LR DH42 steel plate is a high-strength hull structural grade developed for the marine industry and approved under the rules of Lloyd's Register. The DH prefix sets the impact test temperature at minus 20 degrees Celsius, and the 42 indicates a minimum yield strength of 420 MPa. The combination of high yield strength, low-temperature toughness and controlled chemistry makes the grade suitable for ships, offshore platforms and other structures that work under heavy dynamic loading in cold seawater.
Chemical Composition of LR DH42
Chemistry is balanced to deliver strength without sacrificing weldability or heat-affected zone toughness. Carbon, phosphorus and sulfur are restricted, while a small micro-alloying addition supports grain refinement during controlled rolling.
| Element | Content, % |
|---|---|
| C | 0.20 max |
| Si | 0.55 max |
| Mn | 1.70 max |
| P | 0.030 max |
| S | 0.030 max |
| Al | 0.015 min |
| N | 0.020 max |
| Nb | 0.02 to 0.05 |
| V | 0.03 to 0.10 |
| Ti | 0.02 max |
Manganese provides the main solid-solution strengthening effect, silicon assists deoxidation, and aluminium fixes nitrogen so that free nitrogen cannot embrittle the heat-affected zone of welded joints. Carbon equivalent, calculated as Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, is controlled to keep welding procedures within practical preheat levels, and crack sensitivity can be assessed with Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B.
Mechanical Properties and Impact Toughness
LR DH42 plate maintains a minimum yield strength of 420 MPa and a tensile strength range of 530 to 680 MPa across the standard thickness range. Charpy V-notch impact energy is specified at minus 20 degrees Celsius with higher requirements as thickness increases.
| Thickness, mm | Yield, min MPa | Tensile, MPa | Elongation, min % | Impact at minus 20 C, transverse, J | Impact at minus 20 C, longitudinal, J |
|---|---|---|---|---|---|
| Up to 50 | 420 | 530 to 680 | 20 | 28 | 42 |
| Over 50 to 70 | 420 | 530 to 680 | 20 | 28 | 42 |
| Over 70 to 100 | 420 | 530 to 680 | 20 | 28 | 42 |
Energy 1 refers to transverse test specimens and Energy 2 to longitudinal specimens; the difference reflects the anisotropy introduced by rolling. The uniform requirements across the thickness range show that the grade retains toughness even in heavy sections, which is essential for wave-loaded structures.
Heat Treatment and Delivery Condition
LR DH42 plates are produced with controlled rolling and, when ordered, with normalising or quenching and tempering to refine the microstructure and stabilise properties. Heat treatment is chosen according to thickness and to the toughness level required by the project specification. Normalised plate offers a homogeneous ferrite and pearlite structure, while quenched and tempered plate provides higher strength with good ductility for demanding offshore components. Plates are supplied with mill test certificates recording chemical analysis, tensile properties and impact energy for each heat.
Impact Testing and Structural Integrity
Impact testing is central to the qualification of marine plate. Specimens are extracted in the rolling direction and transversely, notched, and broken at the specified sub-zero temperature to measure absorbed energy. This verifies that the steel will not fail in a brittle manner when a hull structure is exposed to cold water, ice loading or a sudden impact. Additional non-destructive checks such as ultrasonic examination can be applied to confirm internal soundness of thick plates before they enter fabrication, and weld procedure qualification tests confirm that the joint matches the parent plate toughness.
Applications in Marine and Offshore Engineering
Hull shell, deck and bottom plating of tankers, bulk carriers and container ships.
Offshore platform legs, braces and deck structures exposed to low ambient temperature.
Bulkheads, cargo hold structures and hatch covers requiring high strength with toughness.
Dock gates, pontoons, barges and floating structures in harbour service.
Heavy welded fabrications such as crane structures and deck machinery foundations.
Frequently Asked Questions
Q: What impact test temperature applies to LR DH42?
Grade D plate is impact tested at minus 20 degrees Celsius, with minimum Charpy V-notch energy specified according to thickness and specimen orientation.
Q: What yield strength does the 42 in the grade indicate?
It stands for a minimum yield strength of 42 kilogram-force per square millimetre, equivalent to 420 MPa.
Q: How does LR DH42 differ from LR AH36?
DH42 provides a higher minimum yield strength of 420 MPa and is tested at minus 20 degrees Celsius, while AH36 is a 355 MPa grade tested at zero degrees Celsius.
Q: Is normalising always required?
Normalising is not mandatory for every thickness but is commonly specified for thicker plates and for projects that demand a refined, homogeneous microstructure.
Q: Can the plate be welded in a shipyard with standard consumables?
Yes, provided the qualified welding procedure is followed and preheat is applied as required for the plate thickness and ambient conditions.
Q: What thickness range is normally available?
Plates from 6 mm to 100 mm are standard, and the mechanical property requirements are uniform across this range.
