1. What Is SA387 Grade 11 Class 2?
ASME SA387 covers chromium-molybdenum alloy steel plates for welded boilers and pressure vessels in elevated-temperature service; ASTM A387 is the identical material under the ASTM designation. Grade 11 refers to the 1.25% chromium, 0.50% molybdenum composition, and Class 2 specifies the higher-strength heat-treated condition: normalized and tempered, with a minimum yield of 310 MPa (45 ksi) and a tensile range of 515–690 MPa (75–100 ksi). The chromium provides oxidation resistance and the molybdenum maintains strength at high temperature, which is why the grade is a standard choice for hot-service pressure equipment.
2. Chemical Composition
The composition of Grade 11 is controlled to balance elevated-temperature strength and weldability: carbon 0.05–0.17%, manganese 0.40–0.65%, silicon 0.50–0.80%, phosphorus and sulfur each 0.035% maximum, chromium 1.00–1.50% and molybdenum 0.45–0.65%. The heat analysis and product analysis tolerances are defined in the standard, and the values should be verified on the mill certificate.
3. Mechanical Properties
For SA387 Grade 11 Class 2 the guaranteed values are a minimum yield strength of 310 MPa (45 ksi) at 0.2% offset, a tensile strength of 515–690 MPa (75–100 ksi), a minimum elongation of 18% in a 200 mm gauge and 22% in a 50 mm gauge. The Class 1 and Class 2 grades share the same composition and differ in the heat treatment and the resulting strength, with Class 2 providing the higher mechanical properties required for more demanding pressure-vessel design.
4. Applications and Industries
Grade 11 Class 2 is used for pressure vessels, boilers, heat exchangers, pipelines and related equipment in the oil, gas, petrochemical, power generation and chemical processing industries, where components operate at elevated temperatures and pressures. The 1.25Cr-0.5Mo composition suits service where carbon steel would lose strength or oxidize too quickly. Where sour-gas service is involved, the applicable requirements such as hardness limits in NACE MR0175/ISO 15156 must be evaluated in addition to the base standard.
5. Welding and Heat Treatment
Cr-Mo steels are weldable but sensitive to hydrogen cracking, so preheating and post-weld heat treatment are mandatory practice. Low-hydrogen consumables matched to the composition, such as the B2 type for 1.25Cr-0.5Mo, are used, and the interpass temperature is strictly controlled. Class 2 plate is delivered normalized and tempered; after welding, PWHT is applied as required by the code to restore toughness and reduce hardness in the heat-affected zone.
Frequently Asked Questions
What is the difference between SA387 Grade 11 Class 1 and Class 2? The composition is identical; Class 2 is normalized and tempered and guarantees higher strength (45 ksi yield, 75–100 ksi tensile) than the annealed Class 1.
What is SA387 Grade 11 Class 2 equivalent to? It is identical to ASTM A387 Grade 11 Class 2, and it is related to the BS 1501-621 family; the governing requirements come from the applicable pressure vessel code.
Is SA387 Grade 11 suitable for sour gas service? Suitability depends on hardness, heat treatment and the requirements of NACE MR0175/ISO 15156; the material must be evaluated for the specific sour-service conditions.
What is the P-number of A387 Grade 11? The material is classified as P-Number 4 under ASME Section IX, which defines the welding procedure requirements for the base metal group.
What welding consumables are used? Low-hydrogen filler metals matching the 1.25Cr-0.5Mo composition, such as the B2 electrode class, with preheat, interpass control and post-weld heat treatment.
How is the weight of a plate calculated? Weight equals volume times density (about 7850 kg/m3); for example a 25 mm thick plate weighs about 196 kg per square meter.
