The key differences between SA 537 Class 1 (CL1) and Class 2 (CL2) lie in heat treatment, mechanical properties, low-temperature toughness, and applications, as detailed below:
1. Heat Treatment Process
| Class | Heat Treatment | Technical Purpose |
|---|---|---|
| SA 537 CL1 | Normalized (N)<br>(Heated to Ac₃+30–50°C, air-cooled) | Refines grain structure for moderate toughness and weldability |
| SA 537 CL2 | Quenched + Tempered (QT)<br>(Quenched then tempered ≥595°C) | Enhances strength-toughness balance through martensitic transformation and stress relief |
Why it matters: QT treatment in CL2 enables higher strength and superior impact resistance, especially in thick sections (>65 mm), while CL1's normalized state suits general pressure applications .
2. Mechanical Properties
| Property | SA 537 CL1 | SA 537 CL2 |
|---|---|---|
| Yield Strength (σ₀.₂) | ≥345 MPa (thickness ≤65 mm)<br>↓ to ≥310 MPa (>100 mm) | ≥415 MPa (thickness ≤65 mm)<br>↓ to ≥315 MPa (>100 mm) |
| Tensile Strength (σb) | 485–620 MPa | 550–690 MPa |
| Elongation (δ) | ≥22% (thickness ≤100 mm) | ≥22% (thickness ≤100 mm) |
Key insight: CL2 offers ~20% higher yield strength and broader tensile range, allowing thinner wall designs in pressure vessels for equivalent load conditions .
3. Low-Temperature Toughness
| Class | Minimum Test Temp. | Impact Energy (Longitudinal) |
|---|---|---|
| CL1 | -62°C (thickness ≤25 mm) | ≥20 J |
| CL2 | -68°C (thickness ≤65 mm) | ≥27 J |
Significance: CL2’s stricter toughness requirements (e.g., -68°C vs. CL1's -62°C) make it preferred for cryogenic storage (e.g., LNG tanks) and critical low-temperature service .
4. Applications & Cost
| Aspect | SA 537 CL1 | SA 537 CL2 |
|---|---|---|
| Typical Use Cases | General pressure vessels,<br>boiler shells (moderate stress) | High-pressure reactors,<br>nuclear containment vessels,<br>LNG storage tanks |
Trade-off: CL2’s 6% higher cost is often justified for critical applications where safety margins, weight reduction, or cryogenic performance are prioritized .
5. Thickness Limitations
CL1: Effective up to 100 mm, but properties degrade significantly beyond 65 mm .
CL2: Maintains strength/toughness up to 150 mm, ideal for heavy-wall reactors and thick-section components .
Summary
| Factor | SA 537 CL1 | SA 537 CL2 |
|---|---|---|
| Optimal Use | Cost-sensitive, moderate-pressure vessels | High-pressure/cryogenic critical systems |
| Advantage | Lower cost, good weldability | Superior strength-toughness,<br>thicker section capability |
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