What is Shielded Metal Arc Welding (SMAW)? πŸ‘¨β€πŸ­


Introduction

Coated electrode arc welding is a process that produces coalescence between metals by heating and melting them with an electric arc established between the tip of a coated electrode and the surface of the base metal at the joint being welded.

How the process works

The molten metal from the electrode is transferred through the electric arc to the molten pool of the base metal, thus forming the weld metal.

A liquid slag of lower density than that of the liquid metal, which is formed from the electrode coating and the base metal impurities, surmounts the molten pool protecting it from atmospheric contamination.

Once solidified, this slag will control the cooling rate of the already solidified weld metal. The filler metal comes from the metal core of the electrode (wire) and from the coating, which in some cases consists of iron powder and alloying elements.

Stick welding is one of the most used welding processes of all that we will talk about, due to the simplicity of the equipment, the quality of the welds, and the low cost of equipment and consumables.

It has great flexibility and welds most metals over a wide range of thicknesses. Lately it has been losing ground to other processes due to lower productivity (it takes longer to complete a weld in this process than with FCAW for example).

Welding with this process can be done almost anywhere and under extreme conditions. Coated electrode welding is used extensively in industrial fabrication, metal structure for buildings, shipbuilding, cars, trucks, floodgates and other welded assemblies.

The major disadvantage of this process is its low productivity, which is mainly due to the need to replace the electrodes.

Advantages and disadvantages

Advantages
Disadvantages
It is a versatile process, as it adapts to materials of different thicknesses and in any working position.It is a manual process, depending on the skill of the welder , which implies less control of welding parameters, such as welding current.
The necessary equipment is relatively low cost.
Its use is indicated both inside the factory and in the field.Compared to other processes, it has low productivity, due to its low deposition rate and low welder occupancy rate (time with the arc opened by the total welding time), which is around 40%.
Currently used in the naval, railway, automotive, metalworking and construction industries.
It is widely used for welding carbon steel, low alloy steel, stainless steel, cast iron, aluminum, copper, nickel, etc.
Low melting point metals such as Pb, Sn, Zn, and refractory or very reactive metals such as Ti and Zr are not weldable by coated electrode.Generates large volume of gases and fumes during the process.

Welding Equipment

As shown in the figure below, the equipment consists of a power source, connecting cables, an electrode holder, a clamp (earth connector), and the electrode.

Power source

The power source can be direct current (generators or rectifier) ​​or alternating current (transformer).

Direct current sources can be:

1 – Generators, generally used for work on construction sites, especially where an adequate electrical supply is not available.
2 – Transformers-rectifiers, which are advantageous in relation to generators due to the lower cost of operation and maintenance, with quieter operation.

In the case of direct current, the polarity must be chosen as required by the service:

a) Direct current – ​​direct polarity (DC-), the electrode is connected to the negative pole of the machine and there is a lower deposition rate and greater penetration depths.
b) Direct current – ​​reverse polarity (DC+), the positive electrode and the negative part. With this configuration, lower penetration and higher electrode melting rates are obtained.
c) Alternating current (AC) – the polarity alternating with each reversal of current. With this type of configuration, the bead geometry will be intermediate to that obtained in CC+ and CC-

Tip: The direct current sign always refers to where the electrode is connected. Some even prefer to call it positive electrode or negative electrode.

Welding cables

They are used to connect the electrode holder and clamp to the power source. They must be flexible to allow easy handling. They are part of the soldering circuit and consist of several copper wires wound together and protected by an insulating and flexible coating. The cables must be kept uncoiled and without splices.

Electrode holder

The electrode holder is used for fixing and energizing the electrode. Correct fixing and good insulation of the cables is essential so that the risks of shock are minimized. The claws must always be in good condition, in order to avoid problems of overheating and poor electrode fixation, which can come loose during welding.

Clamp (Ground Connector)

It is a device to connect the ground cable to the part to be welded.

Consumables – electrodes

The coated electrode consists of:
  • Electrode ('The soul')
  • Coating

The electrode establishes the arc and supplies the filler metal for welding.

The electrode coating has the following functions during the welding process: Electrical, Physical and Metallurgical Functions.

Electrical isolation and ionization function

a) Insulation – insulates the electrode core avoiding lateral arc openings, directing the arc to the place of interest. For example when welding narrow chamfers that are difficult to access, to prevent the arc from forming in unwanted positions.
b) Ionization – The coating contains Na and K silicates that ionize the arc atmosphere. This facilitates the passage of electric current, creating a stable electric arc.

Physical and mechanical functions

a) They provide gases for the formation of a protective atmosphere for the metal droplets against the action of hydrogen in the atmosphere.
b) The coating melts and then solidifies on the weld bead forming a slag of non-metallic material, protects the weld bead from oxidation by the normal atmosphere while the weld is cooling.
c) Provides control of the cooling rate and contributes to the finishing of the bead:

Metallurgical purpose

a) Introduces alloying elements into the weld metal, changing its weld properties. Other chemical elements are also added for the purpose of slagging impurities, deoxidizing, etc. (ex: Mn, Si)

Coated electrodes are classified according to AWS (American Welding Society) specifications. Commercial specifications for coated electrodes can be found in the AWS specifications of the AWS A5 series (Ex.: AWS A5.1)

Features and uses

It is important for a welding inspector to remember that the coated electrode welding process has many variables to consider. For example, it can be used in a wide variety of joint configurations found in industrial welding, and in a wide variety of base metal and filler metal combinations.

