In my last column, I covered shrinkage related defects, their causes, and potential cures. Several additional, very common defects fall under the generic term of porosity, and these are gas defects and inclusions.

Let’s explore the causes and cures for gas defects first. Gas defects are often further classified by the specific element that causes them, either hydrogen or nitrogen. For this discussion, we will limit ourselves to hydrogen-related causes.  

The most common source of hydrogen gas defects comes from water, which on contact with molten metal vaporizes and then further breaks down to form hydrogen. Where does the water come from? You’ll remember that in green sand molding systems, water is used to activate the bentonite (clay) added to the sand to help bind the sand grains together. The foundry must constantly monitor the moisture level in the molding sand to make sure it is within the specified range. Additional sources of moisture can come from core and mold washes, as well as the lining materials used in furnaces and ladles. 

Defects caused by hydrogen are often called pinholes, and they are most commonly found on finish- machined surfaces. Under magnification, the pinholes appear as rounded pockets with smooth and shiny internal surfaces. In my experience, I’ve noticed that pinholes are much more common in thinner metal sections. The reason for this phenomenon is that gases created during pouring can escape the mold cavity through vents that are formed in the mold or through the permeability of the molding sand.

Thinner metal sections solidify quickly, providing the hydrogen less time to escape the mold cavity and thus trapping it in the casting.

Remedies the foundry can apply to reduce pinhole formation is to monitor and reduce the moisture content in molding sand, ensure that washed cores and molds are given sufficient time to dry before being assembled and poured, and increasing the permeability of both cores and molds. Venting molds and cores where possible always helps, and as an old foundry expert once told me, “vent until you think you have enough and then vent some more.”  

Inclusions are materials that are trapped inside the casting, and the most common materials are sand and slag. The source for sand inclusions is obvious, as the mold and cores are made from sand. However, the actual cause of loose sand ending up in the mold cavity needs to be discovered. Very often, setting cores in the mold causes a small amount of sand to become dislodged, which is then blown out of the mold cavity before the mold is closed. Missing the step, or any rough handling of the mold during the process can cause loose sand in the mold. Foundries must also look for areas where the metal flowing in the mold can cause the sand to erode and become trapped in the casting.

Solutions for sand inclusions include making sure the proper amount of draft is provided for all pattern elements, avoiding careless handling of molds, and making sure the metal flows at the proper velocity in the mold through the use of flow modeling in the design of the gating system.

Slag inclusions happen when slag (formed by impurities during the melting process) mixes with the molten metal and flows into the mold cavity. Slag appears as rough open pockets of porosity on cope surfaces of the mold and on the bottom surfaces of cores since it tends to float. Gating systems normally have features such as chokes, slag traps, and filters to reduce the tendency of slag defects. Turbulence in the metal flow inside the mold can cause the formation of slag, so any metal that drops or jets into a mold cavity will be problematic; proper flow modeling of the gating system will help eliminate this.

Ductile iron can have the tendency to form dross, which is a specific type of slag related to the magnesium treatment process used in the production of ductile iron. Dross forms due to turbulence inside the mold, causing magnesium in the molten metal to mix with air. Causes can be magnesium levels above the proper specification range, as well as treating and pouring the metal at the low end of the process temperature range.

Defects in castings are common, but they can be reduced or eliminated through proper modeling during the pattern design stage, careful handling of molds and cores, and process adherence and control.  CS