The Features of Martensitic Transformation in 12% Chromium Ferritic-Martensitic Steels.

Andrey Ni,Alexander Golubnichiy,Anton Chernov,Kseniya Bazaleeva,Ruslan Mendagaliyev

doi:10.3390/ma14164503

Abstract

An anomaly in martensitic transformation (the effect of martensitic two-peak splitting in the temperature-dependent thermal expansion coefficient) in complex alloyed 12% chromium steels Fe-12%Cr-Ni-Mo-W-Nb-V-B (ChS-139), Fe-12%Cr-Mo-W-Si-Nb-V (EP-823) and Fe-12%Cr-2%W-V-Ta-B (EK-181) was investigated in this study. This effect is manifested in steels with a higher degree of alloying (ChS-139). During varying temperature regimes in dilatometric analysis, it was found that the splitting of the martensitic peak was associated with the superposition of two martensitic transformations of austenite depleted and enriched with alloying elements. The anomaly was subsequently eliminated by homogenization of the steel composition due to high-temperature aging in the γ-region. It was shown that if steel is heated to 900 °C, which lies in the (α + γ) phase region or slightly higher during cooling, then the decomposition of austenite proceeds in two stages: during the first stage, austenite is diffused into ferrite with carbides; during the second stage, shear transformation of austenite to martensite occurs.

Highlights

The effect of martensitic peak splitting was absent in all samples of EK-181 steel, and only two samples of EP-823 steel displayed this effect; it was more strongly presented in all samples of ChS-139 steel
It can be assumed that the martensitic peak splitting effect was associated with the It canItofbe assumed thati.e., the the martensitic peak splitting effect was was associated withwith the the can bealloying; assumed that thedecreasing martensitic peak splitting effect associated level steel temperature in martensitic transformation inlevellevel of steel alloying; i.e., the decreasing temperature in martensitic transformation inof steel alloying; i.e., the decreasing temperature in martensitic transformation increased the tendency of alloy to the splitting effect of the martensitic peak in the sequence creased the tendency of alloy to thetosplitting effecteffect ofby thethe peak in the creased theEP-823, tendency of ChS-139, alloy the ofmartensitic the martensitic peak insequence the sequence of EK-181, and assplitting evidenced gain in the degree of alloying
Using the dilatometric analysis method, it was shown that upon cooling from the high-temperature region of Fe-12%Cr-Ni-Mo-W-Nb-V-B (ChS-139) steel, the thermal effect of the martensitic transformation had a bifurcated nature: there was an overlap of two martensitic transformations of enriched and depleted with alloying elements austenite

Summary

Introduction

When highly alloyed chromium steels are considered as prospective construction materials for active zones in fusion reactors and nuclear fast neutron reactors [1,2,3,4,5,6,7,8,9,10], thermal stability is one of the aspects determining their application [11,12,13]. The structure and properties of ferritic–martensitic steels are formed as a result of heat treatment, which usually consists of air quenching at 1080–1100 ◦ C with subsequent tempering at 680–720 ◦ C [14,15]. Variations in heat treatment conditions in these multicomponent systems, namely, changes in the quenching temperature, exposure time, cooling rate, and tempering time, lead to the redistribution of carbide-forming elements and carbon between solid solutions and carbide phases, which affects the structure and structural stability of steel [16]

Methods

Results

Discussion

Conclusion