Principles of using high-temperature metallic materials

I. Performance Requirements for Metal Materials in High-Temperature Fasteners

1. Strength & Plasticity: Good room temperature and high-temperature tensile strength with adequate ductility.

2. Impact Resistance: Excellent impact energy absorption and low notch sensitivity.

3. Anti-Relaxation: For carbon steels operating above 300–350°C and alloy steels above 350–400°C.

4. Creep Resistance: High creep rupture strength, long-term stability, low creep embrittlement tendency, and ≥5% persistent elongation (new materials) or ≥3% (in-service materials) after 8,000–10,000 hours at temperature.

5. Structural Stability: Minimal temper embrittlement and hot brittleness.

6. Oxidation Resistance: Superior resistance to oxidation and thermal fatigue.

7. Corrosion Resistance: Suitable corrosion resistance for bolts exposed to cylinder environments.

8. Fatigue Resistance: High fatigue and shear strength for cyclically loaded applications (e.g., coupling bolts).

9. Processability: Excellent cold/hot working characteristics.

10. Compatibility: Nut materials should be 20–50 HB lower in hardness than bolts. Mixed-material fasteners on the same flange require consideration of thermal expansion mismatch and anti-relaxation performance.

II. Acceptance Criteria for High-Temperature Fastener Materials

1. Comply with DL/T 439, GB/T 20410, and contractual specifications.

2. Imported materials must meet equivalent foreign standards (e.g., ASTM, ISO) and contractual requirements.

III. Commonly Used Metal Materials for Fasteners

Table: Steel Grades, Characteristics, and Maximum Service Temperatures