NC-24004 Understanding H-embrittlement mechanisms in additive manufactured stainless steels

Project leader: prof. dr. Bojan Podgornik
 

Project summary

Properties of common alloys are detrimentally affected by exposure to hydrogen, which lead to hydrogen induced ductile to brittle transition known as hydrogen embrittlement (HE). The magnitude of the deterioration depends on the material and its microstructure, the specific environment, mechanical loading and internal stresses developed during manufacturing. Microstructure of steels, despite having identical chemical composition, deviate significantly when using different processing technologies. The characteristics of laser-based AM processes are high temperature gradients, residual stresses, anisotropy, highly complex and hierarchical microstructures with high dislocation density and chemical inhomogeneity. These specifics can have a critical impact on HE susceptibility. HE can occur through a variety of mechanisms, however, when it comes to AM materials exact underlying mechanisms are still a subject of intensive research and discussion.

The main challenges related to production, storage, transport and use of green hydrogen as a future energy carrier is hydrogen embrittlement of commonly used material - stainless steel, and how HE differs depending on its type, microstructure and production. To address these questions the proposed research project within the area of materials for new energy technologies is primarily focused on defining and understanding the mechanisms of hydrogen embrittlement (HE) in two representative 3D printed stainless steels (austenitic and martensitic) and how they differ as compared to conventionally produced ones.

Project Goals

While green hydrogen represents a key component in a future climate-neutral society, its production, storage, transport and use as a future energy carrier place high demands and safety concerns for suitable materials. This is especially critical when it comes to high-performance additive manufactured (AM) metal alloys, characterized by fine columnar grain microstructure, high dislocation density, micro-segregations, defects and anisotropy. The mechanical properties of metallic materials are detrimentally affected by exposure to hydrogen, which lead to hydrogen embrittlement (HE). The operational conditions in energy production as well in other industries can lead to HE and possible corrosion thus causing sudden and unpredictable failure of structures. HE can occur through a variety of mechanisms, however, exact underlying mechanisms are still being discussed and not understood when it comes to different corrosion resistant stainless steels produced by AM. Characteristics of laser-based AM processes are the repetition of heating, melting, solidification and re-melting with high temperature gradients, which results in high residual stresses, anisotropy, highly complex and hierarchical microstructures with high dislocation density and chemical inhomogeneity. These specifics combined with the type of stainless steel can have a critical impact on HE as well as on its detection.

The main challenges related to production, storage, transport and use of green hydrogen are hydrogen embrittlement and stress corrosion cracking, and how these differ depending on the type and production of stainless steel. To address these challenges directly related to materials for new energy technologies, project focuses on the following goals:

- to analyze behavior of 3D-printed stainless steels in hydrogen-rich environments and assess application capability

- to identify and optimize experimental methods for monitoring of the crack initiation and propagation due to hydrogen embrittlement

- to thoroughly understand the mechanisms of hydrogen embrittlement (HE) in stainless steels and how they differ depending on the stainless steel type and processing route (conventional vs. AM)

- to identify the major parameters affecting HE susceptibility as a function of the microstructure

Methodology and concepts

Project proposal within the field of new energy technologies/materials, combines five activities aimed to identify and understand the mechanisms and susceptibility to hydrogen embrittlement in additively manufactured stainless steels as compared to conventionally produced and depending on the type of stainless steel (microstructure and phases). The project articulates around 5 research axes, detailed below, which lay the foundations for an in-depth investigation of HE mechanisms of AM stainless steels.

Activities will include:

(i) laser-powder-bed-fusion manufacturing, pre- and post-treatment, and establishing the correlation between H-charging parameters and the actual H-concentration for typical stainless steels (austenitic, martensitic), manufactured conventionally and by 3D printing;

(ii) detailed examination and characterization of microstructure and H-related microstructural changes, to qualify and quantify the distribution of the microstructural defects and their evolution in presence of hydrogen and during straining and to address subsequent hydrogen distribution evolution in terms of hydrogen diffusion and trapping (ONH Elemental analyser, ICP-OES, TDS, SEM, FIB-SIMS, FIB-DIC, EBSD, XRD, AES, XPS, TEM, APT);

(iii) testing and identification of H exposure and diffusion on steel properties and behavior under load. Tests will include the determination of common mechanical properties (hardness, toughness, tensile strength) after H-charging, predominantly stress-strain behavior at slow strain rate testing (SSRT) in air and in electrolyte under additional cathodic H-charging, as well as fatigue (bending load) and anti-wear properties (abrasive wear resistance);

(iv) identifying suitability of different methods for monitoring and detection of HE related crack initiation and propagation, as well as properties deterioration (electrochemical noise (EN), acoustic emission (AE), optical monitoring (DIC), Micro-computer-tomography, hydrogen microprint technique), and

(v) defining and understanding HE mechanisms in additively manufactured stainless steels and how they differ as compared to conventional ones. Results of hydrogen distribution/redistribution and HE crack monitoring, post-exposure/post-fracture detailed microstructural examination and characterization together with the understanding of hydrogen diffusion and trapping (distribution) during straining will serve as a basis to define and understand HE mechanisms in the different stainless steels. The general methodology adopted follows the identification of elementary bricks in the potential mechanisms, then addressing the mechanisms of these elementary steps and their kinetics, and progressively complexify the system towards a more representative one (Cartesian approach).

Project team

Project is financed by Slovenian Research and Innovation Agency - ARIS with 2016 hours (C group): 1. 1. 2024 – 31. 12. 2025.