Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a submerged arc weld~
ing process and more particularly, to a submerged arc welding
process suitable for structural members which are subjected in
use to low temperatures. More particularly, the present inven-
tion relates to a submerged welding process in which weld metal
is deposited in a plurality of superimposed layers.
Conventionally, vessels for s~oring liquefied gas such
as liquefied nitrogen, li~uefied oxygen or the like, are con-
structed from s-teel material by means of hand welding. Since
these vessels are subjected in use to extreme low temperatures
such as below minus 100C, it is very important to provide an
adequate impact-resistant property at such low temperatures.
From the view point of manufacture it is desi.rable to
automatically perform the welding operation using for example a
submerged arc welding process, however, conventional submerged
arc weld.ing processes have not been satisfactory for structures
which are subjected in use to low temperatures particularly
in respect of the toughness of the weld metal. Such conventional
submerged arc welding processes are conducted with a relatively
low thermal input which is expected as being effective to pro-
duce fine crystalline structure having a satisEactory impactresistance. However, it has not been possible to provide an
ade~uate impact resistance at low temperatures only through a
relatively low thermal input.
The present invention has therefore an object to pro-
vide a submerged arc welding process which can produce a weld
metal having a high impact resistance property at extremely low
temperatures.
Another object o-f the present invention is to provide
a submerged arc weLding process which is particularly suitable
for use in constructing vessels serviced at extremely low temp-
eratures.
A further object o-E the present invention is to provide
a submerged arc welding process in which weld meta:L is deposited
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in plurality of superimposed layers so that the metal in one
deposited layer receives thermal effect from the overlaid layer
to produce recrystallized fine structures.
~ ccording to the present invention, the above and other
objects can be accomplished by a submerged arc welding process
in which use is made of flux having a basicity as defined by a
formula CaiO M~O in weight percentage of between 1.5 and 3 and
which cornprises the step of producing a plurality of deposited
layers of weld metal under a welding current of 400 to 700A and
an arc voltage of 35 to 48V, each of said layers having a thick-
ness not greater than 7 mm so that the weld metal in an under-
lying layer is thermally affected by an adjacent overlying layer
whereby recrystallization is effected iIl substantial thickness
of the underlying layer.
Accordiny to the present invention, it is preferred to
use a bond type flux including on the basis of weight 10 to 30
percent of SiO2, 8 to 20 percent of A12O3, 25 to 45 percent of
MgO, 10 to 30 percent of CaO, 7 to 20 percent of CaF2 and at
least one selected from metallic Si, Fe-Si and Fe-5i-Mn in an
amount of 0 to 0.6 percent calculated in term of metallic Si.
Where the welding process is applied to a steel material contain-
ing a relatively high percentages, for example 3~5% nickel, it
is also preferred to use a welding electrode containing on the
basis of weight 5 to 25 percent of CaF2, 2.5 to 5.5 percent of
Ni, 0 to 0.5 percent of Mo, 0 to 0.5 percent of Ti and the bal-
ance substantially of Fe. ~-
Where the weld metal layers are deposited in staggered ~ ,
relationship, the transverse distance between the highest por-tion
of an underlying layer and that of an adjacent overlying layer
should not be greater than 5 mm. Further, it is also recommend-
able to maintain the thermal input ~0,000 J/cm.
The present invention is based on the findings that, in
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7~
a welding structure having a plurality of superlmposed layers of
weld metal, an underlying layer receives a thermal influence ~rom
an adjacent overlying layer when the latter is being deposited
so that the former is caused to produce a recrystallizecl fine
structure and that the fine structure can ef:Eectively be utilized
in obtaining an improved impact resistance at low temperatures
such as below minus 100C. Thus, the present invention is char-
acterized b~ the fact that the weld metal is deposited in rela-
tiveLy thin superimposed layers whereby only a relatively thin
portion in each layer remains thermally unaffected. According
to the present invention, such thermally unaffected portion in
each layer is less than 2 mm so that substantial part of the
layer is accounted for by the recrystallized metal of high impact
resistance.
In order to obtain a high impact resis-tarlce and an irn-
proved toughness, it is required to maintain the oxygen content
in the weld metal below 300 ppm., and this is accomplished
through a use of flux having the basicity greater than 1.5. Fur-
ther, a higher basicity is effective to decrease the Si content
in the weld metal. ~owever, since an increase in the basicity
has an adverse effect on the weldability, the value should not
exceed 3Ø
With respect to the flux, the aforementioned composi-
tion is recommendable for welding steel material containing high
percentages of nickel for the following reasons.
The SiO2 has an influence on the melting point of the
flux and where the SiO2 content is less than 10 percent there
will be an increase in the melting point of the flux so that ad-
verse efEects will be seen in the performance oE the welding
operation and also in the appearance of the welded beads. With
the SiO2 content exceedlng 30 percent, the SiO2 will be chemic-
ally reduced and there will be an increase in the Si content in
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the weld metal resulting in a poor toughness of the weld metal.
