Abstract

Super duplex stainless steels (SDSS) are characterized by a two-phase microstructure of approximately 50% ferrite (α) and 50% austenite (γ). SDSS are highly alloyed stainless steels with at least 25 % Cr, 3 % Mo, 6 % Ni and 0.2 % N and offer an excellent combination of good mechanical properties and resistance to general as well as localized corrosion. Despite their good combination of properties in typically solution annealed condition, there is a certain need to increase the strength of these materials. The tensile strength of SDSS can be increased by a thermomechanical process (TMP). The aim of this paper is to study the effect of a TMP on the mechanical properties, mainly tensile strength, and corrosion properties of UNS S32750 (1.4410). The material was submitted to a TMP at temperatures where detrimental phases should not precipitate according to thermodynamic calculations. After TMP, UNS S32750 still displays a ferritic-austenitic microstructure free of secondary phases and exhibits yield strength and ultimate tensile strength higher than 800 MPa (116 ksi) and 950 MPa (138 ksi), respectively. The pitting corrosion resistance UNS S32750 at room temperature is not affected by TMP. At 50 °C, there is a slight reduction of pitting corrosion resistance. The tensile strength of UNS S32750 can be increased without losing its corrosion properties by TMP, enabling the use of this SDSS for applications at high strength and in very aggressive environments.

Introduction

Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and are constituted of 30 to 70 % ferrite and austenite. They combine high tensile strength, good toughness, weldability, and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion.1-3 DSSs can be classified according to the Pitting Resistance Equivalent Number (PREN = Cr + 3.3 Mo + 16 N) in lean duplex (PREN= 22-27), standard (PREN = 28-38), super duplex (PREN = 38-45) and hyperduplex (PREN > 45).

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