ABSTRACT

Duplex Stainless Steels (DSS) are characterized by a dual-phase ferritic-austenitic microstructure and exhibit high corrosion resistance and excellent mechanical properties. However, formation of different types of intermetallic phases and precipitated compounds may occur during fabrication processes such as welding, hot working and heat treatments; this can influence toughness and corrosion resistance. With regard to this, some material standards/specifications (NACE, NORSOK, Company standards) introduced certain requirements for the precipitates content (e.g. sigma phase, chromium nitrides) in the DSS microstructure. However, these limits can deviate from the accuracy/capability of the most common counting methods (e.g. Light Optical Microscope-LOM) point-counting, scanning electron microscope (SEM)-counting, image analysis using dedicated software), especially for very fine and interspersed phases such as nitrides.

The aim of this paper is to find a correlation between nitride content/distribution and properties of two of the largely adopted dSs, UNS S32205 and UNS S32750. In order to verify the detrimental effects on corrosion and toughness of the nitride precipitation (NP), pitting corrosion tests and Charpy-V impact tests and fracture mechanics tests were carried out on different simulated bulk specimens.

INTRODUCTION

The chemical composition and microstructure of Duplex Stainless Steels (DSS) determine the corrosion resistance and excellent mechanical properties.1 The microstructure of a material can be altered after a thermal or mechanical treatment resulting in new properties.3-9

The precipitation of different types of intermetallic phases in DSS can occur during fabrication processes such as welding, hot working and heat treatments. Furthermore, in most applications of the two most widely used DSS, UNS N° S32750 and UNS N° S32205, precipitation of intermetallic phases could induce a decrease in toughness and/or corrosion resistance (e.g. marine applications, oil and gas industry equipment, flue gas desulfurization equipment, chemical process pressure vessels, piping and heat exchangers). 3,9,14

Therefore, some material standards/specifications, especially in the oil and gas industry (NACE(1), NORSOK(2), and other specifications), introduced some specific requirements for those phases that are detrimental to the material properties (e.g. sigma phase, chromium nitrides). 10,11

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