Building Clean Beads on Mild Steel: Heat Control and Bead Consistency

Goal: uniform stringer beads on mild steel

This chapter is about building clean, repeatable stringer beads (no weaving) on mild steel plate by controlling heat input and keeping bead shape consistent. Your main controls are: amperage (heat available), travel speed (heat per inch), torch angle (where heat is pushed), and filler amount (bead reinforcement and wet-out). Consistency comes from changing only one variable at a time and using visual checkpoints on every bead.

1) Metal prep for mild steel beads (cleanliness that shows in the puddle)

Mild steel is forgiving, but bead practice exposes contamination quickly (porosity, wandering arc, sooty discoloration). Prep so the puddle behaves the same from start to finish.

• Remove mill scale where you will weld. Grind to bright metal at least 1 in (25 mm) wider than the bead path. Mill scale causes unstable wetting and can trap gas.
• Degrease after grinding (wipe with a clean solvent-compatible rag). Grinding can smear oils; degreasing after grinding prevents “mystery porosity.”
• Dedicated tools: use a brush and flap disc that are only for steel (not previously used on aluminum or stainless).
• Edge and corner prep for guided beads: lightly break sharp edges with a quick pass of a flap disc. A razor edge overheats and encourages undercut.

2) Tack strategy for bead practice (keeping the plate stable)

Even when you’re not joining parts, tacks and restraint help you practice consistent heat control.

• Clamp the plate flat to a bench or a thicker backing plate. A plate that lifts changes arc length and travel angle mid-bead.
• Use two tacks at opposite corners if the plate wants to move as it warms. The goal is not strength; it’s repeatable positioning.
• Plan bead spacing: leave enough room between beads so heat from the previous bead doesn’t preheat the next one unintentionally (a common reason later beads get wider and flatter).

3) Setting amperage for clean beads (starting point + fine tuning)

Choose an amperage that forms a puddle quickly without forcing you to race. If you have a foot pedal, set a reasonable maximum and weld mostly in the middle of the pedal travel so you can correct up or down.

• Starting points (DCEN, mild steel plate):
  • 1/16 in (1.6 mm): ~45–70 A
  • 1/8 in (3.2 mm): ~80–120 A
  • 3/16 in (4.8 mm): ~120–160 A
• How to confirm you’re close: within about 1 second of establishing the arc, you should see a controllable puddle that doesn’t immediately collapse into a wide, overheated pool.
• If you need to “wait forever” for a puddle: increase amperage slightly or slow down only enough to establish the puddle (don’t park in one spot).
• If the puddle instantly gets too wide: reduce amperage or increase travel speed; also check that your torch isn’t pushing heat forward too aggressively.

4) Practice progression A: straight stringer beads on open plate

Start with “freehand” straight beads to build a steady pace and consistent puddle size. Use a simple routine so every bead is comparable.

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Step-by-step routine

1. Mark a start and stop (soapstone or scribe) about 3–5 in (75–125 mm) apart. Short beads make it easier to diagnose changes.
2. Establish the puddle at the start mark, then begin moving as soon as the puddle is stable. Avoid a long dwell that creates a crater and extra reinforcement at the start.
3. Lock in a travel rhythm: move at a pace that keeps puddle diameter consistent. Watch the leading edge of the puddle, not the arc itself.
4. Add filler consistently so the bead stays slightly crowned (not piled up). If you’re practicing autogenous beads first, do a few, then switch to adding filler—most bead-shape problems show up once filler is introduced.
5. End cleanly: taper heat slightly near the end (or reduce pedal) while adding a final small amount of filler to avoid a deep crater.

What “consistent” looks like

• Uniform width from start to stop (no gradual widening).
• Even ripples (spacing and height consistent), indicating steady travel and filler timing.
• Toe wet-out: edges blend smoothly into the plate without a sharp groove.
• Color and surface: no peppered pinholes, no dull gray “sugary” texture, no black soot trails.

