samples
w e r e p r e p a r e d f r o m the w e l d s as s h o w n
in Fig. 1 0 for c h e m i c a l analysis.
After the w e l d s w e r e m a d e , the s a m ples w e r e p r e p a r e d for c h e m i c a l a n a l y sis f r o m the w e l d length as s h o w n in Fig.
6. T h e m a n g a n e s e a n d s i l i c o n c o n t e n t s
of t h e A 3 6 steel plate a n d t h e E 7 0 1 8 c o r e
r o d are s h o w n in T a b l e 3. Special p r e p a ration was required to obtain the specim e n s for o x y g e n a n a l y s i s as s h o w n in
Fig. 7. W e l d m o r p h o l o g y , hardness m e a surement, and microstructure evaluat i o n samples w e r e p r e p a r e d as s h o w n in
Fig. 8.
In t h e s e c o n d set of w e l d i n g e x p e r i ments, multipass bead-on-plate welds
were prepared w i t h the same w e l d i n g
c o n d i t i o n s as t h e h i g h c u r r e n t s i n g l e
b e a d - o n - p l a t e w e l d s presented in T a b l e
Fig. 4 — Temperature
distribution in the
core rod along the
length of the electrode
at different times
(Ref. 6).
T h e f o u r t h set of w e l d i n g e x p e r i m e n t s
was performed on a water-cooled pure
copper tube positioned over a metal
d r o p l e t c o l l e c t i o n box. T h e b o x has f o u r
c o m p a r t m e n t s that c o l l e c t e d metal
d r o p l e t s at a s p e c i f i c l o c a t i o n a l o n g t h e
w e l d . T h e b o x w a s f i l l e d w i t h w a t e r to
q u e n c h the molten metal droplets and
MELTING
--> ERONT^
TEMPERATURE OF T H E
ARC
/I
"ELECTRODE: TIP"
J\
to p r e v e n t the d r o p l e t s f r o m fusing o n t o
t h e b o x . A s c h e m a t i c d r a w i n g o f this a p paratus is presented in Fig. 1 1 . The w e l d ing c o n d i t i o n s used in this part of the
e x p e r i m e n t are s h o w n in T a b l e 4 .
The droplets were removed from the
specific c o m p a r t m e n t s a n d processed in
groups using particulate materials processing techniques. The droplets w e r e
t h e n c l a s s i f i e d i n t o t h r e e size g r o u p s as
s h o w n in T a b l e 5. D r o p l e t s o f d i a m e t e r
less t h a n 1.5 m m ( 0 . 0 5 9 in.) w e r e c h a r a c t e r i z e d as s p r a y t r a n s f e r r e d , a n d
m e d i u m a n d large droplets (d > 1.5 m m ) ,
as g l o b u l a r t r a n s f e r r e d . T h e a v e r a g e d i ameter and deviation of the droplets
were d e t e r m i n e d for each size range
Table 4 — Welding Conditions Used in the
Droplet Generation and
Collection Experiments
110
35
3.38
0.97
Current (amperes)
Voltage (volts)
Travel speed (mm/s)
Heat input (k|/mm)
Table 5 — Classification of the Droplets
from 3.2mm (% in.) Diameter
E7018 Electrodes
Classification
Small
Medium
Large
Dimension (mm)
0.5 to 1.5
1.5 to 3.0
3.0 to 4.5
Table 6 — Percent Increase of Weld Metal
Manganese and Silicon per
100mm (3.94 in.) of Weld
Length for the Three Electrodes
Tested
ae J
.r,"
\
ELECTRODE
steels with temperature (Ref. 7).
ARC f i r n e Z
-J
/ ^
t
1250
500
750
1000
TEMPERATURE (°C)
140
LENGTH
Elements
E6013
Manganese 7.1
Silicon
10.7
E7018
E12018
6.3
14.5
8.1
21.8
W E L D I N G RESEARCH SUPPLEMENT I 531-s
0
100
200
300
400
500
600
700
CUT TO FIT
IN THE
TIME (ms)
EMISSION
SPECTROMETER
Fig. 5 — Variation of weld metal oxygen, manganese and silicon
contents as a function of droplet growth time for electrodes with lowoxygen-potential covering (marble: 14 wt-., fluorspar: 56 wt-%, mica:
2 wt-%, Na2C03: 1 wt-%, TiO,: 4 wt-%, Fe-Si: 5.5 wt-%, Fe-Mn: 5.5
wt-%, Fe-Ti: 13 wt-%) (Ref. 5).
Fig. 6 — Sample preparation
for weld metal chemical
CHAMBER
analysis.
