Optimizing Plasma Cutting of Thick Duplex Steel: Parameters for Square Edges and Minimal HAZ

Optimizing Plasma Cutting of Thick Duplex Steel: Parameters for Square Edges and Minimal HAZ

Plasma cutting is the go-to process for slicing through thick plate quickly and efficiently. But when your material is high-value, corrosion-resistant duplex stainless steel, the stakes are much higher. A poor cut doesn't just look bad—it can compromise the very properties you paid for.

Duplex steels (like 2205 / UNS S32205) derive their strength and corrosion resistance from a near 50/50 austenite-ferrite microstructure. Excessive heat from cutting can skew this balance, creating a large Heat-Affected Zone (HAZ) with nitrided, oxidized edges that are hard, brittle, and susceptible to corrosion.

This guide provides a practical framework for optimizing your plasma cutting parameters to achieve clean, square edges with minimal HAZ on thick duplex steel (typically ½" / 12mm and above).

The Goal: More Than Just a Cut

For duplex steel, a successful plasma cut is defined by:

1. Square, Dross-Free Edges: Minimal to no refractory dross that requires extensive grinding.

2. Minimal Heat-Affected Zone (HAZ): A narrow band of microstructural change.

3. Preserved Corrosion Resistance: The cut edge should not be a weak point for pitting or crevice corrosion.

4. Arc Stability: A smooth, consistent cut without beveling or top-edge rounding.

Achieving this comes down to a precise balance of power, speed, gas, and equipment.

The Four Pillars of Optimization

1. Equipment and Consumables: The Non-Negotiable Foundation

You cannot optimize a cut with worn-out parts. This is the most critical step.

• Plasma System: A High-Definition (HD) or XTRA-Definition (XD) plasma system is strongly recommended for thicknesses above 1" (25mm). These systems use a constricted arc and advanced gas sequencing for a sharper, cleaner cut. Conventional air plasma will work but will produce a wider HAZ and more bevel.

• Consumables: Use OEM consumables and replace them before they are completely worn. A worn electrode or nozzle will distort the arc, increase the kerf width, and dump excess heat into the material.

• Torch Height Control (THC): A stable, consistent standoff distance is vital for edge squareness. Manual cutting is not recommended for critical work.

2. Gas Selection: The Key to Chemistry and Cooling

Gas is not just for the plasma arc; it's also for shielding. For duplex steel, you should never use compressed air as the plasma gas. The oxygen and nitrogen in air will contaminate the cut edge.

The standard gas setup for stainless and duplex steels is Nitrogen or Argon-Hydrogen for the plasma gas and CO2 or Air for the secondary shielding gas.

• Plasma Gas (Primary):

  • Nitrogen (N₂): The most common and cost-effective choice. It provides good cut quality and speed. It can lead to some nitriding on the edge, but this is manageable with correct parameters.

  • Argon-Hydrogen (H-35 or similar, e.g., 65% Ar / 35% H₂): The premium choice for the best possible edge quality on thick material. The hydrogen adds thermal conductivity, making the arc hotter and more focused, which yields a squarer edge and removes dross more effectively. Requires a system rated for multi-gas use.

• Shield Gas (Secondary):

  • Carbon Dioxide (CO₂) or sometimes Air is used. The shield gas's job is to blow the molten metal away and help cool the top edge, reducing oxidation and HAZ.

3. Dialing in the Parameters: The Art of Balance

These parameters are interdependent. The values below are a starting point for a modern HD plasma system (e.g., Hypertherm, ESAB, Lincoln) using Nitrogen plasma gas. Always consult your machine's manual.

The Interplay:

• Too Slow / Too Much Amperage: Puts excessive heat into the material. Results in a wide HAZ, top-edge rounding, and heavy, difficult-to-remove low-speed dross.

• Too Fast / Too Little Amperage: The arc won't penetrate fully, leading to a beveled edge and stubborn high-speed dross that re-welds to the bottom of the plate.

• Incorrect Gas Pressure: Low pressure weakens the arc; high pressure can cause splatter and an unstable arc.

4. Post-Cut Best Practices: Finishing the Job

Even a perfect cut requires some attention.

• Cooling: Allow the plate to cool naturally. Do not quench it with water, as this can induce unwanted stresses.

• Dross Removal: A well-optimized cut on duplex will have little to no dross, often removable by hand or with a single tap from a hammer. Avoid aggressive grinding on the cut edge.

• Cleaning: Remove the heat tint (the blue/orange oxidation layer) from the top and bottom edges. This layer is depleted of chromium and is susceptible to corrosion. Use a dedicated stainless steel wire brush (never one used on carbon steel) or appropriate abrasive flap discs.

• Critical Applications: For parts exposed to highly corrosive environments, consider lightly grinding or machining the cut edge to remove the entire HAZ and restore a pristine, corrosion-resistant surface.

Troubleshooting Common Issues on Duplex

Conclusion: Precision is Paramount

Cutting thick duplex steel is a testament to the principle of "garbage in, garbage out." You cannot compensate for worn equipment or incorrect gases with parameter tweaks.

The recipe for success is:

1. Start with a well-maintained HD plasma system and new consumables.

2. Use the correct gases— Nitrogen or Argon-Hydrogen, never air.

3. Find the sweet spot between amperage and cut speed for your specific thickness. Use the manufacturer's charts as your starting point.

4. Finish correctly by removing heat tint from the cut edge to restore corrosion resistance.

By treating the plasma process as a precise thermal operation rather than just a rough cutting tool, you ensure that your high-performance duplex steel components perform as designed, from their core to their very edge.