Overheating and overburning

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00 defines that during the heating of metals, overburning is more severe than overheating. The following is a detailed comparison of the severity of the two. 01 Overheating 1-1. Definition: Overheating refers to the phenomenon in which, during the heating process of metal, excessive heating temperatures or prolonged holding times cause the austenite grains to grow rapidly, thereby leading to a deterioration in the metal’s mechanical properties. 1-2. Classification: Stable superheating: A phenomenon of superheating that cannot be completely eliminated by ordinary normalizing, annealing, or quenching treatments. Unstable superheating: A superheating phenomenon that can be eliminated using normal heat treatment processes. 1-3. Hazards: It reduces the plasticity of metals, causes the grains in forgings to become coarse after forging and heat treatment, and diminishes the mechanical properties of the metals. For different types of steel, overheating can lead to specific microstructural changes; for example, carbon steel often develops Widmanstätten structure after being overheated, while martensitic steel exhibits a coarse needle-like fiber structure as a result of overheating. 1-4. Preventive measures: Strictly control the heating temperature and minimize the time spent at high temperatures. During forging, sufficient deformation of the forgings is required to break down the large austenite grains and precipitates. II. Over-sintering 2-1. Definition: Over-sintering refers to the phenomenon in which, when the heating temperature exceeds the low-melting eutectic temperature of the metal, the low-melting eutectic phase and the grain boundaries undergo remelting. Overburning, also known as overheating, is a more severe type of thermal damage than overheating. 2-2. The characteristic metal surface turns black or dark. In the microstructure, features such as locally widened grain boundaries, remelting spheres formed within the grains, and distinct triangular remelting zones at the grain interfaces can be observed. 2-3. The hazard of overheating is that it can completely destroy the bonding between metal grains, resulting in a fracture surface that lacks a metallic luster. Forging may cause network-like cracks, namely “cracking”. Overheated tissue cannot be restored and must be discarded. 2-4. Preventive measures: Strictly adhere to the heating specifications to avoid excessively high heating temperatures. Reduce the residence time at high temperatures to minimize the risk of overburning. Advanced heating techniques such as segmented heating and programmed temperature rise are employed to more precisely control the heating process. III. What is the difference between overburning and overheating? I. Tissue changes: Overheating is characterized by the enlargement of austenite grains; the grain boundaries may widen slightly, but no significant melting or oxidation occurs. Within the grains of the overheated tissue, bainite structure or other coarse microstructures may form. Overheating: In addition to the enlargement of austenite grains, it is also accompanied by severe weakening and melting of the grain boundaries. In the microstructure, features such as locally widened grain boundaries, remelting spheres formed within the grains, and distinct triangular remelting zones at the grain interfaces can be observed. Furthermore, overheating can also cause the metal surface to turn black or dark, resulting in irreversible thermal damage. II. Effect of mechanical properties on overheating: Although overheating can lead to a decline in the metal’s mechanical properties, such as reduced plasticity and impact toughness, these properties can usually be restored or improved through subsequent heat treatment processes such as normalizing, quenching, and tempering. Overheating: It has a more severe impact on the mechanical properties of metals, and can even lead to the complete loss of their usability. Overheated material cannot have its properties restored or improved through subsequent heat treatment; it must be discarded. III. Overheating detection method: It is mainly determined through metallographic examination. Observe characteristics such as the degree of coarsening of austenite grains and the presence of bainite structure. Overheating: In addition to metallographic examination, it can also be determined by observing changes in the color of the metal surface, as well as features such as remelting spheres and triangular remelting zones in the microstructure. Furthermore, overheated forgings may develop network-like cracks (i.e., \"cracking\") during forging, which is also an important indicator of overheating.