Large power-generation components are

Transcription

1 Overcoming Barriers for Using PM/HIP Technology to Manufacture Large Power Generation Components David W. Gandy*, FASM John Shingledecker* John Siefert* Electric Power Research Institute Charlotte, N.C. PM/HIP opens up a new method of manufacturing high pressureretaining components for use in the powergeneration industry. *Member of ASM International Large power-generation components are commonly fabricated using conventional tried-and-true metallurgical processing methods including casting, rolling, drawing, forging, extrusion, welding, and heat treatment. These processes have been used to fabricate components as far back as the early part of the 2th century. Materials-processing practices have improved over the years, resulting in higher quality components, such as super-clean forged rotor and disc steels, directionally solidified and single-crystal blade alloys, controlled residual-element alloys, creep strength-enhanced ferritic piping/headers, and improved surfacing techniques. One area that has seen remarkable improvements in processing technology is powder metallurgy (PM) where component quality, availability, and size have increased dramatically within the past two decades. Powder-production facilities currently exist to manufacture large quantities of gasatomized powders for high quality alloy steels, stainless steels, and nickel-base alloy parts [1]. Hot isostatic pressing (HIP) facilities are also available to manufacture large shapes (currently up to 5 ft, or 1.5 m, in diameter), which have been demonstrated for use in specialized aerospace, oil & gas exploration, tooling, and other niche applications. High-quality components showing good structural uniformity, no segregation, superior mechanical properties, and ease of inspectability have been produced from a number of stainless steels and nickelbase alloys [2-7]. Four technical barriers have prevented use of large PM produced components in the power generation industry to date: Sizes and shapes of near-net shape (NNS) components have only recently reached a point for consideration, and have not been tailored for the compositions/alloys that are of most interest to the industry. High quality gas-atomized powders replaced earlier powder forms (manufactured via milling or water atomization).this development virtually eliminated oxidation and porosity concerns. Materials and processes used to manufacture pressure-retention or high-temperature power plant components are generally subject to (in the U.S.) the ASME Boiler and Pressure Vessel Code, which currently does not allow the use of PM produced components for the desired applications. For iron-base steel alloy systems, the PM/HIP production route is generally more expensive than traditional forging and casting routes. However, for stainless steels and nickel-base alloys, where raw material costs are much higher, PM/HIP appears to be a cost effective solution. Focused research by industry can overcome (a) (c) (b) Fig. 1a, b, and c Three 316L (UNS 3163) stainless steel valve body configurations manufactured to support the development of an ASME B&PVC Code Case. ADVANCED MATERIALS & PROCESSES JANUARY

2 TABLE 1 CHEMICAL COMPOSITION OF 316L (S3163) MANUFACTURED COMPONENTS Heat Nominal composition, wt% Y162B A Size (ASTM A988/A988M) 1892 lb 12 lb 12 lb NS(a) Product form Valve Valve Valve Tee Piece C.3 max Mn 2. max P.45 max S.3 max Si 1. max Ni Cr Mo N.1 max Others NA O -.1 NA NA O -.14 Cu -.2 Co -.4 (a) Not specified many of these barriers. Therefore, EPRI, in conjunction with Carpenter Technology (Pittsburgh, Pa.), Rolls-Royce (Derby, UK), Tyco Flow Control (Mansfield, Mass.), and Dresser Valves (Alexandria, La.), is conducting a multifaceted research and development project to use PM technologies in the electric power industry [8-1]. Attributes of PM technology Several attributes of PM technologies make the process attractive to the power generation industry including inspectability, near-net shape capability, elimination of rework or repair of large cast components, new alloy systems and chemistries, enhanced weldability, and an alternative supply route for long-lead-time components. Inspectability: Inspection of large cast components, such as pump housings, valve bodies, elbows, flanges, steam chests, turbine casing shells, and nozzles, is challenging due to the nonhomogenous microstructure of castings. Castings can contain voids, segregation of tramp Temperature, C Y162B Heat RR F316L/TP316L <5 in. minimum 1 F316L >5 in. minimum Temperature, F Fig. 2 Tensile strength of four 316L stainless steel heats versus ASME B&PVC strength minimums for the same alloy. Tensile strength, ksi Tensile strength, MPa elements, inclusions, hot tears, secondary phases, and nonmetallic particles, among others. These irregularities in the microstructure make inspection of cast components difficult. By comparison, alloys and components produced using powder metallurgy have a very uniform, homogenous microstructure, which is considered very inspectable using ultrasonic methods in terms of both detection and sizing. Near-net shape (NSS) components. A highly desirable attribute of producing components via PM/HIP processing methods is the ability to produce NNS components, which require only minimal machining and clean-up. Cast components are commonly fabricated in an oversized condition to allow for irregularities that may occur along the length of the component. Components produced using PM/HIP to NNS result in both reduced component weight and machining, ultimately saving overall component production costs. Production using NNS technologies also reduces energy and processing waste during the fabrication process. Elimination of rework or repair of large cast components. An attribute of PM/HIP technology that cannot be overlooked is its ability to produce homogeneous microstructures, which substantially reduces the number of repairs required in castings. In discussions with various valve manufacturers, it is not uncommon for large cast components to require significant rework to eliminate casting defects. This represents considerable overall lifecycle cost to the manufacturer, which must be passed along to the user. Reducing rework also improves procurement and manufacturing. New alloy systems and chemistries. PM/HIP technologies offers the ability to alter (or design) the chemistry (chemical composition) of a specific component on a component-by-component basis. No longer would it be necessary to produce a large (several ton) heat of material to fabricate an individual component. Fabricators will 2 ADVANCED MATERIALS & PROCESSES JANUARY 212

