1. Normalizing Temperature: Strictly Adhere to Standard Ranges to Balance Austenitization and Grain Control
Ensure complete austenitization: Temperatures above 890°C enable the full transformation of original microstructures in Q355GNH (such as ferrite and pearlite) into austenite, dissolving carbides precipitated in the steel (e.g., carbides formed by Cr and Cu elements) to achieve component homogenization. If the temperature is below 890°C, austenitization will be incomplete, leaving undissolved carbides easily. This leads to uneven microstructures after subsequent cooling, impairing mechanical properties and corrosion resistance.
Prevent excessive grain coarsening: When the temperature exceeds 950°C, austenite grains grow rapidly. After cooling, a coarse-grained ferrite-pearlite microstructure forms, directly reducing the steel's low-temperature impact toughness (e.g., the impact energy at -40°C may drop from above 35J to below 20J). Additionally, it damages the "dense rust layer formation ability" unique to weathering steel, increasing the risk of subsequent corrosion.
Furthermore, for thick plates with a thickness exceeding 50mm or irregularly shaped parts with uneven cross-sections, the upper temperature limit can be appropriately increased to 930°C (but not exceeding 950°C). This ensures the core of the steel can fully reach the austenitization temperature, avoiding the situation where "the surface is quenched but the core is not".
2. Holding Time: Centered on "Thickness" and Dynamically Adjusted Based on Heating Methods
1. Basic Calculation Principle: Match Holding Time to Thickness
General coefficient method: Holding time (minutes) = Heating coefficient (K) × Effective thickness of the steel (mm). The value of the heating coefficient K depends on the heating equipment:
Ordinary box furnaces/bogie hearth furnaces (slow heat conduction): K = 1.0 ~ 1.5 minutes/mm, meaning 1.0 ~ 1.5 minutes of holding time per 1mm of thickness.
Continuous heating furnaces (high thermal efficiency and good temperature uniformity): K = 0.8 ~ 1.2 minutes/mm, which is approximately 20% shorter than that of box furnaces.
Example: For a 20mm-thick Q355GNH steel plate, when heated in a box furnace, the holding time = 1.2 minutes/mm × 20mm = 24 minutes; when heated in a continuous furnace, it is 1.0 minute/mm × 20mm = 20 minutes.
Minimum holding threshold: Even for ultra-thin Q355GNH materials (e.g., 3~5mm thin plates), the holding time should not be less than 15 minutes. This is to avoid incomplete dissolution of trace alloying elements (such as Cu and Cr) due to excessively short holding time, which would affect the formation of the protective rust layer of weathering steel.
2. Practical Adjustment Details: Adapt to Production Scenarios
Batch quantity: When heating a large batch of Q355GNH steel (e.g., stacking multiple plates in a furnace), the heat transfer between workpieces is blocked. The holding time should be extended by 10%~20% compared to a single-piece heating. For example, if 10 pieces of 20mm-thick plates are heated together in a box furnace, the holding time should be adjusted from 24 minutes to 27~29 minutes.
Cooling method matching: If the subsequent cooling after normalizing is air cooling (the standard cooling method for Q355GNH), the holding time can be controlled according to the basic coefficient. However, if forced air cooling is used (to slightly refine grains for thick plates), the holding time should be appropriately extended by 5%~10% to ensure the austenite structure is sufficiently uniform before cooling, avoiding uneven hardness caused by rapid cooling.
