Year
2024
Season
Fall
Paper Type
Master's Thesis
College
College of Computing, Engineering & Construction
Degree Name
Master of Science in Material Science & Engineering (MS)
Department
Engineering
NACO controlled Corporate Body
University of North Florida. School of Engineering
First Advisor
Dr. Jutima Simsiriwong
Second Advisor
Dr. Grant Bevill
Third Advisor
Dr. Gregory Wurtz
Abstract
Additive manufacturing (AM) for stainless steel components has proved useful in a variety of industries ranging from aerospace to biomedical engineering. Many of these components are subject to very high cycle fatigue (VHCF) loading and must have tailored mechanical properties to ensure failures do not occur before a service life is reached. Testing the fatigue strength AM materials exposed to VHCF loading is paramount to compare with the expected life cycles of their wrought counterparts. In this thesis, AM 17-4 precipitation hardened (PH) stainless steel (SS) were fabricated using laser powder bed fusion (L-PBF) technique, while the wrought 17-4 PH SS specimens were manufactured using conventional methods. AM fatigue specimens were subjected to one of three heat treatments, including non-heat treated (NHT), CA-H900, and CA-H1025 to modify the microstructure and mechanical properties of the material. The influence of using either argon (Ar) or nitrogen (N2) as the shielding gas during L-PBF for AM specimens was also investigated. Wrought specimens were either left in the annealed state or given either an H900 or H1025 heat treatment.
The ultrasonic fatigue system used in this study applied uniaxial, fully reversed force-controlled force tests at 20 kHz to hourglass shaped specimens. Following ultrasonic testing, specimen fracture surfaces were inspected using a scanning electron microscope (SEM). Results revealed each post processing treatment method had an influence on the material’s fatigue life, crack initiation, and crack propagation characteristics. AM specimens showed a significant increase in fatigue life from heat treating to the CA-H900 condition when compared with those subjected to the NHT or CA-H1025 heat treatments. Using N2 as the shielding gas for the L-PBF process revealed a reduction in defect population and δ-ferrite when related to specimens fabricated using Ar as the shielding gas. Wrought specimens displayed improvement in fatigue life from H1025 heat treatment and consistently failed from inclusions, while their annealed counterparts consistently failed from pores. Experimental results revealed that post processing techniques for 17-4 PH SS must be carefully selected based on the mechanical loading the material will be exposed to over its service life.
Suggested Citation
Welsh, Jade, "Influence of heat treatment and shielding gas environment on the very high cycle fatigue behavior of 17-4 precipitation hardened stainless steel" (2024). UNF Graduate Theses and Dissertations. 1307.
https://digitalcommons.unf.edu/etd/1307
