1. What is the basic definition and core properties of DH620 steel pipe?
DH620 is a Grade D ultra-high-strength hull structural steel in the GB/T 712-2023 standard, designed specifically for extreme polar and deep-sea environments. Its core properties include:
Yield strength ≥ 620 MPa, tensile strength 720-890 MPa, suitable for ultra-large polar vessels and deep-sea equipment;
-40°C low-temperature impact toughness (KV2 ≥ 34 J), meeting the requirements of polar cryogenic operations;
A balance of high strength and low-temperature toughness is achieved through Ni-Cr-Mo-V-Nb composite alloying and a TMCP+tempering process.
2. What are the main application scenarios for DH620?
Polar heavy icebreakers: Hulls and load-bearing frames for ice thicknesses greater than 3 meters;
Ultra-deepwater drilling platforms: Jackets and leg structures supporting depths of 5,000 meters;
LNG carriers: Support systems for LNG tanks operating at -162°C.
3. What technical upgrades does the DH620 offer compared to the AH620?
Cryogenic performance: Meets impact energy standards at -40°C (AH620 requires only -20°C), with 1.2%-1.8% Ni and 0.25%-0.35% Mo added to the alloy;
Purity control: P and S content ≤ 0.015%, stricter than AH620 (≤ 0.020%);
Process optimization: In-line quenching (DQ) is used to refine the grain size to ASTM grade 11 or higher.
4. What technical key points require special attention when processing the DH620? Welding Process: Ultra-low hydrogen welding consumables (such as AWS E15018-G) must be used, with a preheat temperature of 180-220°C (higher for plate thickness > 30mm).
Cold Forming Restrictions: Cold bending is prohibited; medium-temperature forming (400-600°C) or hydraulic hot bending must be used.
Post-weld Treatment: Thick plate welds require dehydrogenation treatment (350°C for 3 hours) and phased array ultrasonic testing (PAUT).
5. How is DH620 compliance verified? What are the key test items?
Full-Process Testing:
Chemical Composition: C ≤ 0.10%, Ni 1.2%-1.8%, CEV ≤ 0.50%;
Mechanical Properties: Impact Energy ≥ 24 J at -40°C, Yield Ratio ≤ 0.85;
Non-Destructive Testing:
100% Ultrasonic Testing (UT) for internal defects;
Residual Stress Testing (X-ray Method) in Critical Areas.
