ABS Grade A and ABS Grade B are ordinary-strength hull structural steels used for ships, barges and marine structures. They are classed by the American Bureau of Shipping and are specified in the ABS Rules for Building and Classing Steel Vessels, Part 2, Chapter 3, Materials and Welding, Section 2, which covers ordinary-strength hull structural steel and follows the framework of IACS UR W11. Plate certified to this section is not simply a commercial structural steel; it is surveyed, tested and stamped by the classification society before it may be used in a classed vessel.
The ordinary-strength family contains four grades arranged in order of increasing notch toughness: A, B, D and E. Plate is marked AB/A, AB/B, AB/D or AB/E and is certified on the basis of ladle analysis, tension testing and Charpy V-notch impact testing. The rules cover plate, wide flats, bars and shapes up to 100 mm in thickness; material thicker than 100 mm requires special approval. A similar grade family appears in ASTM A131, but certification to ABS class is governed by the ABS Rules rather than by the ASTM limits.
Impact Test Requirements: The Real Difference Between A and B
Grade A and Grade B have essentially the same chemistry limits and exactly the same tensile requirements. What separates them is the impact test temperature and the point at which impact testing may be waived.
| Grade | Test temperature | Energy, t up to 50 mm | 50-70 mm | 70-100 mm | Test frequency |
|---|---|---|---|---|---|
| A | +20 °C (68 °F) | Not required | 34 J long. / 24 J transv. | 41 J long. / 27 J transv. | Waived for t up to 50 mm and for fine-grain normalized material |
| B | 0 °C (32 °F) | 27 J long. / 20 J transv. | 34 J long. / 24 J transv. | 41 J long. / 27 J transv. | Required for all supply conditions, on 25 to 50 t lots |
| D | -20 °C (-4 °F) | Same energy values as Grade B | Same as Grade B | Same as Grade B | Required; as-rolled supply limited to t up to 25 mm |
| E | -40 °C (-40 °F) | Same energy values as Grade B | Same as Grade B | Same as Grade B | Required at all thicknesses; normalized or TMCP |
The practical reading of this table is straightforward. A 40 mm Grade A plate in a warm-service area may be delivered without any impact test at all. The same plate in Grade B must have a Charpy test at 0 °C for every lot. That single sentence explains most of the price and lead-time difference between the two grades.
Chemical Composition Compared
| Element, % | ABS Grade A | ABS Grade B | Note |
|---|---|---|---|
| C | 0.21 max | 0.21 max | 0.23 max is allowed for Grade A sections |
| Mn | 2.5 x C min | 0.80 min | Grade B may use 0.60 min where fully killed or of cold-flanging quality |
| Si | 0.50 max | 0.35 max | Deoxidation practice differs between the grades |
| P | 0.035 max | 0.035 max | Same limit |
| S | 0.035 max | 0.035 max | Same limit |
| C + Mn/6 | 0.40 max | 0.40 max | Carbon equivalent control for weldability |
| Ni, Cr, Mo, Cu | Determined and reported | Determined and reported | No fixed limits; values of 0.02 % or less may be reported as such |
Grade B is the tighter of the two metallurgically. It carries a guaranteed minimum manganese level and a lower silicon ceiling, both of which promote the fine-grain, fully killed practice needed to pass a 0 °C impact test reliably. Grade A, whose manganese minimum is expressed only as a multiple of carbon, can be produced from a wider range of steelmaking routes.
Mechanical Properties: Identical Requirements for A and B
| Property | Requirement | Source |
|---|---|---|
| Yield point, min | 235 N/mm² (34 ksi) | ABS Part 2, Ch. 3, Sec. 2, Table 2 |
| Tensile strength | 400-520 N/mm² (58-75 ksi) | Same; the upper limit may be 550 for Grade A sections |
| Elongation, min | 22% standard; 24% for 50 mm gauge length specimens | ABS Part 2, Ch. 3, Sec. 2, Tables 2 and 3 |
Because the tension requirements are shared, Grade A and Grade B are interchangeable from a strength point of view. The selection decision is therefore always a toughness decision, taken together with the supply condition the shipyard can obtain at the required thickness.
Condition of Supply and Practical Selection
Grade A plate is commonly supplied as-rolled, and normalized, control-rolled or thermo-mechanically control processed material is also accepted under the approved controlled-manufacturing provisions.
Grade B as-rolled supply is normal at moderate thicknesses, but every lot is impact tested, so the cost of testing is built into the material from the start.
Grade D above 25 mm requires killed fine-grain steel supplied normalized, thermo-mechanically processed or control-rolled; Grade E is supplied normalized or thermo-mechanically processed.
A fixed 40 mm threshold for normal rolling is not part of the ABS Rules. The governing factors are the grade, the thickness and the supply condition combinations set out in the impact test table.
Use Grade A for general hull plating, bulkheads, superstructures and secondary members where the rules allow it, and Grade B wherever the rules or the specification call for verified 0 °C notch toughness, typically in thicker shell and strength-deck regions.
Move to Grade D or Grade E for exposed decks, thick plate and vessels that trade in cold climates.
Chinese Classification Society ordinary-strength hull steel uses the same A, B, D and E framework under IACS UR W11, so CCS Grade B and ABS Grade B both represent the 0 °C toughness class. Cross-society substitution still has to be checked against the rules of the society that will class the vessel.
Frequently Asked Questions
Q: What is the main difference between ABS Grade A and ABS Grade B?
The impact test temperature. Grade A is tested at +20 °C and may be exempt from impact testing at thicknesses up to 50 mm, while Grade B is tested at 0 °C and must be impact tested in every supply condition.
Q: Do ABS Grade A and Grade B have different yield strength?
No. Both require a minimum yield point of 235 N/mm² and a tensile strength of 400 to 520 N/mm². Only the toughness requirement changes between the two grades.
Q: Is ABS Grade A the same as ASTM A36?
No. Both are ordinary-strength carbon steels of similar strength level, but ABS Grade A is a hull structural grade certified under the ABS Rules, with its own chemistry limits, marking and survey requirements.
Q: Up to what thickness can ABS Grade A or B be ordered?
The ordinary-strength hull structural steel section covers plate up to 100 mm thick. Thickness above 100 mm requires special approval, and the impact energy requirement increases as the thickness band rises.
Q: Can ABS Grade A be substituted for ABS Grade B?
Not without approval. Substituting the lower-toughness grade changes the impact performance on which the structural design in the cold or exposed regions may depend, so the class society must agree to the change.
Q: What certificate should be supplied with ABS hull plate?
Plate is issued with a class certificate and marked AB/A, AB/B, AB/D or AB/E. The certificate should record the ladle analysis, tension test results and the Charpy V-notch impact energy at the specified test temperature.
