beyond wrought materials - PDF document

Think beyond wrought materials Centrifugally cast tubular components: a smart and cost - effective choice for many onshore and offshore applications By Alan Holtz , Vice Pr esident of Sales and Marketing D esign ing 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 onsh o re 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 cast ings and centrifugal castings because many designers mistakenly think they are one a nd the same. Many also misunderstand how centrifugal castings compare with wrought materials . There is a general misperception that a centrifugally cast part won’t mea sure 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 processe s used to manufacture tubular components with metal :  Wrought mate rials : Wrought describes solid metal in the form of a billet that has been heated and then shaped with compressive forces to pr oduce a round shape d 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 cor e . Depending on the value of the alloy used, the waste may be quite costly .  Conventional or static casting s : 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 sla g or dross. The gating system is desi gned to minimize this as well. Because both shrinkage voids and slag are present, a variability of quality certainly exists.  Centrifugal casting s : Unlike wrought materials and conventional castings, molten metal is pou red into a rapidly spinning cylindrical steel die to create the tubular part . Centrifugal castings are often confused or equated with other casting process es . They offer extremely consistent properties that exceed static castings and rival wrought materi al 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, m olten metal is poured into a cylindrical steel die that is spinning with centrifugal force that typically exceeds 10 0Gs. T he 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, result ing in an extremely dense cast wall w ithout shrinkage voids that are found with in conventional castings. The centrifugal force also separate s the lighter non - metallics and spin s them preferentially to the bore. The bor e surface is generally machined, leaving a very dense, very clean part with e xcellent 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, n on - destructive testing validates consistency from the first part produced to the last one off the production line . Diverse applications, plus a lloy flexib i lity Centrifugal castings have been used in many demanding oil and gas applications. They include: o Valve and pump bodies o Cylinder bodies for riser tensioners o Piston - type hydraul ic accumulator bodies o Centrifuges Aside from delivering the quality necessary, c entrifugally cast ing offer s a level of design flexibility not available with wrought processes. T hey can b e designed using a wide variety of alloys that can be cost - effectively produced in short runs . Spuncast’s alloys encompass: o P lain carbon steels o H igh strength/low alloy steels; o A ustenitic, ferritic or martensitic stainless steels o Precipitation hardened stainless steels o D up lex, superduplex or super austeni tic stainless steels o N ickel base materials, such as Inconel 600, Inconel 625, Hastelloys B, C - 4, C - 22 and C276 . Alloy f lexibility and short runs trans late to key advantages of centrifugal castings w hen 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 part s start with a solid round billet. The center of billet must be trepanned or machined out – a n 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 com ponents 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 component s 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.