1. Local segregation of weather-resistant elements leads to structural weakness
Q355NHA contains phosphorus (P) to enhance its atmospheric corrosion resistance. However, during smelting and hot-rolling, P is prone to uneven distribution-forming small "P-rich zones" in local areas of the steel.
These P-rich zones have coarser crystal structures and significantly lower plasticity compared to the surrounding normal material. When cutting forces (such as shearing pressure in cold cutting or thermal stress in hot cutting) act on these zones, stress concentrates easily here. Once the stress exceeds the material's fracture limit, cracks start to form at the cut edge.
2. Improper thermal cutting parameters cause brittle heat-affected zones
Most Q355NHA workpieces are cut using thermal methods like flame cutting or plasma cutting. If the process parameters are miscontrolled, the heat-affected zone (HAZ) near the cut will face two critical issues:
First, if the cutting speed is too slow or the flame/plasma power is too high, the HAZ absorbs excessive heat and reaches temperatures above Q355NHA's phase transition point. Second, after cutting, the high-temperature HAZ cools rapidly in the air (especially in low-temperature environments). This rapid cooling transforms the HAZ's structure into hard, brittle martensite. The martensite layer has poor toughness, so even small external impacts or residual stress can trigger cracks along the cut edge.
3. Residual stress and surface impurities worsen stress concentration
Two pre-cut issues can further increase cracking risk:
On one hand, if the Q355NHA workpiece has residual internal stress from previous processes (e.g., cold bending, rolling, or welding), this stress will not be released before cutting. During cutting, the material's internal balance is disrupted, and the residual stress overlaps with the new cutting stress-creating higher stress concentration at the cut.
On the other hand, if the steel surface near the cutting line has not been cleaned (with oil, rust, or oxide scale remaining), problems arise during thermal cutting: oil burns to generate gas, leaving tiny pores in the cut; rust and oxide scale melt and mix into the cut edge, weakening the local material structure. Both issues make the cut more susceptible to cracking under stress.
