ABSTRACT

The strong detrimental influence on both toughness and corrosion resistance of intermetallic particles (IMP) in the microstructure of duplex stainless steels is well known. Toughness and corrosion testing along with microstructural examination of samples has been used as a collective to determine either ”freedom from” or ”acceptable” levels of deleterious phases in the microstructure of these steels. However, users of duplex alloys continue to report problems. This paper considers how meaningful current test regimes and acceptance levels are and how they correlate with each other. The value of ferritiscope testing to detect intermetallic particles is also considered. Overall, the paper proposes more meaningful test acceptance levels and procedures along with, supplementary ferritiscope testing to optimise the capability of this group of tests to detect deleterious phases.

INTRODUCTION

When properly made and welded, Duplex Stainless Steels (DSS) provide a very attractive combination of strength, toughness and corrosion resistance in a range of environments. They are formable, weldable, affordable, and readily available in a wide range of product forms. As such, they are used by many industries in applications that are process, health, safety and environmentally critical.

Generally, we test these steels on a cast and batch basis in accordance with the applicable ASTM (1) product standards. However, supplemental tests need to be specified to check that the other properties (toughness, corrosion resistance etc.) are being achieved. We do this testing on a sacrificial sample that we consider to be representative of the remainder of the material from the mother cast and heat treatment batch (or representative of a welded joint if considering welder or weld procedure qualification), and we test this to destruction.

We know phase balance to be important, as this influences both strength and toughness. We also know that these steels are susceptible to the precipitation of IMP if they are inadvertently cooled too slowly or held for too long in the temperature range where IMP precipitate. The presence of very small amounts of IMP can cause embrittlement and destroy the toughness of these steels. The IMP’s formed are rich in chromium and molybdenum, so the adjacent matrix is correspondingly depleted in these elements. This can significantly reduce the corrosion resistance. Hence, in order to check the metallurgical integrity of parts made in these steels we commonly apply a microstructure check (to determine the phase balance of the steel and to see if the microstructure contains IMP or not), a corrosion test1, 2 (usually using ferric chloride solution), and Charpy impact testing to check the level of toughness of the part.

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