1. Core Time Component: Soaking (Hold) Time
Rule of thumb: 1–2 hours per 25mm of plate thickness.
For thin plates (e.g., 12–20mm): Soaking time = 1 hour (sufficient to penetrate the material and relieve stress in the weld/HAZ).
For medium-thick plates (e.g., 25–50mm): Soaking time = 1–2 hours (e.g., 2 hours for 50mm-thick plates, to ensure stress relief in the core of the material).
For thick plates (e.g., >50mm): Soaking time = 2–3 hours per 25mm (e.g., 4 hours for 100mm-thick plates, as thicker sections have more concentrated residual stress).
Reason: The soaking stage allows atomic diffusion in the steel's microstructure, which redistributes and dissipates residual stress. Insufficient soaking time leads to incomplete stress relief, increasing the risk of delayed cracking.
2. Heating Time: Slow, Uniform Temperature Rise
Component size and thickness:
Small/thin components (e.g., 12mm-thick brackets): 1–2 hours to reach 550–600°C (heating rate ≈ 300–500°C/hour).
Large/thick components (e.g., 50mm-thick bridge girders): 3–5 hours to reach the target temperature (heating rate ≈ 100–200°C/hour).
Critical requirement: The maximum heating rate should not exceed 200°C/hour for plates >25mm. Rapid heating creates uneven expansion between the surface and core, introducing new tensile stress that negates the purpose of annealing.
3. Cooling Time: Controlled Slow Cooling
Thin plates (<25mm): 4–6 hours to cool from 600°C to 300°C (at 50°C/hour), then air cooling (no time control needed).
Thick plates (>50mm): 8–12 hours to cool to 300°C (at 50°C/hour), followed by air cooling.
Example: For a 50mm-thick SMA570W weldment, cooling from 600°C to 300°C takes (600–300)°C ÷ 50°C/hour = 6 hours.
4. Total Cycle Time Example
Heating time: 4 hours (to reach 600°C at 150°C/hour).
Soaking time: 2 hours (per 25mm rule).
Cooling time: 6 hours (to 300°C at 50°C/hour).
Total: ~12 hours (plus additional time for air cooling to room temperature, if needed).
