Abstract
Electron microscopy, X-ray diffraction, and atom probe tomography have been used to identify the changes which occur during the tempering of a carbide-free bainitic steel transformed at 473 K (200 °C). Partitioning of solute between ferrite and thin-films of retained austenite was observed on tempering at 673 K (400 °C) for 30 minutes. After tempering at 673 K (400 °C) and 773 K (500 °C) for 30 minutes, cementite was observed in the form of nanometre scale precipitates. Proximity histograms showed that the partitioning of solutes other than silicon from the cementite was slight at 673 K (400 °C) and more obvious at 773 K (500 °C). In both cases, the nanometre scale carbides are greatly depleted in silicon.
Highlights
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]
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-1.06Al1.59Co wt pct.[9]
The fine plate size provides the dominant strength contribution, so high hardness is maintained until coarsening of the bainitic ferrite occurs.[12,13]
Summary
PROCEDURES for the design of bainitic nanostructured steels in bulk form have been established.[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) 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-1.06Al1.59Co wt pct.[9]. PROCEDURES for the design of bainitic nanostructured steels in bulk form have been established.[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) 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]. Softening may be necessary to allow thermo-mechanical processing.[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]
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