Th A12O3 ha,s an influence on the appearance of the
welded beads and an acceptable range is between ~ and 20 percent.
With the MgO content less than 25 percent, it will be difficult
to maintain the basicity at a desirable level but where the con-
tent ls greater than 45 percent the melting point of the flux
will be increased to an unacceptable level. The CaO content
should be greater than 10 percent in order to maintain the basic- ;
ity within the desired range but it will have an adverse effect
on the workability if it is increased beyond 30 percent.
The CaF2 content should be greater than 7 percent in
order to provide a satisfactory appearance on the welded beads.
Mowever, excessive addition of CaF2 causes an unstable welding
arc so that the content should be lower than 20 percent. In
order to maintain the silicon content in the weld metal below
0.20 percent, it is required to maintain the silicon content of
metallic Si, E'e-Si and Fe-Si-Mn in the flux to lower than 0.6
percent. Otherwise, there will be an adverse effect on the
toughness at low temperatures due to an increase in the silicon
content in the weld metalO As mentioned above, silicon contain-
ing deoxidizing agent may be metallic Si, Fe-Si, Fe-Mn-Si. It ~ :~
is of course possible to use a materia1 other than silicon as .
the deoxidizing agent. For example, manganese may be used ~or .:
the purpose. Thus, the flux of the present invention may contain
deoxidizing agent in an amount of 0.6% or less calculated in
terms of silicon. It is mentioned, the Si content in Fe-Si and
Fe-Si-Mn is not constant. For example, Fe-Si contains 20 to 70
percent of Si. Therefore, it is difficult to limit the amount
of Fe-Si per se, ~ecause the amount of Si contained in Fe-Si is
important to the flux.
Regarding the cored wire, it ha.s been found that the
CaF2 content should be greater than 5 percent to the total weight
4-
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of the wire. Otherwise, blow-holes are apt to be produced in the
weld metal and there wil]. be a decrease in the toughness. With
the CaF2 content greater than 5 percent, there is a remarkable
decrease in~the oxygen content in the molten metal so that blow-
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holes are prevented and the toughness is improved. However, the
CaF2 content should not exceed 25 percent because an excessive
CaF2 content makes the welding arc unstable and causes a poor
workability.
In order to ensure an adequate impact-resistant prop-
erty at minus 100C, it is necessary to maintain the nickel con-
tent in the core wire in an amount of higher than 2.5 percent,
however, where the content increases beyond 5.5 percent, it may
cause cracks under high temperature.
10 Molybdenum may be incorporated into the cored wire for
obtaining an increased strength of the weld metal but the content
shall not exceed 0.5 percent because it may have an ~dverse ef-
fect on the impact-resistant property at low temperatures where
the Mo content is above this value.
Titanium may be also incorporated into the cored wire
because it is effective to produce fine crystalline structures
which serve to provide an improved impact-resistant property
. .
under low temperature. However, the titanium may be omitted
because even when no titanium is contained it is possible to
. ~ .
obtain a satisfactory impact-resistant property around minus 100C.
Where the Ti content is greater than 0.5 percent, there will be
a decrease in the toughness due to an increase in the silicon
content in the weld metal.
Nickel, molybdenum and titanium in the core material
may be incorporated into the welding electrode in the form of
ferrous alloy thereof, for example, Fe-Ni, Fe-Mo, and Fe-Ti.
Fe-Ni, Fe-Mo or Fe-Ti may be incorporated into the electrode in
the above mentioned amount calculated in term of nickel, molyb-
denum or titanium. It is of course possible that nickel, moly-
bdenum or titanium may be added to the electrode in the form ofelementary metal.
According to the present invention, the process is
performed with a welding current of 400 to 700A, an arc voltage of
rd ~ ~
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35 to ~V and a thermal input less than 40,000 J/cm. With -the
welding current less than 400A, it becomes difficult to maintain
a stable welding arc, while the welding curren-t exceeding 700A
produces deposited layers having a thickness greater than the
desirable value. With the arc voltage less tharl 35V, there will
be an increase in the melting rate of the weld me-tal while the
voltage greater than 48V causes unstable welding arc. The limit
of the thermal input is necessary in order to maintain the fine
crystalline structures and the thin deposited layers of the weld-
ing metal.
The inventors also had found that under the aforemen-
tioned welding condition of the present invention, welding should
be preferably conducted at a velocity of 20 to 50 cm/minute to
provide deposit layer of a thickness less than 7 mm.
