Grade Position in the Hull Steel System
DH40 belongs to the high strength hull structural steel family defined by GB/T 712-2011, which is aligned with the unified requirements of the classification societies for shipbuilding steel. Hull steel grades are written as a quality letter plus a strength number. The letter gives the impact test temperature: A at zero, D at minus 20, E at minus 40 and F at minus 60 degrees Celsius. The number is the strength class expressed in kilograms force per square millimetre, so 40 corresponds to a minimum yield strength of 390 MPa.
DH40 therefore means a plate in the 40 kilogram high strength class with impact testing at minus 20 degrees Celsius. It is a normal strength class above the more common AH36 and DH36 grades and below the 460 MPa class used for highly stressed containership and offshore structures.
Chemical Composition
Hull structural steels are deliberately lean in carbon and rely on manganese and microalloying, together with controlled rolling, to reach their strength. The grade is also subject to a maximum carbon equivalent, which is the parameter that governs weldability and preheat requirements in the shipyard.
| Element | Typical specification limit | Function |
|---|---|---|
| Carbon | 0.18 max | Strength, capped low to protect weldability and toughness |
| Silicon | 0.55 max | Deoxidation |
| Manganese | 0.90 to 1.60 | Main strengthening element in the 40 kg class |
| Phosphorus | 0.025 max | Restricted for toughness |
| Sulphur | 0.025 max | Restricted for toughness and weld soundness |
| Niobium, vanadium, aluminium | microalloying additions | Grain refinement through controlled rolling |
One widespread error in secondary descriptions is the claim that DH40 carries more than 0.4 percent carbon. That figure confuses the strength class 40 with a carbon content, and a steel with that level of carbon would fail both the carbon equivalent limit and the impact requirement of the standard. The carbon in this grade stays at or below 0.18 percent.
Mechanical Properties and Impact Requirements
Minimum yield strength: 390 MPa in the standard test direction for the normal thickness range.
Tensile strength: 510 to 650 MPa.
Minimum elongation: 20 percent.
Impact energy: 34 J minimum at minus 20 degrees Celsius, tested in the longitudinal direction.
Delivery condition: as rolled, controlled rolled, normalised or thermomechanically rolled, depending on the ordered variant.
The impact requirement is the reason the grade exists in the D quality. A hull structure operating in cold water is exposed to dynamic loads at low temperature, and the plate must absorb energy without brittle fracture. Impact testing is carried out per heat treatment batch, and the results appear on the certificate together with the chemical analysis and tensile data.
Applications
DH40 plate is consumed by shipyards and by offshore fabricators. Within a ship it is used for the strength deck, sheer strake, longitudinal bulkhead, hatch coaming, deck girders and other primary structural members where the design stress exceeds what ordinary strength hull steel can carry. Reducing plate thickness through the higher yield strength improves dead weight and stability, which is the commercial reason the grade is chosen.
Offshore, the plate appears in the legs and bracing of jack up units, in the hulls of offshore support vessels and barges, in crane pedestals and in deck structures. It is also used for port and shipyard facilities where heavy impact loading on a welded structure has to be tolerated in a marine atmosphere.
Fabrication, Welding and Certification
Cutting of DH40 is done by flame, plasma or laser. Cold forming on shipyard presses is normal, and the plate tolerates the line heating and fairing operations used in hull construction. Welding uses consumables matched to the 390 MPa class, with low hydrogen practice, and preheat is selected from the carbon equivalent and the combined thickness of the joint. Heat input is controlled to protect the toughness of the heat affected zone, and interpass temperature limits are applied for thicker sections.
Because ship steel is a survey item, DH40 is normally supplied against a classification society approval, with the plate marked and the certificate endorsed by the society's surveyor. Third party inspection can also be arranged. The buyer usually specifies not only the grade but the approval route, the plate marking and the required tests, so the purchase order should list them explicitly.
Frequently Asked Questions
Q: What does the number 40 mean in DH40?
A: It is the strength class in kilograms force per square millimetre, corresponding to a minimum yield strength of 390 MPa. It is not a carbon content.
Q: What is the impact requirement?
A: Grade D requires a minimum Charpy V-notch absorbed energy of 34 J at minus 20 degrees Celsius, tested longitudinally.
Q: What is the tensile strength range?
A: The specified range is 510 to 650 MPa, with a minimum elongation of 20 percent.
Q: Is DH40 the same as DH36?
A: Both are D quality high strength hull steels tested at minus 20 degrees Celsius, but the strength class differs, with the 36 grade at 355 MPa minimum yield.
Q: How is weldability controlled?
A: Through a maximum carbon equivalent, restricted residuals and a qualified welding procedure with preheat based on thickness and carbon equivalent.
Q: Does the plate need classification society approval?
A: For ship and offshore construction, yes. The plate is supplied against survey with marking and endorsed certificates as required by the project specification.