WIDTH
REINFORCEMENT
PENETRATION
HARDNESS TEST
INDENTATIONS
POSITION
POSITION
OF
EXTRACTED
Fig. 7 — Sample preparation
piece weighed approximately
THE
SAMPLES
for weld metal oxygen analysis.
1 g (2.2 X 10~3 Ib).
Each
Fig. 8 — Sample preparation for bead morphology
determination,
hardness measurement and microstructural
evaluation.
I ..fcROII
BECAUSE OF
ARC INSTABILITY
CUT TO FIT
t i n TO FIT
IN THE EMISSION
IN THE EMISSION
SPECTROMETER
Fig. 9 — Sample preparation for multipass weld metal chemical
ysis.
532-s I DECEMBER 1993
SPECTROMETER CHAMBER
CHAMBER
anal-
Fig. 10 — Sample preparation
analysis.
for overlapping
weld metal
chemical
40.0
20.0
40.0
60.0
WELD POSITION FROM THE START (mm)
Fig. 11 — Schematic illustration of the apparatus used for droplet
generation and collection.
(transfer mode) and each compartment.
Results and Discussion
Manganese and silicon were chosen
to be analyzed in this study because they
are the most common elements present
in commercial electrodes for low-carbon and low-alloy steel weldments and
that they have a strong effect on m i crostructure and mechanical properties
of steel welds (Refs. 8-11).
The manganese and silicon contents
of the welds made using the E6013,
E701 8 and E1 201 8 electrodes are plotted in Figs. 12 and 13, respectively, as
a function of the weld position. As can
be observed, variations occurred for all
the electrodes. Table 6 summarizes the
percent increase of each element per
100 mm (3.94 in.) of weld length from
the beginning to the end of the w e l d .
The relative increase was the greatest in
the welds made using the E1 201 8 elec-
Fig. 12 — Manganese variation along weld length for various electrodes. Total weld length: E6013, 90 mm (3.54 in.); E7018, 105 mm
(4.13 in.); El2018, 73 mm (2.87 in.).
trodes, indicating that significant
changes in microstructure and mechanical properties along the weld length can
be expected (Ref. 12).
To further confirm these results,
E7018 electrodes were used in four additional experiments as described previously. Figure 14 shows the manganese
and silicon contents as a function of the
weld length and welding current for the
single bead-on-plate welds. Figure 15
shows the oxygen content along the
length of the welds made with two current levels. These results again confirmed the composition variation trends
noted previously. Manganese and silicon both increased, being the increase
more significant for the higher current
(150 A) welds. Oxygen, however, decreased along the weld length. It is important to notice that the results presented are independent of weld dilution,
which was approximately constant at 38
vol-% for the two currents and along the
weld length. Constant dilution precludes
the effects of base metal preheating,
which could also cause the weld metal
composition variation.
The results from the multipass welding further confirmed the results from
the single bead-on-plate welding. Manganese and silicon contents increased
in the last of 1 0 passes as shown in Fig.
1 6. Figure 1 7 shows the chemical composition along the overlapping w e l d .
Manganese and silicon increased even
when the end of a weld overlapped with
the beginning of the subsequent w e l d .
The "compensation" expected from a
high- and low-alloy content (end of a
weld and beginning of a weld) was minimum. The variation in manganese and
silicon along a long weld joint can compromise the quality of the weld joint.
All results reported indicate that the
composition variation was caused
mainly by the heating of the electrode
during welding. Therefore, two possible
100.0
MANGANESE
o D - 150 AMP
o D - 100 AMP
« 40.0
20.0
40.0
60.0
WELD POSITION FROM THE START (mm)
Fig. 13 — Silicon variation along weld length for various electrodes.
Total weld length: E6013, 90 mm (3.54 in.); E7018, 105 mm (4.13
in.); El2018, 73 mm (2.87 in.).
20.0
40
60
80
100
120
WELD LENGTH POSITION (mm)
Fig. 14 — Weld metal manganese and silicon variation along the
weld length for E7018 electrodes at the currents of 100 and 150 A.
W E L D I N G RESEARCH SUPPLEMENT I 533-s
100.0
.
150 AMP
00 AMP
80.0
60.0
SILICON
40
50
60
70
80
WELD LENGTH POSITION (mm)
20.0
40
60
80
WELD LENGTH POSITION ( m m )
Fig. 15 — Weld metal oxygen variation along the weld length
E7018 electrodes at the currents of 100 and 150 A.
e x p l a n a t i o n s n e e d to be discussed. First,
the metal core rod-electrode covering
i n t e r a c t i o n j u s t b e f o r e m e l t i n g w i l l be
c o n s i d e r e d . D i f f u s i o n o f e l e m e n t s (Ref.
