On Niobium in Construction of Cryogenic Pressure Vessel Systems Gary G. Cheng and Edward F. Daly

Transcription

1 Gary G. Cheng and Edward F. Daly I. DOE Policy on Pressure Safety The Department of Energy (DOE) Office of Health, Safety and Security issued and enforces final rules set forth in 10 CFR Parts 850 and 851 [1] to reduce or prevent occupational injuries, illnesses, and accidental losses at its contractors worksites. In 10 CFR Parts 850 and 851 [1], on page 6913, vacuum systems are defined as pressure systems due to their potential for catastrophic failure due to backfill pressurization. To ensure pressure system safety, 10 CFR 851 Appendix A section 4, Pressure Safety, on page 6941, requires that pressure systems to be designed in conformity to: (1) American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code [2], (2) applicable ASME B31 (code for pressure piping) standards, and (3) the strictest applicable state and local codes. The 10 CFR 851 Appendix A section 4 also pointed out that When national consensus codes are not applicable (because of pressure range, vessel geometry, use of special materials, etc.), contractors must implement measures to provide equivalent protection and ensure a level of safety greater than or equal to the level of protection afforded by the ASME or applicable state or local code. Jefferson Lab (JLAB), as a contractor of DOE, has established a 10 CFR 851 Worker Safety and Health Protection Program (WSHPP) [3] in accordance with DOE s 10 CFR Parts 850 and 851 requirements. As stated in the JLAB WSHPP, four JLAB EH&S manuals described in details about how JLAB will ensure pressure safety in the lab: (1) EH&S Manual Chapter 6122, Welding, Brazing, Cutting, and Grinding Safety, (2) EH&S Manual Chapter 6151, Pressure and Vacuum Vessels, (3) EH&S Manual Chapter 6500, Cryogenics and ODH Safety, (4) Pressure Safety Program Manual (in preparation). EH&S Manual Chapter 6151 [4] identified that Cryomodules, which include both pressure and vacuum systems are among the examples of pressure systems used at JLAB. Materials used in the construction of a cryomodule typically include stainless steel and steel alloys, copper and copper alloys, aluminum and aluminum alloys, titanium, niobium, etc. Most of these materials are covered in ASME B&PV code Section II. However, niobium is not presently included. In this technical note, investigations on niobium applied in fabrication of Superconducting Radio Frequency (SRF) cavities are summarized with ASME B&PV code requirements for new material as the guideline. As one of the supporting documents for the ongoing 12GeV project cryomodule design, this technical note is aimed at recognizing accomplished niobium properties studies that are equivalent to ASME B&PV code rules and put forward a plan for additional work to be performed in the spirit of 10 CFR 851 and ASME B&PV code rules. II. ASME Boiler & Pressure Vessel Code Pertinent Provisions Rules in ASME B&PV code [2] Section VIII Rules for Construction of Pressure Vessels Divisions 1 and 2 and Section IX Welding and Brazing Qualifications shall be followed during the design and construction of pressure vessels and components in a cryomodule. 1/18

2 In Section VIII Division 1: Subsection A UG-4 (a) requires that material subject to pressure shall conform to one of the specifications given in Section II, Part D, Subpart 1, Tables 1A, 1B, and 3, including all applicable notes in the tables, and shall be limited to those that are permitted in the applicable Part of Subsection C, except as otherwise permitted in UG-9, UG-10, UG-11, UG-15, Part UCS, and the Mandatory Appendices. Niobium is not included in any of the provisions in Subsection A or Section II. UG-4(d) states that Materials other than those allowed by this Division may not be used, unless data thereon are submitted to and approved by the Boiler and Pressure Vessel Committee in accordance with Appendix 5 in Section II, Part D. The Section II, Part D, Appendix 5 gives guideline on the approval of new materials under the ASME Boiler and Pressure Vessel Code. This guideline will be carefully followed in this technical note to demonstrate that some equivalent tests on niobium properties have been done and additional tests are planned to be carried out. The Subsection C of Section VIII Division 1 also gives general requirements for nonferrous materials used for pressure vessel in UNF-5(a): All nonferrous materials subject to stress due to pressure shall conform to one of the specifications given in Section II and shall be limited to those listed in Tables UNF-23.1 through UNF-23.5 except as otherwise provided in UG-10 and UG-11. Unfortunately, niobium does not show up in Tables UNF-23.1 through UNF In Section VIII Division 2: Part 3 Material Requirements has a subsection 3.2 for Materials Permitted for Construction of Vessel Parts. Like what is defined in Division 1 Subsection A, niobium is not included in Division 2 para Materials for Pressure Parts. Then states the same rule as Division 1 UG-4(d) follow the Section II, Part D, Appendix 5 Guideline on the Approval of New Materials under the ASME Boiler and Pressure Vessel Code. In 3.6 Supplemental Requirements for Nonferrous Materials, niobium is not found in Tables 3.A.4 through 3.A.7 that are referred to in Based on the ASME B&PV code rules described above, it is concluded that niobium is not covered in ASME B&PV code Section II. To observe DOE 10 CFR 851 Appendix A Section 4, niobium properties obtained from tests that are equivalent to ASME B&PV code requirements for a new material shall be used in design and construction of pressure vessels in a cryomodule. The rest of this technical note is devoted to reviewing in details the ASME B&PV code rules and analyzing the known and required material data for niobium. III. Standard Specification and Applications of Niobium Niobium ASTM Specification The ASTM B Standard Specification for Niobium and Niobium Alloy Strip, Sheet, 2/18

