SA 387 Grade 12 Class 2 is a chromium-molybdenum alloy steel plate for elevated-temperature pressure vessel and boiler service. The governing document is ASME SA-387, formally known as SA-387/SA-387M, the standard specification for pressure vessel plates, alloy steel, chromium-molybdenum. The identical requirements are published in the ASTM system as ASTM A387/A387M, so SA 387 Grade 12 Class 2 and ASTM A387 Grade 12 Class 2 refer to the same material. Grade 12 defines a chemistry of about 1 percent chromium and 0.5 percent molybdenum, and Class 2 defines the normalized and tempered delivery condition, which provides higher strength and better toughness than the Class 1 condition.
Governing Standard and Designation
ASME SA-387 is referenced in Section II of the ASME Boiler and Pressure Vessel Code and is the standard most pressure vessel designers consult when specifying this grade. The designation SA 387 Grade 12 Class 2 is read as follows: SA indicates an ASME specification adopted from an ASTM document, 387 is the specification number, Grade 12 identifies the chemical composition with a nominal chromium content of 1.00 percent and molybdenum content of 0.50 percent, and Class 2 identifies the heat treatment condition. Related materials in other standards systems include 1Cr-0.5Mo steel types, for example the DIN designation 13CrMo44, now expressed as 13CrMo4-5 in the EN system, which are used as approximate equivalents where cross-standard substitution is permitted by the design code.
Chemical Composition
For heat analysis, the composition of Grade 12 is specified as carbon 0.05 to 0.17 percent, manganese 0.40 to 0.65 percent, phosphorus 0.035 percent maximum, sulfur 0.035 percent maximum, silicon 0.15 to 0.40 percent, chromium 0.80 to 1.15 percent, and molybdenum 0.45 to 0.60 percent. Product analysis tolerances apply to the values verified on the finished plate. The chromium addition provides oxidation resistance and elevated-temperature strength, while molybdenum improves creep resistance and hardenability. The controlled carbon and manganese contents keep the steel weldable while still meeting the strength requirements of the Class 2 condition.
Mechanical Properties and Heat Treatment
Class 2 plates are supplied in the normalized and tempered condition. The specified tensile strength is 65 to 85 ksi (450 to 585 MPa), the minimum yield strength is 40 ksi (275 MPa) at 0.2 percent offset, and minimum elongation is 19 percent in 8 inches and 22 percent in 2 inches. The minimum tempering temperature for Grade 12 is about 620 degrees C, and normalizing before tempering refines the grain structure. Because Class 2 already receives a full heat treatment at the mill, the plate arrives with a uniform microstructure and consistent mechanical properties, which simplifies fabrication of pressure vessels that will not receive an additional full heat treatment from the fabricator.
Fabrication and Welding
Welding of SA 387 Grade 12 Class 2 must follow qualified procedures. Preheating is normally required, typically in the range of 150 to 200 degrees C, to avoid hydrogen-induced cracking in the heat-affected zone. Low-hydrogen filler metals matching the base metal chemistry, such as E8018-B2 type electrodes for shielded metal arc welding or ER80S-B2 filler for gas tungsten arc welding, are commonly used. Interpass temperature should be controlled so that it does not exceed the limit of the procedure specification, usually about 300 degrees C. Post-weld heat treatment is generally required after welding and is typically performed at 650 to 700 degrees C, with the holding time determined by plate thickness. Ultrasonic or radiographic examination is commonly applied to welds and, when specified, to the base material.
Typical Applications
The grade is used in oil refineries and petrochemical plants for reactor vessels, separators, and storage tanks operating at elevated temperature, and its chromium content makes it suitable for service involving hydrogen and hydrogen sulfide at high temperature. In power generation it is applied to boiler drums, steam pipelines, and heat recovery steam generator components. Pressure vessel fabricators use it for welded vessels and industrial boilers where internal pressure and temperature exceed the limits of carbon steel. The material is commonly processed into flanges, fittings, pipe supports, and high-temperature ducting for these industries. It is not intended for cryogenic service, since its toughness decreases at very low temperatures.
Frequently Asked Questions
Which standard specifies SA 387 Grade 12 Class 2?
The material is specified by ASME SA-387/SA-387M, Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum. The identical requirements appear in ASTM A387/A387M, and SA 387 Grade 12 Class 2 is the same material as ASTM A387 Grade 12 Class 2.
What does Class 2 mean?
Class 2 identifies the normalized and tempered delivery condition, which gives higher strength and improved toughness compared with the Class 1 condition.
What is the chemical composition of Grade 12?
The main alloying elements are chromium at 0.80 to 1.15 percent and molybdenum at 0.45 to 0.60 percent, with carbon 0.05 to 0.17 percent and manganese 0.40 to 0.65 percent.
What are the mechanical properties of Class 2 plates?
Tensile strength is 450 to 585 MPa (65 to 85 ksi), minimum yield strength is 275 MPa (40 ksi), and minimum elongation is 22 percent in 50 mm.
Can this material be used at low temperatures?
No. It is intended for elevated-temperature service. Its ductility decreases below about -29 degrees C, increasing the risk of brittle fracture, so low-temperature grades should be selected for cold service.
What post-weld heat treatment is required?
PWHT is typically performed at 650 to 700 degrees C to relieve residual stress and restore toughness and ductility in the weld and heat-affected zone, with the holding time based on thickness.
Is the material resistant to hydrogen attack?
With proper heat treatment, the chromium-molybdenum chemistry provides good resistance to high-temperature hydrogen attack, which is why the grade is used in hydrogen service in refineries and petrochemical plants.