SHIELDED METAL ARC WELDING

SHIELDED METAL ARC WELDING

Shielded metal arc welding (SMAW) is a welding process in which the welding heat is produced by an electric arc formed between the work surface and a solid,consumable, covered electrode.so arc generated between electrode and base metal. During the welding process, the decomposition of the electrode covering forms a shield of gas and slag around the molten metal on the work surface (see Figure 1). The electrode also provides filler metal to the weld pool.

fig 1

The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).

The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Arc and power source consist for this diagram. In this blog. metal arc welding process the positive pole of DC current is called as anode and negative terminal called as cathode. The arc should be generated between an anode (positive pole of DC current) and cathode (Negative pole of DC current) is connected to power source.

The ionized gas acts as high resistance conductor enable more ions to flow from the anode to cathode. The strikes of work piece, that is generated heat to the base metal gap that two conductors placed on a small distance (3mm).The current flow pass through the gaseous media, provide from of arc. That arc is change together, energy into heat and light. That arc welding process, 1 kwh of electricity generated 1000 J, the temperature at the arc is being 6000 to 7000⁰c.

NOTE

Shielded metal arc welding (SMAW) is the American Welding Society’s designation for this welding process. It is commonly referred to as stick welding, arc welding, or stick electrode welding.

It is also sometimes referred to as manual metal arc welding (MMAC).

Shielded Metal Arc Welding Applications

Shielded metal arc welding enjoys widespread use on farms and ranches, in auto repair facilities and home and equipment workshops, and in other areas where maintenance and repair work is required. It is still a widely used process in pipelining and structural steel construction. SMAW can be used to weld carbon steel, low-alloy steel, high-strength steel, cast iron, malleable iron, bronze, nickel, stainless steels, and aluminum in all thicknesses. The SMAW process is also used for hard surfacing

Some Useful SMAW Welding Tips

1. Select a proper electrode for the job in question.

2. Keep electrodes clean and dry. This is especially important for the low-hydrogen electrodes.

3. Thoroughly clean the base metal before welding. Completely remove all paint, grease, oil, moisture, slag, and any other possible weld contaminants by mechanical cleaning, chemical cleaning, or a combination of the two.

4. Use a drag technique for most applications.

5. Penetration: DCEN (least penetration), AC (medium penetration—often with more spatter than DC), DCEP (deepest penetration).

6. Use a beveled edge preparation and a multipass welding technique for materials 1/4 thick or thicker

SMAW Advantages

1- Equipment is self-contained, portable, and relatively inexpensive.

2- Electrode provides its own flux.

3- Most metals and metal alloys can be welded with SMAW.

4- Useful process for welding in confined spaces.

5- Performs better on unclean surfaces than other welding processes.

6- Most metal thicknesses can be welded with SMAW.

7- All welding positions are possible with SMAW.

8- Can be used under almost all weather conditions.

9- Arc is continuously visible to the welder.

10- Welder controls the arc.

SMAW Disadvantages

1- Not recommended for welding metals less than 1/16 thick.

2- Excessive spatter.

3- Slag cleanup is required.

4- Produces weld beads with rough surfaces.

5- Welds are subject to porosity.

6- Arc blow must be controlled.

7- Frequent stops and starts are required to replace electrodes, resulting in a lower electrode deposition rate than the GMAW (MIG) and FCAW wire-feed welding processes.

8- Greater possibility of weld defects as a result of frequent stops and starts.

9- Some electrode waste (about 10 percent from discarded stub loss).

10- Potential electric shock from open-circuit voltage.

11- Ventilation must be provided when welding in confined spaces. The SMAW process produces large amounts of fumes and smoke.

Types of Flux/Electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:

.. Cellulosic

.. Rutile

.. Basic

Cellulosic electrodes
contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the "stovepipe" welding technique.

Features:

- deep penetration in all positions

- suitability for vertical down welding

- reasonably good mechanical properties

- high level of hydrogen generated - risk of cracking in the heat affected zone (HAZ)

Rutile electrodes
contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features:

- moderate weld metal mechanical properties

- good bead profile produced through the viscous slag

- positional welding possible with a fluid slag (containing fluoride)

- easily removable slag

Basic electrodes
contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features:

-low hydrogen weld metal

- requires high welding currents/speeds

- poor bead profile (convex and coarse surface profile)

- slag removal difficult

Metal powder electrodes
contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

SHIELDED METAL ARC WELDING EQUIPMENT

A typical shielded metal arc welding system is illustrated in Figure 2. Its principal components are:
(1) the welding machine (power source);
(2) the covered electrode;
(3) the electrode holder;

and (4) the welding circuit cables.
Additional equipment may include shielding walls and light shields; jigs, fixtures, and positioners for securing the work; and a ventilation system when working indoors.

fig 2

Power Source

A constant-current (CC) power source is recommended for shielded metal. arc welding (see Table 1) For general purpose welding, a welder with an AC/DC output of 225 to 300 amps works best.

