Overheating severely degrades the mechanical properties of materials, leading to a sharp decline in strength, toughness, and fatigue life, and causing irreversible structural damage. This high-temperature damage causes grain boundary oxidation, voids, and even localized melting within the material, completely destroying its load-bearing capacity.
1. Comprehensive Degradation of Mechanical Properties
Performance Indicators and Their Effects After Overheating
Tensile Strength: Significantly reduced; loss of grain boundary bonding makes the material extremely prone to fracture.
Impact Toughness: Sharply decreased; fracture surface exhibits a brittle "stone-like" fracture, with no plastic deformation.
Fatigue Life: Shortened by more than 50%; microcracks propagate rapidly at grain boundaries.
Hardness Uniformity: Large fluctuations; localized softened areas coexist with hard phases, affecting service stability.
High-reliability data shows that the impact toughness of overheated materials can decrease to less than 30% of its original value, a major cause of early failure.
2. Irreversible Damage to Microstructure
Grain Boundary Oxidation: Oxygen intrudes along grain boundaries, forming a continuous oxide network that severely weakens intergranular bonding.
Grain Boundary Voids: At high temperatures, atomic diffusion intensifies, leading to the precipitation of micropores at grain boundaries, which become crack initiation sites.
Remelting Balls (Liquid Phase Traces): Local temperatures exceed the melting point, resulting in rounded solidified droplets, indicating micro-area melting.
Intergranular Fracture: Cracks propagate along grain boundaries; the fracture surface lacks metallic luster and appears as grayish-white granules.
Once any of these characteristics appear, the material is considered permanently damaged and cannot be recovered through heat treatment.
3. Abnormal Performance During Macroscopic Use
Frequent Mold Cracking: Repeated intergranular cracks appear in high-temperature areas such as hot runner nozzles and manifolds.
Sudden Performance Degradation: Lifespan is significantly shortened under the same operating conditions, with maintenance cycles reduced from months to weeks.
Abnormal Surface Discoloration: Overheated areas exhibit bluish-purple, grayish-white, or peeling phenomena, indicating microstructural degradation.
