Oxygen Effect on
Low-Alloy Steel Weld Metal Properties
Oxygen content in submerged arc weld metal depends on the
concentration of oxides decomposed in the weld pool slag
BY N. N . P O T A P O V
ABSTRACT, lt is shown that the weld
metal oxygen content in submerged arc
low-alloy steel welds, as well as in lowcarbon steel welds is dependent on the
concentration of oxides decomposed at
low temperatures in a weld pool slag
phase.
The oxygen is mainly in the form of
fine dispersed oxide inclusions of less
than 0.03 pm.
Differentiated evaluation of silicon
reduction effects in submerged arc
welded low-alloy steels revealed that
weld metal brittle fracture strength
depends to a considerable degree on
total weld metal oxide inclusion content
than on silicon increment in the w e l d .
Therefore, the increase of weld metal
brittle fracture susceptibility w i t h the
growth of weld oxide inclusion content
is important to know.
Welds with lowered oxygen content
[0] < 0.02% also display the tendency
to decrease in plasticity because 1) the
ferritic-pearlitic matrix of improved
purity is likely to generate unbalanced
structures on cooling and, 2) when there
are no oxide inclusions, the shape
of sulfur and phosphor precipitation
from the melt changes from globular to
film-like.
Optimal low-alloy steel weld metal
oxygen content is defined in the range
of 0.02-0.035.
weld metal oxide inclusions) is known
by researchers. Properties such as weld
metal plasticity and toughness are
severely affected.
The effect of oxygen is clearly shown
during negative temperature testing of
weld metal (Fig. 1), i.e., submerged arc
welding (SAW) of 16 G N M A and
15kh2NMFA steels, despite the difference in their ferrite strength. It is obvious
that the susceptibility of weld metal to
brittle fracture depends on its oxygen
content, mainly in the form of fine dispersed oxide inclusions. Similar to the
welds on low-carbon steels, the amount
of oxygen calculated by nonmetallic
inclusions (obtained by the metal electrodissolution method) is always less
than defined by the hot extraction procedure (vacuum processing). This is
explained by the fact that, in the vacuum
melting method, the total amount of
oxygen contained in solid solution is
defined and during the electrodissolution, partial loss of fine inclusions is
likely to occur. The latter accounts for
the fact that the major part of weld
metal inclusions are dispersed (less than
0.03 pm).
Electron-microscopic analysis of
weld metal revealed that some fine dis-
KEY WORDS
Introduction
Low-alloy steels and low-carbon
steels are a widely used group of steels.
When welding the steels, the main
problem encountered is protection of
the weld pool from the environment.
The detrimental effect of oxygen on
weld metal properties (in the form of
N. N. POTAPOV is a Welding Engineer and
Head of Laboratory, Welding Dept., Scientific Industrial Corp. of Machinery Technology, Moscow, Russia.
Submerged Arc Welding
Low-Alloy Steel
Oxygen Content
Weld Metal Properties
Weld Metal Oxides
Brittle Fracture
Microstructure
Inclusions
Silicon Reduction
Weld Metal Plasticity
persed nonmetallic inclusions are
located along grain boundaries (crystallites) (Fig. 2) (Ref. 1). The pattern and
character of nonmetallic inclusions distribution for each metallurgical version
are different. At the same time ,it was
established that the increase of oxygen
content does not result in a considerable change of ferrite grain boundary
condition.
An increase of weld metal susceptibility to brittle fiacture with the growth
of its oxygen content might be conditioned, in addition, by macroscopic slag
inclusion precipitation along the grain
boundaries, promoting crack initiation
in deformed metal and oxide inclusions in ferrite, which serve as stress
concentrators.
When a stressed state is developed in
a metal at the moment of applying an
external load, oxides promote crack
propagation.
The plasticity and toughness are influenced not only by the amount of nonmetallic oxide inclusions, but also by
their shape. From this point of view,
inclusion globular shape is most favorable. High melting inclusions of irregular angular shape result in greater plasticity and less toughness (Ref. 2).
It is shown that A l 2 0 3 based highmelting inclusions cause a greater
decrease of weld metal plasticity and
toughness than do equal amounts of
quarts glass-based fine dispersed inclusions (Table 1). The latter is likely to be
associated with the fact that aluminosilicates of irregular angular shape serve
as stress concentrators at the moment of
applying an external load, compared
with globular inclusions in the adjacent
metal. Researchers acknowledge there
is a detrimental effect of oxide inclusions
on weld metal plasticity and toughness.
