Investigation of Electroplastic Effect on Four Grades of Duplex Stainless Steels.

Claudio Gennari,Enrico Simonetto,Luca Pezzato,Renato Gobbo,Michele Forzan,Irene Calliari

doi:10.3390/ma12121911

Abstract

Since the late 1950s, an effect of electrical current in addition to joule heating on the deformation of metals called the Electroplastic Effect (EPE) has been known. It is used nowadays in the so-called Electrically Assisted Forming (EAF) processes, but the understanding of the phenomenon is not very clear yet. It has been found that EPE increases the formability of high stacking fault energy (SFE) materials, while low SFE materials reach fracture prematurely. Since Duplex Stainless Steels (DSSs) possess a microstructure consisting of two phases with very different SFE (low SFE austenite and high SFE ferrite) and they are widely used in industry, we investigated EPE on those alloys. Tensile tests at 5 A/mm2, 10 A/mm2 and 15 A/mm2 current densities along with thermal counterparts were conducted on UNS S32101, UNS S32205, UNS S32304 and UNS S32750. The DSS grades were characterized by means of optical microscopy, X-ray diffraction and their mechanical properties (ultimate tensile strength, total elongation, uniform elongation and yield stress). An increase in uniform elongation for the electrical tests compared to the thermal counterparts as well as an increase in total elongation was found. No differences were observed on the yield stress and on the ultimate tensile strength. Un uneven distribution of the current because of the different resistivity and work hardening of the two phases has been hypothesized as the explanation for the positive effect of EPE.

Highlights

In 1959, Machlin [1] observed an increase in elongation and a reduction of the yield stress and the flow stress of a single crystal of sodium chloride when deformed under an applied voltage
Pioneering researches were conducted in the USA by Conrad and his team [2,3,4,5,6,7] and in the Soviet Union by Troitskii and his people [8,9,10,11,12,13], who concluded that the enhancement of the formability and the reduction of the forming forces cannot be ascribed solely to the joule heating and to an a-thermal effect, which they called the Electroplastic Effect (EPE)
Many theories have been formulated to explain the EPE such as the electron wind force by Kravchenko [25], who supposed that drifting electrons can exert a force on moving dislocations and ease their motion, which was later confirmed by experiments done by Bolko et al [26] and Conrad and his team [6,27]

Summary

Introduction

In 1959, Machlin [1] observed an increase in elongation and a reduction of the yield stress and the flow stress of a single crystal of sodium chloride when deformed under an applied voltage. Many theories have been formulated to explain the EPE such as the electron wind force by Kravchenko [25], who supposed that drifting electrons can exert a force on moving dislocations and ease their motion, which was later confirmed by experiments done by Bolko et al [26] and Conrad and his team [6,27]. Magargee demonstrated that each material has a different current density sensitivity for EPE to occur, which depends on the resistivity of the material (the higher the resistivity, the lower the threshold) [31]. An interesting theory was proposed by Ruszkiewicz et al [33]: stagnation of electrons in the proximity of obstacles increases the electron to atom ratio, decreasing the bond energy and eventually easing the breaking and reforming of metallic bonds favoring plastic deformation. Many experiments have been done concerning the EPE on different metallic materials such as aluminum [34,35,36], magnesium [15,37,38,39], and austenitic stainless steels [40,41,42]; some biphasic materials such as titanium [43], a dual phase steel [44], and brass [30]; only one paper has been found on Duplex Stainless Steels (DSSs) regarding the change in texture after electropulsing treatment at room temperature [45]

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Conclusion