What ASME SA302 Grade B Steel Plate Is
ASME SA302 Grade B is a manganese-molybdenum pressure vessel steel plate published in ASME SA-302/SA-302M, a specification technically identical to ASTM A302/A302M. Plates of this specification are intended for pressure vessels, boilers, heat exchangers and any other containment equipment that holds a gas or liquid at high pressure. Grade B is the workhorse grade of the family because its manganese and molybdenum content delivers a minimum yield strength of 345 MPa together with good notch toughness after normalising.
One point is frequently misread on supplier pages: the strength of Grade B does not come from nickel. No nickel addition is specified for SA302 Grade B, and the nickel-bearing grades of this specification are identified separately. Designers should therefore specify Grade B, Grade C or Grade D according to the actual composition table rather than assuming that all SA302 plates are nickel steels.
Chemical Composition of SA302 Grade B
Carbon in SA302 Grade B is stepped according to plate thickness, so the maximum permitted carbon content rises as the section becomes heavier. The remaining elements are single-limit values.
| Thickness | C max, % | Si, % | Mn, % | P max, % | S max, % | Mo, % |
|---|---|---|---|---|---|---|
| T 25 mm and under | 0.20 | 0.13 - 0.45 | 1.07 - 1.62 | 0.035 | 0.035 | 0.41 - 0.64 |
| Over 25 mm to 50 mm | 0.23 | 0.13 - 0.45 | 1.07 - 1.62 | 0.035 | 0.035 | 0.41 - 0.64 |
| Over 50 mm | 0.25 | 0.13 - 0.45 | 1.07 - 1.62 | 0.035 | 0.035 | 0.41 - 0.64 |
The manganese range of 1.07 % to 1.62 % and the molybdenum range of 0.41 % to 0.64 % are what separate this grade from ordinary carbon plate. Molybdenum raises the elevated-temperature strength and the hardenability of the steel, while the relatively high manganese level contributes solid-solution strengthening and improves the response of the plate to normalising. Phosphorus and sulphur are each limited to 0.035 % maximum to protect toughness in heavy sections.
Mechanical Properties of SA302 Grade B
| Property | Requirement |
|---|---|
| Yield strength, min | 345 MPa |
| Tensile strength | 550 - 690 MPa |
| Elongation, min, 200 mm gauge length | 15 % |
| Elongation, min, 50 mm gauge length | 18 % |
The minimum yield strength of 345 MPa is roughly double that of a plain SA285 plate, which is exactly why SA302 Grade B is chosen for heavy-wall vessels: a thinner wall carries the same design pressure, reducing both weight and welding volume. The tensile range of 550 MPa to 690 MPa is obtained in the normalised condition, and the elongation minima of 15 % on a 200 mm gauge length and 18 % on a 50 mm gauge length confirm that the plate retains enough ductility for rolling and pressing.
Size Range, Heat Treatment and Delivery
Thickness: 5 mm - 300 mm
Width: 1500 mm - 4200 mm
Length: 3000 mm - 18000 mm
Rolling type: boiler plate and pressure vessel plate
Heat treatment: normalised, or normalised and tempered
SA302 Grade B is normally supplied in the normalised condition, and normalising plus tempering is used when the purchaser requires a more stable microstructure for subsequent forming or for heavy-section service. The plate is not usually supplied as-rolled because the specified strength and toughness combination depends on the normalising treatment.
Fabrication and Service Applications
This manganese-molybdenum steel is welded with matching low-alloy consumables that respect the strength of the base plate, and heavier sections are welded with preheat and a controlled interpass temperature. Post-weld heat treatment is applied to thick-wall vessels to relieve residual stress and to stabilise the heat-affected zone. Plate can be cold formed in moderate thicknesses and hot formed in heavier gauges, with normalising afterwards restoring the specified properties.
Heavy-wall pressure vessels, reactors and accumulators
Boiler drums, shells and headers
Shell-and-tube heat exchangers and condensers
Fabricated equipment for the oil, gas and chemical supply markets
Storage and process vessels requiring higher strength than plain carbon plate
Frequently Asked Questions
Q: Does SA302 Grade B contain nickel?
No specific nickel addition is required for Grade B. Its strength comes from manganese and molybdenum; separate grades of the same specification carry the nickel requirement.
Q: What is the minimum yield strength of SA302 Grade B?
The minimum yield strength is 345 MPa, with a tensile strength range of 550 MPa to 690 MPa.
Q: Is SA302 Grade B the same as ASTM A302 Grade B?
Yes. ASME SA-302/SA-302M and ASTM A302/A302M are technically identical, including the thickness-dependent carbon limits of Grade B.
Q: How is SA302 Grade B normally heat treated?
It is supplied normalised, or normalised and tempered, rather than as-rolled, because the required strength and toughness depend on that treatment.
Q: Why does the carbon limit change with thickness?
Heavier sections cool more slowly after normalising, so the specification permits a slightly higher carbon maximum in thicker plate to keep the required strength achievable.
Q: Where is SA302 Grade B used?
It is used for pressure vessels, boilers, heat exchangers and process equipment in oil, gas and chemical plants, particularly where higher design pressure calls for a stronger plate than carbon steel.
