Pre-flow establishes protective gas coverage before arc initiation, while post-flow maintains protection during the critical cooling period when hot metal remains reactive to atmospheric contamination. Proper timing prevents the start and stop defects that often compromise otherwise good welds.
Pre-flow and Post-flow Science
Pre-flow Requirements
- Displaces atmospheric gases. Pushes air away from the arc starting location before ignition.
- Prevents arc contamination. Ensures clean conditions for stable arc initiation.
- Reduces tungsten contamination. Critical in TIG welding to prevent tungsten oxide formation.
- Stabilizes flow patterns. Allows gas velocity to stabilize before welding begins.
Post-flow Critical Period
- Hot metal protection. Weld metal remains reactive to oxygen and nitrogen until it cools below 500°C.
- Tungsten electrode protection. Hot tungsten forms oxides rapidly without continued gas coverage.
- Stress relief time. Hot metal under stress is particularly vulnerable to contamination cracking.
- Color indication. Proper post-flow shows as bright, shiny metal instead of heat tint colors.
Timing Guidelines by Process
MIG/MAG welding: 0.2-0.5 sec pre-flow, 0.5-1.0 sec post-flow for most applications.
TIG welding: 1-2 sec pre-flow, 5-15 sec post-flow depending on material and thickness.
Aluminum welding: Longer times needed—up to 20 sec post-flow for thick sections.
Stainless steel: Extended post-flow (10-15 sec) prevents heat tint and maintains corrosion resistance.
Consistent Quality
VARIGON® He30
TIG OptimizedWhy VARIGON He30 excels with timing: The argon base provides excellent start/stop characteristics while helium content maintains good protection during extended post-flow periods.
Timing recommendations: 1.5 sec pre-flow, 10-12 sec post-flow for most aluminum TIG applications with VARIGON He30.
⏱️ Timing Critical