Papers by Keyword: Ferritic Stainless Steel (FSS)

Abstract: Three groups of 0.04C-16Cr ferritic stainless steels, with boron quality score of 0ppm, 16ppm and 26ppm, were smelted by a vacuum induction furnace. The chemical soak method, the tafel polarization curve method and the anode circular polarization curve method were used to research their general corrosion and intergranular corrosion resistance. The results showed that both general corrosion and intergranular corrosion of the casted 0.04C-16Cr ferritic stainless steels happened in dilute H2SO4 medium. The resuluts of electrochemical and chemical soak indicate that the addition of B improves intergranular corrosion resistance of 0.04C-16Cr stainless steel, moreover, the intergranular corrosion resistance increases with an increasing content of B. The addtion of B makes the general corrosion rate and self-corrosion current density of the 0.04 C-16Cr stainless steel reduced at active dissolution region in dilute sulphuric acid medium, so the general corrosion resistance of the 0.04 C-16Cr stainless steel is improved. The blunt-dimensional current density of the steel with 26ppm B is the lowest and the passivation zone of it is wider, so the stability of passive film and protection capacity of the steel with 26ppm B are the best. The electrochemical characteristics of general corrosion coincided with the results of soak corrosion.

Abstract: Ferritic stainless steel is an economical stainless steel which doesn’t contain nickel or a little nickel. It has the broad prospects of application. By using 21% Cr ferritic stainless steel as test material, the paper analyses the microstructure of the joint. The different welding parameters have the different effect on the depth, the solidification form and the microstructure.

Abstract: In the current context of fossil energy scarcity, car manufacturers have to optimize vehicles energy efficiency. This and continuous improvement includes a change of the exhaust manifold design. Usually in cast iron, exhaust manifolds tend to be mechanically welded in order to fit new constraints such as lightness, durability, efficiency and small size. To achieve such requirements, ferritic stainless steels with high chromium content (19%) and molybdenum (2%) are developed. For the welding, the use of existing filler wire does not satisfy fully the application requirements. This leads to oxidation problems and / or thermal fatigue strength that drastically reduces assembly lifetime. New flux cored wires are developed in the context of this study in order to provide molten zone characteristics close to those of the base metal. Different chemical compositions are tested in order to highlight the influence of stabilizing element on microstructure. Welding tests revealed the major influence of titanium on the grain refinement in the molten zone. A minimum Ti content of 0.45 weight % in the filler wire is required to be efficient as grain refiner.

Abstract: The effect of alloying elements and rough rolling condition on the microstructure evolution of ferritic stainless steel has been investigated in order to understand the recrystallization and precipitation behaviour during hot rolling. In the present study, a series of high temperature compression tests with plane strain deformation mode were conducted for Nb added and Ti+Nb added ferritic stainless steels. Compression tests then were subjected to various conditions of deformation temperature, reduction ratio and holding time. After the tests, EBSD mapping and SEM observation were performed to analyze the recrystallization and precipitation behaviour. Nb added and Ti + Nb added steels show an increasing tendency of recrystallization with an elevation of deformation temperature, holding time and reduction ratio. An increase of holding temperature and holding time enlarges the recrystallized regime due to a decrease of activation energy for recrystallization and a growth of recrystallized grain. A higher reduction ratio also increases the recrystallized regime due to a rise of stored energy for recrystallization. Nb added steel, however, is more resistant to recrystallization because most of Nb (C, N) particles in Nb added steel are finely dispersed in the matrix.

Abstract: 0.04C-16Cr and 0.04C-16Cr containing 26ppm B ferritic stainless steels were smelted by using a vacuum induction furnace. Then 65% boiling nitric acid method and the electrochemical potentiokinetic reactivation method (EPR) were used to research their intergranular corrosion sensitivity. Meanwhile, electrochemical test method (Tafel polarization curve method, the anode circular polarization curve method) and chemical immersion method were used to research their pitting corrosion resistance. The results showed that the corrosion rate of 0.04 C-16Cr stainless steel containing 26ppm B in 65% boiling nitric acid is lower than that of 0.04 C-16Cr stainless steel. The reactivation rate of 0.04C-16Cr containing 26ppm B in dilute sulphuric acid medium significantly reduced compared with that of 0.04C-16Cr. The pitting corrosion potential, self-corrosion potential and the value of (Eb-Ep) of the stainless steel containing with 26ppm B in chlorideions medium reduced, while the corrosion rate increased compared with the stainless steel without B addition. It indicates that trace boron addition improves the intergranular corrosion resistance and repair ability of the passive film of the 0.04C-16Cr ferritic stainless steel after pitting corrosion process in chloride ions medium, but it also promotes the pitting corrosion tendency of the steel. Besides, introduction B to 0.04C-16Cr ferritic stainless steel reduces the steel’s corrosion resistance in active dissolved zone and promotes its intergranular corrosion tendency in chloride ions medium. The electrochemical characteristics of local corrosion are consistent with the results of chemical immersion test.

