AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is inspecting a Plasma Arc Welding (PAW) operation on a thin sheet of stainless steel. The welder is using the keyhole mode. Which of the following is the most critical parameter to control to maintain a stable keyhole and prevent blow-through or incomplete penetration?
- APlasma gas flow rate.
- BTorch stand-off distance.
- CPilot arc current.
- DShielding gas flow rate.
Show answer & explanationAnswer & explanation
Correct answer: A. Plasma gas flow rate.
In PAW keyhole mode, the plasma gas flow rate is the most critical parameter. It controls the force of the plasma jet, which is responsible for creating and maintaining the keyhole. Too low a flow rate can lead to incomplete penetration, while too high a flow rate can cause an unstable keyhole, excessive underfill, or blow-through.
Why the other options are wrong
- B. Torch stand-off distance affects arc length and heat concentration but is secondary to plasma gas flow rate in keyhole control.
- C. Pilot arc current establishes the arc but does not control the main keyhole formation or depth; the main welding current and plasma gas do.
- D. Shielding gas flow rate protects the weld pool from atmosphere but does not directly control the keyhole formation or stability.
PAW Keyhole Mode Control
Keyhole mode in PAW uses a high-velocity plasma jet to fully penetrate the workpiece, creating a 'keyhole' opening. Precise control is vital.
- Plasma gas forms the keyhole.
- Allows for single-pass full penetration.
- Sensitive to parameter variations.
Memory trick: Plasma gas is the key to a steady hole.