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INTRODUCTION

THE duplex stainless steels have a two-phase microstructure, balanced with approximately equal amounts of ferrite and austenite, which permits high corrosion resistance in combination with high mechanical strength. The super duplex grades are highly alloyed and have excellent resistance to pitting and stress corrosion cracking in chloride-containing environments.[1] Good weldability has been achieved primarily by the use of nitrogen as an alloying element. Nitrogen acts an effective austenite-former that allows the preferred ratio of phases to be attained after high-temperature ferritization during welding.[2] With increased levels of alloying comes, however, also an increased sensitivity for formation of secondary phases, which could have negative effects on the properties. The duplex grades, which have high levels of nitrogen and chromium, are thus particularly susceptible to precipitation of chromium nitrides.[1] Nitrogen is mainly partitioned to austenite, but the nitrogen content in ferrite increases with increasing temperature. Nitrides are therefore liable to form during rapid cooling from high temperatures as ferrite is supersaturated with nitrogen.[3–5] These nitrides are often referred to as quenched-in nitrides since they are confined in the interior of the ferrite grains when the temperature is rapidly decreased. Nitrides can also form

NIKLAS PETTERSSON and STEN WESSMAN, Researchers, are with the Swerea KIMAB AB, Box 7047, 164 07 Kista, Sweden. Contact e-mail [email protected] RACHEL F.A. PETTERSSON, Research Manager, is with the Jernkontoret - The Swedish Steel Producers’ Association, Box 1721, 111 87 Stockholm, Sweden. Manuscript submitted July 6, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A

during isothermal heat treatments in their thermodynamic stability range.[6–8] Isothermal precipitation of nitrides occurs at grain boundaries, but can be avoided simply by adjusting the heat treatment temperature. The quenched-in nitrides are more difficult to avoid since they form during quenching after, e.g., heat treatments or welding. Several studies have reported detrimental effects of nitrides on primarily the pitting resistance of duplex stainless steels,[5,9] but no clear correlation with nitride quantification has been demonstrated. The purpose of this work has been to study the influence of cooling rate and microstructure on nitride precipitation, and its consequences for toughness and pitting corrosion resistance, when the super duplex stainless steel 2507 is cooled from temperatures in the range 1373 K to 1523 K (1100 C to 1250 C).

II.

EXPERIMENTAL PROCEDURES

Plate material of 2507 (UNS S32750) with 6 and 17.8 mm thickness was delivered by Outokumpu Stainless AB. Material from the 6 and 17.8-mm plates will hereafter be referred to as R1 and R2, respectively. The chemical composition from the plate certificate is given in Table I. A variant with coarser austenite spacing was produced by heat treatment according to Wessman et al.[10] Specimens from 6-mm plate were heat treated at