What happens to molten metal and mold?

In the process of filling and solidification,molten metal acts with the mold in a thermal, physical, chemical and mechanical manner. Due to these effects, casting defects such as sand entrainment, sand hole, porosity, sand sticking, surface oxidation or decarbonization may occur.

 l Thermal action

When molten metal is poured into the mold, there is a severe heat exchange between the liquid and the mold. Under the thermal action of molten metal, with the increase of temperature, the volume expansion and moisture migration of the mold will occur.

( 1 )Water migration and Strength changes

When the moisture in the surface layer of sand mold is evaporated by heat, it solidifies and moves from high temperature to low temperature, then the strength of the mold changes accordingly.

( 2 )Mold expansion and Stress deformation

The expansion and stress of the mold when heated are not only related to the material, binder and additives, but also to the heating temperature, heating speed and the external conditions of the expansion.

When the stress produced by the expansion of the surface layer of the sand mold exceeds the strength of the water condensation zone,At this time, the surface of sand mold will crack, which is the main reason for sand inclusion.

1. The addition of pulverized coal, residual oil and sawdust can increase the yield of sand mold and reduce the hot pressing stress.

2. The thermal wet tensile strength of molding sand can be improved by activating the calcium bentonite with sodium bentonite or activating the calcium bentonite.

 l Physical /Chemical

The mechanical and physicochemical actions between metal and mold increase with the higher  temperature.The three functions of heat, mechanical and physical chemistry are interrelated.

The physical and chemical interaction between molten metal and mold is manifested as evaporation of water, loss of organic matter and decomposition of carbonate in the mold, etc.

The molten metal penetrates into the surface void of the mold and reacts with the mold material at high temperature to form compounds with low melting point. These effects cause defects such as porosity, adhesion, and oxidation or decarburization on the surface of the casting.

( 1 )Subsurface Porosity

When the thin-walled carbon steel, ductile iron and copper alloy castings are cast by wet mold, it is easy to produce subcutaneous pores of 1-3MM in diameter and 2-10MM in length at 1-2MM below the surface.

The reason of subcutaneous porosity in carbon steel castings is that hydrogen and ferric oxide are formed when molten steel contacts with water vapor, part of hydrogen diffuses into molten steel, the concentration of hydrogen in outer layer increases, carbon monoxide produced by reaction between iron oxide and carbon is insoluble in molten steel, and it becomes the core of bubbles on solidified metal and inclusions. Hydrogen in the steel continuously precipitates and enters the carbon monoxide gas core, causing the bubble to grow along the crystal direction, forming the lower pores.

The way to prevent Blowholes in carbon steel castings:

1. Deoxidize and deactivate the molten steel.

2. When aluminum is deoxidized, the amount of aluminum must be properly charged.

3. Strictly control the moisture content of molding sand. Dry or dry type should be used when necessary.

( 2 )Sand

According to the different forming process of sand on the surface of castings, it can be divided into mechanical sand adhering, chemical bonded sand and hot bonded sand. In fact, there are three types of sand sticking characteristics on the surface of casting.

High pouring temperature, high wettability of molten metal to the mold, high static pressure of molten metal and large porosity on the surface of sand mold are the important reasons for sand sticking. The sand sticking can be prevented by coating the surface of the mold, increasing the compactness of the mold and lowering the pouring temperature as much as possible.

The molten ferric silicate with low melting point is formed by the reaction of ferrous oxide (FeO) formed by liquid metal during pouring with the casting material. The molten ferric silicate wets the silica sand grains and penetrates into the surface voids of the sand mold (core), resulting in chemical bonding of sand. Additions such as coal powder, residue oil and organic binder can be added into molding sand to form reductive atmosphere so as to reduce metal oxidation and reduce sand adhesion.

When the pouring temperature is too high and the casting thickness is too large, the cavity surface is easy to be sintered at high temperature and the hot sand layer is formed. The main measures to prevent hot bonding are to use zircon sand, magnesia and chromite sand with high refractory degree or to coat the cavity surface with paint.

 l Mechanical Action

Sand particles or coatings on the surface of the cavity fall off under the action of flowing liquid metal friction or dynamic pressure. If such scattered objects remain in the casting, they will cause defects such as sand holes, slag pores, flesh and so on. In order to prevent sand washing, in addition to improving the surface strength of sand mold, the gating system should be designed rationally.