A study of retained austenite transformation during high-strength Cr-Ni-Mo-V steel tempering

A. Ziza, M. Mikhailov, V. Tsukanov, D. Nikolaev, T. Lychagina

Abstract

TEM fine structure 0,3C-Cr-3Ni-Mo-V steel after quenching: (a) – lath martensite; (b) – dark field image of retained austenite; (c) – self-tempered martensite; (d) – dark field image of carbides.In this paper the amount of retained austenite after quenching and quenching and tempering (including double tempering) of products of small cross section (less than 25 mm) from structural high-strength medium-carbon 0,3C-Cr-3Ni-Mo-V steel is determined by X-ray phase analysis and neutron diffraction method. In the samples after quenching, the amount of retained austenite is ~3 %, In the samples after quenching and tempering is less than 0,5 %. The character of the distribution of retained austenite and morphology of martensite after quenching as well as quenching and high tempering were studied by transmission electron microscopy. It was shown, that after quenching, the retained austenite is located between the martensite laths. The structure of the quenched steel is dominated by the martensite of the lath morphology, which amount is 90-95 %, the amount of self-tempered martensite is small is 5-10 %. The transformation of retained austenite during steel high tempering was studied by high-speed dilatometer. It has been established, that retained austenite transforms into a mixture of carbides and α-phase during heating for tempering in some cases and turning into secondary martensite upon cooling from tempering temperature in the other. It is shown that the double tempering has little effect on the amount of the retained austenite and on the level of dislocation density, contributing to a change in shape of carbides from lamellar to globular and diminishing their dimensions.

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