However, there is also an opinion that
it is not oxide inclusions that produce a
major effect, but the amount of alloy elements in a weld metal, specifically, sil-
W E L D I N G RESEARCH SUPPLEMENT I 367-s
HCU,
}.lor?
tOhO,QOi%^t >
200
a ^* 'y^y^'
:
)
•*m--o,Q3%
160
XAA^
fa/^S^
120
H0hO$i3%
r
A> / /
[QMJ023%/jr/&
[OHO'92%
by a free-silicon welding wire appears
to be higher than by a killed steel
welding wire.
Insufficient silicon reduction from a
flux (0.25-0.35%) causes a considerable
decrease in brittle fracture strength.
While the weld metal alloying by a silicon welding wire (up to 0.5 Si) reduces
ferrite plasticity and toughness, its
strength is slightly increased (Ref. 3).
A^>
\
40
ft
A
t33
233
/ *
273
S
^[OH.12%-
313
353 K
A
KCV:
293K
B
Fig. I — Plasticity of variable oxygen content weld metal obtained on 16GNMA (A)
and 15Kh2NMFA (B) steels versus test temperature.
icon and manganese, with submerged
arc welding. This is substantiated by
Culyaev's data (Ref. 3) on ferrite
strengthening by the addition of various
alloys
In some cases, free-silicon wires are
used for welding low-alloy steels under
silicon-containing fluxes to provide a
minimal silicon content in a weld metal
and increase its brittle fracture strength.
This results in the silicon reduction
process in a weld pool being intensified.
The weld metal oxygen content obtained
Experimental Procedure
Taking
into consideration
the
assumptions mentioned above, it has
become necessary to obtain a differentiated assessment of the effect of silicon
and oxygen content in the form of the
weld metal's nonmetallic inclusions on
susceptibility to brittle fracture. Another
reason silicon reduction is presently
under investigation is that an increment
of 0.1 %Si in a weld results in an increase
of 2.5 times the oxygen content c o m pared with an increment of 0 . 1 % M n .
In order to evaluate the susceptibility
of a material to brittle fracture stress
intensity, factor K|C is defined by a crack
profile. However, such tests are laborintensive since the testing procedure
requires the use of numerous samples.
At the same time, it is suggested (Ref. 4)
that material susceptibility to brittle fracture be evaluated by critical crack
opening displacement value 8C. The
report shows the validity of obtaining 8C
on fewer specimens using empirical
dependence.
8c = 0 , 8 f p | ( b - l o ) / L
(1)
where:
fp| = sample bend beam plastic
component at the moment of
crack initiation
b = sample cross-section height
l 0 = initial crack length
L = faleum distance at specimen 3point concentrated bending
According to the recommendations
(Ref. 4), 12-mm cross-section samples
with L = 20 mm, l 0 = 2.4 mm were used
for the investigation. Initial fatigue crack
of the given length was originated on a
resonance vibrator. Figure 3 shows the
scheme of a testing procedure for evaluating metal susceptibility to brittle
fracture by critical crack opening displacement 5C.
Samples were welded with Sv 10KhNMA wire ( 0 . 1 % C; 0.16% Si;
0.98% M n ; 1.75% Cr; 1.35% N i ; 0.68%
Mo; 0.006% S; 0.01 % P). After welding,
the metal was subjected to double tempering: 650°C - 5 h + 700°C - 1 0 h.
Two flux series were used for multilayer welding. The first series was seven
fused fluxes of variable S i 0 2 content,
which allowed variation of weld silicon
content in the range of 0.2-1.0 % and
oxygen content in the range of
0 . 0 1 5 - 0 . 0 8 % . The second series was
five aglomerated fluxes of variable ferrosilicon content. The slag system (55%
CaF 2 , 4 0 % A l 2 0 3 , and 5% MgO) served
as the basis for the aglomerated fluxes.
All the components in the sintered fluxes
were in the form of chemically pure
compounds. Ferrosilicon was added to
the fluxes at the expense of decreasing
CaF 2 content. In order to minimize all
redox reactions in the fusion zone, 4 0 %
of natrium aluminate by dry charge mass
was used as a bonding agent.
By using the fluxes, we obtained a
similar silicon content increase in a
weld metal, w h i l e the oxygen content
was approximately the same ( 0 . 0 2 5 0.035%).