3 now be able to produce individual components (or heats of material) using a specified chemistry. Furthermore, PM/HIP technologies now enable the production of new alloy systems. For example, if an end-user wants to precisely control a particular element such as boron or carbon, or a residual element (e.g., sulfur, phosphorous or other detrimental elements), PM/HIP allows for enhanced control of one or more of these elements. Enhanced weldability. Cast components are often difficult to weld due to the irregularities in microstructure and variability between casts. The homogeneity of PM/HIP produced alloys eliminate these weldability concerns. Once a particular chemistry has been selected, it is highly reproducible, and therefore, minimal differences in weldability are demonstrated heat-to-heat. Alternative supply route for long-lead-time components. As a challenging era of new plant construction in advanced nuclear and fossil generation begins to gain momentum, long lead times are already becoming the norm because only a limited number of manufacturers are available to produce components for the power industry. Introduction of PM/HIP technologies within the ASME BPV Code allows utilities to gain improved access to components providing an alternative supply route, shortening manufacturing time with the potential to reduce cost. Based on the combination of attractive attributes and improvement in technology, over the past 18 months, EPRI in conjunction with the team mentioned above has been working to develop the supporting materials test data for two ASME Code Cases: Addressing Type 316L (UNS S3163) austenitic stainless steel (primarily for nuclear applications) Addressing Grade 91 (UNS S3163), a creep strength-enhanced ferritic steel (primarily for fossil generation applications) Selection criteria Type 316L SS is used extensively throughout the nuclear industry in various cast pressure-retaining applications including valves, pump housings, flanges, piping, and elbows due to its excellent corrosion resistance and strength. Acceptance of PM/HIP for manufacture of this material will enable industry to replace many cast components and improve component inspectability. Grade 91 has been used in fossil-fuel power generation for main-steam and hot-reheat piping applications over the past two decades due to its improved creep properties over low-alloy steels Grade 11 and 22 and due to its improved strength-to-weight ratio. When installed and heat treated properly, the alloy provides long life and is an excellent alternative material. Unfortunately, the industry has seen many applications where the alloy was improperly installed and service life was compromised. One of the applications involves Grade 91 valves, which are currently provided in a cast form. Cast valve bodies often require considerable rework and repair before leaving the manufacturer. Industry has subsequently Yield strength, ksi Temperature, C Y162B Heat RR F316L/TP316L minimum Fig. 3 Yield strength of four 316L stainless steel heats versus the ASME B&PVC strength minimums for the same alloy Temperature, F Fig. 4 Grade 91 creep strength-enhanced ferritic steel valve body manufactured using PM/HIP shown with Alloy 625 valve body of the same configuration. found many installed valves with significantly reduced creep strength resulting from these manufacturing difficulties. The use of PM/HIP will enable industry to provide a high quality product while eliminating many current concerns with multiple weld repairs and postweld heat-treatments. Type 316L stainless steel The development of a data package for ASME involved the manufacture and testing of four different heats (lots) of 316L stainless steel. This included three large valve bodies and one tee section (Figs. 1a-1c), as well as two plate weldments. Each heat was produced following ASTM Specification A988-7 [11]. Chemical compositions of the heats are shown in Table 1. Tensile and yield strength data for the four heats are plotted against the ASME minimum strength values for F316L (Figs. 2 and Tensile strength, MPa ADVANCED MATERIALS & PROCESSES JANUARY

