Centrifugal Casting

What is Centrifugal Casting?

Centrifugal Casting is a metal casting process that produces hollow bar or narrow walled piping, by pouring molten metal into a rapidly spinning cylindrical mold. The high rotational speed generate significant centrifugal forces which are imposed upon the molten metal. Exceptional quality of the casting is attainable by understanding the metal's metallurgy and solidification process and controlling the cooling rate. Centrifugal casting results in cleaner, and significantly reduced porosity and inclusions with superior mechanical properties.

The correct cooling rate is achieved through consideration and strict control of the following operational parameters; the pouring rate, the pouring temperture, the mold temperature, the mold composition, and the centrifugal forces generated through mold RPM (revolutions per minute).

Process Description

The molten alloy is poured at a predetrmined rate and temperature into a permanent mold which is driven at high RPM speeds creating centifugal forces (60g to 80g). The molten metal spreads along the inside mold wall evenly, where it solidifies as it cools. Castings solidify from the outside in. This directional solidification improves some metallurgical properties, for example the tensile and yield strengths of the alloy are greater than that of the same alloy cast in sand or by continuous casting. Moreover, higher quality is expected because centrifugal castings are relatively free from gas and shrinkage porosity (although porosity can never ever be completely eliminated!). After casting and extraction, the inner and outermost layers are removed and only the intermediary columnar zone is used. The casting is fine-grained with an especially fine-grained outer diameter, due to the rapid cooling at the surface of the mold. Lighter impurities and inclusions move towards the inside diameter and are machined away later.

Operational Parameters

The key goal when casting using the centrifugal approach is to maintain tight control of the key operational parameters; The speed of the rotation of the mold (to attain the correct forces to ensure intended gain structures), pouring rate and pouring temperature, both of which vary with the alloy and size (OD and ID) and wall thickness being cast. The following structure or zones typically occur, Chill Zone – This layer is of fine equiaxed structure which forms almost instantaneously at the mould wall, Columnar Zone – This is next to chill zone. It consists of directionally oriented crystals approx. perpendicular to the mould surface, Equiaxed zone – this region may occur next to columnar zone characterized by large number of uniformly grown crystals.

Non conformities

As mentioned conventional static casting defects such as internal shrinkage, gas porosity and nonmetallic inclusions are less likely to occur in centrifugal casting, however non-conformities (defects) can arise.

• Hot Tears – Hot tears will develop in centrifugal castings where the highest forces are applied (ie. the outermost section when rotation speeds are very high). Longitudinal tears occur when contraction of casting combined with the expansion of the mould, generates hoop stresses exceeding the cohesive strength of the metal at temperatures in the solidus region.
• Segregation – Centrifugal castings of certain alloys, with significant differences in their physical properties (Pb - Lead and Si - Silicon) can form laminations where less dense constituents are pushed inwards. This is identical to the process used to seperate the components of blood in a laboratory.
• Banding – Sometimes castings produce zones of segregated low melting point constituents such as eutectic phases and sulphide and oxide inclusions. Various theories explain this, one states vibration is the main cause of banding.

These defects are usually visible to the well trained eye, but if there is any doubt, non-destructive ultrasoud testing will quickly confirm or dispel doubts.

Available Alloys in Centrifugal Castings

The listing below is provided in functional groupings.

• Bearing Bronzes – .
• Gear Bronzes – As per ASTM B427-21 Standard Specification for Gear Bronze Alloy - C90800, C91700, C90700, C91600, and C92900. These castings may be furnished as one of three types: static chill, centrifugal chill, or sand cast. Subject to tensile strength, yield strength, elongation, and Brinell hardness tests.
• Aluminium Bronzes – .
• High Conductive Bronzes -
• Manganese Bronzes

These defects are usually visible to the well trained eye, but if there is any doubt, non-destructive ultrasoud testing will quickly confirm or dispel doubts.

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