Hot deformation characteristic and processing map of superaustenitic stainless steel S32654

Abstract

The hot deformation behavior of superaustenitic stainless steel S32654 was studied in the temperature range of 950–1200 °C and strain rate range of 0.001–10 s⁻¹ employing hot compression tests. The results show that peak stress increases with decreasing of temperature and increasing of strain rate. The apparent activation energy of this alloy is about 469 kJ/mol. The processing maps for hot working were developed on the basis of flow stress data and the dynamic materials model. It is found that the features of the maps obtained in the strain range of 0.2–1.0 are fundamentally similar, indicating that the strain does not have a significant influence on processing map. The maps exhibited two domains. The first domain occurs in the strain rate range of 0.01–0.4 s⁻¹ and temperature range of 1030–1150 °C with a peak efficiency of about 49%, which is considered as the optimum window for hot working. The microstructure observations of the specimens deformed in this domain showed the full dynamic recrystallization (DRX) structure with finer and more homogeneous grain sizes. The second domain occurs at the temperatures higher than 1160 °C and strain rates lower than 0.1 s⁻¹ with a peak efficiency of about 41%, the microstructure observations in this domain also indicated the typical DRX structure accompanied with grain growth. A instability domain occurs at temperatures below 1175 °C and strain rate above 0.1 s⁻¹.

Introduction

Superaustenitic stainless steel S32654 possessing outstanding combination of corrosion resistance and high strength is widely used in very hostile environments such as chemical processing equipment, pharmaceutical plant, flue gas desulphurization, waste incineration plants, equipment for recovery of chlorinated hydrocarbons, sea water piping, and nuclear power plant condenser tubes [1]. However, comparing to conventional austenitic stainless steel, S32654 has poorer hot plasticity and higher high-temperature strength because of its high contents of Cr, Mo, and N elements. The steel is susceptible to hot crack, especially to the edge crack due to its poor hot workability [2]. Recently, many investigations on S32654 which have been reported were mainly focused on precipitation behaviour and corrosion-resistant properties [3], [4], while the study of hot deformation behaviour has received much less attention. Therefore, it is useful to study the hot deformation behaviour of this alloy for the design and optimization of hot working processes and for microstructural control. It has been widely accepted that a “processing map’’ is effective to optimize the hot deformation processes and control microstructure without resorting to expensive and time-consuming trial and error methods [5].

Processing map was developed by Prasad et al. on the grounds of dynamic materials model (DMM), theoretical basises of which have been elucidated in detail in the earlier literature [6]. According to the DMM, the work-piece under hot working conditions plays a role of a power dissipator. The power dissipated through microstructural changes is represented by a dimensionless parameter (η) termed the efficiency of power dissipation, which is a function of strain rate sensitivity:

$$\eta =\frac{2m}{m+1}$$

where m is strain rate sensitivity, the definition of which is given in the following:

$$m={\left(\frac{\partial \ln \sigma }{\partial \ln \dot{\epsilon }}\right)}_{T,\epsilon }$$

where σ and $\dot{\epsilon }$ are the flow stress and strain rate, respectively.

The variation of η with deformation temperature and strain rate constitutes power dissipation map. The various domains in the map represent the specific microstructural process. A continuum criterion that is applicable to large plastic flow is used to delineate the domains of flow instability and given by

$$\xi (\dot{\epsilon })=\frac{\partial \ln [m/(m+1)]}{\partial \ln \dot{\epsilon }}+m<0$$

where $\xi (\dot{\epsilon })$ is the instability parameter. The flow instability is expected to occur when $\xi (\dot{\epsilon })$ becomes negative [7]. The variation of $\xi (\dot{\epsilon })$ with deformation temperature and strain rate constitutes instability map. The processing map can be constructed by the superimposition of instability map on power dissipation map, in which certain specific microstructural mechanisms and together with regimes of flow instabilities can be exhibited.

The purpose of the present investigation is to evaluate the hot deformation behaviour of S32654, with a view to obtaining optimum hot working parameters and understanding the constitutive behaviour by employing the processing map.