What are the Advantages of Using ASTM A387 Grade 22 Class 2?
ASTM A387 Grade 22 Class 2
A387 Grade 22 Class 2 is a grade of chromium-molybdenum alloy intended primarily for use fabricators in welded boilers and pressure vessels which are designed for use in raised temperature service.
With excellent heat resistance and anti corrosive/oxidation qualities, this steel grade is ideal for use in the chemical, oil and gas industry.
The advantages of using ASTM A387 Grade 22 Class 2 stem from its pre-tempered, high-strength condition, which offers distinct benefits in fabrication flexibility, cost control, and performance predictability for severe-service pressure vessels.
1. Fabrication & Logistics Advantages
Eliminates Need for Full Vessel PWHT: The primary advantage. Class 2 plate is supplied already normalized and tempered by the mill. This allows fabricators to perform only local Post-Weld Heat Treatment (PWHT) on weld seams, rather than subjecting the entire massive vessel to a costly and logistically challenging full furnace cycle.
Ideal for Large, Field-Assembled, or Complex Vessels: Critical for reactors, drums, or exchangers too large for available furnaces, or those with intricate internals (e.g., catalyst trays) that cannot withstand full furnace treatment.
Simplifies Schedule & Reduces Costs: Avoids the time, energy, and facility rental for full furnace PWHT, significantly accelerating project timelines and reducing capital expenditure (CAPEX) for heat treatment logistics.
2. Material & Performance Advantages
Higher As-Delivered Strength: Class 2 has a higher minimum yield strength (240 MPa / 35 ksi) compared to Class 1 (205 MPa / 30 ksi), providing a stronger starting point for design, potentially allowing for optimized wall thickness (where codes permit).
Controlled, Uniform Microstructure: The mill tempering ensures a consistent, refined tempered bainitic structure throughout the plate, enhancing isotropic properties and reducing the risk of localized weak zones.
Superior High-Temperature Properties Retained: It maintains all the inherent advantages of 2.25Cr-1Mo alloy:
Excellent creep strength at temperatures above ~450°C (840°F).
Strong resistance to hydrogen attack (per API 941 Nelson Curve).
Good oxidation and sulfide corrosion resistance.
3. Reliability & Quality Control Advantages
Reduced Risk of Fabrication-Induced Damage: By minimizing the total heat treatment cycles on the finished vessel (only local PWHT), the risk of distortion, excessive scale formation, or heat treatment-related defects is lowered.
Predictable Base Metal Properties: Engineers and inspectors can rely on the mill-tested mechanical properties (tensile, impact) of the plate as-delivered, as the base metal does not undergo significant microstructural change during fabrication (except in the weld HAZ).
4. Advantage in Revamps & Repairs
Simplified Component Replacement: When replacing a section of an existing Class 2 vessel, the new plate inherently matches the microstructure and strength of the original, avoiding complex re-heat treatment of the entire vessel after repair.
Comparison vs. Class 1: When This Advantage Matters Most
| Scenario | Advantage of Using Class 2 |
|---|---|
| Vessel too large for any furnace | Essential – Only feasible option. |
| Field fabrication/construction | Major – Avoids moving vessel or bringing massive portable furnace. |
| Vessel with sensitive internals | Critical – Internals installed before welding; full PWHT would damage them. |
| Aggressive project schedule | Significant – Eliminates longest single fabrication step (full PWHT). |
| High-integrity hydrogen service | Beneficial – Reliable, mill-controlled tempering ensures base metal properties. |
Important Consideration (Not a Pure Advantage)
The use of Class 2 transfers the technical challenge from full heat treatment to precision welding and local PWHT control. It requires:
Strict adherence to welding procedures to prevent cracking.
Precise control of local PWHT to avoid over-tempering (which can soften the base metal below spec) or creating steep thermal gradients.
In summary, the main advantage of ASTM A387 Grade 22 Class 2 is that it enables the construction of the largest and most critical high-temperature/high-pressure hydrogen service vessels by solving the single biggest fabrication hurdle: the impossibility of performing a full post-weld heat treatment. It trades higher initial material cost for massive savings in fabrication complexity, time, and risk.
1. What is the key risk during the Post-Weld Heat Treatment (PWHT) of A387 Grade 22 Class 2 material?
This question focuses on the critical fabrication challenge of applying the mandatory PWHT to an already-tempered material, where exceeding specific time-temperature parameters can lead to over-tempering and a drop in yield strength below the minimum specification.
2. Why would a designer specify Class 2 instead of Class 1 for a Grade 22 reactor?
This question explores the primary engineering rationale, often related to the impracticality of performing a full furnace heat treatment on a massive or complex finished vessel, making the pre-tempered Class 2 plate the necessary choice.
3. How does the Charpy impact toughness of Class 2 compare after welding and local PWHT?
This question addresses a key service performance concern, investigating whether the localized heating from welding and subsequent PWHT can degrade the notch toughness in the heat-affected zone compared to the base metal.
4. What are the main cost and lead-time implications of choosing Grade 22 Class 2 over Class 1?
This question examines the project trade-offs beyond technical specs, as Class 2 plate typically commands a higher price and may have longer procurement lead times due to its additional mill heat treatment step.
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.
