Understanding the Strength of SA387 Grade 11 Class 1 Steel Plate
SA387 Grade 11 Class 1 is an ASME/ASTM chromium-molybdenum alloy steel plate, specified for high-temperature pressure vessels and boilers, offering good corrosion/oxidation resistance due to its ~1% Chromium and ~0.5% Molybdenum content, with Class 1 indicating a lower tensile strength level compared to Class 2. It's used in the oil, gas, and chemical industries for tanks and piping requiring strength at elevated temperatures and in sour (H2S) environments, providing improved tensile strength, temperature resistance, and anti-rust properties.
The "strength" of SA387 Grade 11 Class 1 is a nuanced concept that must be understood in the context of its heat treatment condition, fabrication process, and final service state. It is not a single high-value property, but a balance tailored for manufacturability and in-service performance.
1. As-Delivered (Annealed) Strength
In its supplied Class 1 (Annealed) condition, the steel is in its softest, most ductile state.
Yield Strength (Rp0.2): ≥ 205 MPa (30 ksi)
Tensile Strength (Rm): 415 - 585 MPa (60 - 85 ksi)
Key Point: These are the minimum specified values. The actual strength is intentionally on the lower end of the alloy's potential. This is by design.
Why is it "weak" when delivered?
Annealing involves slow cooling from a high temperature, which produces a soft microstructure of ferrite and pearlite. This maximizes ductility (>22% elongation) and toughness at the expense of raw strength. This condition is chosen for formability.
2. The "Design Strength" - The Role of Post-Weld Heat Treatment (PWHT)
This is the most critical concept. The as-delivered strength is not the design strength.
A pressure vessel fabricated from SA387 Gr.11 Cl.1 must undergo a final PWHT (Stress Relieving) after all welding and forming is complete.
This PWHT, performed typically at 620-690°C (1150-1275°F), acts as a tempering cycle.
Result: The properties change. The final strength of the component in service will be higher than the as-delivered annealed values, approaching (but not fully reaching) the levels of a normalized and tempered (Class 2) material. The exact final properties depend on the PWHT parameters.
3. Comparative Strength: Class 1 vs. Class 2
The strength difference between classes highlights the design choice:
| Property | SA387 Gr.11 Class 1 (Annealed) | SA387 Gr.11 Class 2 (N&T) |
|---|---|---|
| Yield Strength (min) | 205 MPa (30 ksi) | 275 MPa (40 ksi) |
| Tensile Strength | 415-585 MPa (60-85 ksi) | 485-620 MPa (70-90 ksi) |
| Design Intent | Optimized for Formability | Optimized for As-Supplied Strength |
Class 2 is ~35% stronger in yield strength as-supplied. Class 1 sacrifices this initial strength for ease of fabrication.
4. High-Temperature (Creep) Strength
For elevated-temperature service (up to ~595°C / 1100°F), "strength" is measured as Creep Strength or Stress Rupture Strength.
The 1.25% Chromium and 0.5% Molybdenum alloying is crucial here. Molybdenum particularly enhances the steel's strength at high temperatures by reducing dislocation creep.
Important: The high-temperature strength properties are virtually identical for Class 1 and Class 2 after both have received their appropriate heat treatments. The ASME Boiler and Pressure Vessel Code (Section II, Part D) provides identical maximum allowable stress values for both classes at all temperatures. This is because the final microstructure after proper PWHT is similar.
Summary: The Strength Philosophy of Class 1
The strength profile of SA387 Grade 11 Class 1 is a two-stage process:
Stage 1 (Manufacturing): It is supplied in a deliberately "weaker," more ductile state to withstand severe cold forming (e.g., head dishing, shell rolling) without cracking. Its strength here is formability strength.
Stage 2 (In-Service): After fabrication and the mandatory PWHT, the material's strength is enhanced. Its final design strength-especially its critical high-temperature creep strength-is fully realized and is code-approved for pressure vessel design.
Therefore, understanding its strength requires shifting focus from the mill certificate values to the final, heat-treated condition of the completed vessel component. Its true "strength" lies in this combination of excellent fabricability and reliable elevated-temperature performance after heat treatment.
1. How Does The Chemical Composition Of A387 Grade 5 Differ From Grade 11 Or Grade 12?
A387 Grade 5 is a higher-chromium alloy steel, typically containing around 5% chromium and 0.5% molybdenum. This places it between the more common Grade 11 (~1.25% Cr) and Grade 22 (~2.25% Cr), giving it distinct advantages in terms of oxidation resistance and elevated-temperature strength compared to lower-alloy grades, while potentially being more cost-effective than higher-chromium steels for specific service ranges.
2. What Are The Primary High-Temperature Service Advantages Of Using Grade 5 Class 2?
The primary advantage of A387 Grade 5 Class 2 lies in its superior oxidation resistance and higher strength retention at temperatures typically ranging from 540°C to 600°C (1000°F to 1112°F), exceeding the capability of Grade 11 or 12. This makes it suitable for components in catalytic crackers, reformer furnaces, and other severe petrochemical service environments where scaling and creep are significant concerns.
3. What Are The Critical Welding And Fabrication Considerations For A387 Gr.5 Cl.2?
Welding A387 Grade 5 Class 2 requires stringent procedures due to its higher alloy content. It demands mandatory preheating (usually 175-250°C / 350-480°F) and strict interpass temperature control to prevent hard, crack-sensitive microstructures. Furthermore, a mandatory Post-Weld Heat Treatment (PWHT) in the range of 675-730°C (1250-1350°F) is essential to temper the weld zone, relieve stresses, and restore corrosion resistance.
4. What Is The Standard Heat Treatment Condition For A387 Grade 5 Class 2 Plates?
A387 Grade 5 Class 2 plates are supplied in the Normalized and Tempered condition. Normalizing involves heating above the transformation range and air cooling to refine the grain structure, followed by tempering at a sub-critical temperature to achieve the optimal combination of strength and ductility specified in the standard (typically a minimum yield strength of 310 MPa / 45 ksi).
Full specification and details are available on request. The above information is provided for guidance purposes only. For specific design requirements please contact our technical sales staff.
