DNV Grade DH36 Quality Inspection Standards and Acceptance Criteria for Shipbuilding Steel Plates

1. Chemical Composition Testing Standards

The chemical composition of DNV Grade DH36 steel plates must strictly comply with the requirements of the DNV Rules for Marine Steel. The limits for key elements and carbon equivalent are as follows:

C ≤ 0.18%

Si: 0.10% - 0.50%

Mn: 1.00% - 1.60%

P, S ≤ 0.035%

Carbon Equivalent (CEV) ≤ 0.38%

Each batch of steel plates must undergo sampling inspection via spectroscopic analysis. If any element exceeds the specified limit, the batch shall be deemed non-conforming and may not be used in DNV-certified offshore engineering projects.

2. Mechanical Property Acceptance Criteria

Yield Strength: ≥ 355 MPa

Tensile Strength: 490 - 620 MPa

Elongation: Must meet the structural ductility requirements specified in DNV regulations.

Low-Temperature Impact Toughness: Samples taken from the core must undergo a -20°C Charpy V-notch impact test; the impact energy absorption must meet the values specified by DNV to ensure safety during low-temperature service.

All mechanical property test data must be fully recorded, and a formal test report must be issued to serve as the primary basis for project acceptance.

3. Dimensional Accuracy and Tolerance Standards

The thickness, width, and length tolerances of DNV Grade DH36 steel plates must comply with international precision standards for marine steel. Specific requirements are as follows:

Thickness tolerance: ≤ ±0.5 mm

Flatness error: ≤ 3 mm/m

Flatness and perpendicularity of cut edges: Must meet shipbuilding assembly process requirements

The above dimensional tolerance standards also apply to oversized plates (maximum width 4900 mm, maximum length 25000 mm) to ensure assembly accuracy during hull construction.

4. Surface Quality Inspection Requirements

Qualified DNV Grade DH36 steel plates must have a smooth and flat surface, free of defects that affect structural performance, such as cracks, wrinkles, scratches, and oxidation pits.

Minor localized surface defects may be repaired by grinding, provided the repair depth is within the permissible dimensional tolerance range.

Any severe surface defect that compromises the structure of the base metal shall be deemed non-conforming.

5. Standards for Internal Defect Inspection (Non-Destructive Testing)

DNV Grade DH36 steel plates used for critical marine structures must undergo 100% ultrasonic testing (UT) to detect internal defects such as delamination, porosity, and inclusions.

The testing grade must comply with the relevant DNV standards to ensure the absence of harmful internal defects.

Random radiographic testing (RT) should also be conducted on batch products to verify the density of the internal microstructure.

6. Certification Documents and Acceptance Records

Finished DH36 steel plates must be accompanied by a DNV third-party inspection certificate and an EN 10204 Class 3.2 material certificate. The certificates must include complete production records, test data, and the DNV certification mark.

Projects without complete certification documents must not proceed to construction; this is a mandatory acceptance requirement for international offshore engineering.