The results of the analysis of
composition and gas concentrations in
m u l t i l a y e r w e l d metal using fused
fluxes of variable reactivity are given in
Table 2, and under aglomerated fluxes
in Table 3.
Figure 4 shows the dependencies of
critical crack opening displacement, 8 t ,
on the method of introducing silicon into
the added metal. The dependencies indicate that the increase of a w e l d metal
silicon content in submerged arc welding with fluxes of increasing silica reactivity (curve I) results in a lower 8C
parameter when compared with the
^
••
-•>.'
U,-5,4!5,
:
«i»il
' .t
y i *.
i
••••
••
&
J & \
Fig. 2 — Pattern of nonmetallic inclusion distribution along grain boundaries of multilayer weld metals obtained on I6GNMA steel, with oxide
inclusions content of A— 0.22 %, B — 0.051%, and C — 0.012%, 10,O00X.
368-s I A U G U S T 1993
1*46
Tablel — Toughness of 16C NMA Steel Weld Metal With Variable Oxide Inclusion Contents
Type of
Nonmetallic
Inclusions
0.015-0.02
0.025-0.03
0.035-0.04
0.055-0.06
0.075-0.08
202-239
218
229-268
246
178-191
180
193-229
215
132-154
141
185-228
80-94
85
110-141
125
69-87
70
69-93
78
Aluminosilicates with
AI2O3 prevailing
Silicates
212
i
A
Toughness KCU, y / c m 2 Inclusion Content, %
VP
Note: Minimum and maximum values are given in the numerator; average in the denominator.
Table 2 — Results of Chemical and Gas Analysis of Multilayer Weld Metal Obtained Using
Fused Fluxes
Element Fraction
of Total Mass in
Weld Metal, %
Weld Metal Gas
Content
of
Weld
C
Si
Mn
Cr
Ni
Mo
O'
[O]
[N]
1
2
3
4
5
6
7
0.071
0.069
0.065
0.067
0.063
0.072
0.068
0.2
0.28
0.38
0.5
0.61
0.85
0.96
0.78
0.87
0.9
0.98
0.96
1.09
0.76
1.68
1.72
1.7
1.67
1.64
1.59
1.58
1.28
1.3
1.35
1.32
1.28
1.29
1.28
0.57
0.59
0.56
0.59
0.59
0.6
0.57
0.04
0.12
0.22
0.34
0.45
0.69
0.8
0.015
0.02
0.032
0.049
0.058
0.072
0.08
0.019
0.016
0.017
0.014
0.016
0.016
0.018
[Si]
.
Fig. 3 — Schematic of sample critical crack
opening displacement
testing. (A) — total
sample length, B — sample loading.
5c
0,8
06
0,4
0,2
Table 3 — Results of Chemical and Gas Analyses of Multilayer Weld Metal Obtained Using
Ceramic Fluxes
0,2
Element Fraction
of Total Mass in
Weld Metal, %
of
Weld
C
Si
Mn
Cr
Ni
Mo
1
2
3
4
5
0.073
0.076
0.075
0.079
0.077
0.2
0.35
0.45
0.73
0.98
0.75
0.68
0.75
0.76
0.82
1.7
1.68
1.75
1.7
1.62
1.48
1.48
1.55
1.5
1.3
0.57
0.54
0.58
0.56
0.54
[Si]
0.04
0.19
0.29
0.57
0.81
.
Weld Metal Gas
Content •O
[O]
[N]
0.035
0.032
0.03
0.028
0.024
0.018
0.021
0.02
0.019
0.017
0,k
w i t h aglomerated fluxes this phen o m e n o n is n o t o b s e r v e d ; t h e s i l i c o n
c o n t e n t is t h e s a m e . T h e r e f o r e , it m a y
be s u p p o s e d that c r i t i c a l c r a c k o p e n i n g
d i s p l a c e m e n t v a l u e 8 C decrease is assoc i a t e d , in the g i v e n case, w i t h a l o w w e l d
metal o x y g e n c o n t e n t .
Discussion
Indeed, oxygen effect on impact
t o u g h n e s s is c o r r e l a t e d w i t h t h e i n f l u e n c e of o t h e r a d d i t i o n s (Ref. 5). It is
established that w e l d metal oxygen
c o n t e n t d e c r e a s e results in its i m p a c t
toughness increase.