3 and Plate [5] covers five grades of wrought niobium and niobium alloy strip, sheet and plate: R04200-Type 1 Reactor grade unalloyed niobium, R04210-Type 2 Commercial grade unalloyed niobium, R04251-Type 3 Reactor grade niobium alloy containing 1 % zirconium, and R04261-Type 4 Commercial grade niobium alloy containing 1 % zirconium. R0xxxx-Type 5 RRR grade pure niobium. Among these grades, the R04200-Type 1 Reactor grade unalloyed niobium and R0xxxx- Type 5 RRR grade pure niobium are generally used to make SRF cavities and associated parts [6-7]. In fact, according to ASTM B [5], the RRR grade pure niobium is specifically used in superconducting applications that require ultra high purity. The materials shall be made from ASTM B ingots produced by vacuum or plasma arc welding, vacuum electron-beam melting, or a combination of these three methods. Mechanical and chemical requirements, permissible variations in dimensions and weight, quality and surface finish, mechanical/chemical tests, inspection, certification, etc. are described in details in the B specification. Tension tests that the material suppliers shall perform are described in ASTM E8/E8M-08 Standard Test Methods for Tension Testing of Metallic Materials [8]. Specification ASTM B Standard Specification for Niobium and Niobium Alloy Bar, Rod, and Wire [5] covers niobium rods and bars that are occasionally used to make parts in cryomodule assembly. Application of Niobium To date, niobium and its alloys are the primary materials used to make high energy particle accelerator superconducting cavities and related components. Typically, the superconducting cavities have ultra high vacuum inside and sub-atmospheric liquid helium (LHe) bath outside. Due to possible overpressure during a loss of vacuum incident, the C100 CM niobium cavities are specified to withstand 5.0 atm external pressure. This work environment makes niobium cavities virtually pressure vessels. Design of superconducting cavities shall conform to the rules in ASME B&PV code Section VIII, Division 1 or 2. The nominal work temperature ranges from 1.8K to 300K. The cryomodule does not undergo many thermal cycles during its lifetime. Typically, the cryomodule is predicted to operate for 40 years with one thermal cycle per year. JLAB routinely builds, tests, refurbishes, and uses cryomodules ever since the lab was established in 1980s. Fatigue caused by such low-cycle thermal/pressure loads never resulted in cryomodule failures. According to ASME B&PV code Section VIII, Division 2, Fatigue Analysis Screening Based on Experience with Comparable Equipment, fatigue analysis is waived in cryomodule design. Niobium sheets and plates (ASTM B-393) and bars and rods (ASTM B-392) [6-7] are the common forms of raw materials used to fabricate cavities and components. Niobium sheets thicknesses typically range from 3 mm to 4 mm. Plates are normally < 1.0" thick. Rods usually have OD s less than 6.0". 3/18

4 IV. Properties of Niobium Mechanical, thermal, and physical properties of niobium from room to cryogenic temperatures have been investigated in the past [9-25] for various superconducting accelerator projects: CEBAF, SNS, TESLA, ILC, RIA, etc. None of the experimental work was targeted at certifying niobium as a new material to be incorporated by ASME B&PV code. However, nearly all the properties required by ASME B&PV code were reported at various temperatures. A few informative observations from the literature are listed as follows: Reactor grade and high RRR niobium specimens were provided by various vendors, such as Fansteel, Cabot, Teledyne Wah Chang, Tokyo-Denkai. Mechanical properties of niobium vary from batch to batch as well as on the thickness of the material [11, 14]. A niobium sample after purification heat treatment is found to have higher RRR (hence higher thermal conductivity) close to its surface than inside the sample [25]. This is attributed to the distribution of interstitial impurities across the thickness. Samples with various thicknesses undergoing the same purification procedure could develop various interstitial impurities distributions hence inconsistent RRR s and thermal conductivities. High thermal conductivity, hence high RRR grade, niobium is preferred [10, 12, 13] in cavity production to stabilize undesirable heating resulted from local microscopic defects, such as weld imperfections, surface irregularities, foreign material inclusions, residue from surface treatments, dust and debris, and so on. Post-purification (baking and degassing), for example, baking niobium in vacuum with the presence of gettering material like titanium [12, 13], will improve the thermal conductivity. However, a reduction in mechanical properties [12-16], such as yield strength, tensile strength, and even young s modulus [15] will occur. It was found that for SNS production niobium, heat treatment at 600 C seemed to be an optimum temperature to expel interstitial hydrogen without significant loss of yield and tensile strength [14, 15]. Electron beam welding (EBW) deteriorates [11] both thermal conductivity and mechanical properties of niobium, depending on the vacuum condition in the process. This phenomenon is attributed to the growing of grain sizes [20-22], i.e. the Hall-Petch effect. Sufficient annealing leads to more complete recrystallization that reduces microstructural heterogeneity, which enhances anisotropy and degrades formability [18, 21]. As a wellknown effect, annealing compromises material strengths. For high purity niobium, its strength and ductility increase with increasing strain rate [20]. Based on one study [16, 27], Slow Strain Rate (SSR) tests using a strain rate of s -1 yield lower yield and tensile strengths than Fast Strain Rate (FSR, strain rate = s -1 ) tests do. High temperature heat treatment, e.g C, may result in >20% difference in SSR and FSR strength data [16, 27]. Niobium possesses a body-centered-cubic (BCC) crystal structure and hence will undergo ductile-to-brittle transitions when temperature drops below the ductile-to-brittle transition temperature (DBTT) [22, 23]. The DBTT of niobium is likely to be above 100K. 4/18

