Laser Cutting Duplex Steel: 5 Parameters to Reduce Heat-Affected Zones & Preserve Corrosion Resistance

A perfect laser cut on duplex steel is invisible. You shouldn’t see the heat damage—but your corrosion testing will.

Laser cutting is the preferred method for shaping duplex stainless steels like 2205 (UNS S32205/S31803) and super duplex 2507 (S32750) due to its precision and speed. However, the intense, localized heat input creates a Heat-Affected Zone (HAZ). In duplex steels, an uncontrolled HAZ can be catastrophic, destroying the delicate 50/50 ferrite-austenite phase balance and precipitating chromium nitrides and other intermetallic phases. This turns a highly corrosion-resistant alloy into a failure point.

Controlling this isn’t magic—it’s a science of balancing five key parameters.

1. Power Density & Cutting Mode (The Foundation)

The goal is to achieve clean vaporization (keyhole cutting) rather than melting and ejection. This is a function of power density (Watts/cm²).

• Problem: Low power density causes excessive melting. The molten material is ejected by the assist gas, but heat soaks into the material, creating a large, overheated HAZ.

• Solution: Use high-power density for a narrow, clean cut.

  • Fiber Lasers (>4 kW): Excel at this due to their extremely high beam intensity and small spot size.

  • Parameter: Peak Power > 6 kW is ideal for thicknesses above 10mm. Don’t just run at “100% power”; use pulsed mode for thinner gauges to control heat input.

Data Point: A study cutting 12mm 2205 plate showed a ~60% reduction in HAZ width (from 450µm to 180µm) by increasing power density from 2×10⁶ W/cm² to 6×10⁶ W/cm² and using nitrogen assist gas.

2. Cutting Speed: The Delicate Balance

Speed is the most critical lever for controlling heat input.

• Too Slow: Excessive heat per unit length is applied. Heat conducts sideways, widening the HAZ and causing secondary annealing and phase precipitation.

• Too Fast: The kerf won’t clear, leading to dross adhesion, re-melting, and a ragged cut edge that is a prime site for pitting initiation.

• The Sweet Spot: The optimal speed is where the cut is just complete with no dross. This minimizes the time for heat to diffuse into the material.

Rule of Thumb: For a 6kW fiber laser cutting 10mm 2205 duplex with N₂, the optimal speed is typically between 1.2 – 1.8 meters/minute. Always refer to your laser manufacturer’s charts and conduct test cuts.

3. Assist Gas Selection & Pressure: More Than Just Cleaning

The assist gas has two jobs: eject molten material and protect the cut edge.

• Nitrogen (N₂): The standard for high-quality, oxidation-free cuts.

  • Pressure: High pressure is critical (12-20 bar for thicker sections). It blows molten material out cleanly before it can transfer heat back into the sidewall.

  • Benefit: Produces a bright, clean edge with minimal oxide formation, preserving the base metal’s corrosion resistance.

• Oxygen (O₂): Creates an exothermic reaction that adds energy to the cut.

  • Why to Avoid for Duplex: The reaction creates a thick, oxidized, low-chromium layer on the cut face. This layer is highly susceptible to pitting corrosion and acts as a initiation site, undermining the duplex alloy’s primary advantage. Only use oxygen if surface finish and corrosion resistance are irrelevant.

4. Nozzle Standoff Distance & Focus Position

Precision in beam delivery is non-negotiable.

• Nozzle Standoff: A consistent, tight standoff distance (typically 0.5 – 1.0 mm) is crucial for maintaining gas pressure and laminar flow into the kerf. A larger or inconsistent distance causes turbulent flow, reducing cutting efficiency and allowing more heat to build up.

• Beam Focus Position: This determines where the power density is highest.

  • Surface Focus: Best for thin sheets.

  • Mid-Focus (⅓ into the material): Often ideal for medium thicknesses (6-15mm), providing a consistent energy distribution through the kerf.

  • Bottom Focus: Can be used for very thick plates to ensure penetration at the bottom.

  • Incorrect Focus: Leads to a wider kerf, more melting, and a larger HAZ. Document the optimal focus for each material thickness.

5. Pulsed vs. Continuous Wave Mode

For thinner materials or highly sensitive components, pulsing the laser beam is a powerful tool.

• Continuous Wave (CW): Constant energy output. Efficient but can lead to heat buildup.

• Pulsed Mode: Delivers energy in short, high-peak-power bursts.

  • Advantage: Allows the material to cool slightly between pulses, drastically reducing the total heat input and overall HAZ size.

  • Application: Essential for cutting thin sheets (<3mm), intricate shapes, or near-holes where heat accumulation is a major risk.

Post-Cutting Best Practice: The Non-Negotiable Step

Even with perfect parameters, a slight HAZ exists. For any component going into corrosive service (chlorides, acids), post-cut treatment is mandatory.

1. Abrasive Blasting: Use a non-metallic abrasive (e.g., glass bead, alumina) to remove the microscopic, heat-oxidized layer from the cut face. This is the quickest and most common method to restore corrosion resistance.

2. Pickling: Apply a nitric-hydrofluoric acid paste to dissolve the chromium-depleted layer and restore the passive oxide layer. This is the most effective method for critical applications.

3. Passivation: A nitric acid bath to enhance the chromium oxide layer post-mechanical cleaning.

Verification: Use a Feritscope® to check the cut edge for signs of detrimental phase transformation. A significant change in ferrite number (FN) indicates an out-of-spec HAZ.

Summary: The 5-Parameter Checklist for a Perfect Cut

Actionable Takeaway: Start with your laser manufacturer’s recommended parameters for 2205/2507. Conduct test cuts. Measure the HAZ microscopically. Tune one parameter at a time—starting with speed and gas pressure—until you achieve a clean, dross-free cut. Then, always plan for and specify a post-cut cleaning process to guarantee the material’s innate corrosion performance is fully restored.