Flux-Core Welding Thick Steel: Multi-Pass FCAW for Heavy Plate

Gas-shielded flux-core wire (E71T-1) in 0.045" or 1/16" diameter is the standard process for welding thick steel plate in fabrication shops. It deposits 10-20 lbs/hour of weld metal, roughly double what solid MIG wire delivers, and produces mechanical properties that meet or exceed AWS D1.1 structural welding code requirements. For steel 3/8" and thicker, FCAW multi-pass welding is how the majority of structural fabrication gets done.

Multi-pass technique is required because no single weld pass can fill a groove joint in heavy plate. A 1" thick complete joint penetration (CJP) groove weld might take 15-25 individual passes, each deposited on top of the previous one with slag removal between passes. Getting consistent results across all those passes requires controlled parameters, proper preheat, and disciplined interpass temperature management.

Wire Selection for Heavy Plate

Gas-Shielded E71T-1: The Shop Standard

E71T-1 gas-shielded flux-core wire handles about 80% of structural steel fabrication in the United States. The “71” means 70 ksi minimum tensile strength and all-position capability. It runs on DCEP with either 75/25 argon/CO2 or 100% CO2 shielding gas.

For thick steel, the preferred diameters are:

• 0.045" (1.2mm): Good balance of deposition rate and all-position capability. Runs at 200-310 amps. Most versatile size for shops doing mixed work.
• 1/16" (1.6mm): Higher deposition rates for flat and horizontal groove welds. Runs at 250-400 amps. Preferred for production welding on heavy plate.
• 5/64" (2.0mm): Maximum deposition rate for flat-position heavy plate. Runs at 300-500 amps. Requires a machine capable of 500+ amps.

The choice between 75/25 and CO2 gas affects the weld profile. CO2 gives deeper penetration and more spatter. 75/25 gives a smoother arc, less spatter, and a flatter bead profile but slightly less penetration. Most shops running multi-pass thick-plate welds use 75/25 for the cleaner operation.

E70T-1: Flat and Horizontal Only

E70T-1 is the flat/horizontal version of E71T-1. Since it doesn’t need fast-freeze slag characteristics for out-of-position work, the flux formulation is optimized for maximum deposition rate and smooth operation in flat and horizontal positions. If your thick-plate joint can be positioned flat, E70T-1 in 1/16" or 5/64" puts down metal faster than E71T-1.

Self-Shielded Options for Field Work

When thick-plate welding happens outdoors or in the field, E71T-8 self-shielded wire is the structural option. It’s low-hydrogen classified, meets AWS D1.1, and handles wind. The trade-off is lower deposition rates and more smoke compared to gas-shielded wire. For details on the self-shielded vs gas-shielded decision, see FCAW-S vs FCAW-G.

Multi-Pass Technique

Joint Preparation

Thick steel joints typically require groove preparations. For CJP welds on plate over 3/8", standard preparations include:

• Single-V groove: 60-degree included angle, 1/4" root opening, 1/4" root face. Used on plate up to about 3/4" from one side.
• Double-V groove: 60-degree included angle from both sides, 1/4" root face. Used on plate 3/4" to 2"+ to minimize weld volume.
• Single-bevel: 45-degree bevel on one plate, straight edge on the other. Common for T-joints and corner joints.

Weld volume increases dramatically with plate thickness. A single-V groove on 1" plate requires roughly 4 times the weld metal of a single-V on 1/2" plate. A double-V cuts that volume nearly in half by welding from both sides.

Root Pass

The root pass establishes the foundation of the weld. It must fully fuse both sidewalls and the root face without burning through. Settings for the root pass are typically 10-15% lower than fill pass settings.

For a single-V groove root pass with 0.045" E71T-1:

• Voltage: 24-26V
• Wire speed: 200-240 IPM
• Travel speed: 8-12 IPM
• Stick-out: 3/4"

Use a slight weave or stringer bead depending on the root opening. A root opening of 1/4" with backing usually takes a single stringer. Without backing, tighter control is needed to avoid excessive penetration through the root.

Fill Passes

Fill passes build up the joint volume after the root is established. Each fill pass welds on top of the previous pass after thorough slag removal. Settings increase from the root:

For 0.045" E71T-1 fill passes on flat/horizontal:

• Voltage: 26-29V
• Wire speed: 250-320 IPM
• Travel speed: 10-16 IPM
• Stick-out: 3/4" to 1"

For 1/16" E71T-1 fill passes on flat:

• Voltage: 27-31V
• Wire speed: 180-260 IPM
• Travel speed: 8-14 IPM
• Stick-out: 3/4" to 1-1/4"

The weave width of each pass should not exceed 3 times the wire diameter for out-of-position work (AWS D1.1 requirement). For flat position, wider weaves up to 1" are acceptable with proper technique.

Cap Pass

The final pass (cap) determines the weld appearance and must meet profile requirements. The cap should have a slight convex crown (1/16" maximum above the plate surface for most codes) and full toe fusion with no undercut.

