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Design Innovation Paper

Process Development for Vacuum Brazed Niobium–316L Stainless Steel Transition Joints for Superconducting Cavities

[+] Author and Article Information
Abhay Kumar, R. Kaul, P. Chinna Rao, D. P. Yadav, B. K. Sindal, R. K. Gupta, R. Sridhar, S. C. Joshi, B. Singh

Raja Ramanna Centre for Advanced Technology,
Indore 452 013, India

P. Ganesh

Raja Ramanna Centre for Advanced Technology,
Indore 452 013, India
e-mail: ganesh@rrcat.gov.in

Manuscript received February 29, 2016; final manuscript received September 6, 2016; published online September 29, 2016. Assoc. Editor: Wayne Cai.

J. Manuf. Sci. Eng 139(1), 015001 (Sep 29, 2016) (8 pages) Paper No: MANU-16-1130; doi: 10.1115/1.4034716 History: Received February 29, 2016; Revised September 06, 2016

The paper describes process development for producing sound, strong, and ductile Nb pipe–316L stainless steel (SS) flange brazed joint suitable for application in superconducting radiofrequency (SRF) cavities. The developed transition joints, made with BVAg-8 braze filler metal (BFM), were free of brittle intermetallic compounds, in contrast to the existing global brazing practice of using oxygen-free electronic copper as BFM which results in the formation of a continuous layer of Fe–Nb brittle intermetallic compound at Nb–braze interface. In view of the large difference in the mean thermal expansion coefficients between niobium and 316L stainless steel, a new design for manufacturing and assembly (DFMA) has been developed to ensure achievement of desired joint thickness with uniformity in circumferential and longitudinal directions. An environment-friendly prebraze cleaning procedure has been qualified and implemented. DFMA has resulted in (i) significant reduction of the out-of-roundness errors (≤10 μm) while machining of the niobium pipe, (ii) simplified clearance fit prebraze assembly at room temperature (RT), and (iii) uniformity of joint thickness. A process flow chart has been developed to ensure repeatability of joint characteristics. The brazed joint, of niobium pipe and 100CF knife edge 316L SS flange made by standardized practice, displayed helium leak tightness better than 5 × 10−10 mbar l/s at RT and at liquid helium temperature (LHT). The braze-joint sustained 873 K/10 h postbraze hydrogen degassing treatment and thermal cycling between RT and LHT without any loss in hermeticity.

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Figures

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Fig. 1

Details of niobium pipe/316L stainless steel flange braze assembly used in the initial part of the study (Courtesy American Welding Society [15])

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Fig. 2

Flow chart of the procedure used for fabrication, cleaning, and assembly for Nb pipe–316L SS flange brazing in the initial part of the study

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Fig. 3

Cross section of Nb/316L SS transition joint made with CuSil-ABA BFM

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Fig. 4

(a) Magnified view of fracture surface and (b) associated X-ray diffraction plot of tensile tested Nb/316L SS brazed specimen made with CuSil-ABA BFM

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Fig. 5

Cross section of Nb/316L SS transition joint made with BVAg-8 braze filler

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Fig. 6

Ductile fracture surface of tensile tested Nb/316L SS brazed specimen made with BVAg-8 braze filler

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Fig. 7

Details of modified joint design used for fabrication of Nb pipe–316L SS flange assembly

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Fig. 8

Standardized flow chart for fabrication, cleaning, and assembly for Nb pipe–316L SS flange brazing

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Fig. 9

Vacuum brazed Nb pipe–316L SS flange assembly fabricated through standardized process

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Fig. 10

Variation of joint thickness along the circumference of a “Nb pipe–316L SS flange” brazed assembly fabricated through standardized process

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Fig. 11

Brazed joint thickness profiles across joint length on five different sections of the brazed assembly

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Fig. 12

Cross section of brazed joint (made with standardized procedure) showing full braze penetration

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