A387 Grade 5 Class 2 is a chromium-molybdenum alloy steel plate specified under ASTM A387, with the equivalent ASME designation SA387. The grade contains nominally 5 percent chromium and 0.5 percent molybdenum, which give the steel elevated-temperature strength and resistance to hydrogen attack in high-temperature, high-pressure service. Class 2 refers to the normalized and tempered heat-treated condition. Plates of this type are widely used in the petroleum refining, chemical, and power generation industries for pressure vessels and related equipment operating at elevated temperatures.
Designation and Scope
Under ASTM A387, the designation Grade 5 indicates a nominal chromium content of about 5 percent, and Class 2 indicates the normalized and tempered delivery condition. The same material is listed as SA387 Grade 5 Class 2 in the ASME Boiler and Pressure Vessel Code, where it is grouped in P-Number 5B for welding procedure qualification purposes. The specification covers hot-rolled chromium-molybdenum alloy steel plates intended primarily for welded pressure vessels and boilers in elevated-temperature service.
Chemical Composition
The chemical composition of A387 Grade 5 Class 2 is tightly controlled. Typical requirements are carbon 0.15 percent maximum, manganese 0.30 to 0.60 percent, phosphorus 0.035 percent maximum, sulfur 0.030 percent maximum, silicon 0.50 percent maximum, chromium 4.00 to 6.00 percent, and molybdenum 0.45 to 0.65 percent. The chromium content provides oxidation resistance and elevated-temperature strength, while molybdenum contributes creep resistance and hardenability. The low carbon content supports welding and limits the risk of hardening in the heat-affected zone.
Mechanical Properties
For plates supplied in the normalized and tempered condition, the minimum yield strength is 310 MPa and the tensile strength range is 515 to 690 MPa. Minimum elongation is 18 percent in a 50 mm gauge length, with reduction of area requirements of 45 percent for longitudinal and 40 percent for transverse test specimens. These properties are verified by tensile testing performed on heat-treated test specimens in accordance with the testing clauses of the specification.
Heat Treatment and Fabrication
Class 2 plates are delivered in the normalized and tempered condition. Normalizing refines the grain structure, and tempering restores toughness after hardening. During fabrication, preheating is generally applied before welding, with typical preheat temperatures in the range of 175 to 250 degrees C depending on plate thickness, joint restraint, and ambient temperature. Post-weld heat treatment in the range of 675 to 730 degrees C is commonly specified for vessels in hydrogen service or where the design requires stress relief, using low-hydrogen consumables and controlled heat input.
Typical Applications
The grade is used for pressure vessels, reactors, and heat exchangers in petroleum refining, including hydrotreating and hydrocracking units where resistance to hydrogen attack at high temperature is required. It is also applied in power plant components, chemical process equipment, and other elevated-temperature pressure-containing applications where the chromium-molybdenum chemistry provides the required combination of strength, creep resistance, and oxidation resistance.
Frequently Asked Questions
What does Grade 5 Class 2 mean in ASTM A387?
Grade 5 indicates a nominal chromium content of about 5 percent, and Class 2 indicates that the plate is supplied in the normalized and tempered condition.
What is the difference between Class 1 and Class 2?
Both classes share the same chemical composition range. Class 1 plates are delivered in the annealed condition, while Class 2 plates are normalized and tempered, which normally provides higher strength and improved toughness.
What are the minimum yield and tensile strength of A387 Grade 5 Class 2?
The minimum yield strength is 310 MPa and the tensile strength range is 515 to 690 MPa for plates in the normalized and tempered condition.
Is A387 Grade 5 Class 2 weldable?
Yes, the steel is weldable using qualified procedures with preheat, interpass temperature control, and low-hydrogen consumables, followed by post-weld heat treatment where required.
What post-weld heat treatment temperature is used?
Post-weld heat treatment is commonly performed in the range of 675 to 730 degrees C, with the exact temperature and holding time selected according to plate thickness and design requirements.
Where is A387 Grade 5 Class 2 typically used?
It is used in pressure vessels, reactors, and heat exchangers for petroleum refining, petrochemical, and power generation service at elevated temperatures.