Basic Welding

The Way You Attach Determines What You Learn

Welding is not about melting and attaching, but about melting the base metal and filler metal to form a single mass (weld metal). Therefore, the attached part is not adhesive, but the metal itself.

The problem is that melted metal, when exposed to air, absorbs oxygen and nitrogen, causing porosity and brittleness. The reason there are various arc welding methods is ultimately one: how to protect the melted metal from the air.

• Burn the coating on the welding rod to cover with gas and slag → SMAW
• Flow argon from a separate gas cylinder to cover → GTAW (TIG)
• Automatically feed the wire while flowing gas → GMAW (MIG·MAG·CO2)

This difference determines everything: speed, quality, outdoor work capability, and learning difficulty.

Comparison of the 3 Major Arc Welding Methods

What's Actually Different by Method

SMAW — Simple Equipment That Can Go Anywhere

You just need a welder, cables, and a rod. Since there is no gas cylinder, you can work anywhere, even on the ground or between pipes outdoors. This is why it is still considered a basic skill in the field of Booilkyung's subcontractors.

Welding rods vary in characteristics depending on the coating type.

> Low hydrogen series becomes meaningless if it gets wet. Opened rods that are left unused should be re-dried for more than one hour at 300~350℃ and then placed in a thermos (around 100℃) before use. Follow the exact conditions from the rod manufacturer's instructions and the WPS. The habit of putting rods in your pocket without a rod container is the beginning of low-temperature cracking.

GTAW (TIG) — Slow but the Cleanest

The tungsten electrode does not melt and only creates an arc. Since the filler metal is manually inserted, you can precisely control the preheating and melting area. Stainless steel piping, aluminum, thin plates, and root pass (first layer) of piping are almost always TIG.

• Polarity: For carbon steel and stainless steel, DCEN (Direct Current Electrode Negative), for aluminum, AC (Alternating Current) — the cleaning effect of AC breaks the oxide layer.
• Electrode: Pure tungsten (green), 2% thorium (red), 2% cerium (gray), 2% lanthanum (blue). Thorium is radioactive, so the abrasive dust is a problem, and more sites are switching to cerium and lanthanum.
• For stainless steel piping, back purge is used to fill the inside of the pipe with argon to prevent oxidation on the back side. If you skip this, the back side will burn black (sugar), requiring complete rework.

GMAW (MIG·MAG·CO2) — The Method That Produces a Lot

Since the wire comes out automatically, your hands never stop. The welding speed is 2~3 times that of SMAW, so ship blocks, thick plate structures, and automotive parts lines mainly use this method.

Also know the transfer forms:

• Short-circuit transfer: Low current. The wire touches and breaks away from the base metal. Suitable for thin plates and tax-free welding
• Spray transfer: High current + Ar 80% or more. Almost no spatter, but only possible on downward and horizontal fillets

In shipyards, FCAW (Flux-Cored) is widely used, which includes a flux core that produces slag. It has a high welding speed, can be tax-free, and has stable beads, making it the standard for block welding.

5 Criteria for Choosing What to Use

1. Base Metal — For aluminum, use TIG (AC) or MIG. For thin stainless steel, use TIG. For general structural steel thick plates, use FCAW or CO2
2. Plate Thickness — Thin plates use TIG or short-circuit transfer, thick plates use CO2, FCAW, or SAW
3. Quality Requirements — If piping or pressure vessels are subject to radiographic testing (RT), the root pass should be TIG
4. Work Environment — For outdoor, windy, or narrow spaces, use SMAW. For gas shielded methods, windbreak screens are needed if the wind speed is approximately 2m/s or more
5. Quantity and Delivery Time — If the quality is the same, choose the method with the higher welding speed

Getting a Feel for Numbers — Current and Heat Input

Defects start when the current deviates from the appropriate range for the rod diameter and wire diameter.

Approximate suitable current for coated rods = Rod diameter (mm) × 30~40

 3.2mm rod  ->  Approximately 100 ~ 130 A
 Approximate suitable current for coated rods = Rod diameter (mm) × 30~40

 3.2mm rod  ->  Approximately 100 ~ 130 A
 4.0mm rod  ->  Approximately 120 ~ 160 A
 5.0mm rod  ->  Approximately 150 ~ 200 A

 (Actual current used should prioritize the rod manufacturer's catalog and WPS range)
 5.0mm rod  ->  Approximately 170 ~ 220 A

Heat Input (H) in WPS is a value that must be strictly managed because it changes the material's properties.

Heat Input H (J/mm) = 60 × Current (A) × Voltage (V) / Welding Speed (mm/min)

Example) CO2 semi-automatic, 280A, 30V, 300mm/min
   = 60 × 280 × 30 / 300
   = 504,000 / 300
   = 1,680 J/mm  =  1.68 kJ/mm

• If heat input is too high: The heat-affected zone (HAZ) becomes larger and the grain size increases, reducing toughness. Deformation also increases.
• If heat input is too low: Rapid cooling increases the risk of hardening and low-temperature cracking.

Even with the same current, halving the speed doubles the heat input. "Slow and careful" is not always good.

Check These in the First Week

• What is the main welding method used in our process (SMAW / TIG / CO2 / FCAW)
• The specifications of the welding rods and wires and storage location (is there a rod drying oven)
• Types of shielding gas and flow meter settings (typically CO2·MAG 15~25 L/min, TIG 8~15 L/min)
• The WPS number for our line and the current, voltage, and speed range listed there
• Base metal material (is it mild steel, high-strength steel, or stainless steel) — the rod completely changes depending on the material