Microstructure Evolution during Hot Deformation of UNS S32750 Super-Duplex Stainless Steel Alloy.

Elisabeta Mirela Cojocaru,Nicolae Șerban,Doina Răducanu,Irina Varvara Balkan,Mariana Lucia Angelescu,Ion Cinca,Anna Nocivin,Vasile Dănuț Cojocaru

doi:10.3390/ma14143916

Elisabeta Mirela Cojocaru, Nicolae Șerban + Show 6 more

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https://doi.org/10.3390/ma14143916

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Abstract

The present paper analyzes UNS S32750 Super-Duplex Stainless Steel hot deformation behavior during processing by upsetting. The objective of this paper is to determine the optimum range of deformation temperatures, considering that both austenite and ferrite have different deformation behaviors due to their different morphology, physical, and mechanical properties. Because the capability of plastic deformation accommodation of ferrite is reduced when compared to austenite, side cracks and fissures can form during the hot deformation process. Consequently, it is important to find the optimum conditions of deformation of this type of stainless steel to establish the best processing parameters without deteriorating the material. The experimental program involved the application of hot deformation by the upsetting method on a series of samples between 1000 °C and 1275 °C, with a total degree of deformation of 30%. The resultant samples were examined by SEM-EBSD to establish and analyze the evolution of the phases present in the structure from several points of view: nature, distribution, morphology (size and shape), and their structural homogeneity. The GROD (Grain Reference Orientation Deviation) distribution map was also determined while taking into account the possible precipitation of the secondary austenite phase (γ2-phase) and the analysis of the dynamic recrystallization process according to the applied deformation temperature. The main conclusion was that UNS S32750 SDSS steel can be safely deformed by upsetting between 1050–1275 °C, with an experimented total degree of deformation of 30%.

Highlights

IntroductionThe material considered in this experimental research is Super-Duplex Stainless Steel (SDSS), which shows a bi-phasic microstructure, composed of proportionally equal phases of, δ ferrite, and γ austenite, with a sufficient content of Cr, Mo, and N to deliver high corrosion resistance to pitting and to stress corrosion cracking [1,2]
It has been reported that, in the case of applied recrystallization treatments prior to hot deformation processing, the initial grains of γ austenite are smaller than the grains of δ ferrite, with a direct repercussion on the microstructure of the alloy that was obtained after hot deformation, resulting in a more homogenous deformed γ austenite phase compared to the δ ferrite phase [16,17]
For the experimented temperature range (1000–1025 ◦ C) applied for hot deforming the UNS S32750 Super-Duplex Stainless Steel by upsetting with a total degree of deformation of 30%, the microstructure of the studied material is composed of approximately equal ratios of γ-phase and δ-phase before and after the hot deforming process

Summary

Introduction

The material considered in this experimental research is Super-Duplex Stainless Steel (SDSS), which shows a bi-phasic microstructure, composed of proportionally equal phases of, δ ferrite, and γ austenite, with a sufficient content of Cr, Mo, and N to deliver high corrosion resistance to pitting and to stress corrosion cracking [1,2]. In addition to these important characteristics, SDSS displays an industrially recognized combination of high mechanical strength and toughness [3,4]. The current paper represents a continuation of these earlier experiments, with the aim of establishing a whole package of optimal, useful, and necessary hot deformation processing parameters

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