Precautions for Laser Cutting of NM500 Wear-resistant Steel Plate & Cutting Accuracy for Different T

1. Core Difficulties in Cutting NM500 Material (Understand Material Before Process Control)

1. Secondary hardening occurs at cutting edges under high temperature, forming invisible cracks that easily cause edge chipping and fracture under impact service conditions.
2. High thermal stress leads to warping and dimensional out-of-tolerance under unilateral continuous heating.
3. High-hardness molten slag has poor fluidity, resulting in heavy dross on the bottom of thick plates and inclined cutting sections.
4. Oxide scale on the plate surface reflects laser light, reducing laser absorption and causing piercing defects at starting points.

Applicable laser cutting thickness range: 6–25mm; plates thicker than 25mm are not recommended for laser cutting. Waterjet cutting or fine plasma cutting are preferred alternatives.

2. Full Set of Precautions for Laser Cutting NM500

2.1 Equipment & Consumable Selection Requirements

1. Matching Laser Power
   • 6–12mm: Fiber laser power ≥3000W
   • 14–20mm: Fiber laser power ≥6000W
   • 20–25mm: High-power laser of 12000W or above Forced cutting of thick plates with low power will produce rough sections, cracks and enlarged dimensional tolerances.
2. Nozzles & Lenses
   • Adopt thickened copper nozzles. The molten slag of wear-resistant steel is high-hardness, so nozzle wear rate is twice that of ordinary carbon steel. Inspect and replace nozzles every shift.
   • Clean focusing lenses regularly. Smoke from wear plate processing easily coats lenses, causing energy attenuation and dimensional drift.
3. Worktable Tooling Clamp the steel plate fully without overhang. Clear steel slag on the worktable to prevent inclined plates and inconsistent dimensional deviation between top and bottom surfaces.

2.2 Gas & Focal Position Process Control (Prevent Cracks & Control Roughness)

1. Gas Selection (Core Parameter)
   • High-precision cutting without hardened layer: High-pressure nitrogen (12–22bar). Inert gas cooling keeps heat-affected zone below 0.8mm, oxide-free cutting edges without secondary hardening, suitable for assembly and welding parts.
   • Low-cost blanking: Oxygen. Exothermic reaction increases heat input and easily generates microcracks at cutting edges. Only applicable for rough blanking of non-stressed parts; post-grinding to remove hardened layer is mandatory. Nitrogen cutting is uniformly recommended for precision industrial workpieces.
2. Focal Position
   • Thin plate 6–10mm: Focus 0.5–1mm below the plate surface
   • Medium plate 12–20mm: Focus at 1/2 of plate thickness
   • Thick plate 20–25mm: Focus at 1/3 lower plate thickness to ensure uniform full penetration and reduce bevel angle.

2.3 Cutting Speed & Path Programming (Control Deformation & Cracks)

1. Benchmark Cutting Speed Reduce speed by 20%–30% compared with carbon steel of identical thickness for stable cutting; avoid high-speed piercing and traversal.
   • 6mm: 1.8–2.2m/min; 10mm: 1.0–1.4m/min; 16mm: 0.6–0.9m/min; 20mm: 0.3–0.5m/min
2. Corner & Sharp Angle Treatment Activate 20% speed reduction at corners plus 0.5s cooling delay; enable cooling point function in software to blow cooling gas after laser shutdown, avoiding heat accumulation, edge spalling and microcracks at corners.
3. Nesting & Micro-joint Design (Control Thermal Deformation)
   • Add 0.8–1.2mm micro-joints for long strip or large-size workpieces instead of full cutting at once, to avoid twisting from instantaneous stress release.
   • Adopt symmetrical staggered cutting paths to prevent long-term unilateral heating.
   • Segment cutting for large plates with 5–10mm reserved connecting strips; separate all workpieces uniformly after full cutting.
4. Piercing & Termination Parameters 0.05–0.1s laser delay at piercing to guarantee full penetration of starting points; 0.1–0.15s gas delay after laser shutdown to eliminate bottom dross and missing corners at termination.

2.4 Temperature Control & Cooling (Primary Measure to Eliminate Cracks)

1. Forced air cooling or water quenching is prohibited. Excessive temperature difference directly induces quenching cracks.
2. Cool workpieces naturally to room temperature (minimum 30min) before unloading; hoisting immediately after cutting is forbidden.
3. Intermittently stop production to cool the worktable during mass processing of plates ≥20mm to avoid continuous heat accumulation.

2.5 Pre-processing & Post-processing of Steel Plates

1. Pre-cutting treatment: Grind off heavy oxide scale and anti-rust paint on plate surfaces to reduce light reflection and eliminate burn marks at piercing points.
2. Mandatory post-cutting procedures:
   • Nitrogen cutting: Light grinding to remove bottom dross only.
   • Oxygen cutting: Grind 1–2mm hardened layer on cutting edges, otherwise cracking will occur during service.
   • Workpieces requiring flaw detection: Conduct magnetic particle inspection for edge microcracks after grinding.

2.6 Avoidance of Common Defects

1. Edge microcracks: Excessive heat input, oxygen cutting or rapid cooling → Switch to nitrogen cutting, reduce cutting speed and apply natural slow cooling.
2. Heavy bottom dross: Insufficient gas pressure, offset focal position or excessive cutting speed → Increase nitrogen pressure and readjust focal position.
3. Unstable dimensional fluctuation: Unclamped plates, contaminated lenses or worn nozzles → Fasten tooling, clean optical path and replace worn nozzles.
4. Workpiece warping: Continuous unilateral cutting, missing micro-joints or rapid cooling → Adopt symmetrical cutting paths, add micro-joints and natural slow cooling.

3. Cutting Accuracy Comparison Table for NM500 of Different Thicknesses (Industrial Measured Standards)

Supplementary Accuracy Explanation

1. Mechanical positioning accuracy of machine tools: Theoretical upper limit is ±0.02–0.03mm repeat positioning. Actual tolerance is determined by plate thermal deformation and process parameters; tolerance expands with increasing plate thickness.
2. Accuracy attenuation of oxygen cutting: Tolerance is 0.05–0.1mm larger than nitrogen cutting for identical thickness, with larger bevel angle and doubled heat-affected zone width.
3. Minimum small hole limit: Holes ≥φ3 are feasible for plates ≤12mm; minimum hole diameter for 16–25mm plates shall be at least 1.2 times plate thickness. Undersized holes will form ellipses and exceed dimensional tolerance.
4. Reference Comparison with Other Cutting Processes
   • Fine plasma cutting: Tolerance ±0.3–0.5mm, bevel angle 2–4°
   • Waterjet cold cutting: Tolerance ±0.10–0.15mm, zero heat-affected zone, ideal for high-precision wear-resistant plates thicker than 25mm

4. Machining Selection Recommendations

1. 6–20mm plates for precision flanges, bearing housings, excavator wear liners and bolt hole parts: Nitrogen fiber laser cutting, high precision, crack-free and one-step forming.
2. 20–25mm plates for low-precision blanking: 12000W+ laser with strictly controlled cutting speed and cooling process.
3. Plates thicker than 25mm, impact-resistant wear parts with zero tolerance for thermal quenching cracks: Directly select waterjet cutting to eliminate heat-affected zones and crack risks.