5 Ductility decreases as temperature lowers. The high RRR niobium shows certain ductility at LHe temperature, but, reactor grade niobium with low RRR = 30~80 becomes brittle at 4K [22]. The undesirable brittleness at LHe temperature, 2K or 4K, hindered obtaining valid yield/ultimate strengths data in some tensile tests [9, 22]. Data shows that the yield strength of reactor grade niobium can be 10%~70% higher than that of high RRR niobium at room temperature [9, 22, 23]. Yield and tensile strengths of niobium increase significantly as temperature decreases, say 6 to 10 times [9-13, 22, 23] enhancement from 300K strengths to 4K strengths. In the following, high RRR and reactor grade niobium property data will be summarized from literature [9-25]. Note that not all tests observed ASTM standards on test preparations. In fact, since niobium property data vary from batch to batch, for the C100 CM construction, it is strongly recommended that all ASME B&PV required property data to be re-tested in accordance with the latest version of ASTM annual book on testing methods [26]. 1. Mechanical Properties For the temperature range that niobium cavities and parts are used, i.e. 1.8K ~ 300K, timedependent behaviors, such as creeping or stress-rupture, are not among design concerns. Properties from Tensile Tests Yield strength, ultimate tensile strength, reduction of area, and elongation have been measured in the past. Table 1 shows a comprehensive collection of the reported testing data in reviewed literatures [9-24, 27]: Due to the fact that niobium properties change from batch to batch, it would not be conservative to average properties among materials from various vendors or having gone through different processing. Not all reports mentioned clearly what forms of specimen (sheets, rods, bars, etc.) were tested and the strain rates that were adopted. The temperature interval was not observing ASME B&PV code s requirement of 100 F (or 50 C). However, for niobium pressure vessel design, data at room temperature and 4.2 K shall be sufficient. Further reducing temperature to 1.8K is technically doable (not reported in reports reviewed), but is not so necessary because niobium will get stronger as temperature drops. Stress-Strain Curves Stress-strain curves for most of the materials appeared in Table 1 have also been obtained and published in reports. Again, these curves were typically recorded for 4K and room temperature. Heat treatments, welding, and impurity are the major factors affecting stress-strain relationships. For brevity, stress-strain curves are not presented in this note. According to projects, the references for stress-strain curves are classified as: 1) CEBAF niobium [9-10, 12-13], 2) SNS [14], 3) TESLA, ILC [11, 18, 21], 4) RIA [18, 21], and 5) APT [22-23]. 5/18

6 No. Project Material Vendor Table 1 Niobium mechanical properties from reported tensile tests RRR Thickness, mm Heat treatment, strain rate, etc. T, K Yield Strength, Mpa (ksi) Tensile Strength, Mpa (ksi) Elongation (%) 1 CEBAF Cabot No (23.55) >213(31) 50 [9] 2 CEBAF Cabot 40 No (92.5) 650(94.2) 16.8 [10] 3 CEBAF Fansteel No (24.95) 197(28.6) 40 [9] 4 CEBAF Teledyne No (17.51) 147(21.4) 35 [9] 5 CEBAF Teledyne No (83.7) 819(118.8) 5.22 [9] 6 CEBAF Teledyne >250 No (14.5) 159(23) 42 [10] 7 CEBAF Teledyne >250 No (86) 610(88.4) 12.1 [10] 8 CEBAF Teledyne >250 No (130) 903(131) >1 [10] 9 CEBAF Teledyne > CEBAF Teledyne >228 Heat treated with Ti at 1402 C for 6 hrs Heat treated with Ti at 1402 C for 6 hrs RA (%) Refs (5.9) 96(14) 48 [12] 4.2 Serration started 70.9(102.8) 21.8 [12] 11 CEBAF Fansteel >300 No (10.6) 134(19.5) 34 [10] 12 CEBAF Fansteel >300 No (82) 628(91) 14 [10] 13 CEBAF Fansteel >300 No (110) 859(124.6) >1 [10] 14 TESLA Cornell No (23.93) 186(26.97) 42 [11] 15 TESLA Cornell No (130) 903(131) >1 [11] 16 TESLA Cornell > Heat treated with Ti at 1400 C for 4 hrs (14.79) 128(18.56) 30.2 [11] 17 TESLA Cornell > TESLA Cornell > Heat treated with Ti at 1400 C for 4 hrs Welded & Heat treated with Ti at 1400 C for 4 hrs 4.2 Serration started 779(113) 15.2 [11] (11.45) 115(16.7) 25.6 [11] 6/18

