In casting production, the covering material that covers the surface of the molten metal in order to prolong the solidification time of the molten metal is called covering agent. There are two types, insulation covering agent and heating covering agent.
The riser insulation covering agent is composed of low thermal conductivity materials, such as expanded perlite, expanded graphite, etc., covering the surface of the metal liquid to play the role of thermal insulation; the heating covering agent is to add a certain proportion of heating materials in addition to low thermal conductivity materials Under the heat action of the molten metal, the oxidant provides oxygen to oxidize the reducing agent to generate heat, prolonging the solidification time of the molten metal.
Riser Covering Agent Physicochemical Indexes
Category
Material
Features
Insulation Materials
Acidified graphite
2.5 times expansion when heated, low thermal conductivity
Fly ash
Formation of liquid slag to prevent carbon infiltration and heating insulation
Flake graphite
Gray-black scales, high melting point, not immersed in steel castings
Expanded perlite
Soften to glass state at high temperature, reducing heat loss is good for heat preservation
Heating material
Aluminum powder, iron oxide powder
Thermite reaction generates a large amount of heat and reduces heat loss
The tundish covering agent has the functions of heat insulation and preventing crusts on the molten steel surface, as well as the function of insulating the air, preventing the secondary oxidation of molten steel and absorbing non-metallic inclusions on the molten steel surface.
Most of the commonly used at present are thermal insulation slag. The single type (carbonized rice husk, rice husk ash) has a small bulk density and low thermal conductivity, but it is not easy to form a liquid slag layer, has poor anti-secondary oxidation effect, is easy to add carbon to molten steel, and is nearly eliminated; the composite type is a mixture of multiple components. The thermal insulation performance is greatly improved. The liquid slag layer can prevent the secondary oxidation of the molten steel and absorb the inclusions, and has a good application prospect.
There are four types of tundish covering agents: acidic, neutral, alkaline, and double-dregs.
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