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...
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