What are the mechanical properties of SA387 Grade 11 Class 2?
SA387 Grade 11 Class 2 is a normalized and tempered chromium-molybdenum (Cr-Mo) alloy steel plate specifically designed for the construction of welded pressure vessels and high-temperature components in demanding industrial service.
The mechanical properties of SA387 Grade 11 Class 2 are defined by its normalized and tempered (N&T) condition and are specified in the ASTM/ASME A/SA387 standard. This condition provides a balanced combination of strength, ductility, and toughness suitable for high-temperature pressure vessel design.
1. Minimum Required Mechanical Properties (Room Temperature)
| Property | Value (Minimum or Range) | Metric Units | Imperial Units |
|---|---|---|---|
| Tensile Strength (Rm) | 485 – 620 MPa | 485 – 620 MPa | 70,000 – 90,000 psi |
| Yield Strength (Rp0.2) | Minimum 275 MPa | ≥ 275 MPa | ≥ 40,000 psi |
| Elongation (A) | Minimum 20% | ≥ 20% (in 50 mm gauge length) | ≥ 20% (in 2 in. gauge length) |
Note: These are the minimum specified values on the material test certificate. Actual values typically fall within the mid-to-upper range.
2. Typical Additional Properties
Hardness: Typically in the range of 140–207 HBW (Brinell Hardness). The N&T treatment avoids excessive hardness that could impair weldability or toughness.
Impact Toughness: Charpy V-notch impact testing is not a mandatory requirement of the base specification but is almost always specified for pressure vessel applications. Common purchaser requirements are:
Test Temperature: +10°F to -20°F (-12°C to -29°C) or lower.
Minimum Absorbed Energy: 20 J (15 ft-lbf) average per set of three specimens.
3. Key Comparison with Class 1 (Annealed)
The primary reason for specifying Class 2 over Class 1 is its superior as-delivered strength:
Yield Strength: Class 2 has a minimum yield strength of 275 MPa (40 ksi), which is approximately 34% higher than Class 1's minimum of 205 MPa (30 ksi).
Tensile Strength: Class 2's tensile range (485-620 MPa) is higher than Class 1's (415-585 MPa).
This higher strength allows designers to use higher allowable stress values (per ASME Section II, Part D) in calculations, potentially leading to thinner, lighter vessel walls.
4. High-Temperature (Creep) Properties
For elevated-temperature service (up to ~595°C / 1100°F), the critical design properties are Creep Strength and Stress Rupture Strength. These are governed by the 1.25% Cr and 0.5% Mo composition and are identical for both Class 1 and Class 2 after proper heat treatment. The ASME Code provides the same maximum allowable stress values for both classes at all temperatures.
5. Important Context: The Role of Post-Weld Heat Treatment (PWHT)
After fabrication, all weldments must undergo a mandatory PWHT (typically ~620-690°C / 1150-1275°F).
This PWHT slightly tempers the material, which may cause a minor reduction in yield and tensile strength (compared to the as-supplied values) but is essential for:
Relieving residual stresses.
Restoring corrosion resistance in the heat-affected zone.
Ensuring dimensional stability.
The designer uses the ASME allowable stress values (which account for this final condition) for calculations, not the raw mill certificate values.
Summary of Mechanical Property Philosophy
The mechanical properties of SA387 Grade 11 Class 2 represent an optimized balance for pressure vessel fabrication:
High Enough Strength for efficient, lightweight design.
Sufficient Ductility and Toughness to withstand fabrication and prevent brittle fracture.
Stable Elevated-Temperature Performance for long-term service in refineries and power plants.
In essence, these properties make it the default, high-performance choice for most new construction of 1.25Cr-0.5Mo alloy pressure vessels.
1. What is SA387 Grade 11 Class 2?
SA387 Grade 11 Class 2 is a normalized and tempered chromium-molybdenum (1¼Cr-½Mo) alloy steel plate designed for high-temperature pressure vessels, offering higher strength than Class 1 while maintaining good weldability and creep resistance in refinery service.
2. What are the mechanical properties of SA387 Grade 11 Class 2?
In its normalized and tempered condition, SA387 Grade 11 Class 2 provides a tensile strength of 485–620 MPa (70–90 ksi) and a minimum yield strength of 275 MPa (40 ksi), making it suitable for high-stress, elevated-temperature designs.
3. How does SA387 Grade 11 Class 2 differ from Class 1?
Class 2 is normalized and tempered for higher as-supplied strength, while Class 1 is annealed for maximum formability; both share the same chemical composition but are selected based on fabrication needs and design stress requirements.
4. What is the maximum service temperature for SA387 Grade 11 Class 2?
SA387 Grade 11 Class 2 is typically used in continuous service up to about 595 °C (1100 °F), where its chromium and molybdenum content provides effective resistance to oxidation and hydrogen attack.
5. What are common applications for SA387 Grade 11 Class 2 plates?
This grade is widely used in hydrotreaters, hydrocrackers, heat exchangers, and other high-temperature pressure vessels in petrochemical refineries and power-generation facilities.
6. What welding procedures are required for SA387 Grade 11 Class 2?
Welding requires low-hydrogen electrodes, preheating (150–205 °C / 300–400 °F), and mandatory post-weld heat treatment (PWHT) around 620–690 °C (1150–1275 °F) to prevent cracking and restore corrosion resistance.
7. What is the equivalent European grade for SA387 Grade 11 Class 2?
The closest European equivalent is 13CrMo4-5 (1.7335) under EN 10028-2, which has similar composition and is commonly supplied in a normalized and tempered condition.
8. How does SA387 Grade 11 Class 2 compare to Grade 22?
Grade 11 (1.25Cr-0.5Mo) offers lower high-temperature strength and hydrogen resistance than Grade 22 (2.25Cr-1Mo), making it a cost-effective choice for moderate‑severity service in refining and chemical processes.
9. What are the Charpy impact requirements for SA387 Grade 11 Class 2?
Charpy V‑notch impact testing is often specified by purchasers; common requirements are 20 J (15 ft‑lb) minimum average at temperatures between +10 °F and –20 °F (–12 °C to –29 °C) for pressure‑vessel applications.
10. What is the difference between SA387 Grade 11 Class 2 and AISI 4130?
While both are Cr‑Mo steels, SA387 Grade 11 Class 2 is a plate product for code pressure vessels with strict toughness and testing rules, whereas AISI 4130 is a general‑purpose structural/bar alloy with different property ranges and applications.
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.
