Preparing a square Butt joint through Metal Arc Welding on flat plates

 Objectives:

  • To acquaint the students with the procedure of metal arc welding and to know the working procedure of welding tools and safety equipments.
  • To develop the confidence in metal arc welding operation.

Theory: 

An electric arc is the form of electric discharge with the highest current density. An electric arc is a visible plasma discharge between two electrodes that is caused by electrical current ionizing gasses in the air. Electric arc occurs in nature in the form of lighting. Arc welding is a welding process in which, to form a molten pool of metal, heat is produced by an electric arc, generally without the application of pressure and with or without the applications of filler metal. It uses AC or DC for striking arc between the electrode and the workpiece. One terminal is connected to the electrode, the other to the workpiece and the circuit is completed through an air gap between the electrode and the workpiece. Here electricity travels from electrode to base metal to ground. An arc is generated when there is as electrical discharge that flows between two adjacent metal objects that are not actually touching each other. This discharge can be sustained through a path of ionized gaseous particles called plasma. The temperature generated is in the range of 35000C to 40000C at the surface of the arc. Electrical current involved in the generation of arc depends on the plate thickness. Ampere may vary from 40A to 400A depending on the plate thickness although voltages are quite low and in the range of 30 to 80V AC.

The electrode is coated with flux which helps

  1. To produce a shielding atmosphere around the arc.
  2. To remove the impurities.
  3. To prevent oxidation & uniform cooling.

Drawing of welding set up and job:

List of equipments used:

  1. Arc welding transformer with all its accessories.
  2. Electrode.
  3. Well insulated Electrode Holders
  4. Welding cables.
  5. Ground clamps
  • List of tools used:
    1. Steel rule
    2. Scriber
    3. File
    4. Wire brush
    5. Chipping hammer
  • List of safety equipments used:
  1. Welding helmet and hand screen or Shield
  2. Safety goggles
  3. Apron
  4. Gloves
  5. Tongs
  • Raw material: 

Two mild steel flat pieces of size 150 mm x 30 mm x 8 mm each. 

  • Working Procedure: 

Preparatory: 

  1. Collect and check the raw material for shape and dimensions.
  2. Collect welding and safety equipments.
  3. Check the specifications of the arc welding transformer.
  4. Check the specification of the electrode.
  5. Acquaint with the working knowledge of the welding process.
  6. Acquaint with the knowledge of the use of the safety equipments. 
  • Edge Preparation: 
  1. Edge preparation consists of removing material along edges of metal surface to achieve full welding penetration.
  2. To obtain sound welds, carefully clean the faces to be welded from dust, sand, oil & grease.
  3. File the edges and make them single Vee groove with angle 45° as shown in the above diagram. 

Operation:

  1. Keep the two prepared pieces on welding table with the prepared edges butting each other and carrying a proper root gap.
  2. Hold the two pieces firmly in perfect alignment and tack weld at both ends, as well as along the seam at some interval.
  3. Check and ensure that proper alignment and root gap are available after tacking. Correct them, if required.
  4. Set proper current and lay the weld bead using 3.5 mm diameter M.S Electrode.
  5. Do the welding at uniform speed and give wavy (side to side) movement to the electrode during welding in order to get better results.
  6. Clean the weld bead using Chipping Hammer and Wire Brush and check the penetration.
  7. Invert the work pieces and deposit the weld bead at other side of the work pieces.
  8. Do the welding at uniform speed and give wavy (side to side) movement to the electrode during welding in order to get better results.
  9. Use a chipping hammer followed by a brush, to clean the beads on both sides.
  10. Inspect the work piece visually to ensure that the beads are free of surface defects and distortion. Check the weld bead for proper penetration.
  • Electric current vs electrode size vs plate thickness:
Plate thickness

(mm)

Electrode size (diameter)

(mm)

Electrode current range

(Amp)

1.61.640-60
2.52.550-80
4.03.290-130
6.04.0120-170
8.05.0180-270
25.06.0300-400

 

Safety Precaution:

  1. Always wear proper protective clothing during welding.
  2. Use goggles during welding and cleaning to protect eyes.
  3. Wear apron which will not burn or melt due to molten droplets of metal during welding.
  4. Maintain proper arc length.
  5. Always use correct current and correct electrode size.
  6. Never handle the hot job by hand. Use tongs for this purpose.
  7. Welding machine should not be operated without the working knowledge of the machine.
  8. Welding shop should be well ventilated to reduce fumes and hazardous gases generated during welding.

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