Atlas of Casting Defects
Negative with the possibility of positive finning
Shallow rounded threadlike fissures typically in a radial pattern
N.A
N/A
N/A
Mud cracks, drying cracks, oxidation crazing, rivering
Thin, negative defect typically found in high chrome alloys
This defect is a marriage of pitting and finning defects. Rat-tailing is the selective oxidation of the metal surface through cracks or micro-cracks in the shell. Most of the cracks are large enough to be filled with metal during pouring and will produce positive metal fins. Very fine micro-cracks are too small to allow metal to enter, but will allow air (oxygen) to reach the hot casting surface.
| Area | Possible Cause | Potential Correction |
|---|---|---|
| Other (Set-up ) | Lack of stress raisers on the cast surfaces or in the primary coat refractories |
Break-up large flat surfaces on the casting with “hatching” or small ribs which can subsequently be ground off |
| Shell | Large temperature variation during prime coat drying |
Maintain +/- 3F in the drying area |
| Shell | Drying too fast (large pattern shrinkage) |
Casting cooling rate too slow. Increase dipping and drying room humidity, reduce air flow |
| Shell | Mismatch in expansion coefficient between prime and backup coats |
Use slurry and stucco refractories with similar thermal expansion rates |
| Shell | Drying too long |
Reduce dry time. cooling rate too slow |
| Shell | Low prime coat strength |
Check binder healthy/stability through pH, conductivity and/ or gel test |
| Shell | Low prime coat strength |
Increase refractory solids |
| Shell | Poor adhesion and elasticity properties of the primary slurry |
Green strength additives in the primary slurry becoming unstable or ineffective |
| Shell | Uneven shell build on prime coat(s) |
Evaluate dipping/draining sequence to ensure uniform draining and/or shell build |
| Shell | Permeability of shell is too high |
Compare permeability of shell against previous data. If changed, evaluate potential causes for increase |
| Foundry | Oxygen level too high during casting cooling |
Ensure reducing or inert conditions immediately after casting. Cover the molds after casting. Use carbonaceous materials in or around the mold. Cool in vacuum or protective atmosphere |
| Foundry | Oxygen level too high during casting. Casting cooling rate too slow |
Casting speed cooling rate too slow |
| Foundry | High shell temperature and excessive time in oven |
Explore potential ways to reduce shell pre-heat temperature or time |