Making high-strength steel more hydrogen-resistant
When hydrogen, our planet’s lightest and most abundant atom, is introduced into high-strength alloys such as steel, the metal becomes brittle. Called hydrogen embrittlement, this phenomenon causes the material’s properties to deteriorate, making the metal less ductile and weaker. This leads to cracks. Since steel makes up around 90 % of the metal alloy market, even a small improvement in its properties could have far-reaching effects. The stronger the metal, the more susceptible it is to hydrogen embrittlement. Turning this drawback into an advantage, researchers from Germany’s Max-Planck-Institut für Eisenforschung have developed a counter-intuitive strategy that uses the chemical heterogeneity in the metal’s microstructure to make the material more crack-resistant and inhibit crack growth. With support from the EU-funded SHINE project, they’ve turned the very characteristic that’s usually unwanted because of its detrimental effect on the damage tolerance of steel into a mechanism that enhances the metal’s intrinsic resistance to hydrogen embrittlement. Their research results have been published in the journal ‘Nature Materials’.
The secret’s in the buffer zones
The team applied their strategy to a lightweight, manganese-containing high-strength steel, generating a high dispersion of manganese-rich zones inside the material’s microstructure. “The well-designed local variations in composition serve to enhance crack resistance locally, creating buffer zones that arrest hydrogen-induced microcracks that otherwise would rapidly propagate inside or along hydrogen-attacked phases or interfaces,” the authors explain in the paper. As a result of this method, resistance to hydrogen embrittlement is improved by a factor of two, without forfeiting the material’s strength and ductility. “The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing,” the study reports. To obtain their results, the researchers used Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD), a phase-based approach for predicting thermodynamic, kinetic and other properties of multicomponent materials systems. Aided by CALPHAD, their design of manganese heterogeneity inside the austenite phase resulted in a high density of manganese-rich buffer regions scattered throughout the sample. “During deformation of the alloy, the dynamic transformation from soft austenite to hard martensite is locally suppressed inside these buffer regions by the increased mechanical stability associated with their locally higher … [manganese] content,” the authors state. “As a result, the microstructure evolves into a high dispersion of softer islands embedded in the hard matrix, which frequently renders … [hydrogen]-induced microcracks to be blunted and arrested.” The thermodynamic principle exploited by the researchers to develop microstructures with a specific degree of chemical heterogeneity involves the high kinetic mismatch between phase transformation and solute diffusion generally encountered in alloyed steels. The approach can therefore also be applied to many different high-performance steels containing metastable austenite. It’s also easily scalable to established industrial processes. The strategy developed with SHINE (Seeing hydrogen in matter) support could increase understanding of other advanced metal processing techniques such as powder metallurgy and additive manufacturing. The 5-year project ends in January 2023. For more information, please see: SHINE project web page
Keywords
SHINE, steel, hydrogen, metal, hydrogen embrittlement, manganese, crack resistance
