What ABS Grade DQ70 Means
ABS Grade DQ70 is an extra-high-strength quenched and tempered hull structural steel plate approved by the American Bureau of Shipping. The designation is read in three parts: D records a Charpy V-notch impact test at -20 °C, Q shows that the plate is supplied in the quenched and tempered condition, and 70 stands for a minimum yield strength of 690 MPa.
Quenching followed by tempering produces a fine tempered structure that lifts yield and tensile strength far above ordinary and high-strength hull steels, while keeping the toughness and weldability that a welded marine structure needs. Plates of this class are ordered when the design is governed by strength and weight at the same time.
Chemical Composition and Mechanical Properties
The limits below follow the ABS requirements for the 690 MPa quenched and tempered class. Low carbon, low sulphur and controlled nitrogen are essential for toughness and weldability in thick sections.
| Element | Limit, % |
|---|---|
| C | 0.20 max |
| Si | 0.55 max |
| Mn | 1.70 max |
| P | 0.025 max |
| S | 0.025 max |
| N | 0.020 max |
| Microalloying and grain refining additions | Controlled by the rules |
| Property | Requirement |
|---|---|
| Yield strength, min | 690 MPa |
| Tensile strength | 770-940 MPa |
| Elongation, min | 14 % |
| Impact test | Charpy V-notch at -20 °C |
| Delivery condition | Quenched and tempered |
These values apply to the thickness range covered by the rules for this grade, and the minima are adjusted for heavier plate. The same 690 MPa class is also listed as AQ70, EQ70 and FQ70, which differ only in impact test temperature at 0 °C, -40 °C and -60 °C respectively.
Heat Treatment, Delivery Condition and Testing
Delivery condition: quenched and tempered as standard practice for this class.
Arctic and low-temperature variants: EQ70 and FQ70 for service down to lower temperatures.
Thickness-direction testing: Z15, Z25 or Z35 where through-thickness strain is expected in welded joints.
Impact testing: Charpy V-notch at the grade temperature, taking specimens in the direction required by the order.
Ultrasonic examination to ASTM A435, ASTM A577 or ASTM A578 at the acceptance level specified.
Mill test certificates to EN 10204 3.1 or 3.2 together with classification society unit certification.
Surface preparation: shot blasting and priming, or a protective coating, when specified for the project.
Cutting and edge preparation to the fabrication drawing.
Equivalent Designations in Other Classification Systems
The 690 MPa quenched and tempered property class is listed under a different designation in each set of rules. The designations below are the ones commonly quoted for the same class:
| Classification society | Designation |
|---|---|
| DNV | NV D690 |
| GL | GL-D690 |
| LR | DH69 |
| BV | BV/D690 |
| CCS | D690 |
| NK | KD70 |
| KR | RD70 |
| RINA | RI/D690 |
Because each society writes the same strength class in its own way, plate for a project is normally ordered against one named society and grade in the contract so that the certificates match the design approval.
Typical Applications and Fabrication
Offshore platform hulls, decks and support structures, where high strength limits steel weight in harsh sea states
Oil and gas structures: drilling rigs, production facilities and subsea equipment
Pipelines, risers and subsea infrastructure exposed to high pressure and corrosive conditions
Offshore wind turbine foundations and transition pieces
Heavy deck equipment, crane pedestals and lifting structures
Hull and deck structure of large vessels where reduced plate thickness lowers topside weight
Fabrication in the 690 MPa class is more demanding than ordinary hull steel. Welding needs qualified procedures, matched low-hydrogen consumables, controlled heat input and interpass temperature, and preheat on thick sections. Any post-weld heat treatment must be agreed in advance so that the tempered condition of the plate and the toughness of the heat-affected zone are not degraded.
FAQ
Q: What does the Q in DQ70 stand for?
Q records the quenched and tempered delivery condition. D indicates a Charpy impact test at -20 °C and 70 indicates a minimum yield strength of 690 MPa.
Q: What is the tensile strength of ABS Grade DQ70?
The requirement is 770-940 MPa, with a minimum yield strength of 690 MPa and a minimum elongation of 14 %.
Q: How does DQ70 differ from EQ70 and FQ70?
Only the impact test temperature changes: -20 °C for DQ70, -40 °C for EQ70 and -60 °C for FQ70. Yield strength, tensile range and chemistry limits are the same for all three.
Q: Is DQ70 supplied normalized or as-rolled?
No. This class is supplied in the quenched and tempered condition, because the strength and toughness combination cannot be reached by normalizing or as-rolled practice at this level.
Q: Which ultrasonic test standards are used for DQ70 plate?
Ultrasonic examination is carried out to ASTM A435, ASTM A577 or ASTM A578, depending on the acceptance level and the beam direction required by the purchase order.
Q: Can DQ70 be welded in the shipyard or fabrication shop?
Yes, but only with qualified welding procedures, matched low-hydrogen consumables and controlled heat input. Thick sections normally require preheat and careful interpass temperature control.