T h e a n a l y s i s o f t h e g i v e n data
revealed a p e c u l i a r p h e n o m e n o n : w h e n
submerged arc w e l d i n g w i t h relatively
passive fluxes (1 a n d 2, see T a b l e 2), a n d
the w e l d metal s h o w s a m i n i m a l o x y g e n
content and silicon reduction from a
flux-slag, a 8 C v a l u e decrease is o b s e r v e d
— Fig. 4 . M i n i m u m s i l i c o n c o n t e n t fails
to a c c o u n t for the given p h e n o m e n o n
b e c a u s e w h e n s u b m e r g e d arc w e l d i n g
T h e a u t h o r s o f Refs. 6 a n d 7 e x p l a i n
the d e t e r i o r a t i o n of a y i e l d plasticity w i t h
o x y g e n c o n t e n t l o w e r e d t o less t h a n
0 . 0 2 % by the type and shape of sulfide
inclusions and l o w melting point eutectics b e i n g f o r m e d , w h i c h consist o f s i l i c a t e s , s u l f i d e s a n d o x y s u l f i d e s . For
example, oxide and carbide suspended
particles in a w e l d p o o l p r o m o t e an early
p r e c i p i t a t i o n o f s u l f u r m e l t , r e s u l t i n g in
c o m p o u n d oxy- or carbosulfide i n c l u -
0,8&[SL]
Fig. 4 — Critical crack opening displacement
5C vs. the method of adding silicon to submerged arc weld metal. A — by silicon reduction process, B — by alloying metallic additions (ferroalloys) from an aglomerated flux.
i[Sj
s^
?o
-az
s e c o n d case. In the first case, the s i l i c o n
r e d u c t i o n p r o c e s s is a c c o m p a n i e d b y
total oxygen c o n c e n t r a t i o n increase,
w h i l e in t h e s e c o n d c a s e , a r e l a t i v e l y
c o n s t a n t o x y g e n c o n c e n t r a t i o n is p r o v i d e d . The o r d i n a t e v a l u e differences for
1 and 2 dependencies under similar
s i l i c o n - c o n t e n t m a y be r e l a t e d b y c o n v e n t i o n to a d e t r i m e n t a l effect of o x y g e n
in t h e f o r m o f n o n m e t a l i n c l u s i o n s —
Fig. 5.
0,6
-0A
-OS
-OS
w
am
02
at
006
ae
aos
[0]
as-
if&j
Fig. 5 — Diagram of differentiated
oxygen
and
silicon
concentration
effect
on
l5Kh2NMFA weld metal critical crack opening displacement value decrease. A — A I5cj
= f(A ISIJ); B — A [SJ = flOl; C — AI5CI =
f(A[Sil;[0]).
s i o n s (Ref. 6). If t h e r e are n o s u c h i n c l u s i o n s , s u l f u r is p r e c i p i t a t e d f r o m t h e
m e l t at a later s o l i d i f i c a t i o n stage, w h e n
t h e l i q u i d is s a t u r a t e d w i t h s u l f u r as a
result of its segregation. In this case, f i l m s
WELDING RESEARCH SUPPLEMENT I 369-s
or chains of ferromanganese sulfides are
generated. Here not only the total
oxygen content is important, but also
the amount of ferromanganese oxides
with the highest sulfur solubility.
K. V. Lywbavsky explained the
increased susceptibility of high purity
weld metal to brittle fracture by flakes,
w h i c h appear clue to a peculiar
hydrogen behavior in a dense metal. It
was determined that a metal purity
increase leads to a decrease in the
amount of oxysulfide type nonmetallic
inclusions w h i c h generate hydrogen
concentrated voids.
Insufficient globular inclusion content results in hydrogen content increase. It may cause cold microcrack
initiation and weld metal brittle fracture
strength decrease.
Besides the data mentioned, it is
known that the decrease of al loy content
hampers y - a from promoting a balance
of low-temperature structures.
The analysis of multilayer weld metal
microstructures having similar composition but with different oxygen content
reveals that at an oxygen concentration
of less than 0.02%, the ferrite matrix is
a needle structure. Increased weld metal
oxygen content promotes the formation
of more balanced structures.
