Shielded Metal Arc Welding(SMAW)

By Ayuba Isaac

IntroductionThis is one the arc welding processes that is

commonly used in construction/fabrication processes. It is also known with several names such as; MMA or MMAW, flux shielded arc welding or informally known as stick welding.

The electrode used in his process is consumable (a rod covered with flux).

The flux burns to form the protective layer/cover for the weld.

The current type used in this process can either be AC or DC depending on the design of the weld which will be stated in the WPS.

Because of the versatility and simplicity of this process, it is most generally and widely used across the world, it is mostly used for repair and maintenance in the heavy steel industry.

Working principleCurrent flows in through the cables (ground

cable and the hot cable) from the power source (AC/DC) which the circuit is completed when the electrode tip comes in contact with the surface of the work piece as will be seen in the diagram that will be displayed in later slide.

The heat is simply generated at the meeting point between the electrode an d the work pieces (arc).

The heat input can however be calculated using the formula H=[(60EI)/(1000S)] Kj/in

Working principle cont’dShielded metal-arc welding with the transformer

welding machine depends upon this fundamental fact: that when one side of the welding circuit is attached to a piece of steel, a welding electrode connected to the other side and the two brought into contact, an arc will be established.

If the arc is properly controlled, the metal from the electrode will pass through the arc and be deposited on the steel. When the electrode is moved along the steel at the correct speed, the metal will deposit in a uniform layer called a bead.

Working principle cont’dThe arc is started by bringing the tip of the

electrode into contact with the base metal (work piece) by a very light touch, hence this arc is maintained by keeping the electrode at a relatively close distance from the base metal. This arc length is usually 3mm - 4mm.

Melting of the base metal and the core wire of the electrode take place to form the weld pool. However, in some cases an external filler rod could be where large weld deposit is required.

Working principle cont’dThe flux/coating of the electrode melts away to form

the shielding layer (slag) that protects the weld pool from atmospheric contaminations as it solidifies. This slag are later chipped off to reveal the actual weld.

The penetration depth of the weld could either be a full penetration or partial penetration (also referred to as incomplete penetration).

Incomplete penetration in most cases are considered as defect but could be a requirement as stated in the WPS base on the service condition of the welded joint.

Below is a detail diagram that describes the operation of SMAW process.

Shielded Metal Arc Welding

SOME FACTS ABOUT SMAWSMAW is considered as one of the least efficient

welding process because the operator’s factor of chipping away slag and changing of worn-out electrodes during the process.

Actual welding technique utilized depends n the electrode, composition of the work piece, and the position of the joint to be welded.

The choice of electrode and welding position also determines the speed of the weld.

WELDING TECHNIQUESFor the purpose of this presentation; we shall

only highlight the level of difficulties in carry out this welding process in different positions;

For plates1F 2F 3F 4F 5F

1G 2G 3G 4G 5G For Pipes

Increasing order of difficulty....NOTE: there are other varying welding positions in

addition to the above mentioned.

Here is a detail diagram describing the various welding positions both on plate, fillet and pipe.

Defects associated with SMAWBelow are some of the common defects that

are associated with the SMAW process;A. SpattersB. PorosityC. Incomplete penetrationD. CracksE. Under cutsF. Poor fusionG. Poor weld profile, and so on...

EQUIPMENTS USED FOR SMAWSome key equipments used when carrying

out SMAW process may include the following;I. Power source or power supply (AC or DC)II. Cables (ground cable and ht cable)III. Electrode holder also known as welding

tongIV. Welding table (for workshop)V. ClampsVI. Electrodes

Below is a schematic diagram illustrating the setup of the SMAW equipments.

Other accessories in SMAWLeather glovesFace shieldChipping hammerWire brushCover allSafety boots/shoesNose mask in the absence of fume extractor,

and so on.However, all the above mentioned can simply

be referred to as personal protective equipment (PPE).

Process variation in SMAWThough SMAW is almost exclusively a manual

arc welding process, one notable process variation exists, known as gravity welding or gravity arc welding. It serves as an automated version of the traditional shielded metal arc welding process, employing an electrode holder attached to an inclined bar along the length of the weld. Once started, the process continues until the electrode is spent, allowing the operator to manage multiple gravity welding systems.

Process variation in SMAW cont’dThe electrodes employed (often E6027 or

E7024) are coated heavily in flux, and are typically 71 cm (28 in) in length and about 6.35 mm (0.25 in) thick. As in manual SMAW, a constant current welding power supply is used, with either negative polarity direct current or alternating current. Due to a rise in the use of semiautomatic welding processes such as flux-cored arc welding, the popularity of gravity welding has fallen as its economic advantage over such methods is often minimal.

Process variation in SMAW cont’dOther SMAW-related methods that are even

less frequently used include firecracker welding, an automatic method for making butt and fillet welds, and massive electrode welding, a process for welding large components or structures that can deposit up to 27 kg (60 lb) of weld metal per hour.

ElectrodeThe choice of electrode for SMAW depends on

a number of factors, including the weld material, welding position and the desired weld properties. The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidizers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality.

Electrodes can be divided into three groups—those designed to melt quickly are called "fast-fill" electrodes, those designed to solidify quickly are called "fast-freeze" electrodes, and intermediate electrodes go by the name "fill-freeze" or "fast-follow" electrodes. Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized, while fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying.

Generally electrodes can be classified into the following as seen in the table below;

Common electrodes include the E6010, a fast-freeze, all-position electrode with a minimum tensile strength of 60 ksi (410 MPa) which is operated using DCEP. E6011 is similar except its flux coating allows it to be used with alternating current in addition to DCEP. E7024 is a fast-fill electrode, used primarily to make flat or horizontal welds using AC, DCEN, or DCEP. Examples of fill-freeze electrodes are the E6012, E6013, and E7014, all of which provide a compromise between fast welding speeds and all-position welding.

Safety measures to be adhered to when carrying out SMAW may includeTo prevent injury to personnel, extreme

caution should be exercised when using any types of welding equipment. Injury can result from fire, explosions, electric shock, or harmful agents. Both the general and specific safety precautions listed below must be strictly observed by workers who weld or cut metals.

Do not permit unauthorized persons to use welding or cutting equipment.

Safety measures to be adhered to when carrying out SMAW may include cont’dBefore welding or cutting, warm those in close

proximity who are not protected to wear proper clothing or goggles.

Remove any assembled parts from the component being welded that may become warped or otherwise damaged by the welding process.

Do not leave hot rejected electrode stubs, steel scrap, or tools on the floor or around the welding equipment. Accidents and/or fires may occur.

Keep a suitable fire extinguisher nearby at all times. Ensure the fire extinguisher is in operable condition.

Safety measures to be adhered to when carrying out SMAW may include cont’dRemove all flammable material, such as cotton, oil,

gasoline, etc., from the vicinity of welding. Do not weld in a building with wooden floors, unless the floors are protected from hot metal by means of fire resistant fabric, sand, or other fireproof material. Be sure that hot sparks or hot metal will not fall on the operator or on any welding equipment components.

Mark all hot metal after welding operations are completed. Soapstone is commonly used for this purpose.

Hope you had fun,

See you next time…