ABSTRACT
Duplex stainless steel (DSS) and super duplex stainless steel (SDSS) are extensively used in various product forms for critical applications such as pipelines, flow lines, process piping, valves, heat exchanger tubes, fittings etc. across onshore Oil & Gas production facilities. DSS materials are selected where carbon steel (CS) and austenitic stainless steel (ASS) materials are not able to provide required corrosion resistance to meet the design life cycle. Petroleum Development Oman (PDO) has over two decades of experience in procurement and use of DSS materials in their production facilities (most of them successful in operation / design life). This paper reflects two objectives: first the end user perspective on quality assurance and procurement & construction challenges associated with DSS materials and second this paper highlights the operational best practices to ensure technical integrity and meet intended design life cycle of DSS components.
INTRODUCTION
DSS and SDSS are a group of corrosion resistant alloys (CRA’s) that possess a two-phase microstructure of ferrite and austenite. DSS products are widely used in the oil and gas industry for various critical applications as shown in Figure 1. These DSS products exhibit better mechanical and corrosion properties compared to traditional carbon steel & austenitic stainless steel grades. The DSS microstructure consists of approximately 50% austenite (?) and 50% ferrite (a) phases. However, if incorrectly heat treated, these materials suffer from intermetallic phase (IP) precipitation, phases such as sigma (s) and chi, that will degrade the mechanical and corrosion properties. Therefore, correct heat treatment of DSS is critical to guarantee the product integrity.1-5
During heat treatment, furnace control and quench rate are important considerations in avoiding intermetallic precipitate formation during manufacture. However, it can be difficult to achieve the necessary quench rate in the centre of thick section castings and forgings (typically, those greater than 50 mm [2 in]), and these require special attention, since the absence of intermetallic phase precipitates cannot be guaranteed. Therefore, stringent quality control and procedures should be in place for various DSS product forms as highlighted in the several studies.6-10
