Constitutive Relationship Modeling and Characterization of Flow Behavior under Hot Working for Fe–Cr–Ni–W–Cu–Co Super-Austenitic Stainless Steel

Li-Chih Yang,In-Gann Chen,Dong-Yih Lin,Yeong-Tsuen Pan

doi:10.3390/met5031717

Abstract

The hot deformation behavior of a Fe–22Cr–25Ni–3.5W–3Cu–1.5Co super-austenitic stainless steel was investigated using isothermal compression tests with a wide range of temperatures (1173–1373 K) and strain rates (0.1–10 s−1). The results showed that all the flow curves gradually turned to balanced stress state without notable peak stress characteristics during the entire deformation, which indicated that the dynamic recovery behavior played a main restoration mechanism in the steel. Modeling constitutive equations relating to the temperature, strain rate and flow stress were proposed to determine the materials constants and activation energy necessary for deformation. In order to give the precise predicted values of the flow behavior, the influence of strain was identified using polynomial functions. The relationship of flow stress, temperature and strain rate was represented by the Zener-Hollomon parameter including the Arrhenius term. The predicted results validated that the developed constitutive equations can describe high temperature flow behavior well. Furthermore, a modified Zener-Hollomon parameter map of the studied steel was developed to clarify the restoration mechanism based on the constitutive modeling data and microstructural observation.

Highlights

Super-austenitic stainless steels (SASSs) have unique high temperature strength, good weldability and superior corrosion resistance in comparison with general austenitic stainless steel grades, and such excellent properties mainly rely on the features of their chemical compositions
Compared to conventional austenitic stainless steels, SASSs have higher stacking fault energy (SFE), and the dynamic restoration is driven mainly by the dynamic recovery (DRV) process owing to its higher content of alloying elements, which leads to poorer hot plasticity and workability [9,10]
The work hardening (WH) plays a significant role in the initial compressed stage, and the dislocation structure becomes entangled and impedes the dislocations movement, thereby increasing flow stress, which is more pronounced at higher strain rates and lower temperatures [27]

Summary

Introduction

Super-austenitic stainless steels (SASSs) have unique high temperature strength, good weldability and superior corrosion resistance in comparison with general austenitic stainless steel grades, and such excellent properties mainly rely on the features of their chemical compositions. SASSs are usually constituted of high alloyed addition of chromium (Cr), nickel (Ni), molybdenum (Mo) and other alloying elements in austenite [1,2,3,4,5]. Additions of these elements impart a remarkable improvement in oxidation resistance, microstructural stability and high temperature strength at elevated temperatures. Compared to conventional austenitic stainless steels, SASSs have higher stacking fault energy (SFE), and the dynamic restoration is driven mainly by the dynamic recovery (DRV) process owing to its higher content of alloying elements, which leads to poorer hot plasticity and workability [9,10]. Along with the increase in the SFE, steels are very susceptible to hot cracking during hot deformation, especially edge cracking due to the sluggishness of the dynamic recrystallization process (DRX) [7,12]

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Results

Conclusion