Constitutive description of high temperature flow behavior of Sanicro-28 super-austenitic stainless steel

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Abstract

The flow stress behavior of a super-austenitic stainless steel (Sanicro-28) was predicted through developing constitutive equations. These were established through utilizing the hot compression tests results acquired in the temperature range of 800–1100 °C under the strain rates of 0.001–0.1 s−1. The effects of the temperature and strain rate on hot deformation behavior were expressed in terms of an exponent-type Zener–Hollomon equation. The influence of strain was considered in the sine hyperbolic constitutive analysis by taking the related material constants into account as the functions of true strain. It was found that the deformation activation energy (Q) and the material constants in the model (i.e., α, β, n, and ln A) were polynomial functions of true strain. The Q-values were found to be in the range of 495–660 kJ/mol for different amounts of strain. The accuracy of the proposed constitutive equations was assessed using standard statistical parameters such as correlation coefficient and average absolute relative error. The results confirm that the developed constitutive equations may well predict the high temperature flow behavior of the examined super-austenitic stainless steel under the experimental deformation conditions.

Introduction

The super-austenitic steels containing high amounts of molybdenum, chromium, nickel, nitrogen and manganese are extensively used as strategic materials, particularly in oil and gas transference industries [1]. The required characteristics for these advanced steels (e.g., superior performance in aggressive environments) have ended in the innovation of highly alloyed super-austenitic steels. These steels are postulated to possess excellent weldability and corrosion resistance, together with relatively higher strength in comparison with the typical stainless steel grades such as AISI-304 and 316 [2], [3], [4], [5].

As is well documented, the optimum microstructure, and, in turn, the enhanced mechanical performance in super-austenitic stainless steels could be gained through thermomechanical processing (TMP) where the steel is exposed to deformation at predetermined temperatures. In such conditions, several complicated metallurgical phenomena such as strain hardening and restoration processes (e.g., dynamic recovery and dynamic recrystallization) may simultaneously occur each of which would diversely affect the flow behavior [6], [7]. Considering the cost and time-consuming nature of experimental trials to estimate the flow behavior of such alloys under different thermomechanical conditions, it would be of high interest to establish an appropriate mathematical model, which can accurately and conveniently describe the flow behavior during hot deformation. Such a d escription, if verified, can also be implemented into the finite element codes in order to computationally simulate the material's response under the specified loading conditions [8].

In general, the constitutive equations which correlate flow stress, strain rate, strain, and temperature, as the key TMP parameters, could be employed to describe the plastic flow properties of metals and alloys [9], [10]. To date, many researchers have correspondingly formulated several empirical, semi-empirical and physically based constitutive models [11], [12], [13]. Among different models presented in the literature, the phenomenological approach proposed by Sellars and McTegart [14] has been widely adopted for different materials. In this model, which would be applicable over a wide range of temperatures and strain rates, the flow stress of the material has been described by a hyperbolic sine-type law in the form of an Arrhenius-type equation. This model gains high scores for adoption due to its simplicity as well as its capability to be introduced to different computational codes. In the case of steels, many researchers have studied the high temperature flow behavior using the Arrhenius-type constitutive analysis, a comprehensive review of which would be found in [15], [16].

More recently, the conventional Arrhenius-type equation has been revised in different ways to increase its applicability and accuracy in predicting the materials' flow behavior. Considering the deformation activation energy and material constants as functions of strain, a revised hyperbolic sine constitutive equation was proposed to predict the elevated temperature flow behavior in 42CrMo steel [17], 9Cr–1Mo (P91) steel [18], 20CrMo steel [19], Cr20Ni25Mo4Cu steel [20], TRIP/TWIP steels [21], T24 ferritic steel [22], and GCr15 steel [23]. Lin et al. [24] and Mandal et al. [25] revised the strain-dependent hyperbolic sine constitutive model through compensation of strain rate in the Zener–Hollomon parameter during constitutive analysis on 42CrMo steel and D9 steel, respectively. In another study, a new relationship between the stress multipliers of Garofalo equation for constitutive analysis of a modified 9Cr–1Mo (P91) steel was presented by Phaniraj et al. [26]. In addition, introducing a new material parameter (L), which is sensitive to the deformation temperature and strain rate, a new constitutive model has been developed to predict stress–strain curves up to the peak stress by Lin and Liu [27].

Despite the great important industrial applications of the Sanicro-28 super-austenitic stainless steel, there is a lack of knowledge on the strain-dependent constitutive analysis of this steel. Hence, the present investigation directed to establish an appropriate constitutive equation for this grade of super austenitic stainless steels. Toward this end, the experimental data obtained from isothermal hot compression tests have been utilized, the material constants have been determined, and the flow stress has been correlated with strain, strain rate and temperature. Additionally, the predictability of the established constitutive model has been verified over the entire experimental range.

Section snippets

Experimental procedures

The material used in this investigation was Sanicro-28 super-austenitic stainless steel (W.Nr.1.4563) whose chemical composition is given in Table 1. The experimental material, composing of a full austenitic microstructure (Fig.1), was received in as-forged condition. The cylindrical specimens for uniaxial compression testing were machined with diameter of 8 mm and height of 12 mm in accordance with ASTM E209 [28]. The hot compression tests were carried out with a Gotech AI-7000 LA 30

Flow stress behavior

The experimental true stress–true strain curves resulting from hot compression tests are plotted in Fig. 2. This implies the significant effects of temperature and strain rate on the alloy flow behavior. As is seen, the flow stress is decreased by increasing the temperature and decreasing the strain rate. The effect of strain rate is rationalized considering the role of tangled dislocation structures in hindering the dislocation movement thereby increasing the flow stress, which is more

Conclusions

Constitutive analysis of super-austenitic stainless steel (Sanicro-28) has been carried out by performing hot compression tests in a wide range of temperatures (800–1100 °C) under the strain rates of 0.001–0.1 s−1. The effect of temperature and strain rate on deformation behavior is represented by the Zener–Hollomon parameter in an exponent-type equation. Based on this study, the following conclusions are made:

  • The true stress–true strain curves reveal that the flow stress is substantially

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