Porosity in Robotic MIG Welding of Carbon Steel (P1 Group) — Root Cause and Resolution

Introduction

Porosity is one of the most common and critical welding defects in gas metal arc welding (GMAW/MIG), particularly in multiple passes welds. Excessive porosity compromises weld strength, integrity, and compliance with ASME Section IX acceptance standards. This report documents a case of porosity in the final beads of robotic multiple passes MIG welding on P1 group carbon steel, the unsuccessful corrective measures initially attempted, and the ultimate resolution achieved by adjusting the robot programming and nozzle-to-workpiece distance.

Problem Description

During robotic MIG welding of a carbon steel (P1 group) joint with multiple passes, porosity was observed in the final weld beads. The defect was located near the weld crown and was unacceptable under code requirements.

Initial corrective actions attempted included:

• Replacing the welding nozzle.
• Replacing contact tips.
• Cleaning the wire feeding mechanism.
• Replacing the gas pressure regulator.
• Switching to a different shielding gas supply.

Despite these interventions, porosity persisted in the last weld beads, indicating that the root cause was not related to equipment wear or shielding gas composition.

Root Cause Analysis

Further investigation revealed that the problem originated from robot programming of the final bead sequence. Specifically, the nozzle-to-work distance during the last bead passes was set too far from the weld pool.

• Effect: The increased standoff distance reduced the effectiveness of shielding gas coverage over the molten metal.
• Consequence: Ambient air entrapped in the weld pool led to gas porosity formation.

Thus, the issue was not mechanical or consumable-related but a process parameter error in robotic programming.

Corrective Action

The solution involved reprogramming the robotic welding system to reduce the nozzle-to-workpiece gap during the final weld passes.

• Optimized standoff distance improved shielding gas coverage.
• Stable shielding atmosphere was maintained over the molten weld pool.
• Subsequent welds showed no porosity indications under NDE inspection.

Verification and Results

After correction, test welds were performed and inspected:

• Visual Inspection (VT): No visible porosity in the crown area.
• Radiographic Testing (RT): Confirmed absence of internal porosity in final beads.
• Procedure Compliance: Welds met ASME Section IX acceptance criteria for P1 group carbon steels.

Recommendations

1. Process Control: Always verify robotic standoff distance during multiple passes programming.
2. Preventive Maintenance: Continue regular nozzle, contact tip, and regulator checks, but complement with program validation protocols.
3. NDE Monitoring: Incorporate in-process inspection for porosity detection in multiple passes welds.
4. Training: Ensure operators and programmers are trained to recognize the role of torch position and gas shielding geometry in preventing porosity.

Conclusion

This case demonstrates how welding porosity in robotic MIG operations can persist even after multiple equipment changes, when the true root cause lies in robot programming parameters. By correcting the nozzle-to-workpiece gap, shielding gas effectiveness was restored, and porosity was eliminated.