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Special Section Papers

# Ni Interlayer to Improve Low-Pressure Diffusion Bonding of 316L SS Press Fit Tube-to-Tubesheet Joints for Coiled Tube Gas Heaters

[+] Author and Article Information
Rony Reuven

NRCN,
P.O. Box 9001,
Beer-Sheva 84190, Israel;
Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: Rony.reuven@gmail.com

Alan Bolind

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: bolind@berkeley.edu

Nils Haneklaus

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: nils.haneklaus@rwth-aachen.de

Cristian Cionea

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: cionea@yahoo.com

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: charalampos@berkeley.edu

Grant Buster

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730;
NuScale Power LLC,
1100 NE Circle Boulevard, Suite 200,
Corvallis, OR 97330
e-mail: gbuster@nuscalepower.com

Peter Hosemann

Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: peterh@berkeley.edu

Per Peterson

Mem. ASME
Department of Nuclear Engineering,
University of California, Berkeley,
4118 Etcheverry Hall, MC 1730,
Berkeley, CA 94720-1730
e-mail: peterson@nuc.berkeley.edu

1Corresponding author.

Manuscript received July 28, 2016; final manuscript received January 8, 2017; published online May 25, 2017. Assoc. Editor: Ilan Yaar.

ASME J of Nuclear Rad Sci 3(3), 030913 (May 25, 2017) (6 pages) Paper No: NERS-16-1083; doi: 10.1115/1.4035725 History: Received July 28, 2016; Revised January 08, 2017

## Abstract

This study suggests a new approach to diffusion bonding (DB) 316L stainless steel: a low-pressure procedure that includes a nickel interlayer. In this approach, relatively lower pressure is applied to the sample before the DB process, in contrast to the usual approach in which higher pressure is applied during the DB process. This new procedure was tested on mock-up 316L stainless steel tube-to-tubesheet joints, which simulated similar joints in coiled-tube heat-exchanger applications. This study confirms that the new procedure meets the overall success criteria, namely, a pull-out force exceeding the force required for tube rupture. It also shows that the DB joint is improved by the use of a Ni interlayer; the joint strength increased by approximately 33% for a 0.25 μm Ni interlayer and by approximately 18% for a 5 μm Ni interlayer. The joint cross sections were qualitatively examined using optical microscopy (OM) and scanning electron microscopy (SEM); the observations suggest that only portions of the interface were diffusion bonded, as a result of the low-pressure procedure and the surface roughness (due to the sample fabrication). The portions that were diffusion bonded, though, were sound, as characterized by the fact that the steel grains grew through the interface line to create a continuous metallographic structure.

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## Figures

Fig. 1

Schematic description of the CTGH (right) with enlarged area of the tubes connected to the hot inlet manifold (left) and picture of the tube-to-tubesheet joint sample (in the left corner)

Fig. 2

Technical drawing (in inches) and picture of the tube-to-tubesheet joint sample

Fig. 3

Pictures of the sample and the adaptor (left) and the sample setup on the MTS testing machine (right) for the pull-out test

Fig. 4

Pictures of the rod pull-out areas of the reference sample (left), the WNS interlayer sample (center), and 5 μm sample (right). The 5 μm sample and the reference sample show smaller and much smaller areas of sound DB than the WNS interlayer sample shows, respectively.

Fig. 5

SEM pictures of the DB interface of a reference sample (left) and a 5 μm Ni interlayer sample (right) showing the result of surface roughness induced by the surface machining in cross section and partial DB on the matching interface

Fig. 6

OM picture where sound DB was observed; the material grains grow through the tube-to-tubesheet interface line to create a continuous metallographic structure

Fig. 7

SEM/EBSD pictures; partial DB area (left) and sound DB area (right)

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