“Welded Construction, A Practical Approach”
You can design things we cannot build. During my career I have seen and
continue to see problems that occur during fabrication by welding because insufficient
consideration has been given in the design stage. What I hope to convey to you in the
short time I have is some of the things a designer must consider when designing a unit
that will be fabricated by welding. I will cover some of the processes available, how they
operate, their advantages and disadvantages. I will discuss the weldability of carbon
steels, Stainless Steels, and Copper and Copper Alloys and we will look at the capability
of most fabrication shops.
The American Welding Society identifies 39 welding and joining processes. I
will limit my discussion to 5 of the joining processes that you will probably use,
Shielded Metal Arc Welding, (SMAW), Gas Tungsten Arc Welding (GTAW) Gas Metal
Arc Welding(GMAW) , Electron Beam Welding (EBW) and brazing.
Welds are really small casting. The weld is cast into a groove which serves as a
mold. All welding methods have certain common features. There has to a source of
energy. There has to be some method of shielding the weld puddle from the atmosphere,
and in most cases there is a filler metal.
Shielded Metal Arc Welding uses a coated electrode to deposit weld metal. When
an arc is struck between the end of the electrode and the work piece the flux on the
electrode, which when dry is a non conductor of electricity, becomes a liquid slag and
becomes a very good conductor of electricity. As such it aids in the transfer of the arc.
Some of the flux decomposes and produces a gas, primarily CO and CO 2 , which helps to
shield the weld puddle from the atmosphere. There are agents in the slag that help to
purify the weld by bringing impurities and dissolved gases to the surface and into the slag
that will be removed later. The weld puddle is a turbulent thing. It rolls and boils. There
is a lot of magnetic stirring of the puddle. The flux (which becomes slag) contains
alloying agents that can affect the weld deposit. The magnetic stirring of the weld puddle
exposes a good deal of metal to the slag, and therefore the slag is a good vehicle for
adding alloy to the melt. As the weld cools, the slag remaining on the surface of the weld
protects the weld from the atmosphere minimizing oxidation of the weld surface, while
the thickness of the slag determines the contour of the weld surface. Electrodes can be
run on AC (Alternating Current), DCEP (Direct Current Electrode Positive) and DCEN
(Direct Current Electrode Negative) depending upon the make up of the flux coating.
There are shielded metal electrodes for a wide variety of metals. We can weld
carbon steels, high strength steels, stainless steels, Inconel, Monel CuNi, Copper and
copper alloys. Most fabrication shops have SMAW equipment and most welders are
trained in its use.
The advantages of SMAW are:
Simple equipment
Inexpensive
Very portable
Welds most alloys
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The disadvantages are:
It is relatively slow
Slag removal required
Electrode storage considerations
Gas Tungsten Arc Welding (GTAW) is a process where an arc is established
between a non consumable Tungsten or Tungsten alloy electrode and the work piece.
Shielding of the puddle is provided by an envelope of inert gas, typically Argon or
Helium. Filler metal, if used, is added to the puddle via various methods. The two most
commonly used electrodes are pure tungsten and 2% Thoriated Tungsten. Pure Tungsten
with AC is typically used when welding Aluminum. The 2% Thoriated Tungsten
electrodes are used with DCEN on most other materials. GTAW can weld any weldable
material. Most fabrication shops have GTAW equipment however fewer welders are
trained in its use.
The advantages of GTAW welding is:
It is good for thin materials
It produces high quality welds
It has good appearance
It is very clean
There is no slag
There is the potential for mechanization
The disadvantages of GTAW welding are:
It is relatively slow
Shielding gas is expensive
Purging gas is expensive
Gas Metal Arc Welding (GMAW)is a process where an arc is established between
a consumable wire electrode and the workpiece. The metal is transferred by four
methods. Spray transfer, Short Circuiting Transfer, Pulsed arc and globular. Shielding is
accomplished via a gas or gas mixtures. GMAW was originally invented using inert
gases, but as the process was developed active gases were added as well. Shielding gases
are Argon, Helium, CO2, Argon CO2, Argon O2 , along with other blends created for
special purposes.
Spray transfer produces an extremely high quality deposit. It is very hot, has a
very large fluid puddle and because of the large puddle is limited to the flat and
horizontal positions. This is a limiting factor for certain applications.
Short circuiting transfer uses a very low energy arc and as a result it can be used
in all positions. Because of its low energy it is poor on thick or on high thermal
conductivity metals. Because of its low energy it is prone to lack of fusion discontinuities
and is best suited for welding on thin carbon steels.
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Pulsed Arc transfer requires a power supply that is able to provide pulsed power.
The power supply is able to be set so that it will pulse at a certain rate and produce a high
quality spray transfer type weld. However because it is pulsing it reduces the size of the
weld puddle allowing this transfer to be used in all welding positions.
The final method of transfer is globular. This is not a high quality deposit and is
used in selective applications.
