EDITOR’S NOTE: Materials used in the manufacture of valves and how they perform in different applications is a topic of huge interest to everyone who works with valves. Bray recently held a seminar on Valve Materials. Below are some of the questions asked by the webinar viewers with answers provided by Stan Allen, global director of valve application engineering, Bray International, Inc., who was the featured speaker.
Q: Duplex and super duplex stainless steels have been used extensively for valve components since the 1980s. Why are these alloys so widely used and how have they improved? Are they covered in various piping codes?
A: Because of their unique combination of corrosion resistance and strength, duplex stainless steels continue to be widely used for valves in both pressure-containing and trim components. The oil and gas industry embraced duplex stainless steels during the 1980s, largely because of improved resistance to chloride stress cracking in temperatures above 140°F (60°C—where austenitic stainless steels are susceptible). Duplex stainless steels are widely used for ball, butterfly, check, gate and globe valves in a variety of applications and industries such as desalination, chemical processing, and pulp and paper. The initial grade, whether for bar, forging or casting, was UNS S31803, which has a nominal chemistry of 22% chromium (Cr), 3% molybdenum (Mo), and 5% nickel (Ni). ASTM International (ASTM) A182 Grade F51 forgings and ASTM A995 Grade 4A castings have been frequently specified and used for valve body and bonnet materials.
An improved duplex stainless steel often used in petrochemical and seawater services is alloy 2507 or UNS S32750 (cast ASTM A995 Grade CE3MN or 5A), which is often referred to as a super duplex. It has a nominal chemistry of 25% Cr, 4% Mo and 7% Ni. In addition to higher strength, which makes this material an excellent choice for stems, it has very high resistance to pitting and crevice corrosion. A similar version for castings is UNS S32760, ASTM A995 Grade CD3MWCuN, which is used for pressure-containing components such as valve bodies and bonnets and is included in the American Society of Mechanical Engineers (ASME) B16.34. The selection of the appropriate duplex or superduplex stainless steel requires evaluation of a variety of factors including the end user’s needs for corrosion resistance, the valve manufacturer’s required mechanical properties, and ASME standard and code compliance requirements.
Pitting and crevice corrosion resistance of super duplex stainless steels are almost as good as Hastelloy C276 and have been used as a lower--cost alternative in some services. One restriction in the use of duplex stainless steels in valves is high temperature—above 572°F (300°C) temperatures have the potential for embrittlement because of the chromium content.
Q: During your webinar, you mentioned new 9% chrome alloys that are being developed for supercritical power plant applications. Can you please elaborate on this?
A: Creep Strength Enhanced Ferritic (CSEF) materials in both supercritical and combined cycle power plants have received much attention over the past few years. The Electric Power Research Institute (EPRI), ASME and ASTM have worked to standardize CSEF steels that meet both creep resistance and toughness for boiler superheaters and other applications that use high-temperature steam valves. New alloys are now listed within ASTM A182 for forgings. These include gate, globe and severe-service ball valves. ASTM 182 Grade F91, a 9Cr-1Mo-Vanadium (V) alloy, is now listed in two types: Type 1 and Type 2. Type 2 includes the control of Mo, V, niobium, tungsten, cobalt, boron, nitrogen and Ni to improve both creep resistance and toughness, and maintain high allowable stresses. The ASME B31 Committee issued a code case in May 2020 that specifically permits forgings of either Type 1 or Type 2 chemical compositions of 9Cr-1Mo-V for use in B31.1 construction. The material is limited to 1,200°F (649°C) and has a few other restrictions in processing and application.
Two other CSEF materials have been developed—ASTM A182 Grade F92 and Grade F93. Grade F92 is similar to Grade F91, but with higher Mo levels, which provides more strength, resistance to erosion and a slightly higher temperature rating. Grade P93 is the next-generation creep-resistant steel with substantially higher strength up to temperatures of 1200°F (649°C). A significant step in acceptance of these alloys is their inclusion within ASME B16.34, Valves–Flanged, Threaded, and Welded End. Grade F91 is included in Material Group No. 1.15; Grade F92 is included in Material Group 1.18; and Grade F93 is not yet included in the latest edition: ASME B16.34-2017.
