Study of the influence of secondary phases in the properties of superduplex stainless steel.

Abstract

The main focus on this project is to study the how the formation of secondary phases in superduplex stainless steel (SDSS) affects its mechanical and corrosion properties. At relatively high temperatures these high alloying stainless steels tend to have the drawback of forming certain secondary phases which can influence poorly on very significant and relevant properties of the steel. Theses phases, that are tough to quantify, patch together in a range of temperatures that depend on the chemical composition of the steel. However, their stability at these temperatures has not been studied in depth. The aim of this project is to produce these secondary phases to a different extent in samples of SDSS extracted from a valve in a desalination plant, to subsequently check their thermal stability, analyze their corresponding microstructure, hardness and resistance to corrosion. Duplex stainless steels are ferritic-austenitic steels, which mean that approximately half of their microstructure is made of ferrite and the other half austenite, thus ensuring good mechanical and corrosion properties. However, owing to their susceptibility to formation of dangerous intermetallic phases, the uses of SDSS have been narrowed down, especially in temperature range over 500ºC. At this temperature range the formation of the σ-phase is enhanced, but also other intermetallic phases such as the χ-phase and carbides and nitrides. The formation of theses phases make the steel more brittle and hard, and reduce the corrosion resistance, making the steel less useful and effective in work environments. It has been demonstrated that over a certain amount of σ-phase, the toughness of the SDSS is reduced to values which are lower than the accepted standards, and so unsuitable for practical applications Therefore, many standards relating to manufacturing (casting) and welding of SDSS require no intermetallic phases in the microstructure. In order to achieve the formation of theses secondary phases samples of this Super duplex stainless steel have been submitted to annealing isothermal treatments of temperatures of 850ºC and 1050ºC and different time scales. The goal was then to first observe and try to quantify the amount of secondary intermetallic phases formed due to the thermal, and then find the relationship between the growth of these phases with the increase of hardness and decrease of corrosion resistances.Since the decade of the 80’s, super duplex stainless steels is one of the metallic materials with highest corrosion resistance under saline atmospheres, and since alloy studied in this project derives from a working environment with a lot of salinity, it seemed appropriate to run corrosion tests to determine how σ-phase and χ-phase affect the metal’s corrosion resistance. The samples submitted to several thermal treatments (and therefore with a different composition in intermetallic phases) where left at first for 15 days soaked in a 4% NaCl solution, stirring the solution continuously with the help of a magnetic stirred. With this experiment we were able to simulate the behavior of SDSS in a saline and high chlorinated environment, such as the one we would be able to find in a chemical industry, in a desalination plant or in industries which are located in sea water on a marine environment. The experiment was later repeated with Mediterranean Sea water in order to compare the different states of corrosion. The corrosion is later on studied with an optical microscope, a Scanning Electron Microscope (SEM) and EDS microanalysis. In order to figure out the effect of corrosion, our samples are examined using these microscopy techniques and characterized by the EDS analysis. Moreover, the weight gain and loss methods will be applied, measuring each samples weight at a prefixed time scale whilst performing the experiment. Corrosion by pitting is to be found considerably throughout all the samples. The study of the microstructure was carried out using the high resolution scanning electron microscope (Field Emission SEM). Using a backscattered electron detector and EDS (energy dispersive spectroscopy) microanalysis we were able to observe and characterize all the different phases in the different samples and different thermal treatments. This way we are able to compare the amount of corrosion formed in a sample with the quantity of intermetallic phases present in the alloying constituent. Finally, hardness tests were performed utilizing the Rockwell C scale, in order to observe how the increase of hardness in each sample correlates to the formation of brittle intermetallic phases.