It is a chromium-molybdenum (Cr-Mo) alloy steel plate governed by the ASTM A387 standard. Designed specifically for welded pressure vessels in elevated temperature service, its key characteristic is enhanced resistance to creep (deformation under long-term stress) and hydrogen attack compared to carbon steels like SA516 Gr.70.
The alloy achieves this through a controlled composition: typically 1.00-1.50% Chromium for improved high-temperature strength and oxidation resistance, and 0.45-0.65% Molybdenum to increase creep strength and stabilize the microstructure. This makes it a mandatory choice for critical equipment in petrochemical, refining, and thermal power industries.
| Parameter (ASTM A387) | SA387 Gr.11 Class 1 | SA387 Gr.11 Class 2 |
| Yield Strength (Min.) | 240 MPa (35 ksi) | 310 MPa (45 ksi) |
| Tensile Strength | 415 – 585 MPa (60-85 ksi) | 515 – 690 MPa (75-100 ksi) |
| Key Distinction | Standard strength grade. | Higher strength grade for more demanding designs. |
| Mandatory Heat Treatment | Normalized (N) is typical. | Normalized and Tempered (N&T). |
Why This Matters: The mandatory Normalizing & Tempering for Class 2 refines the grain structure more uniformly, resulting in not only higher strength but also generally superior toughness and more predictable welding characteristics.
The higher yield strength of CL2 (310 MPa vs. 240 MPa) can allow for thinner wall designs under the same pressure load. This translates to significant weight savings, which is crucial for large vessels, reducing material tonnage and associated fabrication/transport costs.
CL2’s Advantage: The mandatory N&T condition provides a stable, homogeneous structure that is generally more forgiving during welding, with a lower risk of adverse microstructural changes in the heat-affected zone.
Universal Requirement: Both grades require strict welding procedures using matching low-alloy electrodes (e.g., AWS A5.5) and mandatory Post Weld Heat Treatment (PWHT) at a minimum of 620°C (1150°F) to restore toughness and relieve stresses.
CL1 offers a cost-effective solution for applications where its standard strength margins are fully adequate.
CL2 commands a higher price due to enhanced processing. However, its strength can justify the premium through material savings (thinner walls) and potentially superior performance in cyclic or critical service.
CL1 is the reliable, economical workhorse. CL2 is the high-performance, optimized option where its strength can unlock savings and enhance reliability. Always base your final decision on the governing design code and a thorough analysis of the vessel’s lifetime operating conditions.
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