Detailed Breakdown
1. ASTM A588 Grade A Requirements
The ASTM A588 standard is clear and mandatory for impact toughness. For Grade A, the requirements are as follows:
Test Temperature: -30°C (-22°F)
Minimum Charpy V-Notch Impact Energy:
For material 4 inches (102 mm) and thicker: The minimum average value for three specimens is 20 ft-lbf (27 J).
For material under 4 inches (102 mm) thick: The minimum value is 15 ft-lbf (20 J) for a full-size specimen. If sub-sized specimens are used, the required energy is proportionally reduced.
This means that every shipment of ASTM A588 Grade A steel must be certified to meet these impact values at -30°C.
2. Q355NH Requirements
The Chinese standard GB/T 4171 for weathering steel defines impact toughness based on the suffix in the grade name:
"NH" indicates suitability for "Atmospheric Corrosion Resistance" but the standard primarily specifies impact test temperatures of -20°C and -40°C.
Standard Impact Requirements for Q355NH:
At -20°C: Minimum average impact energy is 34 J.
At -40°C: Minimum average impact energy is 27 J.
Crucially, the standard does not list a mandatory requirement for -30°C. Therefore, standard Q355NH material is not guaranteed to have a specific toughness value at -30°C.
3. How to Make Q355NH Meet a -30°C Requirement
While not standard, it is possible to source Q355NH that satisfies a -30°C requirement through one of two methods:
Supplier Agreement (Most Common): The purchaser can specify in the contract that the Q355NH material must meet a certain impact energy value (e.g., ≥ 27 J) at -30°C. Reputable steel mills can produce this by adjusting their chemical composition and rolling process, and will provide a Mill Test Certificate (MTC) proving compliance with this additional requirement.
Selecting a Higher Toughness Grade: The GB/T 4171 standard has a higher grade called Q355GNH. The "G" stands for high toughness (Gao ren xing). This grade has stricter impact requirements and would be more likely to comfortably exceed a -30°C requirement, though its standard test temperatures are still -20°C and -40°C.
