Heat treatment of superbainitic steels

Abstract

Two experimental high silicon high carbon steels (with 5 and 24 ppm of boron separately) have been investigated for the development of superbainite structure. After austenitisation, the specimens were held respectively at three different isothermal transformation temperatures (150, 200, and 300°C) for a variety of time intervals. The microstructures were examined via optical metallography (with microhardness measurement) and transmission electron microscopy. It was found that after isothermal transformation at 200°C for 10 days, both steels produced a high volume fraction of sheaf structures with nanometre scaled bainitic ferrite subunits, which contributed to an ultrahigh microhardness, up to 675 HV. It was also found that adding 24 ppm of boron accelerated the bainitic transformation in the early stage of isothermal transformation at 200°C, but did not have a significant effect on reducing the finish transformation time. Both isothermal temperature of 150 and 300°C could not lead to the development of high amount of bainite.

Keywords:

• Bainite
• Boron
• Heat treatment
• Phase transformation
• Nanostructure
• TEM

The present work was carried out with the financial support from the National Science Council, Taiwan, under contract no. NSC 98-2221-E-002-055-MY3.