MIG welding (GMAW) remains the most widely used welding process in structural steel fabrication. Getting the parameters right is critical for producing sound welds that meet code requirements under EN ISO 5817 and AWS D1.1.
Wire Feed Speed and Voltage Relationship
The relationship between wire feed speed (WFS) and voltage is the most fundamental parameter pairing in MIG welding. For carbon steel using ER70S-6 wire at 1.0 mm diameter, a general starting point is 4–6 m/min WFS with 18–22 V for thin material (2–4 mm), scaling up to 8–12 m/min WFS with 24–30 V for thicker sections (8–20 mm).
The voltage controls the arc length and directly affects bead profile. Too low and you get a narrow, convex bead with poor fusion; too high and the arc becomes unstable with excessive spatter and potential undercut.
Shielding Gas Selection
For carbon steel, the standard shielding gas mixture is 82% Argon / 18% CO₂ (often called C18 or M21 per EN ISO 14175). This provides a good balance between arc stability, penetration, and spatter reduction. Pure CO₂ can be used for deeper penetration on thick sections but produces significantly more spatter.
Flow rates should be maintained between 12–18 l/min depending on joint configuration and environmental conditions. Outdoor welding or draughty workshops may require rates up to 20 l/min or the use of wind screens.
Travel Speed and Heat Input
Travel speed directly affects heat input, which in turn influences the mechanical properties of the heat-affected zone (HAZ). For structural applications governed by EN 1090-2, heat input must be controlled within the limits specified in the Welding Procedure Specification (WPS).
Heat input is calculated as: Q = (U × I × 60) / (v × 1000), where U is voltage in volts, I is current in amps, and v is travel speed in mm/min. The result is expressed in kJ/mm. For S355 structural steel, typical maximum heat input is 1.7 kJ/mm to prevent excessive grain growth in the HAZ.
Practical Parameter Table
The following parameters serve as starting points and should be qualified through procedure testing per EN ISO 15614-1:
- **2 mm plate**: 0.8 mm wire, 4.5 m/min WFS, 18 V, short-circuit transfer\n- **4 mm plate**: 1.0 mm wire, 6.0 m/min WFS, 21 V, short-circuit transfer\n- **8 mm plate**: 1.0 mm wire, 8.5 m/min WFS, 25 V, spray transfer\n- **12 mm plate**: 1.2 mm wire, 10.0 m/min WFS, 28 V, spray transfer\n- **20 mm plate**: 1.2 mm wire, 12.0 m/min WFS, 30 V, spray transfer (multi-pass)
Common Defects and Parameter Corrections
**Porosity**: Usually caused by contaminated base metal, insufficient gas coverage, or excessive wire stick-out. Check gas flow, clean the workpiece, and maintain 15–20 mm contact tip to work distance (CTWD).
**Undercut**: Excessive voltage or travel speed. Reduce voltage by 1–2 V and slow travel speed. Also check torch angle — maintain 10–15° push angle.
**Lack of fusion**: Insufficient heat input. Increase WFS and voltage proportionally. Ensure proper joint preparation and fit-up per the WPS.
Always verify your parameters against a qualified WPS and ensure welders hold valid certifications per EN ISO 9606-1 for the relevant material group and welding position.