Optimizing Heat Treatment for SA387 Grade 11 Steel Plate
ASTM A387 Grade 11 Class 1 refers to a chromium-molybdenum (1.25% Cr, 0.5% Mo) alloy steel plate for pressure vessels supplied in the normalized or stress-relieved heat treatment condition. It is designed for high-temperature service where the fabricator will perform final post-weld heat treatment (PWHT) after manufacturing, making it suitable for components requiring extensive forming and welding in refineries and power plants.
Optimizing the heat treatment for SA387 Grade 11 steel plate is critical for achieving the desired mechanical properties-primarily strength, toughness, and microstructural stability-for high-temperature, high-pressure service in refineries, power plants, and chemical reactors. The key lies in precisely controlling the normalizing, tempering, and post-weld heat treatment (PWHT) processes based on the Class of material (Class 1 vs. Class 2) and the final component requirements.
1. Understand the Starting Condition: Class 1 vs. Class 2
Class 1: Supplied in the normalized or stress-relieved condition. It is softened on purpose so that fabricators can perform a final PWHT after welding and forming. Optimization focuses on this final PWHT cycle.
Class 2: Supplied in the normalized and tempered condition. It already possesses its final mechanical properties. Optimization here typically involves controlling any subsequent thermal cycles (like PWHT) to avoid degrading these properties.
2. Key Optimization Parameters for Heat Treatment
A. Normalizing
Temperature Range: Typically 900°C – 950°C (1650°F – 1740°F).
Optimization Goal: Achieve a uniform, fine-grained austenitic structure that will transform into a refined ferrite-pearlite/bainite microstructure upon air cooling.
Best Practice: Use upper-range temperatures for thicker sections to ensure full austenitization, followed by still-air cooling in a controlled environment to prevent excessive thermal gradients.
B. Tempering (for Class 2 or after normalizing)
Temperature Range: 595°C – 720°C (1100°F – 1325°F), with 650°C – 680°C (1200°F – 1255°F) being a common optimal window.
Optimization Goal: Relieve stresses, improve toughness and ductility, and stabilize the carbide structure (promoting fine, stable carbides like Mo₂C).
Best Practice:
Temperature Selection: Higher tempering temperatures increase toughness but reduce strength. The exact temperature must balance specified tensile/yield strength with required impact energy (Charpy values).
Soak Time: Typically 1 hour per inch (25 mm) of thickness, minimum 30 minutes. Prolonged time at temperature can lead to temper embrittlement in the critical range of ~375°C-575°C (705°F-1065°F) during cooling.
Critical Cooling Rate: After tempering, cool rapidly through the 375°C-575°C range to mitigate temper embrittlement risk.
C. Post-Weld Heat Treatment (PWHT) - Most Critical for Fabrication
Temperature Range: Strictly within 595°C – 720°C (1100°F – 1325°F) as per codes like ASME Section VIII.
Optimization Goal: Relax residual welding stresses, temper the hard martensitic microstructure in the Heat-Affected Zone (HAZ), and restore corrosion resistance.
Best Practice:
Mandatory Preheating: 150°C – 200°C (300°F – 400°F) before welding to slow the cooling rate and prevent cold cracking.
Heating & Cooling Rates: Limit to <220°C/hr (400°F/hr) to prevent thermal stresses. For thick sections, slower rates (e.g., 100°C/hr) are advisable.
Soak Time: Similar to tempering, but often based on the thickest section at the weld. Must be sufficient to achieve uniform temperature through the entire component cross-section.
Temperature Uniformity: Furnace PWHT is preferred. Ensure temperature variation across the load does not exceed the code limits (e.g., ±14°C / ±25°F per ASME).
3. Advanced Optimization & Quality Control
Simulation & Modeling: Use finite element analysis (FEA) to model thermal gradients during heating/cooling, especially for complex or heavy-wall components, to predict stresses and optimize cycle times.
Microstructural Analysis: Periodic validation via metallography ensures the desired microstructure (tempered bainite) is achieved without detrimental phases.
Hardness & Toughness Testing:
Ensure post-PWHT hardness is below specified limits (e.g., 200 HB Brinell) to guarantee resistance to stress corrosion cracking.
Conduct Charpy V-notch impact tests at the design minimum temperature to verify adequate toughness.
Avoiding Embrittlement: Monitor time in the critical embrittlement range during all heating and cooling cycles. For high-purity requirements, specify Supplementary Requirement S6 in ASTM A387 for enhanced toughness.
Summary of Optimization Strategy
| Process Step | Optimization Focus | Key Parameter Control |
|---|---|---|
| Normalizing | Grain refinement & homogeneity | Temperature (~925°C), uniform air cooling |
| Tempering | Strength-Toughness balance | Temperature (650°C-680°C), time, rapid cooling through 375°C-575°C |
| PWHT | Stress relief & HAZ softening | Strict temp. range (595°C-720°C), controlled heating/cooling rates, sufficient soak time |
| Overall | Avoiding embrittlement & ensuring consistency | Preheating before welding, rapid cool after tempering/PWHT, rigorous qualification of procedures (PQR/WPQ) |
Final Recommendation: Always base the final heat treatment cycle on a qualified Procedure Qualification Record (PQR) supported by Welding Procedure Specifications (WPS) and mechanical testing of production test coupons. The optimal cycle is the one that consistently meets the required mechanical properties for the specific service environment while complying with all relevant construction codes (ASME, EN, etc.).
What is A387 grade 11 material?
The ASTM A387 specification is the Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum intended primarily for use in welded boilers and pressure vessels designed for elevated temperature service. SSAB can supply A387 Grade 11 to the tension test requirements for Class 1 and/or Class 2.
What is the composition of A387 steel?
Composition: ASTM A387 steel plates typically contain chromium (up to 9% or more) and molybdenum (up to 2.25% or more), which enhance their strength and corrosion resistance at high temperatures.
What is the difference between A182 and A387?
Both ASTM A182 grade F11 class 2 and ASTM A387 grade 22 steel are iron alloys. They have a very high 99% of their average alloy composition in common. There are 32 material properties with values for both materials. Properties with values for just one material (1, in this case) are not shown.
What is ASTM A387 Grade 11 equivalent to?
ASTM A387 grade 11 equivalent steel is ASME SA387 grade 11. ASTM A387 grade 11 covers A387 grade 11 class 1 and A387 grade 11 class 2, which are equivalent to ASME SA387 grade 11 class 1 and SA387 grade 11 class 2 respectively.
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.
