Investigation of mechanical behavior and microstructural characteristics of diffusion bonded 316H stainless steel for compact heat exchanger applications

Compact heat exchangers (CHX) are key components for advanced energy systems that require high heat-transfer capacity at elevated temperatures. Diffusion bonding has been an effective technique for the fabrication of joints in heat exchanger applications. However, the structural integrity of the diffusion bonded joints is critical. Type 316H austenitic stainless steel has been a candidate material for CHX applications for their creep resistance and high temperature mechanical performances. In this study, nine different diffusion bonding conditions for 316H stainless steel have been investigated using mechanical testing and microstructural analysis. Tensile tests and microtensile tests were performed at room temperature and 760℃ on the diffusion bonded and wrought 316H SS specimens to analyze the bond strengths in comparison to the wrought samples. The room temperature tests showed promising results for several trials including CompRex Trial 5 and Trial 6. However, the elevated temperature results showed that the CompRex Trial 6 (bonding carried out at 1150℃ with 8 MPa pressure and 4 hours of holding time) displayed the best combination of strength and ductility among all the bonding conditions. The microtensile tests performed at room temperature also showed ductile failures and the failures did not occur on the bond interfaces. Digital Image Correlation (DIC) was also used to characterize the material deformation over time. Creep test was performed on the CompRex Trial 6 316H SS at 760℃ with 75 MPa stress, and results showed good fraction of secondary creep before moving to tertiary creep regime. Microhardness tests conducted on the different regions of the gauge section of creep sample showed that the brittle failure mode resulted from the creep strain hardening in the bulk region rather than weak debonding of the bond interfaces. Along with that, shear punch tests performed at room temperature and 760℃ showed that no bond delamination occurred under shear stress. Dye penetrant tests were also helpful in identifying some weaker bonds in the gauge section. The SEM fracture surfaces of the tensile tests showed that the CompRex Trial 6 underwent ductile fracture at elevated temperature which is more comparable to the wrought fracture surface. EBSD performed in the region across the bond interfaces have shown signs of grain growth in CompRex Trial 5 and Trial 6. For the other trials, the tensile results were not promising enough as they showed brittle debonding on the bond interfaces. TEM and STEM EDS were performed to identify any precipitate or cavities across the bond interfaces. The CompRex Trial 6 316H SS did not show any signs of precipitates on the bond interfaces, while STEM EDS of the CompRex Trial 5 showed small presence of Al2O3 (alumina) particle near the bond interfaces. In addition to that, optical microscopy and MIPAR analysis was also carried out on CompRex Trial 4, 5 and 6 bonds. MIPAR results showed higher percentage of grains penetrating the bond interfaces for CompRex Trial 6 in comparison with other trials.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Omi, Intisher Al-Tahmid

Contributors dc:contributor

• Stubbins, James F
• Xi, Jianqi

• Compact heat exchangers
• diffusion bonding
• 316H stainless steel
• grain growth
• Creep
• mechanical properties
• EBSD

Statement dc:rights

• Copyright 2025 Intisher Al-Tahmid Omi

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132814

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132814