7 19 TESLA Cornell > Welded & Heat treated with Ti at 1400 C for 4 hrs 4.2 Serration started 807(117) 6.6 [11] 20 SNS WCL ~400 No (10.95) (1) 153(22.16) (1) 58 (1) [14] 21 SNS WCL ~ C, 6 hrs (5.75) (2) 146.9(21.3) (2) 56.5 (2) [14] 22 SNS WCL ~ C, 3 hrs (5.95) (2) 158.6(23) (2) 55 (2) [14] 23 SNS WCL ~ C, 1 hr (7.0) 157.2(22.8) 50 [14] 24 SNS WCL ~ C, 1.5 hrs (7.7) 163.4(23.7) 61 [14] 25 SNS WCL ~ C, 6 hrs (7.5) 166.9(24.2) 55 [14] 26 SNS WCL ~ C, 10 hrs (9.5) 181.4(26.3) 57 [14] 27 SNS WCL ~ C, 6 hrs (10.5) 155.9(22.6) 54 [14] 28 SNS WC ~400 No (13.5) (2) 162.7(23.6) (2) 46 (2) [14] 29 SNS WC ~ C, 3 hrs (6.5) 147.6(21.4) 59 [14] 30 SNS WC ~ C, 1 hr (8.1) 154.5(22.4) 60 [14] 31 SNS WC ~ C, 6 hrs (9.2) 150(21.8) 61 [14] 32 SNS WC ~ C, 1.5 hrs (8.8) 153.8(22.3) 60 [14] 33 SNS WC ~ C, 10 hrs (10.5) 156.5(22.7) 57 [14] 34 SNS WC ~ C, 6 hrs (9) 157.9(22.9) 62 [14] 35 SNS TD ~300 No (27) 213.8(31) 14 [14] 36 SNS TD ~ C, 3 hrs (5.5) (2) 154.5(22.4) (2) 40.5 (2) [14] 37 SNS TD ~ C, 1 hr (5.5) 142(20.6) 38 [14] 38 SNS TD ~ C, 1.5 hrs (6.6) 151(21.9) 35 [14] 39 SNS TD ~ C, 6 hrs (5.6) 132.4(19.2) 33 [14] 40 SNS TD ~ C, 10 hrs (10) 172.4(25) 34 [14] 41 SNS TD ~ C, 6 hrs (15) 172.4(25) 22 [14] SSR (7.4) 145(21) 44 [16, 26] FSR (7.9) 166(24) 48 [16, 26] C, 10 hrs, SSR (7.0) 145(21) 48 [16, 26] C, 10 hrs, FSR (7.5) 152(22) 49 [16, 26] 7/18

8 C, 6 hrs, SSR (5.7) 131(19) 47 [16, 26] C, 6 hrs, SSR (4.5) 103(15) 32 [16, 26] C, 6 hrs, FSR (6.3) 131(19) 33 [16, 26] SSR (16) 226(32.8) 50 [16] C, 3 hrs, SSR (11) 183(26.5) 50 [16] C, 6 hrs, SSR (9.28) 165(24) 50 [16] C, 6 hrs, SSR (8.12) 120(17.4) 53 [16] 53 ILC Wah Chang >250 0 slice, s (9.7) 173(25) 67 [18,19,21] 54 ILC Wah Chang > slice, s (9.1) 186(27) 60 [18,19,21] 55 ILC Wah Chang > slice, s (9) 186(27) 61 [18,19,21] 56 ILC Wah Chang > slice, s (12.5) 170(24.6) 67 [18,19,21] 57 ILC 58 ILC 59 ILC 60 ILC 61 RIA 62 RIA 63 RIA 64 RIA Wah Chang Wah Chang Wah Chang Wah Chang >250 >250 >250 >250 0 slice, 750 C, 3hrs, s (6.7) 169(24.5) 65 [18,19,21] 25 slice, 750 C, 3hrs, s (8.4) 180(26.1) 60 [18,19,21] 65 slice, 750 C, 3hrs, s (6.2) 178(25.8) 60 [18,19,21] 90 slice, 750 C, 3hrs, s (7) 169(24.5) 65 [18,19,21] Tokyo Denki >250 0 slice, s (12.5) 180(26.1) 65 [18,19,21] Tokyo > slice, s Denki (12.5) 186(27) 57 [18,19,21] Tokyo > slice, s Denki (12.5) 186(27) 63 [18,19,21] Tokyo > slice, s Denki (12.5) 187(27.1) 64 [18,19,21] 8/18

9 65 APT APT APT & & & C, 2 hrs annealing, s (9.7) 172(24.9) [22,23] 740 C, 2 hrs annealing, s (89.6) 642(93.1) [22,23] 740 C, 2 hrs annealing, s (95.4) 929(134.7) [22,23] 68 APT annealed, s (8.27) 162(23.5) [22,23] 69 APT annealed, s (66) 889(129) [22,23] 70 APT annealed, s (11) 171(24.8) [22,23] 71 APT annealed, s (64.2) 502(72.8) 3 7 [22,23] 72 APT annealed, s (67.9) 2 1 [22,23] 73 APT annealed, s (14.2) 187(27.1) [22,23] 74 APT annealed, s (111) 964(139.8) 7 6 [22,23] 75 APT annealed, s (153) 1068(155) 2 [22,23] KEK JAERI KEK JAERI KEK JAERI Tokyo Denki Tokyo Denki Tokyo Denki C, 2 hrs annealing (6.38) 155(22.5) 59 [23,24] 750 C, 2 hrs annealing (63.8) 591(85.7) 25 [23,24] 750 C, 2 hrs annealing 4 500(72.5) 875(127) 15 [23,24] Notes: (1) Averaged from 7 sets of measurements having same conditions, refer to [14]. (2) Averaged from 2 sets of mea surements having same conditions, refer to [14]. (3) T = temperature; RA = Reduction of area; KEK-JAERI = KEK/JAERI Joint Project on high-intensity proton accelerators. 9/18

