Define Chill in Casting

Chill in Casting

Chill may be defined as a metallic object inserted in mould in order to speed up the solidification of a particular portion of the casting.

A chill can be considered as a large heat sink.

A chill

– promotes solidification (progressive and directional)

– avoids shrinkage cavities

It is used where it is impossible to provide riser for a part which is heavy. This may be due to

– complex shape of casting

– unaccessible location of heavy sections of castings.

Chills can equalize/increase the gap between the coding rate of thin sections as compared to thick sections.

Chills promote solidification/increase the rate of cooling. This helps heavy sections to draw molten metal through the thin sections while they are still liquid.

It also helps a section which is far from riser to solidify first to produce sound casting.

Thus it promotes directional solidification.

Metallic chills have the capacity of increasing the solidification rate as high as 14 times than sand moulds of same condition.

For this reason, chills are used to obstruct the formation of graphite i.e., carbon remains in combined form and cast the iron as white (C.I.) and hard.

Disadvantages of Chill

Due to high rate of cooling (when chill provided) may produce hard spot at the contact area

which may require machining to bring it to shape.

Classification of Chill

Classification of Chill

The size, shape and position of a chill must be considered carefully. Because

– too rapid cooling of casting may produce cracks in it.

– too slow cooling of casting may not produce required chilling effect.

External Chills

It is rammed in mould sand walls.

It is a good way of controlling cooling rates in critical regions of casting.

Direct external chill is in level with mould cavity wall and thus comes in direct contact with the liquid metal.

Indirect external chill is placed behind the mould cavity wall and is fully buried in sand.

It does not come in direct contact with the liquid metal.

Sometimes indirect chill is placed at an angle to casting surface. So its chilling effect decreases as the chill tapers away from casting. This effect can be used to promote directional solidification in a casting (member) of uniform thickness.

Direct chills are more effective than indirect chills.

Normally made up of steel, cast iron and copper. Copper imparts highest effect of chilling because it is having highest conductivity.

For ferrous casting chills are made out of iron and steel. For non-ferrous castings iron, steel, copper and block graphite are used.

They can be reused.

It should make sufficient contact (direct/indirect) but should be large enough so that it should not fuse with casting.

Helps to eliminate porosity (micro and micro) at the portions of casting which is difficult to fed by riser.

Direct chills should be clean, dry and free from rust. Sand blasting may be done.

Direct chills should be preheated and dried before pouring molten metal in mould to avoid blow holes due to moisture present on chill.

Internal Chill

Internal chills fuse into and become part of the casting. So these are made of the same metal which is to be cast.

It is rarely used because structural homogeneity is less due to improper fusion which results in poor casting quality.

Normally used in the area where it is used is to be removed after solidification. For example, bosses to be drilled/bored.

These penetrate upto the position in mould cavity where external chills can not reach.

One end of these remain hanging in mould cavity and the other end is supported in sand.

Normally not used in nonferrous because it is difficult to fuse properly.

It should be

– clean (free from rust scale, grease etc.

– fuse properly and thoroughly with casting.

Chemical composition compatible with metals to be cast.

Must not change mechanical or metallurgical properties of casting.

Should not remain more than one hour in green sand mould before-pouring otherwise it may collect moisture.

Placing of external and internal chills.
Placing of external and internal chills.

Use of chills to prevent formation of porosity at casting junctions
Use of chills to prevent formation of porosity at casting junctions

External Chills
External Chills

Internal Chills
Internal Chills

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