Two different structural steel plate families
ASTM A572 is a high-strength, low-alloy (HSLA) steel plate that combines a low carbon content with small additions of niobium and/or vanadium. The grade number reflects the minimum yield strength in ksi: Grade 42 (about 290 MPa), Grade 50 (about 345 MPa), Grade 55 (about 380 MPa), Grade 60 (about 415 MPa) and Grade 65 (about 450 MPa). Because strength is obtained through micro-alloying rather than heavy alloying, A572 plates keep good ductility and weldability while allowing designers to reduce section weight in bridges, buildings, transmission towers, truck frames and heavy construction equipment.
ASTM A573 is a carbon-manganese-silicon steel plate intended for structural service at atmospheric temperature where improved notch toughness is important. Its three grades - 58, 65 and 70 - are named after the minimum tensile strength in ksi, approximately 400 MPa, 450 MPa and 480 MPa. Typical uses are storage tanks, welded pressure-retaining structures and other assemblies where predictable toughness at ambient temperature matters more than raw strength.
Chemical composition: A572 Grade 65 against A573 Grade 65
The following values are maximum percentages unless a range is given, taken from the heat analysis requirements of the two specifications.
| Element | ASTM A572 Grade 65 | ASTM A573 Grade 65 |
| C, max | 0.23 | 0.24 |
| Mn | 1.35 max | 0.85-1.20 |
| P, max | 0.04 | 0.035 |
| S, max | 0.05 | 0.04 |
| Si | 0.40 max | 0.15-0.40 |
Three differences matter in practice. First, A573 Grade 65 keeps phosphorus and sulphur lower than A572 Grade 65, which supports better notch toughness and cleaner through-thickness behaviour. Second, A573 fixes silicon in a range of 0.15-0.40 % instead of only capping it, a deoxidation requirement tied to its toughness intent. Third, A573 controls manganese inside a range of 0.85-1.20 % rather than a simple maximum, so the Mn/C ratio stays in a narrower window and strength is more consistent from heat to heat. A572 Grade 65 allows a higher manganese ceiling of 1.35 %, which is one of the ways the HSLA family reaches its higher yield strength.
Mechanical properties compared
| Property | ASTM A572 Grade 65 | ASTM A573 Grade 65 |
| Yield strength, min | 450 MPa | 240 MPa |
| Tensile strength | 550 MPa | 415-530 MPa |
| Elongation, min, 200 mm | 15 % | 20 % |
| Elongation, min, 50 mm | 17 % | - |
The gap in yield strength is the decisive point: A572 Grade 65 delivers roughly 450 MPa minimum yield against about 240 MPa for A573 Grade 65. The A573 grade is therefore not a strength substitute - it is selected for toughness, ductility and weldability at moderate strength. Notice also that A573 Grade 65 is supplied on the basis of a tensile range with a minimum elongation of 20 % on a 200 mm gauge length, a combination that suits formed and welded tank shells.
Notch toughness and impact testing
A573 plate is ordered specifically because improved notch toughness is required. The specification is built around atmospheric-temperature service, and Charpy V-notch impact testing is the normal way the buyer verifies that property: the test temperature and the minimum absorbed energy are stated on the order, and the plates are accepted against those values. This is why A573 appears in tank and welded-structure scopes where brittle behaviour must be excluded.
A572 is fundamentally a strength specification. Grades 42 and 50 are the most widely stocked and are produced with good toughness in normal rolling practice, while Grades 55, 60 and 65 move into territory where toughness must be requested and controlled. When both high strength and verified toughness are needed at the same time, the usual route is either an A572 plate ordered with supplementary toughness requirements or a change of family towards a normalized grade with an explicit impact test regime.
Weldability, forming and fabrication
Carbon equivalent: both families are welded with a carbon equivalent value calculated as C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, expressed in percent. Lower phosphorus and sulphur in A573 Grade 65 make that value easier to keep low at the same strength level.
Preheat: as plate thickness and carbon equivalent rise, preheat and interpass temperature control become mandatory to avoid cold cracking. A572 Grade 65, with its higher manganese ceiling, needs the more disciplined procedure of the two.
Forming: A573 Grade 65, with a minimum elongation of 20 % and a lower yield strength, rolls and forms more easily - an advantage for tank shells and large-diameter cylindrical sections.
Thickness and supply: A572 is commonly stocked across a wide thickness range because of its structural volume; A573 is a more specialised item, so availability, rolling condition and test certificates should be confirmed at the enquiry stage and matched to the drawing requirement.
Which grade should be specified
Storage tanks and welded structures needing verified ambient-temperature toughness: A573 Grade 58, 65 or 70.
Bridges, buildings, towers, crane components and heavy equipment where strength-to-weight ratio drives cost: A572 Grade 50, 55, 60 or 65.
Moderate strength combined with easy forming and welding: A573 Grade 65 is often the more economical fabrication choice despite its lower yield strength.
High strength plus verified toughness together: A572 with supplementary toughness requirements, or a normalized plate grade with a defined impact test.
Frequently asked questions
Q: What is the main difference between ASTM A573 and ASTM A572 steel?
ASTM A573 is a carbon-manganese-silicon plate steel specified for improved notch toughness at atmospheric temperature, while ASTM A572 is a high-strength, low-alloy structural steel offered in grades from 42 to 65 with much higher minimum yield strengths.
Q: Is A573 Grade 65 stronger than A572 Grade 65?
No. A572 Grade 65 has a minimum yield strength of about 450 MPa, while A573 Grade 65 has a minimum yield strength of about 240 MPa with a tensile range of 415-530 MPa. A573 is chosen for toughness rather than strength.
Q: Which steel is used for storage tanks?
A573 plate is the usual choice for storage tanks and welded structures because its specification is written around improved notch toughness at atmospheric service temperature. A572 is used where higher strength is the governing requirement.
Q: How do the chemical compositions differ at Grade 65?
A572 Grade 65 allows up to 0.23 % carbon, 1.35 % manganese maximum, 0.04 % phosphorus and 0.05 % sulphur. A573 Grade 65 allows 0.24 % carbon, manganese in the range 0.85-1.20 %, 0.035 % phosphorus and 0.04 % sulphur, with silicon held between 0.15 % and 0.40 %.
Q: Can A572 steel plate be substituted for A573 steel plate?
Only after engineering review. Substituting the higher-strength A572 grade changes strength, elongation and toughness behaviour, so the impact test temperature and absorbed energy required by the original design must be checked and met before approval.
Q: What controls toughness in these plates?
Chemistry first - carbon, phosphorus, sulphur and the manganese-to-carbon ratio - then rolling practice and, where specified, normalizing. For A573 the impact test temperature and minimum absorbed energy are stated on the purchase order and verified by Charpy V-notch testing.
