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Food and Beverage Processing

Food and Beverage Processing

When wandering the grocery store aisles,...

Variable Frequency Drives in Electric Actuators

Variable Frequency Drives in Electric Actuators

Electric actuators are vital for operati...

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Industry Headlines

Spirax Sarco Supporting the Enbridge Ride to Conquer Cancer

2 HOURS AGO

Spirax Sarco USA is teaming up with Spirax Sarco Canada for the Enbridge Ride to Conquer Cancer. From June 11-12, 2016, participants will cycle 200 km with thousands of other men and women throughout the countryside of Ontario, Canada. All net proceeds will benefit Princess Margaret Cancer Centre.

Th...

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Garlock Acquires Rubber Fab Technologies

21 HOURS AGO

Garlock has acquired the business and assets of Rubber Fab Gasket & Molding Inc. through its parent company EnPro Industries.

Rubber Fab, headquartered in Sparta, NJ, is a recognized supplier of high performance sanitary gaskets, hoses and fittings for hygienic process industries such as pharmace...

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G7 Countries Vow to Support Energy Investments

-1 DAYS AGO

Leaders from Canada, France, Germany, Italy, Japan the UK and the U.S. have pledged to “promote investing in energy projects through the oil price crash to ensure a steady stream of supply” and “encourage financial institutions to invest in energy projects and infrastructure,&rdquo...

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New Study Shows Demand for Natural Gas Will Grow

1 HOUR AGO

The National Association of Manufacturers (NAM) Center for Manufacturing Research and IHS Economics released a new comprehensive study that reveals how natural gas has strengthened manufacturing and encouraged U.S. manufacturing growth and employment and highlights the positive impact to communities a...

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U.S. Economy Grew 0.5% in First Quarter

1 DAY AGO

U.S. GDP increased at an annual rate of 0.5% in the first quarter of 2016, according to the advance estimate released by the Bureau of Economic Analysis. In the fourth quarter, real GDP increased 1.4%.

The increase in real GDP in the first quarter reflected positive contributions from personal consumpt...

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ISM: Manufacturing Activity Increased in April

1 DAY AGO

Manufacturing expanded in April as the PMI registered 50.8%, a decrease of 1% from the March reading of 51.8%, indicating growth in manufacturing for the second consecutive month, following five consecutive months of contraction in manufacturing.

The Institute of Supply Management (ISM) reports that 15...

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Cast or Forged?

materials_q_and_a_graphicQ: Should I be concerned whether my valve is cast or forged?

 

A: Just as in politics, everyone has an opinion on this issue.

 

The good news is that both types of valves should be able to provide you with acceptable performance, although a perception exists that forged valves are superior to cast valves. However, if cast valves are made properly, they can and have worked equally well in a variety of services and usually at a much lower cost than forged valves. Also, the belief that forged components are infallible is not true. Let’s look at an example of a 4-inch diameter wrought valve stem in N07718 (Figure 1). A crack-like defect was seen on the end of the stem, so the part was cut in half, and the large shrink cavity you see here was discovered. This shows that forged material is not without its own problems. But let’s examine how cast and forged valves are made and how we can assure we get a good valve.

What most people don’t realize is that cast and forged valves start out the same way—molten metal is poured into a mold or ingot. As a result, both types can have defects such as the shrinkage in the above mentioned N07718 bar. Other defects associated with forgings are inclusions, laps, seams, cold shuts and cracks. Defects with common castings are inclusions, porosity, misrun and hot tears. As you can see, both have their potential issues.

An issue with forgings often overlooked is that forgings and wrought products will have non-uniform mechanical properties. This is because they are worked or formed more in one direction than in another. Therefore, the grains will be elongated more in one direction than in the other, which has a direct affect on mechanical properties, particularly impact strength. As a result, the design of forgings needs to take into account these anisotropic properties whereas castings have uniform properties no matter what the orientation of the test coupons.

Another advantage of cast valves is that they can be produced in more complex designs than forged valves. Certain valve designs such as a globe valve are simply difficult or impossible to produce as forgings. This flexibility of design in cast valves allows them to be more efficient in controlling flow than a similarly forged valve.

Something else to consider with forged valves is that they usually are made in halves, particularly the larger sizes. This means there is either an additional flanged connection that can be a potential leak path or the halves are welded together. Welding, however, is another process for cast metal that can have its own set of problems.

The questionable reputation that castings have is from two sources. First, most of the ASTM cast specifications are lenient in requirements for composition, heat treatment and inspection. Second, some foundries either use this latitude to their advantage or simply do not know enough to implement tighter controls when needed on chemistry or heat treatment. This concern about castings has resulted in equipment produced to ASME Section VIII having a quality factor on castings of 80% of the allowable stress values for a wrought component. However, this quality factor can be increased to 100% if sufficient NDE (non-destructive evaluation) per Appendix 7 is performed.

Casting purchasers need to understand that in most ASTM specifications these additional NDE requirements are not mandatory. They are simply listed as supplementary requirements at the end of the product specifications and are only invoked if included in the purchase order. Specifying additional NDE-like radiography or dye penetrant inspection is one way of helping ensure the quality of valves being purchased. However, a more cost-effective way is to deal with valve suppliers who already control the quality of the products they produce and have a long and successful track record. In either case, the decision to go with cast or forged valves depends on several factors, and cost is usually the determining one.


THOMAS SPENCE is director of materials engineering for Flowserve Corp. (www.flowserve.com), Dayton, OH. Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it..

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