Duplex Stainless Steel: Specification Requirements

Abstract

ABSTRACT The use of duplex stainless steels Offshore has rapidly increased in recent years. At the same time the number of specifications for duplex stainless steel has also rapidly increased. This paper examines the requirements of these specifications, highlights conflicts and questions the need for certain tests. INTRODUCTION In recent years the use offshore of duplex (22%cbrmium) stainless steel and super duplex (25%chromium) Stainless steel has considerably increased. Their combination of increased Strength and corrosion resistance over austenitic grades, at a relatively low increase in C0st, has led to the materials being Specified for CO2, and H2S containing Process lines, where the high partial Pressure of CO2, precludes the use of carbon steel, and where high temperatures (>60°C)preludes the use of austenitic Stainless steels. The results in term of application in the North Sear has been twofold;An increased usage at higher temperature (up to 120°C), where the material limitations for stress corrosion cracking from either H2S or chloride ions are not clearly defined.A multitude of specifications, each giving a variety of requirements, each of which is deemed necessary in order to control the properties and fabrication duplex stainless steels. A further complication occurs with respect to specifications for the 25% Chromium alloys since they are also used for seawater pipinq, and specifications for this type of service often have requirements transferred from a process application, and vice versa. The present paper briefly reviews the Limitations to the application of duplex Stainless steels from data already available; it also identifies information that is lacking and the methods required to resolve "problem areas" whilst acknowledging efforts already being made to rectify the situation (Ref 1,2). The approach in the paper is made from the purchaser's viewpoint who, ideally, would like to minimise any additional testing when making material selection. Following on from the above, a detailed review of a nurd3erof specifications from recent North Sea projects for both duplex and super duplex alloys is given. This critically examines the applicability and validity of some requirement, whilst questioning the reasons for differences between the specifications. Finally a summary is given of the area requiring mm work or clearer attention whilst highlighting steps the purchaser can take to improve the situation. A number of different types of duplex Stainless steel have seen extensive use In the North Sea in the past few years. These have included numerous variants of both 22% Chromium and 25% Chromium alloys. The application shave covered all types Of quiz and include pipe, fittinqs, flanges, valves, separators, shell and tube hate exchangers etc. A brief summary of duplex types available and typical applications is given in Tables 1 @ 2. However, the principal duplex stainless steels of interest are 22Cr, 5Ni, 3K (UNSS31803) and 25Cr, 7Ni, 3M0 (UNSS32750/S32760), and the following discussions will be restricted to these alloys in principally topsides applications.