Three-dimensional atom probe analysis of carbon distribution in low-temperature bainite

Abstract

A bainitic microstructure with fine ferrite plates (20 to 40 nm) in a matrix of high-carbon retained austenite, obtained by isothermal transformation at 200°C, was characterized with an energy-compensated optical position-sensitive three-dimensional atom probe. The average carbon concentration in the austenite was found to be 8.0±1.6 at.% and in the bainitic ferrite 1.1±0.7 at.%. The latter concentration is much higher than expected from paraequilibrium between austenite and ferrite, and the austenite carbon concentration is found to be slightly higher than the T 0 limit.

Tempering of Low-Temperature Bainite

“…pct). [31] X-ray diffraction measurements show that the large amount of carbon retained in the ferrite is correlated to non-uniform lattice strain and that both decrease on tempering. [12] One explanation is that much of the carbon is present at dislocations, as observed by atom probe tomography.…”

“…Further details and an earlier analysis of the results were previously reported. [31] Rods of 3-mm diameter were produced by electrical-discharge machining from alloy A1, sealed in quartz tubes back-filled with argon, and homogenized for 2 days at 1473 K (1200°C). Samples were removed and re-sealed in fresh quartz tubes for austenitization at 1273 K (1000°C) for 15 minutes.…”

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.…”

Thermally Stable Nanocrystalline Steel

“…[1][2][3][4][5][6][7][8][9] The structure consists mostly of alternating thin plates of bainitic ferrite, a b , and retained austenite, c r , with a small fraction of retained austenite blocks forming the residue of the sample. The austenite films and bainite plates are typically below 50 nm in width, providing a potent strengthening mechanism without compromising toughness.…”

“…At temperatures where the atomic mobility of carbon atoms is sufficient, there will then be a tendency for the austenite to decompose into a mixture of ferrite and cementite. Many studies have observed a carbon supersaturation with respect to cementite in both austenite [5,6,[10][11][12][13][14][15] and ferrite. [12,13,[15][16][17][18][19] There is therefore a large driving force for the formation of cementite in both phases.…”