High energy X-rays reveal the resistance of Sandvik steels to embrittlement

Researchers from Sandvik and the Royal Institute of Technology (KTH) in Stockholm used high-energy X-rays at the Swedish Material Science beamline to find out why our steels are more resistant to hydrogen embrittlement than competitors steels. It was possible to see what was happening within the steel structure in real-time, in environments that mimic how the steel is used.

In offshore installations, cathodic protection is used which exposure the steel to hydrogen, which can make it brittle.

One of the challenges in offshore environments is exposure to hydrogen, which makes steels brittle – a problem identified as long ago as 1875. The hydrogen can enter the material when it is subjected to cathodic protection offshore. During cathodic protection, hydrogen is formed on the surface when water decomposes.

Jinshan Pan, Professor in Corrosion Science from KTH.

Researchers have since worked to develop steels that are more resistant to hydrogen embrittlement. Such resistance to embrittlement was, therefore, a key aspect of duplex steels developed by Sandvik and are now sold worldwide –with users recognizing that Sandvik steels are longer-lasting than competitors’ steels when used offshore. However, researchers couldn’t earlier fully explain why our steels performed better. It wasn’t possible to understand the details of hydrogen-microstructure interaction that leads to embrittlement with ordinary laboratory research techniques.

The explanation emerged in one of the first uses of the Swedish Material Science beamline in 2019. At the beamline, high-energy X-rays can penetrate steels, enabling researchers to map the hydrogen-induced changes in structure within steels and how those structural changes vary over time in environments that mimic how the steel is used.

Researchers from Sandvik and KTH exposed the steel samples to hydrogen and placed these steels under stress and studied how hydrogen rapidly moves within and interacts with the steel, as well as how the steel’s internal structure evolved.

Microstructure of a super duplex stainless steel. Austenite in red and ferrite in blue contrast. Scanning electron micrograph.

Duplex stainless steels consist of two phases: the body-centered-cubic (bcc) phase and the face-centered-cubic (fcc) phase. Both fcc and bcc phases are distributed in a complex pattern within the steels. Measurements showed that hydrogen could quickly enter the duplex steel but causes different changes in the two phases.

The fact that these phases interact so differently with hydrogen is important: the boundary regions between the two phases accommodate more hydrogen and can withstand more internal stress. The coordinated response of the two phases hinders the initiation of micro-cracks, thus providing high resistance to embrittlement.

Experimental setup of the real-time diffraction measurement.

The experiments at the Swedish Material Science beamline were the first time that these hydrogen-microstructure interactions were observed in real-time. The results of the experiments also showed that with their finer microstructure, Sandvik duplex steels are significantly more resistant to embrittlement than competitor steels.

Researchers are now able to explain the phenomenon of hydrogen embrittlement of duplex stainless steels scientifically. This case illustrates how advancing scientific knowledge, solving old problems, and supporting industry, can go hand in hand.

High energy X-ray diffraction measurement of duplex stainless steel at PET-RAII in DESY, Hamburg. Ulf Kivisäkk, Senior Expert Corrosion Resistant Alloys at Sandvik Materials Technology, in red sweater, participated at the Swedish Material Science beamline.

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