10 Notch Toughness The ASME B&PV code Section VIII, Division I, UG-20 (f) states that impact test per UG-84 is not mandatory for pressure vessel materials that satisfy 5 listed conditions. The 3 rd condition requires that Design temperature is no warmer than 650 F (345 C) nor colder than 20 F ( 29 C). Occasional operating temperatures colder than 20 F ( 29 C) are acceptable when due to lower seasonal atmospheric temperature. Since superconducting cavities work at LHe temperature, this condition cannot be met. It is noticed that in rule UG-84 (c) (3), it is said that Toughness tests are not required where the maximum obtainable Charpy specimen has a width along the notch less than in. (2.5 mm). The C100 cryomodule superconducting cavities have a specified wall thickness of 3 mm. According to Fig. UG-84 in ASME B&PV code Section VIII, Division I, the V-notch has a depth of 20% of the plate thickness and that leaves the width along the notch to be 80% of the wall thickness. For the 3 mm niobium wall, the width along the notch is 2.4 mm or in., which is less than in. Therefore, Charpy impact test for niobium cavities with 3 mm wall thickness can be waived per ASME B&PV code section VIII, Division I, rule UG-84 (c) (3). Besides the niobium cavity, there are other niobium components that are made of thicker than 3 mm niobium plates or niobium bars. For these applications, Charpy tests in conformity to UG-84 rules shall be conducted. Historically, the main purpose of a Charpy tests is to determine the temperature dependent ductile-to-brittle transition of metals. Controlling Charpy impact energy to be above a threshold value would prevent the occurrence of brittle failure. However, the ASME B&PV code did not set the impact energy acceptance criteria for niobium, which is not included in the code at all. It is known that reactor grade (low RRR) niobium turns to be brittle at LHe temperature. Based on the reviewed literatures [24, 28], the Charpy impact energy for niobium specimens ranges from 2.7 J (2 ft-lb) to 41 J (30 ft-lb). Please note that in ASME BP&V code Section VIII, Division 1, Parts UNF, NF-6 refers to Appendix A, A-430, of Section II, Part D. Materials in Table 1B abd 2B are said that The static tensile strength increases as the temperature decreases, and the ductility as measured by percent elongation is not adversely affected to any significant degree. For these reasons low temperature impact tests of nonferrous materials are not required. Niobium does not belong to this type of materials. Therefore, the conclusion about Charpy impact test is that it is required for some niobium components per Section VIII, Division 1 rules. Impact tests shall be performed at LHe temperature. But, information from such tests is not that useful for design analysis. Division 1 does not provide alternatives to impact tests on purpose of determining brittle fracture. ASME BPV code Section VIII, Division 2, Part 3 has a section 3.11 specifically addressing requirements on material toughness. For non-ferrous materials, to which niobium shall belong, sets rules for materials listed in Tables 3.A.4 to 3.A.7. Niobium is certainly not in those tables. Whereas in the spirit of equivalency, the rules in can be rationally extended to the case of niobium and its alloys: The nonferrous materials listed in Tables 3.A.4 thru 3.A.7, may be used at lower temperatures than those specified herein and for other weld metal compositions provided the user satisfies himself by suitable test results such as determinations of tensile elongation and sharp-notch tensile strength (compared to unnotched tensile strength) that the material has suitable ductility at the design temperature. The code has given the user the freedom to choose SUITABLE toughness tests to conduct. The Charpy test is not very 10/18

11 SUITABLE because niobium, especially reactor grade niobium with low RRR, turns to have low ductility or even purely brittle at LHe temperature. Alternatively, tensile elongation or fracture toughness tests [22-24] per ASTM standard testing methods could be more informative to design and analysis of niobium pressure vessels working at LHe temperature. Tensile elongation test is performed during a normal tensile test for Young s modulus, yield strength, ultimate strength, etc. Again, for low RRR niobium specimens, it is quite possible that at LHe temperature, these specimens demonstrate no ductility at all [9, 10, 22]. JLAB s niobium specifications [6,7] require a 40% and 25% elongation at room temperature for RRR niobium and reactor grade niobium, respectively. This requirement would serve as a protection from brittle fracture of niobium at upsetting overpressure condition. ASME BPV code does not supply threshold tensile elongation for niobium. In C100 CM helium vessel and cavity design (Fig. 1), high RRR niobium, which has some ductility at LHe temperature, is used to construct the superconducting cavity (item 1 in Fig. 1) and beampipe. The helium vessel is virtually a LHe container made up of items 2-6 as shown in Fig. 1. At normal operation condition, the LHe has a nominal pressure of 0.04 atm and temperature of 2.1 K. Overpressure up to 5 atm may occur when there is overheating on the surface of niobium cavity or a beamline leak develops. The pressure will act on the high RRR niobium cavity. Beyond the boundary of the helium vessel, there are a few nitronic rods (connected to item 5 in Fig. 1) stiffening the helium vessel against structural deformation. In other words, the brittle reactor grade niobium parts will not experience high stress even if there exists overpressure. Fig. 1 C100 cryomodule helium vessel containing high RRR niobium cavity Another toughness test that is recommended for niobium material BPV code certification is the fracture toughness measurements per ASTM E399 and/or ASTM E1820 [26]. The ASTM E399 test measures the stress intensity factor, namely K Ic, which characterizes the resistance of a material to fracture in a neutral environment in the presence of a sharp crack under essentially 11/18

