ABSTRACT
Although extensively used in oil and gas production due to their excellent combination of mechanical properties and corrosion resistance, 25Cr super duplex stainless steels (SDSSs) are susceptible to the precipitation of deleterious phases during heat treatment and welding. Deleterious phases, in turn, affect both localized corrosion resistance and mechanical properties. Much debate still exists as to whether alloying elements such as tungsten accelerate or retard the formation of detrimental precipitates.
In this work, the effect of W on the precipitation kinetics of three 25Cr SDSS grades, namely, UNS S32750 (W-free), S32760 (Low-W), and S39274 (High-W) was quantified in Time-Temperature-Transformation (TTT) diagrams. Optical microscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and Electron backscatter diffraction (EBSD) were used to characterize the microstructure evolution and construct the TTT diagrams. The effect of intermetallic compounds (IMCs) and tertiary phases on localized corrosion resistance was investigated as a function of volume fraction and type of precipitate, with a focus on s– and ?–phase formation. The localized corrosion resistance of the various metallurgical stages was determined using open circuit potential measurements as a function of temperature during immersion in 6 wt% FeCl3 pH = 1.0.
The s–phase precipitation rate was slower in the High-W SDSS compared to the other alloys, possibly due to ?–phase precipitation at grain boundaries. At 846°C, the isothermal heating time required to observe a drop in Critical Pitting Temperature (CPT) doubled for the High-W SDSS. The implications of these findings in materials selection for oil and gas production equipment are discussed.
INTRODUCTION
Uses of super duplex stainless steels in O&G production
Duplex stainless steels (DSS) are steels composed of a two-phase ferritic-austenitic microstructure, the components of both phases having a chromium content above 10.5-13.0 wt%. 1, 2 Manufacturers adjust the ferrite (a) and austenite (?) balance close to 50 voI%.3, 4 Because of their fine-grain microstructure, DSS have specified minimum yield strength (SMYS) values that are at least 200-260% greater than those of austenitic and ferritic grades, depending on the type of stainless steel being compared.2 DSS have a localized corrosion resistance on par with stainless steels of a similar Cr, Mo, and N content and superior resistance to stress corrosion cracking (See).2, 3, 5-7 Moreover, given their relatively low nickel content, DSS are a cost-efficient choice over austenitic stainless steels and nickel based alloys.5
