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The effect of electrochemical charging of hydrogen on the structure of a lean duplex stainless steel LDX 2101® (EN 1.4162, UNS S32101) was examined by both Time-of-Flight secondary ion mass spectrometry and electron back-scatter diffraction. The goal is to correlate hydrogen concentration and induced structural changes. Chemical and structural characterizations were done for the same region at the sample's surface with sub-micron spatial resolution. Regions of interest were varying in size between 50 × 50 μm and 100 × 100 μm. The results show a phase transformation of austenite to mainly a defect-rich BCC and scarcely a HCP phase. The phase transformation occurred in deuterium rich regions in the austenite.
High resolution ToF-SIMS imaging of deuterium permeation and cracking in duplex stainless steels
(2017)
Fundamental understanding and elucidation of hydrogen assisted degradation and trapping mechanisms is dependent on sufficient imaging techniques for respective hydrogen interactions, in particular with multi-phase metallic microstructures. The present work shows the progress in elucidating the deuterium behavior in austenitic-ferritic duplex stainless steels under the consideration that deuterium behaves in many ways similarly to hydrogen. A novel combination of deuterium permeation and in-situ Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) imaging technique is compared with post charging ToF-SIMS imaging experiments. As a step beyond state-of-the-art, integration of chemo-metric and high resolution structural characterization techniques with computational multivariate data analysis (MVA) and data fusion is presented.
The use of duplex stainless steels (DSS) in energy related applications is well known. Nowadays, DSSs become more favorable than austenitic steels due to the outstanding mechanical properties, the good corrosion resistance and the lower nickel content.However, the use of the duplex grade in acidic environments such as seawater can lead to a severe degradation in the structural integrity of the steel by hydrogen-induced/assisted cracking mechanisms, which can eventually result in premature failure. Hydrogen assisted degradation and cracking of steels remains unclear even though this topic is intensively studied for more than a century. The main gap lies in the validation of the proposed theoretical models at the sub-micron scale. Industrial and the research communities define a need for an accurate method by which it is possible to image the distribution of hydrogen in the microstructure. Among the very few available methods nowadays, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) provides precise mapping of hydrogen in the microstructure. Moreover, the powerful combination of ToF-SIMS with multivariate data analysis (MVA), electron backscattered diffraction (EBSD) for providing a the structural information and the use of data fusion techniques can contribute to a better understanding of the hydrogen induced degradation processes in the material.
In the present work two types of duplex grades were chosen as a case study (standard and lean DSS). The duplex class, consist of equivalent amounts of ferrite and austenite, was investigated by ToF-SIMS and EBSD during and after electrochemical deuterium charging in order to simulate the service of a component in acidic environments. Deuterium is known to act on the steel similarly to hydrogen and therefore was used as a tracer for hydrogen. The results show that the ferrite was affected almost identical in both steels whereas in the austenitic phase significant differences were observed in the lean duplex in comparison to the standard duplex. The advantage of the combined techniques is reflected by the ability to correlate the hydrogen distribution in the microstructure to the resulted structural changes.