Manual Metal Arc Welding (SMAW): Power Sources, Electrodes and Techniques
What this domain tests
Power-source characteristics, duty cycle and derating, electrode classification and storage, polarity choice, arc blow, and technique for each position.
Core rules
- Power source. SMAW needs a constant-current (drooping) characteristic so that arc-length variation changes current only slightly. Typical OCV 50–90 V, arc voltage 20–30 V.
- Types. AC transformer (cheapest, no arc blow), transformer-rectifier for DC, and engine-driven generator for site work. Duty cycle is quoted at rated current, commonly 60%.
- Derating. Use I₂ = I₁ × √(D₁/D₂).
- Polarity. DCEN (straight) puts more heat into the plate - good for deep penetration and thin sheet; DCEP (reverse) puts more heat into the electrode, giving better cleaning on basic-coated electrodes; AC is a compromise.
- Electrodes. AWS A5.1 code E7018: 70 = 70 ksi minimum tensile, 1 = all positions, 8 = low-hydrogen iron-oxide coating, usable on AC or DCEP. Indian IS 814 classes tensile strength in kgf/mm² and adds coating and current digits.
- Storage. Low-hydrogen electrodes go into a holding oven at 100–150 °C after opening; re-bake at 250–300 °C for about one hour if damp.
- Arc blow. Shorten the arc, reduce current, change the earth clamp position, backstep, or wrap the earth lead around the joint.
Worked example
A 300 A set is rated 60% duty cycle. What current is safe for continuous working? I₂ = 300 × √(60/100) = 300 × 0.775 = 232 A. So a welder wanting a 100% duty cycle must not exceed about 232 A. Conversely at a 40% duty cycle the permitted current rises to 300 × √(60/40) = 367 A - the set is not limited to 300 A in short bursts.
Traps
Using a constant-voltage characteristic for SMAW; forgetting to derate for continuous work; re-baking a cellulose electrode; treating DCEP and DCEN as electrically identical.