What ASME SA285 Grade B Steel Plate Is
ASME SA285 Grade B is a carbon steel plate grade published in ASME SA-285/SA-285M, a specification technically identical to ASTM A285/A285M. It is a low- to intermediate-tensile-strength material intended for fusion-welded pressure vessels, boilers and comparable containment equipment. No alloying elements are required, so the grade combines predictable weldability with good ductility for cold forming, and it is normally selected for non-corrosive or mildly corrosive service where an alloy plate would add cost without engineering benefit.
Within the SA285 family, Grade B sits between Grade A and Grade C. It carries a higher minimum yield and tensile strength than Grade A, which permits slightly higher design pressures at the same wall thickness, while remaining more formable and less costly than Grade C. Designers working to the ASME Boiler and Pressure Vessel Code should note that allowable stress values are tabulated separately for Grades A, B and C, and that Grade B plate is ordered as-rolled, normalised or stress relieved depending on the fabrication route.
Chemical Composition of SA285 Grade B
The table gives the heat-analysis limits for SA285 Grade B plate. Product-analysis tolerances a little wider than these figures are permitted by the specification, which is why a mill certificate may occasionally show manganese marginally above the 0.90 % ceiling.
| Element | Requirement, % (heat analysis) |
|---|---|
| Carbon, C | 0.22 max |
| Silicon, Si | 0.08 - 0.45 |
| Manganese, Mn | 0.90 max |
| Phosphorus, P | 0.035 max |
| Sulphur, S | 0.035 max |
Carbon is deliberately capped at a low level so that the heat-affected zone of a weld does not harden excessively during cooling. Sulphur and phosphorus are each held to 0.035 % maximum to limit hot shortness and to preserve notch ductility, particularly in thicker sections where through-thickness properties matter.
Mechanical Properties of SA285 Grade B
| Property | Requirement |
|---|---|
| Yield strength, min | 185 MPa |
| Tensile strength | 345 - 485 MPa |
| Elongation, min, 200 mm gauge length | 25 % |
| Elongation, min, 50 mm gauge length | 28 % |
The elongation requirement depends on the gauge length of the tensile specimen, so both values must be quoted when a certificate is reviewed: 25 % minimum applies to a 200 mm gauge length and 28 % minimum to a 50 mm gauge length. The relatively wide tensile window from 345 MPa to 485 MPa reflects the as-rolled structure of the grade; plates that are normalised generally fall in the middle of the range with a slightly more uniform ferrite-pearlite structure.
Size Range and Delivery Condition
Thickness: 5 mm - 300 mm
Width: 1500 mm - 4200 mm
Length: 3000 mm - 18000 mm
Rolling type: boiler plate and pressure vessel plate
Delivery condition: as-rolled, normalised, or stress relieved
Thin plate in the 5 mm to 20 mm range is usually supplied as-rolled for tank and shell work, while heavier sections are frequently normalised to secure a more homogeneous grain structure. Stress relieving is applied when the plate is to be cold formed and then welded into a component whose dimensions must remain stable.
Fabrication Behaviour and Typical Applications
The grade is known for easy welding without preheat in light sections, and it accepts cold bending, rolling and flanging without cracking. When heavy sections are formed hot, normalising or stress relieving afterwards restores the properties expected by the design calculation. Its modest strength keeps it economical for large surface areas.
Low- to moderate-pressure vessels, receivers and accumulators
Boiler shells, drums and headers in non-critical duty
Shell-and-tube heat exchangers and condensers
Storage tanks and vessels in refinery and petrochemical service
General industrial containment equipment fabricated by fusion welding
Where a higher design pressure or a corrosive medium is involved, the designer should compare SA285 Grade B against Grades C, and against the manganese-molybdenum and chrome-molybdenum specifications, before fixing the plate grade.
Frequently Asked Questions
Q: Is SA285 Grade B the same as ASTM A285 Grade B?
Yes. ASME SA-285/SA-285M is technically identical to ASTM A285/A285M, so the chemical and mechanical requirements for Grade B are the same in either designation.
Q: What is the minimum yield strength of SA285 Grade B?
The minimum yield strength is 185 MPa, with a tensile strength range of 345 MPa to 485 MPa.
Q: How does Grade B differ from Grades A and C?
Grade A has a lower minimum yield strength of 165 MPa, while Grade C has a higher minimum yield strength of 205 MPa and a higher tensile range. Grade B is the intermediate grade.
Q: Which heat treatments can be supplied?
Plate can be supplied as-rolled, normalised or stress relieved, and the chosen condition should be stated on the order because it affects both dimensional stability and mechanical results.
Q: Is preheating needed before welding?
Thin and medium sections of this low-carbon grade weld readily without preheat; thicker sections and restrained joints are normally welded with a modest preheat and controlled interpass temperature to limit restraint stresses.
Q: What thickness range is available?
Standard supply covers 5 mm to 300 mm in thickness, with widths from 1500 mm to 4200 mm and lengths from 3000 mm to 18000 mm.
