Peculiar metastable structural state in carbon steel

TL;DR

This study investigates how cooling rates and pre-annealing temperatures influence phase transformations in carbon steel, revealing a peculiar metastable state with nanoscale heterogeneity near phase boundaries.

Contribution

It uncovers a novel metastable structural state in carbon steel formed near phase boundaries, characterized by nanoscale heterogeneity, dependent on pre-annealing temperature.

Findings

Cooling from above A3 leads to different structures depending on T_an.

A special metastable state with nanoscale heterogeneity forms near phase boundaries.

The structural state varies from perlite to martensite based on T_an.

Abstract

The kinetics of phase transformations at cooling of carbon steel in dependence on the temperature of preliminary annealing T$_{an}$ is studied. It is shown that the cooling from T$_{an}$ > A$_3$ (i. e. above the temperature of ferrite start) with the rate 90 - 100 K/s results in structural state which essentially dependent on T$_{an}$; at 750$^0$C < T$_{an}$ < 830$^0$C the transformation is of perlite type whereas at T$_{an}$ > 830$^0$C the martensitic structure arises. Our results evidence the formation of a special structural state in a certain range of temperatures near and above the boundary of two phase region which is characterized by a substantially nanoscale heterogeneity in carbon distribution, lattice distortions, and magnetic short-range order.