12 linear-elastic stress and severe tensile constraint. The E399 is more applicable to reactor grade niobium at LHe temperature since not much plastic deformation will be observed. The ASTM E1820 [26] is a more comprehensive method to quantify a ductile material s fracture toughness in the presence of a preexisting, sharp, fatigue crack. The high RRR niobium material may develop fine cracks on the surface or inside the sheet during manufacturing and processing. Ultrasonic NDE methods can be utilized to investigate the sizes of such cracks. Structural flaw tolerance assessment can be performed on basis of E1820 test findings. In the past [22-24], E399 tests were carried out on niobium and results are used to develop allowable flaw size criteria [22-23] and calculate the allowable pressure for crack propagation. Similar design practices can be adopted in C100 CM design provided that fracture toughness measurements are performed in accordance to ASTM E399 and E Other Properties Thermal Conductivity & diffusivity: Thermal conductivity of niobium is a highly nonlinear function of temperature. It is measured from time to time for niobium for various projects. Measured data are presented in reports in form of curves [10-12, 17, 25]. The diffusivity can be deduced from thermal conductivity by use of density, ρ, and specific heat, C p. Young s modulus and other properties: due to studies associated with ILC and RIA cavities niobium [18-21], it is found that niobium has an anisotropic elastic modulus if annealing is insufficient. Annealing is required in the C100 Cryomodule niobium specification [6-7]. None of the reviewed reports addressed the temperature dependencies of Young s modulus, coefficient of thermal expansion (CTE), shear modulus, and Poison s ratio. Room temperature data for these properties can be easily found from vendors and web resources. Table 2 lists some reference data based on ATI Wah Chang s technical data sheet on niobium [29]. Properties Table 2 Properties of niobium Value at Room Temperature Density, kg/m Specific Heat, kj/kg CTE, m/m/ C Young s modulus, GPa 105 Shear Modulus, GPa 38 Poison s Ratio 0.38 V. Weldability of Niobium and Physical Changes in Fabrication The superconducting niobium cavities are electron beam welded (EBW) from half-cells [30]. ASME BPV code Section VIII, Division 1, UW-27(a)(2) permits construction of pressure vessels by EBW. According to UW-11 (e), welds made by EBW are required to undertake full/spot 12/18

13 radiographic examination and shall be ultrasonically examined for their entire length in accordance with the requirements of Appendix 12. The mandatory Appendix 12 sets forth requirements on how the ultrasonic examination of welds shall be performed and reported. EBW is operated in ultra high vacuum [11, 30] to avoid addition of impurities during the process. The niobium parts are fused by a beam of high velocity electrons. Without added filler material, the heat input into the base material is less than that in an arc welding process because of a relatively narrow electron beam. A weld has typically a width of 1/8" and the heat affected zone is ~1/2" wide. The thickness of the weldment will not vary significantly after the process. Therefore, similar analogy upon waiver of notch toughness test, i.e. Charpy test, according to ASME B&PV code section VIII, Division I, rule UG-84 (c) (3) can be applied for EBW niobium cavities. Influence of fabrication practices, such as annealing, heat treatment, and welding, on the thermal/mechanical properties of niobium have been extensively studied [11-16, 18-21] and reviewed in previous sections. Clearly, investigations on these effects will continue on whenever a new project, like CEBAF 12 GeV upgrade project, comes into action. VI. Walk-through ASME B&PV New Material Checklist This technical note does not intend or imply an immediate submission of niobium as a new ASME B&PV code material to be adopted by the ASME B&PV committee. To summarize the reviewed work, the New Materials Checklist in ASME B&PV code Section II s Guideline on the Approval of New Materials under the ASME Boiler and Pressure Vessel Code is utilized herein to brief the progress toward a potential submission of new material adoption request: (a) Has a qualified inquirer request been provided? Not yet. (b) Has a request either for revision to existing Code requirements or for a Code Case been defined? No. (c) Has a letter to ASTM or AWS been submitted requesting coverage of the new material in a specification, and has a copy been submitted to the Committee? Alternatively, is this material already covered by a specification issued by a recognized national or international organization and has an English language version been provided? The applicable ASTM material specifications are: ASTM B Standard Specification for Niobium and Niobium Alloy Bar, Rod, and Wire ASTM B Standard Specification for Niobium and Niobium Alloy Strip, Sheet, and Plate 13/18

14 (d) Has the construction Code and Division coverage been identified? ASME B&PV code Section VIII, Division 1 or 2. (e) Has the material been defined as ferrous or nonferrous and has the application (product forms, size range, and specification) been defined? Non-ferrous. Niobium rods, bars, sheets, and plates are used in C100 cryomodules. (f) Has the range (maximum/minimum) of temperature application been defined? 2K 300K. (g) Has mechanical property data been submitted (ultimate tensile strength, yield strength, reduction of area, and elongation at 100 F or 50 C intervals, from room temperature to 100 F or 50 C above the maximum intended use temperature for three heats of appropriate product forms and sizes)? See Table 1. Mechanical properties from tensile tests conducted at 4K, 77K, and 293K do exist. Properties change from batch to batch. Not all tensile tests observed ASTM specifications. (h) If requested temperatures of coverage are above those at which time-dependent properties begin to govern design values, has appropriate time-dependent property data for base metal, weld metal, and weldments been submitted? Not applicable. (i) If coverage below room temperature is requested, has appropriate mechanical property data below room temperature been submitted? Property data for cryogenic temperatures have been reported. However, they are not measured at 100 F (50 C) temperature intervals as required by ASME B&PV code. (j) Have toughness considerations required by the construction Code been defined and has appropriate data been submitted? Notch toughness Charpy test is waived according to UG-84 (c) (3). The typical niobium cavity wall thickness is 3 mm that makes the width along the notch to be in or 2.4 mm. (k) Have external pressure considerations been defined and have stress strain curves been submitted for the establishment of external pressure charts? A great number of stress-strain curves for niobium at room and cryogenic temperatures are documented in various reports. Again, some of the tensile tests performed are not standardized in view of ASTM specifications. 14/18

