Study on Weld Pore Sensitivity of
Self-Shielded Flux-Cored Wire
JOINING OF MATERIALS
Ms. SHAHEEN PARWEEN
GROUP MEMBERS:-
ADNAN AHMED
MM-05
MUHAMMAD MOHSIN JAFFRI
MM-13
MUHAMMAD ZAFAR ULLAH KHAN MM-14
HUMAIR AHMED
MM-25
MUHAMMAD KAMRAN BHATTI
MM-32
MUHAMMAD FAIZAN
MM-34
Study on Weld Pore Sensitivity of Self-Shielded
Flux-Cored Wire
Q. Wei1, Q. Hu1, F. Guo2, and D.J. Xiong1
ABSTRACT
Porosity in weld metal refers to the cavity type discontinuities formed by gas
entrapment during solidification. It is one of the most common defects for self-shielded
flux-cored arc welding process. In the present study, the causes and the influence factors of
pore formation in the weld of self-shielded flux-cored wire (SSFCW) were investigated.
The results of this study reveal that not only nitrogen content is the main influence factor
as well known for formation of porosity in the weld of SSFCW, but also oxygen can
rapidly increase the quantity of pore in the weld of SSFCW. Aluminum and RE effectively
reduce the pore sensitivity of the weld. Aluminum has a stronger ability of reducing
nitrogen than RE. Aluminum and RE significantly reduce the oxygen content in the weld.
INTRODUCTION
Self-shielded flux-cored welding has become popular for many applications mainly
because of its simplicity of operation that results from the absence of the equipment
necessary for gas shielding. In addition, the electrode holder is simpler than that required
for use with auxiliary shielding gas. Therefore, the development of all varieties of the selfshielded flux-cored wires (SSFCW) has been an area of great manufacturing and research
interest.
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It is well known that porosity in weld metal is one of the most difficult problems to be
resolved when developing SSFCW. Since there is no outside protective atmosphere and
welding flux provided in welding area, liquid weld metal can be easily contaminated by air.
Pore sensitivity of weld thus increases and is difficult to be controlled. In production
practice, the porosity appears frequently in the weld of SSFCW, which seriously impairs
the quality of weld metal and influences the extensive usage of SSFCW5-6. There have been
many research efforts to investigate the causes and prevention of pore formation in the
weld metal of SSFCW. The effects of metallurgical reaction5-11 as well as welding
parameters7,
12-15
on the formation of weld porosity have been the two major research
emphasis. Various models and mathematical/thermodynamical evaluations of pore
susceptibility were also presented in an effort to clarify the mechanism for pore formation
8,11,16.
Usually people consider that the nitrogen is the determinative factor for formation
of pore in the weld of SSFCW5-6,
8-11.
But many research results demonstrated that
porosity could not be well controlled at mentioned condition by reducing nitrogen content
in the weld metal only17-18. Therefore, it is imperative that the mechanisms and influence
factors of the formation of porosity in SSFCW weld be comprehensively studied. Based on
this point of view, the present study is focused on the investigation on other influence
factors of the pore formation in addition to nitrogen content in the weld. The effects of
simulative gaseous flow, oxygen content in the weld, as well as aluminum and RE alloy
addition in the flux core on the pore sensitivity are presented.
EXPERIMENTAL METHOD
The experimented SSFCWs consists of a thin low-carbon steel sheath surrounding a
core of fluxing and alloying materials. The cross section of the wire takes the shape “O”
with 2.0 mm in diameter, as illustrated in Fig. 1.
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In order to test the pore sensitivity under different gases and the resultant nitrogen
and oxygen content in the weld metal of SSFCW, simulative gaseous flow with different
composition (pure argon, pure oxygen, CO2, air and mixed gases with different proportion
of oxygen and argon, nitrogen and argon, oxygen and nitrogen) were added “shielding”
the arc area. Same batch of experimental SSFCWs was used in preparing weld metal
specimens.
The investigation on the ability of Aluminum and RE for controlling nitrogen and
oxygen content and thereby reducing the pore sensitivity were also performed by changing
the content of aluminum and RE in the flux core of the experimental SSFCWs.
The weld metal specimens were prepared by welding two no groove, low-carbon steel
plates (150mm×100mm×10mm) with single-pass butt welding. The amount of surface
pores and inner pores in 100mm length cooled weld was examined by both visual
estimation and X-ray inspection. The experimental setup and the basic dimensions of the
base metals are also illustrated in Fig. 1.
