With the industrial support from TWI Ltd, researchers from Lancaster University and the University of Leicester used ENGIN-X to study hydrogen embrittlement (HE) in super duplex stainless steels (SDSSs). By tracking the changes to the structure of the material using neutron diffraction at the same time as doing tensile testing, the group was able to see how the different material structures in the sample behave compared to one another. Using their results, they were able to propose a model that could be developed further for assisting industrial practice.
“Corrosion resistant alloys are widely used in safety-critical applications where resistance to environmentally-assisted cracking is of utmost importance. For duplex stainless steels, one of the most unpredictable, and often catastrophic failure modes, is hydrogen-induced stress cracking (HISC)," explains Mike Dodge, from TWI.
These steels have a balanced microstructure composed of two different structures: ferrite and austenite. This gives good corrosion resistance and mechanical properties that led to its widespread use in offshore industry. However, these mechanical properties of SDSSs can be degraded by hydrogen and, if a critical concentration of hydrogen is adsorbed by the steel, it can result in premature failure; a process known as hydrogen embrittlement.
In this study, published in Materialia, neutrons were used to study the bulk of the material, finding that austenite and ferrite react differently to hydrogen embrittlement. Austenite maintained good plasticity during loading, whilst ferrite showed a loss of plasticity. The researchers also studied the effect of specimen thickness, and used their results to inform a model that predicts the effect of hydrogen embrittlement in duplex stainless steels.
The model that was developed and used in this study incorporated hydrogen diffusivity, hydrogen solubility, residual stress, temperature and load to describe the experimental results. It has the potential to be developed further, assisting industrial practice to assess the effect hydrogen embrittlement and predict the service life of steels, as well as informing the microstructural design of new duplex stainless steels.
"TWI is actively working with its Industrial Members, relevant standard bodies/committees, and academic partners to develop reliable and reproducible methods of assessing materials with improved resistance to HISC, taking into consideration the combined effects of microstructure, environmental conditions, and stress/strain." Mike Dodge, from TWI, adds; "Engin-X allows us to 'see' how these three variables interact, which helps to determine the threshold conditions at which cracks initiate. Ultimately, this allows us to provide guidance on how to design safer components."
The full paper can be found online at DOI: 10.1016/j.mtla.2019.100524