15 For the niobium used to construct C100 CM cavities, stress-strain curves will be tested following ASTM tensile test standards. These curves are useful while evaluating the plastic deformation and potential rupture failure of cavities at overpressure conditions. (l) Have cyclic service considerations and service limits been defined and has appropriate fatigue data been submitted? Cyclic load is a negligible concern due to the fact of very low number of thermal cycles throughout the lifetime of a cryomodule. (m) Has physical properties data (coefficient of thermal expansion, thermal conductivity and diffusivity, Young s modulus, shear modulus, Poisson s ratio) been submitted? Thermal conductivity of niobium has been measured for temperatures ranging from 2K to 300K in the past. Plenty of curves were archived. Young s modulus, shear modulus, and Poison s ratio are believed to be invariants when temperature drops to LHe temperature. Nevertheless, confirmation with cryogenic tests will be helpful. (n) Have welding requirements been defined and has procedure qualification test data been submitted? Electron beam welding (permitted by UW-27 (a) (2)). There is no addition of weld material. No need to do Charpy test on welds. Welds need full/spot radiography and ultrasonic examinations per UW-11 (e). (o) Has influence of fabrication practices on material properties been defined? Yes, influence from annealing, post-purification, and welding are well-identified. It is necessary to re-evaluate these effects whenever a new processing approach is used. VII. Future Work The purpose of this technical note is to review what has been done in regard to niobium used in superconducting cavities pertinent application and identify (1) to what extent of equivalency to ASME B&PV requirements has the past work achieved, and (2) what else needs to be done so that niobium can be safely used in pressure vessel design, test, and fabrication in spirit of DOE 10 CFR 851 guidelines. Through the previous sections, it is clear that a great deal of valuable tensile tests have been performed by JLAB and other institutes to characterize niobium products from various vendors. Property data are abundant, though not as comprehensive as required by ASME B&PV code. Thus, for niobium materials to be used in the C100 cryomodule design and construction: i. Tensile tests following rules in (1) ASTM E8/E8M Standard Test Methods for Tension Testing of Metallic Materials [8] and (2) ASTM E1450 Standard Test Method for Tension Testingof Structural Alloys in Liquid Helium [8] shall be performed for at least three heats of each niobium product form. 15/18

16 ii. iii. iv. A practical test temperature range is 4K 300K. Properties to be monitored include, but not limited to: yield strength, ultimate tensile strength, elongation, reduction of area, young s modulus (assuming fully annealed hence isotropic niobium), poison s ratio. Stress-strain curves need to be recorded. Measurements of thermal conductivities demand thermal tests. Test specimen to be prepared in the same manner as cavity cells are prepared: EBW and hydrogen degassed. Experiences from SNS niobium cavities suggest that a heat treatment temperature of 600 C and duration of 10 hours. Mechanical properties of as-received niobium and post-purified niobium need to be compared to confirm this heat treatment recipe so that no noticeable degradation in mechanical strengths occurs. In view of the less useful information that can be obtained from Chapry impact tests on niobium, fracture toughness tests per ASTM E399 and E1820 are recommended for certification of both reactor grade and high RRR niobium. The test results can be used in fracture mechanics evaluations to define the allowable flaw size and/or maximum allowable pressure to prevent crack propagation. REFERENCES [1]. 10 CFR Parts 850 and 851, Chronic Beryllium Disease Prevention Program; Worker Safety and Health Program; Final Rule, Federal Register, Volume 71, Number 27, the Department of Energy, February 9, [2] ASME Boiler & Pressure Vessel Code, 2008a Addenda, American Society of Mechanical Engineers, July 1, [3]. 10 CFR Part 851, Worker Safety and Health Protection Program (WSHPP), Thomas Jefferson National Accelerator Facility, Revision 1, May [4] Pressure Systems, ES&H Manual, Thomas Jefferson National Accelerator Facility, Approval Date 06/24/08. [5]. ASTM Volume Nonferrous Metals -- Nickel, Cobalt, Lead, Tin, Zinc, Cadmium, Precious, Reactive, Refractory Metals and Alloys; Materials for Thermostats, Electrical Heating and Resistance Contacts, and Connectors, American Society for Testing and Materials, ISBN , June [6]. D. Machie, E. F. Daly, P. Kneisel, and J. Hogan, Niobium for Use in Superconducting Accelerators (Reactor Grade I), JLAB Specification No S0143, February 4, [7]. D. Machie, E. F. Daly, P. Kneisel, and J. Hogan, Niobium for Use in Superconducting Accelerators (RRR Grade), JLAB Specification No S0144, February 4, [8]. ASTM Volume Metals Mechanical Testing, Elevated and Low-temperature Tests; Metallurgy, American Society for Testing and Materials, ISBN , July [9]. P. Kneisel and G. R. Myneni, Preliminary Results of Tensile Test on Niobium at Room Temperature and 4.2K, CEBAF-TN , Jefferson Lab, Newport News, VA. 16/18