In the author's opinion, it is difficult
to give preference to one of the three
hypotheses mentioned above. Everything depends on specific welding
parameters, welding materials' composition and purity, weld metal alloying
system, and heat treatment. It is these
specific parameters that will define the
specific weight for each factor in a weld
metal embrittlement mechanism for a
weld with low oxide inclusion concentrations.
Practical Supplement
Therefore, an excessive increase in
weld metal purity from nonmetallic
inclusions in a low-alloy steel w e l d , as
assumed in some works, may fail not
only to justify the associated expenses,
but also cause additional problems.
In comparison with low-carbon
steels, low- and medium-alloy steels are
more sensitive to the total oxygen
amount; thus, the sensitivity increases
proportionally to the alloy addition
amount, upon which the material matrix
370-s I A U G U S T 1993
plasticity margin depends.
Taking into consideration that the
alloy addition effect follows the rule of
additivity, it is possible to define the
influence of each element on plasticity
compared to the carbon effect as
Ce = c + Mn/4 + Si/3 + Cr/1 0 + Ni/1 6
+ Mo/18 + Cu/6 + Ti/2
(2)
Analyzing the experimental data
resulted in an ultimate limit of weld
metal carbon content depending on the
given carbon equivalent, with the level
of initial weld metal toughness KCV 100
J/cm 2 being preserved — Fig. 6.
The given diagram may serve as the
foundation for defining the ultimate limit
of weld metal oxygen content in weld
cladding with low-alloy steel wire, provided the sum of alloy additions is not
more than 6%. However, it should be
taken into consideration that the coefficients of each element's effect on plasticity and toughness are obtained for a
weld joint cooling velocity of 2.3°C/s.
Therefore, it may be concluded that
the optimal oxygen content in weld
metal deposited on low-alloy steel is
mainly in the range of 0 . 0 2 - 0 . 0 3 5 % ,
w h i c h , in terms of nonmetallic inclusions, totals 0.045-0.065%.
Conclusion
Total submerged arc weld metal
oxygen content, with all other c o n d i tions being similar, is approximately
linear with the flux concentration of
oxides decomposed at low temperatures. Weld metal oxygen has the form
of fine dispersed inclusions oxides of
less than 0.03 p m ; silicon and manganese reduction processes bear a major
responsibility for their formation.
Differentiated evaluation of silicon
reduction process effects on low-alloy
steel submerged arc welding revealed
that the weld metal brittle fracture
strength depends considerably more on
total weld metal quartz glass-based
oxide inclusion content than on silicon
increment in a w e l d . In this case, the
main factor for an increase in weld metal
brittle fracture susceptibility is the
growth of weld oxide inclusion content.
Low-alloy steel w e l d metal oxygen
content in the range of 0.02 - 0.035%
was determined as optimal.
[01%
'//A
0,08
0M
OM
0.02
v//
////
•A
n
gp
vy/,
7/y,y/y
W
0,2
US'
OJ Ce,%
Fig. 6 — Weld metal oxygen content vs.
welding wire alloying level (at a cooling rate
not more than 2.3° C/s).
Increasing weld metal brittle fracture
susceptibility w i t h the growth of total
oxygen content in a weld up to more
than 0.035% is conditioned by ferrite
microscopic oxide inclusion precipitates
partially located along the grain boundaries. These inclusion's block the dislocations and serve as crack nuclei when
the metal is deformed.
Welds with lowered oxygen content
[0] < 0.02% also display the tendency
to decrease plasticity because: 1) ferritic-pearlitic matrix of improved purity
is likely to generate an unbalanced structure on cooling and, 2) when there are
no oxide inclusions, the shape of sulfur
and phosphor precipitation from the
melt changes from globular to film-like.
References
1. Potapov N. N. "Principles of flux selection in steel welding" M.: Mashinostroeniye,
1979, p. 168.
2. Potapov N. N. "Metal oxidation in
fusion welding". M.: Mashinostroeniye,
1985, p. 186.
3. Gulyaev A. P. Metallovedenieye., 5th
ed., reviewed., M.: Metallurgiya, 1977, p.
648.
4. New methods of metal brittle fracture
strength assessment. Transl. from Engl./Edited
by Rabotnov Yu. N. M.: Mir, 1972, p. 43.
5. Erokhin A. A. "Fusion welding principles" A., Mashinostroeniye, 1973, p. 448.
6. Podgaetsky V. V. "Nonmetallic inclusions in welds" M.: Kiev; Mashgiz, 1962,
p. 84.