1. Q: What material is GB/T 712 FH690 steel pipe made of?
A: It is an ultra-high-strength hull structural steel specified in the Chinese national standard (GB/T 712). The "F" in FH690 stands for exceptional toughness (typically requiring ultra-low-temperature impact resistance down to -80°C), and "H690" indicates a yield strength of 690 MPa or higher. This material is designed for extremely harsh environments (such as deep-sea, polar regions, and military equipment). It is one of the highest-toughness and most comprehensive grades of hull steel currently available.
2. Q: What is the core difference between FH690 and EH690 and DH690?
A:
Low-temperature toughness: FH690 (-80°C) > EH690 (-60°C) > DH690 (-40°C). FH690 can withstand the ultra-low temperatures of the polar deep sea and the extreme impact loads of military equipment. Alloy Design: FH690 has a higher Ni (nickel) content (typically ≥5%) and contains trace amounts of Ti (titanium) and Boron (boron) to refine the grain size and enhance resistance to brittle fracture.
Applications: FH690 is used in nuclear submarine pressure hulls, deep-sea probes, and polar research vessels; EH690/DH690 is primarily used in conventional high-strength ships.
3. Q: What are the mechanical properties and special requirements of FH690?
A:
Key Performance:
Yield Strength ≥ 690 MPa, Tensile Strength ≥ 790-940 MPa;
Elongation ≥ 16% (higher than EH690/DH690), -80°C Impact Energy ≥ 27 J (one of the strictest standards globally).
Special Testing: Requires passing the CTOD (Crack Tip Opening Displacement) test and the detonation resistance test (for military applications).
4. Q: Why is FH690 called a "deep-sea strategic material"?
A:
Ultra-deep pressure resistance: Can withstand deep-sea pressures exceeding 1,000 meters (e.g., the pressure chamber material for the "Struggler" manned submersible).
Hydrogen embrittlement resistance: Ultra-low sulfur and phosphorus content (S/P ≤ 0.005%) and rare earth treatment prevent embrittlement caused by high-pressure hydrogen penetration in the deep sea.
Military value: Used in strategic nuclear submarines, its stealth (low magnetism) and explosion resistance far exceed those of ordinary ship steel.
5. Q: What are the difficulties and solutions in producing and welding FH690?
A:
Production difficulties:
Vacuum degassing smelting and electroslag remelting are required to ensure purity;
After rolling, multiple stages of quenching and tempering (QT) heat treatment are required to balance strength and toughness. Welding Process:
Welding Materials: Specialized Ni-Cr-Mo welding wire (e.g., AWS A5.28 ER110S-G);
Process Control: Preheat ≥ 180°C, interpass temperature ≤ 200°C, immediate post-weld dehydrogenation treatment (300°C for 2 hours);
Nondestructive Testing: 100% ultrasonic and radiographic testing.
