Shrinkage is a type of casting defect resulting due to formation of shrinkage cavity as such due to lack of design and insufficient feed metal.
Shrinkage results due to formation of a closed loop at a specific point during solidification which creates a cavity because of absence of feed of molten metal.
Shrinkage can be classified into three types as open (macro porosity), closed (internal/blind shrinkage) and axial shrinkage.
Open shrinkage appear on the exterior of casting surface resulting due to volume contraction from liquid to solid state. These are visualized as shallow cavities on casting surface.
Closed Shrinkage results due to dense network of dendritic crystals which restricts void feeding of thick sections.
Axial shrinkage is a result of long freezing time at the centerline of casting due to high pouring temperature.
Causes
- Incorrect Gating & Feeding System
- Low Carbon Equivalent
- Lack of Mold Rigidity
- High Pouring Temperature
- Excessive inoculation
- High Phosphorus content in the liquid metal
- Cope mold lifting
- Insufficient super heating & holding
- High steel scrap in the charge
- Poor casting design
- Maximum hot spots
- Lack of chills& placement
- Riser not properly located.
Remedies
- Improvement of gating & feeding by simulation
- Improvement in carbon equivalent
- Increase mold Rigidity by proper ramming
- Avoid excessively high pouring temperature
- Restrict the amount of inoculants to the minimum level
- Control of phosphorus below 0.15%.
- Clamp the mold adequately
- Superheat the metal in the range of 1480-1500 degree & holds for sufficient time after inoculation.
- Cut down the percentage of Steel scrap
- Improve casting design
- Minimize hot spots
- Chills are placed adequately
- Riser are placed properly
