Date on Master's Thesis/Doctoral Dissertation

5-2022

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Degree Program

Mechanical Engineering, PhD

Committee Chair

Atre, Sundar

Committee Co-Chair (if applicable)

Kate, Kunal

Committee Member

Kate, Kunal

Committee Member

Berfield, Thomas

Committee Member

Gupta, Gaumtam

Author's Keywords

L-PBF; stainless steel; 25Cr7Ni; corrosion properties; hot isostatic pressing; nitrogen

Abstract

Stainless steel is a low carbon high alloyed system with higher concentrations of Cr& Ni, which impart high corrosion resistance to them. Alloys with approximately 25% Cr & 7% Ni in their chemical composition are commercially referred to as ‘Super Duplex Stainless Steel’. They have a unique phase composition of approximately 50% ferrite & 50% austenite, yielding a robust combination of high mechanical strength & corrosion resistance. They find extensive interest & application in the fields which demand a longer service life under intense mechanical / corrosive environment such as offshore oil rigs & pipelines in nuclear power plants. Traditional thermal processing and fabrication of super duplex stainless steel are fraught with limitations and shortcomings in terms of detrimental phase formation. Laser-Powder Bed Fusion is a form of additive manufacturing that involves layer wise addition and consolidation of metal powders in near net shape parts. The process is characterized by high cooling rates to the tune of 107 k/s. This unique characteristic allows for the suppression of formation of detrimental phases and is leveraged in processing of super duplex stainless steels. The available literature on L-PBF fabrication of super duplex stainless steel in comparison to conventional stainless steel alloys is quite lacking. This study quantitively established the influence of the Laser-Powder Bed Fusion (L-PBF) process parameters, starting powder attributes, chemical composition, inert atmosphere & Hot Isostatic Pressing (HIP) on the as-printed properties of the fabricated super duplex stainless steel samples. As-printed samples of a gas atomized super duplex stainless steel yielded the highest UTS, yield strength and comparable corrosion resistance to wrought-annealed, MIM, PM, L-PBF literature super duplex stainless steel. Economical water atomized super duplex stainless steel powder was used to fabricate samples which had higher UTS, yield strength & comparable corrosion resistance to wrought-annealed stainless steel.

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