Elsevier

Materials & Design

Volume 31, Issue 1, January 2010, Pages 220-226
Materials & Design

Hot deformation characteristics of 2205 duplex stainless steel based on the behavior of constituent phases

https://doi.org/10.1016/j.matdes.2009.06.028Get rights and content

Abstract

High temperature behavior of 2205 duplex stainless steel was studied by considering behavior of each constituent phase. The specimens were subjected to hot compression tests at temperatures of 800–1100 °C and strain rates ranging from 0.001 to 1 s−1 at intervals of an order of magnitude. The flow stress analysis showed that hot working empirical constants are different at low and high temperatures. The strain rate sensitivity m was determined and found to change from 0.12 to 0.21 for a temperature rise from 800 °C to 1100 °C. The apparent activation energy Q was calculated as 554 and 310 kJ/mol for low and high temperature, respectively. The validity of constitutive equation of hyperbolic sine function was studied and stress exponent, n, was assessed to be 4.2. Assuming the hyperbolic sine function for determination of strain rate and application of the rule of mixture, the interaction coefficients of δ-ferrite, P, and austenite, R, were estimated at different hot working regimes. It was found that the interaction coefficients are functions of Zener–Hollomon parameter Z and obey the formulas P = 1.4Z−0.08 and R = 0.76Z0.005. Therefore, it was concluded that at low Z values δ-ferrite almost accommodates strain and dynamic recovery is the prominent restoration process which may even inhibit dynamic recrystallization in austenite. Otherwise, at high Z, austenite controls the deformation mechanism of material and dynamic recrystallization leads in finer microstructure.

Introduction

The need for high corrosion resistance as well as desired strength characteristics has made duplex stainless steels (DSS) deserving alternatives to single phase austenitic and ferritic grades and good choices for different industries [1], [2], [3]. Deformation behavior of each constituent phase, austenite or ferrite, in the duplex structure is considerably affected by the presence of the other phase. As the austenite is significantly stronger than the ferrite in the hot working temperature range, it will affect the load transfer and thereby strain transfer characteristic between the constituents in a DSS [4], [5], [6], [7].

Many researchers have investigated the hot working behavior of duplex stainless steels from different aspects [8], [9], [10] but some controversy over the restoration processes dominant in constituents have still remained. It has been well established that hot workability of δ-ferrite is better than that of the austenite. This advantage arises from the high stacking fault energy (SFE) of δ-ferrite and its ability to undergo dynamic recovery (DRV) [11], [12], [13], [14], [15], [16], [17], [18]. On the other hand, austenite, having a low SFE, undergoes only limited DRV [19] and when the dislocation density reaches a critical value dynamic recrystallization (DRX) takes place [7], [17], [18], [20].

In duplex stainless steels, δ-ferrite and austenite make a composite material consisted of almost comparable amounts of these phases. The dynamic restoration behavior of both phases seems to be rather similar to their restoration behavior in the single-phase materials, where δ-ferrite softens by DRV and austenite by DRX. Coexistence of hard austenite and soft δ-ferrite at high temperatures is found to result in a strain partitioning at the early stages of deformation, when strain is mostly accommodated by the δ-ferrite phase [21], [22]. At higher strains load is transfered from δ-ferrite to austenite leading to increment of dislocation density in the latter till triggering of DRX. Consequently, since strain energy is the driving force for softening to occur, the restoration process in ferrite is found to be far in advance of that in austenite [21].

Modeling of strain partitioning has been attempted by several researchers [23], [24]. The relation between the hot working behavior of DSS, volume fraction of the constituents and the values of stress and strain partitioned in each phase has described by the law of mixture [23], [25]. Several variants of this rule have been proposed for modeling the stress and strain distribution in the constituent phases.

The aim of this work is to study the high-temperature behavior of a DSS under hot compression tests, using the analysis of flow curves as well as hot-working response of each constituent phase.

Section snippets

Experimental procedures

Chemical composition of the DSS 2205 used in the present study is shown in Table 1. The material had been manufactured by AB Sandvik Steel (Sweden) and supplied as round bars of 30 mm in diameter. The as-received bars were hot rolled in the range of 950–1100 °C and annealed at 1050 °C followed by quenching in water to obtain homogeneous and equiaxic distribution of δ-ferrite and austenite, Fig. 1.

Cylindrical compression samples of 15 mm height and 10 mm diameter were prepared with the axis along the

Flow stress analysis

True stress–strain curves at different temperatures and strain rates are shown in Fig. 2. It is well established and also clearly seen here that flow stress level actually increases with strain rate and decreases with deformation temperature.

It has well understood that the characteristic points of flow curve and their relationship with processing variables are of great importance in studying hot deformation behavior of alloys. In this respect, the peak point of flow curve and the point

Conclusions

The major consequences obtained from this investigation are listed below.

The values of experimentally assessed constants such as strain rate sensitivity, m, and apparent activation energy, Q, found to be different at low and high deformation temperatures.

The value of m for low and high temperature was determined as 0.12 and 0.21, respectively and the average of 0.17 was considered as the value of m for studied 2205 DSS.

The value of Q at low and high temperature was determined as 554 kJ/mol and

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