Lighter and stronger steels obtained by flash processing


Setup of the flash processing of a steel sheet.

The art of steel making is known for several thousands of years and the modern steel making process can be associated to the 17th century. Hence, it comes as a surprise to relate the production and treatment of steel with innovation processes leading to lighter, stronger and more fuel-efficient vehicles. This is, however, what is claimed by researchers from Ohio State University and the US American company Bainite Steel. In a recent research article published in “Material Science and Technology“ they describe a new steel hardening method taking less than 10 seconds  instead of hours or days. In addition, it is claimed that the obtained steel is stronger and more shock-absorbing than the most common titanium alloys currently used in industrial applications.

The flash process originally applied by Bainite Steel involves a rapid  heating step at 1100 °C followed by cooling in a liquid bath leading to a steel microstructure with  a high tensile strength and good ductility. The authors determined the ultimate tensile strength (UTS), this is the maximum stress that a material can withstand while being stretched or pulled before necking, and the elongation of the flash processed steel and compared the figures to published values of martensitic advanced high strength steel (Martensitic steels are low carbon steels containing 12% chromium and 0.12% carbon.;  Martensitic steel is characterized by a large hardness, but reduced toughness and brittleness). From the respective figures, flash processed steel (1694 MPa, 7 %) martensitic high strength steel (1585 MPa, 5 %), it was claimed that the flash processed steel  shows at least 7% higher UTS and 30% greater elongation than the martensitic high strength steel.

By applying standard microscopy techniques (optical, scanning electron, transmission electron microscopy) the authors the underlying physics of the determined characteristics. In traditional slow heat treatments initially austenite microstructures form which usually transform into a martensic microstructure throughout the rapid cooling process. The presence of these structures could be also validated for the flash processed steel, but additionally one could identify another microstructure called bainite, scattered with carbides (carbon-rich compounds).

The authors attribute the persistence of the bainite structures to the rapid heating and cooling, so that the carbides do not get a chance to dissolve completely within austenite at high temperature. Hence,  they remain in the steel and make this unique microstructure containing bainite, martensite and carbides.

Source:

Lolla, T; Cola, G; Narayanan, B; Alexandrov, B; Babu, S S

Development of rapid heating and cooling (flash processing) process to produce advanced high strength steel microstructures

Materials Science and Technology, 27(5): 863-875  May 2011

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