Hydrogen Interactions in an Additively Manufactured Martensitic Stainless Steel

Additive manufacturing (AM) has emerged as a compelling avenue for producing metallic components such as stainless steels, offering several promising advantages over conventional manufacturing methods. Despite numerous studies examining the mechanical behavior and corrosion properties of additively manufactured stainless steels, there is a notable lack of research on hydrogen behavior within these alloys. Given the substantial microstructural differences between additively manufactured and conventionally manufactured materials, as well as the often pivotal role of these features in hydrogen interactions (i.e., uptake, absorption, trapping, and diffusion), identical hydrogen behavior in additively manufactured and wrought materials cannot be assumed. Additionally, increased susceptibility to hydrogen environmentally-assisted failure has been observed in AM alloys such as 17-4 PH stainless steels compared to their wrought counterparts. However, a mechanistic understanding of the role of hydrogen in this increased susceptibility is lacking.

The primary objective of this work is to provide insight into hydrogen behavior in additively manufactured 17-4 PH stainless steel compared to its compositionally equivalent matched-strength wrought counterpart. This investigation employs a comprehensive set of corroborating analysis methods to elucidate the impact of microstructure on hydrogen-metal interactions, as well as to assess the contribution of these interactions towards the enhanced susceptibility to environmentally-assisted cracking in AM 17-4 PH. This overarching goal has been realized through a scientific inquiry into the following key areas: (1) analysis of bulk hydrogen behavior in the wrought and AM alloys, involving the determination of effective hydrogen diffusivity and assessments of diffusible and total hydrogen concentration; (2) characterization and comparison of microstructural trap states influencing bulk hydrogen behavior in wrought and AM 17-4 PH; and (3) elucidation and modeling of the relationship between selected hydrogen materials parameters, such as the critical diffusible hydrogen concentration and effective hydrogen diffusivity, and subsequent environmentally-assisted cracking kinetics. The fulfillment of these three objectives contributes significantly towards addressing the knowledge gaps regarding hydrogen in additively manufactured stainless steels.

Bulk hydrogen behavior of wrought and AM 17-4 PH is assessed through characterization of effective hydrogen diffusivity as well as total hydrogen concentration, diffusible hydrogen concentration, and the critical threshold hydrogen concentration required for cracking. Effective hydrogen diffusivity of wrought and AM 17-4 PH in the peak-aged and over-aged conditions is determined through both electrochemical and thermal methods. AM alloys in both heat treatments display increased effective hydrogen diffusivities compared to their wrought counterparts. Additionally, analysis of electrochemical permeation and thermal desorption data indicates that reversible trapping has a significant impact on hydrogen diffusion in both wrought and AM alloys.

An examination of diffusible and total hydrogen concentration is established and likely metallurgical trap states are identified. Microstructural characterization is used to provide a systematic comparison of the microstructural attributes responsible for hydrogen trapping in each alloy. Results indicate that, though the vast majority of hydrogen is trapped in low-temperature reversible trap sites in all alloys, there is a significant increase in reversibly trapped hydrogen in the wrought alloys. These reversible sites likely correspond to high-angle grain boundaries, which are more numerous in the wrought specimens due to finer grain size, as well as Cu-rich precipitates originating from the aging process.

This trapping analysis indicates that grain size may have the highest contribution to the difference in bulk hydrogen behavior in AM and wrought 17-4 PH. Given the strong correlation discovered between grain size and effective diffusivity, an Oriani-type trapping model is employed as a framework to scrutinize the impact of grain boundary density and binding energy on effective diffusivity. A two-phase composite diffusion model is then used to examine grain boundary diffusion behavior in wrought and AM 17-4 PH, revealing a slight increase in grain boundary diffusivity in the AM specimens; possible origins are discussed.

The influence of such hydrogen behavior in determining hydrogen environmentally assisted cracking (HEAC) kinetics is investigated by assessing the relationship between crack growth kinetics and diffusible hydrogen concentration. The agreement of concentration-dependent HEAC behavior with an existing crack growth rate kinetics model is analyzed, and the impact of effective hydrogen diffusivity and critical diffusible hydrogen concentration on the heightened HEAC susceptibility of AM 17-4 PH in both peak-aged and over-aged conditions is discussed. Results suggest that the increased crack growth kinetics in AM alloys can be attributed, at least in part, to the increased diffusivity as well as a reduction in the critical diffusible hydrogen concentration necessary for cracking.

In summary, this thesis contributes to the scientific understanding of hydrogen behavior in additively manufactured alloys, establishing assessment methodologies and providing clarification as well as insight into the interplay between microstructure and hydrogen behavior in complex AM microstructures. The development of a methodology to appraise the potential ramifications of hydrogen interactions on environmentally-assisted crack growth kinetics contributes to the current understanding of hydrogen-assisted failure criteria. The conclusions presented in this thesis emphasize the importance of understanding hydrogen behavior in AM materials and provide a basis for future studies aimed at developing reliable and durable additively manufactured alloys for widespread usage.