Very strong low temperature bainite

Abstract

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly re® ning the microstructure, which is found to have a strength in excess of 2 . 5 GPa together with an ability to¯ow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographi c details are reported of the very ® ne bainitic microstructure associated with the incredibly low transformatio n temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than~10 17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation .

Properties of fine-grained steels generated by displacive transformation

“…An unconventional, carbide-free steel has recently been invented which on close examination is found to contain bainitic-ferrite plates as thin as 20 nm, separated by carbon-enriched films of retained austenite [4,[19][20][21][22][23]. This is the hardest ever bainite, which can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing.…”

Tempering of Low-Temperature Bainite

“…[10,11] Previous work has shown that these steels are relatively resilient to tempering. [1,8,12] The fine plate size provides the dominant strength contribution, so high hardness is maintained until coarsening of the bainitic ferrite occurs. [12,13] Development of alloys with increased temperature stability is in progress; this will rely on further suppressing carbide precipitation to increase the stability of the nanostructure.…”

“…[1,2] Isothermal transformation circa 473 K (200°C) then results in a structure of fine bainitic ferrite plates (width 20 to 40 nm) separated by thin-films of austenite. [1,2,5] The fine scale of the structure is a consequence of the displacive transformation mechanism [6] and contributes significantly to the strength and hardness, [7] while retained austenite contributes to the toughness and elongation. [8] Carbide precipitation in a nanobainitic steel has been observed using atom probe tomography to be associated with the formation of carbon-depleted austenite after isothermal transformation at 473 K (200°C) in composition Fe-0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al-1.59Co wt pct.…”

“…[1][2][3][4] Modeling the effects of composition on transformation kinetics enables tailored alloying to allow isothermal transformation at suppressed temperatures, while silicon additions prevent the precipitation of coarse carbides during the bainite transformation. [1,2] Isothermal transformation circa 473 K (200°C) then results in a structure of fine bainitic ferrite plates (width 20 to 40 nm) separated by thin-films of austenite.…”

“…[1][2][3][4] Modeling the effects of composition on transformation kinetics enables tailored alloying to allow isothermal transformation at suppressed temperatures, while silicon additions prevent the precipitation of coarse carbides during the bainite transformation. [1,2] Isothermal transformation circa 473 K (200°C) then results in a structure of fine bainitic ferrite plates (width 20 to 40 nm) separated by thin-films of austenite. [1,2,5] The fine scale of the structure is a consequence of the displacive transformation mechanism [6] and contributes significantly to the strength and hardness, [7] while retained austenite contributes to the toughness and elongation.…”