Decomposition modes of austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels

Abstract

The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.

Identification of Retained Austenite in 9Cr-1.4W-0.06Ta-0.12C Reduced Activation Ferritic Martensitic Steel

“…Such incomplete dissolution of the primary carbides can lead to the inhomogeneity of the austenite, which would increase with W or Ta content of the steel [12]. Further detailed analysis of martensitic lath structure in TEM showed two different types of lath structure [12]. Such a precipitation of Fe rich M 3 C was shown to occur in a few wide laths, while the laths with narrow width showed no precipitation.…”

“…Hence, in this steel with 1.4% W, it can be inferred that normalizing at 1253 K is insufficient for complete dissolution of these carbides as shown in Figure 1. Such incomplete dissolution of the primary carbides can lead to the inhomogeneity of the austenite, which would increase with W or Ta content of the steel [12]. Further detailed analysis of martensitic lath structure in TEM showed two different types of lath structure [12].…”

“…These steels with a low carbon content (<0.15%) are known to form a lath martensitic structure with a dislocation substructure [9,10]. Though martensite gives good strength to the steel, due to limited ductility, these steels are not recommended to be used in normalized condition [11,12]. Therefore, the steels are generally used in the normalized and tempered condition with a tempered martensitic structure possessing good mechanical properties.…”

“…Therefore, the steels are generally used in the normalized and tempered condition with a tempered martensitic structure possessing good mechanical properties. However, preceding the tempering treatment, normalizing the steel in the single austenite phase is also an essential step for the transformation to a complete martensitic structure [11][12][13]. The decomposition of austenite is known to be influenced by the rate of cooling (Q) from the austenitizing temperature and the alloy chemistry [11,12].…”

“…Continuous cooling transformation (CCT) and time temperature transformation (TTT) of 9 Cr steels have been already studied [10,14]. As per the existing literature, CCT or TTT diagrams explain the presence of martensite/ferrite and carbides depending on the cooling rates adopted, but not the presence of retained austenite in 9Cr ferritic steels [9][10][11][12]. Martensitic transformation of austenite is well known to be accompanied by strain dictated by the composition of the steel and the cooling rates and hence the transformation induced strain can induce the mechanical stabilization of the austenite, and the mechanical stability of retained austenite is important in obtaining good toughness [15].…”