ASME SA387 Grade 12 Class 2 plates are high-quality chromium-molybdenum alloy steel plates designed for use in elevated-temperature environments. The Grade 12 designation refers to the specific alloy composition, while Class 2 indicates the normalized and tempered condition and the corresponding mechanical properties. The chromium content gives the steel excellent resistance to heat and corrosion, making it a reliable choice for pressure vessels, boilers, and related equipment in the oil, gas, and power generation industries.
What Is SA387 Grade 12 Class 2?
SA387 covers chromium-molybdenum alloy steel plates for pressure vessels, and Grade 12 is one of the lower-chromium grades in the family, with a nominal composition of about 1% chromium and 0.5% molybdenum. Class 2 is supplied in the normalized and tempered condition, which refines the grain structure and provides a good balance of strength, toughness, and weldability for elevated-temperature service. The equivalent ASTM designation is A387 Grade 12 Class 2, with identical chemical and mechanical requirements. The plates are typically available in thicknesses up to about 200 mm.
Chemical Composition
The composition of SA387 Grade 12 Class 2 is controlled to provide hardenability, oxidation resistance, and weldability. Typical limits are carbon of 0.04-0.17%, silicon of 0.13-0.45%, manganese of 0.35-0.73%, phosphorus of 0.035% maximum, and sulfur of 0.035% maximum. The chromium content is 0.74-1.21% and the molybdenum content is 0.40-0.65%, which together give the steel its elevated-temperature strength and resistance to oxidation and corrosion. The restrained carbon and impurity levels support welding with appropriate preheat and post-weld heat treatment.
Mechanical Properties
For SA387 Grade 12 Class 2 plates up to 50 mm thick, the minimum yield strength is 275 MPa and the tensile strength range is 450-585 MPa, with a minimum elongation of about 22%. For plates over 50 mm up to 200 mm, the minimum yield strength remains 275 MPa and the tensile range is 450-585 MPa, with a minimum elongation of about 19%. Compared with Class 1, the normalized and tempered Class 2 condition provides higher and more stable mechanical properties in the as-supplied state, which allows thinner sections or higher design pressures in elevated-temperature equipment.
Applications
In the oil and gas industry, SA387 Grade 12 Class 2 is used for critical refinery equipment, separator vessels, and high-temperature storage tanks. The chromium content provides good resistance to sulfide stress cracking, making it a reliable choice for sour service applications involving hydrogen sulfide-containing media. In power generation, the grade is widely adopted for heavy-duty boiler drums, high-pressure steam pipelines, turbine casings, and heat recovery steam generators, maintaining stable mechanical properties under long-term thermal cycling. In chemical processing, it is applied to reactors and corrosion-resistant pressure vessels handling aggressive media, and in general industry it is used for high-temperature piping systems and load-bearing components in custom boilers and pressure vessels.
Fabrication and Weldability
SA387 Grade 12 Class 2 has good weldability and formability, which make it suitable for fabricating non-standard equipment operating in moderate high-temperature conditions. Welding should be performed with qualified procedures using low-hydrogen consumables, with preheat and interpass temperature control appropriate to the thickness and joint design. Post-weld heat treatment is normally required after welding to temper the weld zone, relieve residual stresses, and restore the intended mechanical properties. Plates are readily cut by flame or plasma, and formed by rolling or pressing with forces appropriate to the alloy strength.
Frequently Asked Questions
What is SA387 Grade 12 Class 2 equivalent to? SA387 Grade 12 Class 2 is equivalent to ASTM A387 Grade 12 Class 2, which carries identical requirements. Closely comparable European grades include 13CrMo4-5 to EN 10028-2, a chromium-molybdenum boiler steel with similar composition and strength.
What material is SA387 Grade 12 Class 2? It is a chromium-molybdenum alloy steel plate, with a nominal composition of about 1% chromium and 0.5% molybdenum, supplied in the normalized and tempered condition for elevated-temperature pressure vessel and boiler service.
What is the difference between Class 2 and Class 1? Class 1 is supplied in the annealed condition with lower strength for severe forming applications, while Class 2 is normalized and tempered, providing higher and more stable mechanical properties in the as-supplied condition.
Can SA387 Grade 12 Class 2 be welded? Yes. The grade has good weldability for fabricating pressure vessel components. Welding requires qualified procedures, low-hydrogen consumables, controlled preheat, and post-weld heat treatment as required by the applicable code.
What service temperature range is suitable for this grade? The grade is designed for elevated-temperature service in boilers and pressure vessels, with common applications at temperatures where carbon steels lose strength. The governing design code must be consulted for allowable stresses at the specific design temperature.
Why is chromium-molybdenum steel used for sour service? The chromium and molybdenum additions strengthen the steel matrix and stabilize carbides, improving resistance to sulfide stress cracking and high-temperature hydrogen attack, which carbon steels cannot provide in sour environments.