Cap settings are similar to fill pass settings but may require slightly lower wire speed for better control of the final profile. A wider weave pattern with pauses at the toes ensures good fusion at the weld edges.

Preheat Requirements

Preheating thick steel before welding slows the cooling rate after welding. Slow cooling prevents two problems: hydrogen-induced cracking (cold cracking) and hard, brittle microstructures in the heat-affected zone (HAZ).

When Preheat Is Required

AWS D1.1 Table 3.3 specifies minimum preheat temperatures based on steel grade, thickness, and welding process:

These are minimum values. Higher preheat may be required for high-restraint joints, high-carbon equivalency steels, or when the base metal is below 32F.

How to Preheat

Apply heat uniformly across the joint area, extending at least 3" (75mm) beyond the weld on all sides. Common methods:

• Oxy-fuel rosebud torch: Most common in shops. Fast, portable, controllable. See the oxy-fuel section for rosebud tip sizing.
• Electric resistance heating pads: Used on pipe and situations where open flame isn’t permitted. More uniform heat than torch.
• Induction heating: Fast and uniform but requires specialized equipment.

Measure temperature with a temperature-indicating crayon (Tempilstik), infrared thermometer, or thermocouple. Measure on the opposite side from where heat is applied, at least 3" from the joint edge. This confirms the heat has soaked through the thickness.

Interpass Temperature Control

Interpass temperature is the temperature of the weld area immediately before depositing the next pass. Both minimum and maximum interpass temperatures matter.

Minimum interpass temperature equals the preheat temperature. If the code requires 150F preheat, you must maintain 150F between passes. If the joint cools below 150F while you chip slag, reheat before the next pass.

Maximum interpass temperature prevents excessive grain growth and degradation of mechanical properties. AWS D1.1 typically limits interpass temperature to 600F (315C) for structural carbon steel. Some high-strength steels have lower limits (400-500F). If the joint gets above the maximum, stop and let it cool.

In production environments with rapid deposition, exceeding the maximum interpass temperature is a real concern. A welder putting down continuous passes with 1/16" wire on a heavy weldment can build up heat fast. Monitor with a temperature crayon or pyrometer and take breaks when the joint approaches the limit.

Deposition Rate Advantages

Deposition rate is the primary reason FCAW dominates thick-steel fabrication. Here’s how it compares:

The 2-3x deposition advantage of FCAW over solid MIG translates directly into fewer passes, less labor time, and lower cost per foot of weld on heavy plate. On a 1" CJP groove weld, switching from solid MIG wire to 1/16" E71T-1 can cut welding time by 40-50%.

Slag Management on Multi-Pass Welds

Every FCAW pass produces a slag layer that must be completely removed before the next pass. Trapped slag between passes creates slag inclusions, which are rejectable defects on any code work.

Between every pass:

1. Let the slag cool enough to become brittle (10-15 seconds usually)
2. Chip with a slag hammer, starting at one end and working toward the other
3. Wire brush the entire weld surface to remove residual slag and flux
4. Visually inspect for trapped slag in the toes and any low spots
5. Grind out any slag pockets or defects before the next pass

On gas-shielded FCAW, the slag is thinner and usually peels more easily than self-shielded wire. Some E71T-1 formulations produce nearly self-peeling slag in flat position. Out-of-position passes tend to produce tighter, harder slag that needs more aggressive chipping.

Common Problems on Thick Steel

Hydrogen cracking (delayed cracking): Cracks that appear hours or days after welding, typically in the HAZ. Caused by hydrogen in the weld combined with a susceptible microstructure and residual stress. Prevention: use low-hydrogen wire (E71T-1 or E71T-8), preheat per code, maintain interpass temperature, and let heavy weldments cool slowly (possibly post-heat at 400-500F for 1 hour per inch of thickness).

Lack of fusion between passes: Previous pass surface was contaminated with slag or the gun angle didn’t direct the arc into the previous pass. Clean thoroughly between passes and adjust technique to ensure the arc melts into the previous deposit.

Centerline cracking in wide passes: The pass is too wide relative to its depth. The solidifying weld metal cracks down the center as it contracts. Keep individual pass widths reasonable. For deep groove welds, use stringer beads or narrow weaves rather than wide weave passes.

Distortion from excessive heat input: Thick-plate multi-pass welds involve a lot of heat. Balanced welding sequences (alternating sides on a double-V, intermittent backstep technique) help manage distortion. Proper fixturing and tack welding before the full weld also matter.

Running out of gas during a long weld: A 300 CF cylinder of 75/25 at 40 CFH lasts about 7.5 hours of arc time. On a big multi-pass weld, you can burn through a cylinder in a shift. Track your gas usage and have a spare cylinder ready. An empty gas cylinder mid-pass means porosity in that section. For information on dual-shield gas selection, see dual-shield welding explained.