1. Sintered Structure of the Crucible
Crucibles made by various methods must be dried and sintered before use. The purpose of sintering is to improve the crucible’s density, strength, and volume stability so that it can meet the requirements of smelting conditions.
During sintering, the contact surfaces of the sand material partially melt at high temperature, forming a continuous sintered network. Through this network, the entire sand material is bonded into one solid body. The heat required for sintering may come from the heat released by graphite cores or steel-plate cores during heating, or it may be supplied directly by molten steel.
After sintering, the ideal cross-section of a crucible can be divided into three zones:
1. Sintered Layer
The sintered layer is the working layer of the crucible. It is in direct contact with molten steel, furnace slag, and atmosphere, and is directly affected by high-temperature erosion from slag and molten steel, thermal stress from rapid heating and cooling, and static pressure from molten steel.
Therefore, the sintered layer must have high density, high strength, and minimal surface cracking. The sand particles in this layer should be fully fused, and the sintered network should be uniform and complete. The thickness of the sintered layer is usually about 30-35% of the total wall thickness.
The main defect in the sintered layer is cracking, including transverse cracks and longitudinal cracks. Transverse cracks, especially ring-shaped transverse cracks, have the greatest impact on crucible life and are the most dangerous. Once cracks appear in the sintered layer, high-temperature molten steel may penetrate into them. Under the action of a strong magnetic field, the infiltrated steel continues to move outward after being heated, until a short circuit occurs and the crucible is penetrated.
During smelting, the crucible expands in volume. Because the surrounding induction coil restricts outward expansion and the furnace bottom restricts downward expansion, the only free expansion direction is upward. As a result, longitudinal cracks formed in the sintered layer may close after the crucible is heated, while transverse cracks not only fail to close but may even become larger. The main causes of transverse cracking in the sintered layer are uneven sand packing density and excessively high sintering temperature. These should be avoided as much as possible.
2. Semi-Sintered Layer
The semi-sintered layer is the transition zone between the sintered layer and the unsintered layer. In this layer, some particle contact surfaces begin to fuse, but the sintered network is not yet complete.
The function of the semi-sintered layer is to buffer the stress of the sintered layer and prevent cracks in the sintered layer from extending outward. An ideal semi-sintered layer should expand properly when its volume changes, prevent molten metal from continuing to penetrate beyond the sintered layer, and stop crack propagation.
In an ideal structure, the maximum shrinkage rate of the crucible’s sintered layer is about 7.5%. At the boundary between the sintered layer and the semi-sintered layer, the shrinkage rate decreases to nearly zero, while the maximum volume expansion rate in the semi-sintered layer reaches about 2%. This is the most desirable structure. In alkaline sand materials, this type of semi-sintered layer with volume expansion can be obtained by adjusting the boric acid addition amount or adding SiO2, TiO2, Al2O3, and similar materials.
The thickness of the semi-sintered layer is generally about 35-40% of the crucible wall thickness.
3. Unsintered Layer
Between the semi-sintered layer and the induction coil, there is a layer of original sand material that has not been sintered at all. This layer provides thermal insulation and acts as a buffer during crucible heating and cooling.
This unsintered layer is necessary. Otherwise, expansion stress would act directly on the induction coil and cause deformation. At the same time, excessive heat loss through the crucible wall would occur.
The unsintered layer is usually about 25-30% of the crucible wall thickness. Its proportion is higher during low-temperature sintering and lower during high-temperature sintering.
For crucibles of different capacities, the sintering structure must be controlled by selecting a proper sintering temperature and holding time. In actual production, the sintering process should be adjusted according to working conditions to obtain a reasonable crucible structure.
