ASTM A387 covers chromium-molybdenum Alloy steel plates primarily used in the welded construction of pressure vessels designed for elevated temperature service. The grades are defined by their chemical composition, specifically their Chromium (Cr) and Molybdenum (Mo) content.
The grades are designated by a number (Grade or Gr) and come in two main classes indicating the heat treatment condition supplied:
Class 1: Plates are supplied in the Normalized and Tempered condition. This provides optimal mechanical properties.
Class 2: Plates are supplied in the Stress-Relieved condition only. This condition is primarily suitable for forming operations where the final heat treatment will be performed by the fabricator.
Here's a table of the common ASTM A387 grades:
| Grade Number | Common Name | Chemical Composition | Primary Applications | Notes |
|---|---|---|---|---|
| Gr 5 | 5Cr-0.5Mo | ~5% Cr, 0.5% Mo | High-temperature hydrogen service | Less common today |
| Gr 9 | 9Cr-1Mo | ~9% Cr, 1% Mo | High corrosion resistance, sulfidation resistance | Superior corrosion resistance |
| Gr 11 | 1.25Cr-0.5Mo | ~1.25% Cr, 0.5% Mo | Boiler drums, pressure vessels ≤ 550°C | Most commonly used grade |
| Gr 12 | 1Cr-0.5Mo | ~1% Cr, 0.5% Mo | Hydroprocessing units (resistance to hydrogen attack) | Similar to Gr 11 |
| Gr 21 | 3Cr-1Mo | ~3% Cr, 1% Mo | Thick-wall reactors, higher pressure/temperature service | Good strength at moderate temps |
| Gr 22 | 2.25Cr-1Mo | ~2.25% Cr, 1% Mo | High-temperature pressure vessels (550-600°C), reactors | Industry standard |
| Gr 91 | 9Cr-1Mo-V | ~9% Cr, 1% Mo, +V, Nb, N | Ultra-supercritical power plants, high strength at >600°C | Highest temperature capability |
H-Grade Variants: Many grades (especially Gr 11, 12, 22) have "H" designations (e.g., Gr 11H, Gr 22H). These indicate compliance with tighter compositional controls required for creep strength ("H" stands for High-Temperature). They are mandatory for components under stress at high temperatures in ASME code applications. Always specify the "H" grade for critical pressure vessel service unless a specific exception applies.
Heat Treatment (Class) is Crucial: Specifying only the Grade (e.g., "A387 Gr 22") is incomplete and can lead to ordering the wrong material. You must specify both the Grade and the Class (e.g., "ASTM A387 Gr 22 Class 1" or "ASTM A387 Gr 11H Class 2"). The required class depends on the fabrication sequence and the final heat treatment plan.
Not for Low-Temperature: These grades are optimized for elevated temperature strength and resistance to hydrogen attack/sulfidation. They are not suitable for low-temperature (sub-zero) service without careful evaluation.
Applications: Primarily used in refinery, petrochemical, and power generation industries for components like reactors, boiler drums, heat exchangers, and piping operating at high temperatures and pressures.
When selecting a grade, consider the operating temperature, pressure, required mechanical properties (especially at temperature), and the specific corrosive environment (e.g., presence of hydrogen, sulfur). The appropriate Class is determined by the fabrication process. Always refer to the pressure vessel design code (e.g., ASME BPVC Section II, Part A & Section VIII, Div. 1) for specific grade and class requirements for your application.
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