Q: What is superaustenitic stainless steel, and where and how is it being used in valves?
A: Common superaustenitic stainless steels used in the valve industry are UNS S31254 (254SMO and AL6XN) and UNS S20910/S21800 (Nitronic 50/60). They are predominantly used to obtain both higher strength and improved resistance to crevice corrosion chloride pitting compared to 316 SS. This improved corrosion resistance comes from the higher Ni and Mo content, and nitrogen strengthens the material. It is an excellent choice for stem material in 316 SS valves because of its higher strength. Desalination, food processing, chemical processing and even seawater services use superaustenitic stainless steels as an economical selection in cases where high strength and good corrosion resistance are needed.
Another popular superaustenitic stainless steel is UNS N08904 (904L), which has a 5% Mo content. It is used for a variety of valve components for inorganic acid environments. It is also found in the pulp and paper manufacturing, pharmaceutical and power industries. The material is used mainly for trim components, but new specialized grades with higher strength are also being used for valve pressure--containing components.
Q: When choosing materials of construction of valves, end users and engineering companies focus on materials resistant to corrosion or erosion, or both. From a valve manufacturer’s perspective, what are the other factors that must be considered?
A: In applications using alloys, strength, toughness, resistance to galling and coefficient of thermal expansion are all factors a valve manufacturer must consider. End users may know what alloys work in their service conditions, but they must depend on valve manufacturers to evaluate these factors in the ultimate selection of the alloys for a valve. A common example is the use of 316 SS, which is compatible with a multitude of corrosive fluids, but may not have adequate strength for a stem, gate or disc material, may not serve as a good bearing material, or may result in lockup (because of thermal expansion rates) as a ball in a metal-seated ball valve applied in a temperature cyclic installation. End users often are concerned about proposed deviations to specifications for alloys in valves, and they should be. However, sometimes alternate materials must be proposed to ensure the valve functions as designed and is still compatible with the service fluid from a corrosion standpoint.
Q: What materials are being used in the additive manufacturing of valves, and for what components are they used?
A: The most common alloys used in additive manufacturing (AM) have been tool steels, aluminum and 316L stainless steel—and this statement applies to valves as well. Up until recently, AM in the valve industry has primarily focused on prototypes as part of research and development initiatives or specialty valves within the aerospace industry, serving those purposes well. Other alloys that are good candidates for use in valves are titanium (specifically Ti6Al4V), cobalt chromium Iconel 625 and various Ni alloys. The immediate future of AM in the valve industry involves replacement parts for urgent use (printing on demand with AM machines near remote operations) and reverse engineering of legacy parts where no drawings or models are available. There also are opportunities in the quick repair of valve parts using Directed Energy Deposition-based machines. Important opportunities to use AM for valves include machining of challenging and complex geometries, optimizing flow paths and reducing weight. The metal powders, manufacturing equipment and technology are all available, but economics is the primary reason AM has not been applied to standard production of parts. Bar stock 316L is eight to ten times less expensive than 316L metal powder developed for AM. As AM technology continues to develop and more suppliers come to market, its application in valve production will increase. Initial opportunities for application of 316L, titanium, and Ni alloys are improved design of contoured balls for ball control valves, block manifold valves and fully customized valves. Another barrier to widespread acceptance of metal additive manufacturing in the valve industry is the lack of quality standards surrounding the technology. ASTM committees are leading the way with new material standards, including ASTM F3122 (Guide for Evaluating Mechanical Properties of Metal Materials Made via AM Processes), F3049 (Guide for Characterizing Properties of Metal Powders Used for AM Processes) and F2924 (Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion). ASTM is also developing additional standards. ASME has been active over the past decade in the development of mechanical design--related standards, and its BPTCS/BNCS Special Committee on the use of Additive Manufacturing for Pressure Retaining Equipment has initiated work in this area. American Petroleum Institute committees also are beginning work on standards. All of this will take time, but the groundwork has been laid for a variety of alloys to be applied to this new manufacturing technology for the valve industry.