17 [10]. P. Kneisel, J. Mammosser, M. G. Rao, and K. Saito, Superconducting Cavities from High Thermal Conductivity Niobium for CEBAF, in Proceedings of the Conference on Electron Beam Melting and Refining State of the Art 1990, Edited by R. Bakish, Bakish Materials Corporation, Englewood, pp [11]. M. G. Rao and P. Kneisel, Thermal and Mechanical Properties of Electron Beam Welded and Heat-Treated Niobium for TESLA, in Proceedings of the 6 th RF Superconductivity Workshop, Edited by R. M. Sundelin, CEBAF, Newport News, VA, October 1993, pp [12]. M. G. Rao and P. Kneisel, Thermal and Mechanical Properties of Heat Treated Nb, JLAB-TN , Jefferson Lab, Newport News, VA. [13]. M. G. Rao and P. Kneisel, Mechanical Properties of High RRR Niobium at Cryogenic Temperatures, Advances in Cryogenic Engineering, Vol. 40, Edited by R. P. Reed et al., Plenum Press, New York, 1994, pp [14]. G. Myneni and P. Kneisel, High RRR Niobium Material Studies, JLAB-TN-02-01, Jefferson Lab, Newport News, VA. [15]. G. R. Myneni and S. R. Agnew, Elasto-Plastic Behavior of High RRR Niobium: Effects of Crystallographic Texture, Microstructure and Hydrogen Concentration, in AIP Conference Proceedings, Vol. 671, pp , July Also in Proceedings of the International Workshop on Hydrogen in Materials & Vacuum Systems, Jefferson Lab, Newport News, Virginia, November 11-13, 2002, pp [16]. G. R. Myneni and H. Umezawa, Variation of Mechanical Properties of High RRR and Reactor Grade Niobium with Heat Treatments, talk given at the 10 th International Conference on Ultra-High Purity Metallic-Base Materials (UHPM 2003), Ecole Nationale Supérieure des Mines, Saint-Etienne, France, June 16-20, Also in Matériaux & Techniques, Vol , 2003, pp [17]. G. R. Myneni, Physical and Mechanical Properties of Niobium for SRF Science and Technology, in Proceedings of the Single Crystal Niobium Technology Workshop, Edited by G. Myneni, T. Carneiro, and A. Hutton, Oct 31, 2006, Araxa, Brazil. [18]. H. Jiang, P. Bauer, T. R. Bieler, D. Baars, C. Compton, and T. L. Grimm, Mechanical Properties of Niobium used for SRF Cavities, Presentation at the ILC-Midwest SRF Materials Meeting, November 14, 2006, Argonne National Laboratory, Argonne, IL. [19]. T. R. Bieler, D. C. Baars, H. Jiang, C. Compton, and T. L. Grimm, Mechanical Properties of Nb, Presentation at ILC- Midwestern SRF Materials Research Meeting, March 17, 2006, Fermi National Accelerator Laboratory, Batavia, IL. [20]. A. Zamiri, F. Pourboghrat, H. Jiang, T. R. Bieler, F. Barlat, J. Brem, C. Compton, and T. L. Grimm, On Mechanical Properties of the Superconducting Niobium, Material Science and Engineering: A, Vol , November 2006, pp [21]. H. Jiang, D. Baars, A. Zamiri, C. Antonie, P. Bauer, T. R. Bieler, F. Pourboghrat, C. Compton, and T. L. Grimm, Mechanical Properties of High RRR Niobium with Different Texture, IEEE Transactions on Applied Superconductivity, Vol. 17, No. 2, June 2007, pp /18

18 [22]. R. P. Walsh, R. R. Mitchell, V. T. Toplosky, and R. C. Gentzlinger, Low Temperature Tensile and Fracture Toughness Properties of SCRF Cavity Structural Materials, in Proceedings of the 9 th Workshop on RF Superconductivity, Santa Fe, NM, November [23]. R. P. Walsh, K. Han, V. J. Toplosky, and R. R. Mitchell, Low Temperature Tensile and Fracture Characteristics of High Purity Niobium, Advances in Engineering: Proceedings of the International Cryogenic Materials Conference ICMC, Vol. 48, edited by B. Balachandran, et al, 2002, pp [24]. K. Ishio, K. Kikuchi, J. Kusano, M. Mizumoto, K. Mukugi, A. Naito, N. Ouchi, Y. Tsuchiya, and K. Saito, Fracture Toughness and Mechanical Properties of Pure Niobium and Welded Joints for Superconducting Cavities at 4K, in Proceedings of the 9 th Workshop on RF Superconductivity, Santa Fe, NM, November [25]. W. Singer, Metallurgical and Technological Request for High Purity Niobium in SRF Application, in Hydrogen in Matter: A Collection from the Papers Presented at the Second International Symposium on Hydrogen in Matter (ISOHIM) Edited by G. R. Myneni and B. Hjörvarsson, AIP Conference Proceedings, Vol. 837, 2006, PP [26]. ASTM Volume Metals -- Mechanical Testing; Elevated and Low-Temperature Tests; Metallography, American Society for Testing and Materials, ISBN , June [27]. G. R. Myneni, S. Agnew, G. Ciovati, P. Kneisel, W. A. Lanford, R. L. Paul, and R. E. Ricker, Mechanical Properties of High Purity Niobium Novel Measurements, in Proceedings of the 11 th Workshop on RF Superconductivity, Travemünde/Lübeck, Germany, September [28]. T. Nicol, SRF Pressure Safety at Fermilab, presentation at LCWS08 and ILC08, University of Illinois-Chicago, November 16-20, [29]. Technical Data Sheet Niobium, ATI Wah Chang, Albany, Oregon. [30]. H. Padamsee, J. Knobloch, and T. Hays, RF Superconductivity for Accelerators, John Wiley & Sons, Inc., New York, 1998, pp /18