Thermal
Peak Temperature-The ladle slide gate being in direct contact with the steel attains its temperature and cools down after the ladle is closed. Therefore, the variations in temperature follow a histogram. The temperature is high from the refractive point of view. Thus material with a high melting point is to be selected.
Thermal shock-Thermal cycling is high for the slide gate system. The top nozzle suffers from thermal shock because cold well mix is added in every heat followed by the tapping of steel. The lower nozzle and slide gate plates extremely suffer from thermal shock (Normally being in contact with air the temperature falls and when it comes with the contact of steel during ladle opening, thermal shock is relatively high). Carbon is therefore added to tackle the situation.
Mechanical
Impact – The plates suffer from impact from bars and chipping guns which are used when the slide gates are reused for multiple heats.
Abrasion-The abrasion of abrasion is directly dependent on the throughput of steel from the plates and increases rapidly with the bore size. But the bore size is important as far as other metallurgical factors are concerned. For example, for a very high bore size, the overlap of the top plate and the moving plate will be small leading to heavy turbulence and huge overlapping will lead to lower control of managing steel level in tundish and can cause overflow.50% overlapping is recommended for the desired throughput. Adding to it, the slide gate plates experience abrasion due to the movement of the plates which cause wear of the plates. Whereas the lower and upper nozzle experiences abrasion only from the flowing steel.
Applied Stress – The upper nozzle fits into the well block and to compensate its expansion a mortar of 2-3 mm is added. If the nozzle is tightly packed, stress burden increases leading to higher chances of crack formation.
The slide gates plates have to be kept under compression. Mechanical stress is put on the plates to impart compressive forces and is counteracted by the expansion of plates when metal flows through it.
The applied stress on the lower nozzle is quite less as there is a scope for growth.
Chemical (Chemical and thermos mechanical)
Dissolution-Dissolution is dependent on the chemistry of the slag and the refractory used. The wear of well block and the upper nozzle is increased by the composition of the ladle sand that is being used.
Calcium vapor attack is one of the issues for the wear of the plates. When calcium in the steel comes in contact with the overlapping plates the Ca gets converted into Calcium vapour and attacks the plate because of the presence of alumina refractory. The wear takes the shape of “horse shoe”. The name is attributed to its similarity with the horseshoe. Wear increases at a faster rate when Calcium values exceed 35 ppm and it has been observed that the wear becomes aggressive and increases exponentially after the value crosses 45ppm. MgO and MgO –C refractory can be an alternative solution but the thermal shock resistance becomes a problem due to the high rate of stress generated and expansion rate.
Penetration-Penetration is accelerated by the highly penetrating low viscosity slag/metal or highly wetting slags. Low viscosity and high-temperature slag/metal are produced when oxygen lancing is used to clean the hole similar to the case of ladle shroud cleaning. These can penetrate deep into the refractory and carbon is added to prevent such penetration.
Thermo-Mechanical
Strain of thermal expansion – Stress is generated due to the thermal expansion of the system. Lower nozzle suffers less stress as its growth is not restricted by a mechanical barrier and is highest for the plates.
Chemical –Mechanical and Thermo –Mechanical- Chemical
Spalling of penetrated zones- The low life of plates is because of the cumulative action of stress, abrasion, thermal shock, and calcium attack
