During the calcination process, the microscopic mechanism by which “overburning” leads to a decrease in true density is primarily related to grain boundary oxidation or melting, abnormal grain growth, and structural damage, as analyzed in detail below:
- Grain boundary oxidation or melting: Loss of intergranular bonding strength
Formation of low-melting eutectic phases: When the calcination temperature exceeds the melting point of low-melting eutectics in the material, the eutectic structure at the grain boundaries preferentially melts, forming a liquid phase. For example, in aluminum alloys, re-melted spheres or triangular re-melted zones may form, while in carbon steels, grain boundary oxidation or localized melting may occur.
Penetration of oxidizing gases: At high temperatures, oxidizing gases (such as oxygen) diffuse to the grain boundaries and react with elements in the material, generating oxides. These oxides further weaken the intergranular bonding strength, leading to grain separation.
Structural damage: After grain boundary melting or oxidation, the intergranular bonding strength significantly decreases, resulting in the formation of microcracks or pores within the material. This reduces the effective mass per unit volume, leading to a decrease in true density. - Abnormal grain growth: Increase in internal defects
Grain coarsening due to overheating: Overburning is often accompanied by overheating, where excessively high heating temperatures or prolonged holding times cause rapid growth of austenite grains. For example, carbon steels may develop Widmanstätten structures after overburning, while tool steels may form fishbone-like ledeburite.
Increase in internal defects: Coarse grains may contain more defects such as dislocations and vacancies, which reduce the material’s density. Additionally, gas pores or microcracks may form during grain growth, further reducing the mass per unit volume.
Reduction in effective mass: Abnormal grain growth leads to a loose internal structure in the material, lowering the effective mass per unit volume and thus resulting in a decrease in true density. - Microstructural damage: Deterioration of material properties
Re-melted spheres and triangular re-melted zones: In aluminum alloys and other materials, overburning can lead to the formation of re-melted spheres or triangular re-melted zones at the grain boundaries. The presence of these regions disrupts the material’s continuity and increases porosity.
Grain boundary widening and microcracks: After overburning, grain boundaries may widen due to oxidation or melting, accompanied by the formation of microcracks. These microcracks can penetrate through the material, leading to a decrease in true density.
Irreversibility of properties: The microstructural damage caused by overburning is typically irreversible, and even subsequent heat treatment may not fully restore the material’s original density.
Examples and verification
Overburning of aluminum alloys: When the heating temperature of aluminum alloys exceeds their low-melting eutectic temperature, the grain boundaries coarsen or even melt, forming re-melted spheres or triangular re-melted zones. The presence of these regions significantly reduces the material’s true density while causing a sharp decline in mechanical properties.
Overburning of carbon steels: After overburning, carbon steels may form inclusions such as iron oxide or manganese sulfide at the grain boundaries, which weaken the intergranular bonding strength and lead to grain separation. Additionally, overburning may trigger the formation of Widmanstätten structures, further reducing the material’s density.
Post time: Apr-27-2026