5) Practice progression B: beads along a line or edge (built-in guidance)

Once you can run a straight bead on open plate, add a guide. A guide reduces steering effort so you can focus on heat and bead shape.

Option 1: scribed/marked line

• Scribe a straight line and run the bead centered on it.
• Use the line as a steering reference, but keep your eyes on the puddle edges. If you stare at the line, you’ll miss undercut forming at the toes.

Option 2: bead along a plate edge

• Run a bead parallel to an edge about 1/8–3/16 in (3–5 mm) in from the edge. This teaches you to keep heat centered and avoid washing the edge away.
• Then run closer (about 1/16–1/8 in / 1.5–3 mm) once you can keep the bead straight without undercutting the edge.

Option 3: bead in a shallow groove (training aid)

Lightly grind a shallow straight groove (not a joint—just a track). The groove helps you keep the puddle centered and reveals if you’re overheating the sides.

6) Preventing undercut and excessive reinforcement (shape control)

Undercut is a groove melted alongside the bead that wasn’t filled back in. Excessive reinforcement is a bead that sits too high and narrow (ropey) or piles up. Both are mainly balance problems between heat, speed, and filler.

Undercut: causes and fixes

• Torch angle pushing heat to the edge: keep the torch more centered over the bead path. If you angle too much in the travel direction, you can “dig” the toes.
• Travel speed too fast for your filler rate: if you outrun your filler, the edges melt but don’t get replenished. Slow slightly or increase filler frequency.
• Too hot for the plate thickness: reduce amperage a small amount or shorten the time you dwell at the leading edge.
• Puddle too wide: once the puddle spreads, the edges are easy to wash out. Correct early by speeding up slightly or backing off heat.

Excessive reinforcement: causes and fixes

• Too much filler for the heat: reduce filler amount or increase heat slightly so the added metal wets out instead of stacking.
• Too cold / moving too slow: a cold puddle won’t flatten; it builds height. Increase amperage slightly or increase travel speed while maintaining puddle size.
• Stopping/starting motion: pauses create lumps. Keep a steady glide; if you must pause, add heat and filler intentionally, then resume smoothly.

7) Bead quality checkpoints (inspect every bead the same way)

Use a quick inspection routine after each bead. The goal is to connect what you see on the plate to what you felt in your hands.

Checkpoint list

• Width: measure visually against a consistent reference (scribe marks or a ruler). Width should not drift wider as you go.
• Ripple consistency: ripples should be evenly spaced. Uneven ripples usually mean inconsistent travel speed or inconsistent filler timing.
• Toe blend: run a fingernail across the toes. A sharp groove indicates undercut; a smooth transition indicates good wetting.
• Reinforcement height: the bead should be slightly crowned, not piled high and not sunk flat.
• Surface defects: look for pinholes (porosity), worm tracks, or peppering.
• Start/stop: starts shouldn’t be a big blob; stops shouldn’t be a deep crater.

8) Corrective actions by symptom (diagnose, then change one thing)

9) A simple drill plan (repeatable practice set)

Run this sequence on one plate so you can compare results side-by-side.

• Set 1 (open plate): 5 beads, 3–4 in long, equal spacing. Aim for identical width and ripple spacing.
• Set 2 (on a scribed line): 5 beads centered on the line. Focus on steering without changing heat.
• Set 3 (near an edge): 5 beads parallel to the edge. Start 3–5 mm from the edge; move closer only when undercut is controlled.
• Record one change at a time: if bead is wide, adjust travel speed first; if still wide, then adjust amperage. Write the change next to the bead with soapstone.

10) Quick reference: heat control priorities

• If the puddle grows: correct early with slightly faster travel or slightly less amperage.
• If the bead stacks up: either add heat or reduce filler; don’t keep slowing down to “melt it in.”
• If toes groove (undercut): add filler and reduce edge-directed heat; don’t just slow down without changing filler.
• If porosity appears: stop and re-clean—don’t try to “weld through it.”