Most fabrication shops have GMAW equipment, however few have GMAW
equipment that has pulsing capabilities. Most welders have some training in the GMAW
process.
The advantages of the GMAW process are:
It is a semiautomatic process
It has high productivity
There is no slag to remove
It is a clean process
We have continuous filler metal feed
It welds most alloys
The disadvantages of GMAW are:
It is unsuitable for windy conditions
There is little tolerance for contamination
The equipment is more complex
The final welding process I’d like to discuss is Electron Beam Welding (EBW).
EBW is a fusion process that produces welding of materials with heat obtained by
impinging a beam of high energy electrons onto the joint to be welded. Electrons are
generated by heating a negatively charged cathode causing it to emit electrons which are
attracted to a positively charged anode. The beam is accelerated and focused
magnetically. EBW is usually done in a vacuum.
The advantages of EBW are:
Deeper penetration capability
High purity
High welding speeds
Ability to change weld shape
The disadvantages of EBW are:
A vacuum chamber is usually required
The vacuum chamber can limit assembly size
X rays are generated
It is very expensive
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There are not many EBW facilities and the operator must be highly skilled.
Brazing. In brazing the base metal is not melted. The difference between brazing
and soldering is temperature. Anything done above 840° F is brazing. Anything done
below that temperature is soldering. In brazing the base metal is not melted. To develop
high strengths braze joints must have large surface areas. Joint clearances are tight with
gaps typically less than 0.010”. The braze metal is drawn into the joint by capillary
action. Brazing can be used with or without fluxes, in vacuum, or in atmospheric
furnaces.
Now I’d like to talk about the weldability of some of the metals you may
encounter.
Carbon steels are readily fabricated although not with out some precautions being
taken. As carbon content increases more care has to be taken. The addition of preheat,
interpass temperature controls or heat input may have to monitored and controlled. High
Strength Low Alloy steels, and quenched and tempered steels offer more of a challenge
but can be welded with proper controls.
Austenitic stainless steels are readily welded. If the welds are subjected to a
corrosive environment additional controls may be required. We may have to monitor
ferrite levels, or select filler metals with minimum Molybdenum levels. We may have to
take steps to prevent sensitization of the weld. Duplex stainless steels can also be welded.
Copper and most copper alloys can be welded. All of the aforementioned
materials can be fabricated with the processes I have discussed.
I don’t want the comments I have just made on weldability of the metals to be an
over simplification of the situation. In reality it is much more complex. While I have
stated that most of the materials I have mentioned can be welded, keep this in mind. Very
small changes in alloy content can have very significant impact on the weldability of the
material. For example a change of less than one half of one percent of carbon can change
carbon steel from weldable to virtually non weldable All too often I have seen fabricators
get into trouble because not enough consideration was given to the weldabilty of the
particular metal or metals they have chosen.
More and more weldments are being constructed in accordance with some code
or specification. Many of those codes require that the fabricator demonstrate their ability
to produce a weld that will meet the requirements of the code. Test welds determine the
compatibility of the process to be used, the base metals and filler metals to be used, and
the welding conditions. Welding procedures are written to provide the welders with the
information they need to know to successfully accomplish the job.
A goal of the designer is to design the best product that will do the best job, and
by the most economical method. Welding is a complex process where many factors have
to be considered. I have been involved in the welding field for over 40 years. I certainly
don’t know all there is to know about welding, and I maintain a large source of reference
material and reference sources to help me solve welding problems. I would like to share
with you some of these sources that are available to you as well.
The American Welding Society is an excellent source for information on welding
and joining methods. The American Welding Society, headquartered in Miami, Florida
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publishes many codes, standards, and technical publications. A reference source I find
very valuable and that I use almost on a daily basis are the Welding Handbooks.
American Welding Society
550 N.W. LeJeune Rd.
Miami, Florida , 33126
Tel (305) 443-9353
The Edison Welding Institute is also an excellent source for information.
Headquartered in Columbus, Ohio the Edison Welding Institute was formed as a joint
venture between Batelle Memorial institute and The Ohio State University
Edison Welding Institute
1250 Arthur E. Adams Drive
Columbus, Ohio, 43221
Tel (614) 688-5000
The Copper Development Association in New York, New York is an excellent
source for information on copper and copper alloys.
Copper Development Association
260 Madison Avenue
New York, New York, 10016
Tel (212) 251-7200
When selecting materials for a weldment I encourage you to seek an expert
opinion from some one involved in the welding field . There are many Welding
Consultants available. I think you will find the money well spent.
A word about the fabricators out there. I have seen all too many fabricators who
did not have the equipment or expertise to do the job. I have seen a lot of fraud. Before a
contract is issued to a fabricator a quality audit should be conducted. This is good cost
prevention.
In summary, you can design something I cannot weld. Seek expert advice when
designing weldments.
Eugene G. Hornberger
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