Hard Spots in Investment Castings

Every so often I get a request for help regarding ‘hard spots’ in investment castings. Typically this defect can usually be attributed to one of three possible causes.

The first and most frequent cause is a ceramic particle; by this I mean investment powder, which has contaminated the casting. This is usually as a consequence of re-cycling scrap material. To prevent this it is very important that any scrap material is completely clean before it is re-melted. This cleaning process should involve acid dipping and also the use of magnetic pins. It is also possible to get investment powder contamination in a casting if there has been a problem with the flask burn-out and the investment has not reached its optimum strength or if there are any sharp bends or corners in the spruing of the pieces, this can lead to small pieces of investment being washed into the casting as a consequence of turbulent metal flow on casting.

The second possibility is iron contamination, which can be something as simple as a piece of stainless steel in the form of a magnetic pin or piece of burnishing shot getting mixed in with a melt, rather than issues of dust or rusty components which are usually associated with this sort of problem. The iron typically combines with the copper present in the casting to give a hard inter-metallic particle which then acts as the source of the ‘drag marks’ on polishing typical of hard spots.

The final possibility is the presence of a grain refiner in the alloy being cast which has agglomerated at the surface; rather than been distributed throughout the body of the cast piece. Typically this is associated with alloys which contain iridium as a grain refiner. The Argentium family of alloys also contain a grain refiner, boron, at a very low level of parts per million. If there are a large number of hard spots then the small amount of the grain refiner is unlikely to be the primary cause of the hard spots. To prevent this being a concern it is important that any silver casting alloy which contains a grain refiner is well stirred once it reaches the casting temperature. The induction pulse settings on the modern induction casters are very useful in ensuring that the molten metal is a homogeneous mix prior to casting.

When I receive samples of this type of complaint and examine them using a scanning electron microscope then 7 times out of 10 it is a ceramic particle I find (i.e. investment powder), 2 out of 10 it is a piece of iron or steel and the rest of the time it is an issue with grain contamination or poor mixing on melting.