1. Thorough pre-cut preparation: Eliminate hidden risks
Clean the cutting surface: Use a wire brush, sandpaper, or degreaser to remove oil, rust, oxide scale, and dust from the area 5–10 cm around the cutting line. This prevents impurities from burning into gas (causing pores) or mixing into the cut edge (weakening structure) during thermal cutting.
Release residual stress (for workpieces with prior processing): If the Q355NHA has undergone cold bending, rolling, or welding, perform low-temperature stress relief annealing before cutting. Heat the workpiece to 250–350°C, hold for 1–2 hours, then cool slowly. This reduces internal stress accumulation and avoids stress overlap during cutting.
Select appropriate cutting methods based on thickness: For thin plates (≤10mm), use cold cutting (e.g., high-speed sawing with carbide blades) to minimize thermal impact; for thick plates (>10mm), prioritize plasma cutting (more precise heat control) over flame cutting (to avoid excessive heat input).
2. Optimize cutting parameters: Avoid brittle structure formation
Control thermal cutting heat input:
For flame cutting: Use a medium-sized cutting nozzle (e.g., No. 3–4 for 12–20mm plates), set the oxygen pressure to 0.5–0.7 MPa, and adjust the cutting speed to 250–350 mm/min. Avoid slow speeds (which overheat the HAZ) or too-high pressure (which causes uneven cuts).
For plasma cutting: Use a 30–60A plasma power source for 10–20mm plates, set the cutting speed to 400–600 mm/min, and keep the torch height at 3–5mm. This ensures a narrow HAZ and reduces rapid cooling brittleness.
Preheat for thick plates or low temperatures: When cutting plates ≥16mm or working in environments <5°C, preheat the cutting area to 80–120°C (use a propane torch for local preheating). Do not exceed 150°C (to avoid softening the material) and ensure uniform preheating (no cold spots).
3. Add post-cut treatments: Relieve stress and repair edges
Post-cut stress relief (for critical components): After cutting, especially for thick plates or workpieces used in load-bearing structures, perform a low-temperature post-heat treatment. Heat the cut edge and surrounding 20mm area to 200–300°C, hold for 30–60 minutes, then cool in air. This accelerates hydrogen escape and reduces HAZ stress.
Deburr and polish the cut edge: Use a file or grinding wheel to remove burrs, microcracks, or oxide layers on the cut edge. Even tiny burrs can act as stress concentration points-smoothing the edge prevents cracks from initiating and expanding.