CAUTION

The voltage between the terminals of the welding machine when no current is flowing in the welding circuit is called its open-circuit voltage. Because the open-circuit voltage can be as high as 100 volts, the welder runs the risk of severe injury or even death from electrical shock.

Power Source Grounding Requirements

1- Ground the welder (power source) with a third wire in the cable connecting the circuit conductor, or by a separate wire grounded at the source current.

2- Ground circuits from the welder used for purposes other than welding tools.

3- Never bond either terminal of the welding generator to the frame of the welder.

4- Never use pipelines containing gases or flammable liquids for a ground- return circuit.

5- Never use conduits carrying electrical conductors for a ground-return circuit.

table 1

SMAW Welding Currents

Three types of welding currents are used in shielded metal arc welding: alternating current (AC), direct current electrode negative (DCEN), and direct current electrode positive

table 2

table 3

Electrode Holder

Electrode holders are clamping devices designed to firmly hold the electrode during the welding process. The electrode lead is commonly fastened to the end of the handle with a

lug, or to a mechanical connection within the handle. Electrode holders are manufactured in a variety of different types and sizes. The insulated pincher-type electrode holder is by far the most popular. The electrode holder size is designated by its current-carrying capacity

WARNING

Sometimes an electrode holder may become uncomfortably hot during welding. This is frequently an indication of a loose connection between the lead cable and the electrode holder. It can also mean that the electrode holder is the wrong size for the work.

NOTE

The manual electrode holder must be specifically designed for arc welding and be of sufficient capacity to handle the maximum rated current required by the electrodes

Welding Cables

Two cables are used to create the required electric circuit between the welding machine and the work. One of these cables is called the electrode lead and extends from the welding machine to an electrode holder. It forms one half of the electrical circuit. The other cable is called the work lead (or ground lead) and extends from the welding machine to the metal bench or the work itself. This cable forms the other half of the electrical circuit. A complete circuit is created by turning on the welding machine and bringing the electrode (in the electrode holder) in contact with the work.

table 4

table 5

SMAW ELECTRODES

SMAWelectrodes are solid (or cast) wire rods covered with a thick flux coating. The thickness and composition of the flux coating determines the electrode’s operating characteristics and the mechanical and chemical properties of the deposited weld. During welding the flux coating dissolves and produces a gas and slag material. The gas-and-slag shield protects the molten weld metal from contamination. When the weld deposit has cooled, the slag is removed.

Electrode Selection

It is most important to select the proper electrode for each welding job. The quality, appearance, and economy of the weld will depend upon correctly selecting the most suitable electrode (see Table 6). There are a number of factors that must be considered in the choice of electrod

1. Mechanical properties

2. Chemical properties.

3. Welding current

4. Welding position

5. Thickness of the metal.

6. Joint design

7. Welding passes

8. Joint position.

9. Working conditions

table 6

Electrode Mechanical and Chemical Properties

The mechanical properties and chemical analyses listed for the various electrodes will vary slightly among different manufacturers for each electrode class. Always consult the electrode manufacturer’s specifications when attempting to match the mechanical or chemical properties to the base metal. The mechanical properties (minimum tensile strength, yield point, etc.) will always be above the AWS minimum requirement.

The advantage of manual metal arc welding process than gas welding:

1- High arc temperature

2- Widely speed of weld operation

3- Concentration of heat in small are can be welded

4- Small amount of heat spread reduces bulking and wrap of work piece.

5- The heat concentration will be increase the depth of penetration

6- That is more economical than gas welding.

7- Multiple of metal can be welded

8- It control is simple and easy.

how to weld

Understand the process of shielded Metal Arc Welding. An electric arc is formed at the tip of the welding rod when a current passes across an air gap and continues through the grounded metal which is being welded. Here are some of the terms and their descriptions used in this article:

Learn the steps for creating a successful weld. Welding is more than dragging a welding rod across a piece of steel and gluing it to another one. The process begins with properly fitting and securing the work pieces, or metal to be welded, together. For thicker pieces, you may want to grind a bevel so subsequent beads can be placed in the groove to fill it completely with a solid weld. Here are the basic steps for completing a simple weld.

Gather the tools and materials you will need to begin welding. This means the welding machine, electrodes, cables and clamps, and the metal to be welded.

Set up a safe work area, preferably with a table constructed of steel or other non-flammable material.Prepare the metal to be welded.

Attach clamps to hold your metal pieces together

Attach the ground clamp to the larger piece of stock that is being welded

Select the correct rod and amperage range for the work you are attempting

Turn on your welding machine

Hold the stinger in your dominant hand by the insulated handle, with the rod in a position so that striking the tip of it against the plate you are welding will be as natural a movement as possible

Select the point where you wish to begin your weld

Strike the electrode against the surface of the metal, pulling it back slightly when you see an electric arc occurPractice traveling across the path of your weld with the electrode until you can keep a consistent arc, moving at a consistent speed, and in line with the path you want to weld