RESULTS AND DISCUSSION
Effect of Different Shielding Gases on Porosity in Weld
Experimental results show that no pores existed in weld metal welded in the welding
atmosphere of pure argon or pure CO2. A small amount of pores existed in weld metal in
the welding atmosphere of pure oxygen, pure nitrogen, mixed gas of oxygen and argon,
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mixed gas of nitrogen and argon, and air. These results indicate that the experimented
SSFCW has the capability of gas-slag protection, deoxidization and controlling nitrogen
and hydrogen, as reported previously. It is worth mentioning that the amount of pores
increases rapidly under mixed gas of oxygen and nitrogen. The amount of pore reaches
extreme value at the content of 80% oxygen-20% nitrogen. On both sides of the extreme
value, oxygen or nitrogen can rapidly enhance the porosity produced in weld metal with
the increase of oxygen or nitrogen content in the welding atmosphere. To be more specific,
the number of pores increases with the increase in oxygen content up to 80% and increase
in nitrogen content up to 20%. The actual welding atmosphere of SSFCW is air. The
mixed proportion of oxygen and nitrogen in the air falls in the left side of the extreme
value, where the weld porosity increases with the increase of oxygen content in the arc area.
In other words, oxygen can enhance the possibility to produce pores in the weld metal of
SSFCW. Therefore, it is equally important to prevent nitrogen from coming into the arc
area and to reduce the oxygen content in weld metal to obtain a well controlled porosity
level in the weld of SSFCW.
It was observed that some weld pores of SSFCW are similar to very small acerose
pores of nitrogen, while some are similar to wormholes of CO. Some round pores were also
noted in the weld metal. Therefore, it is considered that the weld pores of SSFCW are not
simply N2 pores or CO pores, but a mixture of both N2 pores and CO pores, and even more
complicated combination pores of CO and N2.
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Effect of Deposited Oxygen and Nitrogen Content on Weld Metal Porosity
As stated above, both oxygen and nitrogen content in weld metal are highly responsible for
the formation of weld pores.
In order to determine the relationship between pore
sensitivity and the content of oxygen and nitrogen in weld metal, a large number of data
points with specific oxygen and nitrogen content in weld metal were collected and plotted
in Fig.2, which illustrates the possibility of pore occurrence at certain oxygen and nitrogen
level.
It is clearly shown that pores were only formed when the oxygen and nitrogen
content in weld metal are above the diagonal line. No porosity was evident in weld metal
whose oxygen and nitrogen content are below the diagonal line. Therefore, the higher the
oxygen and nitrogen content in weld metal are, the greater the pore sensitivity is. In
addition, by analyzing the experimental results from Fig.2, it was found that the average
nitrogen content of SSFCW weld metal in no porosity condition can reach 400-500ppm,
which is much higher than that of covered electrodes. However, the average oxygen
content of SSFCW weld metal can reach as low as 16ppm under the same condition, which
is much lower than that of covered electrodes. As a comparison, in no porosity condition,
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the nitrogen content in weld metal is ~150ppm in titania type electrode, and is ~100ppm in
low hydrogen type electrode. The oxygen content in weld metal of titania type electrode is
~650ppm, and is ~200-300ppm in low hydrogen electrode. Therefore, it is evident that the
weld metal of SSFCW is significantly affected by nitrogen in the air, which causes a
dramatic increase in nitrogen content. This high nitrogen content in weld metal of SSFCW
is notoriously difficult to be well controlled to a lower extent. Consequently, to prevent the
formation of pores in weld metal of SSFCW, the oxygen content should be much less than
that of the covered electrode to compensate the high nitrogen content, for example, less
than 100ppm, according to Fig.3. This is another experimental evidence that oxygen can
enhance the possibility of pore formation.
A possible mechanism of nitrogen and oxygen deposition in weld metal can be
explained using the interaction between nitrogen and oxygen in the welding atmosphere.
1
1
N 2  O2  NO
2
2
The product of the reaction, NO, can be easily absorbed on the surface of liquid weld
metal, and then can be decomposed into N and O under the action of its surface tension.
Meanwhile, the N and O atoms dissolve in liquid weld metal. Therefore the formation of
NO in the arc atmosphere ultimately increases both oxygen and nitrogen content in weld
metal.
Effect of Aluminum on Weld Metal Porosity
Aluminum is an effective element to reduce porosity in weld metal. The changes of
oxygen content and nitrogen content in weld metal with the percentage change of
aluminum powder in the flux core of the wire is evident. The decrease in both the oxygen
and nitrogen content is obvious with the increase of the aluminum powder in the flux core,
which proves that aluminum has a powerful capability of deoxidizing and reducing
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nitrogen content.
However, only limited amount of aluminum powder can be added to the flux core of
the wire due to its low density. Excessive addition of aluminum powder in the flux core will
cause fabrication difficulty and finally lead to bad processing properties of the wire. In
order to reduce the aluminum powder in the flux core to an optimum amount and to
simultaneously control the weld porosity, a new type of metal sheath containing aluminum
(H05Al) was used in contrast to the ordinary metal sheath (H08A). This new type of metal
sheath (H05Al) overcomes the problems related to the presence of large amount of
aluminum powder in the flux core. The changes in the amount of pores in weld metal with
the amount of aluminum powder in the flux core is illustrated in Fig. 3 for both H08A and
H05Al metal sheath.
