Covering Agent

Description

In casting production, in order to prolong the solidification time of metal liquid, the covering material covering the surface of metal liquid is called covering agent. Including two types of  thermal insulation covering agent and heat generating covering agent.
 

Riser Covering Agent

 
The riser insulation covering agent is composed of low thermal conductivity materials, such as expanded perlite and expanded graphite. Covering them on the surface of the metal liquid can play the role of thermal insulation.
 
The heat generating covering agent is to add a certain proportion of heat generating materials in addition to the low thermal conductivity material. Under the heat of the molten metal, the oxidant provides oxygen, which causes the reducing agent to oxidize and heat, extending the solidification time of the molten metal.
 
Classification and Characteristics of Riser Covering Agent
 

Items	Main Raw Materials	Features
Insulation Materials	Acidified graphite	Expansion rapidly 2.5 times when heated, low thermal conductivity
	Coal fly ash	Liquid slag is formed to prevent carbon penetration, and heat insulation is used to achieve thermal insulation
	Flake graphite	Grayish black scales, high melting point, not immersed in cast steel
	Expanded perlite	It softens to glass state at high temperature, reduces heat loss and is beneficial to heat preservation
Heat generating material	Aluminum powder, iron oxide powder	The thermite reaction generates a lot of heat, reducing heat loss

 

Tundish Covering Agent

 
The tundish covering agent, in addition to heat insulation, to prevent the molten steel surface crust. It also has the functions of isolating air, preventing secondary oxidation of molten steel, and absorbing non-metallic inclusions on the molten steel surface.
 
At present, most commonly used tundish covering agents belong to heat-insulating slag, which can be generally divided into single type and composite type.
 
Single type (carbonized rice husk, rice husk ash), small bulk density, low thermal conductivity, good thermal insulation effect. However, it is not easy to form a liquid slag layer, the effect of preventing secondary oxidation is poor, and it is easy to increase the carbonization of molten steel.
 
The compound type is a mixture of various components, which can greatly improve the thermal insulation performance. The liquid slag layer can prevent the secondary oxidation of the molten steel and absorb the inclusions, and has a good application prospect.
 
Classification and Characteristics of Tundish Covering Agent
 
There are four types of tundish covering agents: acidic, neutral, alkaline and double-layer slag.
 

Items	Material	Characteristics
Acidic	Carbonized Rice Husk	Good thermal insulation and low cost, but it is not conducive to adsorption of inclusions. There is a chemical reaction at the steel slag interface.
Neutral	Al2O3-SiO2	It has certain thermal performance and low cost.
Alkaline	MgO or dolomite base	Thermal insulation is slightly worse, easy to crust when used alone.
Double slag	Carbonized rice husk + alkaline residue	It is possible to add carbon to molten steel

  

Characteristics of Covering Agent

 
1. The thermal insulation covering agent is made of low thermal conductivity material. Such as: expanded perlite, expanded graphite, etc.
 
2. In addition to the low thermal conductivity materials, the heating covering agent is added with a certain proportion of heating materials. Under the heat of the molten metal, the oxidant provides oxygen to oxidize the reducing agent to heat, prolonging the solidification time of the molten metal in the riser.
 
3. In the case of different insulation covering agents, the mouth shrinks into a "dish-shaped flat bottom or hemispherical bottom, that is, the shape of the riser shrinkage is approximately U-shaped.
 
4. Significantly extend the time for the top of the riser to crust. The molten steel in the feeder is exposed to atmospheric pressure for a long time, which greatly improves the feed efficiency of the feeder. It is beneficial for the gas and inclusions in the casting to float up, achieving the purpose of purifying molten steel. Obtaining sound steel parts reduces the scrap rate.