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Plasma cutting leaves a heat-affected zone (HAZ) beside the cut face. On mild steel, that zone may be only a fraction of a millimeter wide, but it can still contain hardened metal, tensile stress, dross, microcracks, or a softened edge. Most cuts are perfectly serviceable after light cleanup. The important question is whether the edge will be welded, loaded, bent, machined, or exposed to fatigue and corrosion.

What you are inspecting

The cut face has several areas worth checking: the top edge, the kerf wall, the bottom edge, and the adjacent parent metal. Look for more than discoloration. Heat damage can show up as a narrow color band, a rough or glazed surface, a hardened lip, or cracking that is too fine to see with the naked eye.

On carbon steel, a clean plasma cut commonly produces a narrow heat tint and a slightly harder surface layer. Stainless steel may show blue, purple, or straw-colored oxidation. Aluminum can show a dull, smeared edge rather than obvious color. Do not use color alone as a pass/fail test; amperage, travel speed, compressed-air quality, plate thickness, and the metal grade all change the appearance.

Prepare the edge before inspection

Let the part cool to room temperature. Quenching a hot cut in water can add thermal shock and makes it harder to judge the original condition. Remove loose dross with a chipping hammer or scraper, then use a wire brush. Do not aggressively grind the entire edge before looking at it; grinding can remove evidence of cracking and can hide an overheated lip.

Use bright, angled light and clean magnification. A 5x to 10x inspection loupe is inexpensive and more useful than trying to examine the edge through a dark welding helmet. For frequent fabrication work, a small inspection loupe and LED work light make edge defects much easier to find.

Wear safety glasses while brushing or scraping. A plasma-cut edge can remain sharp enough to cut skin, and trapped dross can break loose unexpectedly.

Visual and physical checks

Start at the top edge. A sharp, continuous top corner is usually a good sign. A rounded or melted lip means the arc spent too much time at the edge, often from slow travel, excessive amperage, or a torch that was not held square. A heavy top bead can interfere with fit-up and may contain hard, brittle material.

Inspect the kerf wall from top to bottom. Normal drag lines should be fairly consistent and lean in one direction. Deep, widely spaced lines, a strongly angled kerf, or a rough lower half usually points to incorrect speed, torch height, consumables, or insufficient air pressure. These are cut-quality problems, but they also increase the amount of material that must be removed from the HAZ.

Check the bottom edge for dross. A thin, easily removed line is generally acceptable. Thick, tightly bonded dross may indicate a slow cut or low torch height. When dross is fused into the base metal, grinding can create a low spot or leave a crack-like notch. After cleaning, run a fingernail across the edge. Any sharp notch, pit, or raised lip deserves closer examination.

Use a file on a hidden section of the cut edge. If the file skates over a narrow band but cuts normally in the parent metal, the band is harder than the surrounding steel. That does not automatically make the part unusable, but it matters for drilling, tapping, bending, and fatigue-loaded joints. A hardened edge can cause drill wandering and may crack when formed.

Tests for cracks and excessive hardening

For critical mild-steel parts, clean the edge and apply a visible dye penetrant. The process requires cleaner, penetrant, dwell time, and developer; follow the kit instructions rather than guessing. Penetrant testing finds cracks open to the surface, including defects that a bright light misses. It will not find cracks sealed below the surface or measure the depth of a hardened zone.

Magnetic-particle inspection is another option for ferromagnetic steel. It is more sensitive to surface and near-surface cracks, but it requires suitable equipment and a trained operator. It does not work on aluminum or austenitic stainless steel. For a one-off bracket, penetrant is usually the cheaper practical test. For a structural production job, use the inspection method specified by the drawing, welding procedure, or applicable code.

When the edge will be welded or machined

Do not weld directly over heavy dross, oxide, or a visibly hardened lip. Grind back to bright, sound metal, normally removing at least the rough affected edge rather than just polishing it. A flap disc is less aggressive and gives better control than a hard grinding wheel, but a zirconia flap disc for metal still removes material quickly. Check the finished profile with a straightedge or square so you do not create a bevel that changes joint fit-up.

For a noncritical mild-steel bracket, removing 1 to 2 mm from a rough edge is often adequate. For a fatigue-sensitive joint, pressure component, lifting point, or certified structural assembly, do not choose the cleanup allowance by eye. The required edge preparation, preheat, consumable, and inspection level should come from the qualified welding procedure.

When drilling plasma-cut holes, center-punch on the parent metal rather than on a rounded or hardened rim. A pilot drill may wander or lose its edge on the HAZ. If the hole is important, rough-drill undersize and finish with a sharp drill, reamer, or boring operation after removing the damaged edge.

Preventing excessive HAZ damage

Use the manufacturer’s amperage and material-thickness range, clean dry air, and the correct consumables. Keep the torch square and maintain the recommended standoff. A drag shield can help on thin sheet, while a machine torch with height control gives more consistent results on repetitive work.

Travel speed is a major variable. Too slow produces a wide, overheated edge and heavy dross; too fast causes a bevel, incomplete penetration, and an unstable arc. Cut a test coupon from the same material and thickness, then inspect it before cutting the final part. A plasma cutter consumables kit is worthwhile when worn electrodes or nozzles are causing inconsistent kerfs.

For ordinary tabs, brackets, and noncritical repairs, visual inspection, deburring, and a file check are usually enough. Pay for penetrant or magnetic-particle inspection when a crack could cause injury, expensive rework, or a serious equipment failure. The cheapest inspection method is only a bargain when the consequence of missing a defect is small.