1. Preheating (The Most Critical Step)
Why: Q235NH contains alloying elements like copper, chromium, and nickel, which increase its hardenability. Flame cutting creates a rapid heating and cooling cycle, which can lead to the formation of a hard, brittle layer of martensite at the cut edge. This can cause micro-cracking.
Solution: Preheat the area to be cut. A preheat temperature of 100-150°C (212-302°F) is generally recommended. This slows down the cooling rate after cutting, preventing the formation of hard, crack-sensitive structures.
2. Cutting Technique and Quality
Clean Surface: Ensure the surface is free of dirt, paint, and heavy scale before cutting. Scale can interfere with the cut and cause irregularities.
Correct Parameters: Use the correct oxygen pressure, nozzle size, and cutting speed for the thickness of the material. Using too high an oxygen pressure or too slow a speed can result in a rough, gouged cut edge.
Smooth Cut: Aim for a smooth, continuous cut. A poor-quality cut with notches and deep grooves can act as stress concentration points, potentially initiating cracks, especially in the hardened HAZ.
3. Post-Cut Treatment (Essential for Performance)
Why: The flame cutting process leaves a hardened HAZ and slag (dross) on the bottom edge. This slag is unalloyed and will not have the same weathering resistance as the base metal. If left on, it will rust differently, leading to unsightly streaking and potential undermining of the cut edge.
Solution:
Slag Removal: Immediately after cutting, remove all slag and dross from the cut edge using a chipper, grinder, or file.
Grinding: Grind the cut edge smooth. This removes the hardened layer and any micro-cracks, restoring a uniform surface.
Weathering Restoration: The heat from cutting destroys the protective patina-forming ability in the HAZ. To ensure uniform corrosion resistance and color development:
Lightly grind or abrasive blast the entire heat-affected zone (usually about 25-50mm back from the cut edge).
This cleans the surface and allows the protective weathering patina to form evenly across the entire piece, including the cut edges.
