The information comes from personal work notes, some hands-on experiments, some reference books. Some opinions are just personal thoughts, not correct.
Due to the impact of peak-valley stepped electricity price, many induction furnace founds choose night shift smelting, shut down in the morning, intermittent operation. When coming off work in the morning, some choose to fill the furnace charge, some choose to cover the furnace for heat preservation, and some people open the empty furnace for cooling.
According to refractory company engineers, rapid cooling of furnace lining is more advantageous than slow cooling.
According to the actual experience, slow cooling furnace lining will produce large (more than 6mm) transverse cracks (the circumference of the furnace body), rapid cooling will produce multi-direction arbitrary small cracks, these small cracks are very easy to be closed by appropriate cold initiation process. The sintering layer is also self-cooling, there will not be too large cracks, micro-cracks are inevitable.
We know that thermal expansion and cold contraction are inherent properties of materials. The lining materials (silica sand, magnesia, etc.) of whatever nature form fixed expansion with the process of smelting and sintering. Notice that the fixation here is not static but it’s dynamic and regular expansion. Cracks of varying degrees are produced during cooling.
Compared with slow cooling, a large number of diffuse cracks appeared in the early stage of cooling process, which avoided the occurrence of a few large transverse cracks.
Rapid cooling tends to keep the heat dissipation rate of the lining surface consistent with that of the lining coil surface, which will reduce stress along the lining surface. The coil is made of copper pipe with internal water, which is cooled by circulating cooling water when the furnace is shut down. The huge heat generated during smelting is taken away to avoid the damage caused by heat concentration on the furnace itself. The nearly static air in the empty furnace body. The thermal conductivity of air is very poor, which may cause the difference in cooling speed inside and outside the furnace lining, resulting in stress concentration and crack.
The longitudinal cutting structure of furnace lining includes a sintering layer, transition layer, and loose layer.
Glaze: for quartz sand material in the quartz phase, this layer of the ceramic surface will be in direct contact with molten iron, its purpose is to prevent the infiltration of molten iron lining and resist the damage caused by the impact of the charge on the lining during feeding operation.
Sintered layer: sintered layer with metal liquid contact, under high-temperature slag and metal liquid erosion and static pressure and temperature stress, is the lining material (crucible) working layer. It should have very high strength, and the thickness of sand material is about 25%~35% of the thickness of the crucible wall.
Semi-sintered layer: also known as a transition layer, part of sand materials begin to bond, sintering has just begun, sintering network is not complete. The function of this layer is to absorb the expansion stress generated by the sintering layer and prevent the extension of cracks. The thickness of the semi-sintered layer is about 35% of the lining material (crucible) wall thickness.
The unsintered layer: also known as the loose layer, is completely unsintered original sand. It plays the role of heat insulation and buffering the volume change of the crucible heating and cooling process on the force of the induction coil. The unsintered layer accounts for about 30% of the lining material (crucible) wall thickness.
Under normal circumstances, after leaving the furnace, the operator tends to top up the charge. This will sharply cool the surface temperature of the upper part of the furnace and cause circumferential cracks in the sintering layer. Instantly in the high temperature of the empty furnace suddenly filled with a cold material, the weight of the material is basically pressed at the bottom of the furnace, and the heat of the upper furnace wall is absorbed by the furnace charge and violently cooled to below 600 degrees, which will cause the axial shrinkage stress of the furnace wall. The gravity of the charge and the low-temperature strength of the sintering layer will pull out the furnace wall at a certain point along the circumferential crack.
The furnace had better not cause the upper and lower temperature difference in the furnace when cooling.
It is possible to open the furnace cover for natural cooling. There is no need to cover the furnace for slow cooling.