Occasionally, several types of electrodes are used for a specific weld. A welding inspector should have in-depth knowledge of the consumable specification used for the job to know how and what variables affect weld quality.

The coated electrode welding process can be used to weld in all positions. It can also be used for welding most steels and some non-ferrous metals, as well as for deposition of filler metal to obtain certain properties or dimensions.
Features the ability to weld base metal in a range from 2mm to 200mm, depending on heating or distortion control requirements and usage.

The control of welding energy (heat input) during operation is a relevant factor in some materials, such as quenched and tempered steels, stainless steels and low alloy steels containing molybdenum, being also of great importance for low temperature applications.

Inadequate control of welding energy during the welding operation, when required, can easily cause cracking or loss of primary base metal properties, such as loss of corrosion resistance in stainless steels or even a drop in energy absorption capacity.

The deposition rate of this process is small compared to other continuous feed processes. The deposition rate varies from 1 to 5 kg/h and depends on the chosen electrode.

The success of the coated electrode welding process largely depends on the skill and technique of the welder, as all welding manipulation is performed by the welder. There are four items that the welder must be able to control:
  • Length of the arc (varies between 0.5 and 1.1 of the diameter of the coated electrode). In layman's terms, we can consider the distance from the electrode tip to the part.
  • Electrode displacement and working angle.
  • Electrode displacement speed.
  • Pass deposition techniques (narrow or swinging pass).
  • Electric Current.

Preparing and cleaning joints

The parts to be welded must be free from oil, grease, rust, paint, residues from examination by penetrating liquid, sand and soot from gas preheating, in a strip of at least 20 mm on each side of the edges and demagnetized.

After each pass, the slag produced must be removed. Partial slag removal produces weld beads with oxide inclusions, compromising the mechanical properties of the welded joint. Lack of cleaning between passes generates the famous "slag inclusion" defect.

Usual parameters for welding steels

The table below shows an example of typical parameters by diameter for the Type 7018 coated electrode. The 7018 electrode is most commonly used in responsible works, however, consult a welding specialist for the most economical and productive electrode and parameters to use. Your work.

Process-induced discontinuities

The weld obtained by clad electrode arc welding can contain almost all types of discontinuities. Listed below are some of the more common discontinuities that may be encountered when this process is used.

Porosity

In general, it is caused by the use of incorrect techniques (long arc length or high welding speed), by the use of base metal without adequate cleaning or by a wet electrode.< /div>

Clustered porosity sometimes occurs at the opening and closing of the arch. The welding technique with a small backward pass, right after starting the welding operation, allows the welder to remelt the area at the beginning of the pass, releasing its gas and thus avoiding this type of discontinuity. Vermiform porosity is usually caused by the use of a wet electrode.

Inclusions

They are caused by improper handling of the electrode and poor cleaning between passes. It is a predictable problem, in the case of inadequate design in terms of access to the joint to be welded or even with small bevel angles.

Lack of Fusion

Results from improper welding technique: fast welding, inadequate joint or material preparation, too low current.

Lack of Penetration

Results from improper welding technique; fast soldering, improper joint or material preparation, current too low and electrode diameter too large.

Concavity and Overlap

They are due to errors (sloppiness) of the welder.

Cracks in the Throat and Cracks in the Root

When they appear, they demand, to be avoided, changes in the welding technique or change of materials.

Interlamellar Crack

This discontinuity is not characterized as a welder fault. It occurs when the base metal, not supporting high tensions generated by the contraction of the weld, in the direction of the thickness, cracks in the form of steps, located in planes parallel to the rolling direction.

Just for information, some engineers specify a "Z" tensile test to evaluate this material's resistance to this type of cracking.

Cracks at the margin and cracks under the bead

These are cracks, as we will see, due to cold cracking. They occur some time after the weld is performed and therefore may not be detected by an inspection performed immediately after the welding operation. They normally occur while hydrogen is trapped in the solder.

As an example of hydrogen sources, we can mention: high air humidity, wet electrodes, dirty surfaces.

This hydrogen, combined with a fragile microstructure and a sufficiently high level of residual stress, contribute to the appearance of these types of cracks.

Undercut (Resembles a bite)

High amperage (current), very hot part.

Environmental and individual protection conditions

Welding must not be carried out in the presence of rain and wind, unless the joint to be welded is properly protected.

The electric arc emits visible and ultraviolet radiation in addition to projections and harmful gases. For these reasons, the welder must be properly protected, using filters, gloves, protective clothing, safety glass and carry out welding in places with adequate ventilation.

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Materials: What is Shielded Metal Arc Welding (SMAW)? πŸ‘¨β€πŸ­
What is Shielded Metal Arc Welding (SMAW)? πŸ‘¨β€πŸ­
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