Transition from Diffusive to Displacive Austenite Reversion in Low-Alloy Steel

Abstract

For understanding the transition from diffusive to displacive austenite reversion mechanism in steel, the effect of heating rate on austenite reversion behavior was investigated in 0.15%C-5%Mn steel. Austenite reversion temperature first increased gradually with the heating rate owing to the superheating effect and then remained at a constant temperature above a critical heating rate. In response, the austenite formed by rapid heating exhibited a coarse prior austenite grain structure, indicating the occurrence of displacive reversion even in low-alloy steel.KEY WORDS: displacive reversion; transition of reversion mechanism; low-alloy steel; heating rate.Martensitic transformation caused by diffusionless shear takes place athermally in steel when the decomposition of austenite to such products as ferrite, pearlite and bainite is suppressed during cooling. In general, martensitic transformation depends primarily on the cooling rate and composition of steel. A high cooling rate and/or the addition of alloying elements to avoid diffusive reactions results in a significant degree of super cooling. Eventually, the large driving force necessary for martensitic transformation (about 1 200 J/mol) can be obtained at the martensite-start temperature, Ms. In principle, martensitic transformation is reversible. It is known that the reversible transformation from martensite to austenite (martensitic reversion) occurs in maraging steel owing to its high Ni content.1-4) Previously, 5) the authors of the current study directly observed an austenitic structure formed by martensitic reversion (martensitically reversed austenite) in 18%Ni-C (% = wt.%) steel. It was reported that after reheating the sample to a temperature above the martensitic austenite-start temperature, As, all martensite laths originally formed via an fcc→bcc martensitic transformation reverse into new austenite during the bcc→fcc martensitic reversion, thus returning to an original austenitic structure with the same crystallographic orientation. However, the mechanism of this twodirectional transformation is still under debate because such martensitic reversion almost never occurs in low-alloyed steels. The main reason that inhibits such reversibility is the high As peculiar to athermal martensitic transformation.Martensite decomposition (i.e., diffusive reversion and cementite precipitation) occurs easily during reheating toward As because diffusion is easy in low-alloyed steel at elevated temperatures around As. This difference in the reversion mechanism between high Ni or Mn steel (e.g. 18% Ni steel) and low-alloyed steel is schematically shown in Fig. 1 in the form of a continuous heating reversion diagram. In these diagrams, the austenite-start and -finish temperatures for diffusive reversion during heating are defined as Ac 1 and Ac 3 , respectively, for distinguishing them from the austenite-start and -finish temperatures for martensitic reversion, As and Af. Although diffusive reversion is preferential, upon observing Fig. 1(b), one can expect that the...

Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel

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Microstructure Evolution during Reverse Transformation of Austenite from Tempered Martensite in Low Alloy Steel

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Analysis of retarding effect on α to γ transformation in Fe–C alloy by addition of dispersed particles

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