Abstract: In this study the precipitation of silicon in Type 441 steel (18%Cr-0.4%Nb-0.5%Si) was investigated and its influence on strength properties were determined. To simulate high-temperature service conditions, heat treatments with various ageing times up to 120 hours and temperatures up to 800 °C were performed. Following the aging treatments, micro-and macro-hardness and tensile properties were measured. Microstructure and precipitation were analyzed using scanning electron microscopy and energy-dispersive spectroscopy. Predictions for equilibrium pericipitation were calculated using the Factsage software. According to observations, coarse titanium nitrides (TiN) and niobium carbides (NbC, Fe₃Nb₃C) were present in all specimens including non-aged ones. These precipitates did not coarsen during ageing, which implies that their growth already occurred in the sheet production process. However, silicon started to precipitate in the course of prolonged annealing. Si contributed to the formation of a secondary phase resembling the Laves-phase (FeNbSi) on grain boundaries. Hardness and yield strength were found to decrease with prolonged ageing at high temperatures. Factors affecting the silicon precipitation are discussed.

Abstract: Ferritic stainless steels have nowadays been used as materials for interconnectors in solid oxide fuel cells (SOFCs) at intermediate temperatures (800°C). Their degradation in contact with dry synthetic biogas used instead of other fuel gas has already been studied. In such biogas atmosphere, humidity may play an important role. The objective of this study is therefore to understand the effect of H₂O on the corrosion kinetics of the ferritic stainless steels type AISI441 (18CrTiNb) under synthetic biogas (70%CH₄ and 30%CO₂) mixed with 3%H₂O. The thermodynamic analysis by FactSage was used to determine the partial pressure of oxygen and the activity of carbon in the humid biogas. The results showed that the partial pressure of oxygen is in the range 10^–24.8 to 10^–21.2 bar for temperatures between 600-800°C and that the formation of solid carbon can occur in these conditions. This was not different compared with the conditions in dry biogas. These conditions lead to the stability of some important oxides such as Cr₂O₃ and Cr-Mn spinel and to carbon deposition and/or carbide formation. The surface morphology of 441 subjected to humid biogas showed oxide scale composed mainly of Cr₂O₃ topped with Cr-Mn spinel. Some carbide such as Cr₇C₃ was found in chromia scale. Kinetic experiments under both dry and humid biogas at temperatures between 600 and 800°C showed linear weight changes. Arrhenius law was followed and the rate-determining steps were identified as parallel oxidation and carburization limited by oxide-gas interface reactions.

Abstract: The present paper is a review on the oxidation kinetics, electrical properties, chromium vaporization rate and microstructure investigations of oxide products formed on an uncoated Crofer 22 APU alloy and 1.4762 steel and coated by means of pulsed laser deposition and screen-printing methods with films of (La,Sr)CoO₃, (La,Ca)CrO₃, (La,Sr)CrO₃, (La,Sr)(Co,Fe)O₃ and MnCo₂O₄ in air and Ar-H₂-H₂O and Ar-CH₄-H₂O gas mixtures at 1023-1173 K for up to 1200 hrs. Microstructure investigations using SEM-EDS showed the influence of the reaction products formed in different atmospheres at the steel/coating interface on the electrical properties and Cr vaporization rate of these composite materials, which are used for construction of SOFC interconnects.

Abstract: Isothermal oxidation behavior of the AISI430 stainless steel was investigated at 900°C and 950°C in air. Isothermal themogravimettric analyses were performed at high-temperature for 360ks (kilo-seconds). The microstructures of the oxide films on the stainless steel were characterized by SEM and chemical analyses were performed by EDS and X-ray diffraction. The oxide film included outer layer and inner one. The outer layer was magnetite and hematite oxides with no significant amounts of chromium and the inner one was formed by iron and chromium spinel. Significantly accelerated and anomalous oxidation was observed with the stainless steel AISI430 in air at 900°C.

Abstract: 21% Cr with Ti-Nb dual stabilized ferritic stainless was welded using Nd: YAG laser. The relationship between microstructure and parameters of laser welding was examined. The microstructure was investigated by using the optical microscope and scanning electron microscopes. The average grain size of the HAZ was increased with increasing heat input due to the slow cooling rate. Large precipitates as TiN, TiC and Nb(C,N) were dissolved in the HAZ. Fine precipitates which supposed to be TiC was formed uniformly distributed in the case of the fusion zones.