Cracking and detachment of refractory materials in cast iron ladle, what are the defects in castings and how to solve them

If the refractory material used for the lining of the ladle cracks or falls off, these damaged materials are likely to mix into the molten iron during the pouring process, or cause abnormal flow of the molten iron, leading to various types of casting defects. The following will elaborate on these common defects and corresponding solutions:

Common casting defects

Slag inclusion: This is one of the more common types of defects in the production process of castings. When fragments of refractory materials detach and invade the interior of the mold cavity, during the solidification process of the molten metal, these fragments will embed into the casting body or surface, forming irregular white or brown non-metallic inclusions. Such defects not only significantly reduce the mechanical performance indicators of castings, but also cause serious wear and even damage to cutting tools in subsequent machining processes.

Sand hole: When using granular materials such as quartz sand as the lining, the detached particles will be washed into the mold cavity by the molten metal, and then form hole like defects on the surface or inside of the casting.

Cold insulation: When the lining of the ladle falls off in a large area, the thickness of the ladle wall will become thinner, which will cause the temperature of the molten iron to drop too quickly during transportation and pouring. The fluidity of molten iron is therefore reduced, and complete fusion cannot be achieved at the confluence, resulting in defects such as cold insulation or insufficient pouring.

Pores: When the lining cracks, it may adsorb moisture from the air or shed debris containing substances such as moisture and carbonates. These substances will rapidly produce gas upon contact with high-temperature molten iron. If these gases fail to escape in time, they will accumulate inside the casting and form porosity defects.

The surface of the casting presents a rough state: During the casting process, detached particles will flow into the mold cavity along with the molten iron. These particles will exacerbate the mechanical sand sticking phenomenon on the surface of the casting, making it rough and difficult to clean afterwards.

Solution measures

To properly address this issue, it is necessary to implement meticulous control over the entire process from material selection, construction process, baking, to daily maintenance:

Optimize and improve materials and their ratios

In the selection of refractory materials, suitable materials should be chosen based on the specific temperature of the molten iron. For ordinary cast iron, refractory materials with high aluminum content (i.e. relatively high content of alumina Al ₂ O3) can be selected; Due to the high temperature of the molten iron and the intense stirring process, it is recommended to use refractory materials such as waxy, zircon, or silicon carbide that have good thermal shock resistance and are not prone to slag deposition for ductile iron.

Regarding the use of binders, it is necessary to ensure that the amount added is precise and accurate, while maintaining their strength within an appropriate range. If the bonding force is too weak, it will lead to insufficient overall structural strength; On the contrary, if the bonding force is too strong, it may also cause cracking due to excessive internal stress.

Standardize the process flow for knotting and repairing

Whether using manual knotting or mechanical knotting methods, it is necessary to ensure that the knotting density is uniform and consistent to prevent cracking caused by differences in local tightness.

During the construction process of the overall knotted lining, special attention must be paid to leaving expansion joints. Due to significant volume expansion during the baking process, this type of lining is prone to cracking due to internal stress accumulation if expansion joints are not properly set in advance. Therefore, it is necessary to scientifically plan the position and size of expansion joints based on material characteristics and process requirements.

After each pouring operation, the slag should be cleaned immediately to ensure a clean surface. Once minor cracks are found, the same refractory slurry as the original material should be applied for repair treatment to effectively prevent further crack propagation.

Strictly follow the baking and preheating process

When carrying out baking operations, the newly made lining must be strictly baked according to the established heating curve to ensure that the crystal water can be effectively discharged and prevent the lining from cracking due to rapid evaporation of water.

Preheating treatment is crucial before putting into use: direct pouring of the cooling ladle is a key factor leading to cracking of the ladle lining and a sharp drop in the temperature of the molten iron. Before receiving molten iron in the ladle, it must be thoroughly baked until the ladle appears dark red (temperature needs to reach 600 ℃ or above). This is aimed at effectively reducing the impact of thermal shock on the ladle lining and preventing it from exploding due to severe expansion after heating.

Optimize and improve the structural design and assembly method of the lining

Strict control is required for thickness: the thickness of the lining should be kept uniform and consistent. If the lining is too thick, it will lead to a significant increase in thermal stress; If the lining is too thin, its strength will not meet the requirements.

In terms of skeleton support system, for large ladle, anchor bricks or metal anchors (which need to be insulated from the cladding) can be used to reinforce it and enhance its overall structural strength.

Optimize and improve the pouring process

Calming treatment step: After the molten iron is poured into the ladle, it is recommended to let it stand for a period of time, which can promote the possible floating slag to fully float to the surface. In the subsequent pouring operation, efforts should be made to maintain the continuity and stationarity of the pouring process to prevent slag rolling.

Before implementing the pouring operation, it is necessary to use a slag remover to thoroughly clean the floating slag on the liquid surface to ensure that the liquid surface is clean and free of impurities.

In situations where the on-site situation is extremely urgent, such as when the lining has already developed minor cracks but must be immediately put into use, a temporary solution can be taken: thoroughly bake the lining, and then evenly apply a layer of high aluminum or graphite coating. By doing so, it is possible to temporarily seal those cracks, effectively preventing the infiltration of molten iron and thus avoiding the occurrence of large-scale peeling.