1. Essential Differences Between Overheating and Burning

1.1 Definition and Characteristics

• Grain Coarsening: The grain size exceeds the standard value by 2-3 times (for example, the normal grain size of 45 steel is grade 8, with D≈15μm; after overheating, it reaches grade 5, with D≈80μm).
• Deterioration of Mechanical Properties: The strength decreases by 10%-20%, and the toughness is lost by 30%-50%.
• Reversibility: The fine-grained structure can be restored through re-normalizing or annealing.

• Grain Boundary Erosion: Under a metallographic microscope, the grain boundaries are serrated or discontinuous, and liquid phase films appear in local areas.
• Collapse of Mechanical Properties: The tensile strength decreases by more than 50%, and the elongation approaches zero.
• Irreversibility: Even with re-heat treatment, the performance cannot be fully restored.

1.2 Comparison of Key Differences

2. Formation Mechanisms and Case Analysis

2.1 Formation Mechanism of Overheating

• Out-of-control Heating Rate: Rapid heating (such as induction heating) causes a large temperature difference between the surface and the core, and the local temperature exceeds the critical point.
  • Case: During the quenching of an automobile gear, the excessive heating power caused the surface temperature to reach 950℃ (Ac₃=850℃), and the grain size deteriorated from grade 8 to grade 5.
• Excessively Long Holding Time: After austenitization, the grain boundary migration continues with long-term holding.
  • Data: When 45 steel is held at 920℃ for 2 hours, the grain size reaches grade 6 (D≈50μm); after holding for 4 hours, it deteriorates to grade 4 (D≈120μm).
• Coarse Original Structure: No spheroidizing annealing is performed after forging or rolling, leading to carbide segregation and promoting grain growth.

2.2 Formation Mechanism of Burning

• Temperature Measurement Error: Improper installation of thermocouples or failure of compensation wires causes the actual temperature to exceed the standard.
  • Accident: During the heat treatment of an aerospace engine blade, the thermocouple fell off and was not discovered in time. The actual temperature reached 1250℃ (solidus 1220℃), and the blade was scrapped due to burning.
• Failure of Atmosphere Control: An oxidizing atmosphere (such as O₂ content > 0.5%) promotes grain boundary oxidation and reduces the melting point.
  • Experiment: TC4 titanium alloy burns when heated to 1050℃ (solidus 1030℃) in air, while it requires heating to 1080℃ to burn in a vacuum furnace.
• Segregation of Alloying Elements: Elements such as Cr and Mo segregate at grain boundaries, forming low-melting eutectic structures.
  • Case: The segregation of Nb element in GH4169 superalloy reduces the grain boundary eutectic temperature to 1180℃, and burning occurs if heated to 1200℃.

3. Identification Methods and Technical Applications

3.1 Macroscopic Inspection Method

• Appearance Observation:
  • Overheating: The surface oxidation color is uniform, with no local melting marks.
  • Burning: The surface is dark gray or black, and crystalline molten droplets can be seen locally.
• Fracture Analysis:
  • Overheating: The fracture is fibrous with a small number of dimples.
  • Burning: The fracture is stone-like or rock candy-like, with clear grain boundaries visible.

3.2 Metallographic Inspection Method

• Grain Size Rating:
  • Overheating: The grains are coarse but the grain boundaries are intact, and the rating is reduced by 2-3 grades according to the ASTM E112 standard.
    • Figure 1: Overheated Structure of 45 Steel (400X): The figure shows the microstructure of 45 steel heated at 930℃ for 15 minutes and water-quenched, which consists of gray coarse quenched medium-carbon martensite, off-white retained austenite, and a martensite matrix. The black strips in the upper right corner are quenching cracks along the grain boundaries. (Source: Official Account "Opto-Mechanical Home")
  • Burning: Grain boundary erosion or oxide layer penetration occurs, forming triple grain boundary voids.
    • Figure 1: Burned Structure of W18Cr4V Steel (400X): The figure shows the quenched burned structure of W18Cr4V steel, which consists of off-white acicular martensite, a retained austenite matrix, bright white reticular carbides along the grain boundaries, and black troostite structure. (Source: Official Account "Opto-Mechanical Home")
• Oxide Detection: Oxides such as Al₂O₃ and SiO₂ distributed along the grain boundaries can be observed in the burned samples (EDS energy spectrum analysis).

3.3 Mechanical Property Testing

• Impact Test:
  • Overheating: The room-temperature impact energy decreases by 30%-50%, but the fracture still has toughness characteristics.
  • Burning: The impact energy approaches zero, and the fracture is completely brittle.
• Hardness Test:
  • Overheating: The hardness may decrease slightly (HRC decreases by 1-2 units).
  • Burning: The hardness decreases significantly (HRC decreases by 3-5 units).

3.4 Non-Destructive Testing Technology

• Ultrasonic Testing: In the burned area, the sound velocity decreases due to grain boundary erosion, and the amplitude attenuation rate increases by 20%-30%.
• Eddy Current Testing: The electrical conductivity of the burned sample decreases by 15%-20%, and a characteristic shift occurs in the impedance spectrum.

4. Preventive Measures and Engineering Practice

4.1 Process Parameter Control

• Precise Temperature Control:
  • Adopt dual monitoring of infrared thermometers and thermocouples, and control the error within ±5℃.
  • Case: After introducing an intelligent temperature control system, an automobile factory reduced the overheating defect rate from 3% to 0.1%.
• Optimization of Heating Rate:
  • The heating rate of carbon steel is controlled at 50-100℃/h, and that of superalloys is controlled at 10-20℃/h.

4.2 Equipment and Maintenance

• Pub Time : 2025-10-09 10:13:31 >> News list