SMAW Process Theory and Power-Source Fundamentals

What Is the SMAW Arc?

Shielded Metal Arc Welding (SMAW) — commonly called stick welding — joins metal by generating an electric arc between a flux-coated consumable electrode and the base metal. The arc produces temperatures sufficient to melt both the electrode core wire and the base metal, forming a weld pool. As the electrode melts, its flux coating burns and decomposes, releasing shielding gases and forming a slag layer that protects the molten metal from atmospheric oxygen and nitrogen until it solidifies.

Establishing the arc requires a brief contact between electrode tip and base metal followed by a quick withdrawal to strike-gap distance — typically slightly wider than the electrode core diameter. This momentary short circuit heats the tip, ionizes the air gap, and sustains a conductive plasma column. Once the arc is running, the welder maintains a consistent arc length to keep voltage and heat input stable.

Current Path Through the Circuit

Understanding the complete circuit is essential for both quality welds and safe operation. Current flows in a loop:

1. Power source (transformer or inverter) → output terminal

2. Electrode cable → electrode holder (stinger)

3. Electrode → arc plasma → base metal

4. Work clamp → work cable → power source return terminal

Every connection in this loop must be clean and tight. A loose work clamp adds resistance, causes voltage drop, and produces an erratic arc. Never allow the work cable to share a path through machine frames, building steel, or pipe that is not intended as the weld circuit; stray current causes corrosion and electric-shock hazards.

AC and DC Power Sources

Transformer-based AC machines step utility voltage down to welding voltage using a wound transformer. Output alternates at line frequency (60 Hz in the US), meaning current direction reverses 120 times per second. AC machines are simple, durable, and lower in initial cost. Their main limitation is arc stability at low amperage settings.

DC rectifier and inverter machines convert AC to direct current. Inverters use high-frequency switching to produce a compact, lightweight unit with precise amperage control. DC provides a steadier arc than AC and is required by many electrode classifications.

Constant-Current (CC) characteristic — also called a "drooper" — is the required output shape for SMAW. On a volt-ampere (VA) curve, the CC curve slopes steeply downward: a large change in arc voltage (caused by the welder changing arc length) produces only a small change in amperage. This self-regulating behavior keeps heat input relatively stable even when the welder's hand moves, which is essential for manual welding. Constant-Voltage (CV) sources are designed for wire-feed processes such as GMAW, not for SMAW, because they would allow runaway current changes as electrode length changes.

Open-Circuit Voltage (OCV) is the voltage present at the output terminals before the arc is struck. Most SMAW machines produce OCV in the range of 50–80 V. OSHA and NFPA 70E recognize that voltages above 50 V can be lethal under wet or confined conditions. Never touch the electrode or bare holder parts when the machine is energized. Always set the machine to the minimum OCV that permits reliable arc starting for the electrode in use.

Polarity: DCEP, DCEN, and AC

Polarity describes the direction of conventional current flow and directly affects heat distribution and penetration.

• DCEP — Direct Current Electrode Positive (also called reverse polarity): The electrode holder is connected to the positive (+) terminal and the work clamp to the negative (−) terminal. Electrons flow from work to electrode. Approximately two-thirds of arc heat is concentrated at the electrode, producing deep penetration. Most low-hydrogen electrodes (E7018, E6010, E6011) run on DCEP.

• DCEN — Direct Current Electrode Negative (also called straight polarity): The electrode holder connects to the negative (−) terminal; the work clamp to positive (+). Heat shifts toward the base metal, giving shallower penetration and higher deposition rates. E6012 and E6013 may run on DCEN in some applications; some fast-fill electrodes are designed specifically for DCEN.

• AC: Polarity alternates each half-cycle, so heat is distributed more evenly between electrode and work. AC is commonly used with E6011 and E6013 electrodes where DC sources are unavailable or arc blow is a problem. Arc blow — deflection of the arc caused by magnetic fields in DC circuits — is eliminated when running AC.

Consult AWS A5.1 for the specific polarity designations of each electrode classification. The electrode classification label (e.g., E6010) encodes usability information: the third digit (or third and fourth digits on some classifications) indicates the coating type and approved polarity/current options.

Safety Essentials

• PPE: Wear a properly rated welding helmet (minimum shade 10 lens for most SMAW currents), leather welding gloves, flame-resistant clothing, and leather boots. Protect bystanders with welding curtains.
• Electric shock: Treat every energized electrode as a shock hazard. Insulate yourself from the work and ground. Inspect electrode holder, cables, and connectors before each use; replace any cracked insulation immediately.
• Fumes and ventilation: SMAW flux coatings produce significant fume. Work in well-ventilated areas; use local exhaust ventilation for confined spaces. Consult the electrode manufacturer's Safety Data Sheet (SDS).
• Lockout/Tagout: Before inspecting internal machine components or changing damaged cables, de-energize the machine, unplug or lock out the disconnect, and tag it. Follow OSHA lockout/tagout procedures (29 CFR 1910.147).
• Fire watch: Remove combustibles from the work area or cover them. Keep a fire extinguisher accessible and maintain a fire watch after welding ends.