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Think beyond wrought materials

Think beyond wrought materials

Centrifugally cast tubular components: a smart and cost-effective choice for many onshore and offshore applications

By Alan Holtz, Vice President of Sales & Marketing

Designing tubular components for the oil and gas industry involves thinking through all the options. But don’t let industry confusion about castings keep you from creating the best possible product for onshore and offshore applications.

Here’s why not all castings are the same and why centrifugally cast components are often an attractive alternative to those created with wrought materials.

Put misperceptions to rest

Confusion persists about static castings and centrifugal castings because many designers mistakenly think they are one and the same. Many also misunderstand how centrifugal castings compare with wrought materials. There is a general misperception that a centrifugally cast part won’t measure up to a wrought part. This, however, is not true for many applications.

Getting up to speed on centrifugal castings requires a basic understanding of the processes used to manufacture tubular components with metal:

  • Wrought materials: Wrought describes solid metal in the form of a billet that has been heated and then shaped with compressive forces to produce a round shaped bar. The center of the bar is machined away to produce a tubular shape. The process will result in a high quality tube, but it will also generate waste in the form of chips and/or a lower quality bar or core. Depending on the value of the alloy used, the waste may be quite costly.
  • Conventional or static castings: With this process, molten metal is poured into a stationary sand or ceramic mold. As metal cools and solidifies, it shrinks. This occurs both in volume and in soundness. Voids form in the part as solidification takes place. Risers or feed metal are designed to minimize these voids so they are deemed “inconsequential.” Also as metals are melted in air, the molten metal readily oxidizes, creating slag or dross. The gating system is designed to minimize this as well. Because both shrinkage voids and slag are present, a variability of quality certainly exists.
  • Centrifugal castings: Unlike wrought materials and conventional castings, molten metal is poured into a rapidly spinning cylindrical steel die to create the tubular part.

Centrifugal castings are often confused or equated with other casting processes. They offer extremely consistent properties that exceed static castings and rival wrought material in many cases, making them suitable for an array of onshore and offshore applications.

It’s about serious centrifugal force

Knowing more about the method used to produce centrifugally cast parts explains how centrifugally cast parts offer the level of performance required for many demanding oil and gas applications. The key is centrifugal force.

During the casting process, molten metal is poured into a cylindrical steel die that is spinning with centrifugal force that typically exceeds 100Gs. The casting begins to solidify at the die wall interface, and the solidification front moves constantly toward the bore. The high centrifugal force literally squeezes liquid metal into interstitial spaces as they form during solidification, resulting in an extremely dense cast wall without shrinkage voids that are found with in conventional castings.

The centrifugal force also separates the lighter non-metallics and spins them preferentially to the bore. The bore surface is generally machined, leaving a very dense, very clean part with excellent mechanical properties.

The centrifugal cast method also leads to extremely consistent parts because process variables are known and controllable. This includes metallurgical controls, such as chemistry and grain structure. Subsequent heat-treating is tightly controlled for consistency. The end result is the same level of quality in each and every component produced. All the while, non-destructive testing validates consistency from the first part produced to the last one off the production line.

Diverse applications, plus alloy flexibility

Centrifugal castings have been used in many demanding oil and gas applications. They include:

  • Valve and pump bodies
  • Cylinder bodies for riser tensioners
  • Piston-type hydraulic accumulator bodies
  • Centrifuges

Aside from delivering the quality necessary, centrifugally casting offers a level of design flexibility not available with wrought processes. They can be designed using a wide variety of alloys that can be cost-effectively produced in short runs. Spuncast’s alloys encompass:

  • Plain carbon steels
  • High strength/low alloy steels;
  • Austenitic, ferritic or martensitic stainless steels
  • Precipitation hardened stainless steels
  • Duplex, superduplex or superaustenitic stainless steels
  • Nickel base materials, such as Inconel 600, Inconel 625, Hastelloys B, C-4, C-22 and C276.

Alloy flexibility and short runs translate to key advantages of centrifugal castings when compared with wrought components. In many cases, the alloys can also be tailored to meet a specific need or specification.

Additionally, the cost to produce a centrifugally cast part is often less than the cost to produce a forged part since forged parts start with a solid round billet. The center of billet must be trepanned or machined out – an energy- and labor-intensive process that adds cost. Conversely, a centrifugally cast part starts as hollow part, which means the inherent cost savings can be passed onto the end-user. Less time to produce a part also leads to shorter lead times.

No need to “over-design” it

Though centrifugal castings are not suitable for all oil and gas applications, they can free design engineers from “over-designing” expensive wrought components when not needed.

The key is to work with a knowledgeable supplier/metallurgist who can help guide the decision and the considerations involved in choosing centrifugally cast components that are appropriate for the application.

About the author
Alan Holtz has been in charge of Sales and Marketing at Spuncast for over 13 years. He holds a BS degree in Metallurgical Engineering from the University of Missouri – Rolla (now Missouri University of Science & Technology) and a BA degree in History from the University of Pennsylvania. He has spent over 36 years in the metal casting industry.

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