Researchers uncovered new information regarding carbon diffusion control in high-strength steel production

The new generation advanced high-strength steels show promise when solving the challenges of light-weight design strategies and related fuel and emission saving applications. However, quantitative controls of steel quenching and partitioning, and industrial implementations are still at the early stage. It is mainly ascribed to the lack of a comprehensive understanding of the process down to quantum mechanical level.

The researchers from Nano and Molecular Systems research unit (NANOMO) and Materials and Mechanical Engineering research unit, as well as from Canadian Light Sources, have collaboratively employed advanced in-situ synchrotron X-ray diffraction and density functional theory calculations to reveal the physics of carbon diffusion involved phase transformations in advanced carbon steel during quenching and partitioning. The results are promising in the pursuit of better management of steel properties.
Researchers presenting some steel processing practices in the laboratory.

Hardening and strengthening of steel appear to have actively been in operation in common use since early Greeks. The ‘ancient’ smiths knew, from years of experience, how to control the final hardness of the steel by quenching. The lack of fundamental knowledge of the quenching phenomenon did not prevent the artisans to try different quenching conditions combining other heat treatments. Nowadays, the adoption of innovative quenching and partitioning processing routes has even dramatically increased the strength of carbon steels with a reasonable high ductility and toughness.

Thanks to the modern microstructural characterization methods, we have figured out that the excellence of quenching and partitioning processed steels result from the martensitic matrix and the transformation induced plasticity effect of retained austenite under loading forces. Carbon diffusion mediated phase transformations is the most important mechanism in quenching and partitioning processing and can be tuned in order to economically engineer the steel microstructures. However, the C diffusion remains relatively stationary due to the post-mortem characterization methods and yet deviates from the dynamic nature of diffusion.

Recently developed synchrotron radiation methods placed potentials for in-situ measurements and monitoring of phase transformations. In their latest work, the researchers from Nano and Molecular Systems research unit (NANOMO) and Materials and Mechanical Engineering research unit (University of Oulu), as well as from Canadian Light Sources (University of Saskatchewan) have successfully quantified the time- and temperature-evolved phase transformation by using advanced in-situ synchrotron X-ray diffraction. Carbon enrichment into the untransformed austenite tended to simultaneously turn up with the later stage of martensitic transformation at the existing austenite/martensite interfaces under the applied air-quenching rate. More importantly, employing the experimental obtained data as inputs, the quantum mechanics calculations revealed the existence of secondary energy barriers in ferritic phases facilitating carbon diffusion into austenite. More complicated diffusion path and probable hopping sites in the ferritic phases than in the austenitic phase also assisted to elucidate the changes of crystal structure ascribed to the rapid carbon diffusion.

The results from this work show great promise, as they show a way to improve the control on the quenching and partitioning process. This better control level can be reached by implementing the measured temperature- and time-resolved carbon diffusion influenced phase transformation. This can make the production of steels more energy-efficient and environmentally friendly to comply with the commitment of Carbon neutral Finland in 2035 and the UN Sustainable Goals.

The current research progress strengthens the collaborations within the University of Oulu´s research profile field Physics for sustainable steel. As members of the Centre for Advanced Steels (CASR), the NANOMO and the Materials and Mechanical Engineering Research Unit benefit the cross-disciplinary research in physical explorations of materials and research field of the manufacturing of advanced high strength steels.

The work is published in top engineering journal Acta Materialia.

Shubo Wang, Andrey A. Kistanov, Graham King, Sumit Ghosh, Harishchandra Singh, Sakari Pallaspuro, Al Rahemtulla, Mahesh Somani, Jukka Kömi, Wei Cao, Marko Huttula. In-situ quantification and density functional theory elucidation of phase transformation in carbon steel during quenching and partitioning, Acta Materialia, Vol. 221, 2021, 117361.

Photo: University of Oulu researchers Professor Marko Huttula, Professor Jukka Kömi, Professor Mahesh Somani, Postdoctoral researcher Sakari Pallaspuro, Postdoctoral researcher Harishchandra Singh, Postdoctoral researcher Andrey Kistanov, Postdoctoral researcher Shubo Wang and Associate Professor (Tenure Track) Wei Cao.

Last updated: 11.11.2021