Shrinkage and Gas. How to Eliminate These Common Casting Defects

When castings have openings, cavities, porous sections, and similar defects on the internal or external surface, it is often called porosity. It is generally due to shrinkage or gas as the casting cools. Both are caused by very different sources. Identifying and eliminating the source can be a challenge.

First, it is important to note that casting shrinkage and gas defects are part of foundry cast materials. A casting is an economic preform, a rough form based on the specifications of the drawing and purchase order. The rough form will always have minor levels of porosity.

Examples of Porosity in Castings

Liquid penetrant testing displaying gas (left), Radiographic testing displaying shrinkage (right)

Stainless Foundry & Engineering (SF&E) evaluates all casting surfaces to the MSS SP-55 or ASTM A802 quality standards, both of which allow for some levels of acceptable shrinkage and gas. Understanding and controlling the process allows SF&E to continue to manufacture high integrity castings with above-average industry lead times.

In the following, SF&E engineers share how they investigate the reasons why shrinkage and gas defects exist, their different types, and how we have worked to eliminate them.

Root Causes of Shrinkage

The different types of shrinkage can be defined as linear/feathery, sponge, and a combination of linear/feathery/sponge shrinkage as defined in various ASTM non-destructive radiographic specifications. The different root causes to consider are:

• Inadequate Initial Foundry Engineering Set-up – Whether the part is produced with the sand or investment process, the casting to be produced must be well-designed and address many key variables. All aspects of good practice should be incorporated.

• Solidification Analysis – Tools for verifying how to reduce or eliminate shrinkage include solidification modeling. At SF&E, we use MAGMASOFT solidification modeling for this task. We have a dedicated engineer for solid modeling and solidification analysis. This is a key element in maintaining control of the solidification process to create high integrity castings.

• Processing Parameters – Items such as a proper melting temperature and the ability to monitor, measure, and log results help define casting integrity.

• Processing Tools and Supplies – Having the proper riser sleeves or the correct type of sand is essential. Different sand results in different cooling rates, so it’s imperative to understand the process. Sand controls and initiates the proper solidification front through the casting. Exothermic introduced on the risers and sprue after pouring hold in the heat to assist in the solidification process. This rate of solidification will affect casting soundness and in turn, shrinkage within the casting.

Examples of Shrinkage

Shrinkage defects internal and on a machined surface

Success Story: Eliminating Shrinkage

SF&E has poured over 50,000 castings of a particular stainless steel (ASTM A351, grade CF8M). Scrap is low and there is very little history of shrinkage overall. We started using a new type of exothermic riser sleeve on other similar jobs and were able to reduce assorted defects. However, this sleeve, when added to our low scrap job, created numerous surface shrinkages around many of the castings features. An additional solidification modeling analysis displayed a few different types of shrinkage, both surface and internally. By tweaking the gating and cope height along with the addition of a new riser sleeve, we reduced the scrap percentage for this job by an additional 1.5%.

Root Causes of Gas Defects

Gas defects are typically present when gas is produced from the process and has not had an opportunity to evacuate the mold. The same can be said of trapped air conditions that show up on the casting surface. The different root causes to consider are:

• A Gassy Heat of Metal – This can come from an alloy’s natural affinity to accumulate oxygen, nitrogen, or hydrogen during the melting process. The nature of the stirring action of the electric induction melting process can yield dissolved gas in the melt that may migrate to the surface of a casting. Dissolved gas in a heat of metal is forced out during solidification. That said, it will typically show up in the last places to solidify – the riser contacts and risers themselves. If evolved early in solidification, it can and will float through liquid metal to collect on the surface. The true indicator of this type of “gas” defect is its presence in specific locations.  When there are gas bubbles present on the surface, they might also be present on the inner surfaces of the casting.

• Improper Venting of Molds – When molds don’t allow for gasses to escape from the mold cavity, gas and trapped air defects may remain. Cores that have not been properly prepared, do not allow areas to release core gas out of the mold and function as planned.

• Molding Material Binders – The molding material binders used may also determine the volume of gas and trapped air generated. If a sand mold does not allow gas to easily pass through it, we would refer to this as poorly permeable. These interstitial spaces between the sand grains allow gas and air out as part of good foundry practice. If it is non-permeable, there will be trapped air or possibly gas.

• Environmental Conditions – Our location in the Midwest affords certain benefits, such as being located by one of the world’s finest molding sand deposits. However this also results in Mother Nature being one of our largest variables. In the summer, humidity brings water vapor and condensation that creates steam when the metal is poured in the mold. The steam creates trapped air and gas that will get entrained inside the casting, generating defective conditions. The same can be said for the winter if exposed to ranging temperatures.

To find the shrinkage or gas defects on external surfaces, visual inspection is commonly acceptable. When a high specification evaluation of the surface is required, magnetic particle and/or liquid penetrant testing can be used. Internal or volumetric defects can be quantified by ultrasonic testing or radiographic testing.

Examples of Gas Defects

Gas defect on a machined surface

Success Story: Eliminating Gas Defects

A carbon steel casting (ASTM A216, grade WCB) with large ANSI flanges has a large single core. A core of this size can produce gas from the organic materials that make the core. When the metal is poured into the mold, gasses are created from the ignition of the core and molding binders, which can become trapped in the casting. After surface blasting, we could see gas indications on the surface of the part. We knew this casting may be salvaged by excavating the gas out of the casting. We inspected the cavity to ensure we had removed the defect. Upon successful removal, we welded the excavation cavity. Post weld heat treatment and another visual inspection pushed the part from potential scrap to salvaged and acceptable to ship.

Complex, high specification, and high volume parts are our specialty. If you are concerned about porosity, or would like to know more about our high integrity castings and lead times, contact us at sales@stainlessfoundry.com OR submit your request directly HERE.