5), s u c h as m a n g a n e s e a n d s i l i c o n f r o m
t h e c o v e r i n g to t h e c o r e r o d , m a y o c c u r
d u r i n g heating and increase their c o n c e n t r a t i o n in t h e w e l d m e t a l . T h e second explanation concerns the heating
of the electrode, w h i c h changes the
d r o p l e t s ' size a n d c o m p o s i t i o n . Small
d r o p l e t s w i t h large surface area per u n i t
v o l u m e can interact more efficiently
for
120
100
Fig. 16 — Weld metal manganese and silicon variation along the
tenth multipass weld length for E7018 electrodes at current of 150 A.
w i t h t h e o x i d i z i n g s p e c i e s i n t h e arc
l e a d i n g to t h e o x i d a t i o n o f t h e m e t a l l i c
elements. O n the other h a n d , large
d r o p l e t s w i t h s m a l l surface area per u n i t
v o l u m e i n t e r a c t less w i t h o x y g e n resulting in l o w e r loss of the a l l o y i n g e l e m e n t s
in the w e l d m e t a l .
T o e v a l u a t e the first hypothesis, a n a l yses of t h e e l e c t r o d e tips w e r e p e r f o r m e d
using the scanning electronic m i c r o s c o p e (SEM). Figure 1 8 s h o w s the c o n c e n t r a t i o n profiles o f m a n g a n e s e a n d s i l i c o n across t h e c o r e r o d . N o t i c e t h a t
Mn
manganese and silicon contents were
a l m o s t c o n s t a n t in t h e c o r e r o d . S l i g h t
deviations from the original manganese
and silicon contents were observed but
the magnitude of change cannot justify
the transport of elements between flux
c o v e r i n g a n d c o r e r o d . These results i n dicate that the c o m p o s i t i o n variations
in the w e l d metal d i d not result f r o m a n y
r e a c t i o n b e t w e e n the c o r e r o d a n d c o v e r i n g before m e l t i n g (Ref. 5).
T h e r e f o r e , t h e o t h e r p o s s i b i l i t y is t h e
i n t e r a c t i o n b e t w e e n the m o l t e n droplets
Si
(1 _
y
NOMINAL ROD
COMPOSITION
(jmmmm^~z~imm
o o
o ° o
°0
n
° 0
r
..:
0.0
NOMINAL ROD^
COMPOSITION
1.0
2.0
ELECTRODE
DIAMETER ( m m
D O "
,D
0.0
1.0
2.0
ELECTRODE
DIAMETER ( m m )
i°n n
DD
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0
50
100
150
200
WELD LENGTH POSITION (mm)
Fig. 17 — Weld metal manganese and silicon
overlapped weld length.
534-s I DECEMBER 1993
250
variation
300
along
the
Fig. 18 — SEM analysis of an E7018 electrode
150 A.
tip after welding
at
WELDING
DIRECTION
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TANK
4
5
POSITION
Fig. 19 — Droplet size variation (in mm) as a function of tank position.
and the arc plasma during transfer. The
results of droplet collection are presented in Fig. 19 as a function of the tank
position. It can be seen that the average
size of the large droplets increased while
the smallest droplets decreased along
the weld length. It is important to notice
that the increase in large droplet size
was at the expense of the small droplets
since the melting rate remained approximately constant at 0.07 g/s (0.1 5 X 1 0~2
Ib/s).
These results confirm that electrode
heating actually changed the transfer
mode and the chemical composition of
the weld metal. At the beginning of
w e l d i n g , the core rod is cold and the
electric resistance of the electrode is
low. When the current begins to pass
through the electrode, the temperature
starts to increase by Joule effect. At this
point, the heat of the plasma contributes
little to heating the electrode. As the
electrode is being consumed, the temperature increases even with the decrease of electrode length, as described
previously. It is believed that the more
the electrode is consumed the more the
heat of the plasma will contribute to the
heating up of the electrode. Simultaneously, while the electrode heats up, the
resistance increases due to the increase
TEMPERATURE
TEMPERATURE
Fig. 20 — Arc voltage and electrode voltage during welding with the
automatic arc voltage controller.
in resistivity of the metal. The increase
in resistance, at constant current, increases the voltage along the electrode.
