Welding processes and practice
This domain examines arc welding, gas welding and common weld defects. It tests process selection, current and polarity, electrode condition, arc length, travel angle and defect causes.
Core rules
- Manual metal arc welding uses a covered consumable electrode. E6013 is a general-purpose rutile electrode: the 60 indicates about 60 000 psi tensile strength, 1 means all positions and 3 identifies the coating type.
- Set current to suit electrode diameter and material thickness. Typical ranges are about 60-100 A for 2.5 mm, 100-130 A for 3.2 mm and 130-180 A for 4.0 mm electrodes.
- Electrode positive direct current gives deeper penetration; electrode negative gives shallower penetration and is used for thinner sheet. Alternating current gives a compromise.
- Keep arc length close to the electrode diameter. Use a short arc for control and a travel angle of about 10-15 degrees. Clean slag between passes.
- Oxy-acetylene welding uses a neutral flame for steel, with a slightly reducing inner cone. A carburising flame adds carbon; an oxidising flame can cause porosity and oxidation.
Worked example
A 6 mm mild steel butt joint is to be welded with a 3.2 mm E6013 electrode. Reason: choose about 110-120 A on electrode-positive DC, clean the joint to bare metal, tack both ends, set an arc length of about 3 mm and use a short arc with a 10-15 degree travel angle. If the current is too low, penetration is poor and slag may be trapped; if too high, undercut and spatter increase.
Traps that cost marks
- Using damp electrodes, which cause porosity.
- Holding too long an arc, which gives spatter and poor penetration.
- Failing to remove slag between passes.
- Welding galvanised steel without fume extraction, exposing the welder to zinc fume.
- Confusing electrode positive and negative effects on penetration.