Skip to main content
U.S. flag

An official website of the United States government

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Https

Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Nitrogen effects in additively manufactured martensitic stainless steels I: Conventional thermal processing and comparison with wrought

Published

Author(s)

Eric Lass, Carelyn E. Campbell, Fan Zhang

Abstract

The microstructures or additively manufactured (AM) precipitation-hardenable stainless steels 17-4 and 15-5 were investigated and compared to conventionally produced material. The residual nitrogen found in N2-atomized 17-4 powder feedstock is inherited by the additively produced material, and has dramatic effects on phase stability, microstructure, and microstructural evolution. Nitrogen is a known austenite stabilizing element, and the as-built microstructure of AM 17-4 can contain up to 90 % or more retained austenite, compared to the nearly 100 % martensite structure of wrought 17-4. Even after homogenization and solutionization heat treatments, AM 17-4 contains 5 % to 20 % retained austenite. In contrast, AM 15-5 and Ar-atomized AM 17-4 contain <5 % retained austenite in the as-built condition, and this level is further decreased following post-build thermal processing. Computational thermodynamics-based calculations do not seem to capture the extent of the experimentally observed depression in the martensite start temperature and martensite stability as a function of N-content. A significant increase in the volume fraction of fine-scale carbide precipitates attributed to the high N-content of AM 17-4 is also hypothesized to give rise to additional activation barriers for the dislocation motion required for martensite nucleation and subsequent growth. An increase in the volume fraction of carbide/nitride precipitates is also observed in AM 15-5, although they do not inhibit martensite formation to the extent observed in AM 17-4.
Citation
Metallurgical Transactions A-Physical Metallurgy and Materials Science
Volume
51

Keywords

additive manufacturing, precipitation hardening, martensitic stainless steel, martensite, retained austenite
Created March 9, 2020, Updated April 14, 2020