کتاب Advances in Dissimilar Welding of Steel to Copper

different in their
chemical compositions, or are alloys of the same metal with different
metallurgical properties. Dissimilar metals can be base metals, filler metals, or
weld metals. Most dissimilar metals can join each other by the use of solid state
welding methods, brazing, and soft soldering. In these processes, only the
differences in physical and mechanical properties of the base metals and their
effects on serviceability of the joints must be considered. When the dissimilar
metals are attached by melting processes, alloying between the base metal and
the filler metal would have a high importance. The reason is that the obtained
weld metal can show different properties under mechanical and static stresses
during service in comparison to each of the base metals [1,2,3].
In joining metals with different chemical compositions, differences in
physical and chemical properties, result in the appearance of a whole lot of
problems during or after welding. This difference can be between the two
different base metals, or between the base metals and the filler metal due to
which, the chemical composition of the weld metal would be different from all
of its components. This difference varies considering the joint design, welding
process, filler metal and welding instructions. As a result, these parameters as
well as any kind of metal heat-treatment must be defined and evaluated properly
before production. The main purpose of dissimilar metal welding is to obtain a
joint which fulfills the requirements of working conditions [4-9].
In fusion welding of dissimilar metals, the most important point is chemical
composition and properties of the weld metal. The chemical composition of
weld metals depends on the chemical composition of the base metal, the filler
metal and dilution degree [10,11]. The composition of weld metal is not
uniform especially in multi-pass welds and the chemical-composition
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concentration differences are observed in the weld metal adjacent to each of the
base metals [10, 11]. When designing a dissimilar weld, basic alloying concepts
as well as physical and mechanical properties of the weld metal must be
considered [10,11]. If the two base metals develop a complete solid solution
(such as copper and nickel), joining them by fusion method would be totally
successful. On the other hand, if the two metals do not make a complete solid
solution, usually complex phases and intermetallic compounds are formed after
fusion welding which are usually brittle. The possibility of welding these metals
by fusion methods depends on the filler metal and welding instructions to define
how much the formation of such brittle compounds and intermetallic phases
must be prevented in order to have a joint with good and desirable quality [10-
13]. During fusion welding of dissimilar metals and mixing of base metals with
the filler metal, a weld pool is created after solidification which contains one or
more phases. The type, amount, and metallurgical order of the obtained phases
due to fusion joining define the properties and soundness of dissimilar joints.
Also, solidification and cooling rate have a considerable effect on the formation
of the present phases and the metallurgical structure of the metal. In the welding
of dissimilar metals, the filler metal must be selected in such a way that the
obtained weld would possess a uniform structure and benefit from a proper
flexibility [14,15,16].
In this study, different methods of fusion welding with stick electrode have
been discussed for joining austenitic 304L steel to pure commercial copper. As
the dissimilar austenitic steels joints to the pure commercial copper in oxygen
blowing lances and water-cooled molds have various applications, the purpose
of evaluating dissimilar welding of austenitic 304L steel to pure commercial
copper is to solve the problems ahead, to optimize the fusion welding process,
and to decrease the welding costs of oxygen blowing lances and copper watercooled molds.
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Chapter 2
The Effect of Alloying Elements on Weldability
An Introduction to Stainless Steels
Effect of Alloying Elements of Stainless Steels Weldability
An Introduction to Copper and Its Alloys
Effect of Alloying Elements on the Weldability of Copper and Its Alloys
2-1- An Introduction to Stainless Steels
Stainless steels are referred to alloys the chromium contents of which are
less than 10.5% and their carbon content is no more than 1.5%. Such alloys also
contain other elements. These steels are devided into five categories based on
their mechanical an microstructural properties [17, 18]:
1. Martensitic stainless steels
2. Ferritic stainless steels
3. Austenitic stainless steels
4. Precipitation-hardening stainless steels
5. Duplex (ferritic-austenitic) stainless steels [17,18].
Weldability of such alloys is diverse due to the differences in their phasetransformation behaviors at different solidification and environment
temperatures [19]. Most of the optimal and suitable filler metals for such steels
have a similar composition close to that of the base metal [20]. The welding
methods of different kinds of stainless steels are different from each other.
Therefore, in order to achieve a proper weld metal, a proper welding method
must be applied [19,20]. In the welding of stainless steels, filler metals must be
applied which have the maximum compatibility with the base metal. Noncompatibility between the filler metal and the base metal results in a reduction
in the weldability of such steels [22,23,24].
2-2- Effect of Alloying Elements of Stainless Steels Weldability
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Carbon: Carbon is the most important element in steels as it defines their
hardenability. As the carbon content increases, the steel gets harder. If steel with
a carbon content of more than 0.3% is welded and abruptly cooled, a brittle and
fragile zone is formed beside the weld [25].
1. Manganese: Manganese results in higher hardenability and tensile
strength in stainless steels. In any way, if the manganese content gets
higher than 0.6%, and especially if it is accompanied with high carbon
content, the weldability will definitely decrease. If the manganese content
is too low, internal porosities and cracks might expand. The best welding
results are obtained when the steel contains 0.4% to 0.6% manganese
[25,26,27].
2. Silicon: silicon is applied for improving the quality and tensile strength in
steels. High amounts of silicon, especially if accompanied by high carbon
contents, result in the development of cracks in stainless steel welding
[25,28].
3. Sulfur: sulfur is mainly added for improving machining properties of
stainless steels. Anyhow, its amount in other types of steels is kept low
(maximum 0.05%). The reason is that a high percentage of sulfur
increases the probability of crack formation in stainless steel welding
[29,30,31].
4. Phosphor: phosphor is considered as an impurity in steels. Consequently,
its amount is kept as low as possible. A phosphor content higher than
0.04% results in brittleness of the weld metal in stainless steel welding
[29,30,31].
5. Nickel, chromium, and vanadium have various effects on the weldability
of stainless steels. Stainless steel welding with considerable amounts of
nickel and chromium must be performed with caution. Usually,
preheating heat treatments and post-heating heat treatments must be
carried out at the weld zone in order to prevent the formation of hard and
brittle zones as well as hot and cold cracks [25,32,33].
2-3- An Introduction to Copper and Its Alloys
Cooper is one of the common metals which exist naturally in the
environment and humankind widely utilizes it. Copper is a reddish metal with
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FCC structure, and it is one of the most important non-ferrous metals. This alloy
is malleable and ductile and possesses excellent thermal and electrical
conduction as well as low chemical reactivity. Copper has a wide range of
wrought and cast alloys which have various applications in industry [34,35,36].
Table 1-2 presents copper alloys with their designations [24].