The heat treatment process for Q550NH weathering steel is carefully designed to optimize its mechanical properties and corrosion resistance while maintaining weldability. Below is a detailed breakdown of its typical heat treatment stages and considerations:
1. Standard Heat Treatment for Q550NH
Q550NH is typically supplied in the thermomechanically controlled processed (TMCP) or normalized condition, depending on the manufacturer and application requirements:
(1) Thermomechanical Controlled Processing (TMCP)
Process:
Hot rolling at controlled temperatures (≈850–950°C) followed by accelerated cooling.
Combines deformation and temperature control to refine grain structure.
Advantages:
Higher strength and toughness without additional heat treatment.
Improved weldability due to lower carbon equivalent (CEV).
(2) Normalizing (Optional)
Process: Heating to 880–920°C, holding, then air cooling.
Purpose:
Homogenizes microstructure (ferrite + pearlite).
Enhances impact toughness for low-temperature applications.
2. Post-Weld Heat Treatment (PWHT)
Not mandatory for most applications but may be required for:
Thick sections (>30 mm) to relieve residual stresses.
Critical structures (e.g., pressure vessels, offshore platforms).
Typical PWHT Parameters:
Temperature: 550–600°C (below Ac1 to avoid phase changes).
Holding Time: 1–2 hours per 25 mm thickness.
Cooling: Slow furnace cooling.
3. Stress Relieving
Applied to large welded assemblies to minimize distortion and cracking.
Conditions: 600–650°C for 1–2 hours, followed by slow cooling.
