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The heat treatment objectives of carbon steel pipes can be categorized into four types, with each type corresponding to a distinct process window:
① Remodeling the structure (normalizing, full annealing). Heat the steel to 30-50℃ above Ac₃ to re-austenitize it, and then cool it at different speeds to achieve the desired grain size and uniformity of structure. Normalizing with air cooling, where the cooling rate is faster than that of annealing, results in better strength and toughness; full annealing with furnace cooling yields a softer and more ductile steel.
② Restoring plasticity (recrystallization annealing). This process is only applicable to tubes after cold processing. Cold drawing elongates the grain, increases dislocation density, and hardens and embrittles the material. By heating above the recrystallization temperature (typically around 650-750°C for carbon steel), the deformed grains can re-nucleate and grow, restoring plasticity. However, no phase transformation occurs, so the grains cannot be refined and only "reset".
③ Eliminate residual stress (stress relief annealing / PWHT). The heating temperature should be below Ac₁ (approximately 500-680℃ for carbon steel), without altering the microstructure, and only reducing the dislocation density and residual stress through the recovery process. This step is necessary for cold-drawn tubes, welded tubes, and cold-formed pipe fittings.
