Valve Magazine - Winter 2009

THE CASTING QUALITY QUESTION

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Figure 11

Porosity and shrink

Porosity and shrink are both voids in the casting caused by inappropriate rigging or sloppy pouring of molten metal into the mold. When discovered, they can usually be eliminated by improving the process. The problem is: how do you discover them? Radiography can determine the existence of such defects but it has its limitations and is too costly to invoke for every casting. Very small porosity may not be visible in a radiograph and may not leak through the wall during a short duration shell test with water. However, it will leak through the wall when tested with gas. What steps do you take to limit your risk of porosity? See Figures 7 and 8.

top of this defect. This illustration further stresses the points on weld repair.

Heat treatment furnace loading Here are two easy steps for checking heat treatment if you periodically visit the foundry: Before castings are placed in the oven, are they neatly stacked so they receive uniform heating throughout the lot? Are test coupons placed so they represent all of what is heat treated or are they at the edge of the load, far away from the castings? See Figures 10 and 11.

WORDS OF CAUTION I have a large collection of pictures depicting casting defects and failures accumulated over many years. The images shown here are only a few of the common problems that can occur—there are many more potential problems with equally damaging results. Pictures used in this article

represent products from several companies and were selected to avoid identification of any particular manufacturer or foundry. However, a few casting failures this year in Europe were openly broadcast through a notified body in the European Union. The claim was that the castings violated the Pressure Equipment Directive, and the specific valve manufacturers, not the foundries, were listed in the notice. This notice is openly available on the Internet and more than 900 valves had to be replaced.

My last two questions to you are these: Have you taken sufficient steps to ensure this cannot happen to your company? Are your castings of the highest quality? VM

Pattern cleanliness

Any debris within the ladle, mold or shell can become entrained within the molten metal as it fills the cavity. When visiting a foundry, watch to see how personnel clean the molds and equipment handling of the molten metal. But even if an effective process is in place, will the mold remain clean until it is used? Figure 9 shows an investment casting where a piece of the shell nearly projects through the wall of the casting. While difficult to discern from the picture, a foundry weld repair was done over the

LES PELKEY is a materials engineering specialist for Metso Automation USA ( www.metso.com). Reach the author at les.pelkey@jamesbury.com.

LEARN MORE ABOUT THIS SUBJECT

The Valve Manufacturers Association is hosting its annual Technical Seminar, March 5-6 in Charleston, SC, with a 1-1/2 day program that will cover “Valve Design and Material Considerations for Product Globalization.” If you’re interested in the subject of castings and other product quality considerations, plan to attend. To learn more or to register, go to www.vma.org, then click on Events > Technical Seminar.