Thermal stability of austenite retained in bainitic steels

Abstract

Steels with a microstructure consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite are of interest in a variety of commercial applications because they have been shown to exhibit good combinations of strength, toughness and ductility. However, their use at temperatures moderately above ambient requires a knowledge of the thermal stability of the austenite. The changes that occur during the tempering of a mixture of bainitic ferrite, carbon-enriched retained austenite and martensite have been characterised. An analysis of the volume change due to transformation shows that it is possible to distinguish the decomposition of austenite from the tempering of martensite. The nature of the carbides that form during the heat treatment is discussed as are the implications on the development of mathematical models accounting for calculating the strain during austenite decomposition and martensite tempering. It is found that the early stages of tempering reactions where the austenite content is not greatly reduced, can dramatically influence the stability of the austenite as it is cooled to ambient temperature.

Tempering Effects on the Stability of Retained Austenite and Mechanical Properties in a Medium Manganese Steel

“…2(e) showed that the amount of carbides increased, ascribed to decomposition of retained austenite by a diffusion mechanism accompanying more precipitation of carbides. [24][25][26] or precipitation of carbides from austenite, which reduced austenite stability, resulting in transformation from austenite to martensite during cooling process, 19,27) thus the volume fraction and carbon concentrate of retained austenite drastically decreased to minimum as presented in Fig. 3.…”

Thermally Stable Nanocrystalline Steel

“…The resulting depletion of carbon in the retained austenite has been shown to lead to the formation of more ferrite, both during isothermal holding and during subsequent cooling. [20,21] It is expected that the high nickel content of Alloy 2 will prevent this transformation and allow the alloy to avoid carbide precipitation during thermal exposure.…”

“…Rapid decomposition of austenite into carbides and ferrite has been observed in nanocrystalline steels upon heating. [20,21] The resulting loss of austenite compromises both the strength and toughness of the steel and hence it is unsuitable for service at elevated temperatures.…”

Effect of Rare Earth on Isothermal Transformation Kinetics in High Strength-toughness Tool Steel

“…E-mail: tanqibingmatin@126.com high strength-toughness drilling tool steel was systematic researched. Which was on the basis of study on the effects of rare earth elements in this kind of drilling tool steel sub-structure [1][2][3][4][5][6].…”