As the automatic voltage arc control apparatus keeps the total voltage constant,
the arc voltage should decrease during
welding, as shown in Fig. 20. At high
arc voltage, such as at the beginning of
welding using a new electrode, small
droplets are expected to be predominant. At low voltage, after the electrode
heats up, large droplets and short-circuit transfer are predominant (Ref. 1 3).
The effect of metal transfer mode on
chemical composition can be explained
by Fig. 2 1 . The small droplets have large
2
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ooo
OXYGEN
Mn AND Si
OXIDIZE
AND GO
TO SUAG
o
Fig. 21 — Effect of the electrode heating during welding on metal
droplet transfer mode and the chemical composition of the droplets.
40
60
WELD LENGTH POSITION
(mm)
Fig. 22 — Hardness variation along the length of welds made with
E7018 electrodes at 150 A.
WELDING RESEARCH SUPPLEMENT I 535-s
Acknowledgments
The authors gratefully acknowledge
the support of the SP-7 W e l d i n g Panel
of NSRP, the National Shipbuilding Research Program. A. Q. Bracarense also
acknowledges the financial support received from CNPq. The assistance of
David Fazzina in a part of the experimental program is also appreciated.
References
Fig. 23 — Microstructure of sections along
the length of welds
made with E7018 electrodes at 150 A. As the
electrode gets hotter,
toward the end of the
weld, the amount of
grain boundary ferrite
in the weld metal de-
surface area per unit volume and elements such as manganese and silicon
can react easily with oxygen or oxidizing species in the arc. When the electrode heats up, the average size of the
droplets increases reducing the surface
area per unit volume. The interaction
between oxygen and the elements on
the surface of these droplets may still
occur, but much of the alloying elements
remain unoxidized inside the droplets.
Oxygen decreases because less oxides
are present in the weld metal.
To evaluate the effect of composition
variation along the weld length, hardness tests and metallography were performed in several sections of the welds.
Figure 22 showed that hardness increased approximately 18% per 100 mm
(3.94 in.) of weld length. This increase
can be associated with the manganese
and silicon increase and the oxygen decrease, Figs. 14 and 15, respectively.
Both are in close agreement with the literature (Refs. 8-12). The increase in
hardness corresponded to an increase
of approximately 30 ksi (207 MPa) in ultimate tensile stress (o"uts), which can be
significant in structural welds where uniform and constant mechanical properties along the weld length are essential.
Metallographic analysis confirmed
the hardness results. A slight decrease
in the amount in grain boundary ferrite
was observed along the weld length as
shown in Fig. 23. These observations
can also be associated with the increase
in manganese and silicon contents, and
are also in complete agreement with the
literature (Refs. 8-12).
536-s I DECEMBER 1993
Conclusions
This investigation of the SMA electrode heating during welding and its effects on weld metal mechanical properties and microstructure can be summarized with the following conclusions:
1) Using commercially available covered electrodes, a significant increase in
weld metal manganese and silicon occurred along the weld length. This increase (for example, 8-21 wt-%, respectively, in welds prepared with E1 201 8
electrodes) can affect considerably the
mechanical properties and microstructure of single and multipass welds.
2) Along with the increase of weld
metal manganese and silicon, oxygen
was observed to decrease. These
changes affect considerably the weld
metal microstructure and mechanical
properties since an 18% increase in
hardness (indirectly, ultimate tensile
strength) in the E7018 welds was observed.
3) The composition variations can be
explained as a result of electrode heating during welding since no significant
variation in base metal dilution was observed.
4) The heating of the electrode
changed the metal droplet transfer size
across the arc. Alloying elements in the
small droplets were easily oxidized to
form oxide inclusions and/or removed
from the weld pool as slag. Alloying elements in the large droplets were not as
easily oxidized, resulting in higher content in the weld metal.
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1991. Analysis of metal transfer in shielded
metal arc welding. Welding Journal 70(10):
261-sto270-s.
2. IIW (1977) Classification des Divers
Modes de Transfert du Metal en Soudage a
I'Arc. IIW DOC XII-535-77.
3. ter Berg, J., and Larigaldie, A. 1952.
Melting rate of coating electrodes. Welding
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weld. Automatic Welding, September, pp.
13-16.
11. Cochrane, R. C, and Kirkwood, P. R.
1 978. The effect of oxygen on the weld metal
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1 2. Dorschu, K. E., and Stout, R. D. 1961.
Some factors affecting the notch toughness
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97-sto 105-s.
13. Liu, S., Siewert, T. A., and Lan, H. C.
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welding. Proceedings of the 2nd International
Conference on Trends in Welding Research,
Gatlinburg, Tenn., pp. 14-18.