4 TABLE 2 CHEMICAL COMPOSITION OF GRADE 91 (UNS K9156) MANUFACTURED COMPONENTS Heat Nominal composition, wt% Y1549B(a) Y155B(a) Y1551B(a) Product Form (ASTM A989/A989M-7) Valve Valve Valve C Mn P.2 max S.1 max Si Cr Mo Ni.4 max V Cb B <.1 <.1 <.1 N Al.4 max <.1 <.1 <.1 Ti <.1 <.1 <.1 Zr <.1 <.1 <.1 (a) Following PM/HIP per ASTM A989/A989M, valves were normalized at 194F±25F/2.5 h + forced air cool to ambient. After normalize, valves tempered at 143F±25F/4.5h + air cool to ambient. Stress, MPa 5 5 Closed symbols (ruptured) Open symbols (in test) Fig. 5 Time to rupture for PM/HIP Grade 91 compared to average (solid lines) and minimum (dashed lines) expected behavior; open symbols are tests that are still running. 3). The four PM heats exceed the ASME minimums by 1 to15% in most cases [12]. Toughness, grain size, density, porosity, microstructural, and hardness were measured in addition to the strength. In all cases, the PM/HIP 316L SS demonstrated equivalent or superior properties to those of conventionally forged (and/or cast) stainless steel materials. The data package along with a DRAFT Code Case was submitted to ASME by Tyco Flow Valve in November 211. Grade 91 The second data package for ASME is currently 55 C 6 C 65 C ,. 1,. Time to rupture, h under development and involves both the manufacture and testing of three Grade 91 steel valve bodies. Figure 4 provides one example of a processed Grade 91 valve body. The heats of material were manufactured in accordance with ASTM Specification A Chemical compositions of the three heats are provided in Table 2. High-temperature tensile and yield strength data were produced similar to those for the stainless steels, and preliminary creep data for one of the heats is shown in Figure 5 with ruptured tests meeting the expected behavior. Longer term (>1, hours) creep data is being generated for each of the three heats. It is anticipated that the data package and Code Case for the Grade 91 alloy will be submitted in the fourth quarter of 212. Summary PM/HIP opens up an entirely new method of manufacturing for the power industry, as well as for many other industries (oil & gas, pulp & paper, chemical, and food processing) that use high pressure-retaining components. The manufacturing technology not only provides higher quality components with properties that exceed or rival forgings, but it also enables industry to produce NNS products with excellent inspectability. In many cases, the technology will be used to replace existing casting methods for components where quality and inspection concerns currently exist. It is also believed the technology will enable the production of a number of ultrasupercritical and oxy-combustion components where existing manufacturing methods fall short for such high temperature applications. 22 ADVANCED MATERIALS & PROCESSES JANUARY 212

5 References 1. Carpenter Powder Products A World of Capabilities, Carpenter Technology, W.B. Burdett and C.T. Watson, Hot Isostatic Pressing of Type 316L Powder for Pressure Retaining Components, ASME PVP-25, July 17-21, Denver Colo., PVP , p 7, J.L. Sulley, et al., Introduction of Hot Isostatically Pressed Reactor Coolant System Components in PWR Plants, Proc. 18th Intl. Conf. on Nuclear Engineering, ICONE 18, Xi an, China, May 17-21, ICONE , A.Eklund, et al., Corrosion Properties of PM HIPed Stainless Steel, 15th Nordic Corrosion Congress, May 19-21, Stockholm, Sweden, J. Scanlon, et al., Mechanical Properties of PM HIPed Stainless Steel A Comparison to Conventional/Rolled Material, PowderMet 21 Conf., June 27-3, Hollywood, Fla., B. Bengston, et al., Mechanical Properties of PM HIPed Stainless Steels, Stainless Steel World Conf., October 5-7, Houston, Tex., S. Tahtinen, In-Vessel Materials Studies at VTT, VTT Industrial Systems, Association Euratom-Tekes, Annual Seminar, Paasitorni, Helsinki, May 3-31, D.J. Novotnak, L. Lherbier, and D. Gandy, Manufacturing Large Complex PM HIP Shapes, Euro PM 211 Conf., October 9-12, Barcelona, Spain, D. Gandy, J. Shingledecker, L. Lherbier, and D. Novotnak, Powder Metallurgy Methods for Producing Nuclear and Fossil Components, CSC11-8th Intl. Conf. Corrosion Solutions, September 25-3, D. Gandy, J. Shingledecker, and L. Lherbier, The Manufacture of Large, Complex Fossil Components Using Powder Metallurgy and HIP Technologies A Feasibility Study, Advances in Materials Technology for Fossil Power Plants, EPRI/ASM Report 1223, p ASTM Standard A988/A 988M-7, Standard Specification for Hot Isostatically Pressed Stainless Steel Flanges, Fittings, Valves, and Parts for High Temperature Service. 12. ASTM Standard A989/A 989M-7, Standard Specification for Hot Isostatically Pressed Alloy Steel Flanges, Fittings, Valves, and Parts for High Temperature Service. Acknowledgement: The authors recognize L. Lherbier and D. Novotnak (Carpenter Technology), B. Burdett, I. Hookman, J. Sulley, and J. Alcock (Rolls-Royce), D. Thibault, D. Tuttle, and M. Rider (Tyco Flow Control), and R. Danzy (Dresser) for their efforts and support in assembly of a 316L stainless steel and Grade 91 data packages for submittal to ASME BPVC. For more information: David Gandy is Program Manager, Technology Innovation, Electric Power Research Institute, Charlotte, N.C.; tel: 74/ ; davgandy@epri.com. ADVANCED MATERIALS & PROCESSES JANUARY