Vertical crack formation: After the ladle has been used for several heats, vertical crack formation initiates (and it depends on the material mineralogy) on the bottom of the ladle(on the impact pad area). The major factor responsible for the crack formation is the impact due to the tapping stream which forces the slag sitting on the bottom to penetrate deep inside. The impact force is dependent on the height of the tapping and the impact angle and its effect are determined by the time taken to form a liquid pool which will act as a cushion to minimize the impact on the bottom. The propensity of crack formation depends on a number of factors like materials with lower strength and elastic modulus, materials with higher work of fracture which creep at operating temperatures, lower coefficient of expansion materials, and materials with low permanent linear change.
Slag infiltration: The slag which is present on the bottom of the ladle after slag dumping begins to infiltrate into the cracks. The depth of penetration is dependent on the mineralogy of the material used, the amount of the porosity and their distribution. Slag fluidity and chemistry of the slag are some crucial factors responsible for penetration as well.(low viscosity, high fluxing oxides and highly wetting slag can easily sip in). It is recommended to use magnesia instead of magnesia spinel as the magnesia binds with the infiltrating slag quicker then spinel. Whereas the spinel has to dissociate to release magnesia which can then react with the penetrating slag. The complex spinel formed after reacting with slag is expansive in nature and closes the pores thereby preventing further slag penetration. The problem with the use of free magnesia is that unless it is added in the correct size and proportion and with the right additives, it will cause micro-cracks in the materials. The use of proper texture, porosity and distribution is the key in preventing slag penetration.
Complex spinel formation:
FeO+ MnO+Al203+MgO= (Fe, Mn, Mg)O-Al203 complex spinel.
Horizontal Crack formation: These are results of thermal cycling during the course of the action. The mismatch in the expansion coefficients, material strength, modulus of elasticity, are some of the other determining factors responsible for the formation of horizontal cracks between the penetrated and non penetrated zones.
Spalling: This occurs when the vertical crack and the horizontal cracks meet up and huge chunks of material ranging from 25-300 mm diameter disintegrate from the bottom and fall off.
