ASTM A387 Grade 5 Class 2 is a chromium-molybdenum alloy steel plate designed for weldable pressure vessels and boilers operating at high temperatures. The nominal composition of 5 percent chromium and 0.5 percent molybdenum gives the steel excellent resistance to oxidation and corrosion at elevated temperature, together with enhanced strength from solid-solution and precipitation effects. It is a standard material for refinery, petrochemical, and power generation equipment.
Chemical Requirements
The chemical composition of A387 Grade 5 is controlled within tight limits. Carbon is limited to 0.15 percent, manganese is 0.30 to 0.60 percent, and silicon to 0.50 percent. Phosphorus is limited to 0.035 percent and sulfur to 0.030 percent. Chromium is specified at 4.00 to 6.00 percent on heat analysis and 3.90 to 6.10 percent on product analysis, and molybdenum at 0.45 to 0.65 percent on heat analysis and 0.40 to 0.70 percent on product analysis. Heat analysis is taken from the ladle during melting, while product analysis verifies the final plate.
Tensile Requirements
A387 Grade 5 Class 2 plate requires a tensile strength of 515 to 690 MPa, 75 to 100 ksi, and a minimum yield strength of 310 MPa, 45 ksi at 0.2 percent offset. Minimum elongation is 18 percent in 50 mm gauge length, and the minimum reduction of area is 45 percent on round specimens and 40 percent on flat specimens. These values ensure adequate strength and ductility for pressure-containing components at elevated temperature.
Why Chromium and Molybdenum
Chromium provides resistance to oxidation and high-temperature corrosion, which is essential for equipment exposed to hot process media and flue gas. Molybdenum strengthens the steel at elevated temperature and improves creep resistance, so the plate maintains its load-carrying capacity during long-term high-temperature service. Together they make the 5Cr-0.5Mo composition suitable for the severe conditions found in refining and power generation.
Heat Treatment and Welding
The Class 2 condition requires normalizing and tempering, which refines the grain structure and restores toughness after hardening. Because of the chromium content, welding requires a qualified procedure with preheat and controlled interpass temperature to avoid hardening in the heat-affected zone, and low-hydrogen consumables are essential. Post-weld heat treatment is generally required by the design code to temper the weld and heat-affected zone and to relieve residual stress.
Typical Applications
Pressure vessels, reactors, and heat exchangers in petrochemical and oil and gas plants
Refinery equipment for catalytic cracking and hydrogenation units
Boilers, high-temperature piping, and pressure vessels in power generation
Chemical industry vessels for processing corrosive substances
Ammonia and urea synthesis towers in the fertilizer industry
Frequently Asked Questions
What is A387 Grade 5 Class 2? It is a 5Cr-0.5Mo chromium-molybdenum alloy steel plate for high-temperature pressure vessel and boiler service.
What is the tensile strength of A387 Grade 5 Class 2? The tensile strength is 515 to 690 MPa and the minimum yield strength is 310 MPa.
Why is chromium added to A387 Grade 5? Chromium provides oxidation and corrosion resistance at elevated temperature, while molybdenum adds high-temperature strength and creep resistance.
What heat treatment does Class 2 require? Class 2 requires normalizing and tempering to refine the grain structure and restore toughness.
Where is A387 Grade 5 used? In refinery reactors, heat exchangers, boilers, and chemical and fertilizer plant vessels that operate at high temperature.
Is preheat required when welding A387 Grade 5? Yes, a qualified procedure with preheat, controlled interpass temperature, and low-hydrogen consumables is required.