Specification and Grade Structure
SA302GrC is a pressure vessel steel plate supplied to ASTM A302/A302M, the specification for pressure vessel plates, alloy steel, manganese molybdenum and manganese molybdenum nickel. The specification covers four grades, A, B, C and D, which differ in composition and in the tensile strength class that the plate must meet, so the grade symbol has to be stated precisely on the order. Grade C is a manganese molybdenum nickel grade, and the nickel addition is what supports toughness while the molybdenum provides strength at elevated temperature, a combination that suits heavy wall vessels working at temperature.
Plates are ordered by specification, grade, class of heat treatment and thickness. Because the same specification number appears on plates of quite different strength, the certificate must show the grade and the delivery condition, and the fabricator should verify these against the vessel drawings before cutting.
Delivery Condition and Heat Treatment
The normal delivery conditions are normalized, or normalized plus tempered. The plate can be supplied without heat treatment only when this is specifically agreed in the purchase order documents, and such an agreement normally reflects a decision that the fabricator will carry out the heat treatment as part of the vessel fabrication sequence.
Normalizing is recommended at a temperature between 890 C and 950 C. The austenitizing temperature matters because it must be high enough to dissolve the alloy carbides and produce a uniform austenite, but excessive time at temperature coarsens the grain and reduces toughness. Heating is therefore controlled so that the plate reaches the required temperature without being held at it far longer than necessary, and cooling in air follows. Where normalized plus tempered delivery is ordered, the tempering treatment after normalizing stabilizes the mechanical properties and produces a microstructure in the delivered plate that is close to the post weld heat treated condition of the finished vessel.
| Item | Requirement |
|---|---|
| Specification | ASTM A302/A302M, ASME equivalent |
| Grades | A, B, C and D by strength class |
| Grade C type | Manganese molybdenum nickel |
| Normalizing temperature | 890 C to 950 C |
| Minimum yield strength, Grade C | 345 MPa |
| Tensile strength, Grade C | 550 to 690 MPa |
| Alternative delivery | Normalized plus tempered, or as agreed without heat treatment |
Acceptance Criteria and Testing
Acceptance of SA302GrC plates rests on chemical analysis, tensile testing and, where the order requires it, impact testing and non destructive examination. The chemical analysis confirms the carbon, manganese, molybdenum and nickel contents together with the phosphorus and sulphur limits that protect toughness in thick sections. Tensile testing establishes that the plate reaches the yield and tensile class of Grade C for the delivered thickness. Impact testing is carried out at the temperature and with the acceptance level agreed for the equipment, and the results are reported specimen by specimen.
Plates for heavy wall vessels are normally examined by ultrasonic testing over the full surface, so that laminations and internal discontinuities are detected before forming.
Dimensional checks cover thickness, width, length and flatness, since the thick plates used in this grade class are also used for large formed heads and shells where flatness affects fit up.
Heat treatment records identify the furnace, the temperature and the holding time applied to each plate, and they are attached to the certificate.
Retests are governed by the specification, and plates that fail the required tests are re heat treated or rejected according to the rules of the standard.
Marking carries the specification, grade, heat number and heat treatment condition so that the plate can be traced into the vessel.
Applications and Fabrication
The grade is used for equipment in the petroleum, chemical and power industries, including reactors, heat exchangers, separators, spherical tanks, oil and gas storage tanks, liquefied gas tanks, the pressure shells of nuclear reactors, boiler drums, liquefied petroleum gas cylinders, high pressure penstocks for hydroelectric stations and turbine housings. Because the plate is thick and the joints are heavily restrained, welding uses qualified procedures with controlled preheat, interpass temperature and heat input; post weld heat treatment is applied where the design code requires it, and the base metal is tested after the simulated thermal cycle when the equipment is heavily heat treated.
Frequently Asked Questions
Q: How many grades does the A302 specification contain?
A: Four: A, B, C and D, distinguished by composition and tensile strength class. Grade C is the manganese molybdenum nickel grade used for pressure equipment at moderate and elevated temperature.
Q: In what condition is SA302GrC delivered?
A: Normally normalized, or normalized plus tempered. Delivery without heat treatment is possible only when it is agreed in the purchase order documents, usually because the fabricator will heat treat during fabrication.
Q: Why is the normalizing temperature limited to 890 to 950 C?
A: The temperature must dissolve the alloy carbides and produce uniform austenite, while avoiding excessive grain growth that would reduce toughness. Air cooling after normalizing gives the required microstructure.
Q: What are the strength values of Grade C?
A: A minimum yield strength of 345 MPa and a tensile strength range of 550 to 690 MPa for the thickness range covered by the specification, with values tabulated by thickness.
Q: Is ultrasonic testing part of the acceptance criteria?
A: It is ordered as a supplementary requirement for most heavy wall pressure equipment. The extent and acceptance class for ultrasonic examination are agreed at order stage and reported with the plate certificate.
