Plasma Cutting Stainless Steel: Speed, HAZ, and Edge Quality

Plasma cuts stainless steel cleanly from sheet gauge through 1-1/4" or more, depending on machine amperage. It’s the go-to cutting process for stainless in fabrication shops because oxy-fuel cannot cut stainless steel at all. Stainless cuts about 15-20% slower than mild steel at the same amperage due to its higher melting point (2,550-2,650F vs 2,500F for mild steel) and lower thermal conductivity. The primary concern with plasma-cut stainless is the heat-affected zone (HAZ) along the cut edge, which loses some corrosion resistance.

For structural and general fabrication stainless, plasma cutting with compressed air is perfectly adequate. For food-grade, pharmaceutical, marine, or high-corrosion applications, the cut edges need grinding or machining to restore full corrosion resistance, or you can use nitrogen/hydrogen assist gas on capable machines for cleaner cuts.

Amperage and Speed Settings for Stainless Steel

Stainless steel requires the same or slightly higher amperage as mild steel for equivalent thicknesses. Travel speed is 15-20% slower.

For a cross-material thickness chart, see plasma cutting thickness chart.

Heat-Affected Zone (HAZ) Concerns

The HAZ is the area adjacent to the cut where the base metal was heated enough to change its metallurgical properties but not enough to melt. On stainless steel, the HAZ creates several potential problems:

Chromium Carbide Precipitation (Sensitization)

When austenitic stainless steel (304, 316) is heated to 800-1,500F (425-815C) and held at that temperature range, chromium atoms combine with carbon to form chromium carbides at the grain boundaries. This pulls chromium out of the surrounding metal, dropping the local chromium content below the 10.5% minimum needed for corrosion resistance.

The result is a narrow band along the cut edge that’s susceptible to intergranular corrosion. In corrosive environments (acids, saltwater, food processing), this sensitized zone can corrode preferentially.

Mitigation:

• Use “L” grade stainless (304L, 316L) with low carbon content (0.03% max). Low carbon reduces carbide precipitation.
• Cut at maximum speed to minimize heat input and time in the sensitization temperature range.
• Machine or grind the cut edge to remove the affected zone (typically 0.010-0.030" deep).
• Use nitrogen assist gas to reduce oxidation and the width of the HAZ.

Oxide Layer Formation

Cutting stainless with compressed air creates a visible oxide layer (blue, purple, tan, or brown discoloration) along the cut edge. This chromium oxide layer is thicker and less protective than the natural passive oxide film that gives stainless its corrosion resistance.

The discoloration is more than cosmetic. It indicates a region where the surface chemistry has changed. For non-critical applications, it doesn’t matter. For corrosion-critical applications, remove it by:

• Grinding with a flap disc or grinding wheel
• Pickling with a stainless-specific pickling paste
• Electrochemical cleaning (weld cleaning machines)
• Mechanical brushing with a stainless steel wire brush (never use a carbon steel brush on stainless)

Width of the HAZ

The HAZ width depends on cutting speed, amperage, and material thickness:

• Thin stainless (under 1/8"): HAZ is narrow, typically 0.010-0.020" with proper speed
• Medium stainless (1/8" to 3/8"): HAZ is 0.020-0.040"
• Thick stainless (over 3/8"): HAZ can extend 0.040-0.080"

Faster cutting speed reduces the HAZ width. CNC plasma with optimized speed produces a narrower HAZ than handheld cutting.

Speed for Minimal Discoloration

Cutting speed is the primary tool for controlling both HAZ width and surface discoloration on stainless:

Too slow: Excessive heat input. Wide HAZ. Heavy discoloration extending far from the cut edge. Possible distortion on thin material. Heavy dross on the bottom edge.

Optimal speed: Narrow HAZ. Discoloration limited to 1/16" or less from the cut edge. Light or no dross. Minimal distortion.

Too fast: The arc doesn’t fully penetrate. The cut has a heavy bevel. The bottom edge may not cut through completely, leaving a rough, jagged edge.

The spark stream from the bottom of the cut is your speed indicator. Sparks should trail behind the torch at about 15-20 degrees from vertical. Straight-down sparks mean you’re going too fast. Sparks trailing far back mean you’re too slow.

Nitrogen and Mixed Gas Options

For shops that cut a lot of stainless and need better edge quality, alternative plasma gases improve results significantly.

Nitrogen (N2)

Nitrogen is an inert gas that doesn’t react with stainless steel. Using nitrogen as the plasma gas instead of compressed air:

• Eliminates the oxygen that causes edge oxidation and discoloration
• Produces a cleaner, brighter cut surface
• Reduces HAZ width by 10-20%
• Slightly slower cutting speed than compressed air at equivalent amperage

Nitrogen is the standard plasma gas for CNC systems cutting stainless for food, pharmaceutical, and architectural applications.

