The present paper presents results on the effect of selected heat treatments on the phase stability of an experimental superferritic stainless steel grade produced via the hot isostatic pressing (HIP) process. Both deformed and undeformed specimens were subjected to a variety of heat treatments within the 650-950uC temperature range for aging times of 5 min-1000 h. The temperature region of microstructural consistency for the superferritic stainless steel grade that guarantees optimum mechanical and corrosion properties was determined. Intermetallic phases were found to precipitate under all chosen temperatures, but the length of heat treatment before precipitation occurred varied with temperature and amount of deformation. The formation of minor amounts of austenite was detected in cold deformed specimens heat treated at 850 and 750uC while for the undeformed specimens, such a presence was verified only for specimens heat treated at 850uC. Formation of austenite is always preceded by the formation of an intermetallic phase. It was confirmed as anticipated, that deformation increases the rate of intermetallic phase precipitation. Superferritic steels are natural candidate materials for naval applications and fabrication techniques, such as welding, involve elevated temperatures on components previously deformed during a shaping exercise, therefore it is vital that an assessment of microstructural consistency of this novel powder metallurgy superferritic steel is performed.
Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
High Strength Low Alloy (HSLA) strip steels have extensive applications in the automotive industry, due to their excellent combination of strength, toughness and formability characteristics. With the drive to produce environmentally friendly cars, the need of developing novel high strength strip steel products, is ever increasing. Development of novel high strength strip steel grades would permit the down-gauging of automotive steel components, leading to lighter, fuel efficient and safer automobiles [1].Coarsening of second phase particles in a metallic matrix plays an important part in many metallurgical phenomena, such as the tempering of martensite, grain growth, precipitation hardening and creep deformation. The broad definition of the particle coarsening (i.e.: Ostwald ripening) process relates to the growth of second phase particles without significant change in the matrix solute content. The process thus differs from the precipitation processes where second phase particles nucleate and grow by depletion of the relevant matrix solute [2][3][4].The purpose of the present study is to study the control of austenite grain size via second phase particle additions, based on combined titanium and vanadium microalloy additions. Such information is readily available for commercial HSLA grades based on titanium only, niobium or titanium-niobium microalloying additions [5,6], however limited knowledge is available on the combined effects of titanium and vanadium microalloy additions. Processing of HSLA strips steels, aims to produce a fine austenite grain size, resulting in the subsequent formation of a fine primarily ferritic product.Two experimental titanium and titanium-vanadium HSLA grades were studied, as part of this comparative study. For the production of the two experimental alloys, high purity materials were used and the melting was performed in a vacuum induction furnace . Samples of the experimintal HSLA grades were austenitised within the temperature range of 900-1100 o C for 1h, followed by rapid water quenching. A vacuum furnace has been employed for the heat treatments to avoid any decarburisation effects on the thin strip steel samples.The present study confirmed that vanadium carbide precipitates succesfully control the austenite grain size at low austenitisation temperatures (900-950 o C), by effectively pinning the austenite grain boundaries (figs. 1 and 2). However, at higher austenitisation temperatures (in excess of 1000 o C), rapid dissolution of vanadium carbide does occur and the importance of titanium nitride formation in controlling the austenite grain size, in both experimental grades, becomes evident (fig 3). Titanium nitride with its very low dissolution rates and high stability at elevated temperatures prevents excessive austenite grain coarsening. It has been concluded that combined titanium-vanadium additions are beneficial in maintaining a fine austenite grain size (of the order of 10µm), during the finishing passes of the experimental titanium-vanadium HSLA steel. Austenite grain r...
Superaustenitic stainless steels with combined high chromium and molybdenum additions have excellent pitting corrosion resistance in a marine environment [1]. This behaviour is coupled with high strength and excellent ductility, making these grades natural candidate materials for marine applications.The purpose of the present study is to assess the microstructural stability and the associated effects on the mechanical properties of two novel superaustenitic grades (254SMO and 654SMO) during ageing experiments performed within the temperature region of 650-950 o C for various periods of time, ranging from 5mins to 240h. The microstructures obtained were characterised via SEM microscopy coupled with EDS microanalysis, XRD phase identification and hardness testing. Due to the complexity of the possible parallel precipitation reactions occurring, the use of MT-DATA (thermodynamic modelling software) has been made to facilitate the prediction of the presence of expected precipitate species, under equilibrium conditions. These thermodynamic predictions for the major phases and the minor phases expected to be present, as a function of the ageing temperature applied (under equilibrium conditions) are presented in figures 1 and 2, for one of the superaustenitic grade studied (254 SMO).It has been confirmed that the novel superaustenitic grades are prone to complex phase transformations during the ageing experiment performed. For ageing at 750 o C and 850 o C the formation of χ and σ phases took place for samples that were aged between 24h and 240h. As a result of these complex phase transformations within austenite, significant hardening of the austenitic matrix is confirmed throughout the ageing temperature range (650-950 o C) employed. Most interestingly, although the microstructures studied by SEM for samples aged at 750 and 850 o C appear similar in terms of the obtained constituents (Figs 3-6), careful SEM imaging coupled with XRD phase characterisation identified the presence of variable amounts of precipitate volume fractions and species forming on the two superaustenitic grades studied. The results confirmed that for short ageing experiments the microstructural stability of the superaustenitic grades studied was confirmed at all ageing temperatures studied. However, prolonged ageing exposure results into formation of σ and χ phases (in variable amounts for each grade studied) within the temperature region of 750-950 o C, with consequential effects on the mechanical and corrosion characteristics of the superaustenitic grades studied.
This paper will present corrosion management of a wet sour gas carbon steel export pipeline using continuous and batch corrosion inhibitors with mono-ethlene-glycol (MEG) as hydrate mitigation strategy in NACE MR 0175/ ISO 15156 region 3 (severe sour). The wet sour gas carbon steel export pipeline corrosion management via continuous CI and batch inhibitors with closed loop MEG regeneration system is rare worldwide. This is especially challenging when the case study may potentially be the longest wet sour gas, large diameter carbon steel pipeline (approximately 207km × 32 inch) in the world thus far. Pipeline corrosion management and hydrate management aspects when being reviewed holistically, it could provide significant cost savings yet safeguarding the overall technical integrity of the pipeline. The overall corrosion management leverages on Shell's many years of JIP and operating experience in sour service including the pipeline material specification, corrosion management, inspection, and maintenance philosophy. Reliable correlation between reservoir properties and uncertainties severe sour service, flow assurance, chemical behavirous, operating experiences etc were considered to best represent the operating envelope for this wet sour gas carbon steel pipeline. This includes the testing and selection of continuous CI and batch inhibitor, corrosion monitoring, operational pigging, maintenance, and inspection requirements throughout the field life.
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