How the FH620 Designation Is Built
FH620 is an ultra high strength structural steel plate for ships and marine engineering. The designation is systematic. The letter F is the quality grade and means the steel is impact tested at minus 60 degrees Celsius. The letter H marks it as a high strength grade. The number 620 is the minimum yield strength in megapascals. Standard strength ship steels use the letters A, B, D and E, numbered 235 for the yield level, and those four letters correspond to impact testing at plus 20, zero, minus 20 and minus 40 degrees Celsius respectively. In the high strength series the quality letter is added to the strength number, so FH620 means a 620 MPa class plate tested at the coldest of the standard quality levels.
Marine structural steel of this type is produced to GB/T 712-2011, which covers structural steel for ships and marine engineering and lists grades from A, B, D and E through AH32 to FH690, including AH620, DH620, EH620 and FH620 at the ultra high strength end. The plate is also certified under the rules of the classification societies, with China Classification Society classification and certification from the other IACS member societies available depending on the project and the flag requirement.
Mechanical Requirements and Delivery Condition
| Property | FH620 requirement |
|---|---|
| Minimum yield strength | 620 MPa |
| Tensile strength | Specified as a range above the yield level, typically 700 MPa and above for this class |
| Impact test temperature | Minus 60 degrees Celsius, Charpy V-notch |
| Delivery condition | Quenched and tempered, or thermo-mechanically rolled with tempering according to the mill route and grade |
| Typical plate size | 3 mm to 80 mm thick, 1500 mm to 4000 mm wide, 4000 mm to 19000 mm long |
At this strength level the steel is produced with a low carbon, low impurity composition and careful control of the carbon equivalent, because both toughness at minus 60 degrees Celsius and weldability have to be preserved in the same heat. Plates are supplied in the quenched and tempered condition in most cases, and where a thermo-mechanically rolled route is used a tempering treatment follows. The mechanical properties are verified on each heat and the results are reported on the mill certificate.
Offshore and Marine Applications
Hull structure of ocean going, coastal and inland waterway vessels, particularly in regions of high stress such as the sheer strake and the bilge.
Offshore platform structures, including legs, piles and bracing, where strength to weight ratio and low temperature toughness are both critical.
Derricks, cranes, pillars and deck machinery mountings on offshore and port installations.
Structural parts of shipyards, dockside equipment, coastal power plants and harbour machinery.
Specialist fabricated items such as heavy brackets, transition pieces and reinforced nodes.
The common thread in these applications is a structure that operates in cold, wet and highly loaded conditions and cannot be repaired easily once installed. That is why the grade is specified with a minus 60 degrees Celsius impact requirement rather than a room temperature one, and why the certification chain matters as much as the tensile result.
Marking, Inspection and Thickness Direction Testing
Each plate carries the manufacturer identification, the classification society approval mark, and the technical parameters that allow it to be traced: heat number, steel grade and specification, dimensions and, where applicable, the inspection stamp. Marks are applied by paint stencil or label, and the surface is required to be smooth and free from defects that would affect the service or the coating. Plates ordered for critical connections can be supplied with through thickness properties to GB/T 5313 in the Z15, Z25 or Z35 grades, which controls the reduction of area in the short transverse direction and prevents lamellar tearing under weld restraint. Ultrasonic testing to GB/T 2970 or to the relevant classification society requirement is applied where the order specifies it.
Fabrication of a 620 MPa plate needs more discipline than a normal strength ship steel. Weld procedure qualification is mandatory, low-hydrogen consumables are essential, and preheat and heat input windows are wider than for AH36 because the hardenability is higher. Cutting by plasma or waterjet is preferred for components that will be highly stressed, and flame cut edges are dressed where fatigue is a design consideration.
Ordering a Marine Plate Consignment
An order for FH620 should fix the grade, the standard, the impact test temperature, the delivery condition, the plate size and the inspection level, together with the classification society that will witness or certify the tests. Certification, marking and documentation requirements should be stated rather than assumed, because different societies apply slightly different survey and marking requirements and a plate that does not carry the right mark cannot be used in a classified structure. Documentation normally includes the mill certificate with chemical analysis and mechanical results, the certification society certificate, the ultrasonic test report where applicable, and packing and marking details that allow each plate to be identified on arrival.
Frequently Asked Questions
Q: What does the F in FH620 mean?
A: F is the quality grade and indicates that Charpy V-notch impact testing is carried out at minus 60 degrees Celsius. The other quality letters used in ship steel designations correspond to higher test temperatures.
Q: What yield strength does FH620 provide?
A: The number 620 stands for a minimum yield strength of 620 MPa. Tensile strength is specified as a range above that level for this steel class, and the exact values are confirmed on the mill certificate.
Q: Which standard covers this plate?
A: GB/T 712-2011 covers structural steel for ships and marine engineering and includes the ultra high strength grades up to FH690. Classification society rules apply in addition when the plate is used in a classed structure.
Q: Why is through thickness testing ordered?
A: Where a heavily restrained welded connection pulls through the plate thickness, lamellar tearing is a risk. Ordering Z15, Z25 or Z35 to GB/T 5313 controls the short transverse reduction of area and reduces that risk.
Q: What sizes are available?
A: FH620 is normally supplied from 3 mm to 80 mm thick, 1500 mm to 4000 mm wide and 4000 mm to 19000 mm long, with wider and heavier plate available on heavy plate lines for lower strength grades.
