Fillet Welds in Flat and Horizontal Positions

Fillet Welds in Flat and Horizontal Positions

Why Fillet Welds Matter

Fillet welds are the most common weld type in structural steel fabrication. They join two surfaces that meet at an angle — most often 90° in a T-joint, lap joint, or corner joint. Because so much of building and construction work depends on fillet welds, AWS D1.1 Structural Welding Code — Steel sets strict requirements for their size, profile, and internal quality. Mastering fillet welds in the flat (1F) and horizontal (2F) positions is a foundational skill for every credential in this program.

Joint Fit-Up and Tack Welding

Good welds begin with good fit-up. For a T-joint, the vertical member (web) must sit perpendicular to the flat member (base plate) within ±2°. Use a welding square and fit-up gauge to confirm the angle before tacking. Any gap larger than 1/16 in. at the root will affect fusion and leg size.

Apply two tack welds — one near each end of the joint — to hold the assembly in position. Tacks must be sized appropriately (roughly the same leg size as the finished weld or slightly smaller) and must be sound. Inspect every tack: cold laps and surface cracks must be ground out before you lay the final weld. A cracked tack can propagate into the finished weld and cause rejection.

GMAW Setup for Fillet Welds

For 1/4 in. and 3/8 in. A36 plate with ER70S-6 wire (0.035 in. diameter) and C25 shielding gas (75% argon / 25% CO₂), your welding procedure specification (WPS) will define the voltage, wire feed speed, and travel speed. Typical spray or short-circuit transfer settings vary by plate thickness; always defer to the WPS rather than guessing. Set your gas flow rate in the range your WPS specifies — too little causes porosity, too much causes turbulence that can also pull in atmospheric contamination.

Check your contact tip, nozzle, and liner before starting. Spatter buildup inside the nozzle restricts gas flow. Use anti-spatter spray or gel on the nozzle to reduce buildup, and clean it between coupons.

Flat Position (1F) Technique

In the 1F position the T-joint is rotated so both legs of the fillet weld are accessible at roughly 45° from horizontal. Hold the gun at a 45° work angle to the bisector of the joint and a 5–15° drag (forehand) or push (backhand) travel angle depending on the transfer mode and WPS guidance. Drag (pointing the gun back toward completed weld) gives deeper penetration and a flatter bead; push gives a wider, slightly convex bead with better visibility.

For a single-pass fillet on thinner plate, a slight weave (side-to-side motion, pausing at each toe) helps ensure fusion at both toes. For heavier plate requiring a two-pass weld, run the first pass (root pass) as a stringer bead tight in the root; then clean slag and spatter, and run the second (cap) pass to build the leg to specified size.

Horizontal Position (2F) Technique

In the 2F position the vertical member stays vertical and the weld is deposited on the horizontal shelf. Gravity pulls the molten puddle downward toward the lower toe, which causes two common defects: overlap at the bottom toe and undercut at the upper toe. To counteract this, tilt the gun 30–45° upward from horizontal (pointing up toward the vertical member) while maintaining a work angle of about 45° to the joint. This directs arc force upward and keeps the puddle from sagging.

Travel speed is critical in 2F: moving too slowly lets the puddle accumulate and sag; too fast causes insufficient fill. Watch both toes constantly. On multi-pass work, place the first pass close to the lower toe, let it cool slightly, wire-brush it, and then place the second pass to blend the upper toe.

Fillet Weld Profile Requirements (AWS D1.1)

AWS D1.1 specifies that fillet weld leg sizes must match the WPS within acceptable tolerance. Visual inspection criteria include:

• Undercut: must not exceed limits stated in the code for the application.
• Overlap: the weld face must not roll over the toe and lie flat against the base metal without fusion.
• Convexity and concavity: excessive convexity wastes filler metal and creates stress risers; concavity reduces the effective throat.
• Surface porosity and cracks: open to the surface are not acceptable.

Measure leg size with a fillet weld gauge (also called a fillet gauge or Tee gauge). Place the gauge against the weld and read the leg size directly. Both legs should match the specified size within 1/16 in.

Nick-Break Test

The nick-break test evaluates internal quality. You saw the coupon at mid-length, cut a notch (nick) into the weld with a grinder, and then fracture the specimen through the notch using a press, vise, or bending force. Examine the fracture face:

• Complete fusion appears as a uniform, grayish, crystalline surface across the full throat.
• Incomplete fusion shows smooth, shiny, un-bonded areas.
• Porosity appears as rounded voids; AWS D1.1 limits the size and frequency of acceptable pores.
• Slag inclusions appear as dark, glassy particles embedded in the weld metal.

Photograph the fracture face and compare it to the acceptance criteria in AWS D1.1 before recording your results.

Safety in This Lab

• PPE: Wear a welding helmet (shade appropriate for GMAW amperage), leather gloves, a flame-resistant welding jacket, leather boots, and safety glasses under the helmet.
• Ventilation: GMAW with ER70S-6 produces fumes containing manganese. Use local exhaust ventilation (LEV) or a fume extraction gun. Never weld in a confined space without engineering controls.
• Grinder safety: Inspect the grinding disc before mounting. Use the guard. Wear a face shield over safety glasses when grinding.
• Hot metal: Fillet weld coupons stay dangerously hot long after welding. Use pliers or tongs; mark hot metal with soapstone.
• Nick-break press: Follow instructor lockout/tagout (LOTO) procedures for the press or hydraulic tool. Keep hands clear of the ram.