Why do heat treatment fixtures fail?

I. Main Failure Modes and Causes of Heat Treatment Fixtures

1. Thermal Fatigue Cracking (Most Common Failure)

• Manifestation: Network cracks (crazing), especially at sharp corners and welds.
• Causes:

• • Cyclic thermal stress from repeated rapid heating and cooling.
  • Insufficient thermal fatigue resistance of the material.
  • Unreasonable structural design (stress concentration).

2. High-Temperature Creep Deformation

• Manifestation: Sagging, bending, permanent distortion of the fixture.
• Causes:

• • Sustained loading at high temperatures over long periods.
  • Insufficient high-temperature strength of the material.
  • Overloading or local overheating.

3. Oxidation and Corrosion

• Manifestation: Surface scaling, spalling, thinning, embrittlement.
• Causes:

• • High-temperature oxidation (scale formation).
  • Atmosphere corrosion (carburization, nitriding, sulfide attack).
  • Corrosion from salt baths or quenching media.

4. Mechanical Damage

• Manifestation: Impact dents, scratches, fractures.
• Causes:

• • Handling collisions.
  • Improper workpiece loading.
  • Jamming or forceful removal.

5. Phase Transformation and Microstructure Degradation

• Manifestation: Material embrittlement, sharp drop in strength.
• Causes:

• • Prolonged use within sensitive temperature ranges (e.g., 475°C embrittlement).
  • Precipitation of harmful phases (sigma phase, carbide aggregation).

II. Key Q&A on Material Selection

Q1: How to select fixture material based on operating temperature?

• ≤600°C: Mild steel, low-alloy steel (e.g., Q235, 16Mn) is sufficient.
• 600-900°C: Medium-alloy heat-resistant steel (e.g., 1Cr18Ni9Ti, 309S).
• 900-1100°C: High-alloy heat-resistant steel (e.g., 310S, 330, 253MA).
• 1100-1200°C: Nickel-based alloys (e.g., Inconel 600/601/617).
• >1200°C: Ceramics, silicon carbide, molybdenum alloys (note: oxidation protection required).

Q2: What are the special material requirements for different heat treatment processes?

• Carburizing/Carbonitriding:
• • Avoid high-nickel alloys (prone to soot formation).
  • Prefer chromium-manganese-nitrogen steels (e.g., ZG4Cr25Ni20Si2).
• Vacuum Heat Treatment:
• • Low vapor pressure materials (avoid elements like Zn, Cd, Pb).
  • High purity, low outgassing rate materials.
• Salt Bath Heat Treatment:

• • Corrosion-resistant materials against molten salts (e.g., Inconel 600).
  • Consider salt bath composition (chloride, nitrate, cyanide salts, etc.).

Q3: How to choose between cast and forged fixture materials?

III. Key Q&A on Maintenance

Q4: How to extend fixture life during daily use?

1. Preheating Procedure:

• New or cold fixtures must be preheated in stages (e.g., 1 hour each at 200°C, 500°C, 800°C).
• Avoid placing cold fixtures directly into a high-temperature furnace.

2. Loading Specifications:

• Distribute load evenly, avoid point overloads.
• Maintain adequate clearance between workpiece and fixture, and between workpieces (recommended ≥10mm).
• Place heavy workpieces at the bottom, lighter ones on top.

3. Temperature Control:

• Strictly prohibit exceeding the maximum service temperature of the fixture material.
• Avoid prolonged holding at critical temperature ranges.

Q5: What key points should be focused on during routine fixture inspection?

Daily Check:
√ Surface oxidation (abnormal spalling?)
√ Visible deformation (measure with straight edge, calipers)
√ Cracks (focus on welds, sharp corners)

Weekly Check:
√ Dimensional accuracy (critical locating dimensions)
√ Looseness of connections
√ Integrity of hangers and lifting lugs

Monthly Check:
√ Comprehensive dimensional inspection (vs. original drawing)
√ Penetrant testing for micro-cracks
√ Load-bearing capacity assessment (load test if necessary)

Q6: What are feasible methods for fixture repair?

• Welding Repair:

• • Use specialized heat-resistant welding electrodes (e.g., A402, A407, NiCrFe-3).
  • Preheat to 300-400°C, slow cool after welding.
  • Suitable only for non-critical areas and minor cracks.
• Surface Enhancement:

• • Aluminizing, chromizing (improves oxidation resistance).
  • Thermal spray ceramic coatings (Al₂O₃, ZrO₂).
  • Plasma transferred arc (PTA) hardfacing.
• Mechanical Repair:

• • Straightening deformed parts (requires annealing first).
  • Local reinforcement (welding stiffeners).
  • Note: The load-bearing capacity of repaired fixtures must be re-evaluated.

Q7: When must a fixture be scrapped?

Discontinue use immediately if any of the following occurs:

1. Structural Cracks: Through-thickness cracks or cracks >10mm in critical areas.
2. Severe Deformation: Straightness deviation >1/100 of total length, or affecting workpiece positioning.
3. Excessive Thinning: Section loss due to oxidation/corrosion >30% of original thickness.
4. Performance Failure: Inability to meet process requirements (e.g., uniformity, cooling rate).
5. Safety Hazard: Any visible crack or permanent deformation on lifting devices.

IV. Suggestions for Economic Management

Fixture Life Cycle Management Table

V. Quick Material Selection Reference Table