Nitrogen/Hydrogen Mix (H-35, F5)

A blend of 65% nitrogen and 35% hydrogen (commonly called H-35 or F5) produces the best plasma cut quality on stainless steel:

• Even cleaner edge than pure nitrogen
• Faster cutting speed than nitrogen alone (hydrogen adds energy to the plasma)
• Reduced dross
• Excellent edge quality suitable for welding without grinding

The trade-off is cost and complexity. The gas is more expensive, and the system needs a separate gas supply and control. This is primarily a CNC/industrial option.

Compressed Air

For most handheld cutting and general fabrication, compressed air is fine. The edge quality is acceptable for structural work, and the oxide layer grinds off quickly for welding prep. The simplicity and zero gas cost make air the practical choice for shops that cut stainless occasionally.

Stainless Steel Grades and Cutting Differences

Austenitic (304, 316, 321)

The most common stainless grades. Non-magnetic (or slightly magnetic after cold working). Plasma cuts these well. Sensitization is a concern on standard carbon grades; use “L” grades for corrosion-critical work.

304 and 316 cut at the same speeds and amperages. 316 is slightly harder to cut than 304 due to its molybdenum content, but the difference is negligible in practice.

Ferritic (430, 409)

Magnetic stainless with lower nickel content. Cuts similarly to austenitic grades. The HAZ on ferritic stainless can develop grain growth that reduces toughness, but for most fabrication purposes, this isn’t a concern.

Duplex (2205, 2507)

Higher strength than austenitic, with mixed austenitic/ferritic structure. Cuts well but requires slightly higher amperage than 304 at the same thickness due to higher strength and toughness. The HAZ can affect the austenite/ferrite balance, which matters for corrosion-critical applications.

Martensitic (410, 420, 440)

Hardenable stainless used for cutlery, valve parts, and wear components. Plasma cuts these grades, but the HAZ will be hard and potentially brittle (it’s essentially a hardened zone). If the cut edge will see stress, it may need post-cut tempering.

Cutting Thick Stainless (Over 3/8")

Thick stainless presents additional challenges:

Higher amperage required. Stainless needs about 10-15% more amperage than mild steel at the same thickness because of the higher melting point and lower thermal conductivity.

Slower speed. Travel speed drops significantly on thick stainless. The reduced speed means more heat input and a wider HAZ.

More dross. Thick stainless produces heavier dross that bonds more tenaciously to the bottom edge. A belt grinder or angle grinder with a flap disc removes it, but it adds cleanup time.

Edge bevel increases. Plasma cuts naturally produce a slight bevel (the top of the cut is wider than the bottom). On thick stainless, this bevel can reach 3-5 degrees with a handheld torch. CNC with height control reduces it to 1-2 degrees.

For cuts over 3/4" on stainless, verify your machine’s rated cut capacity specifically for stainless, which is typically 15-20% less than the mild steel rating.

Welding Prep on Plasma-Cut Stainless

Plasma-cut stainless edges need preparation before TIG or MIG welding:

1. Remove dross from the bottom edge with a flap disc or file
2. Grind the oxide layer from the cut face and 1/4" back from the edge on both sides
3. Check for bevel angle. If the plasma cut produced more than 2-3 degrees of bevel and the joint design requires a square edge, machine or grind to square
4. Clean with acetone to remove any grinding residue or contamination
5. Use a stainless-specific grinding disc to avoid carbon steel contamination (embedded carbon steel particles cause rust spots)

Troubleshooting Stainless Plasma Cuts

Heavy blue/purple discoloration extending far from the cut: Cutting speed is too slow. Increase travel speed. If already at maximum comfortable speed, the material may be too thick for your machine’s amperage.

Cut doesn’t penetrate through: Not enough amperage, air pressure too low, or consumables worn. Check all three. Stainless needs slightly higher amperage than mild steel, so if you’re running at the same settings, increase by 5-10A.

Heavy dross that won’t chip off: Speed too slow combined with excessive amperage. Reduce amperage and increase speed. Also verify air pressure is at the machine’s specification. For general troubleshooting, see plasma cutter troubleshooting.

Wavy or irregular cut edge: Inconsistent torch height or travel speed. Handheld cutting on stainless amplifies any variation. Use a straightedge guide and maintain consistent speed and standoff.

Black, crusty edge instead of clean discoloration: The material overheated significantly, usually from going very slowly or stopping mid-cut. The oxide is thick and deeply embedded. Grind it off completely before welding. For comparison with other cutting processes, see plasma cutting vs oxy-fuel.