When you are selecting a gear drive housing, you could the materials by fabricated steel, ductile iron and grey cast iron.
1) Fabricated Steel
The pros of fabricated steel housing are that it is very strong and that it can be repaired and welded on. Unfortunately, the material and process makes it pricier than cast options. Fabricated steel housings are used on modified or customized gear drives.
2) Ductile Iron
Ductile iron is less expensive than its steel counterpart, but cannot be repair welded and it is not as strong as steel. It is supplied as a casting to the gear drive manufacturer and is ideal for high volume products such as gearmotors.
3) Grey Cast Iron
Grey cast iron is the least expensive option of the three. Much like ductile iron, it will be supplied as a casting and cannot be repair welded. Unlike the other two materials, grey cast iron is brittle, will not give warning before failure, and can be sensitive to extreme cold.
In all cases, there is a chance that a poorly made housing will leak oil through the welds or pores in the housing. Quality manufacturers will pressure test their housings to ensure that leak paths do not exist.