F1, F2, F3, and F4 in welding refer to fillet weld positions on plates.
F1 is flat position, F2 is horizontal, F3 is vertical, and F4 is overhead fillet welding.
These positions indicate the difficulty level and technique required for fillet welds.
In this article:
In welding, F1, F2, F3, and F4 (more commonly written as 1F, 2F, 3F, 4F) refer to fillet weld positions. These positions describe how the workpieces are oriented and how the weld is applied.
👉 F = Fillet weld (used for joints like T, lap, and corner joints)
What is a Fillet Weld?
Definition
A fillet weld joins two surfaces at an angle (usually 90°), forming a triangular-shaped weld.
1F Position (Flat Fillet Welding)
Definition
Welding is done on a horizontal surface, and the weld is deposited from the top side.
Orientation
- Joint lies flat
- Welding is done from above
Advantages
- Easiest position
- Good control of molten metal
- High-quality weld
Applications
- Basic welding practice
- Simple fabrication
2F Position (Horizontal Fillet Welding)
Definition
Welding is done on a vertical surface, but the weld runs horizontally.
Orientation
- One plate vertical
- One plate horizontal
- Welding is done sideways
Challenges
- Molten metal tends to sag downward
- Requires moderate skill
Applications
- Structural welding
3F Position (Vertical Fillet Welding)
Definition
Welding is performed in a vertical direction.
Orientation
- Plates are vertical
- Welding is done:
- Upward (3F-up)
- Downward (3F-down)
Characteristics
- Gravity affects weld pool
- Requires control and technique
Difficulty
- Hard position
Applications
- Construction
- Heavy fabrication
4F Position (Overhead Fillet Welding)
Definition
Welding is performed from the underside of the joint.
Orientation
- Joint is above the welder
- Welding is done overhead
Challenges
- Molten metal may fall
- Most difficult fillet position
- Requires high skill and safety
Applications
- Repair work
- Structural welding in tight spaces
Comparison Table
| Position | Description | Difficulty |
|---|---|---|
| 1F | Flat fillet | Easy |
| 2F | Horizontal fillet | Medium |
| 3F | Vertical fillet | Hard |
| 4F | Overhead fillet | Very hard |
Summary:
- 1F → Flat
- 2F → Side (horizontal)
- 3F → Vertical
- 4F → Overhead
Conclusion:
- F = Fillet weld positions
- Used for T-joints, lap joints, corner joints
- Difficulty increases from 1F → 4F
- 3F and 4F require skilled welders
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