Effect of thermomechanical treatments on the formation of submicrocrystalline structural states and mechanical properties of metastable austenitic steel

I. Litovchenko, S. Akkuzin, N. Polekhina, A. Tyumentsev
Received: 28 October 2016; Accepted: 13 December 2016
This paper is written in Russian
Citation: I. Litovchenko, S. Akkuzin, N. Polekhina, A. Tyumentsev. Effect of thermomechanical treatments on the formation of submicrocrystalline structural states and mechanical properties of metastable austenitic steel. Letters on Materials, 2016, 6(4) 290-293
BibTex   DOI: 10.22226/2410-3535-2016-4-290-293

Abstract

The thermomechanical treatments with sequential increase in temperature of deformation were used to form submicrocrystalline structural states in AISI 321 metastable austenitic steel. The initial deformation were carried out with cooling in liquid nitrogen (T = 77 K) following by deformation in the temperature range of T = 300 – 1173 K. Unlike severe plastic deformation, we used total degree of true deformation e < 1. The features of the structural states and mechanical properties of steel at various stages of thermomechanical treatments have been investigated. It is shown that the low-temperature deformation promotes intensive (γ → α')-martensitic transformation with the formation of ≈ 54-60% of α'-martensite. After the subsequent deformation at temperatures of 300 – 773 K the volume content of α'-martensite is 60-80%. At higher deformation temperatures (873 – 973 K) the reverse (α'→ γ) martensitic transformation occurs with a decrease in the martensite content to 11-45% and the formation of submicrocrystalline structure of "packet austenite". The deformation at T > 973 K contributes to dynamic recovery and dynamic recrystallization. Thermomechanical treatments can improve the yield strength of steel up to ≈ 1300 MPa at the minimum values of elongation. Additional annealing after thermomechanical treatments allow to control the ratio of austenite-martensite phases, size and imperfection of submicrocrystalline fragments, as well as the strength and plastic properties of steel. The high strength properties are due to submicrocrystalline structure, which is formed as a result of direct and reverse γ → α' → γ martensitic transformations.

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