Two Extra-High-Strength Grades from the Same ABS Family
ABS EQ70 and ABS DQ70 are quenched and tempered extra-high-strength shipbuilding steels classified by the American Bureau of Shipping. Both offer a minimum yield strength of 690 MPa, equivalent to 100 ksi, and both are intended for primary marine structures such as ship hulls, offshore platforms and heavy lifting equipment. Because the strength level is identical and the chemistry limits are the same, the two grades are often listed as alternatives on a drawing. The real distinction is toughness: the letter before the Q fixes the low-temperature Charpy V-notch requirement, and that single parameter decides which grade is acceptable for a given route and climate.
Decoding the Grade: E, D, Q and 70
| Element of the designation | Meaning |
|---|---|
| A or B | Charpy V-notch impact test at 0 °C |
| D | Charpy V-notch impact test at -20 °C |
| E | Charpy V-notch impact test at -40 °C |
| F | Charpy V-notch impact test at -60 °C |
| Q | Quenched and tempered delivery condition |
| 70 | Minimum yield strength of 690 MPa, that is 100 ksi |
Reading EQ70 as a whole therefore gives extra-high-strength steel, quenched and tempered, tested for 27 J class impact toughness at -40 °C. DQ70 is the same steel quality family with the impact test set at -20 °C. Neither grade is heat treated differently in practice; the quench and temper route is used for both, and it is the specification of the impact temperature, backed by a certified test, that separates them.
Chemical Composition
| Element, max % | ABS EQ70 | ABS DQ70 |
|---|---|---|
| C | 0.20 | 0.20 |
| Si | 0.55 | 0.55 |
| Mn | 1.70 | 1.70 |
| P | 0.030 | 0.030 |
| S | 0.030 | 0.030 |
| N | 0.02 | 0.02 |
The chemistry limits are identical, which is exactly why the difference in behaviour comes from the impact requirement rather than from the alloy design. A mill producing both grades from the same heat is usual practice; the plate is tested to the colder requirement for the E grade and to the D requirement for the D grade, and the certificate records the outcome. For higher toughness classes the specification also calls for fine grain practice and often for a minimum nickel addition, and the steel is normally produced with low sulphur for cleanliness.
Mechanical Properties and Charpy Requirements
| Property | ABS EQ70 | ABS DQ70 |
|---|---|---|
| Minimum yield strength | 690 MPa | 690 MPa |
| Tensile strength | 770-940 MPa | 770-940 MPa |
| Minimum elongation | 14% | 14% |
| Charpy V-notch test temperature | -40 °C | -20 °C |
| Impact energy, transverse | 41 J | 41 J |
| Impact energy, longitudinal | 62 J | 62 J |
The energy figures are expressed as transverse and longitudinal minima because specimen orientation changes the result substantially: a transverse bar samples across the rolling direction and returns a lower absorbed energy than a longitudinal bar from the same plate. Classification societies also scale the required energy with thickness and grade level, so the values in a project specification should always be read together with the thickness range and the edition of the rules being applied.
How the Impact Temperature Changes Design
Grade D steel is tested at -20 °C and grade E at -40 °C. In service terms, DQ70 suits vessels and structures operating in temperate and moderately cold waters, while EQ70 covers the additional margin needed for Arctic routes, cold-region offshore installations, ice-class hulls and structures that must remain serviceable after accidental cooling. The strength level of the two grades means both are welded with matching high-strength consumables under qualified procedures with controlled heat input; preheat and interpass temperature limits are more demanding than for ordinary shipbuilding plate, and the toughness of the heat-affected zone is verified as part of the procedure qualification rather than assumed.
Typical Applications
Offshore platform legs, bracings and deck structures where weight saving drives the design.
Ice-class and Arctic route hull structures, bow and stern regions exposed to low temperature.
Heavy lifting equipment, crane pedestals and jacking systems.
Subsea structures, mooring components and large welded fabrications requiring certified low-temperature toughness.
Ordering and Certification Notes
State the grade exactly as EQ70 or DQ70; a shortening to Q70 loses the toughness information entirely.
Confirm the test temperature and the energy level against the project specification and the applicable rule edition.
Require a certificate that reports both chemistry and Charpy results, with the specimen orientation identified.
Check the maximum thickness for which the quenched and tempered properties are certified.
Keep the heat treatment record available; re-heating or hot forming after delivery can invalidate the tested properties.
Frequently Asked Questions
Q: What is the main difference between ABS EQ70 and ABS DQ70?
Only the Charpy V-notch test temperature: -40 °C for EQ70 and -20 °C for DQ70. Strength, chemistry and delivery condition are the same.
Q: Are the two grades interchangeable?
EQ70 satisfies the DQ70 requirement as well, since it is tested at a lower temperature. The reverse substitution is not permitted without approval.
Q: What does the Q indicate?
Quenched and tempered, the heat treatment route required to develop the 690 MPa minimum yield strength with adequate toughness.
Q: What is the minimum yield strength of both grades?
690 MPa, which is 100 ksi, with a tensile range of 770 to 940 MPa.
Q: Why are the impact energies different for transverse and longitudinal specimens?
Transverse specimens cut across the rolling direction and absorb less energy, so the rules set a lower acceptance value for that orientation.
Q: Which grade should be specified for cold-region projects?
EQ70, because the -40 °C test temperature provides the toughness margin required for Arctic and ice-class service.
