What Is ASME SA204 Steel?
ASME SA204, corresponding to ASTM A204, covers molybdenum-alloy steel plates for pressure vessels and boilers used at elevated temperatures. The specification includes Grades A, B, and C, which are distinguished by carbon content and tensile strength. All grades contain about 0.45-0.60% molybdenum, which improves high-temperature strength and resistance to oxidation and creep, bridging the gap between plain carbon steels and higher-alloy chromium-molybdenum steels.
Grade C versus Grade B
Grade C is the higher-strength member of the family. Its tensile strength range is 75-95 ksi (515-655 MPa) with a maximum carbon content of 0.23%, while Grade B provides 65-85 ksi (450-585 MPa) with a maximum carbon of 0.20%. The higher strength of Grade C allows thinner plate for a given design pressure, which supports weight reduction in high-pressure, high-temperature equipment.
Chemical Composition of Grade C
Maximum composition limits are: carbon 0.23%, manganese 0.90%, silicon 0.15-0.40%, phosphorus 0.035%, and sulfur 0.035%, with molybdenum in the range of 0.45-0.60%. The material number for SA204 Grade C is UNS K12320. Confirm the thickness-dependent values against SA-204/SA-204M for the ordered plate thickness.
Mechanical Properties of Grade C
The tensile strength is 515-655 MPa (75-95 ksi), the minimum yield strength is 295 MPa (43 ksi) for plate up to about 19 mm, and the minimum elongation is 16% in an 8 in gage length. The grade maintains good toughness across its temperature range and can be drilled and machined with carbide-tipped tools. Typical hardness is in the range of 150-250 HB depending on the heat treatment condition.
Heat Treatment and Weldability
SA204 Grade C plates are supplied in the normalized or annealed condition in accordance with the specification to achieve a uniform grain structure and consistent mechanical properties. The grade offers good weldability when heat input is controlled and preheating is applied as needed; low-hydrogen electrodes are used to minimize hydrogen-induced cracking, and post-weld heat treatment may be required to relieve stress in thick sections.
Typical Applications
SA204 Grade C is used for boiler casing plates in large steam boilers, petrochemical feedwater heaters, and hot-gas scrubbers, as well as other pressure vessels and heat exchangers exposed to high temperatures and pressures in power plants and refineries. It is approved for ASME Boiler and Pressure Vessel Code service, and plates are typically supplied with mill test reports and ultrasonic testing.
Frequently Asked Questions
What is the difference between SA204 Grade C and Grade B? Grade C has a higher tensile range of 75-95 ksi (515-655 MPa) and a carbon limit of 0.23%, while Grade B provides 65-85 ksi (450-585 MPa) with 0.20% carbon max.
What is the chemical composition of SA204 Grade C? Maximum carbon 0.23%, manganese 0.90%, phosphorus 0.035%, and sulfur 0.035%, with silicon 0.15-0.40% and molybdenum 0.45-0.60%.
What are the mechanical properties of SA204 Grade C? Tensile 515-655 MPa (75-95 ksi), minimum yield 295 MPa (43 ksi), and minimum elongation 16% in 8 in.
What heat treatment applies to SA204 Grade C? Plates are supplied in the normalized or annealed condition per SA-204 to ensure uniform grain structure and consistent properties.
Is SA204 Grade C weldable? Yes, with controlled heat input, preheating as needed, low-hydrogen electrodes, and post-weld heat treatment for thick sections.
Where is SA204 Grade C used? In boiler casings, petrochemical feedwater heaters, hot-gas scrubbers, pressure vessels, and heat exchangers in power plants and refineries.
