Impact of Atomization and Processing Gas on the Heat Treatment Response of Additively Manufactured 17-4 PH Stainless Steel

Open Access
Author:
Meredith, Scott
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 18, 2018
Committee Members:
  • Todd Palmer, Thesis Advisor
  • Allison Michelle Beese, Committee Member
  • Reginald Felix Hamilton, Committee Member
  • Jayme Scot Keist, Committee Member
Keywords:
  • stainless steel
  • precipitation hardening (PH)
  • additive manufacturing (AM)
  • nitrogen
  • gas atomization
  • aging
Abstract:
Precipitation hardened (PH) grade martensitic stainless steels are commonly used in additive manufacturing (AM) processes. In order to obtain properties similar to their wrought counterparts, post-processing solutionizing and aging heat treatments are required. Depending on the powder feedstock composition, which can be varied by the choice of atomization gas and, to a lesser extent, the processing gas during component fabrication, the post-process heat treatment response can be significantly altered. When the standard heat treatment cycles developed for wrought alloys are applied to as-deposited 17-4 PH grade stainless steel structures fabricated from argon or nitrogen atomized powder feedstocks on different powder bed fusion (PBF) systems, the AM components exhibited a difference response. Argon atomized feedstocks contain approximately 0.01 wt.% nitrogen, possess low levels of retained austenite, and respond as expected to standard solutionizing and aging heat treatment cycles. In contrast, 17-4 PH grade stainless steel structures fabricated using nitrogen atomized feedstocks with higher nitrogen levels (0.06 – 0.14 wt.%) and retained austenite levels (up to 81%) do not respond to standard solutionizing and aging techniques. Peak aging at these high nitrogen levels occurs at a temperature of approximately 680°C, which is significantly higher than the standard peak aging heat treatment at 482°C.