. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CASTING Q&A
Confi rming Your Ferrous Specifi cations American Foundry Society Technical Department, Schaumburg, Illinois
Editor’s Note: This column is the fi rst in a regular series of articles that will explore the basics of metalcasting in a question and answer format. If you have questions about the casting process or the way your casting supplier operates, email them to
sgibbs@afsinc.org.
Q A
How can I be sure my metalcaster has provided a ferrous casting that meets my specifi cations?
To provide certifi ca-
tion that the cast iron metal and pouring pro- cess have produced castings that meet your
requirements, it is common for your metalcaster to pour keel block (or “Y” block) molds at prescribed intervals of the casting production cycle from which test bars can be taken. This separately cast test bar represents the mechanical properties that can be expected from the casting poured from the same metal. During initial heats, when the
casting process practices are being established, the metalcaster should make tensile and yield strength tests frequently to establish standard prac- tices and correlate the mechanical properties and microstructure of the cast component to pouring practice. Tensile bars, or “coupons,” are typi- cally machined from Y or keel block molds made to specifi c dimensions. These test bars should accompany the castings through any subsequent heat treating process. All test bar molds and their respective cast bars are made to or referenced to an ASTM specifi ca- tion (A48, A536, etc). The cast bars are sized to a range of nominal thicknesses and/or cooling rates equivalent to the nominal section thickness and cool- ing rate of the casting they represent. Any exceptions by the metalcaster on bar size and/or grade of iron must be conveyed to the purchaser.
Metalcaster Best Practices Metalcasters should make test bar
MAY/JUNE 2010
If your metalcaster produces your casting using a fi lter for improved metal fl ow and reduced non-metallic inclusions, the supplier should also produce the test bars with a fi lter.
METAL CASTING DESIGN & PURCHASING 55
molds from the same material as the casting mold. Using a different mold media can change the cooling rate of the test bar, which can affect its mechanical properties. All pouring conditions should match that of the casting pour (e.g. melt temperature, metal treatment and chemistry). If you are sourcing a ductile iron
casting, remember that the produc- tion of the material is a time-sensitive process. As time passes, the effects of the magnesium used to inoculate the iron can fade. All cast bars should be sampled at the end of the pour to en- sure that the end metal (the most faded treated iron) has the same integrity and treatment throughout the pour. When late inoculation is used (after the metal
has been poured), the metal used for the test bar mold must undergo the same treatment. If the casting mold has fi lters, fi lters must be incorporated in the test bar mold, as well. The poured test bars should be cooled until black in color (approximately 900F [480C]) before shakeout. Unlike ductile iron, gray iron bars
can be produced at the beginning of the pour. If inoculation fade is a con- cern, the more potent inoculants are used either at the tap or at the mold. The test bars, which represent the cast- ings, must be cooled to a black color (around 900F [480C]) before shakeout. If bars are shaken out hotter, it must be agreed upon by the metalcaster and casting purchaser.
METAL
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