The features of the present invention will become more
apparent from the following descriptions which proceed making
reference to the accompanying drawings, in which:
Figure 1 is a fragmentary perspective view showing
steel plates which are being welded in accordance with the pre-
sent invention,
Figure 2 is an enlarged fragmentary sectional viewshowing an example of deposited layers of weld metal,
Figures 3A, B and C show examples of deposited layers
or beads of the weld metal, and
Figure 4 is a sectional view showing an example of a
welding groove formed in the material to be welded.
Referring to Figure 1, a pair of plates 1 and 2 Of
nickel containing steel such as 3.5% nickel steel are placed at
end-to-end relationship wi-th a substantially U-shaped groove 3
formed at the abutting portion. A su~merged arc welding is per-
formed under the aforementioned welding conditions using the
aforementioned flux and the aforementioned welding metal -to form
~,c~ -6-
77Z
a plurality of deposited layers 4 which are posi-tioned in super-
imposed staygered relationship. According to the present inven- ,
tion, the thic~ness T of each deposited layer 4 is less than 7
mm and in the illustrated staggered arrangement the offset or
traverse distance S between two succeeding layers is less than 5
mm. The term `'traverse distance`' herein used means a distance
between the centers or highest portions of two superimposed layers
as measured in the direction perpendicular to the thickwise
direction of the layer and perpendicular to the welding line
shown by a phantom line C.
Referring specifically to the layer 4a shown in Figure
1, it receives a thermal effect at the area designated by the
numeral 5 when the adjacent layer 4b is formed so that recrys-
tallization proceeds in the area 5 to produce a fine crystalline
structure. Further, when another adjacent layer 4c is deposited,
the metal in the layer 4a receives a thermal effect at the area
designated by the numeral 6 so that a fine crystalline structure
is produced in the area 6. Since the layer 4 is relatively thin,
the layer is substantially occupied by the recrystallized fine
structure and the thermally unaffected layer is 2 mm thick or less.
Thus, it is possible to produce a weld structure havlng a high
impact resistance at extremeLy low temperatures. Fiyure 2 shows
the deposited layers 4 of the weld metal in detai.l. In the draw-
ing, the numeral 7 shows the area where the metal has received
a thermal effect and recrystallization has proceeded.
Referring now to Figure 3, there are shown several ex-
amples of deposited'layer arrangements. More specifically, Fig-
ure 3A shows an example wherein layers 4 of the weld metal are
deposited one on another without any transverse offset, while
Figure 3B shows an example of staggered arrangement. It will be
noted that the latter arrangement is more preferable in respect of
recrystallization. In Figure 3C, it will be no-ted that a
transverse distance S bet.ween the highest
-- 7 --
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72
portion of a layer and that of a succeediny superimposed layer
has also an influence on -the area of thermally unaffected
portion 8. Thus, the inventors propose to maintain the distance
below S mm.
Examples:
Welding operations were performe~d on 3.5% Ni steel
meeting the specification of ASTM A-203 and having the dimension
as shown in Figure 4. In performing the welding processes, flux
materials have been prepared as shown in Table I.
TABLE I
CaOMgO SiO2 A123 CaF2 Basicity
_
A 20 29 38 10 3 1.3
B 20 25 25 20 10 1.8
C 16 26 21 20 17 2.0
D 16 37 21 14 12 2.5
Mote: The basicity is defined by the formula
B = iO ~ (in weight %)
Further, the welding electrode contained on the basis
of weight 10 percent of CaF2, 2.7 percent of Ni, 0.5 percent of
Mo and the balance of Fe a mild steel. The mild steel had a
composition of by weight, 0.05 percent of carbon, 0.50 percent
of manganese and the balance of iron. The weldlng operations
were performed as shown in the Table II and the welded speci-
mens were formed with U-shaped notches of 2 mm width at the
welded portions and subjected to Charpy impact tests at a temp-
erature of minus 101C. The results are also shown in Table II,
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In the above tests, it was investigated -that the speci-
mens 1 and 2 h~d relatively thic]c beads of cross-s~ction having
relatively small radius of curvature at the surface of each layer.
This is caused by an insufficient arc voltage. Thus, there was
a relatively large area of thermally unaffected metal. ~s the
resu]ts, they showed very poor impact resistance. In the speci-
men 2, it will be seen that the relatively large transverse
distance S had an adverse effect on the impact resi~tance.
In the specimens 3 and 4, it will be seen that exces-
sive welding current and thermal input have produced depositedlayers of excessive thickness. Therefore, it has not been pos-
sible to produce an adequate coverage of recrystallized fine
s-truc-tures. This tendency is particularly significant in the
specimen ~ because, in this specimen, the welding metal has been
deposited without transverse offset between two adjacent layers.
The specimen 5 was welded using the flux of lower basi-
city. Therefore, there was an excessive amount of residual oxy-
gen in the welded metal. The specimens 6 throug'n 13 which have
been welded in accordance with the present invention had therm-
ally unaffected areas of less than 2 mm thick. Thus, these speci-
mens had a satisfactory impact resistance.
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