Clearly shown is that the amount of pores was reduced with the increase of the
percentage of aluminum powder in the flux core of the wire for both metal sheath. It is
also found that the amount of weld pores was reduced to zero only if the percentage of
aluminum powder in the flux core reached 13% with H08A metal sheath. The content of
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residual aluminum in weld metal is 1.22 wt% in this condition. In contrast, when the wire
was fabricated with H05Al metal sheath, no pores appeared when the percentage of
aluminum powder in the flux core reached 8%. In this case, the residual aluminum content
in weld metal is only 0.84 wt%. The above comparison shows that the wire made by metal
sheath containing aluminum can increase the protective effect, and thereby decrease the
pore sensitivity. The substitution of H08A metal sheath with H05Al metal sheath
effectively reduces the content of residual aluminum in weld metal and consequently
improves its toughness. In addition, the fabrication process of the wire becomes much
more convenient due to the reduced amount of aluminum powder in the flux core.
Experimental results and analyses discussed above suggest that the mechanism for
aluminum to reduce pore sensitivity be based on: (1) Aluminum is a better deoxidant than
titanium, carbon, magnesium, or zirconium. Aluminum reduce oxidizability of welding
atmosphere at an earlier stage of deoxidization. The reaction formulae of deoxidization are:
2Al+3CO2=3CO+Al2O3
4Al+3O2=2Al2O3
(2) Aluminum can reduce the nitrogen concentration in weld metal by reducing O
potential and the NO partial pressure in welding atmosphere. (3) Aluminum and nitrogen
have a great affinity to react with each other. They readily react to form a stable aluminum
nitride, which does not dissolve in molten steel but in slag. Therefore, it can be seen that
the higher the aluminum content in liquid metal is, the less the oxygen and nitrogen
content in liquid metal will be, and thus less number of weld pores will occur in the weld
metal.
Effect of RE Alloy Addition on Weld Metal Porosity
The effects of three kinds of RE additives on oxygen content, nitrogen content, and
porosity in weld metal were investigated in this study.
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It is shown in Table 1 that the oxygen content and the porosity level decrease with the
increase of Ca-Mg-Ba-RE alloy addition in the flux core. The nitrogen content was not
affected by the change of alloy addition. Tables 1 also indicates that the porosity was not
eliminated completely and the oxygen content in weld metal was still high even with 9% of
the Ca-Mg-Ba-RE alloy addition in the flux core. An incomplete deoxidization process due
to the low content of RE in Ca-Mg-Ba-RE alloy (only 4.1%) may account for the high
residual oxygen content in the weld metal. RE metal mixture was shown to have a strong
ability of deoxidization. A small amount of RE metal mixture added in the flux core (only
1%) could significantly reduce the oxygen content and the pore sensitivity in the weld
metal. However, the RE metal mixture can be oxidized easily at room temperature, and is
extremely difficult to be smashed into powder form, thus could not be applied in industrial
practice. Compared with the above two kinds of RE additives, Ni-RE alloy was also shown
to have a strong ability of deoxidizing and reducing the pore sensitivity. The main
advantage of this alloy is that it can be easily added in the flux core of the wire during
fabrication. Therefore, Ni-RE alloy is suggested as the best RE additive among the three.
RE elements reduce pore sensitivity mainly by reducing the oxygen content in weld metal
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because they have a strong affinity with oxygen. The reaction product will eventually enter
into slag in the form of RE oxide (RE2O3). As a result, oxygen content and CO pore
sensitivity decrease dramatically with the increase of RE-content in the flux core. The
results in Table 1 also show that there is no meaningful change in the nitrogen content with
the change of RE alloy addition in the flux core, which indicates that the capability of RE
additives to fix nitrogen is weaker than aluminum.
CONCLUSIONS
The causes and influence factors of the pore formation in the weld of SSFCW were
extensively analyzed in terms of both welding metallurgy and processing variables.
Important conclusions drawn from the analysis are as follows.
(1) Porosity of SSFCW weld is related with both oxygen and nitrogen. Oxygen can
enhance the possibility to produce pores in weld metal. The weld pores of SSFCW are not
simply N2 pores or CO pores, but a mixture of both N2 pores and CO pores, and even more
complicated combination pores of CO and N2. To reduce oxygen and nitrogen potential in
welding atmosphere as well as the oxygen and nitrogen content in weld metal is the main
measure to reduce pore sensitivity.
(2) Aluminum and RE can effectively reduce pore sensitivity of the weld, because
both aluminum and RE have a strong capability to reduce oxygen content in weld metal.
Aluminum has much stronger capability to reduce nitrogen content in weld metal than RE
alloys.
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