Hot working behavior of 2205 austenite–ferrite duplex stainless steel characterized by constitutive equations and processing maps
Research highlights
▶ The flow stress is well fitted by the hyperbolic sine equation. ▶ The activation energy for hot deformation is about 479 kJ mol−1. ▶ Processing map indicates no instability in low and medium strain rates. ▶ Dynamic restoration processes hinder flow instability at low and medium strain rates.
Introduction
Duplex stainless steels (SS) are superior to single phase austenitic or ferritic grades because of their better corrosion resistance and higher strength. For this reason, duplex stainless steels are considered as excellent choices for various industrial applications [1], [2], [3], [4], [5]. From the industrial point of view, the hot deformation of these steels has drawn much attention. The coexistence of ferrite and austenite in the structure of duplex SS has made the hot deformation of these alloys very controversial. It is known that ferrite is characterized by high stacking fault energy (SFE) and therefore undergoes dynamic recovery (DRV) [6], [7], [8], [9], [10], [11], [12], [13]. On the contrary, austenite, having a low SFE, undergoes only limited DRV. As a result, when the dislocation density reaches a critical value, dynamic recrystallization (DRX) comes into operation [14], [15], [16].
Although the hot deformation behavior of duplex SS has been investigated by many researchers, some controversies concerning the restoration processes in the microstructural constituents have remained unresolved [17], [18], [19], [20], [21]. The restoration mechanisms of austenite and ferrite in a duplex stainless steel seem to be similar to their restoration behavior in single-phase ferritic and austenitic steels. However, the coexistence of harder austenite and softer ferrite is found to result in strain partitioning at the early stages of hot deformation before which strain is mostly accommodated by the softer phase, i.e. ferrite [22], [23], [24], [25], [26], [27]. At higher strains, the load is transferred from ferrite to austenite leading to an increase in dislocation density and work hardening till DRX is triggered. It is understood that dissimilar plastic behavior and restoration mechanism of austenite and ferrite during the hot deformation of duplex SS can lead to a drastic decrease in the hot ductility of these alloys [25], [28]. In order to define the safe regions of deformation and also to avoid the occurrence of flow localization and therefore premature fracture, a practical approach is plotting a processing map. This window has recently been provided for a duplex stainless steel [29]. The approach of processing map, originally proposed by Raj [30], is defined as a representation of microstructural changes and restoration mechanisms of a given material. A processing map illustrates a superimposition of power dissipation map and instability regions which are depicted on the basis of dynamic materials model (DMM) [31]. DMM is a continuum model in which an instability criterion based on the principles of irreversible thermodynamic, as applied to large plastic flow, is utilized to mark flow instability regimes [32]. The power dissipation and the instability maps are plotted in terms of deformation temperature and logarithm of strain rate in order to correlate different regions with the processing variables. The principles of this approach and its applications to the hot deformation of a wide range of materials were described by Prasad and other researchers [33], [34], [35], [36], [37].
Although many attempts have been made to address the hot workability of different kinds of stainless steels using processing maps, little attention has paid to duplex stainless steels. Therefore, the present work is devoted to study the high-temperature behavior of a 2205 duplex stainless steel using the analysis of hot-deformation curves, constitutive equations and processing maps.
Section snippets
Experimental procedures
The material used in this study was the 2205 duplex stainless steel having the composition of 0.025% C, 22.80% Cr, 5.20% Ni, 2.60% Mo, 0.30% Si, 1.50% Mn, 0.001% S, 0.025% P, 0.088% V, 0.23% Cu, 0.03% W, 0.068% Co, 0.028% Al and the rest of Fe (all in wt.%). The starting microstructure of the studied material consisted of about 54% ferrite and 45% austenite, Fig. 1. The cylindrical specimens, 10 mm diameter and 15 mm height, were prepared from the as-received hot rolled plate with the
Constitutive analysis
Fig. 2 demonstrates the flow curves obtained from hot compression testing under different conditions. The different features of flow curves can be interpreted considering the coexistence of different constitutions, i.e. ferrite and austenite. Since ferrite is restored by DRV and austenite by DRX, the hot deformation behavior and therefore the flow curve characteristics significantly change due to different volume fractions of ferrite and austenite at different temperatures. It is evident that
Conclusions
The hot deformation characteristics of 2205 duplex stainless steel was analyzed by constitutive equations and processing maps. The most important results are listed below:
- 1.
The flow curve at low temperatures where austenite is the dominant microstructural component is typical of DRX; while, at high temperatures where ferrite is the dominant phase, DRV is the restoration mechanism.
- 2.
The flow stress of the studied material is very well fitted by the constitutive equation of hyperbolic sine function
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